Slow return to idle rpms

What happens when you give a quick blip to the throttle? Is this new or has is been ongoing?

Could be a lot of things from weak return springs to out of sync butterflies (bent throttle shaft) but in the immortal words of Stan (?) “90% of carburetor problems are ignition.”  Check to make sure the advance weights in your distributor aren’t sticking.

It haha started recently and no matter how many times I blip the throttle it does not change the idle slowdown. I will check the distributor weights to see if they are free. The carbs are about two years old.

It could be distributor, sure.

What happens if you let it idle, put your foot on the brake and clutch, put it in first, and drag the idle down with the clutch? Does it stay at idle then? Or does it creep back up in speed?

The throttle cable could simply be too tight. Heat expands some things more than others, could be tightening the cable. Some part of your linkage could be hanging up too.

Or maybe, have you checked fuel pressure? If the pressure is up it could overwhelm the floats and be dripping down the intakes.

Dragging down the rpms will drop the rpms once I slow down the speed

Definitely check the distributor first.

What is your hot idle speed, if I may ask?

1200 rpm due to a hot cam install.

That's your problem. Above 1100, the distributor is advancing keeping the idle up.

My car idled at 800 with a "hot cam" and Weber 44 IDFs and 009 distributor. Webcam 86b with 1.5:1 rockers.

The idle needs to be set LOWER and the carbs need to be jetted for the new cam/engine configuration.

950-1000 hot is a good compromise so it will stay running cold. It will run slow, but should stay running.

When I drop the idle rpms to under 1k it runs rough and shakes the whole car. Is this a jetting issue?

I don’t know about jetting, but I’d suspect something is up between carb sync and timing if it won’t idle smoothly at 900-1000. I can’t imagine any street cam that wouldn’t.

Has it had a good tune up lately? (Points, plugs, wires, condenser, cap, valves checked, carbs sync-ed?)

#1 adjust valves, ice cold. Then do a compression check. Then check for intake manifold and carb base leaks. A plugged idle jet or two will cause VERY rough idling/running.

Basic, but check firing order: 1-4-3-2, it is very easy to swap cylinder 3-4 wires by mistake.

The whole advantage of individual runner carbs is power, throttle response, and lastly will idle smooth with any cam at any rpm you choose. Something is off.

Thank you, I will check this out.

Recent tuneup and electronic ignition. Will check timing.

What kind of electronic ignition? AFAIK, it’s generally a go/no go proposition, but some aftermarket electronic ignition are notorious for failing.

I just replaced the distributor today back to points and condenser. Didn’t help the rpms.

Someone mentioned a possible intake manifold leak.  The symptoms sound like a classic vacuum leak on an intake and with dual throat carbs/intake manifolds it could be straight outside on one runner or in between runners at the base and then it’s undetectable from the outside.  If the heads have large valves and not much metal between ports it is common to suck the gasket out between ports to get symptoms like yours.  Don’t know if something like that would show up with a “Snail” carb sync tool or not.

I am at the point where I am open for any suggestions. I ruled out distributor issues by replacing the distributor so the advance mechanism should be fine. I did notice that on cylinders 1 and 2 when I try to adjust the idle screw on especially #2 cylinder I can turn it completely in and shut off the idle jet and it makes no difference. This is the only symptom that may lead to an answer

@Renfrance posted:

I am at the point where I am open for any suggestions. I ruled out distributor issues by replacing the distributor so the advance mechanism should be fine. I did notice that on cylinders 1 and 2 when I try to adjust the idle screw on especially #2 cylinder I can turn it completely in and shut off the idle jet and it makes no difference. This is the only symptom that may lead to an answer

It's not going to "lead to an answer", it is the answer.

You either have a torn/blown gasket on the 1/2 intake manifold/head junction, or a problem in the carb itself. Everybody is going to say it's a plugged idle jet on #2, but my money is on the gasket.

I’m guessing you’ve got a plugged idle jet or two like Danny suggested.

My money's on the gasket, too.  It's not acting like a clogged jet.

It may be a gasket. I sprayed as well as I could around the carbs but it didn’t seem to make a difference. I will try again tomorrow. Turning the idle screw all the way in on number 2 cylinder on the Weber idf40 did nothing. I still ran the same with it totally closed.

Thank you,

Renfrance

Thank you I will check tomorrow. Can I spray carb cleaner after taking out the idle screw to see if it will unclog?

Just cleaning the idle jet is usually not enough. You have to clean the idle jet circuit.

Watch the video in this link. https://www.speedsterowners.co...he-rest-of-the-story

You might as well go for the whole shebang and check out this link. https://www.speedsterowners.co...-adjustment-and-sync

You will bond with your car even more. Have fun.

Thank you for the video link

Renfrance