Have you checked for vacuum leak? You can use a small handheld propane torch. Turn on engine, turn on propane (don’t light it) run the propane gas along your vacuum hoses and intake manifold. If the RPM picks up, you found your leak.
At higher RPM, your distributor will advance via the vacuum advance. I don’t recall but I think you want 34-36* at full advance. With your timing light, what timing do you read at 3000 rpm?
I suspect that you have too much timing at idle. This will cause the backfire through the carb. This is less of an issue at higher RPM.
Is this a points distributor or aftermarket electronic?
It has been my experience that if you drop in a distributor they way I do by marking the #1 post on the body, if you are a tooth off, when you turn the body to align the points, the rotor does not point as close to the mark as it does on the right tooth. I don’t know why it shouldn’t but for some reason, it doesn’t.
Ideally the center line of the rotor contact would be a few degrees beyond the center of the plug contact in the cap relative to the direction it travels. That results in the best possible alignment of the moving(rotor) and stationary(cap) contacts as the spark advances.
The very large caps used on Fords for several years were somewhat prone to spiking secondary voltage due to excess air gaps especially for the years that Motorcraft rotors used 2 wire ‘cat whisker’ tips.
Interesting. I don’t know enough about the gear configurations used for the distributor shaft/oil shaft pairing to know if that would be expected or not. Could be due to a wearing pattern that developed I suppose. Or maybe you are right, the distributor gear can only mate a certain way and still work correctly. If so that’s news to me. There was an article in Hot Rod magazine recently where Marlan Davis alluded to this possibility, but he wasn’t clear exactly what he was referring to. This was discussed here, but I don’t think it was ever resolved. At least for this sort of engine, Ford, stock, 1970, using the oem points-distributor. Some non-Ford distributors I think are not symmetric, so the angle between the spark plug wire posts is not constant. On those they’d probably have to be installed w/ only one orientation. And electronic ignition distributors may have traits like that too, requiring they be installed with a single specific orientation.
Here are answers to a couple questions. Stock engine with headers. 73000 original motor. I did put a new comp cam roller timing gear set. I have pulled front cover 3 times to ensure timing mark’s are perfect and they are. Intake was off because I put new lifters in. Tried 3 distributor’s, 4 carbs, new point sets as well as pertronix unit, new fuel pump, spark plugs. Car starts right up and idles at 1400rpm for as long as you let it. When you kick off fast idle and car is at 900 rpms is when the fun begins and it will cough through the primaries in the carb. Timing is at 6btdc. I have tried dropping dizzy, finding tdc on # 1 and then adjusting wires to meet 351 firing order. I will try the propane idea for a vacuum leak. That’s one I’ve never heard. I usually use carb cleaner.
Why did you replace the lifters? Did you replace the cam? If not, did you previously have a lifter issue? If you replaced the cam, what cam did you get? If you got something with spunk, the vehicle may not idle well.
Have you re torqued your intake manifold? My experience with these older SBF, you need to go over the torque sequence about 4-5 times.
You have ruled out the timing since you have checked many times. Also the vehicle runs above 1400 rpm. If it was out of time, this would not run smoothly; however, when one of 8 cylinders are misfiring it is more difficult to detect at higher rpms.
Your intake manifold gasket is the most suspect to me. If it did not fit well or moved when installing, you will get a vacuum leak in the intake manifold. If it is leaking, it could be leaking from the inside where the propane or carb cleaner cannot be sprayed and will not be detected. Carb cleaner also works but it can eat paint and damage wires and hoses.
I have this crazy contraption on my ford ranger with a 347.
Replaced lifters because engine sat for 5yrs and I had done everything else so I thought I had a stuck lifter…NOT!! Stock cam and all stock original engine. Numbers matching ShelbyGT350. I know she intakes are always suspect but I really gooped the rtv on but…I could still leak. Going to try your propane trick k when I get home and pray to God it works!!! Nice tunnel ram set up. Wish you were close to me!!
So it was running poorly before you replaced the lifters? As far as gooping up the gasket with RTV, you only need it around the water jackets and corners where the gaskets mate with the cork valley gasket. It also seems like (its been many years since I last replaced them) that the intake gaskets only go on one direction. If they are flipped the ports don’t line up properly.
If you decide to redo the intake gasket, go get some long bolts (i think they are 5/16) and cut the head of the bolt off. Use these as guide pins to keep everything in place. I am assuming you still have the old cast iron intake that is difficult to manage by yourself while leaning way over the fender.
The engine ran fine before I pulled it and replaced with a 408. Lifters were a last dicth effort because I had done everything possible and still the same cough.
Rtv only on jackets and front and rear of block. Felpro copper intake gaskets and yes they only go one way or you block water jackets.
Everything people have been trying to help you with assumed it was a STOCK 351W
Now you’re telling us it’s a 408 stroker crate motor.
Also you want us to believe you went to a 408 stroker and kept the stock 351W cam ?
Depending on what cam you really have, it may not ever idle low enough for a street motor.
Please explain. Do you mean your replaced the camshaft timing chain gear and the crankshaft timing chain gear with a set from a roller cam kit, but you didn’t actually install the roller cam? Just the two gears? If so, I presume you did this b/c you had these two gears on-hand and wanted to eliminate worn timing chain gears as a cause. Is that correct?
The more I read this thread the more I’m thinking one or more of the valves aren’t opening the correct amount for some reason. Or aren’t closing completely. I’m guessing OP already thought of this idea, which is why OP investigated the possibility of a stuck lifter. But maybe that idea is still on the right track.
I know of somebody who could definitely 100% figure this out, if that’s of any value OP. Mark Sanchez at Advanced Engineering, LA area. He appears often as the mechanic in charge of solving a puzzling engine problem in the Hot Rod to the Rescue articles, Hot Rod magazine, and is very familiar w/Ford engines of that era.
Roller timing chains are common on performance engines.
Start by identifying the cylinder that is causing the back fire. Disconnect and ground the spark plug wire one cylinder at a time until you find the cylinder that is causing the back fire. Next remove the valve cover from the cylinder that is causing the problem and observe the rocker arm operation, see if the valves are opening and closing as the should.
It is a possibility that a cam lobe has been “wiped out” or worn flat. The motor oil sold in the last 20 years does not contain the amount of zinc needed for flat tappet engines.
