Hey All!

So I've got a question I've been wondering the answer on for a while now. My truck had a backfire when I got it back, specifically in the exhaust when accelerating. After I got back up north, about 5000 ft elevation gain to 7000 ft, it was backfiring like crazy. Carb was about 4 or 5 turns out, fixed that, then it was backfriring through the carb instead. I advanced the distributer and like magic, the truck stopped backfiring. This was done with a vacuum gauge, like a thread here suggests. When I checked the timing today, she was advacned above 30 TDC, accodording to the balancer marks, but the truck runs great, no ping or anythign whatsoever, even at cusise with vaccum advance. Does anybody have any ideas? It just baffles me and I can't seem to find any explanation as to why it runs beautifully, and has no issues whatsoever. I'm just curious as I'm putting a new distributor in it here soon, and considering an MSD Box too. Deos anybody know why this would be what happens? I'm very curious as to why this is what works. It still runs at 14 BTDC, according to the balancer, but backfires under any load or throttle, even at idle. As is, she has about 11.5" of vacuum at idle, (16 at 2000 ft), which seems about right as it was around 17 or 18 before the cam and everything else, and it ran good then.

For reference, its a 390 FE with a Performer RPM and 4 barrel carb, 204/212 RV cam, and full dual exhaust with headers.

I'd really appreciate y'alls input! This really baffles me! Is the dizzy just not in right? That's all I can really think that could be not together properly. Thoughts?

 

I would continue with diagnosis to try to narrow it down.
Verify all electrical connections to the distributor are tight.
Run a compression check. Only time I have heard of backfire at idle is a valve problem- possibly burned valve, bent push rod or worn cam. A vacuum gauge reading which is jittery is an indication of a compression problem. A low but steady reading could be caused by the timing being off or a radical cam.
Bring #1 cylinder up to top dead center and verify timing mark on the timing cover lines up with 0 on the damper to make sure the damper has not spun.

Unless someone has a better idea I would then change points, cap, rotor and condenser. Better option is replace the points and condenser with an electronic conversion kit.I replaced mine and I could see the timing was more stable when the timing light was hooked up.

Leaning out the idle jets to much could cause possible backfire under acceleration through the carb but not while idling.

If you look into the carb when it is running at low idle, if you see gas flowing from the bottom of the venturi there is a possibility you have a problem with the carb.itself or you leaned out the mixture to much. I have seen folks lean out the fuel to much then crank up the idle speed to get it to idle which starts to draw fuel from the venturi.

 

The distributor was at some point converted to electronic, so no points or condensors to worry about, just one connection to the key, then the dizzy is connected to both sides of the coil. The vacuum is in spec according when you account for the change in elevation (17 inches at sea level).

It inly backfires through the carb and when the timing is pulled out and you hit the gas pedal with any urgency. I want to think that the timing marks are off or the distributor is off a tooth or two. It sounds like backfiring through carbs can be timing too early, which would make sense if it went away when it gets advanced. My dad was the one saying it was backfiring through the exhaust, I only noticed the carb. Especially once she settld in at elevation. Also, even after advacncing the dizzy a lot, I still couldnt get the engine to stumble at all from being over advanced or to have a hard time starting, which makes me think even more that it might be that the dizzy is off by a tooth or the timing marks are wrong. It runs great too, which makes it more baffling now.

 

Welcome to FTE

It is common for old crankshaft balancers to slip, or the heavy outer steel inertia weight actually. It is bonded to the cast part with a rubber "sandwich". With time and heat and ozone etc, the rubber turns to stone and cracks, and it won't be pickin' up good vibrations no mo'. And then it starts to slip, and may even fly off. So here you come along and put a timing light on it, and everything seems to be all whacked out on the timing marks.

There's different ways to check, the easiest most accurate way is with an inexpensive tool called a piston stop. A long 18mm bolt will work on many Fords. Remove the #1 cylinder spark plug and install the piston stop. Turn the engine over by hand till the engine stops. Mark the balancer with a sharpy or whatever. Rotate the engine the opposite direction till it stops again. Mark the balancer again. (Remember to remove the piston stop right away when the test is finished.)

Exactly halfway between those two marks on the balancer is the absolute true TDC for that engine regardless of what the balancer indicates. The pointer should also be aligned with the "0" TDC mark on the balancer, but this is why you make this test.

Very important, we see this problem all the time on the older trucks. It's something that didn't need checking when these trucks were new, but it should be one of the very first things to check now. It's OK to set the initial timing with a vacuum gauge as a kind of expedient but if the balancer weight has slipped, it needs to be rebuilt or replaced.

 

If the distributor is off by one or more teeth, as long as you reset the timing with a light, you will compensate for it.

My son's mustang had a similar problem and he compensated for it by changes to his carburetor. He increased the jet size and changed the accelerator pump cam and setting on the Holley carb so it squirted in more fuel when you depress the accelerator. I believe his idle jets are also set to run on the rich side. He has a radical cam so the most vacuum he can pull at seal level is around 12"..

What ar you running for a carb and when was it last rebuilt?

 

The carb is a brand new in January Holley 600 4160. I just rejetted it because it was fouling plugs and the difference in elevation and temp for what the carb was set up for. (Sea level at 70* to 7000 ft at 45*, so about 3 jets down). So its still basically brand new. Everything esle is untouched, except the tuning screws.

 

I'm starting to think that you could definitely be right about the balancer, espectially if its pretty common. When I first got the truck and it still was factory, it only made about 12 or 13 inches of vacuum. Advanced the timing and like magic, into spec at around 18-19, if memory serves. Does that support the probability that the balancer is off or the dizzy is off a tooth? Thats what the manual indicates from a vacuum reading like that, as well as the guage itself. I'm still pretty new to these trucks and motors, I've only had it since July, so all of this is defintely still a learning process. First truck I've had where these are all things that have and can be adjusted and looked after with nothing more than a single wrench and a screwdriver. I really appreciate all of the input and experience.

 

"Distributor off a tooth" isn't a factor, that won't really change anything, the #1 terminal on the distributor cap is arbitrary, the distributor doesn't care, any cap terminal can be designated as "#1" if the correct firing order is maintained. So long as the distributor housing itself has clearance to be rotated back and forth enough to set the initial timing, it isn't going to matter how the distributor is stabbed.

A loose or worn timing chain set however can cause low manifold vacuum because the valve timing, the point when the intake valve closes, is late. The effective compression is reduced. Makes engine sluggish down low. This is why it's important to start troubleshooting and diagnostics by the book exactly in the order listed. Compression, ignition, exhaust. That way, you can zero in on any defect or problem through a step by step process of elimination. This took me a long time to figure out with my vacuum gauge. I could achieve 20" by giving the dizzy a big ole twist, so everything must be squared away right? Not quite.

Engine manifold vacuum varies depending on altitude, for one thing. In general though a stock, factory engine with a mild camshaft should pull a very high steady vacuum at sea level. The same is also true at altitude, it's just a corrected number for the particular elevation. The higher the better though. Always. 17" might be really good in Denver, but would indicate a big problem in Des Moines. Since there is so much variance in altitude the generic gauge instructions that say "17 to 22 Normal" is kind of misleading. If an engine should pull 20" and it only pulls 17", that is no bueno. Find out why.

Engine should pull 19" to 21" high steady manifold vacuum at the factory ignition timing spec.This is usually 5° to 8° BTDC typically, something like that. So you can see it takes some experience with the gauge to know what you're really looking at, but there is probably no better diagnostic tool.

Most people like to advance the ignition timing a fair bit over factory specs for performance and economy, but for diagnostic or testing purposes the ignition timing must be set back to factory spec before measuring the manifold vacuum and making any fault diagnosis.

Keep at it, measure - don't guess!

Verify the true TDC is indicated correctly on the balancer, so you know the ignition timing will be be set accurately, you can make a determination on the valve timing, vacuum leaks, carb mixture etc. Then using a timing light set it to factory spec. Measure manifold vacuum with the gauge. About 25 different engine and tuning faults will be indicated with this tool if you know what to look for. That's why it's listed in every Tune-Up and factory shop manual ever published, going back a hundred years.

 

My son had same problem with his new Holley carb on a 390 and did the following

Holley 750cfm double pumper 4150 variant.

If it is backfiring under quick acceleration it's not getting enough fuel and pre igniting or the timing is off enough that the intake valve isn't closed.

I changed the Holley accelerator nozzle and the accelerator pump cam. Both impact the timing and size of the accelerator jet spray.

My recollection is he bought a kit Holley sells which includes all the above parts. See page 10 in the attachment.
The "Holley carb service parts" catalog has more details on the type of kits you can buy. One includes the nozzles etc but is expensive. A cam kit is relatively cheap. I was not able to upload the catalog, possibly due to file size. Search online and you should find it.

It is starting to sound like it may be a fuel related issue and could be as simple as properly tuning the accelerator pump circuit.

If you had to change the jets due to altitude, you may have to reset the idle mixture screws. I typically do this by using a tachometer, crank the screw in until the engine speed starts to drop then out it until you reach max RPM. Reset you idle speed screw if needed and do it over again.
Some folks say you should rotate all screws in or out the same amount so you may want to read what others recommend.

 

Since it runs fine with the ignition advanced, maybe when it's set normally it's not advanced enough. The vacuum canister might be shot, a vacuum line might be cracked, the vacuum advance mechanism inside the distributor might be stuck.

Balancers do slip. When mine slipped, it slipped opposite to what you're describing. Setting by the slipped balancer, my ignition was actually way too far advanced causing detonation. A cracked piston was the result. Nevertheless, it ought to be checked as it is a likely cause.