Serious lack of power - Q-jet or timing?
 

I finally have my vette on the road and it really seems to have no power.

First of all, the engine is a 400 sbc, XE 262 cam, ported and polished 462 heads, performer intake with a 1979 Q-jet with a stock HEI distributer. New cap, rotor, wires and mechanical advance.

I took out the L82 to clean up the engine compartment, then re-installed it. When I first put the distributor in, I had it 180 degrees out, and it backfired through the carb BAD. Soon after that, I bought the 400, and swapped it in a week before I moved. Because I was in such a rush, the timing was not set very well, but it ran okay.

The timing is set to 8 degrees BTDC and I have 14 hg of vacuum from the manifold at 900-1000 rpm idle. The carb was rebuilt by a reputable corvette mechanic (Who is now 400 miles away) and I confess, I have messed with some adjustments since then. The vacuum advance is hooked to the port on the front passenger side on the carb plate.

The engine lacks serious power, and will backfire through the carb slightly if I blip the throttle. The last time it backfired, it started to idle a little higher.

Where should I be looking? Power piston, timing, vacuum leaks?

:banghead: :confused:

Re: Serious lack of power - Q-jet or timing? (rexx78)
 

I'm an amateur compared to most of these guys, but I've learned from them to set total timing to 36 degrees at 3000 rpm with vac canister unplugged.
Make sure your secondary air vlave is opening or at least opening at the right time on the qjet. My air horn was making the airvalve stick sometimes, resulting in acceleration on the primaries only. I had to remove the air horn and file away the metal that was causing the flaps to bind.

To adjust the timing of the air valve opening, loosen the set screw, and adjust the windup spring tension to 7/8 turn initially and retighten the set screw. Check some of Lars' tuning papers on the corvettefaq site.


[Modified by isosceles, 12:47 AM 7/2/2004]

Re: Serious lack of power - Q-jet or timing? (isosceles)
 

What do you know about the 400? Which cam, heads, etc/ what's compression like? Maybe it has mechanical problems.

Re: Serious lack of power - Q-jet or timing? (rexx78)
 

I would start with timing, I don't think you have enough advance. Try about 12-16 degrees, but as someone has already said check that your total is around 36 or so.
Another problem that I have seen is spark plug wires in the wrong order. Get out a shop manual and have a car buddy come over and the two of you trace every plug wire again, even if you have already done it. This has solved more than a few lack of power and backfiring problems.
Once those two are done you can start to trouble shoot the carb, if need be.
Jeff

Re: Serious lack of power - Q-jet or timing? (rexx78)
 

I put my money on a vacuum leak. One place to check is between the base of the carb and the intake manifold - ensure you have the correct gasket between the two. Keep us updated with your progress, good luck.

Re: Serious lack of power - Q-jet or timing? (Boofers)
 

I found out that the secondaries are not opening. There is something wrong with the accel pump or linkage; the pump does not go down far enough into the bore from what I can see.

I bought a rebuild kit, and the accel pump is the same. I did find that the carb to manifold gasket is a little different; the old one blocks off some vacuum passages.