I'm experiencing a top end msifire with backfiring out the exhaust. Truck starts an idles fine, lightly accelerates fine. Hits 2200 rpm and tach will not go any higher, needle starts bouncing, truck starts stumbling and backfires out the exhaust.

Truck is 78 f150 with 400 engine. Previous owner did the project bronco buildup verbatim except for the high cr pistons. 351M/400 Performance Build Up
If you haven't read it, 255 dual energy comp cam with matching valve springs, etc, double roller timing chain, edelbrock performer 400 intake, holley 600 cfm carb, hedman headers, k and n filter. Truck was running fine this all started mid drive. I use to have the occasional backfire when I would let off the throttle real fast but very rare.

vaccum test 18 inches hg bounces slightly, drops and recovers with throttle blip, rises and settles when let off from 2000 rp m

determined vacuum advance diaphram was torn, so replaced with new
replaced plugs, wires, coil
number 8 plug had oil, all others looked fine
compression test on number 8 120 psi dry went up with oil
inspected all valves, none appear broken
tried unpluggin no. 8 plug but ran much worse
replaced fuel filter
left side exhaust looks wet and oily inside, smokes (same side as no.8)
right side exhaust looks dry and fine
adjusted timing with vacuum gauge
vacuum advance connected to ported vacuum

Truck starts and idles better with new plugs wires and coil.

Timing light will not light enough to use it. I used the same light on the truck a few years ago. I'm suspecting some type of weak spark. Thinking of getting a drop in hei distributor to eliminate ignition module, pickup coil, cap, rotor and reluctor in one swipe.

I have a new fuel pump, not installed yet, but I don't think that will help. Tested seocndaries they open slighly, but I don't expect them to open much at only 2200 rpm.

I'm out of ideas. What am I missing? Any help would be greatly appreciated. Truck has run like a top since I got it in 2006.

 

Im rusty on gasser stuff, but iirc we have an old IH loadstar that was doing the same thing, ended up being the dizzy / points, i suppose the pickup in the dizzy could mimic the same effect.

 

Yup.......

 

Sounds like a weak coil

 

Valve springs could be weak and floating. Or your fuel pump could be weak. Ive had a number of mechanical fuel pumps wear out not be able to keep up with the engine.

 

Haven't had much time to work on the truck lately. I installed new fuel pump based on little black truck's recommendation, no improvement. I took to a mechanic and he thought it was timing, he replaced the worn distributor gear. Starts great, idle's great with the tuneup, but still cuts out at 2100 rpms. Mechanic was able to time it with his light. He now thinks its the carb, but he doesn't work on Holley's. I spoke with holley and their tech support said it didn't sound like anything a carb could cause. He does work for them, so I take that with a grain of salt.

So mechanic doesn't think its ignition, holley doesn't think its carb, must be in the heads? But, no one can come up with an explanation of what in the heads would cause that. Little black truck suggested weak springs and valve float. My understanding of valve float is that it would occur at higher rpms and as the springs "relax" with age the situation would occur at lower and lower rpms. I don't think weak springs would occur all of a sudden in an engine that has run great with no head work for 6 years. Am i wrong there? I checked all the springs and none appear to be broken.

Someone suggested a sticking valve? How can i check this if it doesn't do it except at high rpms? I have an engine shop nearby. Thinking of just pulling the heads and having them gone through. This guy said he wouldn't recommend a valve job on an old engine without working on the bottom end also. But, he can magnaflux them, vacuum test them and surface them.

I just got bigger tires and a new to me winch. I want to take this thing out and play! Anyone have any other ideas?? Any way to do other tests on the distributor/reluctor/ignition module? If I can eliminate ignition and say for sure its the heads, I can yank them off. Hate to do it if its just a $30 pickup coil or something I'm missing.

I've found a different mechanic that's a little older and builds old hot rods. I'm thinking he may have a little better knowledge of a 30+ year old engine. The mechanic I used is about 50, so he should have seen plenty of 70's and 80's model engines. Also have a neighbor that races mustangs, not sure if he is a holley guy or not, but may be able to eliminate carb from the equation if he's willing to help.

Thanks in advance for any help.

 

I doubt it has anything to do with the heads and don't really think it's fuel related either.
Does it do it everytime, or off and on?
I suspect ignition. Check for worn distributor shaft bushings. Pull the rotor and try "wobbling" the shaft side to side, any play isn't good. Also check the plate that the pick-up coil mounts on. Don't know if it's the same on these, but on the old points version the plate pivots to advance and retard the spark. They used a small plastic pivot that would break up allowing the points to move around.

 

its the distributer or could be as simple as a rotor or cap, i didnt read where you replaced that. I promise you, one of these WILL fix it. I like those one wire HEI billet dizzy on ebay for $45. They put out a hotter spark, look good and you can clean up your engine bay of that coil and Durracrap box/wires. Get your mony back for the coil and pay for it all too (see link below)

http://www.ebay.com/sch/i.html?_trks...m+hei&_sacat=0

 

Thanks for the quick replies.

i replaced cap and rotor a few years ago, but not with the latest work. I only drive a few thousand miles a year though.

It does it every time. Never did it at all, now does it every time. Runs perfect up to that magical 2100 RPM and then craps out. Mechanic thought it sounded like mechanical advance when i dropped it off, so I assume he checked the plate and pivot inside the cap. I'll make sure though.

I actually told the mechanic if he thought it was distributor related to tell me and I would drop in a one wire HEI type. He thinks ignition is all setup and working fine, so he thinks carb is next option.

Anyone have personal experience with the one wire HEI distributors off ebay?

 

Me, they rock even saves a little MPG They are all pro Comp

 

Weak valve springs or a flat cam!!

 

Can weak springs or flat cam happen all at once? Literally drive like a top to nearby town. Parked for 2 hours. Driving home when i turned onto the four lane and started trying to get speed, missed at 2100 rpm. Has missed at that rpm range ever since.

I thought those things were attributable to wear over time, not instant.

 

throw a dizzy on it...

 

Didn't have time to order the ebay distributor, but local parts place had a remanufactured in stock. Mechanic put it in and... no difference.

I told him I didn't think ti was the carb and told him to run a leakdown test. His compression test results confirmed mine that number 8 cylinder was low, about 80 psi and 5,6 and 7 were just a little low, but normal.

He is leaning toward carb even more now. Any Holley experts out there? He doesn't believe the advice from Holley tech support.

The only ignition component not replaced is module. I can't find anything anywhere about a module causing this type of problem.

Mechanic said he heard once of a truck where an entire side of engine was starved for fuel. Not sure how this could happen and still run at idle up to 2200rpm. According to the Holley manual, my secondaries should just be starting to open at around 2300. I did the paper clip test on the secondary rod and it did move slightly. So, i don't suspect a torn secondary vacuum diaphragm. Could the secondary jets be clogged and when the secondaries open, the extra air is leaning out the mixture and causing the backfire? I can try wiring the secondaries shut and driving and see if problem persists.

Does this sound reasonable?

 

Not saying that thats your problem but I had a friend that had an '85 chevy with a 305. He used to pay off employees in cash in the field, he would leave the truck idling with the ac on when he did. After an hour or so of this once he went to pull away and the truck had no power whatsoever, it turned out every lobe on the cam was wiped. Truck had ran fine when he started, no power when he left.