Jetting a Holley carb?

[steves66]

I'm pretty sure that my 440 stroker is running lean because my plugs are not exactly a tan color. They are between white and tan. I am also getting some stumbling from take off and some surging while cruising. My carb is a Holley 850 with mechanical secondaries. The primary jets in there now are #80 and the secondaries are #78, and pump nozzle size is .031. My question is... what size of jets should I jump up to and should I also up the pump nozzle size?

[dodge freak]

Those are good size jets allready but try 81 or 82 in front. Holley says jet up 2 sizes but it seems like 1 size does make a difference. Try that before you increase the shooter size.

The secondaries only really come in to play on WOT, back there you keep jetting up till it starts to slow down then go back down 2-3 sizes.

[steves66]

Thanks df! I have noticed that when the secondaries kick in I don't see any black smoke at all coming from the tail pipes.

[dodge freak]

If you do not have all the jets 79-84 and scine they cost $7-$8 a pair I would try 82 in front , it should help and be very close then you could try those 80 jets in back. Its better to have all the jets and try a few different combs.

[steves66]

I was looking at Summit and they have a jet kit that ranges from #64-#99 for $41. Hopefully if I just bought one pair I would get lucky and guessed right and wouldn't have to buy anymore, but the way my luck has been going I would have to buy 3 or 4 pairs!

[firefighter3931]

Steve, go ahead and buy the kit.    Jet up to 82's on the primaries and take it out for a low speed cruise and keep out of the secondaries. Have a look at the plugs for color....if they're tan/light brown then you're good on the front side. Take it back out and open it up hard for a few seconds then shut it off immediately. Look at the back plugs and jet (the secondaries) accordingly. To tune out the bog you will have to play with the squirters. If it's still stumbleing then go up to a 35 squirter. You might also need to use a 50cc accelerator pump on the front...but try the squirter first. 


Ron

[steves66]

Steve, go ahead and buy the kit.    Jet up to 82's on the primaries and take it out for a low speed cruise and keep out of the secondaries. Have a look at the plugs for color....if they're tan/light brown then you're good on the front side. Take it back out and open it up hard for a few seconds then shut it off immediately. Look at the back plugs and jet (the secondaries) accordingly. To tune out the bog you will have to play with the squirters. If it's still stumbleing then go up to a 35 squirter. You might also need to use a 50cc accelerator pump on the front...but try the squirter first. 


Ron

Will do Ron. 

[myk]

I REALLY need to get a holley...

[crzyc68]

Shouldn't the secondary jets be higher than the primaries?  On the carbs that I have had, Holley and Speed Demon, the secondaries are always bigger.  I have never tried it with the secondaries smaller than the primaries.  Does it make a noticable difference?

[dodge freak]

I believe the 850 DP has a power valve in the secondaries.

[Chryco Psycho]

the rear is still a bit larger due to less restriction with the choke , I would be 2 sizes larger in the rear from what ever you find works in the front

[steves66]

the rear is still a bit larger due to less restriction with the choke , I would be 2 sizes larger in the rear from what ever you find works in the front

That's what I was hoping to do once I got the primaries dialed in. I don't even know if I have a problem on the secondaries or not because when I step on it there is no stumbling or hesitation whatsoever.

[myk]

Whoah, the secondaries are supposed to be larger?  Is that a Holley thing, or does that apply to Carter 'carbs also?

[dodge freak]

Have you check to see what the power valve restriction holes are in the front and back ? Those can be .040 or .060 from holley - at least thats what I seen so far. If the front are .040 and the back are .060 then it is jetted richer in back allready. When a power valve is being use you can not tell how rich or lean it be just by looking at the jet size.

[firefighter3931]

the rear is still a bit larger due to less restriction with the choke , I would be 2 sizes larger in the rear from what ever you find works in the front

Generally, that is where they end up after tuning. A 2 size step from primary to secondary is where you want to be. 

Ron

[cuda66273]

Huh??

You guys have got me confused?

The Holley should have a Power Valve in the front only.
The P/V feeds fuel at about the rate of 7-8 Jets sizes.
When the manifold pressure drops 1.5 to 2" below the P/V size it opens and in theroy it will balance the carb for equal fuel from both the P and S ends of the carb.

So with these assumptions the primary shoud be jetted 6-8 sizes smaller to balance the fuel at 1/3 throttle and above or as the manifold vacuum signal drops.

There's several reasons for hesitation or tip in stumble:

1. Incorrect balance between the primary and secondary idle speed screws.
2. Not enough initial timing
3. Too much carb for the application
4. Too much manifold

Changing squirters or going to bigger pumps and or jets is a band aid for a PP tune up.

You may want to look here:
http://www.4secondsflat.com/Demon_Tuning_Guide.html

[dodge freak]

Well tell Holley that power valves should only be in the front cause on the 850 DP there is a power valve front and back. Not sure why but thats how those carbs come.

Yes if I was him I would just plug the power valve.

[firefighter3931]

Well tell Holley that power valves should only be in the front cause on the 850 DP there is a power valve front and back. Not sure why but thats how those carbs come.

Yes if I was him I would just plug the power valve.

I've seen a few 850dp carbs with a rear power valve. 


Ron

[Chryco Psycho]

most 850 DP Holleys have PV both front & rear

[cuda66273]

Then you jet them same front and back.

Road Race carbs are set up with P/V in Both

Street and Drag set-ups normally use a P/V in the front only to assist in a cleaner idle to transition.

[firefighter3931]

Then you jet them same front and back.

Road Race carbs are set up with P/V in Both

Street and Drag set-ups normally use a P/V in the front only to assist in a cleaner idle to transition.

The newer 850dper's have both front and rear power valves. The inlet restriction created by the choke horn accounts for the staggered jetting, front to back. Anybody who's set up a holley knows this to be the case. Square jetting a 4781 will leave you either fat on the primaries or lean on the secondaries. 


Ron

[dodge freak]

It depends on the power valve restriction holes what size they are Holley uses .040 and .060 holes and anybody can take a pin vise drill and open it up more. Scine that carb seem to come from the factory with 80 in front and 78 in back I think those PV restriction holes maybe .040 in front and .060 in back. You need to remove the PV and check it with some wire drills to know for sure.

What you want is to be able to use the smallest jet poss. and have the power valve kick in to give the best power at WOT. This is not so easy to do , if you make the PV restriction holes too big then you need a new metering block.

[myk]

Rather than start a new thread I figured I'd ask this here:  I've gone 1 stage richer for the primary jets and metering rods for my 'eddy.  I noticed an improvement and my plugs are still clean.  I would like to try one more size up on the primaries, but wanted to know if I had to change the metering rods to a corresponding size also, or can I just change the jets?  In the 'eddy manuals they always change jets with rods, as they have these pre-determined combinations-I guess at this point I'd be guessing what sizes to use for the metering rods, as I've already gone as rich as possible, according to the manuals...

[dodge freak]

Don't know but don't see why you just can not try and find out. I would think you can but you might find that it needs bigger jets and smaller rods than you have now.