I may have used the incorrect term but the idea of backlash is the amount of "gear slop" between the output of the engine and the output of the transmission. There is a dampener in the bell housing that has springs and those springs cushion the changes in torque from the engine output. The flywheel is not heavy enough to appreciably smooth the output torque. Over time the springs in the dampener can become week or break allowing the 2 parts of the dampener to bottom out on one another. You may only notice that the engine bangs into gear a little harsh.
When the engine is running at a normal rpm the counter torque created by the compression of one cylinder is offset by the ignition of an adjacent and since there are three cylinders each is 120° from the other and a relatively smooth operation can be achieved. If a misfire occurs the counter torque will have a more significant effect, momentarily slowing the shaft until a total of 240° of rotation is achieved before the next ignition. In reality since these are 4 stroke engines every cylinder has to travel up and down twice for each ignition so instead of being simply 120° away from the next ignition it is actually 240° on a normal cycle and when you have a misfire it pushes to 480°. This is exacerbated at lower rpm where there is less inertia to carry the engine through the compression stroke. Compound a worn or broken dampener with an irregular misfire or a low RPM and suddenly a pretty harsh condition presents. Hope that explains it well enough. Josh Muckley S/V Sea Hawk 1989 C&C 37+ Solomons, MD On Fri, Jun 1, 2018, 8:21 AM Kevin Paxton via CnC-List < cnc-list@cnc-list.com> wrote: > Thanks John. I'll definitely have to look at that next time we are out. > Not sure if it's related but our RPM's don't get up to the manuals > recommended range either. I'm not sure if it's the Tach isn't reading > right, or the engine itself just won't get up there. I can only really get > it up to maybe ~2200-2400 rpms according to tach. I want to say the manual > says to get it up to 3k? > > I did get down on Wednesday to try it out in the slip with more fuel. I > first ran it without engaging the prop at ~1800-2000 rpms (according to the > tach) for a good 5-10 mins and didn't hear any abnormal noises. Then I > engaged the prop and ran again at ~1800 rpms for another 10-15 mins and > again no abnormal noises. So maybe it was just dirty/low fuel with all the > rocking and rolling around stirring it up. > > I did notice at higher rpms the vibration was minimal, at lower rpms it > does seem to vibrate quite a bit. I'm assuming that is normal. > > Josh, What do you mean by backlash? Just a large jump or something to > cause more vibration? > > Thanks everyone, > Kevin Paxton > '82 34 #473 > Japhys Spirit > Cornfield Creek, Pasadena, MD > > On Sun, May 27, 2018 at 12:05 PM John and Maryann Read via CnC-List < > cnc-list@cnc-list.com> wrote: > >> Have Yanmar 3GM and Kanzaki transmission. Had similar issue of only one >> blade opening after sailing, boat shaking and continuing problem after >> trying varying speeds, reverse / forward etc. Worked fine if starting from >> dead stop. Not excessive wear on blade pins. In our case turned out to be >> worn / polished cones in transmission. Seems an earlier rebuild was not >> done correctly, resulting in shaft not getting fully engaged. Diagnosed by >> watching prop shaft as crew engaged in gear at idle, then slowly increase >> throttle. Shaft rpm’s did not increase corresponding to engine. Would >> occasionally “grab” when higher engine rpm’s were applied. Pulled >> transmission (not that hard), sent to expert on Yanmar / Kanzaki >> transmissions (VERY important), had the unit back in a week, reinstalled >> all is well. Some pointers picked up along the way. 1) In our case the >> fitting attaching the shifting cable to the shift lever on the transmission >> is a double spring push/pull fitting which must be adjusted so when in >> either forward or reverse there is additional movement possible on the >> binnacle. We attach the fitting to the inner hole of the lever. >> Lubricating spring is normal maintenance each year. 2) The mounts are OK >> provided you can get a finger between the base and top plates. 3) Check >> alignment >> >> >> >> Hope this helps >> >> >> >> John and Maryann >> >> Legacy III >> >> 1982 C&C 34 >> >> Noank, CT >> >> >> >> *From:* CnC-List [mailto:cnc-list-boun...@cnc-list.com] *On Behalf Of *Kevin >> Paxton via CnC-List >> >> >> *Sent:* Sunday, May 27, 2018 10:46 AM >> *To:* cnc-list@cnc-list.com >> *Cc:* Kevin Paxton >> >> *Subject:* Re: Stus-List 3GMF knocking sound >> >> >> >> Pretty sure it's a martec, folding 2-blade. I try to power up and it >> vibrates like crazy. I power down to try and get it to reset so that I >> don't damage anything with it vibrating so bad. Then I try again. If I >> rotate the shaft manually I can hear each blade folding open but for some >> reason it is tough to open it after sailing for a bit. >> >> >> >> Coming out of the slip it opens without a problem. >> >> >> >> The sounds came this time though while already under power and the prop >> was open. >> >> On Sun, May 27, 2018, 10:27 AM Gary Nylander via CnC-List < >> cnc-list@cnc-list.com> wrote: >> >> I’m curious. What kind of prop do you have that requires you to slow down >> to get it to open. Most folding ones ask for more speed to get the blades >> to open. Non opening would produce lots of vibration, which would get lots >> of noise as the engine bounces around on (particularly) old motor mounts. >> Shake the motor. >> >> >> >> Gary >> >> Flex-o-fold geared >> >> >> >> *From:* CnC-List <cnc-list-boun...@cnc-list.com> *On Behalf Of *Kevin >> Paxton via CnC-List >> *Sent:* Saturday, May 26, 2018 8:05 PM >> *To:* cnc-list@cnc-list.com >> *Cc:* Kevin Paxton <kpax...@paxdesigns.com> >> *Subject:* Re: Stus-List 3GMF knocking sound >> >> >> >> Yea I did that today and started it up and it sounded fine. However I >> didn't put it in gear or take it out of the slip. The racor was full and I >> didn't see any water in it either. >> >> >> >> Now it does vibrate pretty good under operation. I looked at the engine >> mounts though and none seemed to be cracked or anything. What should the >> mounts look/feel like? How much vibration while running should it have? >> >> On Sat, May 26, 2018, 6:50 PM Josh Muckley via CnC-List < >> cnc-list@cnc-list.com> wrote: >> >> Kevin, >> >> >> >> Fill the fuel tank with 6 to 10 gallons of fresh fuel, try it out, and >> get back to us. Check your racor filter for air. Low fuel/air in the >> line/water in the line can all cause rough operation. These Yanmars have a >> lot of backlash in the engine and when you get rough operations the clutch >> plate springs can bang. If you have a loose/broken/soft engine mount it >> can allow the engine to bang around pretty good too. >> >> >> >> Josh Muckley >> >> S/V Sea Hawk >> >> 1989 C&C 37+ >> >> Solomons, MD >> >> >> >> >> >> On Sat, May 26, 2018, 2:30 PM Kevin Paxton via CnC-List < >> cnc-list@cnc-list.com> wrote: >> >> Hey Everyone, >> >> >> >> Well we were lucky enough to be able to take out our 34 this past >> Wednesday for the Blue Angels show in Annapolis. On our way back our Yanmar >> 3GMF would occasionally make a knocking sound. I'm hoping you guys might >> know what could have caused it. >> >> >> >> When it happened: >> >> * We were about to enter into the mouth of the Magothy and I was taking >> down the main. My wife had the helm and yelled up saying that something >> didn't sound right with the engine. She throttled down and when I got back >> everything sounded ok. She said that it sounded like it was going to die. >> Like it was running out of fuel. >> >> * We kept it running slowly for a bit without issue. Then I tried to >> speed us up again. After about 5-10 mins it started making the noise again. >> To me it sounded like a loud knocking or banging sound. I immediately >> reduced power again and it immediately went away. >> >> >> >> Conditions: >> >> * At the time it started occurring we had about 1/8 of a tank of fuel. >> >> * Heavy, choppy waves for a large portion of our trip due to lots of >> power boats. However, at the time it occurred, seas were calm. >> >> * At start up after we sailed for a bit the prop had trouble opening and >> I had to reduce power a couple of times to get it to open. >> >> >> >> Observations: >> >> * I didn't see anything incorrect or worrysome during operation. >> >> * No leaking oil, fuel, or water. >> >> * Racor water separator doesn't show any water in the bowl. A little gunk >> in the bottom but not much. >> >> * When power was reduced, everything sounded normal and operated normal. >> >> >> >> Thoughts/Questions: >> >> * Could low fuel/dirty fuel/water in fuel make this happen? >> >> * Could this be more of a vibration from the prop not opening properly? >> If that's the case, could it be indicative of something in the strut or >> cutlass bearing? We have in the past had A LOT of problems with the prop >> not opening. >> >> * I did notice that during operation on the way back that occasionally >> the undulation of the waves caused some stress. I'm assuming just because >> of stresses on the prop. >> >> >> >> Today I filled up the tank and started it up in the slip. It started >> immediately and didn't sound like anything was wrong. I brought the >> throttle up and down and everything sounds absolutely fine. So I'm at a bit >> of a loss as to what to check/do. I do know that I still need to change the >> oil and I am planning on changing the fuel filters as well. >> >> >> >> I did notice that I am able to see the notch in the prop shaft where it >> meets the gearbox. There does not seem to be anything on the shaft that >> would indicate that it shifted at all though. Is this normal? >> >> >> >> Would love some help from some of the yanmar experts on here please. >> >> >> >> Kevin Paxton >> >> '82 34 #473 >> >> Japhys Spirit >> >> Cornfield Creek, Pasadena, MD >> >> _______________________________________________ >> >> Thanks everyone for supporting this list with your contributions. Each >> and every one is greatly appreciated. If you want to support the list - >> use PayPal to send contribution -- https://www.paypal.me/stumurray >> >> _______________________________________________ >> >> Thanks everyone for supporting this list with your contributions. Each >> and every one is greatly appreciated. If you want to support the list - >> use PayPal to send contribution -- https://www.paypal.me/stumurray >> >> _______________________________________________ >> >> Thanks everyone for supporting this list with your contributions. Each >> and every one is greatly appreciated. If you want to support the list - >> use PayPal to send contribution -- https://www.paypal.me/stumurray >> >> _______________________________________________ >> >> Thanks everyone for supporting this list with your contributions. Each >> and every one is greatly appreciated. If you want to support the list - >> use PayPal to send contribution -- https://www.paypal.me/stumurray >> >> _______________________________________________ > > Thanks everyone for supporting this list with your contributions. Each > and every one is greatly appreciated. If you want to support the list - > use PayPal to send contribution -- https://www.paypal.me/stumurray > >
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