115 Yamaha running rough

[sidelock]

My 1998 115 Yamaha has been bullet proof and hasn't failed me once yet although I'm the second owner and have only owned it for the last five years. It has just recently started running rough/shaking but ONLY does this between 1000 and 1200 RPM. and anything above or below that it runs perfectly fine and smooth. Had Bass Pro mechanic look at it and do a 34 point inspection but couldn't figure out what is causing it to shake. They removed, disassembled and cleaned the carbs, removed and inspected the reed plates, did a compression test 120 across all four cylinders and suggested I change the fuel pump due to a suspected weak diaphragm , which I have done. They switched carbs around as well as the coils to see if symptom persisted and it did. Told me they had 13 hours into diagnoses but only charged me $450 for four hours.
I don't know about the previous owner but I only run non ethanol fuel and religiously add ring free.
Has anyone experienced similar issues or can offer some advice/suggestions, I just don't want to bounce around from one mechanic to another throwing money away and not find out what the problem is.

[Capt Terry]

Hi. Check around, maybe there is a site for Yamahas similar to the ETEC owners site where one can ask questions and get some good advice. The ETEC site includes regular moderators who either work for BRP or are affiliated in some way and can provide good advice because they get exposed to more information than an individual does- maybe you'll get the same from CSC. The ETEC site requests engine model and serial number in addition to the problem summary. They can advise on service bulletins and sometimes if your engine has received a repair for a bulletin, latest software, etc. .

[McGillicuddy]

Does the shaking happen in idle too, or just in gear?

Have you checked the prop? Looseness or slop in the hub, or even imbalance might cause such a thing.

[caper]

I think you still have a carb problem - rpms where you are having problems are transition from low speed to high speed circuits. Figure out which cylinder is dropping out and go from there.

[sidelock]

Originally Posted by caper

I think you still have a carb problem - rpms where you are having problems are transition from low speed to high speed circuits. Figure out which cylinder is dropping out and go from there.

The mechanic at Bass Pro did say that when he removes the spark plug lead from #4 cylinder, it does not cut out as much as on other cylinders.

[sidelock]

Originally Posted by McGillicuddy

Does the shaking happen in idle too, or just in gear?

Have you checked the prop? Looseness or slop in the hub, or even imbalance might cause such a thing.

I don't believe it does it in idle or at least if it does, its not as noticeable. I'll
check the next time I splash the boat.

[Bushwacker]

Originally Posted by caper

I think you still have a carb problem - rpms where you are having problems are transition from low speed to high speed circuits . . .

I agree. I think you have a lean misfire problem. Outboard carbs are very simple devices, especially the idle and main jet circuits, but the transition between the two systems is pretty sensitive and a little bit of dirt in the idle circuit is all it takes to create your problem. I've been able to create exactly your problem on my old 'Cuda 340 4 barrel Carter AVS carb by adjusting the idle mixture screws too lean, which causes the carb to get VERY lean during the transition stage. Or a slight air leak at a gasket might cause the same problem, where both the idle and main circuits are rich enough to mask a slight air leak, but not during the transition stage.

If your motor is like my old V-4 Evinrude, it will have two 2-barrel carbs, one barrel to feed each cylinder. If you can do a cylinder drop test at 1000-1200 rpm(short out the ignition on individual cylinders; if no rpm drop on one cylinder, that's the one with the problem!) you can narrow it down to which carb needs attention. A quick fix, IF your motor has idle mixture adjustment screws (many just have fixed jets) would be to just richen up the idle mixture on the problem cylinder. But the problem carb really needs a good cleaning.

Personally, I think the old carb motors are so simple and easy to work on that it's hard to justify paying someone $100/hr to do anything on them that doesn't require special tools or the acetylene wrench, so you can save yourself big $ by getting a factory service manual and learning to do your own work! And the ability to diagnose and fix your own motor when you get out in the boonies with no experts around is priceless!

[sidelock]

Thanks for the input Bushwacker. The mechanic did say he switched the carbs around and the issue still persisted so if it is the barrel that feeds the #4 cylinder, wouldn't there be a noticeable difference when he switched the carbs ? BTW, I did purchase a shop manual for the motor however educating myself in the subject is going to take a very long time.

[caper]

The miss would just follow the carb around if you switched it - so no noticeable difference.

[sidelock]

Originally Posted by caper

The miss would just follow the carb around if you switched it - so no noticeable difference.

That makes sense. When you guys talk about low and high speed circuits, am I right to assume you mean fuel flow circuits as opposed to electrical circuits ?