acador Posted February 7, 2012 Share #1 Posted February 7, 2012 I pulled off my mikuni 44s last night to do some head work and I noticed one air filters was fuel soaked. There was also a bit of fuel on multiple air horns and possibly some on the valves. My fuel pressure is set at 3psi and I recently adjusted the floats to spec. What are some possible causes for fuel leaking into the venturi on these carbs? Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/ Share on other sites More sharing options...
Travel'n Man Posted February 7, 2012 Share #2 Posted February 7, 2012 Check your needle valve for each of your carbs - in your float bowl there may be some debris locking the needle valve open and allowing more fuel than what should be in there. Do you have a fuel filter back at the gas tank and before your fuel pump? If not - go ahead and spend the $$ to get the best one you can. You do know what is directly under your air filters don't you? A hot header and raw fuel do not make up for a pleasant drive! Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380096 Share on other sites More sharing options...
acador Posted February 7, 2012 Author Share #3 Posted February 7, 2012 I do have a pre-pump filter and I am planning on adding a higher end filter after the pump. I'm going to take the carbs apart tonight so i'll check to make sure they aren't being stuck open. Can an overfilled bowl allow gravity to pull the excess fuel into the venturi when the engine is not running? A fuel leak above the header is terrifying. Plus, all that fuel wash into the cylinders can't be good either. Thanks for the info. Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380099 Share on other sites More sharing options...
beermanpete Posted February 7, 2012 Share #4 Posted February 7, 2012 Yes, if the fuel level (in the float bowl) is higher than the discharge nozzle the fuel will run out into the venturi. This will only occur while the engine is running however. Once the engine stops and the fuel pump in not pumping the fuel level will equalize at the top of the nozzle and the leak will stop until starting the engine again.Another source of fuel leaking from the venturi is heat soak after shutdown. The exhaust heat can boil the fuel in the fuel line and float bowl, pushing some of it out into the venturi. If the fuel is not actively leaking when the engine is running this could be the cause. Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380102 Share on other sites More sharing options...
acador Posted February 8, 2012 Author Share #5 Posted February 8, 2012 I checked the carbs and the float level is correct and the needle valve seems to be functioning properly. I have never seen fuel leaking while the car is running or not but one filter created a puddle when I set it on the floor so I'm not sure when its leaking. The car runs excellent, the A/F is near stoic at idle/cruise, and low 12s at full throttle. I'd like high 12s at full throttle but I couldn't get it there with the jets I had. If the carbs were leaking while running, would there be a noticeable difference in the drivability/AF readings?Great thought on the heat soak. I have actually been suspecting a heat issue. When I AutoX it runs great during the runs, but it has trouble idleing and launching after the first run. Oddly this is never an issue hot-lap drag racing or driving it for hours. I have a ceramic coated header, header wrap, and an aluminum heat shield I built out of a double-walled aluminum baking sheet. Is there something I can add to the heat shieild to make it more effective? Would a stainless steel one work much better? Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380154 Share on other sites More sharing options...
Travel'n Man Posted February 8, 2012 Share #6 Posted February 8, 2012 I purchased from Summit Racing a heat wrap and put on the underside of my heat shield to keep the heat dispersed.http://www.summitracing.com/parts/THE-13590/ Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380163 Share on other sites More sharing options...
LeonV Posted February 8, 2012 Share #7 Posted February 8, 2012 If you can comfortably put your hand on the carbs after hard driving, you don't have a heak-soak problem. I'd be surprised if your carbs got very hot with header wrap and a heat shield. Do you have rubber isolators between the carbs and manifold? Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380221 Share on other sites More sharing options...
acador Posted February 8, 2012 Author Share #8 Posted February 8, 2012 I do have the rubber isolaters. I was able swap jets by hand immediately after driving when I was tuning and they weren't too hot to the touch then. I'll try to get it back together tonight to do some more testing. I found some 1500 degree heat wrap tape at oreilly for $17 a roll that I may stick to the bottom of the heat shield. Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380235 Share on other sites More sharing options...
Travel'n Man Posted February 8, 2012 Share #9 Posted February 8, 2012 I pulled off my mikuni 44s last night to do some head work and I noticed one air filters was fuel soaked. There was also a bit of fuel on multiple air horns and possibly some on the valves. My fuel pressure is set at 3psi and I recently adjusted the floats to spec. What are some possible causes for fuel leaking into the venturi on these carbs?Was there a problem before you adjusted your floats? Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380237 Share on other sites More sharing options...
acador Posted February 8, 2012 Author Share #10 Posted February 8, 2012 I bought them used and I adjusted the floats before installing them. I don't know if they have always leaked or if this is a recent problem because the fuel doesn't seem to leak out the K&Ns. Only the front carb had fuel on the filter. The others just had a drop on the inlet horn. Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380242 Share on other sites More sharing options...
Travel'n Man Posted February 9, 2012 Share #11 Posted February 9, 2012 You may want to get more detailed info for your float setting from Todd at:http://www.wolfcreekracing.com/http://www.scribd.com/doc/60490072/Mikuni-PHH-Service-Manual Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380244 Share on other sites More sharing options...
acador Posted February 9, 2012 Author Share #12 Posted February 9, 2012 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. Link to comment https://www.classiczcars.com/topic/42211-mikuni-fuel-leak/#findComment-380248 Share on other sites More sharing options...
Wilbur
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