Idle mixture advice?

[Broncobowsher]

I wonder if poor flow from the fuel pump could be a factor in this?

No. Idle is the lowest demand for fuel. If the fuel pump couldn't keep up at idle it would never supply enough fuel to drive anywhere.

 

[Bilgamesh]

OK, what are some other possibilities? Could there be a carb gasket or spacer leak? What should the torque be on the studs/nuts between the carb and intake?

If I tighten the mixture screws down even a half turn there is a noticeable difference. If I get anywhere near 2 turns out, the engine misses and stumbles.

Thanks for taking the time to consider this.

 

[broncnaz]

I would say you have a vacuum leak if the idle speed screw is all the way out and your idle mixture screws are that far out. I like to use the thick gaskets(1/4) under the carb they insulate from heat and tend to seal better just dont over tighten the carb bolts.

 

[Bilgamesh]

I am using a regular gasket, a spacer and a thick gasket on top. I have to use the spacer and the thick gasket to get enough rise off the manifold to allow the articulation of the throttle linkage without scraping the top of the manifold. I use the regular gasket because I understand you need one gasket on each side of the spacer. I asked about the torque, because this has been a concern for me from the beginning. Early on, I actually cracked one of the corners on my carb over-torquing. That carb became my return core for this Holley remanufactured Motorcraft 2150. I am actually considering returning this carb because the kick-down linkage has too much play and does not look like it will ever work.

So you think there is a vacuum leak even though the vacuum is steady at 19 in of Hg? I thought anything over 18 was a good value, and it idles so nicely where it is. Do you think the leak could be around the line that comes from the PCV valve into the back of the carb, or are you thinking there may be a leak between the carb to manifold gaskets?

How about exhaust leaks, can they factor in to this at all. I do have a clattery leak at my passenger exhaust manifold. I have ordered a replacement manifold from JBG.

I guess I will see how it all plays out once the replacement carb and manifold are installed. Please advise it you can think of anything else. - Thanks

 

[broncnaz]

Exhaust leaks dont affect this. The reason I suspect a vaccum leak is that once your idle speed screw is backed all the way out and is not affecting the idle speed that tells me you either have a vaccum leak or the throttle shaft is sticking might want to check you pedal linkage make sure its not holding the throttle shaft open. You can have high vacuum readings and still have a small vacuum leak. most common places for leaks are unplugged vacuum ports or carb to manifold area. I'm a little confused by your statement of needing to use a spacer and thick gasket to provide clearance for the carb to operate? Been awhile since I've messed with a 2 barrel carb and intake so maybe I'm forgetting something. But this leads me to believe that the carb may be to big for your engine and could cause the rich idle condition.

 

[Bilgamesh]

My engine is a 302. Since the carb is a stock 2150 2V, I assume by too big you mean the physical size, and not the cfm rating? The carb fits perfectly to the manifold, gaskets and spacer, its just that if I do not raise it up a bit with the spacer and gasket, the linkage will not turn all the way as the bottom of it hangs on the top of the manifold. Spaced, it operates perfectly. Maybe it is a newer 2150 and the linkage originally was placed a little further out on the shaft to clear the manifold without a spacer?...

When I unplug a vacuum line, I get a huge drop in vacuum. I am thinking if there is a vacuum leak, it must be minute. I do agree the pedal linkage and a general linkage inspection is a good idea I will plan to do.

Thanks again for sharing your thoughts and expertise.

Incidentally Broncnaz, if I read your personal text correctly, Thank you for your service to our country!

Bill

 

[Bilgamesh]

One more thing:

You mention running rich. I need to be clear that the engine is running and idling great the way it is. I only mentioned it smells rich at the tailpipe. I cannot exactly say it smells like gas, just a rich exhaust smell. Maybe this means running rich. I simply do not have a basis for comparison as I have not driven an old vehicle in 20 years. - Thanks!

 

[ugly74]

sounds like a vacuum leak to me, since the idle screws are that far out.

the 1st place I'd check is at the intake manifold/cylinder head sealing surface. get a can of carb clean and spray along where those two meet. if the RPMs raise up, then that's where it's leaking.

 

[Bilgamesh]

Do you think I may not have torqued them down enough? Frankly, during my recent rebuild, I was a little surprised at how low some of the torque settings were listed in the Haynes manual. I cannot recall off the top of my head, but I think those manifolds were one such. On the other hand, given my experience with snapping the corner of my carb, I didn't want to push my luck by going tighter. I will check out the carb cleaner approach; thanks!

 

[airman]

I wonder if this carb is even in the idle circuit at this point.

If that throttle is so far advanced by the speed screw you may be into the transition circuit or even the main circuit. Either one of those would make little difference to the mixture screws and likely be very rich. Also it would run fine on the road at cruise speeds.

But if you can't turn the screws in closer than 2 turns I think you are getting air from somewhere or something is wrong with the mixture screws.

I think its time for a guy who does this for a living to take a look at it. No shame in that. I've done it.

 

[Bilgamesh]

I am not ready to turn over the motor I just rebuilt to a pro just yet, but given all the linkage issues I have had with this carb, I plan to exchange it tomorrow. I guess while I have it off, I will pull the intake for inspection and make sure everything still looks good there. Thanks for all the input.

 

[broncnaz]

No thanks needed its what I do. should be home soon can't wait to get back to my broncos.
I just wasnt sure on the needing a spacer thing if it was just to clear linkages then it should be fine all 2150 carbs have the same mounting pattern and will bolt on any engine I'm not fully sure of the exact CFM differances but there are different sizes just want to make sure it wasnt a throttle plate size differance that required you to use the spacer.
You might be right on the old vehicle thing as they usually do smell a little but if its causing tears its to much really you should be able to tune it to the point that you dont smell it or at least barely smell anything.
You could recheck all the torques I usually do after running it up to operating temps those manifold and carb torques are pretty low and I usually find that they have loosened a little after the first heat cycle.