Lesson on Pumper Carbs needed

[DIAMOND]

Hello All,

I have a 91' Mariner 200 2.5 fishin motor that has the usual mods......33cc heads, relieved exhaust, rod slots, Boyesen reeds, balanced piston assemblies, carter fuel pump (5-6PSI) and drystacked. It seems to be slugish out of the hole, kind of like it is rich and loaded up. I have removed the richner air valve and its hose that is locate on the head. I also pulled the fittings out of the carbs, I can't remember why, just a stupid late Friday night thing. I plan on drilling and tapping those holes for a custom jetting option on another late Friday night. The motor is slow out of the hole and then accelerates strong from the 2,500 RPM to 5,000 and then falls flat at 6,000. It's on a 1,600 lb bass boat turning a 25" Tempest prop. I sure could use some help with setting these carbs up to run.

Thanks! DIAMOND

[Raceman]

How do you know the fuel pressure is 5 to 6 PSI? If it's 6 it's getting on the high side for Merc carbs.

[DIAMOND]

Thanks Raceman,

I have a fuel pressure gauge mounted right next to the carbs and it reads 5-6 psi. I was wondering about the pressure myself. The carbs look soaked with fuel when you pull the cowling off. Perhaps I need a fuel pressure regulator and start reducing the fuel pressure? Also more info, the idle air jet is a .040, the back draft jet is .082 and the progression or "off Idle" jets are .034's. Mains 1-4 are .078's and 5-6 are .082's. The idle screws are 2 turns from being seated. Any less and the motor starts to cough when it's cold.

[Raceman]

It was a problem we fought with the early V6's before Merc came with some better needles and seats, even with the stock pump back then. When somebody's got the symptoms you described it's one of the first places I look. It's not always the cause, but one of the easiest to diagnose and fix. I got to the point that I was regulating all my carb motors with electric pumps down to 4 lbs. The carbs need volume, not pressure. I don't know anything about the Carter pump you have, but would think that it would still supply adequate volume for that motor even if regulated.

[DIAMOND]

Thanks,

I will look into a fuel pressure regulator and start there.

[all8up]

sounds like the main jets are WAY too big.. do you know what the stock jet is for that carb? i would find out for sure. most of the 2.5 carbs ran 66's or 68's. Kerry

[DIAMOND]

ALL8UP,


After I had the mods done to the motor I installed an EGT gauge, senders in the top exhaust adaptor plate. I was running .066 mains when stock and went to .072 after the mods. The EGT was reading 1,300 deg almost pegging the gauge so I jumped up to the .078's to get a cooler reading of around 1,150 deg. Do you think I might have been fooled by reading flame fronts from the exhaust ports and now I'm running too rich? I'm going to install a pressure regulator and then start jetting down by reading the #5 piston top.

Thanks for your help! DIAMOND

[all8up]

Yes, I think you're WAY fat, and reading flame front. Is the intake cross drilled? If so you prob need 66's or 68's.. if it is not, you prob need smaller yet, like 60- 64's . that is why you have no heat, the motor is telling you something. Chuck Goodman taught me a long time ago to let the motor tell you what it needs, not the other way around. I always look at how hard i am working the motor by how hot it is getting on the water temp. I am reading actual water, and usually see around 135 on a hard pass. prob more if you are reading head temp. Kerry

[DIAMOND]

Thanks Kerry!


The intake has the passages that run thru the rib between the carbs if that's what you mean by crossed drilled. I bought the motor new, stock, with the .066 mains in it so I will assume I will end up closer to .068-.070 area and put those carbs on a diet. Looks like I will kill to birds with one stone. My other post was about running too cold (water temp). One more question, please........, where do I mount the pressure regulator? Right after the fuel pump or out at the carbs. Seems to me it should be as close as possible to the carbs with the bypass hose running back to the tank. A loop circuit instead of a loop with a "T" branch going out to the carbs.

Thanks alot for your help, DIAMOND

[XTB21]

If thats the case i'm in trouble because mine never moves past 100 degrees WOT or not.

Whats the operating Temp for a 245 anyway?

[all8up]

If the intake is cross drilled, you will have a perfect 1/2 inch hole right in the middle of each intake runner. you can see the hole if you open up the butterflies and look down the front of the carbs. the 245 hp factory HP 2.5 had a cross drilled intake, and the early ones came out with 72 main jets. then Mercury came out with a bulletin saying to change to 68's. that was the nikasil race engine. these motors like to run on the warm side. 1200+ on the EGT'S Kerry

[all8up]

Oh, and i always mount my fuel regulator right after the pump. You can run it up to the carbs, but an even better idea would be a fuel guage up on the dash, coming out of the same loop as the carbs. then always maintain at least 4 psi fuel, and 11 psi water, and you got most problems licked! Kerry

as far as temp guages go, the only one I trust is the digitron unit with a probe like an EGT probe that goes right into the head, right behind the thermostat location. I pull it up just a little so i am reading actual water temp. I think it is more accurate than most of the electrical guages. Just my opinion.

[Steve Pope]

Dump the carbs!!! Your problem is that the pumper carbs and some later merc carbs had idle discharge ports on the floor of the carb throats. When you trim the engine under to take off and the boat goes nose up the fuel in the carbs float bowls run to the rear of the float bowl and up and out the idle discharge ports in the floor of the carb throats giving a very rich mix. The later carbs have the idle ports on the top of the throats.To prove this, try taking off with the engine trimed way out so that the engine cowl is always level or trimed out, hope this makes sense???
Steve

[DIAMOND]

Thanks Guys!


Kerry, my intake doesn't have those holes. So I must be realy on the rich side.

Steve, yes I noticed the motor seemed to come out of the hole with it trimmed up a little. I just didn't have anything to attribute it to. Very good info! Do you have an idea on what a good carb set-up would be for this motor? Man I can't wait for this weekend to get here so I can play with the carbs. Hidden horsepower waiting to be released:D

Thanks again, Dave

[B.Mac]

1989 Motor (06/2003 Rebuild) 2.5 Mercury 7 petal carb motor/ brand new rebuild/ break in time only less than 20 hrs (still has fat 0.68 jets)/ all steel sleeves/ #5 bored .030 over/ 35 cc heads/ 145 lbs compression X 6/ Holley red pump/ WMH pumper carbs/ V-stacks/ pre-mix 40:1 Pennzoil synthetic/ Boyesen dual stage reeds/ mild exhaust chest work/ rod slots cleaned up/ G-Force tuner/ Bob's solid mounts/ 40 amp charging system/ new Rapair switchbox/ Bob's cav plate/ brand new paint and decals on motor/ approx. 220 HP/ new battery/ 12" manual heavy duty jackplate/ 25P SS Tempest Plus prop/ boat runs 71.3 mph GPS @ 6400 RPM.

Exact same motor as '91/2.5 200........
That motor/ carb combination comes stock with 0.66 mains on all 6 holes but I run 0.68's for a margin of saftey due to the compression. Anything bigger is gonna drown that motor.
I'd start at 0.72s and read the piston tops and work downward to 0.68s at least.
Here's a link to my rebuild..........
http://forums.screamandfly.com/forums/showthread.php?threadid=30952&highlight=BL+Skip+Help
B.MAC:D

[all8up]

Just remember, mercury already has a margin of safety built into these carbs. Kerry

[DIAMOND]

Yes, same motor!


B-Mac, we have just about identical motors. I have not installed a cav-plate yet and I'm planning on replacing the switch boxes for a set of Repair boxes. I also incorporated a 2-peice exhaust plate drystack mod. See "Drystack mod, will this work..." Jets are on order and I will try dialing the carbs in. Is there any more power to be made without porting to this motor? Maybe a different set of carbs?

Thanks for everybodies help! Dave

[B.Mac]

Went into full production and should have the port alignment/ block liner problems corrected as mine has an issue of the ports of the sleeves not being perfectly aligned with the liner. My motor was a limited production prototype to the '91.

This design flaw interferes with the smooth flow of the intake charge but I gotta live with it unless I wanna resleeve the whole block for an extra 10 horsepower if that (not).

You're motor is a very good year and is a great platform to build upon. The 7 petal front half with dual reeds gives you the option of tapping into a better power band if you can get your RPM's up there where the powerband is (@ 7000). Of course ya need a lightweight flywheel, get rid of the 40 amp system and go to the 16 amp. Some guys say the exhaust relief slots make it difficult to increase compression but I had no problem making 145 lbs with 35cc heads. They might be refering to BIG compression.

My pumps are disconnected so I dont get flooded on holeshots, but otherwise my carbs are fine for my purpose, but I dont run more than 6400 tops.........
Good Luck
B.MAC:D

[DIAMOND]

Thanks B.Mac!


It was nice to know that someone else is also experimenting with the same motor I have. Most guys on this board seem to be running true HI-PO motors, 245, 260, 280 and so on. I noticed the sleeve alignment problem too. It's bad, when the intake track runs higher than the sleeve, leaving a big lip on the top of the port for the fuel charge to tumble over. I just figured that Mercury uses the same block casting and presses in different sleeves to make the different models of horsepower.
How do you disconnect the pumpers on these carbs? Does it help with the low end bog?

Thanks again for the help and info! Dave

[BLIZZARD]

only it was a 1992 year
it had normal wmh carbs on it without the pumps
with 70 mainjets

i put on 34 cc heads
and boyeson reeds

i did turn it up too 6900-7100 rpm with a 24 pitch trophy

really strong engine for a carb

[O/B Dude]

I agree that the jets are likely way too fat. As for the fuel pressure, i have to disagree. My 2.5 carb runs 7psi with no problems. Sometimes it get to 7.5 psi. I run 66 main jets in all 6 and its still a bit on the fat side. I have relieved exhaust and velocity stacks which tend to lean it just a bit. Run it and do a plug read. You also have idle mixtures screws on the wmh carbs. They are very sensitive. Try leaning and richening to see what happens. Good luck.

[B.Mac]

Yep me too.......Holley Red @ 7 lbs......no problem at all. I would try the jetting before messing with the pumps. They are not your problem, you are running way fat.....BUT...........

Be very careful jetting down and read those pistons carefully. I'm getting ready ta' go down from 0.68s to 0.66s pretty soon myself once I can get BL down here ta' read my pistons for me.

Dont want no screw ups at this point.........
B.MAC:D

[DIAMOND]

Ran the boat with 70's,


Well I put 70's in for 1-4 and 72's in for 5-6. Ran the boat to 5,500 and the EGT's read 1,200. Had to shut it down and put it on the trailer due to those cheap a$$ relays mercury uses for the trim-n-tilt. It felt strong through the RPM's. Just when I got up to speed the motor starts to trim down. Took them apart and found the contacts welded. Off to the parts store I go!
B.mac I'm abit nervous too when it comes to reading piston tops. No experience there.

Thanks guys, I'm taking notes and learning!

Dave