Idle changed on my Holley 650 double pumper

[deathsled]

I took my car up to 70 mph on the weekend.  (The fastest I have driven this car and yeah, I know I know, sounds pathetic, but I was concerned about blowing up the engine the revs sounded high to me.)  But once I got to the light after the 70 mph run (and yes, the engine did sound glorious and robust at that speed), the idle had dropped from its usual 1200 rpm (yes I know it sounds high but that is where I left it so that it wouldn't quit on me at a stop light) to 500 rpm or so and has been running at this lower idle since.  Driving back from the shop today, it seemed to hesitate or slip which made me think maybe there was some clutch slippage but later this evening after another test, I heard a backfire out of the carburetor when I got on it a bit.  Is it possible that something changed on the carburetor in a negative way or maybe the timing?  I am going out to check the distributor to see if it shifted.  As usual, all opinions welcome.

Best regards,

Richard E.
Addendum:  The distributor remains firm and did not shift.  Now focus goes to the carburetor.  I don't believe the linkages were binding. 

[Drew Pojedinec]

With engine idling, climb on top of engine and look into venturi.
Dripping from boosters?
Top of all four t slots dry?
Shut off engine and open bowl sight plugs.

If any of the above are wet, reset float level or check needle/seat o ring.
O ring is 008 size.

If all seems good, remove carburetor. Keep if level so there is no fuel spillage.
Hold it up and observe bottom.  Any leakage?  Inspect PV signal hole, dry?
You can also draw vac on signal hole, should be dry and there should be no fuel in there at all.
PVs  themselves rarely leak, but metering block gaskets and PV gaskets often do.


Any other symptoms?

[Drew Pojedinec]

Another thought.
If running vac advance...
At your idle speed, ported vac should be in action. Assure vac advance arm is still connected. I have seen E clip fall off with resulting arm getting disconnected. This could drop your idle speed and make it run badly, could also explain intake pop.
Recheck initial timing

[shelbydoug]

I just want to add, that an engine set up to run on unleaded pump gas, if you load in leaded high octane racing gas, except for the popping, the engine will drastically lower it's idling speed, SIMILAR to what you are describing.

[SFM5S000]

Richard,

You may want/should to disassemble and check the metering block with the diaphragm. You mentioned the carburetor experienced a "Backfire" through it. It may need a replacement.

Cheers,
~Earl J

[deathsled]

Thank you all for the suggestions and I will execute on them as soon as I can. I don't want to be the guy that parked the car for 30 years because of a minor or even major problem.

[pbf777]

With engine idling, climb on top of engine and look into venturi.

     When doing this just don't choose to snap open the throttle, as if it should choose to back-fire,.............well, it can be a little toasty for the eye brows!        

     Don't ask me how I know!     

     Scott.

[shelbydoug]

 

With engine idling, climb on top of engine and look into venturi.

     When doing this just don't choose to snap open the throttle, as if it should choose to back-fire,.............well, it can be a little toasty for the eye brows!        

     Don't ask me how I know!     

     Scott.