Mis Rough Idle HELP
Thread Starter
Cruising
Joined: Oct 2014
Posts: 13
Likes: 0
Mis Rough Idle HELP
Just joined Corvette Forum and have so enjoyed reading helpful post.
I have a problem with a 80' C3 stock to the bone I purchased last year. Here is what I have so far, very rough idle almost like vacuum leak. I plugged ALL vacuum to test still same rough idle and spray tested. New dizzy, new Rochester carb, new plugs and wires. Nothing helped. Compression test show in low 140's all the way across. I am at a loss. Any and all suggestions would be greatly appreciated. Thanks Steve
I have a problem with a 80' C3 stock to the bone I purchased last year. Here is what I have so far, very rough idle almost like vacuum leak. I plugged ALL vacuum to test still same rough idle and spray tested. New dizzy, new Rochester carb, new plugs and wires. Nothing helped. Compression test show in low 140's all the way across. I am at a loss. Any and all suggestions would be greatly appreciated. Thanks Steve
Racer
Joined: Aug 2006
Posts: 330
Likes: 3
From: Wellington, New Zealand
Check your ignition timing it sounds like you have too much advance, could be either vacuum or mechanical advance at fault. Did you disconnect the vacuum line to the distributor when you did your testing? Did the idle improve when the vacuum line was disconnected?
Thread Starter
Cruising
Joined: Oct 2014
Posts: 13
Likes: 0
I am now thinking I have a valve issue, I put a vacuum gauge on during idle and it was bouncing all over the place. I am pulling the valve covers. Not sure how to verify which may be the culprit.
Thread Starter
Cruising
Joined: Oct 2014
Posts: 13
Likes: 0
No just a bad mis. The compression is good all across however the vacuum gauge fluctuates greatly. I do think I hear a ticking. I think my next test is leak down, but if one valve is not opening will leak down verify anything ?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
10 Things C8 Corvette Owners Hate (But Won't Tell You)
Joe Kucinski
10 Best Corvettes Coming to Barrett-Jackson Palm Beach 2026!
Brett Foote
Every Corvette Grand Sport Explained! (C2, C4, C6, C7, & C8)
Joe Kucinski
Melting Slicks
Joined: Jan 2014
Posts: 2,478
Likes: 254
From: Downers Grove Illinois
The easiest way to check the pushrods would be to remove the valve covers and inspect them. If all the plugs look wet your probably running the carb too rich. Is there any fouling of the plugs?
Thread Starter
Cruising
Joined: Oct 2014
Posts: 13
Likes: 0
Gas for sure. I pulled the valve covers late last night. Now will take a look at valve, rocker and pushrod movement. Then perform leak test but again a bit confused on if compression is good how leak down will help verify anything. If pushrod is bent the valve stays closed and leak down and compression will pass. Thanks
Tech Contributor
Joined: Aug 1999
Posts: 14,356
Likes: 6,332
From: At my Bar drinking and wrenching in Lafayette Colorado
Tech Contributor
Joined: Aug 1999
Posts: 14,356
Likes: 6,332
From: At my Bar drinking and wrenching in Lafayette Colorado
Thread Starter
Cruising
Joined: Oct 2014
Posts: 13
Likes: 0
Lars, Unfortunately the carb that was on it when I purchased was not the original either. The carb I purchased was from Guaranteed Carburetors. I replaced the carb to try and correct this bad idle problem. Can I assume from your comments that the vacuum being bouncing erratically on the gauge is due to the carb also ?? or still having a valve issue also ? Thanks again