235 1978 low compression on one hole

[jy118lfd]

My 78 235 crossflow has low compression on the starboard bottem hole (no. 6) I pulled the head and the ring is broke It did not break the ring land off the piston just bent the top a little bit. I geuss I caught it in time. The cylinder has a small scuff in it. I was wonering if I could just touch hone that cylinder and replace the piston as the other 5 holes are at 125psi. I want to get it back in the water asap. The motor is already .020 over. What causes this? It looks like the ring caught the exaust port and possibly went right out the port as there are no dings on the top of the piston or the head I just want to ensure that it does not happen again. P.S. I am going to bore and add 6 new pistons over the winter but if I can get by just honeing and installing the new piston I would love that. We do this in my auto shop all the time with great results just as long as the cylinder cleans up with a light hone. I just dont know how it will work in a two stroke?

 

[Hooty]

Re: 235 1978 low compression on one hole

You can almost bet the farm the reason the ring busted is because it's coked up. It's caused by high combustion temperatures due to lugging the engine and it usually happens on the starboard side. You may also find the other cylinders are out of spec due to high combustion temps. So sayeth DHadley. If it was me, I'd go ahead and re-ring all six cylinders while you're in there. If you don't, you're just asking for more broke rings and scored cylinder walls.<br />When you get her back up and runnin', prop it so that with a medium load the engine spins up to 5800-6000rpm at wfo. This'll keep it from coking up so bad. Also, do a good decarb job on it every year.<br /><br />c/6<br />Hooty

 

[jy118lfd]

Re: 235 1978 low compression on one hole

Should I bore the whole .030 or just touch hone and re ring the 5 that are good and replace the bad piston

 

[Hooty]

Re: 235 1978 low compression on one hole

If a hone will clean it up and stay in spec, go for it but be sure and check all six.<br /><br />c/6<br />Hooty

 

[Dhadley]

Re: 235 1978 low compression on one hole

I'd almost bet that once you hit it with the dial bore gauge you'll find its out of spec. Probably on all 6. Its just a result of the higher combustion temps. I'd also bet its set up in the low 5000 range.<br /><br />The coking usually shows up on the starboard (1-3-5) side first.

 

[jy118lfd]

Re: 235 1978 low compression on one hole

Dhadley you are right on it was propped with a 14 1/4 22 that made it run 60 mph at 5100. The cylinders are carboned up I am going to rebuild it should I just bore and install .030 pistons. Also what rpms should I set it up for?Will the higher revs change reliabilty?

 

[jy118lfd]

Re: 235 1978 low compression on one hole

What else can be done to stop the carbon problem? I also want to know how reliable are these motors, properly set up how many hours can you get out of one?

 

[Hotrods]

Re: 235 1978 low compression on one hole

I feel your pain...As you already know mine. but Im happy Im doing all 4 of mine..fresh start...why take a chance.. you''ll have the PW out anyway..My shop charged 40$ a hole and your lookin at 85$ a piston and ring set. EEKK that is 650$ for 5 holes I may change my mind..

 

[jy118lfd]

Re: 235 1978 low compression on one hole

I found the pistons for 66.85 each with rings and thats probably what I am going to do. My uncle's friend own a marine machine shop and said he would bore and hone for me for free so Thats what I am doing. I just want to ensue that it does not happen again. I cant wait to get it don and hear it scream again!!

 

[Hooty]

Re: 235 1978 low compression on one hole

If it was mine, I'd set it up to turn 5800-6000rpm. How many hours? With proper care and maintainance, a thousand or so.<br /><br />c/6<br />Hooty