i got a 78 f-150 with a 302 just dropped it in and went out to see how it ran and seems like when i hammer on it, it shutters and shakes and will backfire through the carb. Would this be a bad intake valve? I set the timing advanced 7 degrees idk if that would have anything todo with it or not?

 

Ah, you mean "Shudder"....

Did your rebuild the engine or simply pirate it from another vehicle? How does it run at idle and cruising RPM? More info please: stick or auto, etc....

C'mon doc, you should really to do some troubleshooting by process of deduction or elimination.

I think you may have a couple of issues: 1) clutch chatter ; 2) Ignition timing or advance; 3) carburetor adjustments

 

yes shudder.... i just took it outta another 78 i had, its a 4 speed. just idleing it sounds good but if you just punch it and let of really quick it will sometimes back fire outta carb and going down road at cruising rpm its not bad at all its when u got your foot into it but when you get to high rpms you dont notice it as bad its like it misses then fires (shudders). ive got a new clutch in it, what do you mean by ignition timing or advance? the carb is off the old 302 i had in there it is different then the donner motor had on it?

 

Sounds ignition related to me, but that's just a guess. How are the plugs, wires, cap, rotor, points (if it somehow has them though it shouldn't if it came out of a 78)?

What he means by ignition timing and advance, likely, is as follows:

1. You set the timing, did you disconnect the vacuum advance like you were supposed to when you did that? Secondly, have you manually applied vacuum to make sure that the vacuum advance is working? Thirdly, have you checked to make sure the mechanical advance is working?

 

yes got new plugs wires cap rotor no does not have points. i did not disconnect the advance. Y does that effect it? (great stuff to learn cause i have no idea on some of this stuff)

 

Well, when you disconnect the vacuum advance, be sure to plug that vacuum port or you'll be setting the timing with a vacuum leak.

It makes a difference because the vacuum advance can come into play at idle and can actually make the timing flutter a little. You get a much more steady, and accurate reading, hence the term "base timing" It is the base that is set before anything else impacts it, such as mechanical or vacuum advances.

 

alright thanks for the info! ill have to do some tinkering. would that cause the backfire through carb?

 

Incorrect timing can cause all kinds of problems.

I have seen cars (mine included) throw a flame out of the tailpipe when you let off at high RPM. Typically for this to happen you have to be running rich enough to have unburned fuel in the exhaust, an exhaust leak to admit some fresh air into the exhaust. You let off from wide open throttle and during that split second it just so happens that the exhaust valve is opened while the charge is still burning and boom... it's actually quite spectacular.

I would imagine that a backfire through the carb would happen something like this:

Imagine that you're at a high RPM so you've got a lot of mechanical advance but wide open throttle produces little vacuum so the vacuum advance is minimal. You've also got a ton of air and fuel flowing through the intake. You suddenly let off the throttle completely, the throttle blades slam closed which creates a huge spike in vacuum, which suddenly advances the vacuum advance all the way. You couple this with the full mechanical advance and you've got a spark that is gonna occur VERY early. If it happens to occur early enough that the intake valve is open on one of the cylinders, it can ignite all the air and fuel in the cylinder, back up through the cylinder head, and into the intake.

If you have your base timing set too far, or you're getting too much mechanical advance, or too much vacuum advance, or any combination of these you could potentially cause this.

If wikipedia can be trusted I wasn't too far off!
http://en.wikipedia.org/wiki/Back-fire

It looks like it could also be caused by running lean. You said the carb came from something different, is it jetted and set up properly for a 302?

 

dang, I thought he was talking about something like this:



That would be cool in our trucks

 

so should i try a new vacuum advance? or just keep tinkering with timing?

 

Check the vacuum advance by pushing the little lever and holding your thumb over the metal pipe and see if it holds vacuum.

 

You can also test it with one of those hand held vacuum pumps. Start the engine and manually apply vacuum to the diaphragm. If it is working you'll be able to hear the change in the engine.