Dominator carb tunning help . . .

2937 Views 16 Replies 8 Participants Last post by mustange70, Jun 29, 2010

mustange70

Discussion Starter · Jun 27, 2010

I know this probably ain't the best forum for this, but I need some awesome quick so i can get parts in for a race i have on thursday, that and the carb in question is on ford's big small block lol, anyway any help will be greatly appreciated:

Some background info first though, motor is in a mud bogger and its a 400 ford, big billet solid roller cam, 4v quench heads, 12.5ish:1 cr, roush b351 intake, 2.25 custom built zoomies, and all the hardware and parts to keep it together, all factory iron (heads, block, crank, rods, etc) and its putting out a conservvative 600ponies.

Anyway carb is a 750 dominator, i've installed a pair of percy's high performance adjust a jets on the carb to help with the tuning (tearing a carb apart to do jets when its caked in mud isn't the best lol) and right now the jets are set at 65 on both sides. So right now my issue is that when i get into the secondaries it stumbles out, acting like its flooding out (you can smell it go rich), now when cruising and running around it runs awesome and only stumbles only on hard accrelation IE when getting into the accrelator pumps (its worse when into the secondaries). Now i don't know the exact size of the the nozzles off the top of my head (anyone know what they would be?), but i'm going to assume they're whatever the carb came with from holley. Now we've adjusted the size of the pump shot down to minimum the linkage will let us and it's helped the problem some, but not enough. So anyway just curious if any of the holley guru's would be able to lend some advice so i get the truck running in tip top shape.

thanks muchly for any help.

1 - 17 of 17 Posts

rmcomprandy

· #2 · Jun 27, 2010

With those "Percy's" plates, you're on your own or call Percy's 'cause they are the ones who claim they will work perfectly.

I have tried them twice on a competition carb and had nothing but problems twice. Finally used one on the primaries of a street 4160, #1850 and it seemed to be OK.

bigblok86ranger

· #3 · Jun 27, 2010

Did the carb work fine before you installed the plates on the carb on your engine? If so I would think the jetting numbers are off. If the carb ran fine on your engine before the Percys plates I wouldn't start changing the other stuff on the carb I would try to get the plates to work with the way the carb was originally. If you start changing other things that worked fine before you are gonna just start compounding your tuning problems.

BOSS 429

· #4 · Jun 27, 2010

what you could be smelling is toooooo lean, 65 is too small,does it have power valves still?

what fuel?

mustange70

Discussion Starter · #5 · Jun 27, 2010

it was running rich before (exhaust had a nice hint of black in the exhaust, you could smell the fuel in the oil, jets were 70's when we took them out, i could bump them back up another couple numbers), and the problem existed before we installed the plates. Power valves are still in it and should be 6.5's. Fuel is 110oct.

fordrka1975

· #6 · Jun 27, 2010

Lean

BOSS 429 said:
what you could be smelling is toooooo lean, 65 is too small,does it have power valves still?

what fuel?

I agree too lean. 600 horspower needs more fuel than that. jets are not the only tuning required. You may have other issues you are trying to correct with jets, but in any case 65s are too small. I'd go up to about 73 in front with the power valve and about 84 in rear without. Randy.

mustange70

Discussion Starter · #7 · Jun 27, 2010

humm, i'll give it a try, but need to get the stumble figured out as well. excuse some of my carb dumbness lol, but what would the purpose of removing the power valve serve?

you can hear the stumble in this vid from my last race, little hard to see anything but you hear everything just fine:

http://www.youtube.com/watch?v=vDYEQFxunsg

bigblok86ranger

· #8 · Jun 27, 2010

The power valve adds fuel beside what the jet supplies it makes a transition at a vacuum point which is what the number stands for. The primaries usually need them to make the engine driveable and secondaries are usually good without them. Some people run no powervalves in the front when they have very high stalls or the engine just doesn't need the leaner side. I would suggest buying a holley tuning book so you understand them better.

mustange70

Discussion Starter · #9 · Jun 27, 2010

thanks, i'm aware of the function of the pv, just wasn't aware of why you would remove it, thanks.

DFI429

· #10 · Jun 27, 2010

mustange70 said:
...right now the jets are set at 65 on both sides. So right now my issue is that when i get into the secondaries it stumbles out, acting like its flooding out (you can smell it go rich)...

Another thing to keep in mind, 'Percy's' 65 probably isn't an equivalent to 'Holley's' 65... Don't assume some other company's number matches Holley's and you're in the same ballpark. You very well could be way off from where you started...

fordman59

· #11 · Jun 27, 2010

The Adjust-A-Jet is supplied with a chart that compares jet sizes to particular number of turns open, this of course is only to get the engine running, after that you no longer need the chart because you are making a rich/lean adjustment, instead of a jet change.

mustange70

Discussion Starter · #12 · Jun 27, 2010

'Percy's' 65 probably isn't an equivalent to 'Holley's' 65

yep, more or less a ballpark/reference number.

DaveMcLain

· #13 · Jun 28, 2010

The reason for removing the PV from the circuit is because sometimes the engine will make more horsepower without the PV. Having all of the fuel enter the main well at one location(jet) tends to make the emulsion leg work more effectively. This is only true when running at WOT and very often having the PV in the circuit can make drivability at part throttle a LOT better so it's a compromise.'

fordman59

· #14 · Jun 28, 2010

Might help to know what it started with.
FYI, Don't know your list # but the Holley 0-80186-1 750 Dom (not modified) should have 70 jets front and rear, power valve front and rear, .028 front and .035 rear squirters, .073 idle air bleed and .036 high speed air bleed.

mustange70

Discussion Starter · #15 · Jun 28, 2010

fordman, yeah thats what i started with: http://www.holley.com/data/TechService/Technical/Carb Numerical Listing.pdf, only difference says it has .028 squirters on the primary/secondary, and as of right now nothing has changed on the carb yet.

Having all of the fuel enter the main well at one location(jet) tends to make the emulsion leg work more effectively. This is only true when running at WOT a

Seems like this will be worth a try, as the truck spends majority of its time at WOT, but the more i dive into and taking in what you guys have said it seems that one or both of the pv's are bad from how low the jets are set (ie sucking fuel through/past the p/v). So i'm going to replace the primary and remove the secondary pv and see if theres an improvement, but i'll do that after i replace the pv and see if the problem goes away. that said is there anything extra i need to do with the removal of the pv, or is it just a yank it out and its good to go?

fordman59

· #16 · Jun 28, 2010 (Edited)

Removing the power valve and using a plug requires jetting up by the same area of the power valve channel holes. Not sure what size ya got in that 750. Maybe go up 6-8 sizes. Could be more. I would also put in a secondary squirter at 35.

mustange70

Discussion Starter · #17 · Jun 29, 2010

Thanks for a ll the help guys, its been muchly appreciated, found some good videos from holley on youtube and a few other places and i definately need to go up in squirter size, and i'm almost 100% certain now that the pv is bad and cause the overly rich condition at idle and such, also i'm going to check the vacuum as i'm also fairly certain that i need to drop the pv down as well (motor is a fairly low vacuum powerplant).

Thanks again.

1 - 17 of 17 Posts

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