misfire - The 1947 - Present Chevrolet & GMC Truck Message Board Network

norwegianc10 · 10-19-2009, 06:16 AM

Engine is a 350. It tends to misfire when idling or at low rpms or low load situations. Tends to get back on cylinders at app.2300-2400rpm. Varies with load, more load- less misfire. Sounds like only one cylinder is misfireing. I have check all sparkplugs but they look all the same. Any suggestions where to go next??

'72customdeluxe · 10-19-2009, 08:09 AM

I was going to say a fouled plug but you already have that covered. Do a compression and leakdown test, that one cylinder might have bad rings or a burnt/stuck valve or something

coloradosean2009 · 10-19-2009, 08:24 AM

Is your ignition system in good working order? Wires, Rotor, Cap, Points? If you are at all unsure, I would spend the $120 to replace with new HEI

68gmsee · 10-19-2009, 07:10 PM

I'd do pretty much what has been recommended... Try to isolate the cylinder if you can. If you can't then this is what I did. Of course you don't have to do them in this order. The misfire started about 2-3 months after I repaced the plugs.

Did a compression check- checked good on all cylinders.

Replaced dis cap and rotor-checked for burn marks or arcing on the old one but wasn't sure of condtion so got a quality replacement.

New points- again just wanted to be sure. Got the better ones.

Plug wires- I actually tested the old ones for arcing by misting them with a water bottle at night. You can see the arcing if they're bad. I replaced them anyway because they looked old.

Checked for vacuum leaks and adjusted the carb for to make sure it wasn't too lean.

After all of the above it still would misfire. I put the old plugs in and the misfire stopped. Went and replaced the plugs with a different brand and no more problems. One of the new ones had gone bad...

dwcsr · 10-19-2009, 08:17 PM

I would change the timing up or down a few degrees and adjust the throttle accordingly. See if the miss fire changes rpm. That would let you know if its carb or ingnition or valve train. If it stays the same then Its either ignition or valve train. If it changes its carb. if not back off the rockers 1/4 turn and check again. If it changes its vales or adjustment
If its still there its ignition.

Test the wires with an ohm meter.
Check that the gap is .035 for points or .045 for electronic.

Put a vacuum gauge on it and see if the needle bouces. If so its a valve issue.

dwcsr · 10-19-2009, 08:22 PM

I would change the timing up or down a few degrees and adjust the throttle accordingly. See if the miss fire changes rpm. That would let you know if its carb or ingnition or valve train. If it stays the same then Its either ignition or valve train. If it changes its carb. if not back off the rockers 1/4 turn and check again. If it changes its vales or adjustment
If its still there its ignition.

Test the wires with an ohm meter.
Check that the gap is .035 for points or .045 for electronic.

Put a vacuum gauge on it and see if the needle bouces. If so its a valve issue.

10-19-2009, 06:16 AM	#1
norwegianc10 Registered User Join Date: Jun 2008 Location: Norway Posts: 90	misfire Engine is a 350. It tends to misfire when idling or at low rpms or low load situations. Tends to get back on cylinders at app.2300-2400rpm. Varies with load, more load- less misfire. Sounds like only one cylinder is misfireing. I have check all sparkplugs but they look all the same. Any suggestions where to go next?? __________________ 67 c10 Short Step 350/th400 daily driver 69 Camaro work in progress

10-19-2009, 08:09 AM	#2
'72customdeluxe Registered User Join Date: Jan 2008 Location: Tejas Posts: 691	Re: misfire I was going to say a fouled plug but you already have that covered. Do a compression and leakdown test, that one cylinder might have bad rings or a burnt/stuck valve or something __________________ '72 cheyenne super step, '05 long bed gmc

10-19-2009, 08:24 AM	#3
coloradosean2009 Registered User Join Date: Aug 2009 Location: Denver, CO Posts: 291	Re: misfire Is your ignition system in good working order? Wires, Rotor, Cap, Points? If you are at all unsure, I would spend the $120 to replace with new HEI __________________ __________________________________________ Sean 1972 C20 2005 4Runner

10-19-2009, 07:10 PM	#4
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: misfire I'd do pretty much what has been recommended... Try to isolate the cylinder if you can. If you can't then this is what I did. Of course you don't have to do them in this order. The misfire started about 2-3 months after I repaced the plugs. Did a compression check- checked good on all cylinders. Replaced dis cap and rotor-checked for burn marks or arcing on the old one but wasn't sure of condtion so got a quality replacement. New points- again just wanted to be sure. Got the better ones. Plug wires- I actually tested the old ones for arcing by misting them with a water bottle at night. You can see the arcing if they're bad. I replaced them anyway because they looked old. Checked for vacuum leaks and adjusted the carb for to make sure it wasn't too lean. After all of the above it still would misfire. I put the old plugs in and the misfire stopped. Went and replaced the plugs with a different brand and no more problems. One of the new ones had gone bad... Last edited by 68gmsee; 10-19-2009 at 07:10 PM.

10-19-2009, 08:17 PM	#5
dwcsr Hollister Road Co. Join Date: Apr 2005 Location: Houston Posts: 6,134	Re: misfire I would change the timing up or down a few degrees and adjust the throttle accordingly. See if the miss fire changes rpm. That would let you know if its carb or ingnition or valve train. If it stays the same then Its either ignition or valve train. If it changes its carb. if not back off the rockers 1/4 turn and check again. If it changes its vales or adjustment If its still there its ignition. Test the wires with an ohm meter. Check that the gap is .035 for points or .045 for electronic. Put a vacuum gauge on it and see if the needle bouces. If so its a valve issue.

10-19-2009, 08:22 PM	#6
dwcsr Hollister Road Co. Join Date: Apr 2005 Location: Houston Posts: 6,134	Re: misfire I would change the timing up or down a few degrees and adjust the throttle accordingly. See if the miss fire changes rpm. That would let you know if its carb or ingnition or valve train. If it stays the same then Its either ignition or valve train. If it changes its carb. if not back off the rockers 1/4 turn and check again. If it changes its vales or adjustment If its still there its ignition. Test the wires with an ohm meter. Check that the gap is .035 for points or .045 for electronic. Put a vacuum gauge on it and see if the needle bouces. If so its a valve issue.



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