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Discussion Starter · #1 ·
I'm not sure were to go from hear. This problem started two days ago. I drive a nearly stock 70 Chevelle SS 396. When I accelerate hard off the line it pulls strong until 51-5200 rpm and then it loses it's power and back fires though the carburetor. When I let up on the gas it runs fine, better than fine it runs great. If I hit it hard again it pulls strong until 51-5200 rpm and then no power and back fires though the carburetor. It's weird but this car has never run as good as it has the past month. I took the car up to 5,300 very slowly with out the secondaries opening and no problem with power loss or back fire, it played hell with my nerves but I can't figure it out. The following is information that may help diagnose the problem and the particulars on the car.

Stock Q-Jet Rebuilt 1 year ago
Pretronix ignition module in original distributor It's been in about a year
1" spacer under Q-Jet It's been on about a month
Vacuum 20 Hg measured at the front of the Q-Jet for the timing advance & 22 Hg measured at the back of the intake were the power brake hose is attached
Fuel Pump about one year old
Timing: 15 initial 7 vacuum 43 total all in by 28-2900 rpm
Plugs: R44T nearly new

I've check the coil, plug wires and plugs for spark and it appears to be strong. I have good clean plugs and that's all I know. I'm at a total loss as to what the problem could be. Any help would be greatly appreciated from the newest official member of Team Chevelle.

Thanks

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Texas SS
1970 SS 396 350HP
Unrestored & Almost Perfect
Houston, Texas
Take a Look http://www.geocities.com/MotorCity/Street/6320
 

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TXSS, Your probably running out of fuel(fuel pump)I've seen this happen a lot.Try one of carters HO pumps.I think you are running to much total timing too. I would stay around 38 degrees total timing.

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Don Edmonds
T.C.Member 1033
1969 Chevelle
2000 Harley softail deuce
www.bottlerats69.homestead.com
 

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I aggree with bottlerat.
Better fuel pump or check the float levels.
Also, timing should be around 36 total. Timing will not cause the backfire, but is a little high.
 

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I triple Don's assesment. If you dont find it there, you might want to redo the valve setting.

Did you, btw, use one of the Pertronix LSC units? If so, there might be another issue at hand here. If the distributor that thing is put on is loose ( like a typical 30 yr old one that hasnt been rebuilt), there could be a problem.

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Gene Chaas
Gold Member 62/ACES 3112
67 SS 427
"Be big. Be a builder."
 

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Discussion Starter · #5 ·
Thanks for the responses.

Bottlerat & 69454Malibu I'll reset the timing back to 36-38 total and check floats. The fuel pump is pumping at 8-9 psi

427L88 The Pretronix unit is about a year old and has never given me any problems. If the LSC unit is one of their newer models then probably not. However I'll have to check to be sure. What's the problem with the LSC unit?

I'm running stock secondary rods and it was suggested to me that the 1" spacer under the Q-Jet creates enough additional fuel velocity that I run out of gas up-top and may need to change to a smaller rod size for additional fuel. Does this sound possible and if so what rod size would you recommend

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Texas SS
1970 SS 396 350HP
Unrestored & Almost Perfect
Houston, Texas
Take a Look http://www.geocities.com/MotorCity/Street/6320
 

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Rods? Rods?
I have 8 in my motor.
I don't know diddly about Q-jets.

they have rods?
Any pistons?
 

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Are you holding 8-9 pounds of fuel pressure at the rpm your car starts to pop or just at idle? I'm not up on Qjets either. but that sounds like it could be a problem if you are holding good fuel pressure.

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Don Edmonds
T.C.Member 1033
1969 Chevelle
2000 Harley softail deuce
www.bottlerats69.homestead.com
 

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What about valve float? It would seem that the valves would float whether you were into the secondaries or not, but maybe the problem is not as pronounced when there's less fuel being dumped into the cylinders. Check all the above items first. Always troubleshoot by working your way, in order, from the simplest possible cause to the most difficult.
 

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I'll second Adam Lym - replace the fuel filter, all of them if you have more than one. I just had this happen on my son's Oldsmobile, and the old filter didn't look dirty. Fixed the problem.
If that doesn't get it, I'd go with a fuel pump.

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Make it the way you like it, forget what the other guys say!
 
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