I recently changed the camshaft and the carb on my engine (496 low deck, 12-1 compression, indy R2 camshaft)

It is hard to start, you have to hold the throttle wide open, as well as it won't idle at anything under 2000 or it trys to bog down and eventually dies. It absolutely wont idle in gear.

It has an out of the box 1050 ultra dominator with 3 circuit metering.
I believe it has 5.5 pv front and back with 78 primary jets and 84 secondarys. Fuel pressure is 6.5-7 psi.

It rev very crisp after idle fwiw.

Any help would be appreciated as this is my first experience with a 3 circuit style carb.

First thing to check is the 4 corner idle screws. Check them to make sure they are not turned the entire way in. You want them turned out about 1 1/2 turns.

How far open are your throttle blades?

Also did the carb come with a sheet with the air bleeds that in the carb? If not you can see a number stamped in them on top of the carb, what sizes are they?

I would also pull both power valaves and plug them. Jet the carb around 88 squared up. Once you get a good tune with them blocked you can add the front valve again for cruise and jet down.

Lastly check to make sure there is no vacuum leak around the intake or carb.

The idle mixture screws are all 1 1/2 turns now. So I will proceed to check for vacuum leaks and block those power valves off.

Thanks

Surprised Dom hasn't jumped in on this thread, yet.

Dom doesn't care to make 3 circuit carbs work and convert em all to 2 with excellent results including 2 8 second cars. Aj will hook ya up as he's helped me bunches.

Are the gains from switching from a 2 circuit to a 3 circuit strictly ease of tuning?

If you think of it, the next time you have the metering blocks off,would you please see if the idle feed supply tube is in the main well? Thanks!

It's deeper than that but yes that's one of the reasons for me personally.................And what Mark said.

If the carb worked fine before you made the other changes to the engine then I would suspect the other changes. Perhaps the cam was installed one tooth retarded or something like that?

An out of the box Dominator should start and idle okay. The 3 circuit carbs tend to be dead rich off-idle but they should start and run okay.

it just aids the guys who can't tune a 3 circuit..

I am thinking that there is too little vacuum at idle and it is allowing the PV to open while idling, but I could be wrong i'm gonna switch to the block offs and check how that works along with going over the whole engine for vacuum leaks.

Also this carb is brand new out of the box, I put it on when I switched the cam.

Too funny right there and thankxx 4 your support........

hell I read it backwards anyway. lol

Like was said already.. plug the PV till you get it running
right then put in the PV and reduce the pri jetting 7 sizes..
make sure your timing is set first

don't you mean from a 3 to a 2?

It doesn't really matter if the PV is open at idle since very little fuel is going to flow thru the PVCR channel at idle. If the PV is torn then the fuel will leak in the intake and that will cause a problem.

You can block off the PV's just to eliminate a variable, but an open PV shouldn't cause a rough idle.

If the vacuum is really low due to the new cam then the problem usually is getting enough air into the engine. The throttle shafts need to be in the correct location. If the engine needs more air at idle then you need to provide idle bypass air. You do not open up the throttle since that messes up the location of the throttle blades.

Dominators are easy to provide idle bypass air. You can drill and tap holes on the top of the carb that will let air into the manifold via some small jets.

You mean drill it where the air cleaner stud is and put main jets in it?

The new gen 3 Dominators have something similar

Has nothing to do with being able to tune it. Has to do with a couple different issues with the factory style 3 circuit metering blocks. One is the inability to lower the idle feed restriction. Early 3 circuit Dominators had the idle feed tube pinched at the bottom, this was your idle feed restrictor. However it made tuning it impossible without pulling the tubes. Later Dominators use an open tube and made the restriction at the top, above fuel level. This is not as stable for fuel delivery as discussed in the link below.

Next is the obstruction the idle feed tube becomes when either higher HP or combination of higher HP and oxygenated fuels are used, you can reach a point where adding more main jet has no effect. Some companies throw a bunch of intermediate fuel to compensate, that is a bad way to do it. You get more liquid fuel, and it will usually negatively impact distribution. You can open up the main wells, but that means pulling the tubes out.

And last is the need to even have intermediate fuel. If you tune it correctly the need for intermediate fuel goes away for most applications. It's like having dirt hauled in to fill a hole you don't have. You end up with more fuel than you need and that can foul plugs.

I use two types of metering blocks, BLP 5124 2 circuit or 5120 3 circuit wide body blocks for gas. I will almost always plug the intermediate circuit on the wide body blocks, I use them sometimes because I can control the size of the kill bleed. For methanol I use a methanol version of the wide body block, and will use a little from the intermediate circuit only to supplement the mains if needed.

http://racingfuelsystems.myfunforum.org/viewtopic.php?f=25&t=18

I just bought my first 3-circuit 1250 G3 and this thread is not helping any...Luckily I called someone that races a similar combo and got a baseline.