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I 2nd Travel'n Man's comments on David Premo. I called David looking for help with a heat soak issue I've been fighting for years and not only did he immediately nail the fix he also suggested a jet change that really helped the top end power. This guy clearly knows how to tune Mikuni's.

I do have a fuel return line to the tank with an electric pump. I ran it that way to help prevent fuel boiling but apparently that is not enough to prevent boiling from impacting the pilot operation. I would be willing to run a drag race style cool can inline if that would solve the problem.

Thanks for the response John. I have noticed fuel being in the filters when removing them but to the point of dripping. Is that caused from the boiled fuel vapors settling in the filter or do you sustpect fuel slosh? The carbs were modified by the PO (came off a dedicated autox race car) to prevent fuel from spilling into the jet blocks. Install longer float bowl vents? Is that so the sprayer water doesn't get in or some other purpose? I'll ditch the aluminum heat sheilds and make a new one of stainless. BTW. I LOVE the Giken LSD you sold me! It totally transformed the cornering abilities of the car. I can precisely steer the car with the throttle now and go full throttle out of turns. I'm very impressed with it. Thanks again.

Hey guys. I would like some advice on what I believe to be a Mikuni jetting or tuning issue. Sorry it’s long but here goes… The issue is as consistent as clockwork. At our local AutoX events, we do 3 runs in the morning (day 1) and 3 runs in the afternoon for (day 2). On warm or hot weather events the following occurs: 1st Run – Car runs excellent 2nd Run – Have trouble launching off idle at the starting line, once going 10+mph the car runs excellent 3rd Run – Slightly erratic idle, hard to launch, once going 10+ mph the car runs excellent Lunch Break (Cool Down) 4th Run – Car runs excellent 5th Run – Have trouble launching off idle at the starting line, once going 10+mph the car runs excellent 6th Run – Slightly erratic idle, hard to launch, once going 10+ mph the car runs excellent Trouble Launching – The engine sputters when attempting accelerate off idle (like the pump jet system is inadequate). Heavily revving the engine before launch seems to help. Car Details: L28, mild cam, Mikuni 44s, ceramic header, good exhaust, MSD, aluminum radiator, electric fan. I assume heat soaking is the root cause but I can't seem to overcome the impact. Heat Soak Prevention: Ceramic Header + Exhaust wrap, home-made double-walled (with air gap) aluminum heat shield covered in DEI heat shielding, & a home-made DEI wrapped aluminum heat shield mounted to the bottom of each carburetor. The top of the carb gets hot but is not too hot to touch when the issue occurs. I can drive the cars for hours in 100 degree Arkansas heat and hot lap it at the drag strip and this issue never occurs in those scenarios. It only happens on my 2nd and 3rd runs on track day. Unfortunately my wide-band broke so I can’t use it to troubleshoot but I assume the car is going Lean. I have done extensive tuning with a wide-band, G-tech, and large pile of jets combinations and I’m pretty sure the tune is good but obviously it’s not perfect. I think I need to modify the pump system or step up to the next larger pilot jet. Before I order jets I thought I would create this post to get some input. Any help would be greatly appreciated.

Here is an pic of my linkage setup which is a little different than the one above.

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.

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.

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.

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.

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.

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.

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.

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.

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?

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.