Hot Rod Forum banner
1 - 4 of 4 Posts

·
Registered
Joined
·
418 Posts
Discussion Starter · #1 ·
Guys,

I'm working on my 69 vette restoration and recently got the rebuilt motor running. It's the L-46 (350/350HP) basically, the 69 precursor to the 70 LT-1. I rebuilt it stock except for a comp cams 280 HR (hydraulic roller) cam. It has the original Q-jet carb and the carb has been professionally rebuilt/restored. It seems to be running fairly well but there is a bog off idle when I just tap the pedal as if the accelerator pump isn't working but I can see it squirting when I look in the carb. Any thoughts?

Mike
 

·
Registered
Joined
·
15,298 Posts
Most likely this is the common experience of having to use too much curb idle opening to maintain the idle. This exposes too much transfer slot which results in an off idle bog. You need to check this which involves removing the carburetor and using to measure how much primary barrel transfer slot is exposed at idle between the bottom of the slot and the bottom of the throttle blades as the throttle bell crank is at rest on the curb idle adjustment screw. This should be between .020 to .040 inch, more will than the max will result in a bog when the throttle is opened actually you might see that happening as early as .030 inch.

Solutions are more idle advance or more idle air without cracking the throttle more open. More advance speeds the idle, factory recommended is pretty conservative. Explore 12 to 16 degrees without the vacuum advance hooked up. You can game this further by playing with full time or timed vacuum ports to see which combination works best between making the engine easy to start for the starter motor and what combination of base timing and vacuum advance gets a nice idle with the need base advance for that idle.

Pistons and deck height are another consideration often on rebuilds so called rebuilder pistons are used without zero decking the block, if this was done you have very low compression which forces abnormal amounts of ignition advance and carb idle opening.


Bogie
 

·
Hates: Liver. Loves: Diesel
Joined
·
6,303 Posts
Agreed with Bogie. Another possibility is a worn throttle shaft bore. Common problem with Qjets. The throttle shaft wears the hole in the baseplate and allows a puff of vacuum leak when you hit the throttle. The common fix is to use a common press-in sleeve to fix the wallowed hole. Even if a pro did it and installed the sleeve, I have had several that it didn't do the trick. If the hole is worn too badly, the sleeve won't do it.
 
1 - 4 of 4 Posts
Top