Low Compression, Low Fuel Flow, Recent Rebuild

[DCSLADE]

I have a 1973 50HP 2 cyl Evinrude.
Brief history:
Motor has been in the family since new.
Had a piston fail last summer which blew a hole through the side of the block. Despite the repeated advice to drop it off the back of the boat I decided to rebuild. I found a used block with a good piston and decent cylinder walls which I honed. I replaced rings, many bearings, all seals, gaskets, etc. To my delite the engine started up in the spring, had good compression in both cylinders, and ran well (better than I can ever remember) up until last week.
I took it out after sitting for about three weeks and within 5 minutes the bottom cylinder began misfiring. This grew steadily worse until it I could feel the second cylinder kick-in only every now and again.
Brought it home to check things out. Good spark but low compression in the bottom cylinder. (I dont have a compression tester but it is obvious by turning the flywheel) It starts and runs ok in the test tank and both cylinders fire.
Took it back out on the water with the same results except this time I noticed that at full throttle the engine will cycle from 2 cylinders firing to 1 firing every 10 seconds or so. At very low speed I can get both to fire with only an occasional misfire which tells me there is a fuel flow problem. The fuel pump is driven off the bottom cylinder and my thought is that if I fix the compression problem the fuel flow problem may correct itself.
So the real question is why would I suddenly loose compression in an engine with only about 10 hr after rebuild? Could the rings be stuck?
Thanks for the advice.
(Sorry for the long story)

 

[Silvertip]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

A compression test is essential here. If the cylinder has again gone bad, its probably due to the same thing that caused it to go bad on the other engine -- lean mixture due to gummed up carbs. When rebuilding a damaged engine it is essential to determine what caused it to fail. Unless you do that, the same cause and effect occurs. Pull the head and see if there is evidence of cylinder scoring, ring breakage, detonation, etc.

 

[DCSLADE]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

A compression test is essential here. If the cylinder has again gone bad, its probably due to the same thing that caused it to go bad on the other engine -- lean mixture due to gummed up carbs. When rebuilding a damaged engine it is essential to determine what caused it to fail. Unless you do that, the same cause and effect occurs. Pull the head and see if there is evidence of cylinder scoring, ring breakage, detonation, etc.

I will get real numbers on the compression within the next couple days. As for the original failure, a large chunk of the piston casting (near the wrist pin) physically broke off, I assumed due to metal fatigue. I did clean the carbs at the time of the rebuild but did not do a complete carb rebuild. Thanks

 

[DCSLADE]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

Very bad news. I pulled the head and found the bottom piston and head to be severely pitted and bright silver. The top looks great and measured 130 on a compression test. What happened?

 

[Silvertip]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

Detonation! Can be caused by lean fuel mixture. Carb rebuilding is first step. Timing too far advanced at WOT is another. Wrong heat range plug, is another. Overheating is another.

 

[DCSLADE]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

I have the powerhead pulled and will be cracking it open tonight. I have a new 30 over piston that I will put in but will need to get it bored out. What should I expect to pay for that?
I will definately rebuild the carbs on this go around. Now that I think of it the bottom carb was far less responsive when I set the low speed adjustment. Should have suspected something then.
I had no means to adjust the timing so this could be it also. Is there any way without a test wheel?
Plugs are per spec and I cleaned all water passages and replaced impeller on the last rebuild. I don't see any other signs of overheating.
Thanks for your advice.

 

[Dhadley]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

Rebuilding the carbs is a standard part of every rebuild as is a cooling system overhaul. Now you know why.

The #1 most over looked part of a rebuild is to find and correct the cause of the original problem. In your first post you mention the piston failed. I'd bet it did not fail but was damaged because of something else. A damaged piston is rarely the cause but rather the part that's affected.

Find and fix the cause, don't simply fix the affected part.

 

[DCSLADE]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

Any help figuring out what caused the original failure would be greatly appreciated. From the beginning:
I inherited the motor from my family. Its always been tempermental when starting but ran pretty well otherwise. This was the story for the first 32 or so. Last spring I had the motor tuned up by a local shop and it ran ok. After a couple days back on the water I was running WOT and boom.. big hole in the side of the block, lower cylinder. Thought I had thrown a rod but it wasn't until I tore it down that I saw a chunk (size of a grape) missing from the piston casting where the wrist pin passes through. The pin was still firmly in place so it would turn over and even run on the top cyclinder. I choked it up to metal fatigue. I did notice right after the failure that both plugs had not been tightened properly when the tune up was done but couldn't see how that could have caused the failure. There was also 1 screw missing in the carb air intake box. I thought maybe it got sucked in, jammed up and caused excessive force on the piston, etc, etc. At any rate it seemed like the original failure was a 1 time freak thing. Could I have missed something?
Thanks again, I need to get it right this time.

 

[DCSLADE]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

Well, I pulled the block apart and found a broken ring in the bottom piston, at least what was left of it. The ring on the plug side was still intact. It appears from the condition of the top of the piston that the other pieces bounced around a while and then got ejected out the exhaust. The head is also pitted but I think it is still usable.
Incidently, when the piston failed last summer (read above) all the rings appeared to be fine.
So... What can cause a new ring to break? (I looked closely and could not see a place where it snagged on a port.) Could this and the original failure be related?

 

[DCSLADE]

Re: Low Compression, Low Fuel Flow, Recent Rebuild

What can cause a new ring to fail within first few hours of operation?
I can't afford to make the same mistake twice.
Thanks for any input.