More carburetor questions and opinions..

-
RustyRatRod

RustyRatRod

I was born on a Monday. Not last Monday.
FABO Gold Member
Joined
Jun 7, 2010
Messages
104,718
Reaction score
102,216
Location
Georgia
This is on my 75 Ford F250, 351M, 4 speed, 3.73 Dana 60.


Ok. Since I hucked the AVS in the scrap barrel it's history. lol I have been trying to find out some information on this Holley 7002-1 4175 spread bore vacuum secondary, and I have somewhat.

It is a carburetor that is completely foreign to me. I have seen them on things before, but have no experience working on one.

Some of the things I have found out. They came originally with a 2 stage power valve. Evidently the first stage opens at one vacuum level for enrichment and the second stage opens at another level for yet more enrichment.

Also, it seems this carburetor may have reverse air screws from the "normal" Holley.

I am having two problems with it. One, the air mixture screws do not work right. The engine continues to run with the screws seated. In fact it gets better with the screws seated, as if I had some more threads and could turn them in more I could get it adjusted correctly.

Two, it has a pretty bad part throttle tip in hesitation or bog. This is only on the primary side. The secondaries come in fine. If I roll into the gas, it does ok.

I have the floats right, even though they are not adjustable. I have the accelerator pump right. The transition slots are not overly exposed.

So after all this blabbering, here is my question. Since I was unaware it had a dual stage power valve, it was replaced with a single stage 6.5. I have read that if you do that, you need to jump up a few sizes with the jets. Is this correct?

Could this also cause the tip in hesitation? It really runs great other than these two problems. I could drive it like this anywhere...and do, but it would be great to get it worked out.

I really need to try to get this worked out if possible because I do not have the money to just throw at a new carburetor, even though that's what I would love to do. Thanks.
 
many years ago a friend put one on chey 350. replacing the a GM carb. it got bad MPG. and i changed the power valve and accl nozzles. MPG went up 2-3.
 
krazykuda said:
Why not put a two stage power valve in it???
Click to expand...

Because for one, as of now, I cannot find one. Two, if I can, I don't know which one it takes, as I cannot find any information on how the 4175 came stock. Three, I want to actually verify that a single stage power valve will make it act like it is, before I start dropping money on it as money is very limited and I don't want to throw parts at it without knowing if it will help.

That was my whole purpose for asking about it here. I was hoping someone might know more about these than I do.
 
Well I'm sure as you know. On most carbs if you screw the mixture screws all the way in and it runs better it usually means you have vacuum leak? I have used a few of these in years past but I really don't remember much about them. Reverse acting mixture screws? Are you sure?
 
I imagine the first stage works like metering rods; suppling fuel under light to moderate acceleration.
What kind of manifold vacuum numbers is your combo posting?
Does it hesitate in neutral, at tip-in?
Does it hesitate immediately, right off idle?

A think the biggest clue is the mixture screw behavior. If they are in fact reverse operating, then it sounds like all the way in is richest, which leads me to think that they may be air screws. In which case the engine is craving more idle fuel, which is manifesting as a hesitation.I would then conclude that the carb is failing to deliver enough idle fuel.In which case I would take it apart,figure out that circuit, and mod it as might be required.I suppose theres a real good chance that the missing fuel is supposed to be coming from the 2-stage.In which case that two stage could be functioning in an anti-diesel capacity;shutting off the fuel when the vacuum collapses at engine shut-down, and allowing idle fuel at start-up, with rising vacuum.
This is all speculation. Ive never had one apart.
 
I pulled a plug and it's certainly lean and not rich like I thought. That could explain the hesitation.
 
It could be set up to run like the early TQs with the mixture screws being air bleed...in which case, turning them out would lean the idle mixture. I will dig a little more in my Holley book and see what I can find for ya.

For starters:

[ame]http://documents.holley.com/199r8340-1rev.pdf[/ame]
 
And then there's this:

Holley Technical Support: 1-270-781-9741

Also: Did not view it, but it might be useful.

[ame="https://www.youtube.com/watch?v=EsIoho5cLIM"]Holley 4175 Spreadbore carburetor - YouTube[/ame]
 
Thank you. Can you give me the link so I can get out of that little window? That's the most info I have seen on them so far and I have looked my eyes out.
 
RustyRatRod said:
Thank you. Can you give me the link so I can get out of that little window? That's the most info I have seen on them so far and I have looked my eyes out.
Click to expand...

Rob, if you hover your cursor up in the top right corner there will be two options, "zoom" and "pop out", click on pop out and it will open in a larger browser window.
 
I know and I saw that, thank you. But true to form, it doesn't work for me. LOL
 
Rusty, according to my Holley book the idle screws are indeed air screws and operate backwards from normal. In is full rich.
 
Which is what my gut suspected all along. That means it is running lean. Everything I have read says to jet up a few sizes when using a single stage power valve in the place of a two stage. But my question is, since the jets are not on the idle circuit, how can jetting up help to achieve getting the idle mixture right? Are these different in that aspect as well?
 
RustyRatRod said:
Which is what my gut suspected all along. That means it is running lean. Everything I have read says to jet up a few sizes when using a single stage power valve in the place of a two stage. But my question is, since the jets are not on the idle circuit, how can jetting up help to achieve getting the idle mixture right? Are these different in that aspect as well?
Click to expand...

Not sure in this instance, but some carbs draw fuel from the idle circuit after it is metered through the primary jets. I will look this evening for a flow diagram......
 
Have you seen this document?

[ame]http://documents.holley.com/199r10645.pdf[/ame]
 
Interesting. No, I have not seen that. Where are yall comin up with this? I have looked my eyeballs out using every word configuration I can think of to no avail. Much appreciated.

Although I still don't know which 2 stage power valve this carburetor came with to get a good baseline of which single stage to use.

And I am sure my truck would fall under the "other vehicles" it spoke about needing more tuning due to the weight. Thanks yall.
 
You must log in or register to reply here.
-

Back
Top