A Tale of Two Carbs

[james singleton]

I have two carbs for my 66'-200 (with L.O.M. dizzy). Both carbs are SCV type carbs; one is a Motorcraft 1940, and the other is a rebuilt Autolite 1100 that I bought recently. Here is the "Tale": The 1940 seems to run smooth at idle, and for that matter runs well at higher RPM's also. The only real problem is that I do not get any vacuum readings @ higher rpm's (between 2,000 & 3,000rpms); so the dizzy is not advancing my timing when I put my foot into it. I installed the new/rebuilt carb today (the Autolite 1100) and it runs a little rough @ idle, and when I screw the air/fuel mixture screw all the way in the engine does "not" die!??? Bad jet??? The vacuum reading on the 1100 is around 4.5" @ idle ("not" normal; should be 0" @ idle for L.O.M., correct?), but will go up to around 7.0" at higher rpm's of 2,000 - 3,000rpm's (which should be normal). Would the 4.5" of vacuum @ idle cause the engine to run rough????? I put the 1940 carb back on the engine since it seems to run the best of the two, but I am a bit baffled by both. Any comments on either carb appreciated! Thanks

 

[Bort62]

When you talk about vacuum, I assume that you are talking about vacuum @ the SCV port. I don't think you are pulling 4.5" of manifold vac at idle.

If your idle mixture screw doesn't do anything, the most likely culprit is idle speed setting too high, causing the motor to idle on the main jet.

 

[Patrick66]

hey, hope you can finally get this fixed.

maybe you just need to replace the hose to the dist. (?) could be leaking and thats why your not getin vaacuujm to the dist.other than that if the 1940 is running good at idle and thro the rpms why bother messing with it?

 

[james singleton]

Bort62, You are correct; I am referring to port vacuum (off the side of the carb) not manifold vac. Since this was the first time this carb (1100) had been on the engine, I adjusted the idle mixture screw out about 2 turns before I started the engine. The engine would only keep running if I kept pumping the accelerator a bit, otherwise it would die. I then adjusted the throttle adjusting screw high enough to keep the engine running so that I could mess with the idle mixture screw. I did not have my tach hooked up so I don't know exactly what rpm's it was set at; but probably a bit higher than spec.! I kept trying to back down the throttle adjustment a little at a time, while trying to get the idle mixture screw correct in order to keep the engine from dying. The idle screw was somewhat responsive to the turning it in & out, but it would not kill the engine like when you normally screw the screw all the way in! The engine runs, but it runs a bit rough, and the idle rpm's are probably a bit higher than they should be. I will hook up the tach tomorrow and see! You mentioned that it was probably idling off the "main jet". Could you explain that a bit. Are you referring to the adjustment of the float inside the bowl?? Thanks for your input!

 

[james singleton]

Patrick66, I hooked up a short piece of "new" rubber hose directly to the vacuum port on the side of the carb and I am not getting any readings @ idle rpm's (that should be normal); but I am not getting any vacuum @ higher rpm's (between 2,000 & 3,000 rpms)and that is not normal. I should get between 5-7inches of vac @ high rpm's! You are right about using the 1940 carb instead of the 1100, since I can live with it. But it just bothers me that I am not getting the vacuum advance to my dizzy that I should "if" I step on it! Thanks!

 

[Harte3]

If your idle speed screw is turned up high enough you are probably getting flow from the transition slot in which case the idle mixture circuit is no longer in play and that is why the engine doesn't kill.

 

[james singleton]

I'll try backing off the idle adjusting screw to where it is hardly making contact, and see if the engine will run long enough for me to make adjustments to the idle fuel mixture screw. Yesterday I could not get the engine to run long enough to adjust the fuel mixture screw without setting the idle adjustment screw higher (higher rpms than spec). Thanks!

 

[Bort62]

The idle circuit pulls fuel from a different jet than the main circuit.

If you crank the idle up too high, it will create sufficient velocity in the venturi to start sucking fuel out of the main jet.

At this point, you are no longer idling on the idle circuit, but on the main circuit. This will cause you all sorts of problems.

Usually, you can look down the carb throat and see this. You will see gasoline dribbling out of the main. If it is running on the idle circuit as it should, the carb will appear bone dry.

Until you get this fixed, you are going to have all hell of problems with everything else.

 

[james singleton]

Bort62, Thanks for the explanation! I will tinker some more tomorrow and see what I come up with! Thanks!

 

[66 Fastback]

Ian is correct in that the idle will not be stable until you get the throttle plates adjusted such that they fully closed at idle.

The load-a-matic and SCV is different than other distributors.
Are all of the vacuum readings taken in your driveway? Also, unless you have a tee in the line to the distributor when reading the vacuum readings, then the distributor is not able to advance the timing at the higher rpms. Increased spark advance increases the engine speed which also increases the engine vacuum. So unless the vac guage is teed into the line, then the spark can't advance. Sometimes the stock steel vacuum line makes it difficult to get the vac gauge tied into the line.

The Load-a-matic requires a load to operate as designed, hence the name. In the driveway, the throttle plates do not have to open much to achieve 3000 rpm's since there is no load on the engine. Since the SCV senses off of a ported source with a valve in it that also bleeds in venturi vacuum, I do not think the vacuum readings you read in the driveway are representative of what takes place on the road. I think the vacuum readings are somewhat of a blend of ported vacuum and venturi vacuum. In the driveway, the manifold vac does not drop enough for the SCV to switch to the Venturi vacuum source, so the readings are a bit confusing. I would rather just read pure manifold vacuum and see how it is behaving.
In order to verify that the system works, I would suggest you get your timing light and watch the timing mark as you rev the engine. If you see the timing advance, then you know the system is functioning.
Doug

 

[LEROY POLL]

HEY JAMES


.....JAMES IF YOU LIKE E-MAIL ME. I'M RETIRED ALSO AND HAVE THE TIME TO SPEND ON THIS PROBLEM. I'VE WORKED ON THESE DIST. SENCE 1954.

.....DOUGS RIGHT ON HOW TO TEST, AND WHERE TO TEST.

.....ARE YOU HAVING A PROBLEM OR DO YOU JUST WANT TO KNOW IF IT'S WORKING RIGHT?

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