Mikuni Trouble: Bowl Vent Diaphragm

Hello,

I just bought my first Ram 50 a month ago. It's a 1988 with a 5-speed and the 2.0 L. It's also got the stock Mikuni carb.

It was running fine, and then not--it really started to sound lousy, running rough and hesitating. After a lot of research, I had the problem about 80% diagnosed as a bad bowl vent diaphragm. After DAYS of searching, I was finally able to locate one of these diaphragms over the internet. It's a little rubber ring on about a 3" shaft, and "pops" out of the carb sort of like the axle on a front-wheel-drive car. Anyway...

The new part arrived today, but when I tried installing it, I encountered resistance. Turns out the gasket on the receiving end (DEEP in the carb!) had become unseated, and was rattling around loose--thus, it wasn't grabbing the shaft like it should, and me forcing the shaft in was only making things worse.

It seems like my only options now are to A) take the carb completely apart, hoping to re-seat the gasket, or B) take it to a shop and throw money around. I hate to do the latter, but I've never opened a carb before and I'm not sure I'd be able to get it back together. Help!

Thanks,
Adam

Adam, you live in Washington as myself does. If your going to throw money, Throw a 32/36 weber on it. You have no more smog checks on your truck and we are exempt. Enjoy some extra power and a better carb. Those Mikuni carbs are pretty much garbage once they go bad. The weber will also relieve you some of the other smog stuff.


Welcome to the Forum also. :)

Thanks for the welcome. I've thought of going the Weber route, but hate to throw money, as you say. I've also heard (on this forum) that buying a Weber also means buying a fuel pump, regulator, etc., which adds up. I just need to figure out if the cost of a Weber is a better return that the cost of a Mikuni repair, which could be anything.

Welcome to the site AdamW. If you've never pulled apart a carby before my recommendation would be to source a reasonably good second hand unit, get a manual and pull it down on a nice open bench that won't get disturbed in a hurry. It's too risky to take apart the only carby you've got and end up getting stranded. The Mikuni's are not the most forgiving thing to disassemble but if you pay attention and ask for advice, you'll be able to work through it. They are nicer to work on than a Nikkei or a Hitachi carby (both of which have some nasty design issues) but they are complex. Oh, and get your hands on a rebuild kit too - there's no point in taking one of these things apart and trying to re-use junk gaskets, seals and diaphragms. It'll grief you no end... Good luck.

I appreciate the advice. I actually managed to install the new bowl-vent diaphragm, which was nice, and when I tested it all seemed well. (Disconnect vacuum hose running from vapor canister to carb, at the canister, and blow through it--if you hear air running through to the carb, the diaphragm is shot. I heard air with the old part, but not with the new one!)
So I was very surprised when that didn't do anything to fix the problem--the truck still runs poorly, with much hesitation and vibration throughout the drivetrain. The shift knob and cab shake so much I'd be sure it was a loose mount somewhere, if not for the poor performance. Though the engine at idle doesn't sound too bad--only when driving does it really bog down. Maybe it could be as simple as a bad fuel filter or something along the line. I guess I'm back at square one!

If you have the g63b motor, it could be that the timing belt skipped a tooth or lost one or more. Check out the filters, all the vacuum lines, especially the ones going into the metal box on the drivers side inner fender. A vac leak will cause that poor performance and shaking.

So, I did the "blow into the bowl vent hose" test, which instantly killed a running engine, so that seemed to confirm a bowl vent issue for my recent emissions woes.

Got the diaphragm part today (Tomco 7404, mis-listed as a "choke pull off") and went to install, but the old diaphragm appeared intact when I removed it. There was a bunch of crusty deposits around the outside face of the diaphragm and around the carb face it seals against, so I thought maybe that could be breaking the seal. I cleaned those deposits off the carb and was gonna install the new diaphragm anyway, but the o-ring midway along the old diaphragm shaft started to tear when I tried removing it to transfer to the new one. It's not a typical o-ring, more cylindrical than donut-shaped, with sorta ridges on the outside.

Could I replace that with a regular o-ring (maybe doubled up in series?), or is there some known replacement part for it, or is it something only available in a full carb-rebuild kit? :scratchheadyellow:

Unfortunately, this isn't good. The diaphragm shaft should have come out cleanly, but what you got is the center of a rubber disc that is the vent valve. Here's a diagram of the bowl vent system. The circled part is the rubber valve:

Attachment 21928

The center of the rubber valve tore out because it was brittle or degraded with age. To replace it now requires removing the carb and it's top portion. But if you're going that far into it, and given the amount of internal crud seen in the pictures, it may be wisest to replace the carb. The Mikuni is very difficult to rebuild successfully, and rebuild kits don't include the rubber valve, which I haven't seen available in decades.

Most site members recommend going to a Weber which is about $300 new. A rebuilt Mikuni is another $150, at least. If those are out of the question, I have a good used Mikuni that might work, but I need to know if your truck has an automatic or manual trans.

A carb in that condition needs a full tear down and rebuild. If it hasn't been taken apart in 15 years, it's due for it. FSM has offered alternatives but IDK if AZ emissions would be happy with a Weber retrofit on your truck. Parts for the Mikuni are still around on ebay but you have to play detective to get what you're looking for (these carbs get stuck to all kinds of different platforms so you'll need to track a part number down and work from there) Rebuild kits are not complete and parts like choke pull off diaphragms, bowl vents etc have to ordered separately.

Uff da. Looks like I'll be doing the Weber conversion I was planning on anyway, just sooner than anticipated. Fortunately, AZ only sniffs for HC and CO, and only inspects for gas cap, cat, and secondary air injection, so NOx, EGR and carb type are not a concern.

Meanwhile, I need to pass emissions now to renew my past-due registration. Reckon I could just reinstall the diaphragm (for sealing purposes only) and cap-off the external vent barb, so the bowl only ever vents internally? That should solve the rich-running problem due to a malfunctioning external bowl-vent valve, no?

Quote:

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Originally Posted by SubGothius

I could just reinstall the diaphragm (for sealing purposes only) and cap-off the external vent barb, so the bowl only ever vents internally? That should solve the rich-running problem due to a malfunctioning external bowl-vent valve, no?

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You can cap or plug the barb, but if they do a visual inspection, make it look stock. If the rich condition is due to the failed vent valve, resorting to internal venting only should lean it out a little. It might make hot starts more difficult, but as a simple, short-term fix, it's worth a try.