Hello all, I have come across an issue with my 352 FE rebuilt last year. I rebuilt myself, having a shop do the machine work. Installed a roller cam, non adjustable rockers, custom cut pushrods. Now due to my job, in the maritime industry, I am not home much to drive it, however the few times I have driven it, occasionally I will get a backfire from the carb right off idle. This occurs when the engine is warm, and the truck has been idling for a few minutes at a stop light. Now this does NOT happen every time, but say I am at a light for five minutes and blip the throttle, I get a pop, a small stumble, and then it is fine. Aside from this it runs strong, no issues. Pertronix distributor with vac advance hooked up to the carb. (Eddy) initial timing at 16°. I have tried richening up the idle, increasing the power valve pump, advance the timing a little more, and I still have this same issue. I am thinking that I have made a mistake measuring the pushrods, I was rushed when I measured and kind of just ran through everything when I put the engine together. I may have cut them too long, so when it gets hot, the lifters get pumped up and everything expands, a valve is hanging open slightly. Valvetrain: Oregon Cam roller cam, .520 lift (roughly) stock rockers with end stands, rockers were shimmed to move them so that the rocker sits correctly on the valve stem. Any ideas of what it would be? I am still on a ship in the Atlantic for the next few months, but would like to get a lot of opinions of what it would be.
Have you checked the accelerator pump? Or is that what you mean by power valve pump? I wouldn’t worrying about pushrod length with your symptoms. I’d expect an error there to be evident all the time.
I would also check that the distributor is functioning properly, as in advancing when you have timing light on it. Do the mixture screws do anything when you turn them in or out?
You have a lean tip-in hesitation. It is usually caused by a vacuum leak (running too lean) (you may not notice it when the engine is cold as the mixture is enriched by the choke), carb accelerator pump adjustment or incorrect timing advance curve. You say PERTRONIX. Is it the complete DIST or just the module? That module only controls fire, not DIST ADVANCE. That is internal to the DIST. DIST vacuum source, ported or manifold?
Idle speed too low? How much total timing without the vacuum advance? FEs don't need more than 30 degrees total.
Yes, I meant the accelerator pump, I had a mind blank while typing that one. I have it set to have the largest shot possible. As far as I know the distributor is functioning properly, it goes up to 24° total, and is just the module inside. It is using manifold vacuum, the port on the right side of the carburetor. Idle is set at 750, maybe this cam/engine set up needs more?
If it’s “Not” every time, and only when stopped and idling, it kind of sounds like a heat soak problem. Maybe just boiling enough fuel out of the accelerator pump? Maybe right on the edge of vapor lock? What’s the ambient temperature when it happens?
You say you set the accelerator pump to the longest stroke, but did you look down the carb throat (engine not running) and make sure you have a smooth even squirt? It should start squirting with the initial movement of the throttle. I would also try ported vacuum on the vacuum advance to see if it made any difference.
Ambient temps are 70° yes, I did check to see that it was squirting out as soon as the throttle was touched. When I am home I will switch the vacuum
Sounds like a lean condition off idle. Not enough accelerator pump shot. Just a wild guess. When you just start to move the throttle shaft slowly it should dribble immediately when you move it and continue into a solid stream of fuel. JMO. Lippy
I saying same as Mr RMK57 , vacuum advance issue . Be sure the vacuum advance arm is attached the the breaker plate correctly and nit binding during movement
Will do, it has been a while, about a year, but when the engine was assembled and I went over the distributor, just real quick, I had a vacuum pump (hand pump) on it to check for movement and all was good. I will try this and see, or have someone at the house try it
It certainly may be carb issue, but if I read your post correctly you indicated couple things I would question. First off, manifold vacuum is at base of carb, not on the side. Ported vacuum is on side. Try switching to manifold vacuum at base of carb. I assume youre running iron heads on your FE as you didnt indicate any head changes, and FE iron heads take a ton of total timing. and it just won't run at 26 total timing. Even with that mild cam you need 34 to 38 total timing. Unless you changed the balancer to an aftermarket one, you wont be able to read total up that high so you can carefully measure balancer timing grid and mark balancer up to roughly 40 degrees. When you hook vacuum to manifold vacuum it'll raise the idle so you may have to idle it down. You may have a valve thats sticking open when engine warms up or a lifter thats not bleeding off. You are correct in adding spacers to rocker stands to get rocker arm geometry right. Push rods have no affect on rocker geometry on FE engines or any engine with rocker shafts. At mid lift, the valve tip to rocker point should be 90 degrees to the rocker shaft centerline. Thats something you can check when home and have a valve cover off. You can also look for bent pushrods and broken rockers and run a leak down test on each cylinder to see if a valve is hanging open. You could also check to see if you have heavy mechanical springs in distributor thats slowing the "all in" total timing. Good Luck with your FE, they are awesome engines but require s bit of TLC to be happy.
A few mistakes, I am currently running manifold vacuum, on the right hand side of the Edelbrock Carb. I will look into bumping the timing up to have more total and see if that fixes anything. I think it’s around 26 total but I cannot exactly remember as it has been a while. I will look into that as well. The plugs and the wires are new as well, so I doubt that they are bad. Leak down test may be in order as well. That will show easily the issues
You say the accelerator pump is connected via the 'longest stroke'? then that is the least aggressive setting, it is typically set at the middle position, if you need it more aggressive (like you were a pickup pulling a boat etc) then set it at the shortest stroke. The longest stroke has a lot of travel to go thru while the throttle blades are tipping open ahead of the accelerator pump shot.
If there's one thing I know about Pertronix distributors, they're a crap shoot. You never know how much total timing they'll have. Advance your timing a few degrees and see if this helps or hurts. Running a cam that big you'll likely want to be closer to 20 BTDC at idle. I generally find most engines prefer ported vacuum, until you get up to 400 cubes - or it has a particularly heavy rotating assembly like Olds that shares the "big block" platform with the 350. If this was a carb issue, it'd have to be a linkage problem since its simply not consistent. Is it an AVS by chance?
Now that the accelerator pump was pointed out, I realize that I was thinking of the linkage backwards. I will put it on the shortest stroke to have a larger shot, I will try the middle, then the closest to the carb. I will change the timing to ported, and bump the timing up to 20° or so. My balancer does not have markings that high, so I will get a stick on timing marks. carb is an edelbrock 1406, around 600cfm
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