Advance timing not steady/backfire.
Thread Starter
Intermediate
Joined: Mar 2006
Posts: 48
Likes: 1
From: Mt. Prospect IL
Advance timing not steady/backfire.
I have a '73 L-48 and have just put on a new performer manifold, long tube headers and had the carb rebuilt by Lars (it already has a little bigger cam and rebuilt, slightly decked stock heads). I also followed Lars' distributor timing instructions and added his VAT. It idles great. The problem I am seeing is when you increase rpm to about 2000 to 2800 (in park, open headers) the timing mark increases as it should to a steady 36 degree advance. After 2500 or so the timing mark gets a little jumpy in the advance direction and it sounds like it is breaking up a bit with some exhaust back fire.
FYI did the same before and after new plugs, points, coil and Mr Gasket advance kit with silver springs. Dwell is right at 30 degrees. I don't know what happens with load because it is still on jack stands.. I am going to get the exhaust put on next week. Will back-pressure and load help?
It has been 25 years since I used a timing light, (but is like riding a bike) I just don't remember a "jumpy" advance timing mark.
Normal? Any ideas thoughts? Could my distributor be messed up?
Thanks
FYI did the same before and after new plugs, points, coil and Mr Gasket advance kit with silver springs. Dwell is right at 30 degrees. I don't know what happens with load because it is still on jack stands.. I am going to get the exhaust put on next week. Will back-pressure and load help?
It has been 25 years since I used a timing light, (but is like riding a bike) I just don't remember a "jumpy" advance timing mark.
Normal? Any ideas thoughts? Could my distributor be messed up?
Thanks
Race Director
Joined: Jul 2015
Posts: 13,082
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From: SW Florida.
10 buck piston stop tool from amazon, jegs, ebay, whatever. points. so we know it is not a module or an MSD box acting up. dwell stays at 30. so i assume points aren't bouncing from a worn out spring. maybe it is psychosomatic and the car needs Dr. Phil and not a mechanic...
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
I assume this is a stock distributor. If so.....how are the bushings for the weights and the pins? Are the weights nice and tight on the pins and swing freely?
The dwell is good so the points are not bouncing......
Hard to say what the backfire is at this point.
Jebby
The dwell is good so the points are not bouncing......
Hard to say what the backfire is at this point.
Jebby
Is my vette stock?? HAHA
Joined: Oct 2015
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From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
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have you tried backing off on the timing to see if the sypmtoms changge
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
303 Stingray,
Anytime I see a new or recent Intake installation with a backfire, I question the method of the installation.
Having said that, are you sure you used enough quality sealant on the Intake gaskets? And both sides of the gasket?
Without a good seal, those gaskets can leak top side, around the ports and blindly along the bottom of the gasket. This scenario could result in a very lean condition on some cylinders, thereby causing a backfire issue. A vacuum leak causes a backfire 90% of the time.
The upper portion of the gasket can be checked with the engine running, for leaks. But along the bottom of the gasket, at the lifter valley, its almost impossible to evaluate. A vacuum tester could show a wacky needle, plugs maybe oil fouled.
Anytime I see a new or recent Intake installation with a backfire, I question the method of the installation.
Having said that, are you sure you used enough quality sealant on the Intake gaskets? And both sides of the gasket?
Without a good seal, those gaskets can leak top side, around the ports and blindly along the bottom of the gasket. This scenario could result in a very lean condition on some cylinders, thereby causing a backfire issue. A vacuum leak causes a backfire 90% of the time.
The upper portion of the gasket can be checked with the engine running, for leaks. But along the bottom of the gasket, at the lifter valley, its almost impossible to evaluate. A vacuum tester could show a wacky needle, plugs maybe oil fouled.
Last edited by HeadsU.P.; Jul 16, 2021 at 08:31 AM.
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Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
With a high output coil and small cap distributor the spark can "cross fire"; jump from the rotor to an adjacent post on the cap. A cheap cap-rotor combination with large clearances can exacerbate this problem.
This is demonstrated in this video with a cheap Pertronix distributor.
I replaced it with an MSD Ready-to-Run and solved the issue.
This is demonstrated in this video with a cheap Pertronix distributor.
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Here's the exact same engine with an MSD. This has a multi-spark box wired in and you can see the multiple sparks occuring, but with no cross-fire.
By the way I don't see any real advantage to using the MSD multi-spark box. The Ready-to-Run works with or without it.
By the way I don't see any real advantage to using the MSD multi-spark box. The Ready-to-Run works with or without it.
Thread Starter
Intermediate
Joined: Mar 2006
Posts: 48
Likes: 1
From: Mt. Prospect IL
Thanks guys, New Blue Streak cap, rotor, coil, Ill take the distributor apart and check everything for play - Lars had some good thoughts. I only used Permatex on the intake gaskets at the water jacket and on the china wall. It idles great with no surge with break clean. I'll let you know by next week. Keep em coming.
Race Director
Joined: Oct 2019
Posts: 10,735
Likes: 63
From: Deep in the Pisgah National Forest
Another thing you can check is proper grounding of the engine and distributor. You need zero resistance from the battery negative to the block and heads and distributor. I've gone as far as to add a #10 wire across all that with clean threads for the bolted connections and stainless steel serrated washers.
Is my vette stock?? HAHA
Joined: Oct 2015
Posts: 20,212
Likes: 9,350
From: Im not allowed to tell you
2020 C3 of the Year Finalist - Modified
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
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Oh ya good point Yadkin!!!, Did you replace the ground strap on the engine to the frame?
Thread Starter
Intermediate
Joined: Mar 2006
Posts: 48
Likes: 1
From: Mt. Prospect IL
Same symptoms with new and old points, cap, cond, rotor, coil. Harmonic balancer checked out with TDC. I think if the balancer was "slipping" it would not be spot on TDC at rest again. I would think it would probably make the mark "jump" but the engine wouldn't respond poorly. Brand new Ground strap with very cleaned up contacts (learned that with the the starter). I'll look at distributor ground, but new manifold/oil pump and very clean hold down bracket is probably making good ground. I don't think I'll really know anything until I get the exhaust hooked up and drive it. I don't have much time between work and then it being too too late to start it with little kids at neighbors in bed. I may just buy a new distributor. I could send it out but I want to get on the road.
PSA - Wear Ear protection when you have open headers
this thing is loud.
I'll post when I figure it out. I love detective work.
PSA - Wear Ear protection when you have open headers
this thing is loud.I'll post when I figure it out. I love detective work.
Last edited by 303 Stingray; Jul 16, 2021 at 12:02 PM. Reason: Typo
Thread Starter
Intermediate
Joined: Mar 2006
Posts: 48
Likes: 1
From: Mt. Prospect IL
Jebby,
Yes stock - 70K miles. It has a new Mr Gasket advance kit- acts the same with any combo of springs. I may feel some play where the plate is attached to the vacuum advance. Lars suggested my looking there.
Yes stock - 70K miles. It has a new Mr Gasket advance kit- acts the same with any combo of springs. I may feel some play where the plate is attached to the vacuum advance. Lars suggested my looking there.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
when doing the intake i like to silicone the gasket to the heads using china wall gaskets as alignment tools. no sealer on top of gasket. set manifold on and let sealer harden. pull manifold and check that gaskets stayed where they belonged and are there to stay. remove china wall gaskets, sealer up front and back. your chouce if you want to put sealer between gaket and intake.
Last edited by derekderek; Jul 16, 2021 at 02:23 PM.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Thanks guys, New Blue Streak cap, rotor, coil, Ill take the distributor apart and check everything for play - Lars had some good thoughts. I only used Permatex on the intake gaskets at the water jacket and on the china wall. It idles great with no surge with break clean. I'll let you know by next week. Keep em coming.
Some get away with it. Most do not.
Not me man. I enjoy applying Edelbrocks Gasiniche, Permatex Right Stuff & Permatex Clear on the gaskets.
I do not enjoy having to do it two days in a row.
If the Intake Gasket leaks on the bottom edge, near the lifter valley, a vacuum leak will NEVER show up by spray testing the top side.
Its a blind vacuum leak. And the only indication is sometimes a oil mist fouling the plugs, or wonky vacuum gauge reading.
Last edited by HeadsU.P.; Jul 16, 2021 at 04:50 PM.