i have an 86 f150 with a 95 roller cam 302 bored .30 over with a preform hei dizzy, a stock cast iron intake manifold, a 1" spacer, and a holley 2300 500 cfm 2 bbl with 70 jets and a locked choke. my timing is set at 12 degrees btdc. It backfires through the carburetor and my vacuum is low around 16lbs. I have tried everything I can imagine to fix the back firing. Ive tried timing from 8-14 degrees with no change, went from a points dizzy to an electronic dizzy with no change. Ive ran it with and without the vacuum advance with no change. Ive changes the position of the choke plate with no change. ive changed plugs, wires, power valve 3 times, jets, float, needle and seat, squirters everything and I cannot figure this out. its not a constant backfire only when its under a load when im taking off in gear (manual trans. t18) The only thing I can think of is the dizzy's gear is not matched to the camshaft but I don't know if that would affect the backfiring. I feel like im spinning my wheels here going round and round with this.

 

I'm no expert but I bet your on the right track. Wonder if it jumped a tooth on the timing chain?

 

Have you squirted brake cleaner around while it is running to look for a vacuum leak? It sounds like it is lean, and a vacuum leak would do that.

Or, as Hopsing said, maybe the valve timing is off. Last idea is that your balancer slipped and the timing is off. Have you checked that?

 

Maybe I'm not remembering correctly, but weren't there two ignition firing orders (and cams to match) on some of these engines?

 

I'm assuming the odds of timing jumping teeth are not that good seeing as the engine has less then 250 miles on the rebuild but I suppose its a possibility. And I've re torqued the intake and checked both carb gaskets and no leaks. The plugs are hit and miss too some are covered in black soot and some are snow white

 

And yes one for an ho motor and non ho I have a non ho motor I've tried both firing orders and the non ho is the right one

 

If it's backfiring through the carb, the ignition is setting the mixture on fire before the intake valve is closed. So, either the ignition timing is off, or the valve timing is off, or the valves aren't sealing for some reason.

Who did this engine rebuild, and who assembled it?

 

Hmm very interesting 250 miles since major overhaul. Well I think you should check it out, as it sure will cause that symptom. I know my 302 jumped a single tooth and it showed up as backfiring out the kazoo. If you had a blown intake gasket you'd be running lean on one or more cylinders, but I don't know if that would cause it to backfire. Curious have to run a tank of high octane fuel in it recently?

 

I did the rebuild I used all new parts minus the heads and block rotating and valvetrain are all new. My grandfather in law and I assembled it. He builds circle track motors and has for about 40 years. I've messed with the ignition timing like crazy but not the valve timing. I'm assuming that involves pulling the timing cover to make sure the marks on the timing set arealigned? Now the guy who did the machine work ddidn't really check the camshaft he just looked at it is it possible that there may be an issue such as a worn lobe to cause this? Also come to think about it I believe I used a 72 intake manifold could it be maybe the runners on the intake and heads not aligned?

 

That is a good question about the 72 manifold that would be off a 289 probably. And you are right you'll have to pull the valve cover on the number one cylinder bank to check all that stuff out. I'd give that cam a second look. Please keep us in the loop as you check off the items you mentioned. I know a guy that builds up racing engines, I will ask him what else you might look for. Richard

 

so at work today I did some digging and found out that the intakes on 302 all have the same port sizes so Im leaning against a vacuum leak at least on the manifold. I tried the straw trick on cylinder 1 to find tdc and when I did the rotor pointed to 1 o clock on my distributor. now if I find tdc again and pull the valve cover off my intake valve should be open correct? I also ran my fuel pump for about 10 seconds into a pop bottle and it filled the bottle up about 1 1/2" before and after the fuel filter so I doubt theres blockage. now when im running my fuel pump without the truck running I can hear it gurgling in the bowl on the carb. Im not sure if that's normal or not. This is my first carb by the way. My fuel pump is just a mr gasket 4-7 psi pump.

 

When you are at TDC you are supposed to check the timing marks on the harmonic balancer. The rotor can point practically anywhere if you get the wiring set up to match it. But, 1:00 is about where it should be normally, although it has to be pointing at #1's terminal on the cap.

As for the valves, at TDC on the compression stroke both valves should be shut.

 

at tdc the harmonic balancer was at 0 degrees tdc. and ok that saves me from pulling the valve covers for now. Ive been searching on all kinds of forums everywhere. would vapor lock cause this? it doesn't stall when im driving it just backfires through the carb. my fuel lines are all rubber and they run along side the steering column, up in front of the motor to above the heater hose then to the carb. I want to change the routing regardless but im just wondering if maybe its getting a little bit of boiling because of how close it is to the heater hose.

 

It is possible that too much heat would cause that. I'm betting it is very lean, and the gas turning to vapor leans it out badly. But remind me what carb you are running?

 

its a holley 2300, 500 cfm manual choke 70 jets.