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2_Late_I_Won 03-28-2007 01:10 PM

Carb vacuum diagram
My secondary isn't opening on my old school Holley 625-600 cfm carb. I don't know jack about carbs. but I know the diaphragm is good. I took it off and put a vacuum test to it and it stayed down. So I know that is good. I started it up and didn't feel any vacuum where the diaphragm goes and didn't feel any vacuum. I put the air hose to it to blow out any thing that might be blocking it but nothing came out and it blew freely. Like I said I don't know that much about Carbs so any help would be greatly appreciated.

MIKE FROM SM 03-28-2007 03:05 PM

It won't pull a vaccum unless it is under a load. It won't happen when in park.

NXS 03-28-2007 03:52 PM

I'd be looking at the spring in the diaphram

MI2600 03-28-2007 04:12 PM

Take the vacuum unit off the carb. Push the rod gently up and plug the port with your finger. If the rod stays up, the unit is functioning.

66bowtie 03-29-2007 06:58 AM

re: Carb vacuum diagram
Try this site for answers. You may have to search the site for your problem. If you can't find it send them an e-mail. Holly is good about answering problems. Be sure to know your model#.

2_Late_I_Won 03-29-2007 07:08 AM


Originally Posted by MI2600
Take the vacuum unit off the carb. Push the rod gently up and plug the port with your finger. If the rod stays up, the unit is functioning.

Yeah I took the vacuum unit off and put a vacuum on it and the rod pulled down. I revved it up and could feel any vacuum on the port on the carb.

I went looking to try and find out the model # but could find anything that stuck out to me as a model #. I drove it up to O'Reilly's and had a guy that said he knew about carbs and he had a hard time finding it too. He said it was from about the 1970's or so. ( he also said he knew about carbs. but he didn't tell me how much he knew about carbs. LOL)

Thanks for the reply's

NXS 03-29-2007 07:59 AM

it is a ported vacuum source that only has vacuum when the engine is pulling more air than the primaries can provide. it is weak and often the stock springs in the vacuum pod won't fully open until 7000+ rpm. They are color coded so if you can see what spring you have and the rpm it will take to open it.

66bowtie 03-29-2007 09:08 AM

This is where to find the list #. From the Holly site.

QUESTION How do I find the "list" number on my carburetor?
ANSWER The list number for most performance and factory 2 and 4 barrel carburetors will be found stamped into the upper right hand corner of the airhorn or sometimes called the choke tower. On the 4150 HP models that do not have a choke tower the list number will be stamped into the mainbody behind the throttle linkage. This number is used to identify the carburetor and also used when needing service parts or renew kits.

2_Late_I_Won 03-29-2007 09:36 AM

Thanks 66BOWTIE when I get home I'll have to ck it out.

2_Late_I_Won 04-02-2007 09:27 AM

It said R75 but that doesn't sound like model number to me. Oh well!!! Anyway I took apart the secondary and noticed there was a Black spring in there it seemed kind of stiff. What color should it be and is the black one stock or do you think it's been changed out before?

Thanks again Brian

NXS 04-02-2007 10:37 AM

White, being the stiffest and latest opening

2_Late_I_Won 04-02-2007 10:42 AM

AHWWW HAAAA :welcome: I think I found my culprit :D Sweet thanks I'll have to get another spring and see if that helps.

2_Late_I_Won 04-03-2007 07:16 AM

Well I went home last night and bought a vacuum secondary spring kit. I put the white one in ( the lightest/less tension )and my secondaries open up yeah :D the only problem is it opens up in to high rpm. Right about the time I want to let off the secondaries open up so I don't get to enjoy the extra power. Is there a way to increase vacuum so it will open up sooner or a way to adjust when it opens up? I know usually you would adjust when the secondaries open by getting a little bit stiffer spring but since I have the weakest on in there now I don't know what to do. Do you think I can run it with out a spring at all or do I have to have one to push the diaphragm back down?

Thanks again for all the help now I just need to fine tune this thing :D

NXS 04-03-2007 07:54 AM

you can pull the spring out and either push it inside out to soften it so the secondaries are open sooner and stay open


cut the bottom off, this will give an initial hit at lower rpms then slowly open in the upper rpms

MIKE FROM SM 04-03-2007 09:37 AM

What makes you think the secondaries are opening late?

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