I spent a while checking out the forums to see if I could solve the following problem:

I have acquired a 1971 350 270hp mostly-original corvette. The engine was rebuilt with a new (and different) cam. Still running the Rochester Q-jet carb. When I go to wide open throttle, the engine bogs.

Basically, if anyone could give me some troubleshooting tips, I would appreciate it. So far I am running carb cleaner through the gas, just to make sure it is clean. No other attempts to fix it.

 

cam changes mean fuel changes which means carb tuning
research how to tune a qjet here.
its probably bogging because it needs more fuel

 

Lars papers will probably help you, as they did for many people :

You can find them here :

http://lbfun.com/Corvette/Tech/vettetech.html

Go to "Corvette pages", click on the "Tech section" and then select "Fuel and Intake". Many infos in .pdf format.

Look for and download : "QJet Tuning Tech" by Lars Grimsrud

 

this link may help too

http://www.florida4x4.com/tech/quadrajet/index.php

...if it is a slow bog, as in it slowly dies after running OK at WOT for a while it is running out of fuel. If it never gets there it could be a number of things, could you describe the bog symptoms so we can help?

Not so obvious things to check:-

1/ fuel cap vent, blocked
2/ gasline not crimped where they run under car, stopping adequete amount of fuel getting to fuel pump then carb
3/ Fuel filter at carb fuel inlet blocked restricting fuel supply


hope this helps

 

Start by checking accel. pump & secondary air valve- spring tension may be too loose.

 

If the carb was just rebuilt, I would bet on the secondary air valve spring being set wrong. As mentioned above, if the spring does not have enough resistance, it opens too far and provides too much air for the secondary mixture...thus, tries to stall.

 

Check Lars' paper for instructions and try this first.

 

I checked the ignition per Lars this weekend and that appears to be good. I will get in to the Carburetor next. I also have a couple of questions:

1. I still have the stock "stiff" springs in for mechanical advance. For street application, is this a bad thing? Lars reccomends getting mechanical advance in by 2800 RPM. Is this still acceptable for street or would it cause bad gas mileage, need for premium gas, etc?

2. My vacuum advance is hooked to a device attached to the carb (on the forward driver's side). It is the original way to hook it up, according to the assembly instruction manual. That said, there is some vacuum at idle. Is this correct? And what the heck is that thing? A mechanic told me it wasn't neccessary for a car w/o AC. Any ideas?

Dan

 

Answer is YES, not only it is acceptable but highly recommended, definitely !
Probably better gas mileage with Lars' recommended springs. Need for premium gas depends on many other parameters ( CR, total timing,... ). If your engine is almost stock, probably not. But if you hear your engine pinging, YES !

Your vacuum advance should be hooked to manifold vacuum for best performance / drivability. So YES, you should have vacuum advance at idle. If you're not sure about the device you're speaking about, plug this fitting and hook your hose directly to a manifold fitting.
If you can take a picture of this device, maybe we can tell you more about it...

 

its probably the device that kicks the idle rpm up with the ac on

 

Your problem is in the carburetor secondary windup spring. It needs to be tightened. Check Lars' instructions for that. (That's what I was referring to earlier.)

 

1. Getting all the mechanical advance in at 2800-3000 is a good thing. It is primarily for performance, but your street use will not be negatively affected. The factory put stiffer springs than necessary as a means of reducing potential warranty expenses. More power developed by the engine means a higher probability of engine damage; so, in their interests, the engine was [effectively] de-tuned by the use of stiffer springs in the distributor.
2. You have a vacuum switch on the side of the carb that was part of a TCS [transmission controlled spark] system. The intent was to prevent the vacuum advance can [on the dist.] from advancing the timing...unless the car was in higher gear ranges (3rd for auto; 3rd & 4th for manual) or at WOT regardless of range. I have the same engine as you (auto tranny in mine), and I have left all of that TCS hardware on the engine...just to keep it in original condition. But, for normal use, I remove the hose [on that switch] which comes from the distributor and put it on the carb plate where the other switch hose connects. That bypasses the switch so that the dist. vacuum can runs directly to manifold vacuum on the carb which allows for better idle quality and better economy at idle and in low gear ranges. Whenever I want it to be connected as 'stock', all I have to do is re-route those two hoses. Hope that helps with your understanding of the hardware on your engine.

 

Great. I will connect that as you suggest. It's what I figured I would end up doing, but I'm glad of the confirmation that it's the way to go.

 

I had the same problem. Check the fuel line near the pump. Mine had a rubber hose that wasnt original and it was collapsing or at least impeding the fuel. Starving off fuel at about 4000 rpm. Had it on the dyno and you could see the fuel/air ratio drop off. When I changed line and pump problem was gone.