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Username Post: Backfire now a skip        (Topic#334053)
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-18-16 09:57 AM - Post#2616160    

Background. My sons 1985 K-10. has a mid 70's 4-bolt main, moderate RV cam, edelbrock 650 carb. Engine has run flawless until a few weeks ago. he was just a block or so from home when he heard a loud "pop" as he called it. Obviously the engine backfired thru the PCV. Large black sooty area on Edelbrock carb base where the hose from PCV goes in. Tried to crank and no luck. checked gas and it was good. Checked for spark and none. Had 12 volts going into (new) distributor. removed cap and checked coil, coil was bad. replaced and truck still would not crank due to no spark. replaced ignition module and truck cranked up. idles fine but starts to skip when you idle it up (just guessing I'd say around 1800-2000 RPM's) Plugs look dirty, not oily or overly sooty, but not whitish to gray as they should be. Would the backfire thru the PCV screw up something in the carb?



 
LMC Truck
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
03-18-16 10:27 AM - Post#2616164    
    In response to sassee

It would have caused problems on a holley carb, they didn't have protection (from the factory, but there was a kit to fix them) from a backfire, it would blow out the power valve.

Edelbrocks didn't have this issue, so it isn't AS LIKELY that it caused a problem in the carb. However, a BAD backfire can always cause problems of one sort or another.

You may want to pull the metering rods out of the carb and check the pistons that the rods hang from, to make sure that the backfire didn't cause soot to form underneath the pistons. Make sure the springs under the pistons are in good shape.

If it was bad enough to cause PCV hose to come off, you may have destroyed the carb base gasket(s). You may want to use an unlit propane torch around the base gaskets to see if there is a vacuum leak. You may want to check around the intake-to-head gasket surface also.

Check any vacuum reservoirs for evidence of cracking, a backfire can destroy those also.

Use a timing light, and stick the probe on each cylinder in the engine, while listening for the skip. If the timing light has a "skip" also while the engine is running, you have found the cylinder that is causing the skip. Check the plug wires and the plug, and replace if necessary. If you still have problems finding the skip, watch underhood when it is dark (and it has to be REALLY dark, like no street lights or other lights around). You will see a glow around all the spark plug wires, but the offending cylinder wire's glow will die out for a second.



 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-18-16 10:45 AM - Post#2616167    
    In response to Vaughn

I am now leaning towards some crap/dirt may be in then carb. I cranked it right after the above post. Could not get any reaction from adjusting the mixture screws. Cut it off, pulled them out and cleaned them off. I can now get some reaction when adjusting the screws but cannot seem to get it quite right. Still; has a little popping when letting off of the throttle sometimes and I did get one small backfire thru the carb after wide open throttle. About to go pull metering rods.



 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-18-16 11:21 AM - Post#2616175    
    In response to sassee

Haven't pulled the metering rods yet but did spray some carb cleaner around base of carb. I am getting some idle advance when I spray. not a lot, but definitely there. Would this be enough to cause my issue? Since i have readjusted the mixture screws, I have good idle and doesn't start to run rough until higher RPM's and when I go wide open. I used "run rough" this time since I think "skip" may have been a little misleading.

Ernie



Edited by sassee on 03-18-16 11:25 AM. Reason for edit: No reason given.

 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-18-16 12:36 PM - Post#2616189    
    In response to sassee

Well it is running good and smooth until you kick in the secondaries. Can't tell if it is loading up or fuel starved. (dang hearing aids) Think it def pulled in some goo when it backfired thru the PCV so it will come off after it cools a bit. Clean it out good and go from there.



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
03-18-16 02:04 PM - Post#2616203    
    In response to sassee

If the idle goes up when spraying around the base gasket, it means that you have a gasket leak. You need to replace the base gasket(s).

Messing with the idle jets won't be really useful unless the base gasket is fixed. It causes a vacuum leak near the idle jet's outlet at the base of the throttle plates, which messes with the idle mixture. You will have to adjust the idle jets again when you have the base gasket fixed.

If it didn't have an issue with a bog before you had the backfire, it is probably the base gasket that is causing the disturbance of the fuel mixture, causing the bog. If the bog happened before the backfire, you may need to change the jets or the rods for more fuel during part throttle.

Make sure your timing is right. Backfiring is usually a timing and sometimes lean fuel issue.

Does your engine have a nylon timing chain top gear? How many miles since the engine was rebuilt? You might want to stick a vacuum gauge on the intake manifold and check for late valve timing.



 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-18-16 04:20 PM - Post#2616232    
    In response to Vaughn

Base gasket appeared to be good. I had to use some starting fluid since it was all I could find. it may have been sucking it in thru the top. Removed and cleaned the carb. Did put vacuum gauge on it, 20-21 inches of mercury and steady. Definite popping, not backfiring. Timing was good, both initial and total timing. Idles great and it is smooth up to a point. When you get into the secondary, it starts popping.



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
03-18-16 04:37 PM - Post#2616238    
    In response to sassee

If it is popping and not backfiring, it may be a stuck valve. Pull the valve covers and look down the line of the tops of the valves while running (use some oil deflectors to keep oil from shooting everywhere). Is one of the valve tops stuck down further than all the others? Is there any bent pushrods? Is one of the rocker arms (at the pushrod cup) sitting down further than the others?

Again - what do you know about the timing chain? It could be VERY important.



 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-19-16 03:57 AM - Post#2616302    
    In response to Vaughn

This engine has less then 12K miles on it. All new parts went in/on. I have never seen a backfire thru the pcv valve. How could these 2 things cross paths? Or was the backfire due to the electrical component failure?



 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-19-16 05:35 AM - Post#2616329    
    In response to sassee

I was 99.99% certain it had steel timing chain and steel gears. I went back and found some photos on my phone and they are definitely steel. I am going to pull the valve covers after while and see if ?i can find something else going on.



Edited by sassee on 03-19-16 05:36 AM. Reason for edit: No reason given.

 
64ss409 
Senior Member
Posts: 890

Loc: Montana
Reg: 12-04-02
03-19-16 10:19 AM - Post#2616373    
    In response to sassee

While you have the covers off as Vaughn mentioned, check for a tight intake or broken valve spring.
Ron

1964 SS 409/340 4 spd, bought new Oct '63 from Ken Boggs Chevrolet, Geraldine, Montana


 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-28-16 03:43 AM - Post#2618227    
    In response to 64ss409

I went ahead and ran a compression check. All cylinders good as new. Checked wiring and distributor pick-up, good as well. I didn't really expect to find any issues there since it is all fairly new. Cranked it up and it ran great cold. No popping in the higher RPM's. As the engine warmed up, it started to pop, got worse as the choke opened fully. I do believe I have a vacuum leak in the carb. Any agreement?



 
Vaughn 
"15th Year" Gold Supporting Member
Posts: 18770
Vaughn
Loc: Colorado Springs, CO
Reg: 08-08-04
03-28-16 01:18 PM - Post#2618343    
    In response to sassee

Popping is timing or insufficient valve seal. Carb would have to be pretty lean to have a vacuum leak cause this.

Get a vacuum gauge on it, look for late valve timing or wavering on the needle indicating insufficient valve seal.





Attachment: vacuum_gauge.jpg (15.61 KB) 19 View(s)



 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
03-28-16 04:19 PM - Post#2618382    
    In response to Vaughn

As noted earlier, I have already put a vacuum gauge on it. 20-21 inches of mercury and steady.

Ernie



 
sassee 
Forum Newbie
Posts: 18
sassee
Reg: 10-01-13
04-03-16 04:52 PM - Post#2619404    
    In response to sassee

In case anyone is wondering, it turned out to be a bad plug wire. They were a fairly new set of Accel wires but one came out of the loom and burned on the header. I checked the wires with a mist of water in the dark and it lit up light a Christmas tree. New wires and running great again.



 
gchemist 
Senior Chevytalk Moderator
Posts: 24202
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
04-04-16 03:18 AM - Post#2619457    
    In response to sassee

Use a couple of tie wraps and make wire loom. A loom will help keep wires off hot engine parts.

'95 Jimmy SLT (Bought @131,814)
'83 Silverardo XST - ZZ4 powered
'96 GMC Jimmy LS Ret. @236651 miles


 
93 Silverado 
Forum Newbie
Posts: 8
93 Silverado
Reg: 07-14-14
12-07-17 05:35 AM - Post#2717161    
    In response to sassee

Thanks for the follow-up. (Had an instance once in a '05 vehicle where a wire moved and began burning on the manifold because I didn't properly replace the wire-set after replacing s thermostat. I caught it quickly due to smoke (wire was additionally wrapped with plastic wire looming). It moved ever so little, but it was enough to touch the heat shield (?). I was very surprised how tight-toleranced the wire locations are.) - Just an additional story about additional importance of wire security in later-model, tight-compartment vehicles.



 
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