Need help with Holley carburetor tune.

[338Raptor]

I pulled a 26 year old down leg 850-DP out of my collection of dusty/dirty parts I keep around “just in case”. I cleaned it up, blew out all the orifices, installed new gaskets, PV’s, acc pumps, etc and put it on my engine.  The engine runs perfectly now, no more lean spot.   
So obviously there must be something wrong inside the fancy annular booster custom 850-HP.  I’ll be sending it back to the carb builder. Hopefully he can find and fix the problem.

If nothing else, I now have a very good understanding of how all the circuits in a carburetor work. 

[Drew Pojedinec]

A lot of over engineering happens at carb shops.
I think I spend the majority of my time just setting stuff back to the original setting. Not that original is perfect, but it’s a much closer starting point

[My427stang]

I pulled a 26 year old down leg 850-DP out of my collection of dusty/dirty parts I keep around “just in case”. I cleaned it up, blew out all the orifices, installed new gaskets, PV’s, acc pumps, etc and put it on my engine.  The engine runs perfectly now, no more lean spot.   
So obviously there must be something wrong inside the fancy annular booster custom 850-HP.  I’ll be sending it back to the carb builder. Hopefully he can find and fix the problem.

If nothing else, I now have a very good understanding of how all the circuits in a carburetor work.

Well, both knowing it is the carb and the lesson are big wins in my book.  Lot's of guys don't like the 4781 DPs, but I enjoy them.  I generally pull the big pump off the back (if it has them), lose the rear PV and jet accordingly and they are happy.  Apparently venturi design slows booster responsiveness, but, I have really seen them work well on the street.  Hopefully they can figure out what's killing the transition on the big dollar carb

[gt350hr]

  I ran a 4781 on my drag race Boss 302 for thirty years! It was VERY predictable and rarely needed any changes despite varying quality of engines underneath it. My contemporaries ALL said a 750dp would out run it but NONE did. I dumped the rear power valve too but kept the big pump . I too still have it "just because".

[338Raptor]

I heard back from the carb builder today. His initial answer, without seeing the carb, is that the carb was calibrated for non-ethanol fuel of 17 years ago.  He thinks modern non-ethanol fuel is different enough to cause the problems I experienced.  Personally I don’t believe his theory but I’m sending it back to him for rebuild/recalibration anyway. After I get it back and get it fine tuned to my engine I will do a chassis dyno comparison between the two carburetors. Should be very interesting to see the drivability differences and actual power difference. I’ll update this thread after I get results. 

[gt350hr]

    "Ethanol" laced fuels do require a bit richer calibration. Older #1850 Holleys especially.

[My427stang]

I heard back from the carb builder today. His initial answer, without seeing the carb, is that the carb was calibrated for non-ethanol fuel of 17 years ago.  He thinks modern non-ethanol fuel is different enough to cause the problems I experienced.  Personally I don’t believe his theory but I’m sending it back to him for rebuild/recalibration anyway. After I get it back and get it fine tuned to my engine I will do a chassis dyno comparison between the two carburetors. Should be very interesting to see the drivability differences and actual power difference. I’ll update this thread after I get results.

Hogwash LOL you were rich and lean depending on the circuit, regardless, hopefully he can get it worked out

[machoneman]

2X on hogwash. 100% gas or 90% gas, it should run fine on either.

[338Raptor]

I think I found the problem.
The 4-1/4” Sidepipe mufflers have a 2” diameter hole thru the baffles.  So I have 2” primary tubes flowing to a 3-3/8” collector to 2” ID mufflers.  No wonder I can’t get the motor to run right at high rpm. It runs great down low.  I’ll be replacing my Sidepipe mufflers ASAP. 

[My427stang]

I think I found the problem.
The 4-1/4” Sidepipe mufflers have a 2” diameter hole thru the baffles.  So I have 2” primary tubes flowing to a 3-3/8” collector to 2” ID mufflers.  No wonder I can’t get the motor to run right at high rpm. It runs great down low.  I’ll be replacing my Sidepipe mufflers ASAP.

No doubt sidepipes steal power, sometimes a LOT, so good call, but didn't the carb swap make it run better?

[MeanGene]

Much more often than not, a misbehaving Holley can be spanked by un-experting whatever has been changed, a good cleaning, a genuine Holley Renew Kit, and proper adjustment of the pump(s), secondary throttle blade setting and idle screws. At that point it should run reasonable well, and quite well if it's the proper one for the application. Got a mild 390 in a car or 4x4? Plain Jane #1850 is hard to beat. Put a 4781 DP on it? Not so much. Chasing problems with an "experted" carb that has mods, by making more mods usually just results in tail-chasing. Un-f#$k it first, might get a nice surprise

[338Raptor]

It runs great with the 850DP.  But is does begin losing power after 5000 rpm.  I’m sure the 2” exhaust pipe is responsible for that. 
Below 5k all is well. 

[Drew Pojedinec]

Gene nailed it.
I hear “wow dunno what you did but it runs great now”
And most times all I did was put it back to factory spec

[338Raptor]

It’s definitely not at factory spec.
It started there, but now:
74 primary (stock 78)
75 secondary (Stock 78)
6.5 primary power valve
Plugged secondary PV (stock 6.5)
PVRC .0620 (stock .0665)
35 primary nozzle (stock cam, don’t remember the color)
31 secondary nozzle (stock cam, don’t remember the color)
Idle/low speed jets are .027 stock, next change is to try .026
I tried .0247 and it was a little too lean. I think .026 will be perfect.

I’m going to wait for the new mufflers before I do any more tuning.  The new mufflers have a 3” flow path so I’m sure the carb will need completely retuned.

[Drew Pojedinec]

Idle feed restriction is .027??
I highly doubt that.
For a 750 I’d start with a ifr/iab like .033/.067
For an 850 more like .035/.070
.027 would be lean in a 600 on your engine.