Truck Doesnt Run Right

[ryhan87]

My truck is running really rough. The worst at lower Rpm under load. It sounds like it backfires back through the carb and loses power. I put a new air filter in a week ago and its is already black and dirty. When i was driving home tonight it got really bad. I noticed when I stopped that my exhaust manifold was glowing red hot. Im thinking its a exhaust restriction, am i going in the right direction?

[cparman]

check the basics first! does your truck have points? are they set correctly? Hows the timing? On start up do you have any smoke? If so what color is it

My guess is that the timing is off. popping back thru the carb would make me think the timing is retarded.

And THAT is my .02 worth

[Legolas894]

I agree with previous-Did you accidently switch plug wires? I put new plugs and wires in recently and then it ran terrible, backfired etc. I was checking all sorts of other things and it turned out that plugs on 1 and 3 cylinders were switched!

[special-K]

I`m thinking timing,wires,all the tuning stuff,and possibly a clogged PCV valve.Pull it out and see if it rattles when you shake it.

[68gmsee]

What engine, mileage, and carburetion? Headers?

I've never seen an exhaust glow red hot unless it was headers on a dragster after a race. I'd be concerned about that. Restricted exhaust can cause rough idling and loss of power so I'd check that out too.

There's several issues that can cause idling rough so I'd start with the basics as mentioned above. Once you've eliminated the ignition, carb issues, then you may be looking at worn cam, burned valves, timing chain.

[68_chop_top]

Have you changed anything recently? Something has changed. I have seen headers glowing red once when I rebuilt an engine and on the initial start up I ran the engine for 20 minutes at 2500 rpm for the cam breakin. The timing was set at about 10 degrees after TDC (way retarded). I have also seen timing jump with a worn out timing chain and it jumped a tooth on the cam gear. Made the timing way retarded.
I would suggest putting a timing light on it and making sure of initial timing set.

[69GMCLonghorn]

Sounds like a bad piston return spring on #5 cylinder...

[mr48chev]

Five good posts with honest advise and one smart #$*.
As the other five said, go to the basics and one by one go through them.

I'm with 68 Chop Top in that I would check the timing first as it may be late.

If it has points check the gap to make sure it is right and then check and set the timing. Along with that check the vacuum advance. A vacuum advance with a ruptured diaphragm will also cause a vacuum leak which will lean out the mixture which will cause hot exhaust along with the retarded timing.

Go through the firing order and make sure that each plug wire goes to the correct plug. On a small block V8 it is usually 5 and 7 that we get crossed up because they are beside each other and fire one after the other.

If the engine is a six with a single barrel check to see if the body of the carb is tight to the base by gripping it with your hands and twisting back and forth. Some times the screws holding the body to the base come loose enough to let this happen.

I'd probably just replace the pcv valve and be done with it as they are under five bucks most places. Also take a serious look at the hose going between it and the carb as I have seen a lot of them crack and leak over the years. Also check the other vacuum hoses on the engine for cracks or leaks.

If those steps don't cure things. I would suggest doing a compression test to make sure that all of the cylinders have good and even or near even compression.

On a compression test you have to pull "ALL" of the spark plugs and prop the throttle wide open to get best results. Pull the coil wire or disconnect the power wire from the hei too. I was taught to and taught all of my students to let each cylinder hit five times. Then write down the results and go to the next cylinder. You are looking first for even compression and then having the compression at or near factory specs.
If you don't own a compression gauge and don't want to buy one you can rent one at Autozone through their rental deal.

If it ran good before you swapped air filter elements my guess is that the timing is off or that there is a vacuum leak and that is what I would check first.

[k5hart]

I immediately thought timing. Retainer may have loosened up and timing changed as you drove. Go simple first. Good luck!

[ryhan87]

I have done some work to my truck recently. The head has been redone with new seats and reground valves. I rebuilt the carb and changed spark plugs, checked the points. Also checked timing at 4deg before TDC. Removed the in carb filter and replaced with a inline filter. Ran new vaccum lines.

Everything ran really good after doing all the work untill a few weeks ago it started running ****ty. Im not sure about the timing being off as sometimes it runs good. It only seems to backfire at low rpm well under load. Anywhere from 1500 to 2500 rpm. Does anyone have any more ideas? Also about the exhaust manifold, its a factory manifold, and at was glowing bright red. I would think its a exhaust restriction and its unable to dissipate heat.

[clay68c10]

The hot manifold could be a restriction, but I'm thinking combustion after the exhaust valve. Either late timing or leaking exh valves.

[CCCC]

There is a lot of good, practical advice above. Given the work done on the engine and the onset of trouble, odds are that it is a vac leak (= lean mix) or a timing issue (late fire?) so take the odds and start there. There seem to be literally hundreds of timing/vac/ign component problems for every exhaust restriction issue, so unless you have had the exhaust apart recently or maybe have a really tight rodent nest WAY up in there, get out the timing light. If timing is off, follow the leads noted above. If problem persists once you are certain that the timing is correct and there are no vac leaks, the diagnosis would become much more specific as noted in some of the other comments. Good luck.