74 Chrysler 55hp, motor knock

[Sheetsfa]

I have a deep knock in my engine. It mostly does it when I let off the throttle, when it knocks it actually causes the motor to twist a bit. Is this something I should be concerned about?

 

[tater76]

Re: 74 Chrysler 55hp, motor knock

How is your lower unit? clean gear lube? I am just thinking start simple, check to see if the motor is hanging up somewhere in the drive? Is your fuel old? Worst case is your rod cap is coming loose and smacking the case, ie, the knocking sound. I know this is silly, but you are running premix fuel right?

 

[Sheetsfa]

Re: 74 Chrysler 55hp, motor knock

Yes I am running premixed fuel, the fuel is old I will go ahead and change it out with some fresh gas and oil mix and just cross my fingers, the lower unit gear oil is new. I am leaning towards the rod cap though considering how loud the knock is. I definitely agree to start simple. Thanks and I'll let you know how it goes.

 

[tater76]

Re: 74 Chrysler 55hp, motor knock

If you do indeed think its the rod cap coming loose don't run it anymore. A $45.00 powerhead seal kit is much cheaper than finding a new case. Pull the powerhead, split the case, and you will know for sure. My 73 55hp Chrysler took a beating when the upper crank bearing fell apart. I was very lucky in the minimal damage dept. I got a few scratches in my cylinders, and some wicked dents here and there. However, like I said I was lucky. I recently picked up an 85 50hp force parts motor for its electric ignition. It was a complete motor, and looked clean. The guy said it had a thrown rod from his buddy running it without premix. I inspected the case as he said it was good, and in his dark shop found no issues. When I got it home and removed all of the electronics I found two small holes in the case. It looked as if someone had shot a 30/06 from the inside out! Don't be afraid to do this work yourself, I just completed a rebuild of my 73 in two weeks. I got a manual, alot of help from this site, seal kit, rings, etc.. and a few basic hand tools besides a torque wrench. It runs like brand new, and has much more get up and go now. If you are lucky enough to just need to torque the caps back down, pull them, inspect bearings, careful they will fall out upon removal so be ready to catch every one, notice the orientation of the cages upon removal and install, check the crank bearing surface. If everything looks clean, smooth, and shiny put her back together! Rod caps are torqued to 170 inch pounds, and don't forget to apply locktight to them. Let us know what you find

 

[Sheetsfa]

Re: 74 Chrysler 55hp, motor knock

Thanks for your help, I have also found out that a knock when the motor decelerates could very possibly be from old fuel especially since it doesn't do it on acceleration. I will let you know the results tomorrow evening when I get a chance to change it out. How much did it cost you for your rebuild and where did you get your parts at? Thanks again for you response and help.

 

[tater76]

Re: 74 Chrysler 55hp, motor knock

Under load does it knock? old low octane fuel may play a part in it. My rebuild consisted of new rings from Iboats, powerhead gasket kit from Iboats, no new pistons, or bearings, and no machine work. I took my block into a marine shop for inspection. They measured the bore and piston, said it was okay to go back with stock sized rings and pistons (even with some scoring?). I honed the cylinders with an autozone borrowed hone, cleaned everything spotless, installed new rings, installed new/used 50 force piston in upper cylinder. I had 120psi on both cylinders after honing/rering. I am creeping up on 140 after 1/2hr of run time Total cost including locktite, gasket maker, degreaser, gaseket kit, rings, hmmmm.... maybe $150? Although, I had to replace the top crank bearing $45 bucks at a machine shop, hplus $30 new bearing Oh, and $100 for the 85 force 50hp parts donor. Seriously, it was not too bad for what you get. It took me less than 3 weeks to complete, and most of that was waiting for parts/answers You can do it!

 

[Sheetsfa]

Re: 74 Chrysler 55hp, motor knock

Thanks once again for your help. No it does not knock under a load or at idle it only does it when I decelerate. I have not had it under a good load yet I have only had it running in a trash in neutral. I was reading on a envinrude knock issue here on boats and some of the replies on there said that old fuel is a cause of a knock during deaceleration so hopefully that's the problem if not then I guess I will be tearing down the motor and doing like you did. I enjoy having projects like that anyway Did the electronic ignition make a difference on the performance of your motor?

 

[tater76]

Re: 74 Chrysler 55hp, motor knock

I hope that the fuel is the issue, thats an easy fix The electric ignition makes a little bit of a difference, but not enough to brag about. My main reason for swapping it was for its capability to recharge the battery. The magneto/points ign doesn't do this. It does seem to idle and start much smoother now?

 

[Sheetsfa]

Re: 74 Chrysler 55hp, motor knock

I will definitely have to look into the electronic ignition. What octane fuel do you run? I live in Colorado and the elevation where I usually go is 6000+ ft and the other place I will go with it is around 10000+/- ft and I know that on a carbureuted car motor it makes a big difference and most of the time have to be rejetted for the elevation.

 

[tater76]

Re: 74 Chrysler 55hp, motor knock

I run premium in mine, I think its 91 octane. I don't have much experience in having to rejett due to elevation, I do most of my boating at around 2500-3500ft. Although, I am planning a trip to Jackson lake in Wyoming this summer, and I think its much higher. Now you got me thinking? What part of Colorado are you from? my wife is from Montrose.

 

[Sheetsfa]

Re: 74 Chrysler 55hp, motor knock

The military brought me here last August, I'm from Virginia. I'm not familiar with Montrose what part of the state is that? I haven't had a chance to run new fuel through yet but I did go ahead and replace the fuel line and drained the carb. I'm hoping to pull it out of the garage today and get it running, and hopefully take it to the lake this weekend for a test run. I'll let you know how the elevation does on it. The boat was from Arizona so if the elevation doesn't mess with mine I would say you will be fine.

 

[tater76]

Re: 74 Chrysler 55hp, motor knock

You might check the bottom of the tank for junk. I just emptied my old gas, about 1/2gal, out of the tank and found some small particles. They were definitely big enough to clog a jet. Montrose is sw colorado

 

[Sheetsfa]

Re: 74 Chrysler 55hp, motor knock

Well the new fuel didn't fix the knock if you don't mind I would love to pick your brain on the tear down so I can get to the rod caps. My number is 915-929-1742. thanks