Tranny goes thump in forward

[dmksails]

I was out on the boat and have been motoring at about 2000 RPM for 20 minutes. I then increased RPM slightly. After a minute, there was a series of large thumps begin heard from the engine compartment. Backed down to below 2000 and they stopped. Opened compart and could see engine taking a big jog when the thump occurred. Problem did not happen in neutral nor in reverse.

Decided to head back to the slip. By the time I got to the slip, the transmission was slipping out of gear. I would need to do a hard rev in forward to get it to engage. Again, no problem in reverse.

So there is a tranny problem. Any suggestions on what the issue is?

BTW - prop looks fine. nothing wrapped around it. Eng is a Yanmar 3GM30F. Transmission is a Kanzaki KH3-P

[Sailmonkey]

Sounds like your transmission is coming out of gear, then re-engaging at speed, the shock makes the engine jump on its mounts.

You’ll need to pull the gear and repair or replace it.

[dmksails]

Do you know of any info online on how to go about doing that?

[dfelsent]

Check the shift linkage first. Could be a problem that leads to inadequate throw.

[Sailmonkey]

Originally Posted by dfelsent

Check the shift linkage first. Could be a problem that leads to inadequate throw.

Yes, try this first to be sure the lever is completely engaged., hopefully it’s this simple!!

But if not. There are literally tons of videos about removal of components from engines. Like the gear from a 3gm30.

[Matsubob]

I have a Yanmar 3GMF with the Kanzaki transmission. These are well designed sturdy units. They are simple, utilizing a massive actuator that has been machined to a cone shape on each end. It has a groove machined into its circumference and this is where the shifting mechanism rides, to move the actuator forward or back, effectively selecting direction, forward or reverse. When locating the shifting mechanism, the cone must be carefully be centered exactly between the two gears, then the shifting mechanism must be put in the center detent then the blade of the mechanism must be carefully placed into the groove and then the shifter mechanism plate must be bolted down. This is how the shifting mechanism is located in NEUTRAL position. If it gets moved in one direction or the other upon installation, the blade that rides in the groove of the cone will drag on one side, which will cause uneven wear on the blade and eventually cause it to wear down enough that there will be excessive play, which would not allow the cone to be positioned as far as needed into the receiving gear.
When I rebuilt mine, the blade was worn unevenly and I replaced it. This could be a contributing factor to the transmission "banging", because of the "slop" created.
That is only one possibility, however. The Kanzaki transmission is very easy to disassemble; one must keep in mind, however, that the output shaft is a LEFTHAND thread!! Turning the nut the other way will only tighten it more.
Nor is it heavy; I have pulled mine and done work on it, using the table in the main salon. Downloading the Yanmar shop manual is the surest way to "do it right"!

[senormechanico]

Check the motor mounts as well.

[roverhi]

If your transmission is toast, have a KM2P 2.2 transmission with less than 300 hours that I took off my 3GM, $750. Switched to a 2.6 reduction gear ratio transmission because of a change of prop. AFAIK, there is no difference in the spec's of the KM2 or KM3 other than the gear ratio. Only difference I found was the bracket attachment point for the shift cable and that was no biggy. As Matsubob said, the transimssions are light, less than 50lbs. US mail was less than $70 to get the new transmission to Hawaii. Would love to get it out of my garage.

[dmksails]

Had a mechanic on board to check out the linkage. He made some adjustment, but wasn't hopefully on the outcome. Suggested I run it up to temp while tied up and see what happens.

Today I did that. Warmed the engine, then put it in gear and ran at 2K rpms for 10 minutes forward, then 10 minutes back. No issues. Cycled between Fwd/Bac a few times - no problem. Ran it in FWD for another 10 minutes at 2K then backed down, switched to neutral than back to FWD. Nothing. Tried a few times - still nothing. Left in FWD and Rev'ed above 2500. The gearbox gave a loud thunk and was suddenly in gear.

I see a rebuild in my future.

Does anyone have a link to a place where I could by the parts for this gearbox. So far not having luck with google.

[roverhi]

There has to be a Yanmar dealer in Port Townsend. They should be able to get you the necessary parts when you've torn it down and figured out what you need.

[boatbod]

Cone drive transmissions can benefit from having the cones lapped. Since you are going to have to pull the unit apart anyway, I'd take a close look at the mating surfaces and evaluate if they are worn or just glazed. The latter can be fixed, the former we'll need parts.

[newhaul]

Check with the salvage yard in lynden they may have a good drop in replacement for cheap http://sailboatwreckingyard.com