Yamaha 90 2-Stroke Runs Rough @1800 rpm

[Manwohr90]

My 1998 Yamaha 2-Stroke starts easily and will idle and ***** all day long. Under power, it is smooth and strong up to it's maximum 5200 rpm on my 17' Boston Whaler Montauk.

The problem I'm experiencing comes in "slow speed zones", where the motor is running at 1600-1900 rpm.

The motor will never stall but it vibrates excessively and steadily. It feels as if it's running on just one or two cylinders. However, I have systematically removed each spark plug cap while running at these rpm's and each plug wire changed the running dynamics of the motor. In addition, I've run every gas additive through the fuel, changed plugs, thermostats, sprayed carb cleaner in the carbs with the motor running and with the motor off.

It has done this since new, and the dealership "rebuilt" the carburetors after about 20 hours. They stated that it must have been bad gas through the motor. My feeling is that they didn't do the work as it did not cure, much less, improve the issue.

My thinking is that the problem is one of two possibilities. 1) The mid-speed throttle linkage needs adjusting, if there is such an adjustment, or 2) The mid speed jets are obstructed, again, if there is such a thing.

Any suggestions?

[TownsendsFJR1300]

I know it would be a PIA but if you time your trip correctly, I would at least try the following:

Go out for a run, run the engine for at least 5-10 minutes when its running the absolute roughest. Immediatly drop anchor, let here cool, down and pull the spark plugs checking their condition.

Another thing, if you have an old fashioned timing light, hook it up to each plug wire (one at a time) and keep an eye on it (aimed at the deck or?). Get it to the roughest RPM again checking the flash from the light . If your getting an intermediate flash (it be obvious), you know what direction to look in (possibly a failing coil when hot, etc)...

Should the plugs look lean (whitish), opening up the pilot screw, CC, (part #11)would richen up the mixture some and should help: http://www.boats.net/parts/search/Ya...TOR/parts.html

[99yam40]

My 1998 Yamaha 2-Stroke starts easily and will idle and ***** all day long. Under power, it is smooth and strong up to it's maximum 5200 rpm on my 17' Boston Whaler Montauk.

The problem I'm experiencing comes in "slow speed zones", where the motor is running at 1600-1900 rpm.

The motor will never stall but it vibrates excessively and steadily. It feels as if it's running on just one or two cylinders. However, I have systematically removed each spark plug cap while running at these rpm's and each plug wire changed the running dynamics of the motor. In addition, I've run every gas additive through the fuel, changed plugs, thermostats, sprayed carb cleaner in the carbs with the motor running and with the motor off.

It has done this since new, and the dealership "rebuilt" the carburetors after about 20 hours. They stated that it must have been bad gas through the motor. My feeling is that they didn't do the work as it did not cure, much less, improve the issue.

My thinking is that the problem is one of two possibilities. 1) The mid-speed throttle linkage needs adjusting, if there is such an adjustment, or 2) The mid speed jets are obstructed, again, if there is such a thing.

Any suggestions?

So you have been having this problem for 15 years since it was new in 1998 and just now you decide to find out what the problem is.
Seem strange.

But shooting carb cleaner at a carb does nothing. They need to come off and apart to clean properly and then adjust everything like manual says

[Manwohr90]

Every time I've asked outboard techs about this issue, I've always received the same comment, "Oh, every three cylinder two-stroke has a rough spot somewhere in its rpm range."

It's more of an annoyance rather than a problem. The motor has about 4500 hours on it, and has always started, got me where I've wanted to go, (including a dozen or so round trip crossings to Bimini, a few to Block Island, RI and several Ft Lauderdale to the Keys) and got me back home without fail.

The plugs have always appeared more blackish-brown, which I understand is proper.

[Manwohr90]

I've replaced the fuel pump about a year ago as it failed at the dock. Not bad after about 4000 hours.

I only run non-ethanol without exception.

I have run her up to temperature and got her at the rough spot, then removed each spark plug cap individually. Each cap removal caused the same change in the running of the motor, adding to the roughness but not causing the underlying issue.

Another explanation has been that it's the motor's computer's "fuel save mode".

[99yam40]

how many times has the carbs been pulled apart and cleaned or the 15 years?

Have they ever used a sonic bath?

Some times it takes some doing to get all passages cleaned up.
They have off idle ports that help transition from idle to higher RPM.

The CDI adjusts the timing during start up and when advancing throttle, but no fuel save mode that I have ever heard of on carbed motors.
Timing is easy to monitor with a timing light to make sure it is in spec at problem RPMs

I did have to replace my C40 cdi due to idle timing was way too far retarded

[Manwohr90]

I had the carbs sonically cleaned in the first 20 hours. Whether the work was actually done or not, I don't know.

What I do know is that I paid for that service, and it did not solve the issue.

Where are the transitional areas/ports in the carbs?

Does getting to them require a complete disassembly?

[99yam40]

That is a lot of hours and fuel to run through a motor to only tryed to clean them once.

The ports are in the throats of carbs ,as the throttle plates come by them it sucks from the different passages.
The carbs need to come apart to clean properly and then adjusted like service manual calls for every time they go back on to make sure link and sync is correct

[whaler82]

I had the same motor and it has done the same thing for I cant remember how long. Over the years I have had my carbs done 2X for unrelated issues. It still does it. I believe mine is a 1997 or 1998

[robert graham]

I've had my 90C Yamaha since new in 1999 and don't recall any vibration or roughness in the mid-**** RPM range. My first thoughts for your problem would be to go back and double-check the carb synchronization procedure. Also wonder if the guy who first cleaned your carbs may have accidentally mixed up the jets from one carb to another?....How far are you from a real, trained Yamaha outboard technician?....one who really knows the finer points of tuning these motors....I believe you can locate one by the "Certified Yamaha Outboard Technician" locator...good luck!

[Manwohr90]

An authorized Yamaha tech told me that if she starts right up and idles after sitting for a month or so, with no stalling issues, and gets the boat up on plane quickly to the maximum rpm range, then "DON'T MESS WITH IT!"

Also, I recently had the compression checked; 115 in each cylinder.

[robert graham]

An authorized Yamaha tech told me that if she starts right up and idles after sitting for a month or so, with no stalling issues, and gets the boat up on plane quickly to the maximum rpm range, then "DON'T MESS WITH IT!"

Also, I recently had the compression checked; 115 in each cylinder.

Well, maybe you can just live with it, but I'm sure it's not much fun going through those "idle/slow" zones.....still, somethin'' ain't right in that mid-**** RPM range. Good luck!

[Bilge Rat]

4500hrs., have you looked at the reeds? Are they sealing?
I have no experience with reeds in Yamaha 2 strokes but from Mercs 4500hrs. is a long time without a close look.

[Manwohr90]

Didn't know the motor had reeds. Let me reiterate, this motor had this issue right out of the box. When I took it in for it's 20 hour service, I brought the problem up to the Yamaha tech at Lauderdale Marina. They disassembled the carbs and put them through the sonic cleaning. Although the motor was brand new, this was not covered under warranty. Needless to say, I'm a bit gun shy with having somebody "fix" it. Also, Lauderdale Marina no longer is a Yamaha dealer. This is not a 4500 hour issue that has developed over time. It has always been there.

[TownsendsFJR1300]

Yes it does have reeds, inbetween the carb and crankcase.


Otherwise it would have to have either piston /port (carb on the side of the engine) intake, or on the older 2 stroke motorcycle engines, some had a flat plate attached to crank that had an slot. When it rotated on the upstroke, the piston going up caused a vacuum pulling in fuel( I haven't seen that used in decades) from the carb mounted on the side of the engine near the crank.

The reeds have considerably more torque off idle..

I had an evinrude 140, V4 they called a "loop" (and maybe something else)style engine. From brand new, (I pulled it out of the box and rigged it) it had a rough spot in a certain RPM range..

The RPM range your having issues with is pretty much in between using the idle jets (part #6):

http://www.boats.net/parts/search/Ya...TOR/parts.html

and switching over to the main jet(full throttle). Re-jetting, if its running very rich in that RPM range should help..