Its a tired crate 350 with HEI and a THM350 with a 10 bolt out of an 80's Camaro. We put a new Edelbrock carburetor with electric choke on it and it ran just fine for a few months. Recently, after cold start and a little warm up time in the driveway, take it out and at steady throttle, it would make the 1-2 shift just fine and with the 2-3 shift it would fall flat on its face and backfire thru the carburetor (as if the accelerator pump wasn't working in an off idle situation). It did this multiple times in the span of a couple of miles. After the hesitation, if I got out of it and eased back into it it would drive on. I never tried stomping on it to see what happens. Parked it and let it sit overnight. Dialed in a little more choke and it had no effect. So while wer're diving into this one, I thought I'd see what the peanut gallery might be thinking.
vacuum leak? faulty mechanical advance mechanism in the distributor? basic tune-up stuff? (plugs, wires, etc) Flat cam?
First thing I did was a spray test. My dad is real conservative when it comes to torquing down carburetors so they don't warp. In this case he might have been a little too gentle with it. I cranked 'em down to snug. I also didn't like the way the motor was cranking to start so I'm going to double check the timing. Let it sit overnight and see what happens.
The entire fuel system is new from the the carburetor to the fuel tank. Checked for leaks and restrictions. Checked both sides of the fuel pump and the pump is working correctly. Took it out for a test drive this morning after torquing the carb down. It started and ran great but did the whole bog and backfire deal after it was warmed up. It feels like it has to do with the vacuum signal to the carb when the load changes going into high gear. pressing on.
I’d start looking at ignition. An old timer told me once that most carburetor problems are really ignition problems.
Make sure the choke is fully opened after the engine is up to operating temperature. Check that the power pistons can move freely and smoothly in their bores. While you're at it, you might as well make sure that the tips of the metering rods aren't damaged.
For some reason quite a few ignition problems are caused by the carb too. So without proof of where the problem is it would be foolish to ignore one and just focus on the other.
High gear shift = more load. Either fuel has changed [less supply (pump, filter, lines) or less metered (inside carb)] or ignition is breaking down. Diagnose all components.
One thing we just noticed. in park, when you rev the engine it runs smooth right up the point where the mechanical secondaries start to open. When that happens the motor does a little brrrrrp thing like a series of small rapid back pressure events in the intake... just enough to lift the air cleaner base casket off the carb a touch. Going to pull the valve covers and see what's going on in there. Then we'll start looking at ignition before pulling the top off of the carburetor.
Uh, you are revving the engine without load enough to open the secondaries? "Its a tired crate 350" How hard are you pushing the pedal when going from second to high gear? "take it out and at steady throttle, it would make the 1-2 shift just fine and with the 2-3 shift it would fall flat on its face and backfire thru the carburetor"
100% absolutely certain. The motor is tired, any maybe its just coincidence that it started doing this right after we put a whole new fuel system on it.
You might pull the top off that carb and check to see if the great fuel we get these day's hasn't crapped up the float bowls and secondary main jets especially if it sat for any length of time. If it's truly back firing and not just a bog it very well could be something in the ignition that is unless a more serious mechanical issue hasn't developed. Maybe a compression check might be in order to see just how tired it really is.
The carburetor has been on for 3 months. We did a compression test at the time the carburetor went on and the compression was good across the board. We checked for a sloppy timing chain and it checked out. We're running the valves right now, then we're going to look at the ignition.
If the exhaust valve isn't opening, or isn't opening enough, that exhaust has to get out of the cylinder somehow, and through the opening intake valve would be the easiest route. The next option for the exhaust to leave the cylinder is to force the burned exhaust past the rings. In a best case, you are contaminating the intake charge with hot, burned, exhaust when that #3 intake valve opens, it goes down hill from there.
Gene, it started with a sputter only in high gear, a worn engine that OP revs to WOT in Park and stumbles when doing that, then loose valves and a flat cam. He's going to test drive it after this. It's already in a valley heading toward an abandoned mine shaft... OP might want to remove the signature Complete Diagnostic & Repairs Check engine light on? Hearing a funny noise? Bring it to us! Our mechanics are sharp and our prices are fair.
Which would explain the backpressure event at higher RPM. That's what caused us to go look at the valve train in the first place. If the valves aren't opening enough, the engine (giant air pump) is not working efficiently enough and the vacuum signal to the carburetor is not sufficient to draw enough fuel which would lean the motor out and cause a sputter/backfire.
Yes. I'm going to test drive it after adjusting the valves because, well, they're already adjusted, and I'm simply curious what effect it's going to have. We already know it needs a motor. The question has been since since the beginning, why it was leaning out in high gear. All we knew about the crate motor was that it was in the truck for many years when I got it, but no idea how many miles were on it, so had no idea HOW tired it was. My dad, who has been piddling with the truck, was watching the engine vacuum signal on the road and in the driveway which is why he revved the motor up. The vacuum signal was dropping at high RPM like there was a restriction in the exhaust system. The back pressure event at high RPM would indicate a restriction as well. Which is why we looked at the valve train. And lo and behold. Wouldn't you know it, there were a bunch of loose valves and a flat lobe. Explains a lot. Diagnostics. How about that. I post threads like this for two reasons. 1) Sometimes you just get a weird one and someone might have had a weird one also, so when traditional thinking runs out, there might be a thought I hadn't thought of. and 2) someone else might have a similar issue and they're searching the HAMB and collectively we might be helping someone. Be part of the process.
Well, took it out and adjusting the valves made a huge improvement. Not perfect, but I can limp it around until I can get a motor in it. The bogging and backfiring is gone for now. My gut instinct was a problem with the vacuum signal to the carburetor and it looks like my hunch was right. Discovered by way of further investigation that the motor is running on low oil pressure so not going to push this one too far. My rebuilder wont be available for a couple of months so hopefully it doesn't eat itself before then. Thanks for your input fellas.
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