1957 35HP Big Twin (hitting hard Knocking)

[bigballerg]

My 35hp 1957 Big Twin at idle or even WOT slams at times.... meaning, seems like I hit something, motor will JERK Hard then go like normal for a few minuets, then slam again etc... every once in a while??? What causes this? NOTE, last time I was in the water an it had these symptoms I lost my propeller... Could it just be a coincidence or are they related? I just reinstalled a new prop and don't want to go out there and have to lose another???

 

[RogersJetboat454]

Re: 1957 35HP Big Twin (hitting hard Knocking)

IMHO,
Sounds like something might be going on in your lower unit (bad dog clutches, or something binding). Have you drained the lower unit oil lately? If you haven't, drain it and see if it has metal particles present.

 

[bigballerg]

Re: 1957 35HP Big Twin (hitting hard Knocking)

I did about 3 mths ago and at the time everything looked ok. What is bad dog clutch? Is it easy to replace? How can you tell its "Bad"?

 

[nwcove]

Re: 1957 35HP Big Twin (hitting hard Knocking)

if the clutch dog is bad ,it "shouldnt" jump out of gear at idle, does it make a loud bang and rev up...or just jerk sideways with force? could be a lean sneeze??
heres a pic of a questionable clutch dog from a smaller motor, notice the shiny edge? its slightly rounded off, and should not be! this is usually caused by owners gently shifting in to gear, rather than firmly "snapping" into gear. (sorry for the blurry pic, but you get the idea)

 

[bigballerg]

Re: 1957 35HP Big Twin (hitting hard Knocking)

No it does NOT jump out of gear... Sideways with force for sure. I think it is high speed adjustment?

 

[nwcove]

Re: 1957 35HP Big Twin (hitting hard Knocking)

....quite curious as to how you lost your prop tho? no cotter pin?

 

[bigballerg]

Re: 1957 35HP Big Twin (hitting hard Knocking)

possibly with the jerking that was taking place. now im going to back out the high speed jet more than 1 turn.. possibly to 2 turns because I did do a fuel pump conversion from the pressure tanks that could possibly be supplying more gas than usual settings....

 

[RogersJetboat454]

Re: 1957 35HP Big Twin (hitting hard Knocking)

May be helpful if you made a video of it in action, and then post it here with Photobucket, or even Youtube as a host.

 

[HighTrim]

Re: 1957 35HP Big Twin (hitting hard Knocking)

You are describing classic symptoms of a worn clutch dog as stated.

The lobes have precisely machined edges, when you are out of adjustment, OR the operator likes to slowly ease the shifter into gear, the edges round. Then as you increase RPMS, they are forced apart, giving the sensation that you hit an underwater object, until the shift linkage pulls the dog and gear back together.

That would be a heck of a lean sneeze if that is the case. Possbile backfire?

 

[HighTrim]

Re: 1957 35HP Big Twin (hitting hard Knocking)

possibly with the jerking that was taking place. now im going to back out the high speed jet more than 1 turn.. possibly to 2 turns because I did do a fuel pump conversion from the pressure tanks that could possibly be supplying more gas than usual settings....

Not possible. The float controls the amoung of fuel in the bowl. Once it fills, the needle closes in its seat.

 

[nwcove]

Re: 1957 35HP Big Twin (hitting hard Knocking)

possibly with the jerking that was taking place. now im going to back out the high speed jet more than 1 turn.. possibly to 2 turns because I did do a fuel pump conversion from the pressure tanks that could possibly be supplying more gas than usual settings....

only move the needles in small increments and note the starting point. give the motor time to respond. if opening the needle 1/4 turn makes it worse, go back to the original setting, wait a few or more seconds, and close the needle 1/4 turn.

i agree with RJB, a short video with sound would be a great help!!

 

[bigballerg]

Re: 1957 35HP Big Twin (hitting hard Knocking)

No, not backfire... seems to be more in tune with what you are saying "Clutch Dog". Violent hit of the motor... well, I will read up on some posts about how to find and where to look for signs of bad clutch dog.. Unless you guys already have links

 

[bigballerg]

Re: 1957 35HP Big Twin (hitting hard Knocking)

I will try to get a video up....

 

[HighTrim]

Re: 1957 35HP Big Twin (hitting hard Knocking)

The only way to verify is to open up the skeg.

 

[RogersJetboat454]

Re: 1957 35HP Big Twin (hitting hard Knocking)

Item #24 in the picture is the clutch dog (rides on item 31 which is the prop shaft).

 

[nwcove]

Re: 1957 35HP Big Twin (hitting hard Knocking)

....still find it odd that it jumps out of gear at idle??? are you running it tiller or remote? maybe cable adjustment? shift rod coupler not in the detent? the handfull of experiences ive had with worn clutch dogs are with more modern rbms, and they would run fine at low rpms all day, but jump to neutral when you put a bit of load on the prop.

 

[bigballerg]

Re: 1957 35HP Big Twin (hitting hard Knocking)

No, I don't think it ever jumps out of gear... It just slams the motor ever once in a while. If its not the needle adjustments, then must be clutch dog. Unless Im not conveying my problem correctly.

 

[HighTrim]

Re: 1957 35HP Big Twin (hitting hard Knocking)

That slamming that you are hearing is likely it slamming back into gear bigballer after coming out of gear....It can come out of gear and lower RPMS if the dog is bad enough. If the linkage was faultly it wouldnt always pop back into gear. Cannot hurt to inspect thought before opening up skeg.

 

[lindy46]

Re: 1957 35HP Big Twin (hitting hard Knocking)

My '57 Lark did the same thing - every 30 seconds or so at high speed would jump like it hit an underwater obstruction. Never jumped out of gear. It was a bad clutch dog.

 

[kfa4303]

Re: 1957 35HP Big Twin (hitting hard Knocking)

Nothing worse than a motor that can run, but won't go anywhere. Sounds like a bad clutch dog to me too. Lord knows, I've had to deal with my fair share of them:/ Thankfully, it's relatively cheap and easy to fix, and afterwards she'll run like a champ for another 50+ years. Ask around here in the forums and or place a WTB ad on aomci.org. Ebay has them too, from time to time. It's kind of amazing that all the torque in these old motor depends on those two little lobes/dogs on the clutch. Most modern versions have 3 or more dogs to prevent wear.