I need HELP?? Motor experts please!!
Thread Starter
Racer
Joined: Dec 2000
Posts: 422
Likes: 0
From: Mesa AZ
I need HELP?? Motor experts please!!
I have a mid 70's 350 that was rebuilt about 10000 miles ago that is running very poorly. I have tried everything I know. It starts and idles OK but at about 2500rpm it starts popping back thru the carb and losing power. I have changed carbs, replaced the plugs, plug wires, cap and rotor, checked timing and firing order several times with no help. I'm running a HEI that was new about 2500 miles ago. The distributor itself is the only thing I haven't changed with relation to ignition and carburetor. I also did a compression test and all cyls are within 5psi, about 160. Could I have skipped a tooth (teeth) on timing chain? That seems remote to me. Any Ideas would be appreciated.
Burning Brakes
Joined: Mar 2004
Posts: 816
Likes: 1
From: Finland
Originally Posted by chadman
I have a mid 70's 350 that was rebuilt about 10000 miles ago that is running very poorly. I have tried everything I know. It starts and idles OK but at about 2500rpm it starts popping back thru the carb and losing power. I have changed carbs, replaced the plugs, plug wires, cap and rotor, checked timing and firing order several times with no help. I'm running a HEI that was new about 2500 miles ago. The distributor itself is the only thing I haven't changed with relation to ignition and carburetor. I also did a compression test and all cyls are within 5psi, about 160. Could I have skipped a tooth (teeth) on timing chain? That seems remote to me. Any Ideas would be appreciated.
Timing chain...have you tried like moving the distributos so that you get the advance first 10degrees one way and tested...if no help then then back and 10 degreed the other way and tested...
Le Mans Master
Joined: Apr 2006
Posts: 5,403
Likes: 95
maybe you got some bad gas check to see if the fuel filter is
clogged and the fuel pump is putting out enough pressure.
Kind of sounds like it is getting enough fuel at Idle but not as
rpm goes up.
clogged and the fuel pump is putting out enough pressure.
Kind of sounds like it is getting enough fuel at Idle but not as
rpm goes up.
Last edited by Little Mouse; Aug 24, 2006 at 04:32 PM.
Thread Starter
Racer
Joined: Dec 2000
Posts: 422
Likes: 0
From: Mesa AZ
OK, I am sure it is not the carb. I just tried my Holley street avenger from my vette (that runs great)
. Same popping!! It has to be something with the timing, right? CAn an HEI module go bad and cause something like this? I thought they either work or don't work. I'm really lost, I have never had a car I couldn't fix before.
. Same popping!! It has to be something with the timing, right? CAn an HEI module go bad and cause something like this? I thought they either work or don't work. I'm really lost, I have never had a car I couldn't fix before.
Melting Slicks
Joined: Dec 2000
Posts: 3,363
Likes: 303
From: Lansdale 19446 PA
2024 C3 of the Year Finalist- Unmodified
St. Jude Donor '10-'11, '15, '19
How did it run before you installed the new HEI? Just a guess but it sounds like a distributor issue. I got a new HEI from Summit and had nothing but problems - rebuilt the original distributor and my 76 has run great since.
It could be a weak coil, a short or a bad pickup coil.
It could be a weak coil, a short or a bad pickup coil.
Intermediate
Joined: Aug 2006
Posts: 30
Likes: 0
From: hawthorne florida
it sounds like a lifter is trying to collapse you can remove the valve covers and try pushing on the back of the rockers if you can push it down with your finger perty easy its going bad it may not be bad enough were it wont idle but bad enough when you give it the gas it wont open the valve all the way
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
It didn't skip a tooth(timing chain) because your compression test came out good. Its got to be an ignition problem. It sounds like the timing is retarded. Maybe the distributor is a tooth off? I know you said you checked the timing but...I think its something with the distributor.
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by 73, Dark Blue 454
Broken valve spring(s)?
Originally Posted by novavet
it sounds like a lifter is trying to collapse
Im guessing the timing has jumped somehow
Do you have another distributor you can try?
Did you line up the timing marks and remove the valve cover to see where you are at?
Intermediate
Joined: Mar 2004
Posts: 46
Likes: 0
From: kalamazoo MI
I had the same problem when I installed my HEI. I would idle fine then through the mid throttle range I would backfire and at WOT I was fine again. Turns out I had an adjustable vacuumed advance (small Allen in the vacuum port), and didn't even know it. I backed off the vacuum advance and all is well with.
Good luck.
Good luck.