help with ignition timing L18
help with ignition timing L18
I just finished putting an L18 together for a friend of mine. Got it running good enough to break the motor in but the ignition timing is throwing me through a loop. it doesn't want to stay running unless it has about 30* advanced and it still feels retarded at that point. It seams to want in excess of 40* advance to achieve the highest amount of vacuum. Is this normal for my combination?
the head and short block have been completely rebuilt.
The head is a closed chambered A87 fitted with what appears to be a L20 cam. the machine shop we bought it from said that it was custom ground but I kinda doubt it.
I converted over to a 1976 HEI unit with a GM Module.
The timing chain, gears, tensioner and guides are also new. The valve timing was dead on perfect with the corresponding marks on the cam gear, sprocket and timing chain marks. I used the # 1 hole on the cam gear.
It retains the stock manifold and exhaust.
we are running the stock intake with a Weber that says 4815 on the side. not sure if thats the carb # or not.
I think that my problem may actually be coming from the carb dumping too much fuel and the advance in ignition timing is evening it out, although this is just an assumption. Any thoughts or ideas as to whats going on?
the head and short block have been completely rebuilt.
The head is a closed chambered A87 fitted with what appears to be a L20 cam. the machine shop we bought it from said that it was custom ground but I kinda doubt it.
I converted over to a 1976 HEI unit with a GM Module.
The timing chain, gears, tensioner and guides are also new. The valve timing was dead on perfect with the corresponding marks on the cam gear, sprocket and timing chain marks. I used the # 1 hole on the cam gear.
It retains the stock manifold and exhaust.
we are running the stock intake with a Weber that says 4815 on the side. not sure if thats the carb # or not.
I think that my problem may actually be coming from the carb dumping too much fuel and the advance in ignition timing is evening it out, although this is just an assumption. Any thoughts or ideas as to whats going on?
Re: help with ignition timing L18
This has a lot to do with what distributor you are using, and how much mechanical advance it has.
Also - you should be checking the timing at idle with the vacuum advance disconnected. I would start at 10 deg. BTDC (stock 510 setting) and go from there. My L-18 likes more advance at idle, so I shortened up the slots in the advance mechanism to allow me to run about 18 deg BTDC at idle, and still maintain about 36 deg total advance.
Also - you should be checking the timing at idle with the vacuum advance disconnected. I would start at 10 deg. BTDC (stock 510 setting) and go from there. My L-18 likes more advance at idle, so I shortened up the slots in the advance mechanism to allow me to run about 18 deg BTDC at idle, and still maintain about 36 deg total advance.
Finished is better than perfect......
Re: help with ignition timing L18
When I built my race motor, I was getting about 35 degrees at idle and couldn't for the life of me get the car to run w/out that much advance (wouldn't start). I had to put in different springs to change the curve (Mallory diz).
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Re: help with ignition timing L18
Check, but there should be no vacuum signal at idle from the carb.
With cap off, suck on the vacuum line and observe that the distributor moves clock-wise and returns only when you release it.
If the vacuum/mechanical advance is sticking or there is vacuum from the carb, the distributor won't return to it's idle timing of Zero. Lets say it sticks at 20 degrees mechanical advance. When you set the timing with the timing light it appears slightly advanced so you retard it to the proper 10? degrees. Now when the motor revs up nothing happens until the mechanical advance goes above 20 degrees. If it has a total of say 35 mechanical degrees advance it will top out at 15 because 20 were removed to set the base timing with the light.
With cap off, suck on the vacuum line and observe that the distributor moves clock-wise and returns only when you release it.
If the vacuum/mechanical advance is sticking or there is vacuum from the carb, the distributor won't return to it's idle timing of Zero. Lets say it sticks at 20 degrees mechanical advance. When you set the timing with the timing light it appears slightly advanced so you retard it to the proper 10? degrees. Now when the motor revs up nothing happens until the mechanical advance goes above 20 degrees. If it has a total of say 35 mechanical degrees advance it will top out at 15 because 20 were removed to set the base timing with the light.
"Nissan 'shit the bed' when they made these, plain and simple." McShagger510 on flattop SUs
Re: help with ignition timing L18
Thats exactly what this thing is doing. the only way I can get it started and keep it going is with that much advance. I adjusted the weber today and it seams within the adjustable tolerances that is set on the weber web site. Im beginning to think that the cam is just too big for the motor. Ive got very little power under 3500 rpms, but after that it starts to pull fairly decently untill it runs out of steam because of the smaller intake port runners . So my guess here is that I have a little of both going on. My friend doesnt want high rpm Power, he wants the torque down low being that this motor is in his 69 521 pickup. The distributor is a Cardone # 31-620 for 75-78 L20 engines. Could this be the problem with the mechanical advance weights since this is an L18.
Right now the initial timing is set at 35* advanced without the vacuum advance hooked up, and about 41* with it hooked up.
Right now the initial timing is set at 35* advanced without the vacuum advance hooked up, and about 41* with it hooked up.
defdes wrote:When I built my race motor, I was getting about 35 degrees at idle and couldn't for the life of me get the car to run w/out that much advance (wouldn't start). I had to put in different springs to change the curve (Mallory diz).
Re: help with ignition timing L18
There is vacuum at the carb port at idle, but Im setting the timing with the vacuum advance unplugged and the port caped off. I will have to check to see if the mechanical advance is sticking but I thought that I had already checked that. hmmm. This one is really got my brain going.
So with all said, could too big a cam be causing the timing to be so far off? The motor really wants to go I can feel it, something is just keeping it from doing so. I rechecked with my friend today and He confirmed that the the machine shop he bought the head from said it was indeed a custom ground cam. Now if thats what the cam is, then I suspects that the head was intended for a L20. If its a big cam for an L20 then I would imaging it would be way to big for an L18. This cam lopes very hard at a low idle speeds and has a vacuum signal at about 10 hgs.
So with all said, could too big a cam be causing the timing to be so far off? The motor really wants to go I can feel it, something is just keeping it from doing so. I rechecked with my friend today and He confirmed that the the machine shop he bought the head from said it was indeed a custom ground cam. Now if thats what the cam is, then I suspects that the head was intended for a L20. If its a big cam for an L20 then I would imaging it would be way to big for an L18. This cam lopes very hard at a low idle speeds and has a vacuum signal at about 10 hgs.
datzenmike wrote:Check, but there should be no vacuum signal at idle from the carb.
With cap off, suck on the vacuum line and observe that the distributor moves clock-wise and returns only when you release it.
If the vacuum/mechanical advance is sticking or there is vacuum from the carb, the distributor won't return to it's idle timing of Zero. Lets say it sticks at 20 degrees mechanical advance. When you set the timing with the timing light it appears slightly advanced so you retard it to the proper 10? degrees. Now when the motor revs up nothing happens until the mechanical advance goes above 20 degrees. If it has a total of say 35 mechanical degrees advance it will top out at 15 because 20 were removed to set the base timing with the light.
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Re: help with ignition timing L18
Vacuum advance is all over by 2k to 2.5 k maybe sooner. mechanical advance is starting to kick in at 1.5k and takes over till about 3K when it's 'all in' . Your cam choice has no effect on mechanical advance at all it may cause low manifold vacuum though.
"Nissan 'shit the bed' when they made these, plain and simple." McShagger510 on flattop SUs
Re: help with ignition timing L18
Ok. I was thinking that maybe it could have been ground it in such a way that would retard the timing even with #1 Cam sprocket hole being used. I know kinda unlikely but my mind has been racing.
Sounds like my issue is mainly in the advance curve of the dizzy then.
So I'll start buy checking to make sure the mechanical advance isnt stuck. If thats fine then what? I know that there is initial, mechanical, and vacuum timing going down in there. My question then is where do I start. just how much mechanical and vacuum advance am I looking for? How much total timing is too much? Oddly enough the motor doesnt knock at all, even with 40* of initial timing?
Sounds like my issue is mainly in the advance curve of the dizzy then.
So I'll start buy checking to make sure the mechanical advance isnt stuck. If thats fine then what? I know that there is initial, mechanical, and vacuum timing going down in there. My question then is where do I start. just how much mechanical and vacuum advance am I looking for? How much total timing is too much? Oddly enough the motor doesnt knock at all, even with 40* of initial timing?
datzenmike wrote:Vacuum advance is all over by 2k to 2.5 k maybe sooner. mechanical advance is starting to kick in at 1.5k and takes over till about 3K when it's 'all in' . Your cam choice has no effect on mechanical advance at all it may cause low manifold vacuum though.
Last edited by TranzAm on 23 Feb 2009 23:10, edited 2 times in total.
Re: help with ignition timing L18
So with the stock dizzy I have, can I recurve that as well? I noticed that you said you were using a mallory dizzy.
="defdes"]When I built my race motor, I was getting about 35 degrees at idle and couldn't for the life of me get the car to run w/out that much advance (wouldn't start). I had to put in different springs to change the curve (Mallory diz).
Last edited by TranzAm on 23 Feb 2009 23:09, edited 2 times in total.
Re: help with ignition timing L18
The distributor is a Cardone # 31-620 for 75-78 L20 engines. Is 36* total advance the max. At what rpms does your vacuum and mechanical advance take effect? also whats your engine combo? Anything like this one?
James wrote:This has a lot to do with what distributor you are using, and how much mechanical advance it has.
Also - you should be checking the timing at idle with the vacuum advance disconnected. I would start at 10 deg. BTDC (stock 510 setting) and go from there. My L-18 likes more advance at idle, so I shortened up the slots in the advance mechanism to allow me to run about 18 deg BTDC at idle, and still maintain about 36 deg total advance.
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Re: help with ignition timing L18
If regound cam Does it look like bigger lash pads in there?
I have ran a Schneider 460 lift cam on a Weber DGv and it was fine.
I have ran a Schneider 460 lift cam on a Weber DGv and it was fine.
Re: help with ignition timing L18
the cam looks a lot wider and maybe a bit taller than an L20 cam.
banzai510(hainz) wrote:If regound cam Does it look like bigger lash pads in there?
I have ran a Schneider 460 lift cam on a Weber DGv and it was fine.
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Re: help with ignition timing L18
Bare with me here:
Mechanical advance:
In order for the fuel and air to burn completely and push down on the piston at the correct time on the down swing after TDC the static timing is set at 10 to 12 degrees BTDC at idle. Lets assume full throttle on an engine starting at idle and revving up. Because it takes a finite amount of time to burn the fuel and air in a fully filled cylinder, there is less and less time to do so as the engine speeds up. Do counteract this, the ignition is advanced to start the burn sooner. The distributor uses a mechanical means to add advance to the ignition in direct relation to engine RPM. Zero advance at idle and slowly increasing to about 20 degrees (30 to 35 degrees total with the static timing added) at about 3,000 RPMs. It's there all the time, always the same amount at the same RPMs and is not affected by engine load.
Vacuum advance:
When the cylinder has very little air and fuel in it, (idle and steady cruise speed) even when compressed, the flame propagates slowly from molecule to molecule so lots of ignition advance is needed so that it is fully burned and pressure is at maximum at the proper time to push down on the piston after TDC. So, keep in mind that very little mixture needs lots of advance. Very little air gets past the throttle plate at idle so this would qualify as maximum advance possibly in the 20 to 25 degree range. Now imagine adding throttle (fuel and air). Manifold vacuum drops... advance drops to match it.
Up until the 60's, the vacuum signal to the vacuum advance was taken from below the throttle plate, thus it was very high at idle. Remember the above paragraph stating that a small amount of fuel air needs more time to burn and thus more advance? The static timing was always set with the vacuum line to the distributor disconnected. When connected the timing would jump to the twenty's or higher.
With the coming of tighter idle emissions it was found that the easiest way to reduce them was to lower ignition advance at idle. This was done by moving the vacuum port in the carb to above the throttle plate where it would be zero at idle. (well 10-12 degrees static timing actually) The static timing could be set without removing the hose because there was no vacuum to interfere. As the throttle plate opens, vacuum is felt at the port and sent to the distributor and it advances the ignition to compensate for the very little fuel and air in the cylinder. As more throttle is added there is less vacuum and less advance... more or less just like a 60s vacuum advance. The difference is the zero vacuum advance at idle and sluggish advance just off idle.
Full throttle at idle: Manifold vacuum is almost non existent.... there is no vacuum advance. This is fine because the cylinder is full of gas and air so no advance is needed with the shorter burn time. As RPMs climb the mechanical advance increases the ignition timing to allow full burning in the shortened time available.
Part throttle or cruise speed: Mechanical advance is always present and variable, depending on RPM between idle and 3,000 RPMs. Lets assume 3,000 cruise RPMs light throttle going down the highway. Mechanical advance is maxed out at 33 degrees (which is fine for full throttle) But we are at part throttle with some manifold vacuum and partially filled cylinders. The engine needs more advance... manifold vacuum provides vacuum advance at the distributor, perhaps 15 degrees. This is added to the mechanical and static advance, for a total of 48 to 50 degrees total advance. This gives the perfect amount of combined advance to assure complete combustion to give the maximum push on the piston after TDC. We give full throttle to pass a car... vacuum drops, vacuum advance drops and we are left with the mechanical advance of 33 degrees which is perfect for filled cylinders and 3,000 RPMs.
Coasting down hill very light throttle : Extremely high manifold vacuum, with very little cylinder filling. Total
advance, mechanical and vacuum can top 55 degrees. 33 degrees mechanical + static, plus 25 degrees vacuum.
Zero vacuum advance at idle is an emissions reducing thing only. Engines run cooler with more advance at idle because the idle mixture can be reduced yet there is more power from increased efficiency. When setting the timing, have you ever noticed that if you advance it above the factory setting the engine revs up??? This is because it is more efficient there than at 10-12 degrees where the idle speed and mixture are set to. Vacuum advance could be connected to manifold vacuum but the distributor advance curve is likely too high and would need re-curving.
A modified engine.. big cam, will have lower manifold vacuum which will throw all of this off. Low vacuum means less advance on an engine that already has big overlap, exhaust diluted, late closing intake valve, weak cylinder filling... which begs for more advance at low speed. Direct vacuum from the intake might make a big improvement here.
Mechanical advance:
In order for the fuel and air to burn completely and push down on the piston at the correct time on the down swing after TDC the static timing is set at 10 to 12 degrees BTDC at idle. Lets assume full throttle on an engine starting at idle and revving up. Because it takes a finite amount of time to burn the fuel and air in a fully filled cylinder, there is less and less time to do so as the engine speeds up. Do counteract this, the ignition is advanced to start the burn sooner. The distributor uses a mechanical means to add advance to the ignition in direct relation to engine RPM. Zero advance at idle and slowly increasing to about 20 degrees (30 to 35 degrees total with the static timing added) at about 3,000 RPMs. It's there all the time, always the same amount at the same RPMs and is not affected by engine load.
Vacuum advance:
When the cylinder has very little air and fuel in it, (idle and steady cruise speed) even when compressed, the flame propagates slowly from molecule to molecule so lots of ignition advance is needed so that it is fully burned and pressure is at maximum at the proper time to push down on the piston after TDC. So, keep in mind that very little mixture needs lots of advance. Very little air gets past the throttle plate at idle so this would qualify as maximum advance possibly in the 20 to 25 degree range. Now imagine adding throttle (fuel and air). Manifold vacuum drops... advance drops to match it.
Up until the 60's, the vacuum signal to the vacuum advance was taken from below the throttle plate, thus it was very high at idle. Remember the above paragraph stating that a small amount of fuel air needs more time to burn and thus more advance? The static timing was always set with the vacuum line to the distributor disconnected. When connected the timing would jump to the twenty's or higher.
With the coming of tighter idle emissions it was found that the easiest way to reduce them was to lower ignition advance at idle. This was done by moving the vacuum port in the carb to above the throttle plate where it would be zero at idle. (well 10-12 degrees static timing actually) The static timing could be set without removing the hose because there was no vacuum to interfere. As the throttle plate opens, vacuum is felt at the port and sent to the distributor and it advances the ignition to compensate for the very little fuel and air in the cylinder. As more throttle is added there is less vacuum and less advance... more or less just like a 60s vacuum advance. The difference is the zero vacuum advance at idle and sluggish advance just off idle.
Full throttle at idle: Manifold vacuum is almost non existent.... there is no vacuum advance. This is fine because the cylinder is full of gas and air so no advance is needed with the shorter burn time. As RPMs climb the mechanical advance increases the ignition timing to allow full burning in the shortened time available.
Part throttle or cruise speed: Mechanical advance is always present and variable, depending on RPM between idle and 3,000 RPMs. Lets assume 3,000 cruise RPMs light throttle going down the highway. Mechanical advance is maxed out at 33 degrees (which is fine for full throttle) But we are at part throttle with some manifold vacuum and partially filled cylinders. The engine needs more advance... manifold vacuum provides vacuum advance at the distributor, perhaps 15 degrees. This is added to the mechanical and static advance, for a total of 48 to 50 degrees total advance. This gives the perfect amount of combined advance to assure complete combustion to give the maximum push on the piston after TDC. We give full throttle to pass a car... vacuum drops, vacuum advance drops and we are left with the mechanical advance of 33 degrees which is perfect for filled cylinders and 3,000 RPMs.
Coasting down hill very light throttle : Extremely high manifold vacuum, with very little cylinder filling. Total
advance, mechanical and vacuum can top 55 degrees. 33 degrees mechanical + static, plus 25 degrees vacuum.
Zero vacuum advance at idle is an emissions reducing thing only. Engines run cooler with more advance at idle because the idle mixture can be reduced yet there is more power from increased efficiency. When setting the timing, have you ever noticed that if you advance it above the factory setting the engine revs up??? This is because it is more efficient there than at 10-12 degrees where the idle speed and mixture are set to. Vacuum advance could be connected to manifold vacuum but the distributor advance curve is likely too high and would need re-curving.
A modified engine.. big cam, will have lower manifold vacuum which will throw all of this off. Low vacuum means less advance on an engine that already has big overlap, exhaust diluted, late closing intake valve, weak cylinder filling... which begs for more advance at low speed. Direct vacuum from the intake might make a big improvement here.
"Nissan 'shit the bed' when they made these, plain and simple." McShagger510 on flattop SUs
Re: help with ignition timing L18
wow Mike! you really lay it all out there dont you? Thanks
ok so 33* of mechanical advance 25* of vacuum advance and an initial timing of 10-12. so thats a total of 70* of advance.
ok so 33* of mechanical advance 25* of vacuum advance and an initial timing of 10-12. so thats a total of 70* of advance.
datzenmike wrote:Bare with me here:
Mechanical advance:
In order for the fuel and air to burn completely and push down on the piston at the correct time on the down swing after TDC the static timing is set at 10 to 12 degrees BTDC at idle. Lets assume full throttle on an engine starting at idle and revving up. Because it takes a finite amount of time to burn the fuel and air in a fully filled cylinder, there is less and less time to do so as the engine speeds up. Do counteract this, the ignition is advanced to start the burn sooner. The distributor uses a mechanical means to add advance to the ignition in direct relation to engine RPM. Zero advance at idle and slowly increasing to about 20 degrees (30 to 35 degrees total with the static timing added) at about 3,000 RPMs. It's there all the time, always the same amount at the same RPMs and is not affected by engine load.
Vacuum advance:
When the cylinder has very little air and fuel in it, (idle and steady cruise speed) even when compressed, the flame propagates slowly from molecule to molecule so lots of ignition advance is needed so that it is fully burned and pressure is at maximum at the proper time to push down on the piston after TDC. So, keep in mind that very little mixture needs lots of advance. Very little air gets past the throttle plate at idle so this would qualify as maximum advance possibly in the 20 to 25 degree range. Now imagine adding throttle (fuel and air). Manifold vacuum drops... advance drops to match it.
Up until the 60's, the vacuum signal to the vacuum advance was taken from below the throttle plate, thus it was very high at idle. Remember the above paragraph stating that a small amount of fuel air needs more time to burn and thus more advance? The static timing was always set with the vacuum line to the distributor disconnected. When connected the timing would jump to the twenty's or higher.
With the coming of tighter idle emissions it was found that the easiest way to reduce them was to lower ignition advance at idle. This was done by moving the vacuum port in the carb to above the throttle plate where it would be zero at idle. (well 10-12 degrees static timing actually) The static timing could be set without removing the hose because there was no vacuum to interfere. As the throttle plate opens, vacuum is felt at the port and sent to the distributor and it advances the ignition to compensate for the very little fuel and air in the cylinder. As more throttle is added there is less vacuum and less advance... more or less just like a 60s vacuum advance. The difference is the zero vacuum advance at idle and sluggish advance just off idle.
Full throttle at idle: Manifold vacuum is almost non existent.... there is no vacuum advance. This is fine because the cylinder is full of gas and air so no advance is needed with the shorter burn time. As RPMs climb the mechanical advance increases the ignition timing to allow full burning in the shortened time available.
Part throttle or cruise speed: Mechanical advance is always present and variable, depending on RPM between idle and 3,000 RPMs. Lets assume 3,000 cruise RPMs light throttle going down the highway. Mechanical advance is maxed out at 33 degrees (which is fine for full throttle) But we are at part throttle with some manifold vacuum and partially filled cylinders. The engine needs more advance... manifold vacuum provides vacuum advance at the distributor, perhaps 15 degrees. This is added to the mechanical and static advance, for a total of 48 to 50 degrees total advance. This gives the perfect amount of combined advance to assure complete combustion to give the maximum push on the piston after TDC. We give full throttle to pass a car... vacuum drops, vacuum advance drops and we are left with the mechanical advance of 33 degrees which is perfect for filled cylinders and 3,000 RPMs.
Coasting down hill very light throttle : Extremely high manifold vacuum, with very little cylinder filling. Total
advance, mechanical and vacuum can top 55 degrees. 33 degrees mechanical + static, plus 25 degrees vacuum.
Zero vacuum advance at idle is an emissions reducing thing only. Engines run cooler with more advance at idle because the idle mixture can be reduced yet there is more power from increased efficiency. When setting the timing, have you ever noticed that if you advance it above the factory setting the engine revs up??? This is because it is more efficient there than at 10-12 degrees where the idle speed and mixture are set to. Vacuum advance could be connected to manifold vacuum but the distributor advance curve is likely too high and would need re-curving.
A modified engine.. big cam, will have lower manifold vacuum which will throw all of this off. Low vacuum means less advance on an engine that already has big overlap, exhaust diluted, late closing intake valve, weak cylinder filling... which begs for more advance at low speed. Direct vacuum from the intake might make a big improvement here.
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Re: help with ignition timing L18
Mechanical advance is about 20 plus the 10-12 static timing at idle for the L18 or L20B.
The only way to have both vacuum and mechanical advance at the same time is at cruising speed, (when all the mechanical advance is present) and with the throttle closed maybe coasting down hill, giving maximum manifold vacuum say 20-25 degrees. The two add together to over 50 degrees.
These numbers aren't absolute. All engines are different. The mechanical advance could be 18 or even 24 degrees. Vacuum advance could be more or less. This is just for argument's sake. Most engines do however, seem to like a total mechanical advance in the 33-35 degree range.
None of this applies to a race motor which seldom goes below 3,000 RPM and is at full throttle with very low intake vacuum anyway. A street car should always have vacuum advance even if it is the smog ported vacuum. Otherwise you only have the mechanical which is not load (throttle) sensitive. Using vacuum to vary the timing in relation to load, (throttle) increases the engine efficiency. You get better mileage and the car is much easier to drive.
The only way to have both vacuum and mechanical advance at the same time is at cruising speed, (when all the mechanical advance is present) and with the throttle closed maybe coasting down hill, giving maximum manifold vacuum say 20-25 degrees. The two add together to over 50 degrees.
These numbers aren't absolute. All engines are different. The mechanical advance could be 18 or even 24 degrees. Vacuum advance could be more or less. This is just for argument's sake. Most engines do however, seem to like a total mechanical advance in the 33-35 degree range.
None of this applies to a race motor which seldom goes below 3,000 RPM and is at full throttle with very low intake vacuum anyway. A street car should always have vacuum advance even if it is the smog ported vacuum. Otherwise you only have the mechanical which is not load (throttle) sensitive. Using vacuum to vary the timing in relation to load, (throttle) increases the engine efficiency. You get better mileage and the car is much easier to drive.
"Nissan 'shit the bed' when they made these, plain and simple." McShagger510 on flattop SUs