I am still messing with that carb (Holley 1920 on a 80 block, hydro valve train and 81-83 heads). I noticed that I get no vacuum out of certain ports such as the one for the EGR and purge. It appears that the gasket perhaps plugs those at the base of the carb. Should I drill out the gasket? Also, how much vacuum should I pull and to which port do I connect the gauge? Thank you.

Mike

Not sure about the 1920, not many of those left in my area...
On the Holley 1945 I usually cap everything off I don't need (EGR, etc...), and when the engine is warm and doesn't need the choke any more I pull the pulloff vacuum hose off the carb and plug the gauge hose into there since it's a constant source of vacuum. I've pulled anywhere from 21 (worst) to 26 (high rpm with choke plate in the way) on the gauge, you want to pull the highest most constant reading you can get...if it 'bounces' the needle or you rev it up and the needle goes high vacuum then decreases back then you have some other things to fix...


you got the tools, time to tune it!

-D.Idiot

Certain vacuum sources are ported such that they only see vacuum in the venturi, such as when you are at part throttle cruise. EGR is typically ported that way, so don't be worried if there's no vac at idle (not a good time to open an EGR valve).
Usually, these ports are above the throttle blade, while manifold vacuum is taken below.

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"A mechanic is Somebody."
- Jim Preston

If I putthe vacuum gauge on a port on the intake, with all other vacuum lines on, I run over 15 at idle and no fluctuation. I still don't think I have a vacuum leak but will also check with propane in the AM. I also noticed that with the low idle screw backed all the way out and the high idle screw not hitting the cam, I still can't get to the 750 rpm at idle. What's up with that do you suppose. One other thing, on the 1920, turning the mixture screw in leans the mixture, correct?

15 inches at idle is on the low side for idle vacuum.
Make sure the throttle cable and kickdown linkage is not holding the throttle open and preventing the idle screw from hitting the stop.
DD

I still find no vacuum leaks after using propane to check. I hold at about 16 vacuum at idle. I checked the linkage, etc to see why I cannot lower the idle further. It appears that the linkage that operates the accelerator pump is the culprit..don't know if moving it one hole will make enough difference. I still get difficult starts and cold running with hesitation on acceleration at all temps. I am about to...gasp, gulp...turn this over to someone else but I don't know if anyone in town can do this either. Any other suggestions are greatly appreciated for this 1920. My mileage has bumped up slightly to 15 mpg so that is good. Thanks.

Mike

A stock slant six should pull between 17 and 21 incjes of mercury at idle with a one barrel carb. I would take the carb apart, make sure that you have the correct base gasket, cap everything except the bowl vent valve and distributor advance and go from there.

If you are idling above 750 RPM and your idle speed screw is not contacting the stop on the carb body you have one of three posisble problems: (1) your throttle cable is clapped in such a way that the throttle is held open; (2) your timing is too far advanced; or (3) you have a vacuum leak or are running too rich.

You base timing should be set to about 10 degrees BTDC. Set it with the engine warmed up and the vacuum advance disconnected and plugged at the distributor. Next check your thottle cable.

Reed

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Casually looking for a Clifford hyperpak intake for cheap.

Well, I checked everywhere for vacuum leaks, tightened everything down, made sure linkage was free, etc. I now run 17 in of vacuum. However, when I went to retard the timing a bit back down to 10, I noticed that when I "hit the gas" quickly, it performed with no hesitation. When the vacuum line is connected, it almost quits when accelerating. Do you suppose the vacuum can at the distributor is set too sensitive and I need to change that now? Thanks.

Mike

Sounds like you may have ruptured the diaphragm in the vacuum pod on the distributor.

Here's a cuple questions:

Some of the Holley 1920s had a big hole on the base plate which matched a hole on the intake. I am not positive what this vacuum feed was for but you may need to have that be working. Maybe someone else can provide the fuction of this hole.

Second, are ou still using the OSAC valve? If so, you should bypass it.


Reed

I am not using the oasc valve. It was between the carb and dist and killed response. The dist is brand new. I could transplant the can from an older one. I noticed that the gasket provided with the carb kit completely covers the holes at the base of the carb. I always wondered if I should drill through and match the holes. Anybody have comments on that??? Thanks.

Mike

My holley 1920 does bad things to my vaccume advance. I was told once that there is no true port on this carb for the distributor and that installing a super six setup would be the easy resolution to the problem. I believe my distibutors vac canister is fine.
For the moment, I just depend on the mechanical advance and plug the port on the carb. This gives me only about 10 to 12 degrees advance added to my initial setting. (thats about 22 deg. total) It's no race car, but it's way better than the horrible stumble I was getting with the vac line connected.
I would really like to hear if someone has another answer to the problem.

I do know that I get vacuum out of that port only when opening the throttle. The vacuum gauge shows it to be somewhat responsive. My engine does not hesitate when I open the throttle without the vacuum line to the distributor attached. It hesitates when connected though.

Mike

Hmmm. Whoever said that a Holley 1920 has no port for vacuum advance is full of BS. It does have a port specifically for this purpose.



It is the one labelled "Distributor vacuum tube".

A stumble on accelleration is most likely cause by a faulty accelerator pump or a carb jetted too lean, or remotely possibly caused by a blown diaphragm in the vacuum advance can. To test the diaphragm in the can you will need a long piece of hose (or you can use the one on the car if you don't mind getting your lips dirty) and a timing light. With the motor running at idle, hook up the timing light and disconnect the distributor vacuum advance hose at the carburetor (remember to plug the port on the carb). Now, watching the timing mark with the timing gun, suck on the end of the distributor vacuum advance hose. You should see your timing advance. Now, quickly slip your finger over the end of the hose to trap the vacuum your lungs just created. The timing mark should stay advanced at the point it was at when you were sucking on the hose (it might slip a little bit if some vacuum was lost when you put your finger over the end of the hose). If the timing either doesn't advance or returns to base timing then you have a leak either in the hose or the diaphragm of your distributor.

However, Holley 1920s are notorious for being jetted to lean, especially if you have purchased a "remanufactured" carb from a parts store.

MSzauner- if your manifold has the hole corresponding to the one in the gasket and the base of the carb then I recommend you drill out the hole in the gasket.

Guest and MSz- I strongly recommend you both do a complete rebuild of your carbs and make sure that you have a #63-65 main metering jet installed and that all of the carb specs are set to factory settings.





The 1920 is a pretty good and simple carb, when adjusted properly.

Reed

I just rebuilt mine and set exactly to specs. However, one thing you brought up that I do not know is the main jet. How do I tell which one I have in there and where can I obtain a replacement (if needed)? Thanks much. It has been running better since a rebuild but the poor acceleration and poor cold start has been troublesome. I find it starts better by just turning the key and waiting for it to start than it does by depressing the pedal to set the choke. Not sure what that means. Thanks again.

Mike

The main meterng jet is accessible y removing the float bowl cover (be sure to put the bowl gasket into a jar of gasoline to keep it from drying out and shrinking) then removing the metering block. You will see the jet on the bottom front center of the carb. It unscrews with a flathead screwdriver. You can buy replacement Holley jets at almost any auto parts store for around $2 jet. To see what size you have you simply remove the jet and look at the number stamped on the side.

I see you are in Texas. You probably don't need a choke much down there, and stepping on the gas to set the choke may flood the engine whenyou try to start it. When you rebuilt the carb did you clean the parts in a carb dipping tank? Did you remove the metering block from the main body and remove the metering jet to clean all internal passages?

Reed