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Dominator - can't even get it to idle

General engine tech -- Drag Racing to Circle Track

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Freddie
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Dominator - can't even get it to idle

Post by Freddie »

I'm at a bit of a loss.

Newly rebuilt engine.

2nd hand carb - but barely used.

632 BBC Checv. Mild cam.

QFT 1150 2 circuit dominator.

On my own, no helpers. Cant get it to run. Moment I take my foot off the pedal it stalls.

It came jetted 84 / 92
IABs 45
HSAB 33
PV 3.5 primary
PV blocked secondary

I can get it to run but stupidly rich by opening up the transfer slot (curb idle) and letting lots of air flow in, but it fouls the plugs in 5 mins and I can't handle the putrid black smoke, it's crazy rich.

What have I tried.

1st I set the transfer slot to exactly 0.020 (measured with a feeler gauge). Since I have done that I have NOT touched the curb idle

Jetted primaries to 80 and secondaries to 84
I know these should make no difference to idle.
but off idle (no fuel from the boosters, just the transition slot it's still crazy rich
I also installed 2.5 PVs primary and secondary (this is a street car so PV's will help) it also always had low Vacumm when it was running last.
FYI - old carb was a POS non adjustable. that fouled plugs all the time. That ain't going back on.

Then I changed Idle air bleeds to 55, 65 and 75. I also wound out idle mixture screws from 2 turns to 3 1/2 turns (with each of the different air bleeds).

Regardless of air bleeds and mixture screw settings, it always did exactly the same thing

1. I have to keep my foot on throttle to open up the primary blades (I'm running a progressive linkage), so no fuel coming out the boosters, but it does expose the transition slot and pulls fuel through that. It will sit nicely on 1100 rpm with my foot resting on pedal primary blades open. Note at this point it's still incredibly and crazy rich, spitting unburnt fuel out the exhaust even at just 1100 RPM. Moment I take my foot off gas it drops to 500 RPM and stalls, instantly
2. I tried removing the idle air bleeds completely so it would have gulps of air...backfired out the carby, so I know it's too much air

so my issue is I cannot even get it to idle at all, literally can't step away from the gas pedal, and still stupid rich when low rpm.

I have idle feed restrictors but I've never changed those before.

My guess is it needs more air, not more fuel.

You may be thinking float level or fuel pressure.

Fuel pressure I filmed gauge while I had my foot on gas, it's good at 6PSI
Float level, neve had car run so I can't get to it. But wit it stopped and pump on I have set it to just above bottom of sight bowl.

Thanks in advance for any advice you might be able to offer.
enigma57
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Re: Dominator - can't even get it to idle

Post by enigma57 »

The old addage...... Most carburettor problems are really ignition problems comes to mind. Without static compression ratio, cam specs and manifold vacuum at idle its really a shot in the dark, but if you are running say 10:1 static compression and are cammed accordingly, initial timing (no vac. advance connected) should be in the 22 degree range, give or take a degree or 2 either way. Racer Brown has some good reference info on this along with their cam specs......

http://4secondsflat.com/Racer_Brown_Cam_Specs.html

More initial timing will speed up engine at idle speed and it may be enough to get you where you want to be without uncovering progression holes.

Of course when adding initial timing, you may need to restrict total advance to prevent having too much timing on the other end, as well. Total timing will likely be in the 36 degree range, give or take a degree or 2 either way.

And if working with the ignition and carb doesn't do the trick, you may very well need more air as you say and that means drilling a hole in each primary throttle blade. Do this only if all else fails. Use drill size 2.4mm or 3/32" if you must do this.

Regarding your carb...... I'd suggest beginning with checking needle and seat at fuel inlet, then recheck float level. If that's not right, nothing else will get you where you need to be.

Jetting...... Holley jets these carbs out of the box with 96 mains if power valve blanked and 92 mains if running power valve. 0.35 primary and 0.37 secondary pump nozzle size in either case. Sorry...... Have no idea what size idle, intermediate or high speed air bleeds are used.

Manifold vacuum at idle will determine power valve size. Idle and other air bleed sizes are dependent on jet sizes in order to provide correct air/fuel ratio in each circuit. Increasing idle air bleed size will not make up for drilling throttle blades if needed to allow for correct throttle blade to progression hole relationship at idle (tickover) speed of say, 750 RPMs or so. That must be handled independent of air bleed sizing.

2 circuit versus 3 circuit metering...... I see you have a 2 circuit Dominator on a mildly cammed road car. I recall hearing mention of mods to make the older 2 circuit Dominators into 3 circuit metering to help with drivability. That was a long time ago and I cannot recall the details now. But I'm sure others here will be able to help if you need to explore that option further.

Hope this gives you some ideas,

Harry
Freddie
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Re: Dominator - can't even get it to idle

Post by Freddie »

Thanks Harry, yeah I actually have tried advancing the timing and retarding. As I can't get it running (even to idle) a lot of my timing activity is pure guess work. When i did get 3 mins of run time before the garage filled with black smoke it was 16 degrees. But I've retarded since then, will add a bit more in.

I'm 10.7 : 1 compression with mild cam (no specs but I know from 5,000 miles of driving on it that it's not lumpy)

Historically (old 3 circuit super rich carb) I'd get 8 In/Hg of manifold vac at idle.

I'm reluctant to drill the blades. i know that is an option. But my view is that I should be able to dial this in using the carb.

I'll try some more timing tomorrow
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Re: Dominator - can't even get it to idle

Post by mag2555 »

The idle air bleeds are not big enough for that large a cid motor!
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Re: Dominator - can't even get it to idle

Post by Coloradoracer »

mag2555 wrote: Sun May 31, 2020 5:43 am The idle air bleeds are not big enough for that large a cid motor!
Agreed!!!! Idle air bleeds should be in the 70's!!! That carb is completely out of calibration and needs to be fixed!!!

Try this as a starting point:

Close the throttle plates until you see only about .020" if transfer slot exposed on both primary and secondary
Close the idle mixture screws then reopen them 1 turn. Opening them farther will give it MORE FUEL...
Change the IAB to a 76
Change the MAB to a 26
If plugs in the float bowls, set floats with engine running so fuel just barely trickles out of hole. If sight glass, halfway up the sight glass. Again, engine MUST BE RUNNING to set float levels. With float bowls off, invert them and set floats with 7/16" drill bit...as a starting point.

What are the emulsion bleeds? How many? Locations?

How much vacuum does the engine pull at idle? The PV should be rated below it because if its not, it will be open all the time. Are the restrictors behind the PV?

This is just a starting point....Make sure inlet fuel pressure is between 5.5-7 psi. I've ran Holleys as high as 9 psi without issue but you don't need that...One more thing, when was the last time the needle valves were replaced? I installed brand new ones in mine, and they stuck, flooding my engine...I replaced them again with better quality ones and problem fixed. Don't overlook the simple stuff. Take the float bowls off, turn them upside down, the right side up again. If the needles don't drop open, replace them. Even if it "feels" like they move freely. If they stick even just a tiny bit, they won't move when fuel is flowing. Trust me, I learned the hard way on this.......
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Re: Dominator - can't even get it to idle

Post by Coloradoracer »

On drilling the throttle plates, I've found that most dominators need this. Drill a .070" hole near the idle discharge port in all four throttle plates. This helps with fuel shearing at that point as well. Mine ended up at .110" for my stuff on 1295 cfm carbs....duals at that......and you can easily plug the holes with a small rivet if needed. The location is important though. Can't just drill them anywhere.
Mark Goulette
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Re: Dominator - can't even get it to idle

Post by Coloradoracer »

enigma57 wrote: Sun May 31, 2020 3:02 am 2 circuit versus 3 circuit metering...... I see you have a 2 circuit Dominator on a mildly cammed road car. I recall hearing mention of mods to make the older 2 circuit Dominators into 3 circuit metering to help with drivability. That was a long time ago and I cannot recall the details now. But I'm sure others here will be able to help if you need to explore that option further.

Hope this gives you some ideas,

Harry
Actually the other way around Harry. Three circuit carbs have a serious issue of part throttle over rich conditions followed by over lean conditions on the top end. The intermediate circuit has it's own feed directly from the float bowl and depending on the feed tube placement, draws fuel early in the metering. This is what causes the overrich part throttle problem. The overlean top end is caused from the idle feed tube being in the main well limiting it's capacity. Even with the jets removed entirely, it's difficult to get enough fuel to feed the engine. Converting a three circuit carb to a two circuit fixes these issues, makes the carb more drivable and much easier to tune. I've done the conversion and it was night and day difference. Two circuit is what you want, not the three. Newer design metering blocks removed the idle feed tube from the main well and gave it it's own circuit, which has helped, but the placement of the intermediate was never changed. Two circuit carbs have never fed the idle circuit from the main well, so that problem doesn't exist. It's also better to relocate the idle feed restrictor to the bottom of the metering block instead of the top like Holley has it, as this cures erratic idle mixture/tuning issues.

On custom aftermarket three circuit carbs, they relocate the intermediate so it's not active as soon, and have improved metering of the transition and main circuits so the part throttle issue isn't an issue anymore. They have also removed the idle feed tube from the main well and all use custom metering blocks....
Mark Goulette
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Re: Dominator - can't even get it to idle

Post by JDR Performance »

Have you bolted another carburetor on to confirm that it's not something else?
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Re: Dominator - can't even get it to idle

Post by Firedome8 »

Is fuel issuing from the boosters at idle?
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Re: Dominator - can't even get it to idle

Post by allencr267 »

enigma57 wrote: Sun May 31, 2020 3:02 am The old addage...... Most carburettor problems are really ignition problems comes to mind.
What's the compression?
What's it sound like?
That is not gasoline spitting out.

Freddie
incredibly and crazy rich, spitting unburnt fuel out the exhaust even at just 1100 RPM.
:(

also, 'cannot even get it to idle at all, literally can't step away from the gas pedal'
Idle stop?
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Re: Dominator - can't even get it to idle

Post by Little Mouse »

Never worked on a dominator so school me. Why does a mild cam engine need a 3.5 power valve, why go to 2.5. I would think 632 cu. Engine with a mild cam would have a stong vacuum pull. I know the PV valve has nothing to do with idle.
Last edited by Little Mouse on Sun May 31, 2020 5:17 pm, edited 1 time in total.
Freddie
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Re: Dominator - can't even get it to idle

Post by Freddie »

Thanks all, will pull carb today and check many of the things mentioned.

I have had the IABs with 75's in it made no difference.

it's 10.7 : 1 compression, and even with the old 3 circuit on it it never got above 8 in / hg at idle on vac

I know that would suggest a 3.5 PV but I've thrown 2.5 in there to be sure
Freddie
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Re: Dominator - can't even get it to idle

Post by Freddie »

JDR Performance wrote: Sun May 31, 2020 9:50 am Have you bolted another carburetor on to confirm that it's not something else?
No that is on my list. My old 3 circuit has been gutted as it had a crack in the baseplate. I don't have another 4500 carb, but worst come to worst I'll buy another and sell it if I need to
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Re: Dominator - can't even get it to idle

Post by JoePorting »

Call up the person who had the carb before and ask him if he changed anything. Then reverse whatever he did.
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Re: Dominator - can't even get it to idle

Post by 6.50camaro »

Check to make sure the PV and secondary PV plug gaskets are seating prperly .I had a carb that wouldn't idle good dropping fuel even after shut off . Found the wrong gaskets on PV and Plug .Just a thought . Dan
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