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MGB Troubleshooting Mikune carb

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drooartz

drooartz

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One of my good friends has a MGB LE (79, 80?) running a Mikune carb. The car had this carb on it when he bought the car a number of years ago. It has run well in past, but this summer he's seeing an issue that has cropped up before.

The car starts up and idles fine, but drives badly and makes no power. I drove it briefly, and it stutters badly.

In the past he did have a local shop fiddle with the carb, and they got it to run again for they year -- but now it's back and he's trying to find a permanent fix.

I had him pull the plugs to take a look, and they are below in order from front to back. I'd love some help here. I don't know the Mikune at all (It looks a bunch like a Weber, which I also don't know). To me, this is saying that one side of the carb has an issue of some sort, maybe running too rich on that side.

He did drive the carb with the air filter off and said it improved maybe a tiny bit not not that much.

Thoughts? Ideas?

Plugs:

Plug1.jpg


Plug2.jpg


Plug3.jpg


Plug4.jpg


Carb:
carb1.jpg


carb2.jpg
 
Drew - my totally brain dead guess ... if the carb is adjusted correctly (and that's a *big IF* ), one side of the intake manifold is partially blocked. Two plugs look perfect, two plugs way too rich.

Do you have a manual or guide for that carburetor?

OK - back to my cave.
Tom M.
 
NutmegCT said:
Drew - my totally brain dead guess ... if the carb is adjusted correctly (and that's a *big IF* ), one side of the intake manifold is partially blocked. Two plugs look perfect, two plugs way too rich.

Do you have a manual or guide for that carburetor?

OK - back to my cave.
Tom M.
Click to expand...

I'm with Tom on this one.

Also worth asking what kind of gas he uses. If regular ethanol could be gumming up the carb.
 
Air corrector jet on the front barrel may be restricted with Ethanol "snot", the accelerator pump check-ball could be stuck in the open position, and/or the fuel is leaking past the choke's check-ball on the front barrel.

Those are about the only things I would think would cause the rich condition in the front two cylinders only.
 
Grantura_MKI said:
Looks to me that all of the vacuum is being taken from the number 1&2 cylinders....NOT an ideal situation.
Click to expand...


True.

Addition of a "balance tube" could be helpful. See the BMC "Special Tuning Manual" drawings for a Weber manifold to get an idea.
 
:thumbsup:

Another thought is that the "disk" for the "choke" enrichment device has some schmaze under it and allows fuel to sneak past it on throttle application.
 
Thanks for the thoughts. Gives me some things to chew on.

To answer questions, no I didn't have a manual -- but do now thanks to Tom. :D

There is a soft pipe running between the two legs of the intake, on the top. You can see it in the first pic of the carb in my first post. I'm assuming that's a balance tube. I had wondered when I first looked at it, but I don't have any experience with this sort of carb. Really only dealt with SU carbs.

So it sounds like if we want to keep this carb on the car, it's a disassemble and clean sorta job, focused on the areas y'all mentioned.
 
Balance tubes need to be hard lines on all four intake runners, as rubber will collapse on heavy vacuum.
Would fit a 40 Weber and be done.
Your problem will not be sorted fully with SUs, as this is a later lump.
I don’t see an anti back fire valve on the booster hose?
 
What would I do? Well, way back in the dark ages, I bought a white Mk2 MG TD that some PO had fitted with a gigantic Ford Holley carburetor. It was loading the engine with fuel, giving me 15 mpg and I'm sorry - an LBC needs SUs or Strombolis, er Strombergs. (Well maybe a pair of Webers). I headed out to Long Island, bought a pair of inch and a half SUs, threw out the Holley and that was it. It also had an engine-turned dashboard that looked great but gave off a terrible glare when the sun hit it. I'd love to know if that one is still running.
 
Jay here. This thread is for my 1980 MGB. I run non-ethanol gas, so that's not a contributor. Thanks for the input and keep it coming!
 
I'm not familiar with the Mikuni, but I've always thought it was pretty much a Weber DCOE clone, so I'll base my comments off of that.

Balance tube isn't the issue. Plenty of people use hoses without issue. Even if it were to collapse it wouldn't cause this issue....it would just be like NOT running a balance tube, and would mostly affect idle quality. I don't have a balance tube with my DCOE....it's somewhere on the "to do" list :smile:

Assuming the "DCOE clone" is correct, it's really like two separate carbs sharing a single float bowl. Almost nothing else is shared apart from the throttle shaft and idle speed screw.

If the carb, like the Weber, has an idle mixture screw for each side I'm assuming you've checked to be sure they are both the same number of turns off the seat. IMHO, DrEntropy is on track in post #5. Since the car previously ran fine there is really nothing to go out of adjustment. Something is making the front half of the carb too rich. Something clogging the air corrector on that side would certainly do that. If, like the DCOE, the jet stacks are easily swapped from side to side you could do that and see if the problem moved to the other pair of cylinders. That would isolate the issue to being in that stack, as opposed to elsewhere in the carb. You may even be able to spot the issue with a really close look at each stack. Might be fixed with blowing out the offending air corrector.
 
I've only seen one of the Mikuni side-drafts fitted to a British engine, didn't have to do anything to it, thought it a weird copy of a Weber DCOE.

Grantura_MKI said:
Would fit a 40 Weber and be done.
Click to expand...

Would be my assessment as well, but I think the 'money gremlin' has a say in that. Decades ago we fabricated a manifold using the aforementioned BMC "Special Tuning Manual" as the template. That rig has been on three subsequent MGB's and now resides on this current one. A 40 DCOE is a "set it and forget it" unit. But as money is likely an issue, I'd be for benching the Mikuni and going through it thoroughly.
 
Thanks for the links, Tom. Hoping to have another look at the carb this week.
 
Pulled the carb this morning, it is a 44: N44PHH-410

Jay is coming over today and we'll be diving in.
 
drooartz said:
Pulled the carb this morning, it is a 44: N44PHH-410

Jay is coming over today and we'll be diving in.
Click to expand...

:encouragement:

Great - hopefully the hybridz and Mikuni sites will be helpful.
Tom M.
 
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