I am new to the forum but have been using it extensively over the last few months. My wife and I purchased a 77 Corvette that was in serious need of rescue. It was undrivable and almost nothing worked on the car. But it looked like the 1977 version of the King of the Mountain Corvette so we bought it based on that alone! Now we are about 3 months in and have it to the point that we can drive it with some confidence within a 30 mile radius of home (the radius keeps getting larger with every problem solved). The car starts and runs smooth. Can drive it around (cruising) all day with no problems at all. Here is the problem I am having. I can start the car, pull out onto the road and nail it and it will have tons of power and break the tires loose even from rolling. By the time I am 5 miles down the road and try it again, it will not break the tires loose but still take off ok but noticeably less power. By the time I'm 10 miles down the road, it will stall out when I nail it from a stand still. I can restart it instantly if I hold it to the floor. I can park the car hot, get back in after only about 10 minutes and it again will have full power but then have same problem after driving a short distance. Again, only if I nail it from a stand still, and only if I have driven it first. It will still run fine and smooth cruising around all day long. It's a basic old school build, '68 double hump heads, mild cam, Performer RPM intake, new Summit M2008 600cfm carb, new Summit HEI distributor, new plugs, wires, etc. I've done a lot of searches but can't find anything on this type of problem and I am stumped!

 

Your car would stall when cold, too....except the Q-Jet will not allow the secondaries to open unless the choke has warmed up fully. A big bog when going WOT [warmed up] is most likely due to a defective seal on the accelerator pump rod in the carb. Other possible causes are retarded timing, secondary air valve spring set too tight, and ignition problems.
To check out the accelerator pump issue, remove air cleaner [engine OFF], look into the carb primary bores and give the carb linkage a WOT shot manually. You should see a fuel stream going into each throttle bore. If not, the accelerator pump seal is likely "toast".

 

Charlie -
He doesn't have a Q-Jet. He has a POS Summit M2008 carb, which is a modified re-pop of the Ford Motorcraft 1964 352 Thunderbird carb. That carb was horrible in 1964, and it's equally horrible now. He has added an awful Summit Chinese distributor to compound the problem, and I'll bet my next SS check that the timing curve is so far off that it's a miracle that the engine runs at all. The fuel return line has likely been disabled, so he's now dealing with vapor lock in addition to the horrible parts quality and mismatch problems. There is no one easy solution to this problem other than to advise the obvious: Put the car back to its stock configuration with stock parts and it will run great.

Lars

 

I'm glad my rookie self kept my mouth shut on this one.

Thanks lars for taking the time to help one person while teaching others

 

Thanks for the 'reboot', Lars. Sometimes I scan the original post too fast and miss some of the detail...........

 

Im guessing your choke is working and its still not fully warmed up so when you jump on it at first its tuned less rich with the choke and no secondaires, thats why it runs good, once its warmed up it might be too lean on just the primairies or way too rich with the secondairies and thats why its choking out. You will have to learn to read plugs which is witchcraft and way too involved or buy an AFR gauge and install it just after the headers or on your down tube after your exhaust manifold

You can get the Cliff Ruggles book and try and tune it. It could also be the carb tune in conjunction with the timing. YOur timing might be set that it likes the air fuel ratio with the choke on but when its getting richer with the secondairies it cant keep up. THeres a lot of things it could be. Study up on timing, and reset your timing to about 15 to 16 degrees advanced at idle with the vacuum advance disconnected and plugged at the temp that it does not like to run well at. THat is the temp everything has to be at to figure this out. Slowly rev the motor to 2500 to 3000 and see what the timing is. It should be about 36 degreees to 38 degrees. If you dont have an indicator that goes that high or a dial back timing light, you will either need a timing tape for your diameter harmonic balancer or you have to measure and scribe the 36* mark in it Good luck