Chevrolet12

I have an 850 holley 4781 carb on my 385 stroker trying to get it tuned correctly and I keep hearing that there is something wrong because my jet sizes are real low. I have 72 in the primary and 70 in the secondary. On my wideband it shows in the 14s part throttle and 12.8-13.2 wide open. It will fire right up when you bump the key when you first start it. If it's hot and it sits for a few minutes you have to crank about 6 or 7 times before it hits. Should be able to bump the key and fire up. Timing is 38 total. Primary idle mixture screws have very little affect secondary idle mixture screws have more. Everyone I have talked to say I should be more in the 78-80 jets range.. Any input?

Tom400CFI

IIWM, I'd be starting here. If the primary idle screws are affecting the idle....something is wrong w/the circuit. That isn't necessarily related to the main jet sizing, but it is a more tangible "problem".

I had a main metering block in my boat (Holley 4160) get "f'ed up" -it would lose 4 cylinders at idle, and was totally unresponsive on both sides, to idle mix screw adjustments. I tried every method of cleaning all the passages in the metering block...I even tried boiling the thing! Nothing produced more than temporary improvements. Finally bought a Quick Fuel Products billet mitering block, and the problem vanished. Something had buggered one of the passages or orifices in my original metering block, and I was un able to get it it.

I'd start by diagnosing the idle, it may lead to a new metering block...which may change your main jetting requirements.

cv67

Need more info on your engine ie head cam intake trans gear converter etc etc
Are your primary blades (idle screw) cranked open more than need be?
Those jet sizes sound like something out of a stock 4779 750 dp different models/brands will probably always be a little different (mine were 79/89)
im no expert but with carbs shoot for a general # is ok but pay attention to the engine...give it what it wants

Chevrolet12

Ported 200cc heads 64 cc combustion chamber weiand high rise intake cam is 248 254 @.050 580 562 lift th400 manual valve body 3500 stall msd 6al ignition magnetic pickup distributor 65k volt coil and 11:1 compression.

BOOT77

Every carb even the same size/brand is a little diff from my experience. Are you using a power valve in the rear? Usually the jet spread is so high because there is no power valve in the rear and you need up to ten sizes to make up for not having one. I run a tight jet spread on my homebuilt 750 annular for my c3, but I also set it up for a lot of power valve shot(front only). I set it up that way for a lean cruise and still have good power. Most don't use a rear pv because of how it can stave under hard accel like in drag racing.

These guys can answer all your carb needs and then some http://racingfuelsystems.myfunforum....forum.php?f=25

Chevrolet12

No rear power valve. And if I were to jet up even back to stock in the rear it will be so rich it won't pull itself. Spit and sputter black smoke. Way rich. I had to jet it down this low to get the wideband reading where it should be but I still have the idle problem and starting problem.. I was thinking about billet metering blocks and seeing if they would fix the problem.. Idk

BOOT77

Sounds like maybe your front power valve might be blown. What's your idle vac and what front pv? Is your front transition slot squared? Is your rear open any? Either past square and you'll get a rich idle. How old is the carb? Is it clean(as mentioned earlier)?

Lots of factors

Chevrolet12

Power valve is 3.5 vacuum at idle is 7. Checked the power valve not blown. The carburetor has a blowout protection. The secondary had to be opened a bit for it to idle.. Otherwise the primary would be open too much. How do I measure the transfer slot to make sure it's not open too far? And all that would make sense for the idle problem but it still don't add up for so low jet sizes..

rklessdriver

If the idle air screws make little to no difference at idle that means the transition slot is over exposed because you have the throttle blades open too far.

The transition slot should show "square" with the throttle blade closed (about .030" below the blade).... if its a vertical rectangle your idling partly on the transition circuit - which is why the primary idle air screw has little effect.

You can correct this by drilling the throttle blades or opening the rear throttle blades some or a combo of both.... on my race car I have .100 holes drilled in all 4 blades.

The main jet only works at or near WOT so if the AFR is inline at WOT and yours seems to be fine then the jet sizes are what they are....

I will say that the leanish part throttle and smaller than normal jet sizes to get acceptable AFR at WOT sound like someone screwed up and enlarged the HS air bleeds..... That particular Holley doesn't have adjustable air bleeds so there is not much else you can do except stuff a piece of wire in the HS air bleeds to make them smaller.... after that you'll have to drill and tap the air bleeds for the screw in type.
Will

BOOT77

What are your floats set at, rear esp? Are your accelerator arms set proper and not too tight(always open and not seated)? Your getting extra fuel from one or more places and trying to correct it with jet. PV can be tricky sometimes and seem good, I keep a few to swap just to be sure. I chased a rich problem onetime that was just the pv hitting the screw boss and not letting the m-block tighten down enough, any hiccup and it'd unseat the m-gasket and leak internally. Some brand pv are thicker and so are some pv gaskets, so I always clearance that boss on all my carbs now.

Did you use the correct m-block gasket, a friend of mine used the one with the notch out that the pv passage uses(it came in his rebuilt kit and those are made to work on a wide range of carbs). Older carbs used a tube with O-rings for the pv from m-block to mainbody. If you use the notch gasket it'll leak and always be really rich.

edit: and square is just what it means, you can google a pic.

Chevrolet12

Floats are at the bottom of the sight plug hole. 7psi fuel pressure. I was wrong in the jet sizes.. I forgot what I put in. They are 70 primary and 73 secondary. I just pulled it apart to see what gaskets I had between the block and the main body. They are the gaskets like in the picture. I'm just thinking about going with new billet metering blocks and see what happens. See if I can adjust the idle feed restrictors to help.

Chevrolet12

Also I flipped over the carb and the secondary transfer slot is square and the primary isn't showing.

BOOT77

That's the correct gasket, the notch would be where the little round center hole is.

7psi is highish. I prefer 4.5-5.5, pressure is not volume. Also larger N&S size will be affected more by higher psi I think. The N&S o-ring can be damaged by the threads if you don't lube it on install.

Tune-able metering blocks are nice anyways. I use transition slot restrictors for the mainbody, drill & tap then use set screws that you drill to size you need and I'll mod the billet m-blocks idle circut. But I also prefer to use a m-body with tune-able hsab/lsab.

You can mod the heck out of your carb or sell it and start with something more adjustable.

edit: Check the flatness of your mainbody and metering block.

edit2: Also if your using a plug for the 2ndary m-block pv make sure it doesn't contact the screw boss.

Chevrolet12

Checked the flatness.. All good. I think I will try a set of the qft billet metering blocks and a different air cleaner and drop my fuel pressure a little and see what that does.. I will let you know what happens when I do this..

BOOT77

I like qft & BLP has some nice ones that others will re-label and sell with their custom carbs. Gonna use some BLP in a new carb I'm putting together this winter.

Chevrolet12

Where do you get the blp ones? I'd like to get a set that I won't have to do alot to. The qft are supposed to be pre calibrated so they should be a bolt on thing.. Are the blp like that?

Chevrolet12

What do you mean by contact the screw boss?

BOOT77

BLP is another carb company and they have their own site http://blp.com/

Your prob gonna have to tune w/e blocks you get and I suggest you check out that forum I linked, a few on there would help you with a base tune.

BOOT77

The throttle plate bolts to the mainbody and there is a screw boss in the pv void on the mainbody, that the pv head or plug goes into when you install the metering block.

cv67

aed will custom make you one to yours, real reasonable, after the point service is over the top. Talk to Jeff
Without adjustable IAB its going to be tough to get it right went through that with my last one.