My truck has a slight miss! BUt NOT ALL THE TIME?? Help me out here!
 

I have a 305 in the truck, I did a complete tune up on it about 3 months ago, rotor, cap, wires, plugs. All new, it has been running great. Smooth, but before I did this it had a slight miss, but not all the time, and just at idle, not under a load. Well, now that slight miss is back, but it is not all the time. What I mean by this, is some days it runs very very smooth, then one day I might get in it and it pull up to a light, and there it is, it is not a constant miss either, just every now and then you can feel it. Hell, I might go a whole week with out feeling it, then all of a sudden, there it is again, I have also upgraded my chargeing system, I thought it might have been that at one time, but it's still doing it! I have a 140 amp alt, internal regulated. One wire .Sowhat doyou think it could be? I wish I could some how, exssplain better of what it is doing, but it's hard too.
Sam

sam, I hear ya buudy... I have the same thing in my 95 K1500 with 350. Replaced the coil, wires, cap, rotor, and plugs. Thought the intermittent miss was gone, and a week later it came back!! Sorry I'm no help, but I know what you mean, and will be closely monitering this post for advice also. Drives ya nuckin futs doesn't it!!!

class of 69,
Yeah it will drive you nuts! I don't understand this one, hell there is times when it goes away for like a month, then it's back, then it might do it everyday, for like 5 days, then it goes away, it isn't a big miss, just real slight, enough to feel it.
Sam

The carb (assuming it's carberated) may be running a little off. How do the plugs look? What color? Are they uniform? What color does the inside of the tail pipe(s) look like?

Had the same problem on my 99, I changed the plugs and it went away. The original plugs only had 25K miles on them. I installed the new Bosch Platinum 4, hadnt had a problem in 12K miles.

It is carerated, I have not pulled the plugs out since I put them in about three months ago, they don't have 5000 miles on them yet, they are Auto lite platinum. I have adjusted the carb, to where I recieve max vacuum on a gauge. Way does'nt it do it all the time? It acts like a egnition problem, but hell it does not do it constant enough to try and pin it down.
Sam

They stick autolites in fords don`t they? That could be your problem. :D

miss
 

Running headers? Might want to check plug wires just to make sure they are not arcing on metal somewhere. Even if wires are new doesn't mean they are good.

Speaking of Fords no matter what I do to my 300 straight 6 it still misses at idle, so as far as I am concerned it can miss till the cows come home.

miss
 

If your ignition is up to snuff, check for a vacuum leak, especially around the intake/head areas. Possibly to power brake booster.

I don't have headers, I looked at this the other night, didn't see any arcing at all. Now as far as a vacuum leak, that is what I was thinking also, going to check it out this weekend, probabley change out all the hoses and see how that goes.
Sam

sam's 72, I have to agree the Autolites belong in the F**ds (don't like to swear on the board)

adamls I like the Bosch plugs, I put them in my 89 Harley.

Sam, electric or mechanical fuel pump? On my '91 w/350 I had EXACTLY the problem you are describing until the electric pump quit completely one day. After replacing pump, miss went away.

Class of '69, on your '95, you should be able to hear the pump. It'll be a real high pitched buzz coming from the behind the cab. If it sounds kinda sick or intermittent when the truck is missing, then I'd go to NAPA and get a pump for $30. Make sure you drive most of the gas out before you drop that tank, though. HEAVY.

The fuel pump is mechanical, I have removed the fuel filter from the carb, and installed a inline filter.
Sam

What are your plugs gapped at? My truck was idleing poorly, pulled the plugs, one was at .06?!? regaped them all to .45, smoothed it out alot. maybe too much vac advance at idle? Good luck
Jesse

Mine does the same thing. Someone told me once that it could be the RV cam that I put in it, makes it harder to even out at idle. But I don't know anything about that. I think I might try that idea for checking the gap on the plugs.

Tynee, I didn't feel it was fuel caused, but i may have to rethink that. The miss appears so suddenly, it's like the ignition switch is being turned off and on quickly. There have 3 times now when at a stop light , it just starts running really rough, and allmost feels like it will die. but when the light changes, and i give it throttle, it comes around again and runs smooth like nothing ever happened. Are these TBI's dependant on constant fuel pressure, unlike a float system?? The truck just turned 103 rounds, so any thing is possible. I put in the MSD Blaster coil, and Accel super stock wires, cap, and rotor. Stock AC plugs. It starts much quicker now, and runs very nice... until it starts missing!!

Class, that sounds EXACTLY like my '91's symptoms. Had electronic diagnostics run, and they said the fuel pressure was low, probably due to a bad pump. I think the TBI's must be dependent on that constant pressure. If you've got a buddy with one of those Sun-pro scanners from Snap-On, it'll tell you what kind of pressure you're getting.

Tynee, I'd have to take it to a shop for diagnosis.... may as well use the money for a new pump!! With the mileage on the truck, it won't hurt to just change it out. I'm planning on a 1600 mile parts run next week, And I don't need it dieing some where on the freeway! thanks for the help.. I hope Sams72 got some help from HIS thread!!??

CO69, I replace a lot of GM fuel pumps, some even at 80,000 ticks. I would also look at the TPS. Also clean the fuel injectors and the valves.

Are you still running points? The dwell has a tendency to not stay set very long.

Could be the Idle circuit in your carb is dirty, try cleaning the carb really well, this happened to me before, same sort of thing.

I posted this last week, I have not done anything to the truck, and now this week, it is running smooth as silk! I don't understand why it does this, makes me think that I might be getting bad gas, maybe a little water in it.
Hell, I don't know!
Sam

Sam,

It just warmed up by you didn't it? Your choke might be off, and when it's cold, you could be rich, causing the plugs to foul out. When it warms up, the plugs clean up, and runs well. Sure sounds like that, or a secondary ignition problem of sorts.

No, it does it after it has been warmed up, I can drive it 30 miles and sometimes this miss is there. But this week, it is running smooth. This has been going on for a while now, every since i bought the truck, and the miss is not constant, it is very slight.
dtlilly, what do you mean by secondary ignition problem of sorts.
What can I look for?
Sam

try richening up the mixture a little bit ..... 1/4 turn (counter clockwise) on both of my mixture screws cleared that up

Talking out of ignorance here, but does your truck have an IAC and computer?
If so, remove the IAC and spray it with carb cleaner, then put it back.
If you can find the wire to the computer (might be going directly to the battery with an inline fuse) disconnect the fuse and let it sit for a few minutes. Then reconnect.
Another thing - make sure all your spark plug wires are away from the exhaust manifolds. Check that when it's hot.