Engine backfiring after rebuilding top end
Thread Starter
5th Gear
Joined: Aug 2022
Posts: 5
Likes: 2
Engine backfiring after rebuilding top end
I recently finished rebuilding the top end on my 350. Got rid of all the smog parts, new heads, cam, exhaust, and carb. Every time I try to start it now it cranks for a few seconds and then it backfires. Sounds like a gun going off in the engine bay. Id really appreciate any advice or help anyone can offer. I've tried adjusting timing. I know for sure I have the right firing order and I dropped the distributer in on TDC. Thank you for any help!
Race Director
Joined: Jul 2015
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From: SW Florida.
Plug wires on wrong. First, unhook ignition and crank it listening to it. If one hole has no compression cuz valves too tight you will hear it. Then 18436572. I bought a vortec engine that was junked cuz they couldn’t find the miss. 9 plug wires. One went from cap to atmosphere. One went from plug to atmosphere. The other 7 hooked up correctly. It is so easy to screw up with plug wires.
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The bang sounds like dissy 180 out. You may have dropped it on number 1. But on the wrong stroke. Easy fix. Pull dissy cap back off. If rotor is pointing straight backwards. Pull dissy and restab with rotor pointing straight forward. Another words just give that dissy a 180 turn.
Burning Brakes
Joined: Jan 2003
Posts: 961
Likes: 275
From: Frederick MD
The bang sounds like dissy 180 out. You may have dropped it on number 1. But on the wrong stroke. Easy fix. Pull dissy cap back off. If rotor is pointing straight backwards. Pull dissy and restab with rotor pointing straight forward. Another words just give that dissy a 180 turn.
Thread Starter
5th Gear
Joined: Aug 2022
Posts: 5
Likes: 2
Plug wires on wrong. First, unhook ignition and crank it listening to it. If one hole has no compression cuz valves too tight you will hear it. Then 18436572. I bought a vortec engine that was junked cuz they couldn’t find the miss. 9 plug wires. One went from cap to atmosphere. One went from plug to atmosphere. The other 7 hooked up correctly. It is so easy to screw up with plug wires.
Thread Starter
5th Gear
Joined: Aug 2022
Posts: 5
Likes: 2
I would initially guess the dizzy is 180 out as well but you can get the exact same behavior from a intake valve hanging open. With the rotor pointing toward the number one both the intake and exhaust valves should be closed on the number 1 and the exhaust and possibly the intake valves on the 6 will be open/opening.
Burning Brakes
Joined: Jan 2003
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From: Frederick MD
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From: SW Florida.
Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
antevans245,
If I understand your original post, you are firing up this rebuild for the first time. You need to slow down and put a lot of thought into this.
Number one priority is to NOT crank & crank & crank. I assume this is a new flat tappet cam that needs to be broke-in.
The more you crank on it, the more you remove the break-in lube.
A fresh rebuild with new cam needs to fire immediately to avoid cam lobe / lifter damage.
I would pull the valve covers. Disconnect the coil wire. Then with the drivers window down, reach in and just jog the IGN key. No cranking. No spinning. Just jog.
Watch the rockers especially # 1 exh & Int. Watch for the Int to close. Note where the timing mark is on the dampener. Lift dizzy cap and verify rotor position at #1.
Note any rockers that refuse to loosen up. As others said, maybe over done valve-lash settings.
You should be able to check all 16 with only a few rotations of the crank.
If I understand your original post, you are firing up this rebuild for the first time. You need to slow down and put a lot of thought into this.
Number one priority is to NOT crank & crank & crank. I assume this is a new flat tappet cam that needs to be broke-in.
The more you crank on it, the more you remove the break-in lube.
A fresh rebuild with new cam needs to fire immediately to avoid cam lobe / lifter damage.
I would pull the valve covers. Disconnect the coil wire. Then with the drivers window down, reach in and just jog the IGN key. No cranking. No spinning. Just jog.
Watch the rockers especially # 1 exh & Int. Watch for the Int to close. Note where the timing mark is on the dampener. Lift dizzy cap and verify rotor position at #1.
Note any rockers that refuse to loosen up. As others said, maybe over done valve-lash settings.
You should be able to check all 16 with only a few rotations of the crank.
Thread Starter
5th Gear
Joined: Aug 2022
Posts: 5
Likes: 2
antevans245,
If I understand your original post, you are firing up this rebuild for the first time. You need to slow down and put a lot of thought into this.
Number one priority is to NOT crank & crank & crank. I assume this is a new flat tappet cam that needs to be broke-in.
The more you crank on it, the more you remove the break-in lube.
A fresh rebuild with new cam needs to fire immediately to avoid cam lobe / lifter damage.
I would pull the valve covers. Disconnect the coil wire. Then with the drivers window down, reach in and just jog the IGN key. No cranking. No spinning. Just jog.
Watch the rockers especially # 1 exh & Int. Watch for the Int to close. Note where the timing mark is on the dampener. Lift dizzy cap and verify rotor position at #1.
Note any rockers that refuse to loosen up. As others said, maybe over done valve-lash settings.
You should be able to check all 16 with only a few rotations of the crank.
If I understand your original post, you are firing up this rebuild for the first time. You need to slow down and put a lot of thought into this.
Number one priority is to NOT crank & crank & crank. I assume this is a new flat tappet cam that needs to be broke-in.
The more you crank on it, the more you remove the break-in lube.
A fresh rebuild with new cam needs to fire immediately to avoid cam lobe / lifter damage.
I would pull the valve covers. Disconnect the coil wire. Then with the drivers window down, reach in and just jog the IGN key. No cranking. No spinning. Just jog.
Watch the rockers especially # 1 exh & Int. Watch for the Int to close. Note where the timing mark is on the dampener. Lift dizzy cap and verify rotor position at #1.
Note any rockers that refuse to loosen up. As others said, maybe over done valve-lash settings.
You should be able to check all 16 with only a few rotations of the crank.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
i didn't read first post closely enough. new cam and never fired are different. maybe pull the intake and each lifter and add a good squirt of spread of assy lube to each lobe and lifter face. then make sure 1 is firing when you set dist to 1. (i hate dizzy) get it timed close. make sure you have coolant. and enough fuel. have it outside. and maybe get a set of inner valve springs and just run them for the break-in. that will also let you feel and check each valve for any too tight or too loose issues.
Melting Slicks
Joined: Aug 2001
Posts: 3,041
Likes: 122
From: Wichita KS
I recently finished rebuilding the top end on my 350. Got rid of all the smog parts, new heads, cam, exhaust, and carb. Every time I try to start it now it cranks for a few seconds and then it backfires. Sounds like a gun going off in the engine bay. Id really appreciate any advice or help anyone can offer. I've tried adjusting timing. I know for sure I have the right firing order and I dropped the distributer in on TDC. Thank you for any help!
From you post, cam is flat tappet not roller? Hydraulic or solid lifter? Lifter source?
Procedure you used to set valve clearance?
Did you use assembly GREASE on cam lobes or assembly lube?
Did you use assembly lube or engine oil on SIDES of lifters?
(Assembly lube is thicker and slows rotation of lifter on camshaft)
Perhaps as simple as distributor/wiring problem, but, a friend had similar EXPLOSION start issues that turned out to be wiped cam lobes. That is why I asked my questions above.
Mapman
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Screwing around with different valve springs would require more unnecessary turning of the crank to each cylinders TDC and a lot more work.
Post 1 says new cam. Why would you need to replace already?
I assume the engine never actually started. It would be almost impossible to wipe a cam lobe or lifter at cranking RPMs. It takes several minutes of run time for that to occur.
However, you don't want to remove any lube off of the cam right now.
Try to avoid turning the crank more than needed to get the IGN timing / valve-lash spot on.
Post 1 says new cam. Why would you need to replace already?
I assume the engine never actually started. It would be almost impossible to wipe a cam lobe or lifter at cranking RPMs. It takes several minutes of run time for that to occur.
However, you don't want to remove any lube off of the cam right now.
Try to avoid turning the crank more than needed to get the IGN timing / valve-lash spot on.
Last edited by HeadsU.P.; Sep 4, 2022 at 09:29 AM.
Racer
Joined: Jun 2005
Posts: 399
Likes: 11
From: Lubbock Texas
Its been along time but if I remember correctly but if you set the cam up dot to dot, crank gear dot at 12:00 & cam gear at 6:00 this will be tdc for the compression stroke for #6 not #1. I believe your distributor is 180 out.
Racer
Joined: Jun 2005
Posts: 399
Likes: 11
From: Lubbock Texas
Advanced
Joined: Sep 2022
Posts: 56
Likes: 9
From: Illinois
I have done this before. It is real easy to get the distributor in 180 degrees out of phase.
I would just move the crank to TDC on cylinder one and check the rotor position. The problem is, you really need to watch the rocker arms to know when #1 is on the compression part of the ignition cycle. Knowing which wire goes to cylinder one, the rotor should be pointing to that plug tower on the distributor cap.
I would just move the crank to TDC on cylinder one and check the rotor position. The problem is, you really need to watch the rocker arms to know when #1 is on the compression part of the ignition cycle. Knowing which wire goes to cylinder one, the rotor should be pointing to that plug tower on the distributor cap.
Race Director
Joined: Jul 2015
Posts: 13,082
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From: SW Florida.
i think you can see #6 rocker or rockers through oil fill hole. if you go back and forth through tdc and the rocker moves-#6 rocker-you are firing on 1. if no rocker movement you are firing on 6. as far as the cam is concerned. pulling either valve cover is easier to be sure.