1993 c85 rpm problem

[mrty1761]

I have a 1993 c85 hp with a 13 1/4 x17 solas myth aluminum prop pushing a 1983 skeeter bass boat weighs about 1800lbs total(boat motor 2 guys and equipment) new tach, new fuel, and tank . The motor seems to take a while to get up on plane rpms don't increase quickly . Once it does get up on plane the motor only reads 3800 rpm try trimming up no increase in rpm or speed but does start to porpoise lower trim fixes thatthe boat will run about 27 mph but drinks gas to .the motor is mounted at cavitation plate at pad or just slightly below on boat . Motor starts right up on first crank with little choke idles smooth revs in neutral up max rpms put it in gear and motor has a little hesitation like its gonna die than proceeds . trying to figure out rpm problem to increase . Compression is all good 118-119 on all 3 cylinders spark to all . top cylinder plug is black oily smells of gas 2nd not as bad 3rd looks almost clean but smells of gas readjusted gaps today after taking out 1 and 2nd plugs were .020 3rd .035 which is what i set them all back to could that be problem or do i need to run some ring free through to see if that cleans out carbs and cylinders to get rpms up. looked at prop sites and they all say i should be able to run 21 p prop for this boat and motor and be in rpm range at wot. any help or suggestions would be appreciated thanks

[mitch]

That prop seems large for a 85 hp with that weight i think you would befit from a smaller prop at least get wot operating revs

[99yam40]

Has this boat and motor ever run properly?
Plug gap being too small may not allow for hot enough spark under compression.
My guess would be that you need to pull and clean carbs and decarbon motor to get it back to proper RPM, but that is just my opinion.

[mrty1761]

motor came off a large pontoon boat so i figured it would not be proped right just not sure where to go with rpms at 3800 obviously need to increase them just wasnt sure how the boat and motoe are new to me previous owner was family and died so no info there

[99yam40]

What is the model #
Coming off a toon it maybe a high thrust.
Not sure what all is different on a high thrust, maybe gear ratio.

have you looked at Yamaha's performance web site to see how the C85 normally performs and what prop they used?
Performance Bulletin Home

Never mind I just looked and they do not even list a 85

Looking at a parts breakdown that does not list a C85 in 1993 but they do in 1994.

I still would look at the carbs and clean. then decarb motor.
Lean running cylinders will melt pistons, so make sure it is running properly.

[jb123]

do you see a gas sheen on water ?? check fuel pump

[mrty1761]

no gas sheen on water it does look like it is putting out a lot of smoke under the water has a milky look to water when starting it it may have little too much oil in gas .motor does not get hot no alarms block is warm to touch after running just had new impeller put in last november good strong stream of water thats warm when it comes out.it has model c85tlrr 1993 so that i guess makes it a 94 model or 93? i have been looking at parts on 1993 year on the yamaha parts catalog for it. i do know the motor was bought from a dealer that had it serviced just prior to putting it on the boat that what i was told dont know what that all entailed. someone mention reproping what would anyone suggest diffrent pitch or diameter? im gonna have carbs cleaned and run some ring free through to see how that does

[99yam40]

A Yamaha shop should have a test wheel that they use in the test tank to make sure is making the proper HP at RPM. That way you do not have to guess at the proper prop for right now.
The C85TLRR is a 1993

[sstreet1978]

May be too late

I was searching threads related to my 93' C85TLRR when I came across your problem. I had the exact same problem with mine. I even have it on a Skeeter too. It ran great in neutral, and always turned right over. When I tried to get up on a plane it acted like it wanted to die. Once on a plane it seemed a little better. I looked at the usual suspects as you seemed to be doing - carburators were cleaned, plugs and wires changed, and the compression was checked (came out fine) - turns out the inner plate on the water jacket was leaking into the top cylinder. I found out too late - I replaced the inner plate and the motor ran perfect for a day - but the top piston came apart on the way back to the launch. I suspect that during the process of diagnosing the problem and then running it, I hosed a hot piston with too much cold water and caused it to weaken. When I finaly fixed it and ran it at full capacity, the piston failed. The good news is that I was able to rebore just the top piston to .5 over. I replaced the top piston, had the other 2 honed and replaced the rings. I am 1/2 way through the re-build now with only $300 into it. So far so good.

Moral of the story - if it is still giving you trouble - check the inner plate of the water jacket!!