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Discussion Starter · #1 ·
hey guys, well last nite i put back in my 406 sbc with trickflow heads, a vic jr and comp roller cam. when we went to start it, it would run for about 5 seconds and then backfire thru the carb and die. we did this about 5 times all with the same result. my carb is a 750 speed demon with NO CHOKE. the air temp was about 40*. would it backfire like this because it was cold and has no choke?? we went thru and checked all the wires and distributor, they seemed fine. what could the problem be???
Preston- 1967 Chevelle- [email protected] w/ old 406
11's here i come w/ new 406
 

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First place I would start is double check timing. Then ensure that the dist was not re-installed 180 deg out (although it shouldnt even run if thats the case) or you could be really lean in the carb.
 

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Discussion Starter · #3 ·
whats the best way to get rid of the lean condition?? put in bigger jets??? thanks
Preston
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by Pres'67chev:
whats the best way to get rid of the lean condition?? put in bigger jets??? thanks
Preston
<HR></BLOCKQUOTE>


try playing with the Idle Mixture Screws, im no carb guru so as far as jets and such I cant make any recommendations as to what to change to. I would start with the IMS and adjust for highest vacuum on each screw, or you can go by ear. Idle may just be set too lean. Someone else will jump in and give you some more insight, sorry, thats all I got. GL!

BTW, not to scare you or anything but when my 396 was rebuilt we had this problem and it turned out to be that the cam had gone flat. Drained the oil and it looked like silver paint. Although, thats probably not the case here.


[This message has been edited by Midnight Marauder (edited 12-27-2001).]
 

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Discussion Starter · #6 ·
i dont think the cam has gone flat, as this is a roller cam, and it hasnt been turned over or even started long enough for it to happen, im going to put a diffrent carb on and see if that is the prob, ill keep u guys posted.

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Preston
1967 Chevelle- [email protected] w/old 406
11's here i come w/ new 406
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by Pres'67chev:
i dont think the cam has gone flat, as this is a roller cam, and it hasnt been turned over or even started long enough for it to happen, im going to put a diffrent carb on and see if that is the prob, ill keep u guys posted.

<HR></BLOCKQUOTE>


Sorry, I didnt mean that it ran flat during break in but that it was a manufacturer problem, had a flat lobe. Was bad from the get go. Anyhow, keep us posted, im curious. Good Luck!
 

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Make sure that the firing order is correct. Since you state that the engine will run for 5 seconds, I would doubt that the timing is 180 off.

Also, check the initial timing. It should run at 8-10 deg btdc well enougth to break in your motor.


Tom.
 

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Yes, being that cold with no choke will contribute to backfireing. If all is adjusted correctly, starting a new engine in that cold will take several tries. You will have to keep the gas flowing untile it warms up just a little. Do that with the gas peddle, not a can or squirt bottle.
I would back the timing off just a little.
It would be better to be a little retarded right now. Just watch the headers when you are running to make sure they don't start glowing. If they do, just advance it a little. It will be warmed up by then anyhow.

I don't know if your cam is hyd or solid roller. You will want to make sure the rockers are set correctly. I doubt this is the case, but if you are unsure, check them.

Keep a fire extinguisher handy. If you don't have one, find a heavy blanket or something you can snuff the fire out with.

Let us know what happens.


Dave
 

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next time you are trying to start the car choke the carb with your hand. It won't suck your hand in and it sure will help you start the car. 40* is plenty cool for a new chokeless engine.
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by SS_Dave:

It would be better to be a little retarded right now. Dave
<HR></BLOCKQUOTE>

Actually one of the only times, other than an insanity plea in a murder case where this is soooo true
,
 

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the only way there can be a backfire(flame) in the intake/Carb. is if a spark/flame is getting past the intake valve. so its either in the timing or a problem with a intake valve or seat.

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Justin McIver
Sacramento CA
66-454-th350
 

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Discussion Starter · #13 ·
Thanks for all the help guys, we found the problem and it was an easy fix. turns out that the timing was retarded 20* before tdc, thats why it was running so bad. we tuned it and it runs awesome. im pissed though, because as soon as we got it running it started to rain, supposed to rain for the next 3 days, cant drive her in the wet with hoosier quiktimes, o well, i guess thats what summers for.


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Preston
1967 Chevelle- [email protected] w/old 406
11's here i come w/ new 406
 

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I just started my 396 last week and had the same problem. I advanced my timing, problem went away.
 

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<BLOCKQUOTE>quote:</font><HR>Originally posted by Pres'67chev:
Thanks for all the help guys, we found the problem and it was an easy fix. turns out that the timing was retarded 20* before tdc, thats why it was running so bad. we tuned it and it runs awesome. im pissed though, because as soon as we got it running it started to rain, supposed to rain for the next 3 days, cant drive her in the wet with hoosier quiktimes, o well, i guess thats what summers for.


<HR></BLOCKQUOTE>

Sweet.
 
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