Carb Fuel Percolation

[Dave K.]

Following a one half hour drive in my 66 327/300 hp roadster in 92 deg weather the engine ran fine until I parked the car. I left it idle for about 5 min. and the engine faltered and stalled. It would not restart until I pumped the gas a few times. Upon restart it would again stall and would not idle. It would run only if I pumped the gas again to start it and continued to depress the gas pedal.

I parked it in the garage overnight and it immediately restarted the next morning. The car has a base engine Holley 3367 list, 93 octane fuel, hot idle~ 600 rpm. I have the metallic heat shield installed between the carb and the gasket.

I suspect carb fuel percolation. Can I install the plugs into the manifold under the carb to block the crossover without holding the heat riser open? If these holes are plugged where does the recirc hot gas go?

I want to PV the car and therefore, do not want to effect start, idle or operational characteristics of the engine.

Regards,
Dave K.

[Jack H.]

Why not start...

by confirming your suspicion of fuel percolation? It that is what's happening to you, the situation ought to get worse with the engine running at idle, the hood open and the air cleaner removed (less underhood forced convection cooling).

You ought to be able to see 'spurts' of fuel errupting from the vent stack(s) on the carb OR dribbling through the idle circuit's overflow path into the venturi if percolation is the issue.

On modifying the intake, there are two issues that're sometimes confused. One is the provision for a 'hot slot' feature beneath the carb (some cars/some years) and the other is the basic cross over feature of the manifold that allows exhaust from the RH bank to exit via the LH bank if/when needed, like during cold start when the RH exhaust path is almost fully blocked by the heat riser valve.

On the first issue (hot slot), most agree it's not a bad idea to pull the carb and plug the end holes in the slot. On the second (exhaust cross over), there IS a difference of opinion.

If you totally plug the intake manifold's cross over (plugs are easy to find and most overhaul gasket sets contain them), then you DO rely on the heat riser to operate properly (eventually open fully) and can experience uneven cold engine idle + poor cold engine driveability under truly low ambient temperatures.

But, there's also a version of these intake cross over plugs that are 'partial'.... Instead of being full block-off plates, they have holes in the center to act as restrictors and REDUCE vs. eliminate the intake's cross over path. Those are the ones I prefer.

The added benefit to dropping the intake temp at the carb, is you also reduce the suceptibility of the intake to burn its Chevy orange paint (cast iron intakes only) along the RH-LH band where the cross over paths 'live' (e.g. center spark plug area).... But, that natural discoloration band IS a sign of factory originality in terms of the intake configuration/installation, so it's a matter of personal taste here....

[john daly]

Re: Why not start...

dave,
I had the same problem on my '67 350 hp until I wired the heat riser valve open...thanks to Duke for the tip.

john

[Timothy B.]

Re: Carb Fuel Percolation

Dave,

I used small plugs to seal the hot slot under the carburetor on my 67. Doing this made a difference in the temperature of the carburetor when the engine is running and hot and does not effect the heat crossover on a cast manifold. Make sure your heat riser is working properly and engine timing is 8 to 10 degrees advanced. I have read that the hot slot is there to aid warm up of the carburetor so it does not ice up during cool humid days.

[Dave K.]

Re: Why not start...

Jack,

Where does one obtain the partially open plugs -- or can you just drill a freeze plug to make it an orifice?

Regards,
Dave K.

[Jack H.]

Re: Why not start...

The gasket kits I've used had both styles of cross over block off plates in them. If memory serves the hole in the center of the partial plate was roughly 1/4-inch. So, you could drill a full block off plate...

Ask at your auto parts store what they might have available outside of a full gasket kit. If not available, I'd call around to those who commercially rebuild engines as they either use one of the sets or discard both sets of plates.

[Clem Z.]

Re: Why not start...

jack is talking about the intake gasket themselves not the intake plugs

[Dave K.]

Re: Carb Fuel Percolation

Installed plugs in my 66 327/300 hp cast iron intake over the weekend. Will test drive soon.

Regards,
Dave K.

[Paul L.]

Gasket Block-off Plates

My heat riser is welded open. The pic shows another potential remedy.

[Roy B.]

Re: Gasket Block-off Plates

heat riser has nothing to do with it, that just cuts down heat to the Carb