[StinkiePhish]

The compression readings aren't perfect: the lowest reading was 103psi in one of the cylinders when measured about 2 years ago (the engine hasn't done more than 25 hours since then) but I don't longer have the readings in the other 2, I think they were around 110psi, so will have to re-measure them all now. I asked the mechanics who were overhauling the OB whether it was time to put in +0.015 oversize pistons and the answer was "Not yet" (perhaps because the readings were less than 10-15% away from one another), so I agreed with installing brand-new standard size ones. Now thinking if I should have insisted on the oversize ones installed back then. more

[kupan787]

It'd be great to see a video with as perfect idle as can possibly be achieved with these engines (ideally showing the engine at a close distance so that its vibrations can be clearly seen). here

[nethoncho]

Yes, uneven compression will cause rough idle as well as MANY OTHER THINGS.
Check fuel quality, compression and plug appearance first. Then move on to spark quality, idle timing and carb adjustments.

[kita59]

The CDI website has all the info on how to test. [links]

[sphinx24]

Hope to get it on the water this weekend and will report back if the problem has been resolved. [links]

[forchrissake]

Have you mucked with the idle speed and idle mixture screws? Did this issue just come up? Is the boat new to you? Details - details - details.

[BillRoundsEsq]

I'm having a similar issue with my '93 Merc 40 so I'll be watching this thread to see how you sort it out. The advice given so far seems really sound. You can download a manual then print off the sections that you need. The "link and sync" section is critical so start there. How do your plugs look? Mine are getting pretty oiled down so I'm planning on removing my oil injection and running a good new bit of mixed gas just to confirm that my injection system isn't providing too much oil. This issue has been pretty frustrating for me as well and has ruined a handful of otherwise good boating days. Hopefully we can get to the bottom of it. more

[yydertt]

Re: 199 40HP 2 stroke Merc stalls at low idle

[S3052]

idle circuit is the first thing to check, when a motor "pops" it means its running lean generally caused by gummed up idle circuit passages and you will notice if you choke it the popping will go away. here

[melvster]

The parameters for adjusting the carburettor on a Mercury outboard involve both the idle speed screw that controls the motor's speed at idle, and the mixture screw that controls the fuel-to-air ratio. Adjusting both of these requires that you have the motor running. While the adjustments seem minor, both affect the performance and economy of your motor.

[ydddert]

When you measure the slack in the throttle cable, rock the throttle cam from side to side, measuring the amount of travel between the throttle cam and the link rod ball.

[Iskender]

Attach a shop tachometer to the motor's Number 1 cylinder. Turn the idle speed screw on the carburettor linkage to adjust the motor's speed to between 700 and 800rpm. [links]

[Grei R. S. Walker]

Re: 99 115 2-stroke vibrates/runs rough midrange

[memen]

This Started The End Of Last Season. Shakes And Runs Rough Off Idle And Up Worst At Around 3000 Rpm. Starts And Idles Okay. Will Run 5300 Rpm. Smoother At High Rpm. Prop Good, Carbs Rebuilt, Fuel Pump Rebuilt. 140-145 Compression In All 4. I Kill Each Cylinder Individually And It Runs Worse Equally So I Know All 4 Are Firing.
Only 200 Hours In Fresh Water. Block Cool When Running. Plugs All Look Same/good . . any Ideas?

[Marko]

I'm curious to know what you find because in my case I changed several things at the same time and then my engine ran much better. Therefore I don't really know if it was tightening the exhaust cover bolts, or one of the other things that I did that fixed it up. more

[davout]

Re: 99 115 2-stroke vibrates/runs rough midrange