engine backfire and loss of power

[neilbyrd]

Went up bee canyon today and the farther I went up the worse my jeep ran. It sounded like the gas was popping in the headers and when I stepped on the gas it would backfire and sputter really bad. It got a little better in 4th gear. On the way back down it seemed to get a little better the farther down I got. I am running a 302 with a truck avenger carb. It always smelled a bit rich. I think the carb needs to be rejetted for the elevation. Can I get some input from you guys if I am on track or is there something I am missing.

[6spdYJ]

Had a 350 do this once. It drove fine on the trip out and had to be limped home 70 miles in 2nd gear. The valves were all fried. Hope this ain't it for you.

[Russ Chung]

Carburetor jetting is a plausible theory. What do your spark plugs look like? If you're running too rich, the plugs should show soot.

[RAT]

Is it the aluminum avenger 670 cfm? Jetting is easy on Holly carbs. If it's the new style metering plates you don't have to remove them to re-jet. You can pick up a variety pack of jets from Pep boys or a local speed shop. Russ is right; pull the plugs; if the are white/tan, they are good and there might be something else going on, if they are black and sooty, too rich. And from what you describe it might just be a jetting issue. You can tune the carb for both altitude and sea level, just a bit of a compromise depending on how much performance you actually want when driving around town. You can rich it up little and go a few numbers hotter on the plugs, or lean it down and run a colder plug. But its all easily done.

[neilbyrd]

I am going to go check the plugs this morning.

[neilbyrd]

So I checked the plugs and they have soot on them. I called Holley and they suggested going down 2 sizes. Called Summit and they are on their way. Thanks for the input.

[RAT]

So I checked the plugs and they have soot on them. I called Holley and they suggested going down 2 sizes. Called Summit and they are on their way. Thanks for the input.

Awsome, but remember, going down two sizes at your altitude will lean out the engine at higher altitude, and too lean is not good. So even if the engine starts running better after the change, make sure to check the plugs a couple of times afterward to make sure they are burning correctly. And, don't make any other changes to the carb like changing idle mixture or anything. Change the jets and drive it for a while unless the change makes it worse. Always with carb; one change at a time. Makes going back easier.

[neilbyrd]

ya I believe I need it to lean out in higher elevation that is where I was haveing the problems. If it is to lean what will the plugs look like then?

[RAT]

ya I believe I need it to lean out in higher elevation that is where I was haveing the problems. If it is to lean what will the plugs look like then?

bright white and the insulator may even be pitted if you run it that way too long. A proper burning plug's insulator will be tan either all the way around or slightly lighter on one side. Black is rich, white is lean. You need to drive the Jeep for a few miles under your normal driving conditions to get a good read on the plugs.

[neilbyrd]

ok thanks. I should have the new jets in a couple days, install sunday and take it up the trail a ways see how it runs.

[1 Bad F N Z]

Awsome, but remember, going down two sizes at your altitude will lean out the engine at higher altitude, and too lean is not good. So even if the engine starts running better after the change, make sure to check the plugs a couple of times afterward to make sure they are burning correctly. And, don't make any other changes to the carb like changing idle mixture or anything. Change the jets and drive it for a while unless the change makes it worse. Always with carb; one change at a time. Makes going back easier.

Rat at higher altitutes, the engine will run richer. Less air makes a engine run richer.

[RAT]

Rat at higher altitutes, the engine will run richer. Less air makes a engine run richer.

I stand corrected. Note to self: Proof read before posting... Thanks kevin.

[chris142]

I don't think 2 sizes will make much difference. If it was mine I'd go 4 or 6 sizes leaner, check the plugs after a cruise and listen for any detonation.

[RAT]

I don't think 2 sizes will make much difference. If it was mine I'd go 4 or 6 sizes leaner, check the plugs after a cruise and listen for any detonation.

holley almost always recommends changing no more than 2 sizes at a time. Just for the reason you mentioned; detonation. They have to play safe.

[chris142]

Ya that makes sense but his carb must be running extreemly rich and I don't think 2 sizes will make enough difference.

But I have acess to a chassie dyno too so I can see what the mixture really is.

My pickup spent 8hrs on the dyno trying to jet an Edelbrock carb. We gave up on that piece of junk and went to a Holley.