carb ported and manifold vacuum question

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NukeBass

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No, this isn't to ask which one to connect to my vacuum advance, but it is related (I was searching on the web and that subject is as bad as politics and religion!)

I was looking at my tuning and, in the course thereof, hooked up a vacuum gauge to the manifold vacuum port on my carb and the ported vacuum port on the carb. The manifold vacuum was giving me 4 in Hg at idle and the ported 10 in Hg at idle. I double checked my vacuum gauge with a loaner vacuum pump from Advance and it checked out fine. So, I double checked my measurements today and got 6 in Hg and 12 in Hg from the manifold and ported vacuums respectively.

The carburetor is an Edelbrock performer model 1400 (the 50 state street legal model) and the motor is a 340 with Edelbrock performer RPM intake and comp cams XE274H cam. All of the carb documentation said that ported vacuum should be nearly 0 at idle and the Edelbrock tech line guy said my measurements were impossible.

I checked, as best I could without taking the carb off, that the throttle plates were moving and such and they looked normal. I didn't see any obstructions or anything in the carb. Has any seen this or have any suggestions on what to look for or what it means? The car has been dieseling a little bit when I shut it off so now I'm wondering if the two could be related.
 
NukeBass said:
No, this isn't to ask which one to connect to my vacuum advance, but it is related (I was searching on the web and that subject is as bad as politics and religion!)

I was looking at my tuning and, in the course thereof, hooked up a vacuum gauge to the manifold vacuum port on my carb and the ported vacuum port on the carb. The manifold vacuum was giving me 4 in Hg at idle and the ported 10 in Hg at idle. I double checked my vacuum gauge with a loaner vacuum pump from Advance and it checked out fine. So, I double checked my measurements today and got 6 in Hg and 12 in Hg from the manifold and ported vacuums respectively.

The carburetor is an Edelbrock performer model 1400 (the 50 state street legal model) and the motor is a 340 with Edelbrock performer RPM intake and comp cams XE274H cam. All of the carb documentation said that ported vacuum should be nearly 0 at idle and the Edelbrock tech line guy said my measurements were impossible.

I checked, as best I could without taking the carb off, that the throttle plates were moving and such and they looked normal. I didn't see any obstructions or anything in the carb. Has any seen this or have any suggestions on what to look for or what it means? The car has been dieseling a little bit when I shut it off so now I'm wondering if the two could be related.
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Sounds like you have the primary plates open too far and it's letting vacuum through the ported port.There should be "0", or very close to that when idling warm. 12* at the manifold port sounds about right, for that cam. Try advancing your initial timing a bit and that should allow you to close down the primary plates a bit.

The dieselling is also caused by the throttle plates being open too far at idle.
 
I'll give that a try. Is there a reason the manifold vacuum would read less than the ported vacuum?
 
Two7T3Dart said:
No...they should read the same when the throttle plates are opened .
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That's the part that is making me scratch my head. I don't know if it is possible the carburetor was put together incorrectly and losing vacuum at that point?

I at least now have an idea of where to start and can adjust timing and idle speed to see if that changes my ported side. It may be next week before I get to it, but I'll post an update of my findings.

Another question; Is it possible my carb is too small for my application? It is 600 CFM, but would that affect the amount of throttle plate opening for a given idle speed?


Thanks!!
 
NukeBass said:
That's the part that is making me scratch my head. I don't know if it is possible the carburetor was put together incorrectly and losing vacuum at that point?

I at least now have an idea of where to start and can adjust timing and idle speed to see if that changes my ported side. It may be next week before I get to it, but I'll post an update of my findings.

Another question; Is it possible my carb is too small for my application? It is 600 CFM, but would that affect the amount of throttle plate opening for a given idle speed?
Thanks!!
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600cfm is on the small side...wouldn't go any smaller. I would think you would need to have the throttle plates open farther on a small carb to get the engine to idle properly. You might try a 650 AVS or 750 vac. sec Holley , if you have acess to them for testing.

If you remove the carb, turn it over and see where the throttle blades are in relation to the vac port as well as the idle slots. There was a thread somewhere on here about how to set them correctly. Maybe try a search for it.
 
Are you sure your correct on what port is ported and what one is manifold? It sounds to me like you have them switched because the manifold vacuum should be the higher of the 2. There's no possible way anything in a carb can amplify vacuum. Like 73 Dart pointed out you really shouldn't have any vacuum on the ported port at idle. I ran a 625 Carter on a 360 with a very similar build as yours with no idle problems or vacuum port problems. Also ran a 750 Holley on it and it might have made 10 more hp at the very top end but the smaller carb was more responsive. I'm with 73Dart in definitely not going smaller though.
 
fishy68 said:
Are you sure your correct on what port is ported and what one is manifold? It sounds to me like you have them switched because the manifold vacuum should be the higher of the 2. There's no possible way anything in a carb can amplify vacuum. Like 73 Dart pointed out you really shouldn't have any vacuum on the ported port at idle. I ran a 625 Carter on a 360 with a very similar build as yours with no idle problems or vacuum port problems. Also ran a 750 Holley on it and it might have made 10 more hp at the very top end but the smaller carb was more responsive. I'm with 73Dart in definitely not going smaller though.
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I double and triple checked the ports. The ported vacuum I am referring to is the higher of the two and sits on the passenger side of this Edelbrock carb. The manifold is on the driver side and is lower. That matches with the diagrams that came with it.

Since the manifold port is reading lower than the ported and the ported is more in line with what you might expect the manifold to be, something in the carb is losing vacuum between the two sides, which doesn't make a whole lot of sense to me.

I don't have access to any other carbs for testing right now. This is the carb the shop that rebuilt my car put on it (the first one was a 625 Carter that had issues and then they replaced it with a new Edelbrock performer 1400) and the wife won't be happy at all if I go buying other carbs! :D
 
I'd suspect a restriction in the channel leading to the manifold port. In over 30 yrs. of rebuilding carbs I haven't seen one do that but something must be happening along those lines since your sure you have them correct. It's not big deal if you don't need to use that port. But you need to find out why the ported port is supplying vacuum at idle. Like 340Dart said the throttle blades must be open too far. And that can cause it to run on like you say it's doing.

BTW: for that cam to run properly the distributor centrifugal advance needs to be modified so the engine has 18-20 degrees initial timing and 34 total. Lighter springs will let the advance to come in faster and aid in power production. Lighter springs aren't mandatory but more initial advance is. Did you do that? If not that'll cause you to have to open the throttle farther to get it to idle fast enough.
 
fishy68 said:
I'd suspect a restriction in the channel leading to the manifold port. In over 30 yrs. of rebuilding carbs I haven't seen one do that but something must be happening along those lines since your sure you have them correct. It's not big deal if you don't need to use that port. But you need to find out why the ported port is supplying vacuum at idle. Like 340Dart said the throttle blades must be open too far. And that can cause it to run on like you say it's doing.

BTW: for that cam to run properly the distributor centrifugal advance needs to be modified so the engine has 18-20 degrees initial timing and 34 total. Lighter springs will let the advance to come in faster and aid in power production. Lighter springs aren't mandatory but more initial advance is. Did you do that? If not that'll cause you to have to open the throttle farther to get it to idle fast enough.
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No, I haven't set initial that high yet. The shop set everything up and it is closer to 5 degrees advance. The shop also had the vacuum advance hooked up to manifold, which is why I really started looking at it in the first place. I did have it set up to 15, but it was getting in the 40 degrees with mechanical advance at around 3000 RPM and over 50 with the advance hooked up. I was hoping to not have to go into the distributor, but it looks like I will need to chart new ground for me. This might be...interesting... :)

[As an aside, it seems like the resto shop should have known this about the cam they put in there.]

Thanks for your help!
 
You need waaayyy more than 5* initial....sounds like maybe it was a Chevy shop that did it???... as thats the way Chev's are setup...using manifold vacuum on advance.

You have to limit the mech. advance in the dizzy to run a bunch of initial, or you'll ping like crazy....but you found that out already. Somewhere 34-38* total advance (init. + mech.) , then add about 10-15* vacuum on top of that, should be ballpark, all depending on what your engine likes.
 
The shop claims to specialize in Mopars and he seemed like a pretty good guy so I gave him my project. Lots of time and things since then, I wonder about the specialize part, but the car is together... The original timing on the car would have been 5 degrees advanced, according to the sticker, but that's with all of the 73 emissions and stock motor.

I'll plan on changing the advance in the distributor in the next couple of weeks and seeing how that turns out. Thanks for all of the advice!
 
Getting the distributor dialed in will help a lot. Below is a link to an article that details how it's done. Something else I do that they don't mention is not only check the reluctor gap with the vacuum advance at rest but also when it's pulled to max. I do this because the reluctor plate due to design just floats there and the vacuum advance doesn't always pull it around in a linear fashion so the gap can increase or decrease some. There is no real cure for that (that I know of) so what you have to do is split the difference. In other words if the gap measures .008" with no vacuum applied to the vacuum advance and .004" with vacuum applied set it where it's .006" with vacuum applied. Any tighter than that and it may not always fire right. You also don't want to go looser than .012" or it may not always fire right, especially if your using a MSD box which seems to like a tighter setting.

http://www.moparmusclemagazine.com/...mopar_electronic_ignition_system/viewall.html
 
Well, I thought it was a MP distributor because that's what my receipt says. I took it apart and the mechanical advance is not adjustable like all of the MP pictures I've seen. I don't have a welder or anything and really don't want to take the rest of the distributor apart to mess with the advance plate, so I think I'm stuck for a while. I have an email into the shop that put the car together to see what they will do about it. Ugh!!!!!!
 
Not all the MP distributors are adjustable, I would guess only the newer ones.

One thing you could do is to buy the advance kit from FBO

Go here

http://www.4secondsflat.com/Ignition.html

down the page, J685K

I don't endorse or "not" never used em, some guys have
 
I wouldn't think this distributor was any more than 2 years old. Maybe I got an old one. I saw that plate kit, but wasn't sure if I can get the rest of the distributor apart to use it. But, then again, I got down to the advance plate in the first place, so maybe I can. That kit is certainly cheaper than a new distributor is, too. I'll have to think about it. Thanks!!
 
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