High idle speed to keep running

[ggenovez]

Hi all,

Finished the head gasket on my 72 250 M130 W114, and she has a little trouble. When in park, she needs to run at about 1400 withouth the fast idle or anything, so she can go into drive without stalling. Even then from a stop she has a hard time getting going as well as uphills.

I'm guessing the idle circuit might be blocked. It's late now, but I'll try that first thing tomorrow morning.

Any other ideas?

Thanks

G

[250 Coupe]

Sounds like ignition or valve timing or a big vacuum leak to me. I would lean towards a vacuum leak myself as thats what I have the most trouble with.

Michael

[ggenovez]

Valve timing is spot on.

I've set the timing to 7 BTDC, no vacuum 800 RPM

And replaced the manifold gasket and ensured that the carb gaskets are tight.

[250 Coupe]

I'm still leaning towards a vacuum leak.

You should be pulling at least 14" at idle and the closer to 17" the better.

I would pull the valve cover and watch each rocker and valve, looking for an intake that is not closing all the way altho an exhaust might cause a problem.

Is the brake booster line hooked up at both ends?

Michael

[ggenovez]

Hey Michael,

I take it you mean, each carburate should be pulling at least 14" from the vacuum tubes correct? both are reading 17"

When I did the head gasket I also adjusted all the vavles. None of the ball studs where loose in any way.

Yes the brake booster is sealed both ends.

I guess I could try to block the accessory vacuum on the manifold and see if that help.

[Mike D]

Sounds like classic symptoms of a vacuum leak. Vacuum readings from carb outlets can be deceiving.

Check the vacuum modulator line. Disconnect and plug the vacuum retard to the distributor. Get the can of carb cleaner out and spray all around the intake manifold and carb gaskets. I've forgotten to reinstall one of the lower manifold bolts and driven myself crazy.

[ctaylor738]

Are the idle solenoids working?

Are the carbs in sync?

Are the secondaries closed at idle?

[ggenovez]

Vacuum modulator line? where is that located?

Ι just finished spraing carb cleaner around the carbs & manifold, and didn't see any change in RPM. I had not disconnected the advance line though. Any reason for that? I've always done it with the line connected.


Ctaylor, yes to all.

[ggenovez]

stupid question but can this be caused by an incorrect dwell angle? I've been reading around the forum and I think I have a transistorized ignition system. I know I have a blue coil. Is there a picture of the system to make sure?

I know my dwell is set to 41'. Could that be the culprit?

Thanks.

[ggenovez]

nope. points at 35 and nothing changed. Timing adjusted for new dwell

[250 Coupe]

Originally Posted by ggenovez

Hey Michael,

I take it you mean, each carburate should be pulling at least 14" from the vacuum tubes correct? both are reading 17"

When I did the head gasket I also adjusted all the vavles. None of the ball studs where loose in any way.

Yes the brake booster is sealed both ends.

I guess I could try to block the accessory vacuum on the manifold and see if that help.

If you are pulling 17" at the accessory (vacuum door lock, AC, cruise control) take off, you may not have a leak after all.

I do wonder if you have a stuck float in one of the carbs, look for fuel running into the venturi at idle. Shouldn't be any.

Michael

[ggenovez]

I am getting 17 at the accessory. I've also plugged it and still ran the same

Forgive my ignorance but the venturi is?

Also are there any signs of the vacuum modulator not working properly? any idea where it's located?

Thanks for everybodys help...

[250 Coupe]

The venturi is where the throat of the carb narrows down and opens back up. The narrow part creates a low pressure area that draws fuel from the main and idle jets. If the fuel level is too high, fuel will run down the throat rather than be metered via the jets.

Something else that just popped into my head is that some carbs have idle shutoff solenoids in the base. If they don't work due to clogging, disconnected wires or a blown fuse, the idle circuit will not work and you'll have to crank up the idle speed a whole bunch. Look for silver bits like a fat AA battery at the base of the carb on the front side.

Michael

[ggenovez]

Yes I do have the idle stop solenoids and they do click on. not sure how to check for clogging. I did top off with premium fuel today and added a can of seafoam hoping it would clear out any gunk.

I'll double check the venturi tomorrow. now if the floats are overflowing, wouldn't I have too much gas, no pun intended, and not have this problem with it bogging down when I go from a stop and need to pump the pedal to get more fuel in?

[cth350]

First, the vacuum modulator connection is the metal pipe at the rear of the intake. It connects to your automatic transmission (presuming you have an automatic transmission).

Next, look down the venturies as the car "idles" at 1,400 RPM and I presume you'll see gas dribbling from the those little feeders sitting in the throat of the carb.

You balanced the carbs using the TWO hoods and TWO meters?

On the intake manifold between the two carbs is a small port where blow-by gasses are routed through the air cleaner and back into the motor. Is that port wide-open or closed? Ideally, it should be open, since that's "running conditions".

Now the fun part. Lower the running rpms by making the adjustment between the butterfly plate and the linkage longer. Turn each each one a quarter turn at first and see what happens. Ideally, you should be checking the carb balance at each turn, but that takes a while and is probably extra work (something you've got to be familar with by now).

The front and rear carbs have different mechanisms to push on their throttles to help keep the car idling. They need separate adjustments and have a "neutral and out of the way" setting (hopefully) for now.

-CTH