having a miss?

[white71chevy]

I got a 71 chevy with a 80s 305 in it and I'm getting a miss from it. My question is what's causing it my carb was messed up on it took it to a guy to work on it and got it back. It ran great for awhile but now its doing it again. What he did was change the plugs (he said they were bad I no they were cause they were black and gummed up) and he tuned the carb set the ports and all he set the timing the best he could (I don't have a timing mark on it idk where it went I have never seen it on there) and he set the points on the dissy. So what could be causing my miss? Thanks guys.
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[mr48chev]

The timing mark is most likely straight up at 12 o'clock to the damper on that engine. If you have the long water pump you can look straight down behind the water pump and see the tab on the front of the timing cover if it hasn't been changed. I have the same thing on the 79 350 in my 71. Find the mark on the damper and mark it good and outline it with some light colored paint and then clean off the timing tab. If you used the short pump on the engine to match the truck you will have to figure something else out and maybe bolt on an after market tab on the timing cover bolts and bring number one up on exact tdc and then mark it accordingly.
As for the miss, make sure that the wires are good and none are touching the exhaust. Also make sure that they are pushed in the cap all the way and that they are pushed on the plugs all the way.

Also check the cap for cruddy connectors and cracks.

If that doesn't work check for vacuum leaks.

[white71chevy]

The cap is kinda new and the wires aren't touching the exhuast I think the wires r bad tho
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[truckster]

You can isolate the misfire to a single cylinder by pulling the plug wires one at a time while the engine is running. It's usually easiest to do this at the distributor. When you pull a plug wire, the engine should run rougher. If you pull a plug wire that has a steady miss, the engine idle won't change.

Of course, if it's not an ignition misfire, it could be caused by the carburetor or a leak in the intake manifold. Look for a broken or cracked vacuum line first. You can hook up a vacuum gauge and look for a fluctuation in the gauge at idle, or you can spray carb or brake cleaner around the manifold. If you spray the brake cleaner around the manifold and the engine speed up, that's an indication of a vacuum leak.

[white71chevy]

Ya but the miss fire isn't very often sometimes it happens a lot sometimes it happens every now and then or not at all.
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[geezer#99]

Experiment time!! Pull the vacuum hose off the distributor and plug the hose and see if the miss leaves. If it does you know it's timing related.

[Rooster's 67]

can you narrow down when it misses? like cold engine, under load, light load, heavy load, only warmed up,in gear, in neutral?

did he replace the points or just check/set the gap?
you stated the spark plugs were gummed up, do you mean heavy with carbon or large amounts of black crusty build up(caused by burned oil)?

check these out and see if yours looked liked any of these:
http://www.classictruckshop.com/club...park/plugs.htm

as far as timing is concerned, if there is no pointer it could still be timed by vacuum and "ear" but ya kinda have to have some knowledge of what detonation sounds like too.

[ETsC10]

One thing you could do at night is go start the truck & if it's mis-firing, look under the hood (in the dark) and check for arcing.

Tons of great 'old school' diagnostics have already been mentioned.

BTW-detonation can sound like a ticking exhaust leak or some thing rattling a metal box-o-rocks under your hood. Not good.

[white71chevy]

Originally Posted by Rooster's 67

can you narrow down when it misses? like cold engine, under load, light load, heavy load, only warmed up,in gear, in neutral?

did he replace the points or just check/set the gap?
you stated the spark plugs were gummed up, do you mean heavy with carbon or large amounts of black crusty build up(caused by burned oil)?

check these out and see if yours looked liked any of these:
http://www.classictruckshop.com/club...park/plugs.htm

as far as timing is concerned, if there is no pointer it could still be timed by vacuum and "ear" but ya kinda have to have some knowledge of what detonation sounds like too.

He jsut set the gap on the points. The spark plugs were heavy with carbon they werent black at all.