very slow yamaha 4 hp

Hello everybody. I wanted to see if I could pick your brain for some advice. I have a 1988 yamaha 4 hp. I think the model is SG4. Anyway I had to rebuild it when I bought it. So the bearings, piston, rings, etc etc are all new. And it runs perfect. But the little thing is so slow. I put it on a 16' Lund for a test run and it ran 6 mph. I used it as a kicker on a heavy cc and it pushed it 6 mph. This weekend I put it on a 12' jon and I thought surely it would perform good on this little 90 pound boat. Same thing. 6 mph. It feels like it needs more prop. It's not straining at all. And it doesn't really gain any speed above about 1/2 to 3/4 throttle. But even at wot it sounds like its governed pretty low. Its not screaming at all.

So my question is, is there anything I can do to get more mph out of it. So far I have not been able to find an aftermarket prop for it. I need some advice, please.

Thanks, Terry

Re: very slow yamaha 4 hp

4 hp is not much power and on a 16 foot boat that's asking a little motor to push hard. Actually, it doesn't matter what size boat you put the engine on. If the engine reaches its maximum wide open throttle rpm the prop can only deliver its theoretical speed. If 5000 rpm is WOT throttle, the prop will spin at whatever the lower unit gear ratio permits regardless of boat size. Without a tach you have no idea what rpm the engine is turning. Try a TinyTach so you have some idea what the engne and prop combination are doing.

Re: very slow yamaha 4 hp

That's true. I need to get a little tach and check it. I have heard of 9.9 and 15hp motors that the only difference between them is the jet in the carb. I wonder if a slightly larger jet would make any difference.

Re: very slow yamaha 4 hp

Probably you have a prop slip on hub, check prop, place a Tiny Tach to see max rpm stated for that engine, assume next update will be 5 HP, check carb parts for that model, swap parts on the 4 HP model. On some engines you'll need to change complete reed valves assy and carb as gas passages are restricted compared to next model, changing jets won't do any good and engine will bog down when throttled.

Happy Boating

Re: very slow yamaha 4 hp

I had a Yamaha 4HP on my canoe for a while, and it definitely wasn't a "screamer" type motor...I sold it and went back to my Tanaka 3HP, which was just about as fast and 20lbs lighter. Is your carb opening up all the way with your throttle control at wide open?...has your rebuilt motor had time to get broken in for best compression and power? The tach would at least tell you if you're getting the 5500 RPM/maximum power. Good Luck!

Re: very slow yamaha 4 hp

Well I've done some checking and the next step up in that year was a 6hp. And its twin cylinder while mine is single cylinder. So a carb swap wouldn't work. And the prop is a sheer pin style. So no hub slippage. But I'm not giving up just yet.
Thanks for yalls input.

Re: very slow yamaha 4 hp

wylie catter said:

I have heard of 9.9 and 15hp motors that the only difference between them is the jet in the carb. I wonder if a slightly larger jet would make any difference.

Click to expand...

Totally untrue, this old misunderstanding won't die.

Re: very slow yamaha 4 hp

i agree ondavar, its all about the right air fuel ratio, the carb throat will only supply a fixed amount of air, adding more fuel with a bigger jet isnt a sollution.

Re: very slow yamaha 4 hp

6mph is good for a 4HP anything on a 16' boat. Do not try to hop it up, you will ruin it.

If you absolutely must go faster, get a bigger outboard that is up to the job.

Re: very slow yamaha 4 hp

JB said:

6mph is good for a 4HP anything on a 16' boat. Do not try to hop it up, you will ruin it.

If you absolutely must go faster, get a bigger outboard that is up to the job.

Click to expand...

I agree with you. I do not want to ruin such an excellent little motor. I'm just exploring possibilities. And gathering info.

Re: very slow yamaha 4 hp

Basically you're going as fast as you can go without planing and you don't have the horsepower to get on plane.

I found out some things about this when I got a 9.9 kicker for my 17' Lund. The closest lake to me has a 10 hp limit from May to September, no wake otherwise. Used to be that when the 10 hp limit was in effect you could go as fast as you wanted. I found that I really couldn't go much faster with the 9.9 wide open than I did at "no wake" with the 60.

Checked with the dealer about repropping and he told me it wouldn't help for above reason.

Messing around, I saw that at around 6 mph the nose starts coming up whether I'm using the 9.9 or the 60 and keeps raising as I go faster until I either run out of oomph (9.9) or start planing (60). The 9.9 can get me to about 6.7 or 6.8 if I really put the whip to it but I'm obviously lugging it. It runs 6 mph pretty comfortably at the fast idle mark (a little less than half) on the throttle handle so that's where I run it.

I'm guessing your 4 hp would probably also get me pretty close to 6 mph.

Re: very slow yamaha 4 hp

bassman284 said:

Basically you're going as fast as you can go without planing and you don't have the horsepower to get on plane.

I found out some things about this when I got a 9.9 kicker for my 17' Lund. The closest lake to me has a 10 hp limit from May to September, no wake otherwise. Used to be that when the 10 hp limit was in effect you could go as fast as you wanted. I found that I really couldn't go much faster with the 9.9 wide open than I did at "no wake" with the 60.

Checked with the dealer about repropping and he told me it wouldn't help for above reason.

Messing around, I saw that at around 6 mph the nose starts coming up whether I'm using the 9.9 or the 60 and keeps raising as I go faster until I either run out of oomph (9.9) or start planing (60). The 9.9 can get me to about 6.7 or 6.8 if I really put the whip to it but I'm obviously lugging it. It runs 6 mph pretty comfortably at the fast idle mark (a little less than half) on the throttle handle so that's where I run it.

I'm guessing your 4 hp would probably also get me pretty close to 6 mph.

Click to expand...

Well I'll be derned. That makes perfect sense. I didn't mention it before because I didn't think it was relevant, but, it went a little faster with the motor tilted down. When I raised the adjuster rod up a few holes it just put the bow further in the air and felt slower. Pushing water like a snow plow.

Re: very slow yamaha 4 hp

you would be surprised at the difference 1 hp would make.....up it to a 5hp motor, and you gain 20%!
i have a 1985 4.5, and a 1957 5.5, the old 5.5 makes the "newish" 4.5 look like a slug on the same boat!

Re: very slow yamaha 4 hp

nwcove said:

you would be surprised at the difference 1 hp would make.....up it to a 5hp motor, and you gain 20%!
i have a 1985 4.5, and a 1957 5.5, the old 5.5 makes the "newish" 4.5 look like a slug on the same boat!

Click to expand...

I'm always looking for a good deal. I'm hooked on fooling with these boats. The little jon boat is rated up to 6hp.