Vacuum Question

[james singleton]

Here is what I have: A Motorcraft/Holley 1940 carb w/SCV and a stock Load-A-Matic distributor on a 66'-200cid engine. I have read the "sticky's" and all; and have also read that normally you don't get more than a couple of inches of vacuum @ idle and maybe only about 5 inches of vacuum on acceleration with venturi-vacuum. I hooked up a vacuum gauge almost directly out of venturi/port (side of the carb) to a vacuum gauge and I am not getting a reading at idle, or at high rpm's! I have good manifold vacuum (17-18inches). I am thinking one of two things: 1. I have a bad SCV ??? (would a bad SCV give a zero reading?)??? or 2. possibly something is plugging up in the venturi passage?? Any thoughts greatly appreciated! Jim

 

[gb500]

not sure if this is exactly your problem :

i have a scv carb and distrib

the diaphram in the dist had a leak in it (probably perished) - such that i had no advance (and overheating as well).

anyways - so i replaced the diaphram and still got no advance - traced it back to the small drillings in the carb were blocked .

seems the perished diaphram had allowed the carb vacuum to pull dirt /oil? thru the diaphram and into the carb drillings.

ended up getting fine wire and a can of carb cleaner to clean out the drillings

brett
melb

 

[james singleton]

Brett, thanks for you reply. I probably should have mentioned that I replaced the dist. vacuum bisquet, but since I was not getting any vacuum directly off of the carb I figured it was not necessary to mention. It is possible that like you said that I have gotten dirt or something blocking the passages inside the carb. Here is another question for anyone out there that might know; could you damage anything (like the SCV) if you pulled off the fitting from the venturi/vacuum on the side of the carb and blew air in there from the air compressor???? Thanks, Jim

 

[LEROY POLL]

HI JIM


.....JIM WHAT YOU HAVE IS CORRECT. WHAT YOU CAN'T SEE IS THAT UNDER A LOAD YOU VACUUM WILL BE IN THE 10* TO 14* AREA. THIS IS WHERE IS SHOULD BE.

.....BY ADVANCING THE DIST. YOU WILL CHANGE WHERE THE MOTOR WILL BE RUNNING. ALSO THE SPRINGS ARE ADJUSTABLE. THE WEAKER THE SPRING THE MORE THE ADVANCE WILL STAY ADVANCED. THE STRONGER THE SPRINGS THE QUICKER THE ADVANCE WILL BE PULLED BACK. THE SPRING POST TURN. THIS TURNING WILL CHANGE THE SPRINGS STRENGTH OF THE PULL ON THE ADVANCE.

..... SOUNDS LIKE YOU HAVE NO PROBLEM

LIVE IN GRACE

LEROY POLL

 

[james singleton]

Leroy, so the fact that I am not getting any readings (inches of vacuum) either at idle rpm, or even when I rev the engine, this is normal??? By load you mean actual driving conditions, correct??? Reving your engine while sitting in the driveway will not produce any (or hardly any) inches of vacuum??? Is that correct?? The engine needs to be under load (actual driving) to produce a reading like you mention of around 10inches of vacuum or so? Thanks for the input/info! Jim

 

[66 Fastback]

I am not familiar with the 1940 carb and its vacuum ports. But as I understand the SCV sticky explanation, as long as there is good Manifold vacuum, the SCV will not switch over to the venturi vacuum. There is no load on the engine when blipping the throttle in the driveway, so the manifold vacuum does not fall low enough for the SCV to switch sources.

Do you have a separate venturi vacuum port, or is the port the distributor port shown in Fig 8 of the sticky? If you are connected to that distributor port, you should be seeing manifold vacuum readings when blipping the throttle in the driveway. You should see high vacuum at idle and at high rpm (as long as there is no load on the engine).

But if the carb is truly equipped with a separate Venturi Vacuum port, you should be seeing some venturi vacuum at high rpm's. If a 1940 carb is configured like the one in Figure 8 of the sticky, the slide valve could be stuck in the venturi position. If that is the case, that would explain why you do not get venturi vac at high rpm's.

If you are not getting any vacuum at the distributor port, then I would think you have a stuck SCV valve and a good cleaning would be in order.
Doug

 

[james singleton]

Doug, To try and answer your question about my 1940 carb, I believe that it does not have "a seperate venturi vacuum port"! It has what Fig. 8 in the "sticky" refers to as a "distributor vacuum tube adaptor" with fitting and a vac line that runs directly to the distributor. I have previously removed the SCV and cleaned things out; still not sure if things are functioning as they should or not? I get some hesitation when I step on the throttle a bit, but then maybe that is just a fuel mixture adjustment problem. I was trying to determine if "no" vac @ idle is normal or not, and what I should expect under load and @ higher rpm's? Leroy said that "no" or very low vac at idle (and reving in the driveway) is normal. Maybe when I get it out on the road, and under load, I will see what kind of vacuum readings I get and report back! Thanks, Jim

 

[66 Fastback]

Jim, I have not trouble shooted my Load-a-matic. It had a leaky vacuum motor diaphram and I just replaced it when I went to a different carb. Therefore, I can't tell you for sure what the readings should be at idle. I know Leroy knows the Load-a-matic distributor. So I re-read the SCV sticky. The second paragraph states that "manifold vacuum enters a hole above the throttle plate". This would imply that the Manifold vacuum is accesses through a ported Vacuum port. So at idle there would indeed be low vacuum readings. I mis-spoke on the previous post. I was confused by them using the term Manifold vacuum througout the sticky. At idle Manifold vac is high but Ported vac is low. But at all throttle positions other than idle, Manifold vac and Ported vac are essentially the same.

I think your idea of getting some readings while driving the car under load would confirm if your SCV valve is switching to venturi vacuum under low manifold vac conditions. Because as mentioned, in the driveway, the manifold vacuum does not drop sufficiently for the SCV to change over to Venturi vac.

But something is still troubling in that your vacuum readings at elevated rpm did not show any vacuum either. If the SCV did not switch because the manifold vacuum did fall to zero, then you should be reading high manifold vac (at high rpm and no load). Either way when revving the engine in the driveway, it seems to me that you should be seeing higher vacuum at the distributor because the manifold/ported vac increases with rpm's in the driveway and the venturi vac would also increase with the air flow.
Still seems to me the SCV may be stuck or the passage in the carb is plugged.
Doug

 

[james singleton]

Thanks everyone! I Emailed Pony Carbs a few days ago concerning obtaining a SCV (Spark Control Valve) and a guy from Pony called this morning. That was a surprise; I figured to get an Email back. Anyway I talked with him a few minutes; and I don't know if he was just pushing to sell one of their carbs (the Autolite 1100) but he was sorta of bashing my Motorcraft/Holley aftermarket 1940 carb (with SCV). I told him that I have read mixed reviews on the 1940 carb both pro & con. I told him I was concerned about "not" getting any vacuum @ idle, and just like Leroy and a couple of other posters commented, he said that with the carb I have, and the Load-A-Matic dizzy, that I shouldn't be getting any vac @ idle! He did say that even with my vehicle sitting in the driveway (not under load)that I should get between 5-7inches of vac @ around 1500rpm. I will hook up my tach and see if indeed I do get 5-7inches vac @ 1500rpm. Just trying to narrow things down. I have a very slight hesitaton which may be nothing more than the carb needing a little tweaking. Putting in a plug for Pony Carbs; he did say they have a better (than orginal??) autolite 1100 carbs for sale that he acted like will give you better performance and better fuel economy. I remember reading not long ago about this new and improved carb from Pony; something I believe they modified for better efficiency. Thanks again, Jim

 

[ludwig]

Pony Carbs is very helpful in after sale help and tech. I have even spoken to the owner, John Enyart himself with a tech question. The phone guy bumped me upstairs and I got my answer from the horse's mouth. You need to know two things about them so you can deal effectively.

1) They DO NOT recommend ANY hop-up or aftermarket mods you might want to do. They are into optimizing STOCK configurations.

2) They are UNINTERESTED in any Holley or Carter carb for the Mustang. On their VHS tape for the Autolite 1100 installation and adjustment, the boss himself John Enyart holds up a Holly and says (I quote): "If you have this style carb, it has no core value." You can take that to mean "it has no value whatsoever."

If you can work within those limits, they will be able to help you a lot. Once you go outside the lines, they are no help at all.

And the new carb is their A1100 'annular discharge' throat. It appeared in another string. I'm thinking of getting it when it comes out.

 

[james singleton]

Ludwig, the name of the gentleman I talked with this morning was John. I got the feeling he wasn't that keen on aftermarket carbs, like you mentioned. I told him that I have read good and bad on the Motorcraft/Holley aftermarket 1940 carb that I have. It makes sense now after reading what you posted; he definitely seemed to favor the Autolite 1100 carbs. Jim

 

[ludwig]

he definitely seemed to favor the Autolite 1100 carbs. Jim

Do ya think? It's the only one they support. And 'John' may well have been him. Da bozz.