Fast Idle

[jjbunn]

I've taken out a very old fuel filter that was in the engine compartment ... I now have (in trunk)tank->filter->pump->(in engine compartment)copper tube running over bulkhead->carbs.

After removing the filter, the car is idling too fast: about 1800rpm. Prior to this, it was a little less than 1000rpm ...

Is the fuel delivery system now more efficient, and making everything too rich?

 

[Brosky]

Julian,

The only way to determine that is to test the fuel pressure, which should not be above 3lbs psi. Generally, if this is the case flooding will occur well before fast idle issue.

My bet? You did something that you didn't realize you did. A vacuum line knocked off or hose disconnected creating lean fast idle, moved choke lever to open, something is wrong beside fuel filter elimination.

 

[poolboy]

Do you mean that now more fuel is being delivered to the carbs and overflowing the actual jet?
I doubt it because the floats should be regulating the amount of fuel available to the carb and close the needle valve when that amount is reached.
If you find that fuel is overflowing the jet itself and causing an increase in rpm because the mixture is now too rich, then I'd say that it's possible some debris has gotten into the float chamber needle valve and keeping it from closing and shutting down the fuel flow into the float chamber.
There are a couple of othe things that might cause a rise in idle speed although maybe not that much. One would be a vacuum line that has become disconnected, the other is a Throttle Bypass Valve that is not sealing and as a consequence allowing an excess ammount of fuel to bypass the throttle disc and feed the combustion chambers. But that would probably not increase your speed by more than 300 rpms.
Is the linkage to throttles and/or choke in a bind or somehow affected during the rerouting of the fuel line process?
Just throwing some things out there.

 

[jjbunn]

I'm not sure what is wrong. I took a look and could see nothing detached or out of place.

I spent some time fiddling with the slow idle and mixture adjustment. I'm finding that with the mixture control fully clockwise on both carbs (richest mixture), the "lift the pistons 1/4" and see" test indicates too lean a mixture (the revs drop and the engine wants to stall).

I set the idle adjust screws for ~900 rpm ... as low as I felt comfortable (any lower and the idle was very uneven).

Off I went for a drive, and found that at the lights it was idling at ~1200 rpm %@$$!! And when I got back it was at ~1200.

Perhaps the timing is off. I need to check that, but the car performs very well indeed on the road. I confess I'm out of my depth on this one.

My odometer stopped working after 69.5 miles ... I repaired it once before, by reattaching the pawl, which had come loose. I suppose it needs to come apart again :frown:

 

[poolboy]

Julian, the first thing that came to mind for me is the Throttle Bypass Valve. If the diaphram that surrounds the brass valve is brittle or split, the valve will not seal and a certain amount of fuel will be able to bypass the throttle disc and enter the intake manifold. That little bit of increase in fuel can raise the idle speed by 2-300 rpms.
If the TBV diaphrams are new, then inadequate tension on the seating spring will keep the valve from fully seating, also noticeable at idle.
I found that 6 to7 CCW turns of the adjusting screw after fully CW gave me the right tension adjustment.
If your carbs do not have an externally accessible adjustment screw, you'll have to remove the TBV assembly and make the adjustment from within the TBV housing.
Not saying it is that, just what comes to mind.
Oh, BTW, if you do have external adj. screws you can just push the screws in with the tips of your fingers to make the valve seat. If the idle speed drops then you've found the culprit.
One more thing, 1/4 inch may be a little too much lift on the air valve piston. Try half of that.

 

[NutmegCT]

my 2¢ - your gas pedal linkage is sticking. If the idle is set at 900, but goes way too high during a drive, try pulling *back* on the gas pedal (stick your toe under the pedal and pull it back toward you) and see if the idle drops.

Simple things first ....

Tom

 

[jjbunn]

Julian, the first thing that came to mind for me is the Throttle Bypass Valve. If the diaphram that surrounds the brass valve is brittle or split, the valve will not seal and a certain amount of fuel will be able to bypass the throttle disc and enter the intake manifold. That little bit of increase in fuel can raise the idle speed by 2-300 rpms.
If the TBV diaphrams are new, then inadequate tension on the seating spring will keep the valve from fully seating, also noticeable at idle.
I found that 6 to7 CCW turns of the adjusting screw after fully CW gave me the right tension adjustment.
If your carbs do not have an externally accessible adjustment screw, you'll have to remove the TBV assembly and make the adjustment from within the TBV housing.
Not saying it is that, just what comes to mind.
Oh, BTW, if you do have external adj. screws you can just push the screws in with the tips of your fingers to make the valve seat. If the idle speed drops then you've found the culprit.
One more thing, 1/4 inch may be a little too much lift on the air valve piston. Try half of that.

Thanks poolboy and Tom.

Tom: the pedal and linkage appear to be OK ...

poolboy: I need to check my Bentley manual to see which part you are referring to (TBV). I already checked the big rubber diaphragms in the body of each carb, and they are good, like new. There is a mixture trim screw on the side of each carb, which I believes allows a small amount of adjustment to the mixture ... I have them both in fully CW. The slow idle adjust screw I have backed out to maintain the 900rpm idle (I readjusted down from 1300 after my drive), and the mixture adjustments on both carbs (adjusted using a custom tool I got from Moss, requires the dashpots to be removed) are set maximum CW.

One thing I noticed is that, when starting the engine, I get a guff of grey smoke *out of the carbs*! I also get a guff of smoke out of the exhaust pipes.

 

[DNK]

Have you redone the temp compensaters?

 

[poolboy]

Julian, take a look at this. About half way down the page.
https://www.buckeyetriumphs.org/technical/Carbs/CarbsII/CarbsII.htm

 

[Tinster]

Julian,

Consider the source but here's an easy "see if it works".

When my engine was finally more or less operational, I had a
similar problem. Nice 800 rpm warm idle in the garage.
Then 1200 to 1300 rpms once I was in the streets and sitting
at stop lights.

I tried all kinds of things to get the idle down to a
constant 800 rpm.

Someone finally suggested I lubricate the throttle linkage on
a regular basis because of where I live. So I sprayed lubed
the return springs and all the other little bits.

BINGO!!! A constant 800 rpms in the garage and in the street.

Hey! It's worth a shot. Quick and easy to try.

d

 

[AltaKnight]

One thing I noticed is that, when starting the engine, I get a guff of grey smoke *out of the carbs*! I also get a guff of smoke out of the exhaust pipes.

Julian....
A guff of smoke out of the carbs might indicate ignition timing being too far advanced; this would also manifest itself as a very fast idle that can't be brought under control.
Check yer timing!