Misfire after 20-30 minutes of driving. 2.8L

[GKP71]

2.8L twin plug high compression engine with 46mmPMO carbs. Newly rebuilt with about 1000 miles on it. Ran fine the first 1000, then it started misfiring and losing power after driving for 20-50 minutes. I turn the engine off and open and close the fuel tank and it runs fine immediately. We have checked the venting and it was fine. The fuel pressure regulator is set at 3.5lbs. Will change the fuel pump next. Initially thought it was a vapor lock and then thought it was the fuel pressure. But hard to figure out since as soon as the car is turned off and back on, it is fine. Thanks ahead of time.

[fanaudical]

Welcome to the forum. I've found lots of help here.

Any "venting" or obvious pressurization of the fuel tank when you opened it?

[GKP71]

Originally Posted by fanaudical

Welcome to the forum. I've found lots of help here.

Any "venting" or obvious pressurization of the fuel tank when you opened it?

No. I even drove with the tank open to the air and covered with a cloth to make sure nothing could get in it, while keeping the tank protected. The engine actually started to misfire after just 2 miles.

[timmy2]

Fuel filter(s) full of sediment?

[gamin]

I think Dennis is correct. You should have a filter for each carb. PMO sends 2 filters with new 46's.

[jpnovak]

Check fuel strainer sock in the tank for sediment. Of course, it goes without saying to check the fuel tank itself and make sure its clean.

What ignition system are you running? I would also check each plug carefully with a timing light to make sure it is firing on all cylinders. Is it possible that your ignition boxes or coils are overheating? Is dwell (if you are still running points) set correctly?

[gomezoneill]

How old is the fuel in your tank?

[pampadori]

Have you ruled out the CDI? I recall they fail after driving a while then come back to life after a short shut down.

[GKP71]

Originally Posted by gomezoneill

How old is the fuel in your tank?

Have not checked the fuel filter. All fuel lines have been replaced. Fuel has been replaced multiple times, mainly because I keep driving it everyday to see if the problem is getting better or worse.

[GKP71]

Originally Posted by jpnovak

Check fuel strainer sock in the tank for sediment. Of course, it goes without saying to check the fuel tank itself and make sure its clean.

What ignition system are you running? I would also check each plug carefully with a timing light to make sure it is firing on all cylinders. Is it possible that your ignition boxes or coils are overheating? Is dwell (if you are still running points) set correctly?

Tank is clean. Been emptied and checked. MSD with job racing twin plug set up. Definitely think overheating could be an issue. Boxes are set on top of each other rather than side by side.?

[GKP71]

Originally Posted by pampadori

Have you ruled out the CDI? I recall they fail after driving a while then come back to life after a short shut down.

Car started to misfire today as I was driving and luckily I was right next to a Porsche shop. Pulled right in and they felt it was electrical in nature as well, especially since it was perfectly fine after we turned the car off and restarted it. I have msd boxes. Will keep you posted. The other option may be the distributor cap?

[jpnovak]

What is your plug gap? It is common to open the plug gap with MSD and they can usually handle the extra current driving the larger gap. However, if you go too far, the internal coil drivers can overheat and start to thermally protect themselves.

Place your hand on the MSD box and coils to see if they are hot.

[jmz]

check your plug wires. make sure you don't have something shorting out. but if you have isolated it to fuel problems then maybe you need to check all fittings and connections. ...and as jamie said check the sock.