Hi to everyone in the US from Guernsey in the Channel Islands!

I'm building a stock 502 with a 250 blower on top, currently running a single Holley 1000cfm carb. Boost will be kept very low (3-5psi), it's just to give the motor a little extra kick!

I'm told that a single carb will lean the rear cylinders out and risk killing the motor, that I'd be better off with twin 750's or similar. Is this the case, even at low boost? Do I need to swap to twin carbs?

If so, where can I get marine blower carbs from at a good price? I can find marine carbs or blower carbs, but not the two combined!

I'm on a tiny island with no one who really knows how to set up the carbs other than me, so the easier the setup the better!

All the best and happy boating/fixing,

James

1 carb is fine.. especially if you are not changing anything on the engine except the induction.

Otherwise, keep this thread alive.

Thanks for the fast reply!

When you say not changing anything other than induction, I take it you mean that I'm not doing internals etc?

So what makes it different when the internals etc are changed about? Is it just the higher hp and therefore airflow figures starting to change matters?

James

Do you have a goal in mind, or do you just want to put that blower and carb on top?

You are really getting into a question that requires a lot of experience to answer.

Oh dear...

I'm not planning to do any more work on the engine, the boat is used for heavy offshore racing and is currently only running a bravo drive, so won't take much more power than stock.

My question is this, I'm putting the 250 blower on my stock 502 and just want to know if the single carb will definately be ok or, as I was told locally, I needed to change to twin 750's.

I'm the inquisative type, so I was just further wondering what makes a motor need twin carbs.

Thanks again for the help,

James

twin carbs do a better job of fuel distribution so you get more even afr & egt's. single will work just not as even with fuel distribution.

a single carb is fine, i did it for couple year's, and i ran it hard. you jet it so the outside piston's are good (1,2,7,8) then the inside piston's will be a little rich, but not enough to hurt you. The reason i went to two carb's was when i built more power and new i was going to be close to the motor's limit's, so i needed all the piston's exsact

I second the single carb for you...just make sure to get jetting dialed in and you will be fine...stock merc 525sc used a single 1050...now do not be alarmed if you need to stagger jet the car to get a better balance...Rob

I have heard/read mounting the single dominator sideways helps. When you open your throttle blades, you can see that at say 1/2-3/4 throttle, all 4 blades angle towards the center of the carb. Someone on the board here tried mounting it sideways, and it dropped the rear cylinder temps a couple hundred degrees.

One thing different about the B&M 250 compared to the weiand 177 that merc used, it the center of the carb on the 250 is farther away from the rear cylinders. The carb is closer to being centered over the #1 and #2 cylinders.

I sold my 177's and plan to run the 250's this year on my 454's. Also planning to run dumps from the intake manifold where the rear water jackets are on the heads, to help cool the rear cylinders and prevent any steam pockets. I'll probably experiment with the sideways mounting on the dyno. Goal here is to try and even out the fuel distribution with the single carb, so that the center cylinders wont have to be so rich. I guess it cant hurt to try and see what happens.

Ok, so it seems that everyone was right really then. It is better to have twins, but not necessary. Makes sense.

I guess I'll stick with the single then, I can have a different adapter plate made up to mount it sideways easily enough, that sounds like a plan. Failing that, if a well priced pair turn up then I'll take that route becuse I can.

Cheers, James