Help! 65 corvette misfires

[rick f.]

i have a rough idle and a slight misfire when cruising. sputters when pulling away from a light but seems to run fine under a load. switched carbs and still does it. changed vacuum advance and noticed that the rod that fits into the breaker plate is a little loose. i can move the breaker plate a little bit by hand is that normal? exhaust smells like unburned fuel. gonna adjust the valves my next step...any insight? thanks

 

[charleshwhite]

i have a rough idle and a slight misfire when cruising. sputters when pulling away from a light but seems to run fine under a load. switched carbs and still does it. changed vacuum advance and noticed that the rod that fits into the breaker plate is a little loose. i can move the breaker plate a little bit by hand is that normal? exhaust smells like unburned fuel. gonna adjust the valves my next step...any insight? thanks

Which engine do you have?

 

[rick f.]

Which engine do you have?

327/ 300hp

 

[rick f.]

no valve spring tension

i was adjusting the valves in my 65 327/ 300 and noticed one of the valves has zero tension.....is it the spring or could it be the lifter or the cam?

 

[JohnZ]

i was adjusting the valves in my 65 327/ 300 and noticed one of the valves has zero tension.....is it the spring or could it be the lifter or the cam?

What do you mean by "zero tension"? Is the rocker arm moving, the valve not opening, or ?? Need a better description of the issue.

 

[rick f.]

What do you mean by "zero tension"? Is the rocker arm moving, the valve not opening, or ?? Need a better description of the issue.

had the motor running, warmed up, valve cover off and loosened up rocker nut to clatter. then slowly tightened to no clatter, then 1/4 turn for each valve. one valve#8, second valve from firewall felt extremely loose like collapsed lifter or something. also seemed like the rocker was askew. so, i took the rocker off and pulled out the pushrod to see if it was bent...seemed straight so i put it back together and now it felt like the rest of them. yes, all seemed to be functioning properly but car still has a misfire. could that lifter be sticking? just trying to get it to run smooth.

 

[BLACK MOON]

This may sound strange but if there is a vacum port above that cylinder on the intake, plug it and see if you still have the problem. I chased a misfire on #8 for 2 years and found a bad power brake booster causing the problem. The booster went to a vacum port on the intake at cylinder #8. This caused a weak charge or lean condition and # 8 would pop at different times. I had been driving for 2 years with a blown brake booster and didn't realize it. New booster and misfire went away. This after changing plugs, wires, carb, lifters, push rods, etc. could only happen to me but you may have the same situation.

 

[BLACK MOON]

After a month any update?

 

[charleshwhite]

327/ 300hp

Rcik:

Sorry about taking so long to reply. My experience with the 300 is limited (also, I thought they were hydraulic valves with only the 365/375 and later big blocks having mechanical valve tappets; but one lives and learns). I do have Shop Maintenance/Services Manuals for the C2 + 65 version - somewhere, but just can't find them yet. After reading comments the only thing I can add at this time is to check the compression on each cylinder. Further I think a worn valve guide may also give you a rough runner.

In the interest of full disclosure I don't really work on either of my Vettes (I know a Mechanic that works on them for large portions of my meager net worth). Others can attest that I don't really know very much (LOL); but that has never stopped me from offering suggestions.

I will continue to look for those books to see what they may offer; but, I fear they may have gone walkabout. You may wish to pick up copies from Helm.

Charles

 

[haganml]

Things to check.

Here are some simple checks that can help you diagnose a rough idle. With the proper caution and planning these test are easy and safe. Be sure that you are not wearing any loose or dangling clothing, jewelry or anything else. you will be working and moving around spinning fan blades, moving belts and pulleys.

Lumpy idle could be vacuum leak in the intake side of the cylinder. Do this only in a well ventilated area, not in a enclosed space like a garage. Try a propane enrichment system to check around the intake manifold, carburetor base and any hose that attaches to the air plenum of the intake manifold. A propane enrichment tester has a propane supply and small hose that will supply a small amount of fuel as a gas to a specific area. If there is a lean misfire (air leaking into the manifold between the carburetor and the intake valve also known as bypass air) the engine will smooth out as you supply gas to the area that is allowing bypass air. Propane is a gas that is heaver than air and can pool in low areas. Allow this gas to dissipate (fade into the air) before moving to the next step.

With the engine running at idle you can remove the spark plug wires one at a time (CAUTION this is very dangerous without the proper insulating tool. Wear latex gloves and do not allow any part of your body to touch any metal) and the one with the least effect on the idle is the one you need to check. You MUST be sure that you are confident that you insulated and are aware of the hazards of electrical shock before attempting this.