98 C60 fading away

[Stevek369]

A little over a year ago, I bought a C60 2-stroke to repower my 15ft A/B Inflatable VST. A few months ago, I had a shop do a thorough check-up and found out that one of the cylinders is down on compression. I don't know if it was bad before or after the purchase, but just this Sunday, I had a fuel starved loss of full throttle. This may or may not be due to the low compression, as it would run fine when I pumped the bulb.

In any case, I checked the fuel filter and it was as clean as could be. I also drained one of the carb bowels and nothing but clear fuel came out. I think this problem may be due to a kink in the fuel line where it goes through the hull (I have a lot of hoses and wires going through that 2-inch hole).

Since I was checking everything out, I hooked up my compression gauge and it read: 110, 110, 85 psi on the cylinders. How bad is that? Should I even take this boat out again?

I have been planning to upgrade to a new motor, and wondering if I should go up to the 70HP 4-stroke (F70LA) since the current 60HP never performed as well as I needed (difficult to plane with 4 passengers and max of 30mph). If those compression numbers were always like that, maybe a new 60HP (F60LB) would perform a lot better than the old/bad 60 2-stroke.

Comparison wise, it seems like the F70LA is only around 5 pounds heavier than the F60LB. It is also less than $500 more. That leads me to believe that I should just go for the 70HP.

Should I pull the trigger and buy the 70HP? All advice welcomed.

Thanks,
Steve

[99yam40]

how much more does the F70 weigh than the C60 and what is the boat rated for.

if it will handle it OK I would go with the 70.

but then finding out what is wrong with the C60 and fixing it might be cheaper.

hard to believe you did not test the motor before buying it or right after since it did not do what you wanted to be sure it was doing all it could.

Hard to say if the F60 or F70 is the right fit for your rig. what do others with a boat like your run on theirs?

[boscoe99]

The compression values you mention would not be cause for the motor to be replaced or for major surgery to be performed. Continuing to use the motor will not cause it any particular harm. IMO. If I were you I would find and fix the fuel supply issue.

The weight of the C60 is 218 pounds. Weight of the F60 is 247 pounds plus the weight of the oil. The weight of the F70 is 253 pounds plus the weight of the oil.

I don't see an F60 performing any better than a C60. In fact, I suspect it will offer less performance. Plus you are now having 30 more pounds hanging on the transom. Don't forget the balance of the boat due to the added aft weight whatever new motor you choose to buy.

[Stevek369]

The compression values you mention would not be cause for the motor to be replaced or for major surgery to be performed. Continuing to use the motor will not cause it any particular harm. IMO. If I were you I would find and fix the fuel supply issue.

The weight of the C60 is 218 pounds. Weight of the F60 is 247 pounds plus the weight of the oil. The weight of the F70 is 253 pounds plus the weight of the oil.

I don't see an F60 performing any better than a C60. In fact, I suspect it will offer less performance. Plus you are now having 30 more pounds hanging on the transom. Don't forget the balance of the boat due to the added aft weight whatever new motor you choose to buy.

I was worried about the compression and that I could get stranded somewhere versus having a new reliable motor. The C60 has been around salt water quite a bid and the Yamaha mechanic said he would not even want to try taking it apart for major surgery.

I replaced the fuel pump late last year, and the bulb pumps up and stays firm with no visible leakage. Last Sunday was a very hot day, and I wonder if that helped soften the fuel line enough to bend/crease where it enters the hull. Every time I went over a wake/bump, I would loose a little power. As soon as I pump the bulb, it would run fine for a minute or so before bogging down and running fine if kept at 3500rpms. In any case (even if I get a new motor), I would want a new fuel line. I plan to pick one up after work. If necessary, I may cut out that hole in the hull so the lines won't be so cramped.

As for the motor size/weight - I did pile some soft weights on the C60 to match an F70. I measured the water line and it barely budged (if at all). The transom plate does allow for up to a 70 (or75?) HP motor. The new AB Inflatable VST Oceanus models are running 70HP motors, but these high performance RIBs are so rare (especially in the USA), I haven't found anyone else with a similar year/make/model to compare motors.

As I've rebuilt 2-stroke motorcycle engines, I thought about getting the new one and making a project out of the C60 and rebuilding it over the winter


Thanks for the advice boscoe99 and 99yam40,
Steve

[TownsendsFJR1300]

There's several products that could very well help:

COMBUSTION CHAMBER CLEANER

And this product is supposed to be dealer use only, COMMERCIAL GRADE:

ACC-CLEAN-DI-00 INTERNAL ENG CLNR 1Q $42.11

I believe this one, ^^^, is used by Rodbolt (a regular here) with excellent results.

No tear down. I'd try one or both before springing for anything..

[boscoe99]

Sounds like someone wants a new motor. Nothing wrong with that.

Go for it.

[zenoahphobic]

That one cylinder difference IMO is significant. Try the cleaners but I would not hold my breath they would only unstick rings if that is the problem, the engine is probably worn (rings and/or cylinder) . I would not use it before knowing the cause in drop in compression. E.g. you may have seized that cylinder, it may have broken ring or corrosion scoring; further running may grind out beyond rebore tolerances, and make rebuild/renovation that much more problematic.

Just presenting another view.

[boscoe99]

Yep, it is about to blow.

Maybe a differential pressure test is needed to confirm that the motor is shot.

[TownsendsFJR1300]

If the other two cylinders are generally good and one is slightly lower, I agree, somethings amiss..

If the engine was worn out I'd expect to see lower #'s across the board..

Should this low cylinder have a busted ring, scored cylinder, I wouldn't expect to see the compression that high in the low cylinder...

Carbon build up is normal as is carboned / sticky rings. I personally would look at the most probably cause first..

[zenoahphobic]

Yep, it is about to blow.

Maybe a differential pressure test is needed to confirm that the motor is shot.

Do I detect some sarcasm?

[zenoahphobic]

If the other two cylinders are generally good and one is slightly lower, I agree, somethings amiss..

If the engine was worn out I'd expect to see lower #'s across the board..

Should this low cylinder have a busted ring, scored cylinder, I wouldn't expect to see the compression that high in the low cylinder...

Carbon build up is normal as is carboned / sticky rings. I personally would look at the most probably cause first..

We should be cautious with the actual number or absolute reading displayed. The fact is a 25% or so difference in one cylinder to the two others with exact same readings.

[boscoe99]

Do I detect some sarcasm?

Only some?

My post was intended to be dripping.