I need some help guys! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

sam's 72 · 12-30-2002, 09:03 PM

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

guitarman · 12-30-2002, 09:28 PM

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.

**old Rusty C10** · 12-30-2002, 09:31 PM

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

jku72 · 12-30-2002, 09:55 PM

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.

Gordo · 12-30-2002, 10:15 PM

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?

Huck · 12-30-2002, 10:27 PM

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

**pound41** · 12-30-2002, 11:22 PM

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.

Longhorn Man · 12-31-2002, 12:07 AM

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.

sam's 72 · 12-31-2002, 07:18 AM

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

sam's 72 · 01-01-2003, 09:06 AM

ttt

72K10 · 01-01-2003, 10:10 AM

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

kaycee · 01-01-2003, 12:11 PM

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.

cheatham · 01-01-2003, 03:13 PM

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

12-30-2002, 09:03 PM	#1
sam's 72 Senior Member Join Date: Nov 2001 Location: Pasadena, Texas, USA Posts: 1,005	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 __________________ 72 LWB chevy truck. Pasadena, Texas

12-30-2002, 09:28 PM	#2
guitarman Do it well or don't do it Join Date: Apr 2002 Location: Georgia Posts: 457	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. __________________ Try to be the person your dog thinks you are. Excellent paint and body site: http://www.autobody101.com/ Visit my Web Site http://home.attbi.com/~agfosteriii

12-30-2002, 09:31 PM	#3
old Rusty C10 Robert Olson Transport Join Date: Oct 2002 Location: recent transplant to NC USA Posts: 20,275	could it be a brain ive seen wierd things happen with those things __________________ Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!!

12-30-2002, 09:55 PM	#4
jku72 Registered User Join Date: Jan 2002 Location: cornville, maine Posts: 659	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. __________________ jku Cornville, Maine 68 C10 burb 42 Willys MB 46 Dodge WF-32 1.5 ton 53 & 56 Olds' 60 Cadillac 22 Dodge Brothers

12-30-2002, 10:15 PM	#5
Gordo Registered User Join Date: Jan 2001 Location: Killingworth, CT. USA Posts: 3,378	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? __________________ 1971 C10 swb stepside 350/700R4/3.73posi (retired as of 4/22/03) 1998 S10 short bed 2002 S10 Blazer 1942 Oldsmobile 1958 Massey Harris Pony 1951 Wife Killingworth, Connecticut May those who love us, love us, any of those who do not love us, may God turn their hearts. And if God is unable to turn their hearts, may he turn their ankles so we may know them by their limping. A man who works with his hands is a laborer; a man who works with his hands and his brain is a craftsman; but a man who works with his hands and his brain and his heart is an artist.

12-30-2002, 10:27 PM	#6
Huck Senior Member Join Date: Jun 2001 Location: Shelbyville, KY Posts: 3,269	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

12-30-2002, 11:22 PM	#7
pound41 Registered User Join Date: Dec 2002 Location: Northern Virginia Posts: 1,269	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.

12-31-2002, 12:07 AM	#8
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	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.

12-31-2002, 07:18 AM	#9
sam's 72 Senior Member Join Date: Nov 2001 Location: Pasadena, Texas, USA Posts: 1,005	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 __________________ 72 LWB chevy truck. Pasadena, Texas

01-01-2003, 09:06 AM	#10
sam's 72 Senior Member Join Date: Nov 2001 Location: Pasadena, Texas, USA Posts: 1,005	ttt __________________ 72 LWB chevy truck. Pasadena, Texas

01-01-2003, 10:10 AM	#11
72K10 Registered User Join Date: Dec 1999 Location: New York Posts: 288	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 __________________ RLTW 70C10 72K10 72K10 Cheyenne 72C10 Cheyenne Super

01-01-2003, 12:11 PM	#12
kaycee Registered User Join Date: Jan 2000 Location: Wyoming USA Posts: 2,445	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. __________________ 2001 Silverado K-1500 2005 FXDWG stage3 69 CHEVY Short/Step 327/4L60 373 posi 63 IMPALA 327

01-01-2003, 03:13 PM	#13
cheatham I need a brake! Join Date: Jul 2002 Location: Nashville, TN Posts: 474	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



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