Carburetor or vacuum leak, which one?

[Anonymous]

I'm starting to think that my YF is really a piece of crap. Today, I tried fine tuning the engine and noticed that whenever I turn in the fuel mixture needle all the way in (stopping gas flow to the intake) it does not effect the engine one bit. As a matter of fact, the engine tends to smooth out.

I have rebuilt this thing twice. Could it be the carb or a vacuum leak? About the only thing the engine reacts to is too much gas. If I turn the needle out more than 2 or so turns, the engine will run VERY rough.


Help.


-Chris

 

[CobraSix]

While I'm not sure about the YFs, make sure you have the butterfly adjusting screw correct. If it is set too open, the idle fuel/air mix will not make a big difference...this happened to me on my 1100...threw me for loops for weeks.

Slade

 

[Anonymous]

Okay Slade, dumb question...hold on...what butterfly adjusting screw?


-Chris

 

[CobraSix]

I'll look on my YF when I get home. I have 1 of every 200 1V carb out there...I'll take a look. I'm working on a 1V carb tuning guide...

Slade

 

[Falcon62]

It's the idle speed screw and is usually attached to the throttle arm and the screw contacts the carb base. Turning the screw in will open the throttle a small amount increasing idle speed.

If your idle speed is too high, then you will bypass the idle circuit and the idle mixture screw will have little, to no, effect on idle quality.

Make sure your idle RPM is correct before trying to adjust idle mixture.

 

[Anonymous]

Phil, once again... Man this is awesome. I think what you just described is exactly what is happening to me.

Right now my car is idling at 950 rpm and no matter what I do to the idle mixture screw, the car does not seem to be effected - exactly as you describe!

This is great. I was thinking that carb was bad.

So, what is a good number of turns to start with on the idle speed screw? After, of course, I began to hit the carberator base. And I should start with about 1 1/2 turns of the idle mixture screw, right?


-Chris

 

[CobraSix]

Go back to the idle mix screw, turn it all the way in and then back out 2.5 turns. That will give you a good starting point. Then screw the idle speed screw in until you get to the desired idle speed.

Slade

 

[Falcon62]

Adjust your idle speed to factory specs (I don't know what they are for your car) rather than turning the idle speed screw a set number of turns. I'd set your idle mixture screw 1 1/2 turns out after bottoming it out "softly". Once your idle speed is set, then you should be able to set idle mixture.

Glad I could help! 8)

 

[pedal2themetal45]

HI 200
I think 650 would be a good idle speed to start unless its an automatic it wll need a higher N idle speed but should drop to about 650 - 700 in D.

 

[CobraSix]

I used to run my idle at 600 RPM with a stick tranny. But when the temps were below freezing, and my old carb's choke isn't working, I couldn't keep it running when I first started with an idle that low, so I put it at 650-700 RPM.

Slade

 

[Anonymous]

If the idle is running at 800 or less, then the air adjust screw is the most effective.

Normally, if turning the screw all the way in does not kill the engine, this means the power valve is open, delivering gas. Low manifold vacuum or a bad gasket seal inside the carb (where the vacuum is delivered to the power valve to pull it shut) are conditions that typically cause the problem.

For instance, I once rebuilt my Holley 1946 TWICE and still ended up with the same problem that you are describing. As it turned out, the die that punched out the gaskets apparently broke where it was supposed to punch the hole that passes the vacuum from the bottom of the carb to the top (this was the vacuum that was supposed to pull the power valve closed). I had bought 2 kits at once, just 'cuz they were cheap. Ended up, I went to a different company to buy the next one for the THIRD rebuild and it worked out fine. That's when I noticed the missing hole on the first 2 gaskets!

%^&$! quality....

 

[Anonymous]

Okay, now you have me wondering MarkP. I think I'll go ahead an pickup another rebuild kit. Would that also cause the engine to "surge" on and off?



-Chris