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back firing through the carb

X

xnoahx

1/2 ton status
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Location
Ventura, CA 93004
OK, i just got my vortec heads on and it starts up and it will idle just fine but when i give it gas it hesitates and backfires through the carb. What could cause this, maybe the intake valves arent closing all the way? could it be the carb is too lean? any help would be much appreciated.

Check out my 73 K5 http://members.theglobe.com/xnoahx/73k5.htm
 
i would run a compression check to see if your valvs are closing all the way if the valvs are doing there job check timing the timing may be to far advanced
just my 2 cents


are we having fun yet
 
Check at least two more times to make sure that you don't have two spark plug wires crossed. A V-8 will idle surprisingly well on 6 cylinders.

Tim

70 Blazer CST 4X4 350 SM465 NP205
87 Burb 4X4 350
01 GMC 2500HD 4X4 Duramax/Allison
 
Did you do any work on the cam i.e. replace it or the chain? When you install a cam the 2 dots facing...this is not TDC. If the dots were not pointing up on both cam and crank you installed the distributor 180deg out. Pull up on the distributor but don't turn it tun the crank one full turn and drop the distributor back in.

Diging it in the dirt with my K5's
Grim-Reaper
<A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
 
70 psi is way too low. 150 is more like what I'd expect to see, depending on your altitude. It drops slightly as altitude increases.

<font color=black>HarryH3 - '75 K5</font color=black>
<A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
It's a great day to be alive...
 
Try adjusting the valves they are probably to tight, if you are getting 70psi. You could be running a little lean, does your exhaust have a funny smell now? Remember your flowing a little more air now, so will need to flow a little more gas.

1978 K5, Palisades Park, NJ 07650
 
1st, did you find anything?
2nd, lets start all over, what motor are you running?
You said you put new heads on, did you change manifold, dist., carb., rockers, lifters, push rods, cam etc.? Did you turn the crank at all while apart? Did you put the distributer back in the same direction if not? Proper imformation from you will make it much easier to help you out.

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<font color=red>[/b]"Rid'in HigH in my K5"
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....
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<font color=blue>[/b]"6" Lift -<font color=purple>38.5's"
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I get 180 - 190 psi compression with my motor with Vortec heads and stock type dished pistons.

Tim

70 Blazer CST 4X4 350 SM465 NP205
87 Burb 4X4 350
01 GMC 2500HD 4X4 Duramax/Allison
 
70! wooooo thats big time low. Uhhhhhhhhhhhhhhh back to my question...did you mess with the cam? Now it's sounding the the cam is not indexed or the valves are not closing or you wasted a bunch of time with a motor with bad rings but even bad rings should pull over 100.

Diging it in the dirt with my K5's
Grim-Reaper
<A target="_blank" HREF=http://grimsk5s.coloradok5.com/>http://grimsk5s.coloradok5.com/</A>
 
Here's another thought, did you adjust the rockers properly? If the adjusting nuts are too tight then the valves won't close quite all the way.

<font color=black>HarryH3 - '75 K5</font color=black>
<A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
It's a great day to be alive...
 
its a 350 i did a cam change a while back and it has ran fine so thats not the problem, when i did the heads, i also switched to a edelbrock performer for the vortec heads and put in an HEI. The distributor is in right because i know what cylinder its on when i adjust the valves. Im going to try loosening all the valves a bit. It mights just be my compression tester is bad, ill try to get another reading pretty soon today, im just letting the engine cool a little before i touch it. im also going to play with my vacuum advance a little, its an adjustable unit on the distributor so maybe its set too far advanced.

Check out my 73 K5 http://members.theglobe.com/xnoahx/73k5.htm
 
Well if everything is setup correctly, vacuum advance won't have any effect (ok realistically there is a bit of "leakage" so there might be a tiny bit of advance) on idle. Should be run on "ported" vacuum only, no vacuum to advance cannister at idle.



Dorian
My tech/links page: <A target="_blank" HREF=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</A>
No anti-theft measures on your truck? No pity when its stolen
 
Had this happen on an old Buick I had. Check to make sure the EGR is working correctly. For some reason it started puking flames at me when the EGR crapped out. Wierd eh?

Why do people keep calling it a Bronco!!!!!
mad.gif
 
Just lose the EGR completely.

I'm suspecting valvetrain. Could also be timing or carb related, but I say valvetrain first.

Tim
'84 Chevy K10, lifted, loud, fast, and 3/4 ton axles
 
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