[schwim]

Hello there, internet neighbors!

Bog's got a 400 with what's supposed to be a torquey "RV" cam and it's got what I think I remember to be a 600CFM Holly on it setting on an edelbrock intake. Ever since the rebuild, it has lugged and bucked at very low RPM(about 1k-1750-2k) and then picked up after. I remember the mechanic played with timing, advances and tuning to try to even it out but it never got any better.

I wonder if it's overcarbed. The way I drive and the shittiness of a smog tractor engine just doesn't seem to need a 4BBL, much less one that large. The 2150 is said to have less than half the CFM and aside from the cam and a slightly less restricted exhaust, this is just a stock engine.

I'd love to hear the groups thoughts first, though, before I start looking for a reman 2150 to pick up.

Thanks for your time!

 

[packagerjr]

Unfamiliar with operating a 2150 but I had no driveability problems with a 600 CFM Edelbrock on top of their Performer intake. That was with no smog stuff. 10 degree BTDC timing on a stock 351Mz

Good luck with your next steps.

 

[Doosenberry]

My 78 had no smog equipment on it when I got it. I don't know if the PO did anything to it internally, but it had an Edelbrock Performer 400 intake and an Edelbrock 750 cfm carb on it when I bought it. I felt that was way too much carb for an otherwise stock 351M, but other than smelling of gas constantly and having hard starts if it sat for more than a couple of days, it ran fine. Its a 4 speed as well.

I did buy an adapter to put a 2bbl carb on it, so I could try the 2100 off my F100 to see if a smaller carb would perform the way i wanted before i bought something new. I basically got better throttle response off idle, but once it revved a bit, you could tell it wanted more cfm. My 2100 is very small for what it's worth. I don't know the cfm, but it even starves my F100's 302 a bit. They make larger 2bbl's of course, so that shouldn't be an issue if you do go with a 2150 or something similar.

 

[CrazyBRONCOguy]

Wide Band o2 sensor will tell you exactly what is going on. However that is likely the same $$ as a different carb.

I do feel like yes you'd likely give up a bit on the top side but the 2150 will likely run better down low, tho I don't think a 600 Holley is too large either. Just sounds like it has the wrong combo of jets and such in it. I would also look everything over really well for a vacuum leak, and make sure the distributor is doing what you think it is both mechanically and with the vacuum hooked up (if you run it down the road that way). You may look into getting the duraspark distributor re-curved too.
Parkland Performance Auto Machine - Services (reincarnation-automotive.com)

 

[schwim]

My 78 had no smog equipment on it when I got it. I don't know if the PO did anything to it internally, but it had an Edelbrock Performer 400 intake and an Edelbrock 750 cfm carb on it when I bought it. I felt that was way too much carb for an otherwise stock 351M, but other than smelling of gas constantly and having hard starts if it sat for more than a couple of days, it ran fine. Its a 4 speed as well.

I did buy an adapter to put a 2bbl carb on it, so I could try the 2100 off my F100 to see if a smaller carb would perform the way i wanted before i bought something new. I basically got better throttle response off idle, but once it revved a bit, you could tell it wanted more cfm. My 2100 is very small for what it's worth. I don't know the cfm, but it even starves my F100's 302 a bit. They make larger 2bbl's of course, so that shouldn't be an issue if you do go with a 2150 or something similar.

The 2150's advertised CFM is 287. I'm not sure what the 2100 is rated at.

Wide Band o2 sensor will tell you exactly what is going on. However that is likely the same $$ as a different carb.

Ironically, we wouldn't be having this discussion if I had gone with the EFI kit instead of the AC setup, which I flipped and flopped over. I should have gone with the EFI setup.

I do feel like yes you'd likely give up a bit on the top side but the 2150 will likely run better down low,

Can I ask what you consider top end, RPM-wise? I ask because I almost always shift before 3k RPM. Once in a blue moon I'll rev to 3500. Am I even getting to what you would consider the range in which I'd lose capability?

tho I don't think a 600 Holley is too large either. Just sounds like it has the wrong combo of jets and such in it. I would also look everything over really well for a vacuum leak, and make sure the distributor is doing what you think it is both mechanically and with the vacuum hooked up (if you run it down the road that way).

I should probably clarify the situation some. The reason I thought overcarbed is because if I am light-footed on the throttle, this issue doesn't occur. I only get the shuddering if I am at above have throttle and above 1k but below 2k RPM. That made me think that it was dumping too much fuel. I imagine jets could be the cause for that. I will say I never smell raw fuel from the exhaust.

You may look into getting the duraspark distributor re-curved too.
Parkland Performance Auto Machine - Services (reincarnation-automotive.com)

The Duraspark/distributor was replaced with a knock-off MSD distributor. There has been 3 different distributors in it, first replacement due to defect, second replacement because mechanic was trying to figure out this issue. I've ensured that the advance was operable but I've not done anything to ensure the curve is correct.

 

[CrazyBRONCOguy]

@schwim It sounds like to me (from reading on here on this post) you are right in that spot where it is switching between what feeds the fuel inside the carb. Meaning stuck between the idle and the main feed or in a weird place where the secondary is just starting to open or something like that. On some carbs you can tune this out, but then you might have an issue if you down shifted and floored it. Very hard for me to get feel for what should be next step without seeing/hearing it run in person (me personally I am sure maybe some other board members can help more that know the Holley's well, I'm more of a Demon guy). Is why I suggested wide band O2...

When I spoke to top end I was meaning 3500/4000+ rpm if not higher is where you likely will see some drop off in performance. Sounds like you may not care if you don't drive it there.... In contrasting example the carb 460 that was in my 78 (before went 429 with Sniper), was never below 3k rpm so the idle to 3k rpm transition was critical to me. Enough so that it likely would not run super great at the rpm's you usually drive (Just different end goals is all)

CFM = (Displacement * Max RPM) / 3456

ie: (400 x 5500) / 3456 = ~650 cfm

General rule of thumb / use for reference only above... Rather applied to your situation (400x3400) / 3456 = 393 cfm....

 

[BigBlue 94]

The 600 is not too big for a 351. I know guys running them on 300-6 engines with little more than a 4bbl manifold and efi exhaust manifolds.

You need to likely adjust the accelerator pump circuit, especially the accelerator pump cam.

Also, remember that 2bbl carbs are measured at a different pressure than 4bbls. So a 300 cfm 2 bbl is bigger than a 300 cfm 4bbl.

 

[Stevo440]

The 600 is not too big for a 351. I know guys running them on 300-6 engines with little more than a 4bbl manifold and efi exhaust manifolds.

You need to likely adjust the accelerator pump circuit, especially the accelerator pump cam.

Also, remember that 2bbl carbs are measured at a different pressure than 4bbls. So a 300 cfm 2 bbl is bigger than a 300 cfm 4bbl.

I was to lazy to respond to this post and figured @BigBlue 94 would chime in and give some advise. I concur with all 3 points/statements he made.

What is the model #/part number of the carb? Is it a vacuum or mechanical secondary carb? Is it a dual or single plane intake manifold? What is your idle vacuum reading? There is a transition slot and only a small portion should be showing when the throttle blades are closed at idle about 0.060"? Just some other things to "thinkabout"/look into!!!!

 

[Gondal]

600 is not too big. It’s very good for a 400
If it were me I would try going up or down 2 jet sizes depending on what you have now
I agree it could be accelerator pump adjustment.
I had one that would fall on its face and stall from a dead stop before it ever got going down the road
Jets were too small

Also consider the power valve. Holley has a great video you can purchase on tuning the car b

 

[schwim]

Thanks very much for the help and guidance, guys. I wish I could get you all to witness what it does to help narrow it down but unfortunately, I don't think a video will do any good as it's nothing that would be noticed unless you were driving him. It's also too loud for me to talk it out on a video for you. I do think that if you could have a better idea of what's going on(if I were doing a better job of explaining the issue), you'd be able to narrow it down.

It's been a long time but I think I do remember playing with the accelerator pump without success. It's likely time to just walk through it again.

Thanks again!