Is exhaust back pressure the culprit here?

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'74 Sport

'74 Sport

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The 360 in my son's car was starting, idling, and running well with the timing set with 3" exhaust pipes ending short of the rear axle so we could install a rear sway bar. Today, we had the remaining segments of pipes over the axle and the tips installed. Because of the tight fit between sway bar, axle, fuel tank, and frame rail, they reduced the pipe down to 2.25" the last bit and used our 2.25" A-body stainless tips. Stopped for lunch and when starting the car to leave, it wanted to bog and die, no more nice low idle. Had to play with it to keep it running. Same story when we made two other stops along the way home. All this seemingly following the trip to the exhaust shop.

The question becomes...did the pipe size reduction at the rear end create a back pressure in the exhaust which is now affecting the ignition timing? If so, what is the fix for this? Do we advance or retard the timing?
 
I seriously doubt that is a big enough change to cause these kinds of problem (especially at idle)
More likely it's collected something in the carb it doesn't like.
Or possibly the distributor cap got disturbed?

Is it possible to disconnect what exhaust was added to prove/disprove that if you don't find anything else wrong?
 
Agree, sounds like a fuel/carb problem. Check if fuel is dripping in the carb while running. Check all vacuum hoses also.
 
I'm thinking fuel delivery too, although I question the exhaust being the cause.
Had the car been on a road trip before going to muffler shop ? Or did this road trip get the intake and carb a lot hotter than it had been before ? If I'm right a stone cold start will show no problem.
 
My mistake, Aaron just corrected me. The 3" header collectors were already reduced down to 2.5" exhaust pipes and mufflers. This final trip took 2.25" after the mufflers out the back.
Brief history...
New engine was broken-in on dyno stand, Holley 750 was tuned, and timing was set. Engine was installed and timing was off, apparently due to differences in MSD dyno ignition and the Dart's stock Mopar ignition. Car was running good, but it died on short trip from neighborhood and refused to restart...until three days later when I retried it out of curiosity. Would start every time afterwards, but now I had no idea what the problem was, and certainly no confidence it wouldn't do it again.
Decided to try Trailbeast's HEI conversion and the timing advance limiter plate by 4secondsflat. Engine re-timed where it liked 16 initial and distributor limited to 18, for total of 34 advanced.
Put over 200 miles on car with this configuration in all kinds of traffic conditions and with multiple stops and restarts. Drove it 30 miles to muffler shop and the above described change in idle was noticed. Car runs good once warmed up and going. That's why the question about whether newly introduced back pressure would require slight adjustment in timing or carb.
 
RustyRatRod said:
It's not the exhaust.
Click to expand...

I could see it being the exhaust if he got it back from the shop like this guy did.
He told them he wanted an X pipe, and this is what they charged him for.
Considering what it looks like on the outside I wouldn't be surprised at ANYTHING I found inside.

They might have gotten punched, but definitely not paid. :D
 

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RedFish said:
I'm thinking fuel delivery too, although I question the exhaust being the cause.
Had the car been on a road trip before going to muffler shop ? Or did this road trip get the intake and carb a lot hotter than it had been before ? If I'm right a stone cold start will show no problem.
Click to expand...
You might be onto something. I went out to the garage this evening and it started just fine and idled smoothly like it has been for weeks. Stone cold start...depressed accelerator once to give it a sip and set the choke. Popped right off, had to give it just a little coaxing to settle into a nice even idle. Let it warm up, killed it, waited a few seconds, and it fired right up again.
 
.008 Fuel delivery is the problem. What carb and how thick is the spacer under it ?
 
You will be at this for a while if you don't get a plan. There are many easy checks you should start with. Fuel filter, fuel pump, maybe a wire going to ground intermitently, damaged plug wire, maybe while the anti sway bar and pipes were being banged around back there the fuel line got damaged. Look down the carb with a strong light and see what it is doing. Distributor tight? Poor fuel milage? Just some thoughts. You get the idea.
 
mine did that twice. Thought it was the carb, fuel filter etc. Ended up being bad fuel with way too much ethanol in it.
 
ok, full inch spacer should be plenty, even with a open cross over.
When heat changes things I suspect percolating ethanol first. Unless the fuel is heating up before or on its way to the carb, the usual suspect isn't guilty. So...
Power valve ? Secondary sticking partly open ?
 
Do they have ethanol free as anywhere near you. Try it. I went around and around with this. Changing PCV's etc. Fuel was the culprit and I proved it twice.
 
There are a couple of places on an A-body where the stock fuel line routing and exhaust come very close. The one that comes to mind for this problem is where the fuel line exits the fuel tank and runs over to the passenger side frame rail and then along the inside of the frame rail. "IF" your car uses the stock fuel route it is "possible" the new tail pipe is too close to the fuel line causing the fuel to boil. As a temporary check you may be able to wrap a bunch of aluminum foil around the fuel line to see if it lessens the problem. Thermo sleeve and an exhaust heat shield will eliminate the problem if that is the cause.
 
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