Hi guys,

Still trying to decide what carb to buy for my '69. Just as a refresher, it a 454HO w/a 750 DP. I was pulling about 4-5" of vaccum at WOT (Snap-on guage hooked up to manifold vac.) It was suggested here that the carb is too small.

I like the mechanical secondaries. I'm considering the following:

800 DP
850 DP
BG 850
BG 850 annular

Will an 800 be enough to lower the vaccum to 1-2"?

In the future, I'd like to get a lumpier cam in there, so that may or may not make a diff.

Also, if the 850 is kinda too big, will the annular help?

Thanks

At WOT your vacuum should be ZERO.

Either you dont have full throttle, or your gauge, she's a no good.

Hello,

Interesting amount of vacumn at wide open throttle. You ask about 800 lowering vacumn 1 - 2 inches. Is your goal to lower WOT vacumn 1 - 2 inches? Have not had experience with working on WOT vacumn numbers.

Annular boosters in a carb are designed to react to very low vacumn signal from the engine. A relatively normal 454 like yours would probably start drawing fuel from the annular boosters like crazy, burning a ton of fuel, making your engine run rich, and creating lots of black smoke behind your car.

I have had an 850 Hollley double pumper on my 454 for many years. Nothing fancy. Just a plain old 850, down leg boosters, primary idle adjusters only. No holes drilled in butterflies or special tricks. My car is stick also. Cam is .553/.571 236/246 @ .050. Always ran well with the 850 with about 11 inches at idle (right now car is supercharged, so I am producing boost at WOT and not vacumn, however the 850 was great when the 454 was not supercharged).

I have not touched or know of friends with Barry Grant / Demons so I cannot say good or bad.

I also had a 750 on my car for a bit. It was fine around town. It did not seem that small or far off from being a good choice.

Good luck with your car, Joe

I ran annular booster for a couple years. No overly rich problem. In fact it ran very well. Correct jetting should fix any mixture problems. When I had the car on the street I found that enlarging the PVCR and using smaller jets worked best. This way it has proper WOT mixture and decently lean cruise mixture. They just aren't the right setup for drag racing. For a street engine they give almost EFI response.

At zero vacuum the carb and engine wouldn't work. Ideally, about .5-.75" is about right at WOT.

Hey,

Check my post,(Proper Carburator Recommendation)...

Has 22 posts, It may help. There's some replys that comment on bigger is not better.. Just my .02 cents...

SS454JC,

Does it run good w/the 850 (good response, etc)?

My car runs good w/the 750, its just some carb shop "modified" it. So now it doesn't idle right no matter what you do. I called them up and asked what was their "modifications", they said they grind this, drill that, polish this.

I think BG Tech recommened the 850 annular. I get about 15" to 16" at idle.

Vince,have confirmed that you have WOT?
I'll have to try a vacuum gauge on mine next time out of curiousity.I have a 750 double pump and a 3310 I could try.

Just a refresher, I had my wife floor it and I confirmed I'm getting WOT. Guage is a snap-on tee'd into the vac. advance line. Tests were done in 1st, 2nd, 3rd gear WOT. Idle vaccum is ~15+", cruise/moderate throttle ~10", WOT 4-5". Engine is bone-stock 454HO (211/230* .510/.540 @.050).

I'm actually looking for a very smooth idle (I should be able to get that w/15"+ at idle). I tried a 1850 carb from a friend and this thing idled perfect w/almost no tickering, so this 750dp is suspect.

Chris, you may need even more carb than me!

Hi Vince,

Yes, 850 has good throttle response and idles well. My cam is a little larger than yours (not much) so the engine is choppy at idle due to cam.

My engine has rectangular port heads and intake manifold, 30 lbs flywheel plus heavy clutch setup..........so throttle response was a tad lazy compared to an oval port motor with lighter flywheel / converter. I am describing naturally aspirated, non-supercharged throttle response.

When I had oval port heads and intake on the car, and a cam just like yours, I had a 750 model 3310 vacumn secondary on it. Ran very very well. Fun street car.

When I bumped up the cam and other parts years ago I installed a used 850. I rebuilt it with a kit from Holley. Installed 6.5 power valves, 80 jets front and rear. I did tweak accelerator pump cams and squirters. Car liked more pump shot so I installed a 50cc pump on the primary side with a 35 squirter.

This is the difference between night and day on my carb.........
There is a screw stop for the rear butterflies that is screwed in from the bottom of the base plate. I removed this screw and reinstalled it from the top of the baseplate so the rear butterfly stop position could be adjusted without taking the carb off again. I tweaked the rear butterflies to be open a little more and more at idle until I got idle very nice........in addition to tweaking primary idle adjustment and air/fuel mixture screws. By tweaking the air fuel mixture screws / primary and secondary butterfly positions......I ended up with very crisp throttle response.

It takes a little time to tweak these screws and adjustments. And if I opened the rear butterflies too far at idle my car would spit, cough and stall......requiring me to feather the gas pedal just to keep the car running at low RPM. You literally would think it was a different carb by how bad it would run just by being a little out of adjustment. But by getting the adjustment correct, the carb is perfect and the car runs excellent. The adjustments between awful and running great are very close.

Important....make sure you have plenty of paper towels and a safe place to throw them away. Every time your wife or someone mashes the pedal, or every time you have to get to the rear butterfly screw, you must move the throttle which dumps fuel from the squirters. Place paper towels in the primary and secondary areas to catch the fuel the squirters spray out BEFORE anyone moves the throttle so the squirters do not flood the engine and / or get fuel into your oil. Tweaking the carb takes time and sometimes much movement of the throttle is required. After a 1/2 hour of carb tuning, you could have tons of squirter fuel in your engine if you do not catch it with paper towels.

Do you have a Holley book? Very helpful. Describes the primary and secondary butterflies should expose about .040 inch of the "tranfer slots" which dump fuel at idle. Any more than .040 exposed and you are beyond the carbs' idle parameters making the air / fuel idle screws ineffective / useless at idle. I am doing this from memory and will look in the book tonight for you. Bottom line is once you adjust primary and secondary butterflies to their stock / original positions.........you can install the carb and see what you have, then start your tweaking. You might be pleasantly surprised that your butterflies are just too far out of adjustment right now for the car to idle properly. I have been there and thought there was no hope.

The grinding / drilling by the carb shop you mention might be tough to remedy to get the carb back to original status. If the modifications were simply "bolt on" items you could easily get your carb working well. With your stock 454 and cam the 750 is not bad for crisp response and fun on the street.

Hope this helps a little. Catch you later, Joe

Hey there,

Found the Holley books. I can scan the pages that describe the idle tranfers slots & butterfly position, then email them to you if you like.

Email me if you would like copies.

.......Joe.

Hi Joe,

Thanks for the post(s).

Just for the record this motor has rectangular heads/intake, roller cam, and a manual trans.

I do have a Holley book by Mike Ulrich. I tried everything I could w/this carb (rebuild, float adjust, powervalve replace, set secondary throttle plates so that transfer slots were "squared", and tinkered w/the mixture screws extensively). Will not idle below 1000 cleanly. I put this old 1850 on there and whamo! Ran real nice and I didn't even touch it. I'm wondering if "modifications" to my carb consisted of internal circuit modifing. This is why I'm reluctant to invest anymore on this carb (who knows what was done).

Hey there,

That's a shame. Wonder if idle air bleeds have been reduced in size causing rich idle???? Do metering blocks match the main body? Is the base plate correct for 750? Usually the metering blocks have numbers on them so you can tell what main body they go with. If the car runs well except for idle........I'd guess the idle circuit has been modified in either the main body or in the metering blocks.

Glad to hear other carb runs well and the motor is healthy.

Hope you find the gremlins.

Good luck, Joe