What size should I go with: 750,825,850 or larger? It will be a speed demon or a race demon.

 

Hmmm, for a mild 454 I would go no lower than 800....that's just my opinion though. I don't like vacuum secondaries either, I prefer mechanical secondaries and manual chokes so the Double Pumper series of carburetors are exactly what I like. I'm currently running an 850 DP.

 

Since you already have a 750 try running that and getting your jetting good and running it. If you think it needs more after playing with jets and squirters step up to the bigger unit and retune for fuel mixture again. Dont spend money right away and find you went too BIG.

 

Any oversized problems Bence?

 

She idles a little rich but that can be solved rather easily. I'm probably going to drill one small hole in each of the primary throttle plates to help lean it out some. If that doesn't work I'll jet down the primaries. All in all though I am very pleased with the throttle response of the carburetor....don't ask me about gas mileage.

 

Subboz, I am trying to gain some performance, but my main objective is to retire the old holley. Its leaking internally and externally, the shafts or worn, and its got some hesitation. Thanks for the advice, I do appreciate it... Do you really think its possible to oversize a 454 with a pretty steep cam?

 

I am looking to change my carb to a bigger one also. Mine is a 780 Holley dual feed with vacuum secondaries. I want to go bigger, but first I am re-jetting. My primaries are 72 and secondarires are 76. What are you using in the 850? :confused:

 

For reference I dont think you can't go wrong with an 850 on a monster Rat motor. However I defy convention with my Avenger 770 with 74/78 jets and I am still running lean on my 350 that has a mild cam. Ultimately it comes down to how well your heads flow. Demons rock. Wish I had the money to pony up for it. And worse case scenario you can jet down the 850 easy. You can always control the gas but they only flow so much air.

 

OK then, what is the advantage/difference between a street demon and a race demon.

 

Demons are underrated for flow. I tried a 850 mech speed demon on my 454 71 4spd and it needed bleed restrictors and a bunch of labor, put it on my BB T-bucket that could use the flow better. Much happier with a Bill Mitchel modified holley 850 ANNULAR. The annular raises the signal for street driving and costs very little flow on the top end. For sure go mech.

 

850 DPs are terrible street carbs. I don't know how much cam you have but it's hard to beat a 780 vac or a Qjet.

 

71,454,4spd


I'm about to buy a 850 Dp/ vav secondary speed demon for my 468ci. I plan to purchase the carb here in the next month.

What part of Louisiana are you from?


[Modified by Corey 68, 12:35 AM 6/27/2002]

 

If you go with the Demon I would suggest a 750. The 850 will probably be to much. On my zz4 I am only running the 625 road demon, and it works well.

Mark B.

 

1st choice a rebuilt Q-jet. 2nd choice any other aftermarket 750. Don't make the same mistake a lot of Bubbas on this board make by over carburating the engine

 

Shane The rich idle is also helped by installing a .015 copper wire bent into a "L" shape and placed in the metering block and held in by the gasket. This will lean out the idle and make the idle screws more sensitive. They should be out 1 1/2 turns or very close. The .100 holes drilled in the throttle plates allow you to close the throttle plates with the idle speed screw to cover up the transferr slots.
I like the 850 double pumpers on a big block and weather I am a Bubba or not they work fine and if jetting properly give decent gas mileage.

 

467-Ratman:
The jets are 80 in the primary and 80 in the secondary.

Norval:
Thanks for the tip. I believe it was from you that I heard about drilling a small hole in each of the primary throttle plates. Can you comment any further on this? What size drill bit? This would help lean out the air/fuel mixture right? Thanks Norval

 

Shane, Thanks for the info on the jets. My 72 and 76 must be way to small, no wonder this engine has not kicked my but yet!!! I also have to check my power valves out and I have heard from my engine guy about drilling the hole in the throttle plates. I will keep you posted on how it worked out. Tom

 

You really shouldn't have to move jet sizes around too much. Many folks get WAY out of the ballpark right here. Carbs are jetted/calibrated for a particular airflow.

Typical "hot rod" style DP's are planned to go on relatively serious motors that have weaker signals to the boosters (low vacuum). As a result, they use larger jets to get the required fuel flow. When you put that same carb on a motor with strong signal to the carb you get rich running. That's where lots of folks get in trouble with carb matching and thinking the richness comes from overcarbing. You can bolt a 750 Dp on a stock 302 Mustang and idle fine and then put it on a 454 with similar cam and it will also idle and run fine as well as be jetted pretty close. It's pretty linear. Think of jetting like a vacuum sec. carb. It will move as much fuel as it has signal/airflow to send. Obviously it will take more RPM for the 302 to reach that fuel flow than a 454 will, but it will still be in correct proportion.

Jetting has little to no effect on idle fuel. You are running on the idle circuits up to the time you get enough airflow to start moving fuel through the main boosters. You can look down a carb as you rev the engine slowly (with a mirror- safety!) 'till you start seeing fuel dribble from the boosters. That is the rpm that you are starting to get into main jetting on YOUR engine. Bolt it onto something else or change your cam, heads, intake, ex. and it can all change.

They are right, you want to have the idle screws out 1 to 1.5 turns. If you can't get them out that far with correct mixture, you have to do some work. You are getting fuel from somewhere else. Most likely it is coming from the idle transition slots in the throttle bores. You probably have a pretty healthy cam that requires you to idle it up pretty high. When you do that the slots get uncovered and they start allowing fuel in at idle. They are supposed to help out only in that transition period between idle and main fuel mettering through the boosters.Think of them as a "passive" accelerator pump.

So before you drill holes try a couple of things. Pull the carb and look into the bores from the bottom. You should just BARELY see maybe .030 or less of the slots when the plates are closed. The less the better. Close the primary blades up to achieve that. Now look at the rear ones. They should really be completely covered, but since you closed up the front ones, you are going to have to get air somewhere to get it to idle. Remove the secondary throttle stop screw in the body and re-install it from the other side. Now you will be able to play with it after carb is installed on car.

Reinstall carb and use the REAR barrels to raise idle slightly. Make sure you don't go so far as to start getting those slots uncovered on the rear side.

Now with both throttle plates allowing air in, you can keep the front ones closed more and get your mixture screws working again. This works 95% of the time on most streetable setups.

In extreme cases, you may have to drill the holes. By drilling holes, all you are doing is allowing in more air to allow you to close the throttle plates more, thus covering up those slots and getting the idle screws working again. Make sense?

IF you have to drill holes, start small. Use a 1/16" bit and drill them on the same side as the transfer slots in line with them.

In cases where the signal is still too strong, then you can restrict the idle fuel orifices with the wire strands or new orifices. I've also used lead pellets from a shotgun shell to plug the holes and redrill. One shotgun shell will last you a lifetime of carb work!

Main jets are what cause rich running in the midrange. You work out jetting by driving under cruise conditions to get it to run right.

Next you work out WFO mixture with power valve restriction tuning. That's where your fuel comes from under those conditions. You reach an RPM where the jet is maxed out and the power valves add additional fuel for the top end charge. That's why many times your best jetting setup on a dyno or at the track is too fat to drive without fouling plugs. You got the additional fuel from the jets, not the PV as intended.

So from all this, I hope you have a few tips to get "race type" carbs working right on the street. The airflow rating isn't the issue most of the time, it's the application of a carb designed for one thing being used on another.

If you read the Holley catalog you will be amazed at how many variations there are of say 850's, 750's etc that are all Double Pumpers. Same with Dominators. Be very careful comparing jetting with other folks. Unless you have the same List # on the carbs, there are way too many other variables to consider. Each one uses different air bleeds, idle restrictors, jetting, PV channel restrictors, boosters, etc etc etc. The perfect jetting in one 750 may be way off for another.

Holley even markets a Dominator now with PV's, two idle circuits and annular boosters. Would work great on the street. It looks like the same setup I've assembled for mine and it works great.

Very mild 454...use a 750DP
Pretty hot one.... use a 850DP

Actually I would use a 950 HP in a heartbeat on a 500 hp 454. It would be great. Always Double pumpers. They work great.


Jim

 

Don't use a DP on the street a vacuum sec carb is always better (it only gives you as much carb as you need at lower RPMs). All of my race cars that didn't have trans brakes used vac. carbs they work better even on BBs with 5000 convertors. Holley made a Dominator with power valves 30 years ago and was a POS as a street carb the new one will be no better.

 

427Hotrod that sound exactly like 3 different articles I have read in Hot Rod and Car Craft. Simple concepts but most of us screw up on the application. NICE JOB on articulating this.