Non Mopar carburetor opinion

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RustyRatRod

RustyRatRod

I was born on a Monday. Not last Monday.
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First off, I would like to thank any of yall in advance for offering an opinion. I put a lot of time into trying to help people and time is valuable. That said, here is my question.

1975 Ford F250 Camper Special. 351M (I know, it's a transplant) T18 4 speed, Dana 60 with 3.73 gears.

I am SLOWLY working on building up a .030 over 400 Ford engine. It will be mild. 9.4:1, smallest Thumpr hyd roller cam 227 241 @ .050, on a 107 LSA. Will run headers.

The 351M now is not quite stock. It is except for the cam. I measured lift when I had to do heads on it before Christmas. Looks like it is the tried and true Melling/Sealed Power/whatever 204 214 @ .050 .484 .512 lift. Lift is tall cause of the 1.73 rockers. It bumps slightly when cold and almost not at all when warm. Running manifolds right now.

Has a Holey 4175 650 spread bore vacuum secondary. No choke. I live in Georgia. Weiand Action plus intake, I plan to use it on the 400. Also have the GM style HEI distributor.

Here's the deal. The carburetor "runs ok". I could never get the air screws just right. If I tighten them up all the way, it idles up a little. Like they cannot go in far enough. It HAS popped through the carb some trying to tune here and there. Might have got the power valve.....but IMO it runs too damn good. Probably anybody else but me would be happy with it.

I know the 400 will probably need something different/better/bigger/whatthehellever. So here is what I want to do. I would like to try one of those new Street Demon 3 barrel jobs. I want to do it before I swap engines, because I know there is room for improvement.

I feel like the 351M would probably be best with the 650. I think though with the 400 (they are actually 402 stock .030 over it will be a 408 ), that it will be better with the 750 considering the mods I plan. I also plan to ditch the M block heads and use the better flowing early 70s 351C 2V heads and headers on the 400.

Do yall think the 750 will drown the 351M now? They are like a TQ in that they only flow what the engine needs, but I am worried the primary side might be a little rich. So whaddya think?

Oh and there's this also. Right now, the 351 will suck the 650 for all it's worth if I stand on it. It runs very strong for a probably 7.8:1 engine.
 
I say 750. The larger TQ works great on small blocks, and the street demon is similar design.
If my TQ ever dies I'm buying Street Demon.
 
I have a 625 street demon on my 78 W-150, 4-speed 318 truck.
I really like the carb, cold start is impressive, it will fire right up on the first crank after sitting a couple of weeks even in cold weather.
It runs smooth down the road all I had to adjust on it was the idle.
The main reason I bought it was the phenolic body, I was having heat issues with my holley in temps above 90 deg F.
 
The best thing would be if I could get the Holley to act right. I am going to try another power valve, but if any of yall have any other ideers there it would be muchas grassy ***. The problem with it is described above.

Thank you drive through.
 
The new street Demon sounds like a pretty good idea.
I don't see a 750 cfm carb as to big. It has a lot of inches. (The engine.)
 
Say Rusty; Id like to try and help you out on that 4175s idle circuit, But having gotten to sorta know you over the last few months, I wonder if Im even qualified, as I believe youre probably way smarter than me on carb stuff. I hesitated to respond, but when I see a guy reaching out its hard for me to not reach back.
 
Does the 4175 have traditional idle mixture screws (i.e. screwing them in provides less mixture)?
 
AJ/FormS said:
Say Rusty; Id like to try and help you out on that 4175s idle circuit, But having gotten to sorta know you over the last few months, I wonder if Im even qualified, as I believe youre probably way smarter than me on carb stuff. I hesitated to respond, but when I see a guy reaching out its hard for me to not reach back.
Click to expand...

Now that's frikkin funny right there. I am not way smarter than anybody. I am open to suggestions. Got any ideas?
 
Aaron65 said:
Does the 4175 have traditional idle mixture screws (i.e. screwing them in provides less mixture)?
Click to expand...

I assume so. When I tighten the air screws all the way seated, the engine idles up a tad as if it had more to go it would smooth on out and run good. SO, I assume they are standard direction screws.
 
If they're standard idle screws, it sounds like it's rich at idle. Have you considered opening up the idle air bleeds a couple thousandths at a time? I know that is a step some might not like to take, but it's an option. I guess you could drill and tap for adjustable air bleeds if your Holley doesn't have them. Are the secondaries adjustable on that carb? Could you open them a little to get a little more air into the mix?

Additionally, if it's still running when you close off the mixture screws, it's getting fuel from somewhere...are the nozzles dripping?
 
for an experiment you could put a fine copper wire in the 2 idle feed restriction to see if it helps
 
I had a speed demon once that I could not tune an off idle stumble and sometimes backfire. One of the brass plugs in the top of the metering block was leaking air. Put some light oil on them and see id it gets suck in. Something to try.
 
So; Just sos I understand you right; It idles best with the idle mixture screws fully shutting off all fuel through their circuits.
That indicates, as Im sure you know, that the carb is supplying the idle fuel from somewhere else.Now Im pretty sure that carb has no idle fuel circuits on the secondary side. So we only have to search the primary side.
I can think of only 4 possibilities; 1)transfers, 2)mains, 3) fuel level, and 4) faulty parts
1) the transfers could be doing all the work due to the butterflies being too far open
2) the mains could be dribbling.Sometimes due to plugged anti-drool orifices, faulty accelerator pump shut-off needle, a too far open butterfly, or a damaged powervalve.
3) If the running fuel level gets too high, pull-over becomes too easy at the low speed discharge ports. Conversely; if too low, pull-over gets to be too difficult and a guy gets to cranking open the butterflies in an effort supply the craved fuel, from the transfers.
4) Faulty parts; well 2 little words to cover a multitude of possibilities! All the common areas; like warped blocks/bodies that the gaskets just cant seal,plugged/restricted air bleeds,ruptured powervalve diaphragm, stuff stuck in passages from the factory,porous or faulty passages etc..
Theres only supposed to be 4 ways for fuel to get into the intake; idle ports, transfers,mains, and pump. Barring faulty parts, One of three of those has got to be contributing when it ought not to be.(obviously not the idleports)

My money is on the transfers. And the reason would likely be a too-far open butterfly. It may be that the engine wants more air.It could also be(however unlikely) that the butterfly is too far closed, blocking the idle ports near to completely.The engine could only run this way if its getting a combustible mixture from somewhere else, and Im pretty sure it would suffer from a pretty good sized tip-in hesitation.
What I do When I get a tough case like this, and after Ive proved that the carb is fine, is this; I sync up the transfer port slot under the butterfly to appear about square to a little rectangular, with the idle speed screw. I reset the mixture screws 1.5 turns out. I close the secondaries. Then, with the engine now running, sorta,( I might have to increase the idle rpm with the fast idle cam, or a wedge between the curb idle screw and its stop) I crank in the timing to where it will stay running, and put the butterflies back on the curb idle screw. Then I begin to introduce air under the butterflies to see what the engine wants. Most want air. Unfortunately Holley et al, never included an idle air bypass circuit, so I have to figure out some way to do that. Ive had good sucess with determining the orifice size with the leak method and introducing that amount of air through the pcv system, or cracking the secondaries. Some engines didnt really like this and complained by stinking and making my eyes water and hurt. On those guys, I drilled the butterflies to get the air to pick up some transfer fuel on its way in.That seemed to do it. The important thing is to not stray too far from the initial T-port sync up.Once it gets close you can fine tune same as usual;idle speed and mixtures
Now Im not suggesting that you might have to go through all this rigmarole.Only that it sometimes comes to this.I mostly run into this on small block engines with bigger cams.I dont think your 400 will fall into this zone. I include it more for the newbes which I think will be watching this post.
I think you will find the your issue back in the 2nd paragraph.

There is a 5th possibility. I dont mention it because I know that you know all about late ignition timings.
I suppose I would be remiss,too, if I didnt at least mention the secondary side. I just assumed it was fully closed,short of sticking closed,so as not to introduce nozzle dribble on that side.
I sure hope you can find a solution in here somewhere. Cheers
 
Rob, I have an item that may interest you, and might help solve your dilemma. It's concerning our conversation earlier.

PM me when you're ready to chat, and I'll give ya a call.
 
Aaron65 said:
If they're standard idle screws, it sounds like it's rich at idle. Have you considered opening up the idle air bleeds a couple thousandths at a time? I know that is a step some might not like to take, but it's an option. I guess you could drill and tap for adjustable air bleeds if your Holley doesn't have them. Are the secondaries adjustable on that carb? Could you open them a little to get a little more air into the mix?

Additionally, if it's still running when you close off the mixture screws, it's getting fuel from somewhere...are the nozzles dripping?
Click to expand...

Negative. No fuel dripping. IMO it is rich at idle. I can smell it, but there's no black or grey smoke. I would assume there would be if the power valve was blown. I may yank it off and look at the secondaries.
 
It actually does have a tip in hesitation. At pretty much any speed. If I roll into it, it does ok.


AJ/FormS said:
So; Just sos I understand you right; It idles best with the idle mixture screws fully shutting off all fuel through their circuits.
That indicates, as Im sure you know, that the carb is supplying the idle fuel from somewhere else.Now Im pretty sure that carb has no idle fuel circuits on the secondary side. So we only have to search the primary side.
I can think of only 4 possibilities; 1)transfers, 2)mains, 3) fuel level, and 4) faulty parts
1) the transfers could be doing all the work due to the butterflies being too far open
2) the mains could be dribbling.Sometimes due to plugged anti-drool orifices, faulty accelerator pump shut-off needle, a too far open butterfly, or a damaged powervalve.
3) If the running fuel level gets too high, pull-over becomes too easy at the low speed discharge ports. Conversely; if too low, pull-over gets to be too difficult and a guy gets to cranking open the butterflies in an effort supply the craved fuel, from the transfers.
4) Faulty parts; well 2 little words to cover a multitude of possibilities! All the common areas; like warped blocks/bodies that the gaskets just cant seal,plugged/restricted air bleeds, stuff stuck in passages from the factory,porous or faulty passages etc..
Theres only supposed to be 4 ways for fuel to get into the intake; idle ports, transfers,mains, and pump. Barring faulty parts, One of three of those has got to be contributing when it ought not to be.(obviously not the idleports)

My money is on the transfers. And the reason would likely be a too-far open butterfly. It may be that the engine wants more air.It could also be(however unlikely) that the butterfly is too far closed, blocking the idle ports near to completely.The engine could only run this way if its getting a combustible mixture from somewhere else, and Im pretty sure it would suffer from a pretty good sized tip-in hesitation.
What I do When I get a tough case like this, and after Ive proved that the carb is fine, is this; I sync up the transfer port slot under the butterfly to appear about square to a little rectangular, with the idle speed screw. I reset the mixture screws 1.5 turns out. I close the secondaries. Then, with the engine now running, sorta,( I might have to increase the idle rpm with the fast idle cam, or a wedge between the curb idle screw and its stop) I crank in the timing to where it will stay running, and put the butterflies back on the curb idle screw. Then I begin to introduce air under the butterflies to see what the engine wants. Most want air. Unfortunately Holley et al, never included an idle air bypass circuit, so I have to figure out some way to do that. Ive had good sucess with determining the orifice size with the leak method and introducing that amount of air through the pcv system, or cracking the secondaries. Some engines didnt really like this and complained by stinking and making my eyes water and hurt. On those guys, I drilled the butterflies to get the air to pick up some transfer fuel on its way in.That seemed to do it. The important thing is to not stray too far from the initial T-port sync up.Once it gets close you can fine tune same as usual;idle speed and mixtures
Now Im not suggesting that you might have to go through all this rigmarole.Only that it sometimes comes to this.I mostly run into this on small block engines with bigger cams.I dont think your 400 will fall into this zone. I include it more for the newbes which I think will be watching this post.
I think you will find the your issue back in the 2nd paragraph.

There is a 5th possibility. I dont mention it because I know that you know all about late ignition timings.
I suppose I would be remiss,too, if I didnt at least mention the secondary side. I just assumed it was fully closed,short of sticking closed,so as not to introduce nozzle dribble on that side.
I sure hope you can find a solution in here somewhere. Cheers
Click to expand...
 
You know, you did mention you're running 22 initial in another thread, didn't you? Maybe with that mild cam in there, you're having to back out the idle screw so much to get it to idle down that you're running into problems (which was a possibility AJ mentioned above).
 
Aaron65 said:
You know, you did mention you're running 22 initial in another thread, didn't you? Maybe with that mild cam in there, you're having to back out the idle screw so much to get it to idle down that you're running into problems (which was a possibility AJ mentioned above).
Click to expand...

It does the same whether it has 4*, 12* or 22*. I certainly won't say it ain't timing, but I don't "think" it is.

And yes, it is up on 22 now. It runs the best it's run so far there.
 
Oh and the air screws are not backed out. If anything it runs best with them all the way seated.
 
Oh, sorry, I meant the idle speed screw, not the mixture screws.
 
IMO, the idle screw is close to right. At this point, I am leaning toward a bad power valve. I don't have any extras. I will get one in town this week. It's cheap and easy so we'll see. Normally, I wouldn't remove the carb to install it, but I will to check the transfer slots. Thanks yall I will let you know what I find.
 
FWIW, the engine will idle and drive at low throttle settings just fine with the powervalve removed and a plug installed. I keep one for fine tuning that circuit.I bet you could hit 50-60mph with a sub 3600# car.Maybe more. Might take a long ways to get there though. But not everybody has one of those laying around.
 
I am going to try my little AVS on it tomorrow I have for sale in the for sale section. None of yall are gonna buy the damn thing anyway. I might as well slap it on. It will probably run just as good as the little 3 barrel. We'll see.
 
The AVS is my 2nd favorite carb, right behind the TQ. I think I still got a couple around here somewhere.Im waiting for my Holley to puke, so I can put one on. I got too much time in my Holley to just shelve it, and it is working pretty fine. But one of these days.(been saying that for 10 yrs or more).
 
Hey rusty,
I'm not really up on Fords, but I sold my 69' 390 Torino to a friend of mine, and he has had nothing but problems with the 750Holley on that engine. It is an air vacuum secondary carb. it ran rich, and Indicated overcarburetion. He went to the 650 AVS, and it runs great.
 
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