V.I. Outlaw

I need some help figuring out my afr. Its lean
4100 rpm 11.7
5400 rpm 13.1
6000 rpm 13.5

540 with B/M 250
Dominator with 94 jets front and rear
1.5 power valves front and rear
Should i have boost ref line to both metering blocks?
Motor made good power 605@ 4100 - 779@6000
3 circuit carb
I have spent 3 hours reading old posts about this and this is the third time trying to post this question. the more detailed versions dont seem to want to post for some reason.
Could use some help from the knowledgeable OSO readers

Thanks
James

mcollinstn

Item 1: That's a small blower for a 540. You won't be able to efficiently run much boost out of it without whipping up too much heat.
Item 2: You're rich at 4100. You aren't overly lean at wide open, but I don't like the way it's leaning out with additional throttle. You're either jetted a little lean on the top end, OR you're running out of fuel pressure. You need to measure your fuel pressure AT the carb line AT wide open throttle and make sure you don't drop below 5psi.

Item 3: Your power valve sizes affect your fuel mix with varying vacuum to the carb. Right now, your carb doesn't "see" manifold vacuum, it only sees constant blower vacuum. Big difference in the way you size power valves for a blower WITH and WITHOUT indexing them to vacuum. Many guys don't worry about it and run PV's that give them a reasonable cruise mix. Pretty much any size PV will open (richen) when the throttles are open regardless of whether you are boost referenced or not. So your wide open leaning is not likely to be a PV issue. Part throttle mix is much more likely to be affected by PV sizes.

Stoichiometric (correct) AFR for cruise is 14.7
Best torque is around 13
Best hp (blown) is around 12.5

Roots blowers, though, like a little extra fuel, since it seals up the rotors and gives you a touch more boost (it's a cheat of sorts).

Some setups even like running down into the 11's, but that's a little overly rich unless you're talking about a hot humid summer day (which is unavoidable with a carb).

You're right, though, you are making good power if that's your real numbers.
I'd say your blowers are at their max and don't need to be pushed any faster.

As far as playing with jets, make sure FIRST you have 5 to 7 psi AT WOT and that it "holds" there and doesn't start creeping down after a few seconds.
If you have good pressure, then fatten up the secondary jets. Don't change the primaries since your 4100 number is already plenty rich.

MC

V.I. Outlaw

Thanks for the heads up. I just went and pulled the carb apart to verify whats been done to it. I found out that the secondary PV was open all the time as there wasnt a boost ref line hooked to it. I ran the boat for two seasons with both power valves not referenced and thus open all the time. Now I have the primary referenced and the motor is surgeing and generally running rough at 3000 rpm and above. I am going to the part stone to get fittings to ref the back PV and see if that helps the idle as well. As far as the PV number, should I take vacuume readings at different rpm levels or is vacuume at idle good enough.

V.I. Outlaw

Also, The vacuume ports in the carb base have been plugged so the only way the power valves will close is with boost reference line to intake. I changed plugs last year because they were fouling at idle. Will I need to change them back to a little colder, I guess i just need to be concerned with how they are at WOT. I will pull the plugs after I make a run. I s there any particular way i should adjust the secondary stop screw for idle?

Thanks
James

V.I. Outlaw

forgot to say those AFR numbers were on the dyno so pretty sure there wasnt a fuel starvation probrem (they have dynoed1300hp + motors).

GPM

Sounds like you need an A/F ratio gauge in the boat so you know what the motor is doing and where to tune.

MILD THUNDER

iTrader: (3)

Im dizzy from trying to figure out your power valve situation.

I have ran the setup you have. I ran 6.5PV's front and rear, (as did merc on the 525SC dominator), with #100 jets front and rear. The setup ran great. My current blower engines have twin 850's. I am running 8.5PV's in the primarys, non referenced.

Are you sure you have "1.5" PV's, because I don't think Holley even makes a "1.5''. Are you sure they aren't 10.5?

The 250 blower is definitely small for a 540, especially with a single 1050. I wouldn't run AFR"s leaner than 11.5 at wot, I'd probably fatten it up a little than that, with that small blower, single carb setup. The fuel distribution is really bad with that setup. Going to 12.0 or leaner will melt a piston on a setup like that, and you might gain 10hp before it melts. Not worth it.

Not trying to burst your bubble, but the best thing you can do for that engine, if you must run the 250 blower, is switch to a pair of 750 quick fuel blower carbs, and ditch the 3 circuit dominator.

badluck

Just an idea, set up the carb like 525SC. 6.5 PW (not boost refered), stock jets are 99 you would need to go up in jet size though.

badluck

MILD THUNDER, you beat me to it..

V.I. Outlaw

Good info!! thanks. I pulled the carb apart today and it does have 1.5 power valves. really really light spring. I know... I went to parts store and holley doesent list power valves that low, 2.5 was lowest i saw. Motor was ran the way it is for 4 years without failure. And I know it was "cained on" before I bought it. The AFR numbers were on the dyno, I was told by a friend that they will be fatter in the boat as the headers are wet and smaller plus the flame arrestor as well. Any thoughts? Back to my other question, what rpm should I be taking my vacume reading at, Idle or up around cruise rpm. the power valves in it now will not open until the motor almost starts making boost. I thought they should open a little sooner. I also jetted up the secondaries to 97.