Mikuni fuel leak

[Travel'n Man]

Was there any debris in the float bowl? If you don't remember pop the top and look right down in the bowl - if its clean in the tank you can rule that out. A lot of times you just have to rule things out until you find the cause.

[LeonV]

Have you taken off the filters to see if you can spot the culprit? Loose venturis will demonstrate these symptoms. If the venturis all check out, run the car with the filters off and observe. I always keep an extinguisher nearby when doing any sort of fuel-related work.

[beermanpete]

I have the Mikuni service manual and adjusted them to 12mm per the instructions. I pushed on the needle valve when I was checking the floats last night and they seemed correct. I should probably remove the valve assembly to get a closer look on the inside. Maybe the "push test" wasn't actually proving anything. A stuck float or heat-soak sounds like the only possibilities so I'll keep digging until I can prove one or the other.

If a float valve is bad it will leak despite being adjusted correctly. Also, make sure the floats are not sinks. Brass floats will slosh when they take on fuel (shake it near your ear). Plastic floats are harder to diagnose for fuel absorption. See if all 3 floats weigh the same or simply place them in a container of fuel and see if any sink or float lower than the others.

Run the engine with the air filters off and look for fuel running out of a venturi and/or float bowl vent. A serious leak will flood the engine and it will run badly. If the float level is just a little bit too high the car will seem to run fine but will idle rich and might idle roughly intermittently as a drop of fuel falls from the main discharge nozzle now and then.

[Travel'n Man]

One more thing - have you air balanced each of the carbs to ensure they are all balanced?

[acador]

I did balance the cabs during install using an unisync. They seemed right but I had a tuff time keeping RPMS consistent. I plan on balancing again this weekend. Anyone know a trick to keep the RPMs up? I tried a zip tie but that did not go well . It was early in the tuning process and running way rich. The RPMs went too high so I turned the car off and it let off a LOUDEST shot-gun sounding backfire I had ever heard. Multiple neighbors were looking out their windows to see what happened. It was pretty embarrassing to say the least.

I agree a fire extinguisher is a must. I have one mounted behind my seat.

Tonight I floated the floats and they are consitant; good idea btw. I disassembled all the float valves and they were clean and functioning properly. All the float bowls and banjo filters were clean. I checked the venturis and they are tight. I reassembled everyting and let the electric pump run for a few minutes. So far everything is dry. I'll try to drive it around tomorrow without the filters and keep checking for leaks. I didn't do anything to correct the issue so I should be able to replcate it. We'll see what happens when they get hot.

[Travel'n Man]

I'm betting this is way the pervious seller sold the carbs - can you call the PO and ask what was going on with the carbs - do you know the setup of the carbs - is it possible that you have different size jets in carbs..............at this point you may have to start fresh so you know what you have.

My jetting setup is:

40mm Mikuni Carbs

32mm Venturis

40 Pump Jets

57.7 Pilot Jets

150 Main Fuel Jets

190 Air Connector Jets

[acador]

I got lucky because the carbs actually came with 135,140,145,150 main jets, 170,180,190,200 air jets, 52.5,57,5, & 60 pilot jets, 34 & 37 mm venturis. I installed the 34mm Venturis first. I tried probably 20 different jet combinations tuning with a wideband & Gtech to see which combo worked the best. I think I ended up getting the best A/F & HP results with the 190/145/60 combo. I don't remember what pump jet I used. I do know all the jets match though.

I may check with the PO to see if he remembers the issue but they came off a race car so he may not know. They also sat in a box for 3 years before he sold them to me.

[LeonV]

I did balance the cabs during install using an unisync. They seemed right but I had a tuff time keeping RPMS consistent. I plan on balancing again this weekend. Anyone know a trick to keep the RPMs up? I tried a zip tie but that did not go well . It was early in the tuning process and running way rich. The RPMs went too high so I turned the car off and it let off a LOUDEST shot-gun sounding backfire I had ever heard. Multiple neighbors were looking out their windows to see what happened. It was pretty embarrassing to say the least.

You had trouble keeping engine speed consistent at idle, or were you doing a higher-speed syncronization?

[acador]

The idle is consistent but I figured it would be more accurate to sync at a higher RPM so I was attempting to do it around 2500. I had a few too many clicks with the zip tie and eneded up closer to 4500 RPMs.

[LeonV]

The idle is consistent but I figured it would be more accurate to sync at a higher RPM so I was attempting to do it around 2500. I had a few too many clicks with the zip tie and eneded up closer to 4500 RPMs.

Syncronization should be done at idle, with all linkage disconnected. Once all throats are breathing equally, make sure all linkage bars are parallel to one another and install them one-by-one on the carbs, making sure at each step that airflow remains unaffected. Once all linkage is installed, check all throats again after giving some blips of the throttle. Airflow should be even. The very last part of syncronization is bringing up the engine to a high-idle (get a helper or hold the linkage by hand, Weber STE makes this process simpler) and checking sync. If it's off, then you have a linkage problem.

Sorry for the aside, but I thought it would be a worthy tidbit.

[acador]

Thanks for the balancing procedures. I'll do that as soon as I get a chance to drive it again. The carbs have remained dry after a few days of sitting so the leak must only occur during or after driving.

[acador]

I took the car for a spirited 10 mile drive today and the carbs remained bone dry before, during, and after the drive. It was only 40 degrees outside so the carbs never got hot and actually felt cool to the touch when I got home. I did a quick balance check at idle and they were all the same. At this point I’m starting to suspect that some debris may have jammed the float for a while and then dislodged and passed through the jets.