Falls on it's Face

[Sean Conway]

Hello, I just picked up a 69 Corvette 427 4 Speed that I'm having problems with when I get into the secondaires. Fuel pressure is good. New points, cap rotor Timing is at 29 at 2500 RPM New plugs accel wires with the stainless covers on them. With the Q-jet it would fall on it's face when I got into it. No back firing just loss of power when the secondaries opened. Today I put a Holley on it and it acts the same way. Only now it back fires when I get on it. What am I overlooking?

Many Thanks
Sean

[Greg L.]

Re: Falls on it's Face

Sean

My 69 L36 was doing this as well when I bought it. I also noticed that it would spray back out of the carb when the throttle was quickly opened. The problem ended up to be a broken exhaust valve spring. I replaced the spring and the problem was solved. A few months later it happened again so after checking all the usual items and found them to be good I checked the springs and sure enough I had another broken exhaust valve spring. This time I put in all new springs and haven't had the problem since.

This may or may not be your problem but it's worth looking into.

Greg

[Mark #28455]

Your timing?

Is 29 degrees as far advanced as it will go? If you continue to rev the motor will it go any further?

It probably isn't the cause of your problem but the stock settings were:

Initial Advance: 4 degrees BTDC

Centrifugal Advance :
0 degrees at 800 RPM
8 1250
17 2000
26 3800 (for a total of 30 degrees with the vacuum advance disconnected for the measurement)

Vacuum advance: 0 degrees at 7" Hg, 12 degrees at 12" Hg. ALWAYS run your engine with the vacuum advance functioning (attached to manifold vacuum).

Your engine would likely run better at a total of 34 to 36 degrees (vacuum disconnected for the measurement) if your fuel octane allows.

Mark

[David M.]

Re: Your timing?

The secondary air valve on the rochester is not set right. They weaken with age. Put it back on and try 1/8 turn tight, if its better keep going, worse go the other way in 1/8 steps. Be carefull not to loose the original setting cuz its hard to get back to where you were. Rochesters are a great tunable, ecnomical carb set-up properly. I had one fall on its face and it turned out to be the filter in the Roch carb was causing the problem, restriction. Theres a black rubber piece at the end of the filter(back flow valve)grab it with a needle nose pliers and rip it out than go for a road test. I'm still super tuning dads 68,C3 with 327 and 1911 Edelbrock(nice tunable carb!). Got it close now. Turned the fuel psi down(electric)from 6.5 to 5.5psi and it had a dead spot? Turned it back to 6.5, dead spot gone and hes happy. So verify running W.O.T fuel psi, yours may not be enough for that fat block. Read the plugs too, to see if your rich or lean.