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I have recently had my original 67 400 GTO engine professionally rebuilt and, upgraded with forged pistons, and rods, balanced, Full roller rocker arms and the Milling SP7 cam(068) and lifters. Ram Air cast iron manifolds. Original Qjet intake and 1970 GTO Qjet carb. Original Turbo 400 rebuilt. Today I had it out on a short run. It was up to full
Temp and showing 40-50 PSI oil pressure. As I was going down a slight hill about 40 mph and decided to see if it would downshift into 2nd. It did and started pulling real hard! It got to about 4300 RPM and back fired! I let completely off of it. I was real close to my house and drove it into my garage. When I put it in Park it was idling at 1200 RPM. I shut it off and let it set a little while. It didn't want to restart. It turned over real slow, kind of dragging.
I have been trying different things.
1. Could it have jumped timing when it backfired?
2. The plugs have 300 miles on them. The plug wires are brand new. It has been converted to electronic with an
pertronix module, and their coil.
3. I had to replace the timing cover due to corrosion of the nozzles and had to use a 65 case that has the pointer for the timing mark and the stock 67 balancer so I have to use a Advance type timing light to set timing. I assume that when the pointer is lined up with the mark on the balancer that is TDC Does that sound correct?
4. I have played with the timing and had it running a couple of times. Sounded ok run smooth, but when I cut it off it is turning over slow when trying to restart. It has a High Torque starter and the battery was put in new and only has 300 miles on it, but is 3 years old
Any help would be appreciated. My engine builder is real busy and I will have to wait on him a while.
Thank in advance!
Richard

67
 

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Backfires can be caused by too much timing advance and/or lean mixtures.

It does sound like you need to make sure your timing is correct. You can do that with an advance timing light.

With vacumn advance can disconnected and plugged to manifold vacumn, Set the base timing 8 to 12 degrees BTDC thatshould get you running.

If you can then plug your vac can to full manifold vac. (Shut off the car if you have to find that and set it up) and then restart and read the timing. with vac to full manifold for the dist at idle. Now you will know your full vac can timing. Maybe it is 20 more degrees, they usually are way too much.

Now shut off car and remove dist cap, take one spring off the centrifigal weights on the top of dist, under the rotor. Put the cap back on. disconnect and plug the vac line to dist so no manifold vac leak.

Now start her up, you should be running at your base timing 8 to 12, now rev the motor up to 3 or 4000 RPM's and read the timing advance where it stops and no longer advances.

That reading minus base timing that you set by hand already is your centrifigal advance. Base and Centrifigal should be 36 together total you can adjust the base a few degrees to get it right.

an additional 10 would be ideal from the vac can for 46 degrees,....this is for light throttle cruise.

This will tell you where you are if that centrifigal number is real high, and or the vac can is real high, too much advance will cause a backfire, so see wht you have and then you can work to get it right,

Make sure your parking brake is on, car is in Park, wheels are chocked and good to have a helper. Also do not forget to replace the dist spring when your test is done.

Sounds like you may have way too much advance, at least a possibility. Makes for hard starting as well.

Good luck, you will get it, sounds like a great car!
 

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PS always start this kind of testing with a Battery at full charge,...... no guessing make sure.

A battery at 12.06 volts is at 25% charge....

at 12.24 volts it is 50% charged

at 12.45 volts it is 75% charged.......

.at 12.65 volts it is 100% charged.
 

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I am going with the carb leaning out on you because you said it pulled hard and then backfired at 4300 RPM's. You can suck the fuel bowl dry on a hard run if the float level is set too low or if you don't have enough fuel flowing to keep the carb full under wide open throttle (WOT).

Make sure you have a good flowing fuel filter. Check all lines for kinks, bends, crushing, or other that might be cutting off the fuel supply. Make sure all rubber lines are in good order, no sharp bends. I have had one that separated on the inside and would collapse restricting the fuel flow - ok just puttering around, but not enough fuel to flow when opening up the carb.

Check to see that you have the correct gas cap. I don't know if your system should use a vented cap or not, or if your gas tank has the factory venting, but if the wrong cap is used with a non-vented tank, fuel flow to the carb via the fuel pump can cause a vacuum in the gas tank may not allow enough fuel to flow during WOT. My buddies '70 Z-28 had that issue and he did not know about types of caps used. His electric pump was creating enough vacuum in the fuel tank that it was puckering in like a tin can. Correct gas cap fixed the problem.

I also second what Lemans guy says about your timing. It could very well be an ignition/distributor issue and I would also check this as well.
 
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