MGB Troubleshooting Mikune carb

[drooartz]

SUCCESS!!

Turned out to be pretty simple. I popped the carb off the car this morning so I could work on it on my bench. Using the links here in this thread and the factory manual I downloaded (thanks, Tom!) I was able to look things over and get familiar with where everything is.

Popped off the cover over the main and pilot jets and was presented with this:

The main jet assembly on the front side was loose and almost all the way backed out. At this point I guessed that was the issue, but once Jay arrived we pulled the main jet assemblies, pilot jets, and the pump nozzles out and clean them with some carb cleaner and compressed air to make sure they were all clear. Put it all back together and back on the car, and it runs again!

There's some fine tuning that could be done (I think it may be a tad rich, we are at 6500' elevation) but over all it seems to be working as before, and Jay thinks it drives now as it always has.

So thanks all for the advice and links, it was all super helpful. Prior to this I've only really messed with SU's, so getting into something like this was a new thing for me. Nice to learn some new things and increase my confidence a bit, and great to be able to help a friend get his B back in shape.

 

[NutmegCT]

 

[DrEntropy]

Good news! Congrats to you both.

And once you've been into a thing like that, it becomes more understandable and less a "box-o-tricks." I'd bet you could easily transition the knowledge to Weber DCOE carbs as well. Very similar in design, guessing the Mikuni is an adaptation of Weber's design.

 

[hightower]

A big thanks to Drew for helping me out with my MGB Mikuni carb! The car runs well and is back to its old self. Diving into a carb was intimidating, but now I'm interested to take more apart and learn more of the intricacies.

Also a big thanks to all of you on this forum. Thank you. I'll post photos out and about soon.

 

[hightower]

Mikuni Cooling Body Assembly--#85 in the screenshot, on underside of carb. This has 2 ports which look to accept vacuum hose (1/4" on inside of hose). What are these for? Are they a relief valve to dump fuel/air if pressures get too high? Should there be hose on each? Going where?

FYI, these 2 ports have never had hose on them in my 14 years of ownership. Came this way from PO in Salt Lake City. Any thoughts?

 

[hightower]

Underside of Mikuni Carb.

 

[Joe Reed]

My guess would be it's a provision for coolant hoses to attach...sort of like many of the Weber DGV manifolds that provide heat to the underside of the manifold directly beneath the carb. Are the two pipes connected....blowing through one does the air come out the other?

 

[DrEntropy]

My guess would be it's a provision for coolant hoses to attach...sort of like many of the Weber DGV manifolds that provide heat to the underside of the manifold directly beneath the carb. Are the two pipes connected....blowing through one does the air come out the other?

With ya, Joe. Closed circuit around the accelerator pump for coolant to circulate. Ignore them or block 'em off and press on.

 

[drooartz]

The main jet backed out again... any thoughts on how to keep it in place?

 

[JPSmit]

The main jet backed out again... any thoughts on how to keep it in place?

thread lock? the mild stuff

 

[DrEntropy]

The only chemical way would be green Locktite... BUT(!) both the jet and the carb body will need to be clean and dry, free of any contamination. With the jet all the way tight, back it out one turn and put a drop of the green on the exposed thread of the jet with a sewing needle or similar. A TINY drop. Then screw the jet back in and let it set overnight as an extra precaution. Most chemical thread lockers will soften and melt when exposed to gasoline or ethanol.

We had a similar issue with a 289 Shelby Cobra fitted with Webers, put green Locktite on the jets. It worked, but a bit too well. That's why my 'tiny drop' emphasis. We had a heck of a time unscrewing the jets to change the mains the first time, had to replace the air correctors due to too much Locktite! Found the straight-pin application to work adequately and still be able to back the jets out without boogerin' the air correctors if jet changing was needed. I thought about "plumber's tape" but reluctant to try it in case a bit of it got loose and found its way into a carb passage. Maybe a "thread-sized" piece of it at the last engaged thread of the jet could work, too. Considered a mechanical fix by deforming the last thread of the jets with a small pin punch, but butchering them like that didn't appeal too much.

EDIT: Just a thought, what if you juxtapose the two jets, see if the same jet comes loose in the other port. Or the port with the opposite jet comes loose. That would tell you if it is the jet's threads or the carb body is the problem. If the same jet loosens, try a replacement of that air corrector. Just a WAG of an idea.

 

[drooartz]

Thanks folks. I'll pass these on to the owner and we'll try a few things.

 

[hightower]

Update on my Mikuni sidedraft carb on my 1980 MGB Lim. Ed:

1. I swapped the larger jets right to left, and vice versa. They have stayed fully screwed in and the car is running well. Just topped off rear diff and transmission fluid, so now I'm really feeling good (front armstrong shocks are next to fill!)
2. A big thank you to all on this site for the help (and especially drooartz). My dad had a 1976 MGB in dark red and that's where I learned to wrench a bit. Lots of good memories with him (yes, he's still alive....still doing projects together). Just wanted to give a shout out. Thanks!

 

[DrEntropy]

Good to learn the problem is solved, Jay.