My wee little motor won't run full speed (2000 OMC 3.5hp)

[Joe_the_boatman]

I'm trying to give new life to this little neglected motor that I paid $60 for.

In a bucket, it idles great and revs fine to 1/2 throttle. Past 1/2 throttle it bogs down, and will run no faster. When I intentionally starve the carb of fuel, it revs higher right before cutting out (like most two strokes). This led me to believe there's a fuel delivery problem, but I've gone through the carb multiple times and it's super clean. This is a slide carb, with a needle jet, and everything appears to be in working order there.

I ran a temporary fuel line directly to the carburetor to take the fuel tank, lines, filter, and valve out of the equation, and nothing changed.

I don't see anywhere where there could an air leak downstream of the carb, and it fits snug on the intake.

If the fuel system isn't causing this, maybe it's the ignition not advancing at higher throttle? (Even though it revs higher when it runs out of fuel?)

I have not changed the spark plug, but see no reason to yet. I haven't messed with slide carbs since my motorcycle days 20 years ago, but this is a very simple device. The only adjustment is the idle speed screw (not a mixture adjustment, it adjusts the slide's bottom position).

I searched, but haven't found much info out there on messing with these little engines.

Any thoughts?

 

[clemsonfor]

How are you not blowing the water out of the bucket? I hope your not trying to run even half throttle in Neutral let alone WOT !!??

Are you sure your timing is fully advancing? Is the carb butterfly fully open at WOT?

 

[Vic.S]

Check the compression
Check that the spark is strong enough to jump a 7/16" air gap
Check the carb float height
Fit a new spark plug. There is a high probability this will cure your problem

The mixture is adjusted by fitting the retaining clip in different positions on the needle but this is not your problem .

 

[Vic.S]

How are you not blowing the water out of the bucket? I hope your not trying to run even half throttle in Neutral let alone WOT !!??

Are you sure your timing is fully advancing? Is the carb butterfly fully open at WOT?

I think you need to read the manual

 

[Joe_the_boatman]

How are you not blowing the water out of the bucket? I hope your not trying to run even half throttle in Neutral let alone WOT !!??

Are you sure your timing is fully advancing? Is the carb butterfly fully open at WOT?

It's a big bucket (55 gal) - at half throttle, water's moving pretty good, but it's always deep enough to keep the water intake submerged.

No neutral on this motor, it's fixed gear.

I'm not sure my timing's advancing, that was the original question. I'll pull the flywheel to investigate. Nothing's tied to the throttle linkage, so ignition advance isn't mechanical.

No butterfly either, it's a slide carburetor. Yes, the slide opens fully.

 

[Joe_the_boatman]

I think you need to read the manual

Me? I don't a service manual yet. Just got the motor a week ago.

 

[Joe_the_boatman]

Parts diagrams of ignition system and carburetor for reference.

 

[saltchuckmatt]

I'm trying to give new life to this little neglected motor that I paid $60 for.

In a bucket, it idles great and revs fine to 1/2 throttle. Past 1/2 throttle it bogs down, and will run no faster. When I intentionally starve the carb of fuel, it revs higher right before cutting out (like most two strokes). This led me to believe there's a fuel delivery problem, but I've gone through the carb multiple times and it's super clean. This is a slide carb, with a needle jet, and everything appears to be in working order there.

I ran a temporary fuel line directly to the carburetor to take the fuel tank, lines, filter, and valve out of the equation, and nothing changed.

I don't see anywhere where there could an air leak downstream of the carb, and it fits snug on the intake.

If the fuel system isn't causing this, maybe it's the ignition not advancing at higher throttle? (Even though it revs higher when it runs out of fuel?)

I have not changed the spark plug, but see no reason to yet. I haven't messed with slide carbs since my motorcycle days 20 years ago, but this is a very simple device. The only adjustment is the idle speed screw (not a mixture adjustment, it adjusts the slide's bottom position).

I searched, but haven't found much info out there on messing with these little engines.

Any thoughts?

I have the Evinrude version of that motor that I got a so called deal on too. I messed with that thing for WAY to long and had similar symptoms just like you have. I like to think I'm a pretty good mechanic but could never solve that one. Threw it away. Years later I wondered if it was a overheat situation where it might have vapor locked but it was just a guess. The thing I hate the most about those models is how long it takes to just get the cap off. Good luck!

 

[Joe_the_boatman]

I have the Evinrude version of that motor that I got a so called deal on too. I messed with that thing for WAY to long and had similar symptoms just like you have. I like to think I'm a pretty good mechanic but could never solve that one. Threw it away. Years later I wondered if it was a overheat situation where it might have vapor locked but it was just a guess. The thing I hate the most about those models is how long it takes to just get the cap off. Good luck!

Well, thanks for that input. I've learned my lesson plenty of times on these old motors, and I'd like to think that I know when to throw in the towel.... but I'm still a sucker when I see one for cheap. They're multiplying in my garage somehow.

 

[Vic.S]

Me? I don't a service manual yet. Just got the motor a week ago.

No, not you
My comment was aimed at clemsonfor
As you know there is no is no butterfly valve
The timing is not advanced by the throttle control as it is with larger engines either.

Have you checked the compression yet or the spark or tried a new plug ?

​

 

[Joe_the_boatman]

No, not you
My comment was aimed at clemsonfor
As you know there is no is no butterfly valve
The timing is not advanced by the throttle control as it is with larger engines either.

Have you checked the compression yet or the spark or tried a new plug ?

​

I will tomorrow and post back. My Gamefisher 5hp got all the attention today.

 

[Joe_the_boatman]

I will tomorrow and post back. My Gamefisher 5hp got all the attention today.

Ah, what the heck. Went out to the garage and played with it for a few minutes. 85psi was all I could get using the recoil start, WOT (though it doesn't seem to make a difference either way).

Didn't have a spare (similar) plug to test with. I'll have to source another one somehow.

Put the old plug back in, hooked up the temporary fuel line, and made a quick video (link below) of what she's doing. (If a picture's worth 1000 words, what's a video worth?)

Note how it bogs down past half throttle and revs up as fuel is used up (so ignition, with advance, seems to be working at those speeds). Seems like it's gotta be a fuel problem, I just don't see what it could be with this little simple carburetor.

 

[Joe_the_boatman]

Spark plug tip. Looked OK to me.

 

[saltchuckmatt]

Ah, what the heck. Went out to the garage and played with it for a few minutes. 85psi was all I could get using the recoil start, WOT (though it doesn't seem to make a difference either way).

Didn't have a spare (similar) plug to test with. I'll have to source another one somehow.

Put the old plug back in, hooked up the temporary fuel line, and made a quick video (link below) of what she's doing. (If a picture's worth 1000 words, what's a video worth?)

Note how it bogs down past half throttle and revs up as fuel is used up (so ignition, with advance, seems to be working at those speeds). Seems like it's gotta be a fuel problem, I just don't see what it could be with this little simple carburetor.

I don't remember an online fuel filter on mine. Check the parts breakdown and see if that is stock. Could be restricting, otherwise float bowl could be working incorrectly.

 

[saltchuckmatt]

Spark plug tip. Looked OK to me.

I couldn't find a 3.5 until 2002 and the model number for that is J3rsnf. Anyway, I never saw an online filter. I would bypass it and see what happens.

 

[saltchuckmatt]

Ah, what the heck. Went out to the garage and played with it for a few minutes. 85psi was all I could get using the recoil start, WOT (though it doesn't seem to make a difference either way).

Didn't have a spare (similar) plug to test with. I'll have to source another one somehow.

Put the old plug back in, hooked up the temporary fuel line, and made a quick video (link below) of what she's doing. (If a picture's worth 1000 words, what's a video worth?)

Note how it bogs down past half throttle and revs up as fuel is used up (so ignition, with advance, seems to be working at those speeds). Seems like it's gotta be a fuel problem, I just don't see what it could be with this little simple carburetor.

Also, what's the pinched off tube for? The second time it died the rpm's went way up so I think in leaned out and that why it ran up so fast just before dying.

 

[Joe_the_boatman]

I don't remember an online fuel filter on mine. Check the parts breakdown and see if that is stock. Could be restricting, otherwise float bowl could be working incorrectly.

Also, what's the pinched off tube for? The second time it died the rpm's went way up so I think in leaned out and that why it ran up so fast just before dying.

I couldn't find a 3.5 until 2002 and the model number for that is J3rsnf. Anyway, I never saw an online filter. I would bypass it and see what happens.

I added the inline fuel filter - the OEM petcock with screen had to be replaced.

The clear tube is a temporary fuel supply to the carburetor; this removes the tank, petcock, & filter from the test. However, the engine behaves the same on either fuel supply.

Previous owner said it was a 3.5hp, and the outside rating stickers are long gone. The serial and model sticker on the support bracket show it's a J2RTSSR, 1.5KW, so actually 2hp, not 3.5 like I originally stated.

 

[Joe_the_boatman]

....otherwise float bowl could be working incorrectly.

I thought the same earlier, but the float hinge (#18 in the diagram) is spring steel, and non-adjustable (can't be bent) as far as I can tell.

The plastic float appears sound and doesn't have any fluid in it, and moves freely.

I'll check the fuel height using the clear supply tube and report back.

 

[airshot]

Yep.. I have the same motor but the Mercury 2.2 hp version. These were made by Tohotsu, they made them for many different name brand names. Mine runs great, 1985 model. Even though you have cleaned your carb, still sounds like you have a blockage some where. Try a rebuild kit for the carb so you have new gaskets and the proper settings, hust cleaning rarely works well!! I will tell you this, there is almost no negligible difference in speed from 3/4 throttle to wot.

 

[Joe_the_boatman]

Well everyone, I got too many other projects that I should be working on instead of this little motor, so I sold it yesterday in it's present state.

It was fun to tinker with and I even made a few bucks on the deal as well. Thanks all for your input.