1978 omc 140 hp inboard, model no 990245h1 loss of rpm

[dcbieske]

I have had this 1978 omc 140 hp motor for about 8 years now. It sits in the original glasstron body. the whole boat has been a pain and a huge expense to me sence I got it . I have changed the upper gear case, shift cable, tilt motor ect ect. but I have never had problem with the engine until now.

recently I have had problems with the motor being able to hold rpm,s. at low/trolling speeds it seems to do fine but if I try to get to a planeing speed it will get to 3500 rpm or even 4k and all of the sudden drop rpms on me. sometimes It will not even go long enough to plane. the longer I use it per day the worse it gets. I have cleaned the fuel filter and had no positive results. The fuel pump check bulb is always empty and I cannot reproduce this issue in the yard. it runs great when attached to a hose with no load on it.

I guess im just asking if anyone has any idea what it could be. Thanks ahead of time for your imput.

 

[Howard Sterndrive]

could be a dirty flame arrester atop the carb, or otherwise bad air or lack of air to the motor.
could be tank vent (try a run with gas cap off), or the anti-siphon valve in the fuel line connection at the tank (my bet on that), or a weak fuel pump, or the carb needs cleaning/rebuild or other attention..
could just need a tune up with points and cap, rotor, etc- and/or advance weights in distributor not advancing the timing - common

 

[dcbieske]

well I got what my local boating store called a tune up kit for the distributer. the problem I have with it is the points don't fit, the rotor and condenser fit just fine but the old points had a screw that would screw the wire into it to hold them in place. the new one does not have that and it not compatible with the old screw in type. the new ones I got are acdelco instead of omc as that was all they had in stock.

I started it in the yard with the new condenser and rotor and it ran ok, I still need to find the exact spacing for the point but that's and easy google fix lol. I had the guy at the shop order me the exact omc part ( at 30 bucks more of course) but that's as far as I have got. I am going to space it out tomorrow with the old points and new parts and test the gas cap theory.


I really appreciate the imput that you provided and ill keep you informed of my progress.

Thanks again.

Danny

 

[dcbieske]

ben reading around and this is probably not the problem as mine will advance rpms, just will not hold them. http://forums.iboats.com/forum/engi...a-goes-bad-after-dealer-installed-new-starter correct?

 

[Howard Sterndrive]

The fuel pump fills the bowl in the carb. If the engine is sucking fuel out of the float boawl faster than the fuel pump can fill it, you'll lose rpm, often accompanied by a backfire as the mixture leans out.
As you are on plane or trying to plane, the fuel pump should maintain 4-7 psi pressure to the carb inlet.
Teeing a gauge into the fuel line and going for a run is the way to rule out anything on the fuel supply side.