ideas on carb size for my 502 ???
 

Hey geys ive got a 26' profile with a 502 that the last owner says is running over 600hp. ive been into it far enough to see it has a solid lifter (noisy) roller cam, blue cast alum. roller rockers, and full 4 into 4 marine headers. i have no idea on any specs but it has a beautiful loap on idle so it must be a pretty big stick!!! with the 1050 dominator it would run mid 80s at about 5300-5400 rpm. but that carb was not user friendly and even after a full rebuild made the back of the boat black in one outing!! so i tryed a 850 double pumper with 84 jets at all 4 corners and its a new boat. i like every thing about it but the fact that ive lost 4 mph on top and 400 rpm. does the 502 at 5300 need more cfm or do i start playing with jeting?? the back of the boat still gets a little black after time but that could still be rinsing out of the mega tips and what hopped up motor doesnt make a litte carbon??

That should be plenty of carb. I would start looking at jetting.

Did you switch intake manifolds to accomodate the new 850 also?????

If it's putting soot on the transom at idle that can be adjusted out with the idle air screws. If it's making the transom black at cruise speeds it's waaaayyyy too rich or you have another problem (blown power valve, high float level etc). You need to make a good hard WOT run and take a plug reading.

i did swap manifolds also, as far as the soot it seems to be after a hard run. thats what makes me think im still rinsing out the mufflers?? i see in here that a guy was running 80s up front and more like 90-92s out back, may be i'm starving it on top end. i'll have to read the plugs like you say. by the way whats everybodys favorite plug for this set up, i don't trust the last guy on anything any more!!

Those jets are smaller than the stock secondaries on a carbed HP 500.

Are you running powervalves in both the primaries and secondaries. When a powervalve opens, it is like opening another a jet and adds the equivalent of 6-10 jet sizes. The HP 500 carbs don't run a secondary PV so the secondary jets are much larger than the primaries.

i dont think there is a power valve on the back?? i'm pullin the motor today to do the bottom end i'll check:D

If there isn't one in the secondaries, you need change jetting to about 80's in the front and 90's rear and dial it in from there. Take plug readings at max cruise speed, just before the secondaries open, to adjust primariy jets and at wot for the secondaries.

Get this, the thing has a power valve in the front and not in the back ?? with 84s at all corners!! did he maybe get the fr. and back blocks turned around, the only dif. i see is a plugged vac. port on the frt m. block?? with a big bump stick i'm probably not running much vac. am i better off to get rid of the power valve and just jet the thing??

The jets on an HP 500 are:

Primaries 75 & 80
Secondaries 89 & 93
No power vavle in the secondaries. Holley #9022.

http://www.mortec.com/carbs.htm

cuda do you mean that the carb has all 4 dif. jets in it or are you giving me a range of sizes. it looks like i should try 80s up front with the power valve and jump up to about 90s in the rear with out the valve. but won't the heavely modified engine (650hp) have much dif. needs than stock???

Yes, the jets are staggared like that. I think it has to do with the length of the intake runners on the intakes (Darts).

That is what I have learned. It is next to impossible to perfectly jet a carb without many hours of dyno time.

Just run it fat and put a lot of wax on the back of the boat. It is better to run them fat, than lean.

I would leave the 84's up front for now and put some 92's in the rear.

You do not need more jetting because its got more HP than 500. Its more based on the cubic inches and fuel mixture. More HP will suck more air and in turn, more fuel at the same time.

great stuff guys keep it coming, my intake is a high rise single plane ,victor jr. i think?? i just ordered a stub stack to gain a little cfm, what do ya think of those?? THANK YOU ALL VERY MUCH!!!