Mikuni Rebuild Schedule?

Engine, Transmission and related drivetrain.
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defdes
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Mikuni Rebuild Schedule?

Post by defdes » 18 May 2018 12:51

I have been chasing my tail on what I initially (2 years ago) thought was a jetting problem but now think it is a rebuild issue. My problem is an off throttle bog and popping at idle at low rpms. I have cleaned and maintained my 44's, but have never fully rebuilt them in 10 years+ with the exception of new floats. The car sits more than it's driven, so I would imagine this only exaggerates the problem. If you have them, what is your maintenance/rebuild schedule?

*EDIT*
I am about ready to pull the 510 out after winter storage and need to address what I think is this fuel issue.
Before I put it away, the car sat for about a month. Upon firing it up it cranked and cranked, and after a long while, finally would catch and turn over, but I'd be pumping the throttle like a mad man to try and keep it going. It would just bog out. I pulled the air and fuel jets out, blasted them with carb cleaner and compressed air. I restarted, but the problem didn't really clear up until, until I pumped and pumped the throttle some more then SLOWLY it began to clear out and be able to idle under it's own. I expected to see a giant mouse nest coming out of the tail pipe as it kind of had that type of obstructed sound.
Any ideas on clearing this up? as I said in the original post above, the carbs have never been fully rebuilt and are likely due, but I just don't know if that is the cause of the problem (or if it was, where in the carbs that type of obstruction it would be).

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defdes
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Re: Mikuni Rebuild Schedule?

Post by defdes » 15 Apr 2019 14:28

BUMP, see the above edit.

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duke
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Re: Mikuni Rebuild Schedule?

Post by duke » 16 Apr 2019 15:51

If nothing is leaking, I'm not sure what the purpose of a full rebuild would be. A good cleaning of the jets may be needed, but I don't think a full rebuild would be necessary. Do you have a wideband O2 sensor? I find mine invaluable in figuring out what the carbs are doing and what they need. Also, make sure that ignition system is fully fresh, cause you know the old adage "90% of fuel problems are ignition." (or something like that)
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defdes
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Re: Mikuni Rebuild Schedule?

Post by defdes » 16 Apr 2019 17:07

Thanks Duke, I have a crank fire Electromotive ignition, and unless it's the plug wires (haven't replaced those in about 10 years) I don't know what else it could be. The way it ultimately cleared itself, lead me to think it was fuel.....that or an exhaust obstruction which given the number of critters around here still cant be counted out. But it could be a confluence of things. I don't have a WB O2, but I can see one in my future. I am really wanting to swap over to some Jenvey ITB's, but that is not going to happen this year.
I might just pull them off and give them a good soak and clean, there could be 15 years of varnish in them.

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Byron510
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Re: Mikuni Rebuild Schedule?

Post by Byron510 » 16 Apr 2019 19:21

You may have mentioned a contributing factor - you say it doesn’t move much. Does that imply that it sits for long periods of time? If so, do you cut he pump and drain the fuel by idling the car down?

If not, then fuel evaporating out of that carbs leaving residue behind certainly can be a problem. And the bigger problem is that once dried up, the deposits of don’t get washed down with fresh fuel, and those dried particals can block fuel and air passages, and obstruct proper fuel flow through jets and around needles. Certainly that could cause problems.

To a methodical mind like yours. A carb rebuild is really just a disassembly and very good cleaning.
New gaskets are likely nessesary. It may be your solution.

If it’s possibly to totally drain the fuel before long periods of sitting, it’s a very good thing.

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defdes
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Re: Mikuni Rebuild Schedule?

Post by defdes » 17 Apr 2019 17:39

Yes, the car can sit and did last summer, more than it was driven. I have some rocker rust and faulty horn that didn't allow it to get inspected. I know about the fuel drain, but I don't do it every time and didn't before putting it away this last time either. I am going to get it out this weekend assuming it's warm and dry, then I'll re-access. I think pulling them and a thorough soak and cleaning is in order. What would be the best solution to soak in to break down the varnish in the body passageways?

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Re: Mikuni Rebuild Schedule?

Post by cartel » 17 Apr 2019 20:51

where is best place to get full gasket kits with acc pump? i have 2 that need to be gone thru as well
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Re: Mikuni Rebuild Schedule?

Post by Three B's Racing » 19 Apr 2019 04:50

You can get rebuild kits and parts from Futofab.com. defdes you really should cup your exhaust tail pipe to keep them critters out, I do because one year I start the 510 and it blew mommy and 4 lil ones along with their nest out the back. Alot of engine shops will dip the carb bodies for you or you could buy a gallon of this stuff and do it yourself. I used to back in the day and it works really well, then you wash in water and take compressed air to it and is the reason I went SR, as I got tired of messing with carbs plus I got lazy and wanted easly more NA power and the easibility of FI. For going on 14 years now my S14 SR has never let me down and starts first time every time.
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Re: Mikuni Rebuild Schedule?

Post by JordanTr » 19 Apr 2019 05:37

+1 for binning the carbs in favor of reliability.
Jordan | '72 2 door KA project | '94 240sx RB26DETT | '97 Silvia RB25DET | '90 Audi 90 Quattro 20V (DD)

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defdes
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Re: Mikuni Rebuild Schedule?

Post by defdes » 19 Apr 2019 05:57

When I seasonally park it, I do seal the tailpipe from moisture and rodentia but once it's out I don't.
I hear you on the FI set up, after body work and a respray, it's first on the list.

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Re: Mikuni Rebuild Schedule?

Post by bertvorgon » 19 Apr 2019 08:18

I totally concur with taping up the tail pipe if car is to sit for any length of time. It is amazing how much moisture is in the air, no matter how dry you think your shop might be.

I have watched Andy at Specialty over the years, as he has tried to keep the race motors rust free over a winter, in his heated shops. The fine oxidation that has showed up on the valve seats and cylinders, even when oiled down, is just crazy.

Leaving pump fuel to sit is horrible too. Even with my Avgas, which leaves no residue, I make sure my gas tank is filled to the brim when I park for the winter, this way the tank cannot rust.

Then, when time to fire up, I pop valve cover off and drizzle some oil over the camshaft and down the chain.
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Re: Mikuni Rebuild Schedule?

Post by Tedman » 20 Apr 2019 21:27

Mikuni rebuild kits and parts are available from Upgrade Motoring in Granada Hills, CA:
https://www.upgrademotoring.com/index2.htm

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