Classic Engine Tech 1967 - 1981 Engine Related

SS396 rebuild won't start backfires.. Suggestions?

Old 02-13-2011, 10:36 PM
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SS396 rebuild won't start backfires.. Suggestions?

I think somebody put the pop tops in and swapped the right and left sides thereby bending the valves... Because we did not separate the piston rod assemblies at the shop. So if someone swapped the rights and lefts the pop tops would be upside down...

Is this possible?

Closed chamber 029 heads... .510 lift Lunati 260H cam, hyd lifters, 10:1 Mahle cast pistons.. Zero deck height

Cam/crank dots are at 12 and 12 with distributor firing#1.. They checked the distributor and coil auto zone said it's good.

In a 68 SS

Thanks
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Old 02-14-2011, 11:15 PM
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Re: SS396 rebuild won't start backfires.. Suggestions?

Of course it could have been assembled wrong but it would not turn over. The domes on the piston would hit the head. I'm pretty sure the timing marks go 12 o'clock on the crank and 6 o'clock on the cam. You could have very well bent the valves yourself with an incorrectly installed cam. If thats the case you should have the engine assembled by a reputable shop.
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Old 02-14-2011, 11:39 PM
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Re: SS396 rebuild won't start backfires.. Suggestions?

the dots are supposed to be 6 on cam and 12 on crank... this is your problem, with pop tops you probably did bend your valves.... just as the above post mentions.... this is just a reiteration. \


fix cam and do a leak down and compression check to see if the above is true. if you are lucky you may not have bent anything.

Last edited by chevy_dude97; 02-14-2011 at 11:42 PM.
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Old 02-15-2011, 06:52 AM
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Re: SS396 rebuild won't start backfires.. Suggestions?

If the pistons were installed wrong the heads probably wouldnt have seated being closed chamber, with pop tops.

Doing a compression check today.

12-12 and 6-12 is the same position as the crank turns once the cam turns a half so if you install at 6-12 go one rev on the crank you end up at 12-12.

At 6-12 #6 is firing and at 12-12 #1 is firing. I believe this to be true anyhow if I wrong Im wrong obviously its been a few years so tryin to help these guys out over the phone.

I'll get over there today and time the cam and check compression thanks for the comments

all cylinders at 90psi

Last edited by retromek; 02-16-2011 at 08:50 AM.
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Old 02-16-2011, 07:20 PM
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Re: SS396 rebuild won't start backfires.. Suggestions?

Originally Posted by retromek View Post
Cam/crank dots are at 12 and 12 with distributor firing#1..
Pop the plug off #1 and bump the starter till you feel compression come out of the plug hole. Then check to see if rotor is pointing at #1 wire on cap.

It's not unusual to have timing 180* off on a new build. Even with the dots aligned properly. That is why I check the lifter positions before I pop the intake manifold on to see which cylinder is actually at TDC firing on new builds or cam swaps.

I like to leave an engine at #6 position on an automatic tranny car. That way I can install the torque converter bolts I can get to, then spin it over once, install the rest, and it will be at #1 position ready for distributor install.
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Old 02-17-2011, 01:03 AM
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Re: SS396 rebuild won't start backfires.. Suggestions?

Originally Posted by retromek View Post
12-12 and 6-12 is the same position as the crank turns once the cam turns a half so if you install at 6-12 go one rev on the crank you end up at 12-12.
this is probably why michell on demand has it 12/12.... I was only taught one way... lmao I just thought it was wrong
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Old 02-18-2011, 01:12 PM
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Re: SS396 rebuild won't start backfires.. Suggestions?

So what happened... Installed a hyd cam and lifters ...The original springs for a solid lifter cam were pretty weak.. . once the oil preasure came up enough the valves open and caused a lil backfire and low compression, hence the no start
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Old 02-18-2011, 03:31 PM
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Re: SS396 rebuild won't start backfires.. Suggestions?

ah you set the backlash without oil in the lifters?
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Old 02-19-2011, 07:57 PM
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Re: SS396 rebuild won't start backfires.. Suggestions?

Yep
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Old 03-06-2011, 05:30 PM
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Re: SS396 rebuild won't start backfires.. Suggestions?

Oops. I put solid lifters in once and set them at 3/4 turn down. We all make mistakes. It makes a funnier story 5 years down the road.
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