AutoX problem

[acador]

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.

[John Coffey]

You're boiling the fuel in the float bowls or you're getting fuel slosh causing a rich condition. Look for vapor or fuel dripping out of your air filters.

Between runs open the hood of the car. If you have electric fans, keep them running. If you have the stock fan, keep the engine idling. If you have a water sprayer for your tires, spray the carbs. Install longer float bowl vents.

I would also try removing the heat shields mounted directly to the bottom of the carbs. Aluminum is a very effective heat transfer material. Re-route the cabin air intake on the driver's side to blow air between the carbs and the header.

Edited March 29, 2013 by John Coffey

[acador]

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.

[Zedyone_kenobi]

My advice to you is run a fuel return line to your tank. THat way you can circulate the fuel with your electric fuel pump. You could even run a cool can like some drag racers do to keep the fuel temps cool during those autox days.

[acador]

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.