Did you set the floats while it was sitting on the trailer, or while floating in the water? Wondering if the attitude sitting in the water is markedly different, resulting in the float being too high, which might explain the fuel dribbling issue. Other thing might be fuel pressure issue, if too high could overwhelm the needle/seat, although I would expect extreme flooding with this.

Pressure gauges may tell the tale, although it's also possible that you have an ignition problem in addition to the carb problems. Do you still have the original HP500 carb?

Good luck!

Possible that the fuel dripping in the carb, is dumping fuel, bogging it down, dumping fuel, bogging it down...?


Fuel dripping could be from several things.

#1 Float set too high

#2 dirt or something stuck in the needle and seat

#3 too much fuel pressure (I doubt thats your problem though)

#4 loose bowl screws causing metering block gasket to leak fuel, or simply bad metering block gasket.

Did you have any backfires through the carb? If so, possible that the power valve is blown. That can also cause dripping fuel and rich running.

Set the float levels in the water yes. Have origional carb i could not figure out whats wrong with it, it kept dumping fuel in the primary venturis (i think theyre called the rings when you look down the carb) so bad that it wouldnt idle. Going to swap carbs tmrw am and go for a run. when i remove sec sight plug float level is right at the bottom of the hole. metering block gasket is new, carb is 2 weeks old. ill swap carbs and see what happens

Starting to sound like this might not be the carb's fault. Went back and read all the posts, and it sounds like the original carb had a similar issue. Starting to wonder if somehow it is getting too much fuel pressure on that one engine. Is it electric pumps or mechanical pumps mounted on the seawater pumps?

Has mechancial mounted to the sea pump. I have the metal fuel lines hp500 come with stock. looks like there is a small allen plug i can put a fuel psi guage in. ill let u know what i find

8 lbs of fuel psi. Little high correct?

My brand new merc pump delivers 8 and carb has no trouble anymore

8 psi is totally fine. Actually, 7-8 psi is perfect. You don't want to be higher though.

If the idle speed screw is opened too far you will have enacted the main fuel system and thus fuel will come out of the boosters. I'm assuming these are what the OP is calling the 'ring.'

If there is too much fuel pressure or the float is set too high or the needle and seat is stuck, the fuel will come out of the vent tube.

OP - don't take this wrong but your actions on the new carb (jetting/metering block changed over from old carb) and some of your terminolgy tells me that you don't know all that much and it may be time to look over the shoulder of someone who knows more.

I learned without someone like that but it's not the faster nor cheapest way to do it.

We can only help so much from here. Our hands can not go thru the computer screen - if you know what I mean.

Watching the fuel pressure gage while running , when it bogs, is what we wanted you to do to see if it's changing.

He is almost there . I'm wondering if he had the dreaded off idle fuel pressure creep

The guage I got was an inexpensive mr gasket that I could screw in to the fuel rail. It was flickering a lot under load. I don't trust that guage I'm going to bring down a snap on one. Or maybe get a beter guage my last boat had auto Meter guages in line and they read rice and steady. This gauge flickered a lot maybe because cheap guage ?

Ya I understand thanks. The fuel hasn't come out of the vent tube only the booster. That was in the old carb. I'm pulling my hair out and yes trying to get someone to come look at it with me.

OP I gave myself the same advice in fact I learned that not a lot of modern mechanics really understand carbs. I took it to an old timer that has done carbs for 40 years and he reset everything to factory specs and I re installed it and the boat runs great. A skilled guy can likely tune it some more but its dam hard to find a guy that can tune a carb without O2 sensors and is willing to do it on the water.

You calling me old ? How dare you.

LMAOF.

Here's the update. Hooked up fuel psi gauge and watched while the surge was occurring. It's holds 7 lbs of pressure steady. Swapped carbs and it still does it. It's not the carb. So must be ignition. Ign module? I have the setup with knock sensors I believe. If I keep the boat will probably go to DUI ignitions

Any chance u can point a timing light while cruising and watch and see what the timing is doing during the surge? Swapping the module from the other engine might be a good idea

Ya ill do that next. I think they are all the way in the back and hard to get to. Thanks for all the help ! Does the ignition module in this look like a thundebolt one? I cant see it think is by the black box back of motor left of the distributor if you looking front to back?

Where's your base timing set at? If it's too high the knock sensor could be pulling timing out and that could be your RPM drop.

You might consider swapping the knock sensors too, just for sh!ts and giggles. Do each component swap one at a time to rule them out. I guess that is one benefit of having twins - being able to swap an identical known good component. I have to give you credit for sticking with it. If this was me with my schedule, it would be drawn out over about six months. :p

Are plug wires new? Crossfiring, especially between 5 & 7 could also set off the knock sensor and cause a surge.

You have the Thunderbolt V ?
Oh boy, not only does it have knock sensors, but it has mean best timing. Wonder if that is causing your surging ?

BTW: To set base timing with these requires a different procedure than normal distributors.
See the V6/V8 Thunderbolt V procedure here:
http://www.perfprotech.com/blog/arti...eengine-timing

and:
http://www.boatfix.com/merc/Bullet/94/94_02.pdf

wires havent been touched this year. I have whatever ignition comes with hp500 in 1999. going back down today to check the timing. When i got the boat they didnt have it timed right, i had retimed it was only getting 24 degrees total. Thanks for the above procedure will try it later. i did ground the wire when i timed it. this new issue hasnt been happening very long week or so. thanks again will post results!

Swapped in a known good ign module same thing

Checked timing at dock have 35 degrees at 3500 rpm and held at 3500 to watch it. Of course the problem will only happen at cruise speed

Dam. Maybe try unhooking the knock sensors? Swap coils?

Trying that right now. I can't find the knock sensors because the stupid waste tank I can't lay infront of the motors. Where are they if your standing infront of the motor

Dang Jiff, you are putting more hours on your boat troubleshooting it than I have put on mine all summer! I admire your persistence. Good luck with it. Pretty sure the knock sensors would be located low on the side of the block, but I'm sure someone who knows for sure will chime in.

Maybe this will help - down low, in front of the starter.

http://ww2.justanswer.com/uploads/ol..._mercknock.jpg

Thanks unplugged knock sensor and it ran right for the first time was able to turn 5k and cruise at 3500 no surge for a few miles. Then on the way in it started again. In the process of swapping ign modules i unbolted the coil and move it around to get in there, swapping couils it the only think i have not done yet. will try a new coil and see what happens. I dont think there is anything left! I have eliminated just about everything.

This is about the time I would've just given the boat away in frustration to have the new owner find a loose ground wire. Keep at it!

Narrowed it down to a bad coil!

Well, that was an azz kicker !

So, (just confirming) you installed new coil and it runs perfect now ?

Will you be selling the holley 850 ?