I need some help guys!
 

I have a 72' with a 1986 305 in my truck right now. It has some what of a miss. It is kind of a wierd thing. It runs great while running down the highway, but when you come to a stop, it has a slight miss, not constanc. Like every 30 to 45 seconds, ( something longer intervils ) it misses. It does not do this all the time, some times it is worse then others. Some times it is real bad when it is cold, other times when cold it is smooth as silk. It acts like it miss fires just every now and then.

This is what I have done so far
New rotor
New cap It has an HEI in it.
New wires
New plugs.
Timing is set 8 degree advance intial, the vacuum advance is working.
Checked all vacuum hose's, none disconnected.
Carb is adjusted with vacuumm gauge, I have it adjusted to where it is pulling max vacuum,
New air cleaner.
When under a load it doe's not miss at all, very very smooth.
I am really stumped on this one. If it did it all the time, I would think that a valve is not set right, or maybe a broken valve spring, but it might go two or three days without doing this, then all of a sudden it's back, when it is doing this, if, I rev the engine, it goes away.
What do you guys think this could be?
Sam

Could be a bad valve seat and the valve is burnt in one spot and when the bad spot on the valve rotates to the bad spot on the seat it burps.

could it be a brain ive seen wierd things happen with those things

My best guess would electrical, aka alternator output too low, or some such. When you added the HEI, did you run a new wire to it? The old one is 6 volt, the HEI needs 12. The truck can run on 6 volts, but the HEI is at its lower threshold at 6, and any sort of drop could push it over the edge. Revving the engine brings the volts back up, clearing the problem. If the truck comes to a stop, the alternator output drops a little, and if you are weak on juice, it could cause what you describe. If you have a voltmeter, it would be super handy to monitor it too see what is happening. Unfortunately, most of the trucks only have an ammeter, but a volt meter is what ought to be used.
This is my best guess only.

Double and triple check the basics, such as, possibly a spark plug cracked during installation, plug wire not pushed on all the way, plug wire end separated from wire (not visable under boot), not impossible you may have one bad spark plug even though they are new. Did it do this before or after you changed those parts?

Interesting problem and no help here as I have the exact same thing happening with my 71 350. It happened before I changed it over to HEI and even with:
New plugs
New wires
Timing set at 8+
larger feed wire to the HEI
Carb idle adjusted
etc.
Sounds exactly the same---never on acceleration, only when ideling and not consistant. More obvious if running a little harder then a quick stop to idle---then it feels like one bum cylinder. Haven't put a vacuum gauge on it or checked it at night for spark jump, but would think that whatever is happening, it was happening prior to HEI and after HEI.
Hope someone comes up with a good suggestion on how to find this little gremlin!! Huck:(

Do a compression test. I have 2 blazer a 70 and 71. Went through numerious carbs, dists and plugs. Finally checked comp. 160 across the board except one cylinder had 50. Bought my 70 this summer documented 75,000 miles tuned it up got worse. went right to compression test. One downed cylinder had flat spotted one lifter and that was it.

Do you have a gas cap? Did the truck go without a gas cap for a while? (a red rag is not a gas cap) Does your gas cap seal very well?
Water in the fuel will cause this too, and will make you go crazy tryin to figure it out.
Buy some water remover...or just dump 1/2 bottle of rubbing alcohol (the stuff in the medicine cabnet) into a full tank of gas.
It sure can't hurt, and at a buck or so...it is worth a shot regardles of the cap/condition.

Longhornmail
I have a gas cap, no red rags, LMAO! Andy it has been doing this for a while, I figured if it was water in the fuel, it would have worked itself out by now, I drive the truck everyday, 65 miles round trip to work. And here lately I have been working everyday for two months straight now, I fill the truck up about every 3 rd day. I want to try what jku72 suggested, I did not install the HEI, someone else already did, so now what I need to know, is if it is wired in wrong, what is the correct way to wire it in.

...where should the wire be comming from, from my fuse box etc
... how big of wire ect.
If some one would give me some good details on this, my weakest link to doing work on cars and trucks is wireing, I do not have a lot of knowlege in this area, so details would help me out alot.
By the way, I still have the external regulator on my chargeing system.
Thanks,
Sam

ttt

Sam:

The wire to the "BAT" terminal of your HEI distributor needs a full 12 volts, as mentioned. It's real simple - just ensure you have a straight wire from your fuse box to the HEI. I run mine from an unfused spade in the fuse box. I don't know the guage of the wire - probably 12 or 14 guage; pretty thick. You would know you had a problem if the wire going to the BAT terminal on your HEI came from the wiring harness or anywhere other than a straight shot from the fuse box. I think the old distributors ran off the yellow wire, which doesn't put out enough volts for the HEI. Hope this helps.

Chris

10 or 12 gauge
 

is what u use for the ign. wire I came off the back of my ign. switch I belive it was the pink wire on it .U can crimp a male electrical spade on the end and it will slip right in beside the pink wire,put a inline fuse in it allso.:bowtie:

Mine was doing just about the same thing. 40 mile round trip and everytime i got off the interstate and stopped at the light it was stumbling. It turned out to be the voltage regulator wasnt putting out enough voltage. Replaced it and the alt. and it I havent had a problem since. Except that I had to replace the battery 2 days later.
Jack