My weekend driven 68' with Holley carb has started giving me some confusing problems. Drove 2 weeks ago & once the car warmed & the choke came off the idle suddenly dropped down so low I had to pump the pedal to prevent stalling. The motor sounded almost like it wasnt running on all cylinders and lacked power. i feathered the gas pedal to get back home. I checked plug wires , distributor, carb no sticking choke and primarys were pumping fuel all seemed ok, plugs were black as car was running very rich. Later that day started it up it worked fine. Last week repeat situation but this time I tried adjusting the idle speed screw 1 full turn to bring up idle to 750 rpms Motor sounded and felt normal drove it 20 minutes idle at 750 RPM, parked it for 30', started it and drove 20' idle jumped to 1500 rpm, adjusted idle screw back down 1 full turn & car was running fine at 750 rpm , plugs nice tan color. Today similar situation at initial start up Low idle at warm up, once choke was off car lacked power and sounded again like it wasnt running on all cylinders with black exhaust had to turn idle screw 1 1/2 turns to keep 750 rpm idle but motor still not sounding right. Shut it down came back later in day car started fine but had to adjust idle screw 2 turns because idle running too high. Drove it and it ran fine.

I cant figure out how to trouble shoot this. The fact that the car lacks power and sounds like its not running on all cylinders makes me question if its a carb issue. But the fact that the motor can return to a proper running state is confusing so I'm Not sure if its the carb, ignition or a problem in the motor eg. valve or compression problem thats at fault. Any ideas on this?

 

I would check fuel for commination and fuel pump pressure and vacuum. Did you check you float levels?

 

I havent checked float levels as I'd have to tear down the carb to get at the floats, The first time it happened my fuel was below 1/4 tank so I have filled it up with fresh gas since the first episode. I have a back up electric fuel pump which I have engaged to see if that solved the problem initially but no difference was noticed. Vacuum could be a problem as it has been running low last time I checked about a year ago but I question why the car works fine sometimes but not others if the vacuums the issue.

 

You may want to pull off your main metering plate and check the power valve. Either suck on the hole with the diaphragm or apply vacuum with a brake bleeder. If it does not hold the valve in a closed position or leaks, you need to replace it. I have had to replace a number of these in the past. One backfire can rupture this valve. If bad, check the inches of vacuum stamped on yours so you can get the same one. You may have to replace your metering block gaskets if you tear them when taking them off as well.
Rough idle, and very rich conditions are the main symptom of this problem.
Good luck.
Kirk

 

thanks for the suggestion.

 

Most automotive Holleys have a sight plugs and external adjustment for float level.

 

Unfortunately the 4165 Holley does not have an external float adjustment.