Compression results

Joker2

Joker2

Junior Member
Did comp. test this morning with a brand new screw in type tester. Had to do it with the engine cold, I can't start it. I got 120 lbs. on all three starboard side cyl. Had 100 lbs. on all three port side cyl. Also this is the 3rd time I found black buildup on the lower starboard side plug. I just changed the plugs last week, only ran motor about 2 hours since. I will try seperate fuel, I think those numbers were like that since I got boat, I feel yesterday was fuel related, just wanted to check comp. Is there a problem with these comp. numbers ?
 
I'm scratching my head. Even compression across all cylinders is best. What motor? It should run with 100 lbs of compression. Is it possible that one of the cylinder heads was replaced at some point or maybe surfaced? Either could mean different cc combustion chambers. If that be the case an easy fix would be to have the lower side head cut to make them more even or just find a matching pair. Being 20lbs off from port to starboard seems a little much. All my Mercs and Johnnyrudes are pretty even. +/- 5 lbs or less If memory serves the 90 Johnson only had 100 lbs after break-in with new rings and pistons. That was over a decade ago.. Sharper minds will chime in soon.
 
I'm with Ridgerunner. Sounds like the motor had a rebuild at some point and either one bank was decked or the head was milled. That is the only thing I can think of that would explain such variation between banks when the variation between the bank is +/- 5%. Keep us posted as to what happens.
 
Being this is a 60 degree looper, the 120 sounds alot closer than the 100 reading. I would expect to see around 110-115 out of it. I believe they are finger port engines which makes them slightly higher compression. 20 pounds difference from one side to the other is a bit excessive, but doesn't really surprise me since you've said this engine was recently rebuilt.
 
Any ideas on what a 93 200 Yamaha 2-stroke should be at for comp. When I did mine it was 115-120 on all 6 cylinders. It was a little rough the first time I started it and let her idle tell warm after that all good. She run like a top at all speeds with and without a load.
 
Redneck said:
Any ideas on what a 93 200 Yamaha 2-stroke should be at for comp. When I did mine it was 115-120 on all 6 cylinders. It was a little rough the first time I started it and let her idle tell warm after that all good. She run like a top at all speeds with and without a load.
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I was going to say 110, so it sounds like yours is in good shape.
 
THEFERMANATOR said:
I was going to say 110, so it sounds like yours is in good shape.
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Good to hear now to keep it that way. Looks like the guy who owned befor me took pride in his toys and had some $$$. everything he did to her wasn't cheep stuff of poorly done installation jobs.
Thanks again for the info.
 
Yes I purchased the engine newly rebuilt, in Sep, 2010 I was told that when he got the engine in off of another boat he opened it up there was nothing wrong, he changed the crank and rebuilt it anyway, but I have a problem trusting this guy having the problems I have had so far, I know he just slapped used parts on it to get it working, it has been running O. K. felt like I was a little week on power though. My past posts have told about my RPM / MPH issues. I don't really know what to do next ? Thanks for everybodys responses.
 
normally there are two different head gaskets available for Johnrudes, ones thicker than the other to drop compression for commercial use, some one could have replaced a head gasket at some point
 
never mind, I see thats its a 60 degree motor, I think they use orings. Check the casting numbers on the head and make sure they match
 
I went down today with fresh gas, checked in-line filter inside motor, looked good,checked all plugs, sprayed them with carb cleaner, disconnected fuel line from engine side of primer ball and pumped a gallon into a bucket, it looked perfect, hooked line up to 6 gall. tank, it started right up using the fast idle lever, let it run for about 1 min. RPM's showing about 1500, pushed the fast idle lever down, she kicked like a mule and shut, couldn't get it started again. I don't know, maybe the fuel pump ? any suggestions ?
 
If it were the fuel pump you ought to be able to mash the primer ball hard enough to fill the fuel bowls and keep it going. I lean toward fuel because it died off when you decreased the fast idle. My motor did something similar a little while back. It would idle for a while and shut off but the primer ball didn't feel right. It was kinda squishy and never felt firm like it does when the needle is in the seat. I didn't discover the primer ball was bad until after I took the fuel pump off inspected it and put it all back together. Went to the local marina and purchased a primer ball, installed it in minutes and cured the issue. The old one had a faulty check valve even though it would still move some gas when tested. I left the old one in the splashwell, it hardened up like a piece of plastic. I say ethanol finished it off as it was nearly 10 years old.
Hope you don't mind but I got a question for the gurus about your motor. Assuming all the jetting is the same in the carbs wouldn't having one side lower by 20 psi cause a bit of a nightmare on tuning it to idle smooth? Is there idle air adjustments or something?
 
I would be curious to know the answer to that also ! My whole fuel system I installed last year, all new except the tanks, which i cleaned with pebbles and acetone. Water seperator and all lines including primer ball, new, fuel shut off valves, gas fills, vents. I don't know if the ethanol could work that fast, could it ?
 
I don't have the answer to the ethanol question. I am running non when I can get it. Joker, when the motor died did you try the primer to see if it was still firm?
 
No, I didn't, I am thinking about checking the fuel pump tomorrow, but I would rather not do that yet because I have no trailer and I have to work on this in the water on my mooring ball, I don't even know if I will be able to take off the lower shroud of the engine, it's going to be a nightmare I think. There are no mechanics that will come out to work on the boat on a mooring around here. I would like to exhaust all other avenues before I dive into taking off the fuel pump. Any suggestions ?
 
Went down today, got it to run with portable tank and my built in tank, same results, I don't think it's fuel. I can run it idling it using the throttle linkage at engine, keeping it at 800-1,000 RPM's when I push it close to 1,500 she kicks and shuts down, she is also running a little rough. After I was done running it I inspected a few plugs, they all looked normal except one, the bottom starboard plug was clean and shiny just how I left it after the last time I was there when I cleaned it. That doesn't seem right. Like I said from previous posts that plug was the one that always had black buildup on it whenever I changed plugs, yet the compression was 120lbs. on that cylinder. Didn't have time to check for spark, maybe that's the problem, waht do you think ?
 
I found what I think to be my problem today, Had no spark on the # 1 cyl. tested the 0583740 ignition coil for # 1& 2 on #3 & 5 cyl. had no spark on # 3, bad coil pack. How common is it for these to go bad, I am thinking about changing all 3 ( only $30.00 each )?
0583740 IGNITION COIL
 
if it was one bank(one side of the engine ) that had no spark, its your power pack, if its just one cylinder, or various cylinders on either side, it could be coils and or power pack. The ground strap on the coils are subject to corrosion, you can remove the cols, clean the strap and the coil mounting surface, if the spark comes back, you've found your problems. If the power packs are old, I'd replace them both just because
 
Joker2 said:
I found what I think to be my problem today, Had no spark on the # 1 cyl. tested the 0583740 ignition coil for # 1& 2 on #3 & 5 cyl. had no spark on # 3, bad coil pack. How common is it for these to go bad, I am thinking about changing all 3 ( only $30.00 each )?
0583740 IGNITION COIL
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While I had my old 86 Rude, I found that coils and power packs go out more often than college kids...ask me how I know...
 
It was just the left side cyl. of the top coil (# 1 ) that had no spark, I swapped out the coil with the coil underneath ( # 3 & 4 ) and had spark on #1 with different coil but still no spark on the original one on the left side, now # 3
 
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