Suspected Idle Circuit Blockage - causing idle problems

[dman535]

Having a hard time getting my A4 to idle down. The motor only runs at low rpm with the choke on. The idle mixture screw is almost all the way down - and it still won't idle. When throttling off idle - there is a bit of a bog before the rpm picks up. I rebuilt the carb over the winter - I suspect that the idle circuit if plugged up.

Last night I sprayed carb clean around the carb and no rpm changes - I assume that I have no leaks. Although I did add a PCV system - anything to be concerned about there?

How much vacuum should this engine pull at idle?

Any tips, tricks or tools (brass wire etc) to cleaning out the idle circuit or to verify that it has fuel flow?

Thanks

Derek.-

[roadnsky]

Derek-
This advice from Don posted a while back...

“The following problems within the idle system (listed somewhat in decreasing order of likelihood) can cause uneven or unreliable idling:

IMPROPER IDLE SET SCREW ADJUSTMENT: Your idle RPM may be set – not by the idle set screw – but by the throttle cable itself. In other words, the idle set screw is backed off so far that the lower limit of the throttle cable is reached before the idle set screw. The problem with this scenario is that throttle cables do not have an accurate end point, and they don’t always stay where you put them. If you pull the throttle cable to its minimum position and walk away, it’s likely to spring back up or down a bit, moving the throttle control arm with it.

To check the idle stop, place the throttle lever in the cockpit at idle, then go down to the carburetor (with a flashlight and mirror if necessary) and note whether or not the end of the idle set screw is actually touching the idle stop coming out from the housing of the carburetor. If it is not, reposition your throttle cable so that the idle set screw does touch the stop.

This adjustment will probably result in too slow an idle, and the next time you start the engine, it will probably shut down as you slowly move the throttle to idle. So the next (and final step) is to readjust the idle set screw to the idle RPM that you want. In this new setting, the idle RPM will actually be set by the idle stop and not by the random positioning of the end of the cable.

CARBURETOR ISSUES AFFECTING IDLE:

1) Dirt in the idle jet.

2) A small speck of dirt in or near the upper of two tiny outlet ports of the idle system. This is the port (about the size of a fly speck) that you see just off the edge of the throttle valve when looking down from the mounting flange of the carburetor.

3) A faulty seal around the idle passage way between the upper and lower halves of the carburetor housing. This passageway is in the very center of the carburetor housing and is about the size of a drinking straw. If the gasket between the upper and lower housing is not completely sealed around this passageway, air can be drawn in and break the suction necessary to draw fuel up to the upper idle port.

4) A leaky float valve which provides such a rich mixture that it floods out the engine at idle RPM.

5) There is an air bleed hole in a small brass plug that is pressed into the lower face of the upper half of the carburetor that sometimes becomes blocked. This plug is located between the idle fuel jet and the venturi tube in the main throat of the carburetor and can be easily seen when the upper half of the carburetor is separated from the lower half. Sometimes a small speck of dirt lies on top of this small plug and interferes with the air being drawn in through the bleed hole to mix with the idle fuel.

MANIFOLD LEAK: You could have a vacuum leak in the manifold gasket or carburetor flange gasket. Some folks have been successful in diagnosing a vacuum leak in these areas by spraying some starting fluid around the manifold gasket and carburetor flange gasket while the engine is at idle. If the engine RPM changes at all while spraying the starting fluid, the gasket is almost certainly leaking.”

Hope this helps.

Dave Neptune will have to chime in on the Vacuum numbers. (He's the resident expert on that)
-Jerry

[roadnsky]

Derek-
One other thought...

When you say, "is almost all the way down" do you mean the idle mixture screw is ALL THE WAY IN?
You may have set it too RICH. (Remember the screw adjusts AIR not FUEL)
Which will cause a "Chugging" or "Stumbling" when coming down from higher RPM to idle.
Look at your spark plugs. Black and sooty and your problem is too rich.
It can also point to a possible Exhaust blockage.

Check the plugs...
-Jerry

[dman535]

I just replaced the exhaust system - so I can pretty much guarantee that its not blocked off.

I don't think its running to rich - as it will only run with the choke on. As soon as I take ot off the motor stalls.

Thanks for the in-depth list of things to look at - will be pulling the carb this weekend to check it out.

[MikeB.330]

Well, if it's not leaking air from the carb flange or the pipe fitting for the PVC kit then I agree that you'v missed something in the carb. Take it completely apart and make sure none of the slow speed circuits are blocked.

and yes, you are lean not rich.

you'll get it.

[roadnsky]

I just replaced the exhaust system - so I can pretty much guarantee that its not blocked off.
I don't think its running to rich - as it will only run with the choke on. As soon as I take ot off the motor stalls.

Hmm, good point on the choke. I missed that.
Got too focused on your screw being turned all the way down.

You'll likely fix it by going thru the carb again though.
Also check the crankcase vent kit. (PCV) If it sticks, which is rare, it can cause idle problems too. (The valve has a variable rate of flow to prevent too much air into the manifold. A PCV valve that is not closing might let more air into the manifold and lean out the fuel mixture)

Happy hunting...

[dvd]

Speaking of idling to low. My A-4 will idle to low and sometimes die if I back off the throttle all the way. I simply set the idle where I think it should be by ear and don't let it get too low. What I have noticed is that these motors will idle lower at warm-up than they do after warming up. When I first start the engine I try to get the idle as low as possible for warmup. As the engine idles for a few minutes the idle seems to increase and I will re-lower the idle with the throttle. Im not sure if the engine just naturally starts to idle higher after warming or if the throttle lever moves on its own. I believe the engine just starts to loosen up and idles higher after warmup. Is my throttle adj. not correct or is this a usual scenario. DVD

[MikeB.330]

DVD, chances are that you need to back out the air bleed screw a bit. Start with 1/4 turn and see if that helps your hot idle.

I have mine set so lean that I need the choke to start if the engine is less than warm. Less then warm to me means the engine has be cooling for 3 hours or so after being up to full running temp. At that point, I need to pull the choke out a little to start it.

fully hot I can idle down to 700 rpm in forward gear without ever stalling.

[Marian Claire]

My 1965 will idle lower after warm up. As I have it set now it is 1000 to 1100 RPMs cold and it drops to 750/800 when warm. Makes me wonder if that is normal. Dan S/V Marian Claire

[MikeB.330]

MC,


When you say the your engine will idle lower warm than it will cold, what do you mean? Do you mean that with the throttle in the exact same position the idle speed will be lower warm? Or, do you mean that you are able to close the throttle more (to lower the idle speed) when the engine is warm than when it’s cold?

Most engine (non computer engines) will idle higher once warmed up if the throttle is left in the same position as your cold idle setting. This is normal, if that's what you were talking about.



Mike

[Dave Neptune]

PCV & idle

Derek, I have a PCV set up on my ole' A-4 and it idles at 700 RPM with 18/19" of vacuum. Runs great and will idle in or out of gear as long as I want.

Check the gasket that seals the idle immulsion well jet assembly and/or the bowl gasket at the same circuit intersection between the bowls. A little extra air in either of those places would really reduce the available fuel by adding to much "un-meetered air" leaning the mix.

Dave
1970 E-35 MkII original A-4 still ticking

[roadnsky]

Derek, I have a PCV set up on my ole' A-4 and it idles at 700 RPM with 18/19" of vacuum.

Told 'ya Dave was the resident expert...

[Marian Claire]

Mike. Wish I had a definitive answer. My statements are based on pulling back HARD on the throttle, cold then warm. The MC is an old homemade boat and may have some linkage, throttle control issues not expected on more conventional boats. Heat/expansion in the linkage? Who knows? Thanks for the info. Dan S/V Marian Claire

[Dave Neptune]

Vac side bar

Jerry, was it you or "rigspelt" that was interested in how the Vac/gage can be used as a base-line for general performance. Yes I know this is a sidebar.
Motoring to Catalina Island in the morning and I expect things to go well as usual.
Have a gud weekend!!!
David

[roadnsky]

It was both of us me thinks.
I'm doing a weekend "camping" sail. Let's talk Monday...