Engine Idling to High

[mouser98]

I've finally got my engine running good again after replacing the fuel pump, but now I have a new issue. It is idling at around 2500 RPM and I cannot get it to drop any lower than that. I've got both screws (Pict 34) screwed all the way in and the fast idle adjustment backed out where it's not touching the choke cam but 2500 is as low as she'll go. I don't think it's a vacuum or a timing issue because it has plenty of power. I've searched through the forum here but it seems this isn't a common problem because I didn't find any other post about it. Any advice would be appreciated.

[68IHscout]

I would check the cable adjustment ,maybe the butterfly is stuck ,hard to guess when not there .

[goober]

How's the fit of your throttle shaft?

[mouser98]

[Cusser]

[beetlenut]

If you've got both screws screwed all the way in and it's still running, you have internal problems with your carb. Were you running a fuel filter before you changed the fuel pump? May have some carp clogging an internal passage or two. Is your throttle plate completely closing when the fast idle screw is screwed out and not touching the fast idle cam, guaranteeing the progression holes are covered?
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scrapyards are for quitters
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[mouser98]

[beetlenut]

[miller0358]

Make sure the choke is open all the way when warm If not the throttle will be resting on the step can and not idle down

[mouser98]

Well I pulled out the carb, dismantled it and now it is sitting in carb cleaner waiting for UPS to drop off my carb rebuild kit. This was some of the gas that was in the carb.

In the bottom of the sediment bowl was a thicker sludge than this. Also, the "volume control screw" O ring was kind of shredded.

Because of all that rust in the fuel, I pulled the gas tank out far enough that I could see down into it through the fuel level sending unit hole. I was thinking it was going to be really bad but honestly, all there was was a little bit of surface rust and some very small accumulations of rust sludge here and there. I'm thinking just leave it alone for now.

[mouser98]

Cleaned up carburetor, rebuilt it and installed it. Then I decided to pull the gas tank and clean it, then I decided to replace all of the soft fuel lines and fuel filter. This is all on top of having just installed a new fuel pump 3 weeks ago.

But, even with all of that, I still have to screw the Bypass Screw (the big one) all the way in to get the idle down to 900 rpm.

I only let the carb soak in carb cleaner for 4 hours or so, but I did blow compressed air through all of the passages. So what could be causing my problem?

[Chris913653]

Remember the only variable was that you changed the fuel pump.

Do you have any way to measure the fuel pump output pressure in PSI?

Reason being, if you perhaps have the incorrect fuel pump with the longer fuel pump rod, or just as a result of the terrible quality of new parts these days, it's very likely that your fuel pump is putting out way more fuel than it should at idle. Which can force through the jets in the carb sometimes.

Just a thought, being that was all you changed?

Try kink the hose on the fuel pump a little or put a vice on the hose and see if reducing the pressure from the fuel pump changes anything?

[mouser98]

[ashman40]

The carb is an air-fuel metering device. It adds fuel based on the volume of air flowing down the carb. Since you say the bypass and volume screws are all the way in it means you are limiting the air-fuel that is bypassing the throttle plate, yet the idle rpms are still high. It suggests that the throttle plate is not fully closed.

When you rebuilt the carb did you remove the throttle plate from the throttle shaft? Is the throttle plate centered in the carb throat so it can fully close? If it is not centered it may not fully close. This exposes the progression holes in the side of the throat. The flow of fuel thru the progression holes are not controlled by the bypass or volume screws. Borrowing a pic from glutamodo:

Ignore the descriptions as this pic was about vacuum ports. Above the word “drilling” you can see a stack of holes in the wall of the throat. These are the progression holes. You can see the dark line that shows where the throttle plate rests when closed. You can see the closed plate rests below these progression holes, blocking them off from the intake vacuum at idle. I suspect when your throttle plate is closed it is still exposing these holes to intake vacuum.


See the pic below:

You can see the large bypass hole below the throttle plate. You can see the bypass screw, volume screw and the cutoff solenoid control the flow of air-fuel thru this passage. This is how the idle rpms are controlled. Just above the closed throttle plate are three progression holes. Once the throttle plate is cracked open and air starts to flow past these holes they will pull fuel thru them to enrich the mixture just off idle. But if the throttle plate is not fully closed air will flow past these holes and draw fuel into the intake. No amount of idle adjustment will control this flow since it is not thru the idle circuit. You need to check that when the throttle arm screw is not touching the fast idle cam the throttle plate is fully closed sealing the progression holes off from the intake vacuum. Remove your carb and check this. If the throttle plate is not fully closed you may need to loosen the two screws and re-center the plate in the throttle shaft.
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AshMan40
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'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[mouser98]

[vamram]

[mouser98]

I've got it working right now and I think I figured out what the problem was. In some of the stuff I was looking at they said to screw the fast idle screw in one quarter turn past touching the choke cam to allow the throttle plate to open just enough for idle, but, in other places I've seen them say that the fast idle screw should be all but touching the choke cam when the engine is warm. I believe now that the former advice is for carburetors earlier than the 34 pict 3, because the 34 pict 3 comes with a hole in the throttle plate, presumably to allow just a bit of air and fuel in for idle. I had my fast idle screw turned in a quarter turn past touching so my throttle plate was open just a bit, drawing air by the 3 ports that Ashman40 pointed out, so it was no wonder I couldn't get the idle down. I corrected that and it is running good now. Thanks everyone for your help.