my carb runneth over

[bg241ihc]

Hi to the Mikuni experts out there,

I have had these carbs off and on 4 times now and thought it was time to quit busting my knuckles and brain and ask those who have gone before me.

My issue is that when running the #3 cyl immediately fills up with gas,floods and wont burn (as noted by the cold exhaust header, wet plug and fuel dripping out of the said exhaust header/muffler joint). If I check the spark it is good and hot. If I run the engine with the plug out the engine sounds pretty good and the other 3 headers get hot, so I suppose the other cyl's aqre firing OK. With the plug out while running the #3 cylinder sprays out gas/air mixture for a few seconds and then dries out. If I hold the plug into the hole while running it starts to draw fuel until, (don't try this at home folks) the fuel ignites and singes your arm hair.

I put the carbs on the bench with a small tank attached to the fuel inlet and the bowels filled up and then all the needles seemed to shut off the flow and there was no over filling.

I have adjusted the floats 3 times and checked the movement of them. The float seems to "float" OK and I have cleaned and checked the needle/seat and I cannot see any reason the carb in question would be dumping excessive fuel into the cyl. Is there any other thing I am missing or do I just need to keep adjusting the float height

[natemoen]

Are the jet sizes across the carbs the same (or close, I know the later ones had a slightly richer jet on the inner 2 cylinders)? Is the spark weak on that cylinder?

[bg241ihc]

Jets are new and the same size. I just rebuilt the set with new carb kits. Spark is great. It jumped well over two inches when holding the plug near the head

[DAVINCI]

Make sure the float is actually floating and not hanging up on the inside of the bowl. Some float hinges have too much play and the float can rock side-to-side. Float bobs that are bowed outward can be a problem, too. Examine your floats closely and make sure they're all square and work smoothly. After re-assembly, turn the (dry) carb bank upside-down, and try blowing air into the fuel lines, you'll know if the float valves are closing.

[Ken Talbot]

If this is happening on your '80, it could be a bad o-ring around the body of that float valve. If this o-ring is shot or missing, fuel will continue to flow around the body of the float valve even when the tip of the needle is properly seated.

[DAVINCI]

I just rebuilt the set with new carb kits.

Good point, Ken. Depends on how 'complete' is carb kits were

[bg241ihc]

Yep, all o-rings are new and in place. I would think that the bench test I did using a small fuel container as described in my OP would have shown any leakage.

One thought I had was if the needle jet is somehow stuck or kept in the wide open position at idle, would that feed enough fuel to flood out the cyl and keep it from burning resulting on raw fuel to keep entering the combustion chamber and remain unburned?

[DAVINCI]

The needle jet ( slide/diaphragm ) works on vacuum, and has a spring holding it closed, how would it be stuck open at idle?

[bg241ihc]

Don't know. Would something cause the vacuum at the diaphragm to be present at idle?

Also, I suppose there could be sticky slide or other faulty part in the needle assy.

I am just checking the possibilities to go over more closely now that they are removed for the fifth time

At this point I am not convinced it is a needle/seat problem

[Larrym]

Pass/Fail...

Inverted and Normal Orientation Bench Test

If the carb bank passes these tests then you can rule out "Needle/Seat/O-ring" issues and start looking elsewhere.

Get up close and personal with the carbs in both the inverted and normal position: Look CLOSELY. Go have lunch, come back, and check again.

HTH.

[yamtom]

etc etc

Hi,

Check to be sure everything suggested above is done, then , see if the bowl gasket is too big and can be trimmed such that it cant interfere with the float, if the float has any casting flash on the sides, sand it off if there is any, Ive done these things as a routine when renewing carbs, if the gasket is wider than needed, trim it.

I think you imply that there are entirely new float needle seats by the oring comment, if that carb never leaked before the new parts, try the old needle and seat to see if that stops it. If that stops it, then buy a different new needle and seat to see if that keeps it corrected. Its only five bux. Be aware that the float needle can get cocked and stick the flat down.

If this still doesnt correct it, I would swap all the number three parts to number four cyl and vice versa to see if the problem moves to number four cyl. This is all labor intensive but will be worth it the first time you twist that throttle . I do believe that some here have said that individual carb parts need to remain with the original carb position, so if the prob moves to number four, swap back all the parts and buy a new carb kit for number three.

Please be sure to let us know what your findings are.

[bikerphil]

This hasn't been mentioned, try tapping the carb bowls with a screwdriver handle when initially filling them. Sometimes a float will hang up because it is hanging too low when no fuel is present. There also is an adjustment tang on the float to keep it from falling too low and letting the needle partially come out of it's seat.

[bg241ihc]

Looks like a lot of good suggestions and tests.

I will report back when I find out

Best regards

[planedick]

Sounds like

The diaphram in the petcock may be bad allowing the gas to enter the vac port on #3 cyl.

[cywelchjr]

The diaphram in the petcock may be bad allowing the gas to enter the vac port on #3 cyl.

That's a good point, that's an alternate way that it could be getting in there without there being anything wrong with the carb at all.