Hello all,
Hoping someone can help with me some insight. I swapped out my good old trusty 454 330's for a set of 502 MPI's. One motor runs great and the other not so good. Boat has been laid up for weeks now and if I didn't sell my 330's, they'd be back in!

The motor starts great and idles great and runs strong up to 2400 RPM's and then will just start popping and not going any further. This is with and without load. All data has been checked on computer by a qualified mechanic and everything checks out. Compression test good on all cylinders. Fuel pressure checks good. Firing order checked. Plugs and wires all new. Valves were done before motor went in.

One thing that doesn't check is that plugs show all cylinders running a bit lean. ECU was swapped from the good motor so it's not that. I'm at a loss and so is my mechanic.
Any insight would be greatly appreciated!

Replace the rotor and cap. Take one plug wire off at a time and place on new cap or label each wire so you dont effit up. If it has not been replaced, that is likely the problem.

Chris

Baja,

Once you have successfully eliminated the diagnostic process of fuel/air/spark, the root cause the issue is likely mechanical. In light of the symptoms, a broken valve spring would be at the top of that list. The inability of the spring to control the mass of the valve beyond a finite RPM level, both under load and unloaded, the resulting backfire, are all symptomatic of this condition.

Bob

It could also be clogged fuel injectors.

Rotor and cap has been done
I was thinking fuel injectors, but I've tried a different set from another intake that I know is good, and all plugs look exactly the same - lean. So I don't think that is it.
I'm starting to think that the shop that did the valves did something wrong.

Maybe this has been checked, but what is the timing advance at 2400 compared to the other motor. If they are good then it has to be one of the other suggestions. The popping usually indicates a fuel problem or valve trane. I would check the vacuum and compare between the 2 also, and maybe swap some of the sensors.

sounds like injectors to me...known issues with them...

Thanks for your feedback guys! I do have one set of injectors being tested and cleaned. Still don't think that's it. Like I said, I've tried two sets now. The second were on a boat last year that ran great. And I can't see all eight going bad at the same time. But ya never know.

I did get some input today that if any of the valves were not done right, it would cause a blowout in the cylinder which would in turn trip the ECU to lean out, thus causing all plugs to indicate running lean.

I've heard so many different things at this point, my head is spinning!!!
Any thoughts?

Can you run an oxygen sensor in the exhaust? That should verify what it is doing for sure, then you can run things down until you find the culprit. I read an article about ethanol fuels, They say enough ethanol in the fuel & it can fool the computer by actually making the engine run lean. Randy

The ignition pickup or control module (same thing) can be bad on the distributor. Swap distributors from one to the other and check it.

Also in case you do find out that this was the culprit see my post here:
http://www.offshoreonly.com/forums/g...odule-icm.html