burned valve?

by McCauley5983 Sat Dec 03, 2011 8:51 pm

Ok having issues with motor has a bit of hesitation off idle runs really rough when I press the throttle in the driveway at about 1000rpms then it smoother out and then it misfires.i heard that if you have suction on Tue tailpipe it could be a burned valve so I took a piece of paper and put it against the tailpipe and it tried to suck it in on the 2,4,6,8 side of the motor. What are common symptoms of a burned valve

by 77mali Sat Dec 03, 2011 9:02 pm

If you have recently done anything to the dist or wires/plugs...check & make sure the plug wires are on the correct cap connections according to the firing order. That happened to me recently when I changed put the plugs & had similar symptoms.

by TWBouska AKA Wooderson Sat Dec 03, 2011 11:51 pm

make sure none of the spark plus wires are burnt, also, check out your cap and rotor, because i fried by cap and rotor and it was doing the same things you were talking about for about a month before it failed finally, probably about time for a tune up anyways.

replace everything you see in the pic, (cap, rotor, control module) and see if that doesnt take care of the problem.

by McCauley5983 Sun Dec 04, 2011 12:10 am

Already did its still there:/

by TWBouska AKA Wooderson Sun Dec 04, 2011 12:25 am

and you checked all your spark plus wires? theres no burns or anything on wires were the insulation is gone and its exposed and could possibly arc? also, did you check to make sure ALL of the boots are on as tight as possible? if youve done all these, you may want to borrow a vacuum gauge, or buy a cheap one, and run a compression test on each cylinder, as long as there within 10 PSI of each other, you know compression is good. after that, i cant really think of anything else to look at, but, from personal exp. it seems like its almost always something to do with the ignition system that causes an idle problem in a certain part of the rpm band. usually if it happens at low idle its because you have a wire arcing and the engine runs poorly until the rpm climbs high enough that the distributor is making so much spark the arcing problem is no longer affecting the motor as bad.

by TWBouska AKA Wooderson Sun Dec 04, 2011 12:27 am

and did you follow 77 malis advice and make sure firing order is correct? also, another thing i though of is your idle air control may possibly be sticking, to test that, you will also need a vacuum gauge.

by ant7377 Sun Dec 04, 2011 12:29 am

You can put air into the suspect cylinder with an air hose say from a compression tester ,with the cylinder at TDC it should not escape. If you see it come from the carb -intake problem. From the tailpipe exhaust valve. Really sucks. I had a problem over the summer like that,still not exactly sure what it was but I replaced the heads and its good now. I think I had a receded valve seat .they were late 60's heads and the guy was supoosed to replace the seats with hardened ones when i had them done, but I dont think he did.

by 77mali Sun Dec 04, 2011 2:31 am

If you want to you can also check the valves by removing the exhaust manifold and turning the engine over by hand to check both the intake & exhaust valves on their respective strokes. If your prob is on the even side only as you suspect start there. W/ the engine @ TDC you can just follow the firing order as you turn and take a peak inside w/ a flashlight to see if the valves move up & down as they are supposed to. If you are turning the engine and you notice that one or more don't open or close all the way then you know you have a problem. You can also check for any signs of severe wear & tear too.

by McCauley5983 Sun Dec 04, 2011 1:15 pm

Thanks I appreciate all the advise ill let you know what I find

by BlackChevelleSS Sun Dec 04, 2011 1:31 pm

compression check, if the burned valve is enough to show a misfire while runnning do a compression check, if a cylinder is considerably lower than the rest id start with those valves.

by 77mali Sun Dec 04, 2011 1:50 pm

It will be easier to check with the manifold off if you remove the wheel wheel & tire unless you enjoy gymnastics, LOL. I thought about this also though, it may be worth it to look under the valve cover first and check to see if you have a loose rocker or bent push rod. If you have over 80,000 miles on the engine the valve lash may be looser than it should be if a push rod is set too loose it will cause the valve spring to not compress the push rod down all the way on that stroke. See if you can easily move or turn the rockers by hand, if so the lash is loose.

by McCauley5983 Sun Dec 04, 2011 3:05 pm

No I adjusted the valves less than 2000 miles ago because of The miss its hydraulic cam and since I did the 4 speed conversion the miss is way more noticeable

by ant7377 Sun Dec 04, 2011 9:26 pm

Could it be a bad cam lobe? Try running for a little while with the valve cover off see if they all move.

by 77mali Sun Dec 04, 2011 9:50 pm

billy-ray wrote:No I adjusted the valves less than 2000 miles ago because of The miss its hydraulic cam and since I did the 4 speed conversion the miss is way more noticeable

That's good then, rule that out. I would second Black Chevelle SS & do that compression check. You can rent the kits if you don't have one. It may be the path of least resistance for you at this point if you have an air compressor. What will suck is having to repair/replace the darn thing. Hopefully it's just one of them. Double check those plug wires are on there right, it's an easy mistake to make. When i goofed it the engine backfired so loud I thought I'd soil my shorts!

by texan01 Mon Dec 05, 2011 12:14 am

Another way to check is to do a vacuum gauge test.

first rule out ignition misfire and vacuum leaks and carb adjustment. You might have a plugged idle feed circuit in the carb. I had that happen when you lose half a carb (2bbl) you really notice it.

Then plug the gauge into manifold vacuum port on the intake. If it's got a regular drop then its very likely a bad valve.

I can't tell you what the readings should be if your engine is no longer stock, but the needle should be steady and not rapidly flickering.

by McCauley5983 Mon Dec 05, 2011 12:25 pm

It's not ignition. Related I've ruled that out the carve is working fine. I do have a whistling noise through the garb and it has recently got louder.when I crank the engine with the distributor unplugged it turns over. But gets about 3/4 of the way around and feels like it gives up. When tweaked the timing the other day I got a loud screaming noise through the darn when I retarded or advanced the timing. I'll do a comp check today if I get time

by texan01 Mon Dec 05, 2011 1:36 pm

Ok, so ignition is ruled out.

Compression check time then. I'm not entirely convinced that the carb isn't at fault but the comp check will point the way.

Be sure to set the throttle wide open to get accurate results, and remove all the spark plugs.

I've seen people run compression tests with just one cylinder and the throttle closed, you will get abnormal and low results.

by McCauley5983 Mon Dec 05, 2011 7:43 pm

Are you talking about a compression test or a cylinder leak test

by BlackChevelleSS Mon Dec 05, 2011 10:33 pm

compression gauge, start the car run for a second with gauge on 1 cylinder then check, its more of a cylinder pressure check, should be over 150 depending on cam, 150 is on the low side tho, Ive got an s10 at the shop with 120psi cyl pressure on number 6 that shows a miss at idle.

by texan01 Tue Dec 06, 2011 12:39 pm

billy-ray wrote:Are you talking about a compression test or a cylinder leak test

Compression test.

A leak test would be great if you've got access to all the parts needed to check it. otherwise a cheap $30 gauge and a buddy will do.

A leakdown test will help pinpoint which valve wether its intake or exhaust. a Compression test will just tell you which cylinder is suspect.

The key is that all the cylinders be within 10% of each other otherwise it will run rough. it could have 60psi on each cylinder and still run smoothly and be incredibly worn out.

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