1991 Donzi 16 4.3 ho running tough top end

[OKII]

Hello there- I'm a pretty new member to the site and to catch everyone up to speed (sorry for the long post) I have a 1991 16 classic with a 4.3 H.O. OMC. I recently purchased the boat and had it checked over, however I was unable to run the boat when I bought it 20 degrees below in Syracuse NY in January. We muffed the boat in a service shop, it seemed to run out fine. A few weeks ago I had put the boat in the H2o and it ran like a top... Until I decided to trim up and see how it was top end. After about 4 seconds (4300 RPM) the engine seemed to stall and go into rough engine running mode. I slow the boat down and it seems to run fine below 2800 RPM> I shut it down and immediately restart the boat, and it seems to run fine again, until 4300 rpm. I know that the '91 has a rev limiter but it seems to be cutting out at a lower RPM than it should going by the OMC manual. I had a tech run it out of water on a dyno machine and he got to 5200 RPM with no problems (obviously not under load) I was informed that I may have a knock sensor, oil pressure sensor, or other issues, all of which can be a guessing game. I would love to know if anyone has any suggestions as to what may be causing this or if anyone has heard of this issue? Thanks in advance!!-John

[MOP]

I think you are running out of fuel due to a fuel filter, dirty tank pickup or the anti syphon valve try is a vacuum gauge in the fuel line between the tank and the fuel pump. The reading should be in the 3 to 5 inch range over that pull the pickup out of the tank clean it and also carefully inspect the anti syphon valve they can get debris in them. Fuel problems are harder to detect on a dyno as most of the time the engine even though under load is not run long enough, it take a few minutes. Most shops don't like to run to long as the dynos have no mufflering, we got rid of ours for just that reason.

Phil

[glashole]

I would check the fuel filter as mentioned above however

this may be right out in left field but are you watching your temp gauge.
if the motor gets too hot it will "shut itself down" and will return to normal operating mode when it cools down (ie you slow the boat down and restart it).

[OKII]

Thanks for the info, glashole and MOP, I failed to mention before i replaced the fuel filter as part of my prep before I put the boat in the H2O. I have also checked the temp guage (unless incorrect) but all of the guages seem to be reading correctly?! MOP you dont think this could be a rev limit issue? or is there a way to adjust the rev limiter ?? sorry for the extra questions>> Thanks for the time. --John

[RickSE]

This is probably not it but did you fill your new fuel filter with fuel before you replaced it? I wouldn't think it would effect a carb motor but I just went through this on my EFI motor. Motor ran fine until I really got on it (>4500 RPM) then it would act like it was vapor locking. Turned out I had air stuck in my fuel system from the new filter, which I did not fill w/fuel before installing. I purged the air out of the system and motor ran fine.

Merc. Tech suggested I pull the fuel filter/seperator and top it off with fuel. I ended up purging the air by removing the second EFI filter upstream from the filter/seperator.

[Bad-Tat]

OKII,
Do you have the Spit-fire ignition? I had all the same problems you did and after all new fuel system, looked at the ign. Turns out no one had ever bought the testers for this system. So put a Merc dist setup on it and solved all my problems.

[MOP]

I doubt the rev limiter is the cause, could be ignition as many of the symptoms will mimic fuel. But being as you can slow down which would let the fuel system catch up with the motors fuel usage still has me thinking it is running out of fuel at WOT fuel. Get a T fitting and a regular vacuum gauge you will need a short section of fuel line some clamps and a long about 10 feet of 1/4" hose to be able to watch the gauge at speed. Option you could just go ahead and pull the pickup tube out of the tank and check it almost as easy, you have to start some where to eliminate the possibilities.

Phil

[wiltok]

Damn - I have the same boat (actually a 90) and am experiencing the same problem. I have had a ton of work done to resolve this - including a conversion away from the Spitfire (electronic dist), all new electrical system, fuel pump, carb rebuild (2), and 2 mechanics and $1000.00 later am still having the problem. The one thing that was not looked at was a fuel pickup problem (I have changed the seperator) - I'll have to check that out myself. I hope that's it....

[wiltok]

MOP - where is the anti-siphon valve and what does it do??

[MOP]

The anti syphon valve is screwed into the pickup, pull a little hose slack into the tank area (not an easy task) you do not have room to pull the hose without damaging it so cut it off close to the fitting (yours being a 90 you may want to change the hose anyway) the whole tube and anti syphon come out together. The anti syphon is a ball check with a light spring to shut the fuel supply in case the hose goes, any debris can make them stick part way open which will cause a problem with fuel delivery. Remove it and check it out, the pickup tube usualy has a screen to keep junk from getting through. If any moisture has gotten to it they get gummy and plug up.

Phil

[jstokes8407]

I have a 18 classic and I had my pick up clog on me. Took a little while 2 find the boat would take off and once the fuel bowl was empty it would starve the engine. At lower rpms it was fine but high rpms it couldn't pump enough fuel through the blockage to keep up

[burnsie15]

Has anyone been able to resolve this issue?

I have witnessed this phenomenon myself, but have not heard of how exactly to fix it.