'76 3.0 swap: rough idle, backfiring

Ricks911s · 04-04-2009, 10:32 PM

as some of you may already know, the car is a '76 911S with a 3.0 engine out of an '81 SC. after a couple of weekends of assembly, the motor is in and we're trying to start her up. as a bit of background, we know the motor runs great - we drove it in the donor car previously. so we think the motor CIS is ok.

so this is what happened: the motor fired and caught almost immediately, and settled into a low-sounding idle (tach wire is still not hooked up so can't tell RPMs). then it started idling a little bit better so we double-checked everything, and then took it around the block.

it was driving OK, a little surging throttle, and then at a stop when it was idling, the motor suddenly dropped off and the car seemed like it was running really rich. the idle was lumpy and irregular, it was backfiring. the oil pressure dropped a bit too, but it was still reading on the gauge. after trying to rev it up a bit to see if it would make a difference, we decided to turn it off. seems like something is enriching the mixture. seems like the likeliest candidate is a vacuum leak (as we're using the 2.7 hoses which MOSTLY match up to the 3.0, but not perfectly). i'll try to track that down tomorrow but it might not be the problem.

any ideas what else could be going on?

914efi · 04-05-2009, 04:51 AM

Check that you have proper power to the warmup (control pressure) regulator and the air bypass valve. If they don't do the right thing as the engine warms, it would run strangely. Air valve not working would result in a racing idle, so probably not that. Being an '81 engine, it has an 02 sensor? Maybe that is not wired correctly, but it should not dramatically affect running.

I put a '78 engine in a '75 body and there was surprisingly little 'canned' info on the swap so you will need to chase a few things I'm sure.

Ricks911s · 04-05-2009, 06:57 AM

Thanks for the input 914efi.

I'll double check that there is power to the warm-up regulator. I think there is. But I think you're thinking the same thing . . . that I have a warmup problem.

The engine does have an 02 sensor but I've hooked that up correctly, I think.

I'm going to double check all my vacuum hoses and probably end up taking all the control pressures today.

Cross your fingers for me.

felixcontour · 04-05-2009, 07:56 AM

what about doing a control pressure test? would that help?