low compression

[ebrown999]

I just purchased a 1974 mercury 500 thunderbolt ignition outboard. Compression readings are 160, 160, 40 (yes 40), and 160. Obviously the number 3 hole is dead. Starting with the low buck options, what should I do first??? I am VERY mechanically inclined so please don't offer retarded advice. From what I have read, this is one of the best rebuildable motors available and I have no problem throwing some money its way!!! The motor sat in a garage in the Pacific NW for almost 9 years and was supposedly running when parked. I am looking for sound advice not smart *** comments. I paid the guy $40 for the motor so I did not give up much for the chance at a solid motor that needs some work!!!

 

[JB]

Re: low compression

Hung valve? In a 2 stroke engine? Is that a troll?

Well, no matter. I would say most likely cause is broken ring. Could be a lot worse, but it is only one bad cylinder. You will need to tear it down to find out. If the cylinder is not scored it should be an easy rebuild.

Good luck.

 

[jason1150]

Re: low compression

try squirting some oil in #3 and see if the numbers come up, that will give you some idea? maybe, J

 

[ebrown999]

Re: low compression

Thank you both. I know a hung valve is a prayer but, LOL, anything is possible. The reality is I need to rebuild the power head and I feel this engine is worth rebuilding. Am I wrong???

 

[emckelvy]

Re: low compression

At 40 psi the rings are toast, guaranteed! The only way you'll know the full extent of repairs required is to remove and dismantle the powerhead.

If it were just a wee bit on the low side, it's of benefit to throw some carbon-cleaning chemicals at the innards. But, with compression that low it's a foregone conclusion.

Maybe you'll get lucky and it'll only require a re-ring and (probably) a piston.

Or, even if the cylinder is scored, it can be bored out and oversize piston/rings installed. Definitely a motor worth rebuilding if you do the work yourself.

Cheers and have a Happy Teardown!............ed

 

[ebrown999]

Re: low compression

Hung valve? In a 2 stroke engine? Is that a troll?

Well, no matter. I would say most likely cause is broken ring. Could be a lot worse, but it is only one bad cylinder. You will need to tear it down to find out. If the cylinder is not scored it should be an easy rebuild.

Good luck.

LOL-I deserved that. forgot 2 strokes don't have valves.

 

[ebrown999]

Re: low compression

Thanks for the advice. I am going to dig into it this weekend. I do have a repair manual so I will follow the directions closely. I know I can order parts through this website but I hate paying for shipping. Are the prices incuding shipping typically the same as buying locally? Also, what might I be looking for as far as damage to the piston? Do they always need to be changed or is it possible the piston is reuseable with a new set of rings.

One other thing of note. I saw on a website, maybe this one, some discussion for low buck outboard stands. I would suggest using an engine stand as most people who work on stuff have one. My outboard fits perfectly on the engine stand.

I also apologize if I offended some of the people offering advice. I have asked questions on other websites and gotten some of the most useless answers or been told that I was stupid for buying an older motor. Thanks to the responders on here who are not like that.

At 40 psi the rings are toast, guaranteed! The only way you'll know the full extent of repairs required is to remove and dismantle the powerhead.

If it were just a wee bit on the low side, it's of benefit to throw some carbon-cleaning chemicals at the innards. But, with compression that low it's a foregone conclusion.

Maybe you'll get lucky and it'll only require a re-ring and (probably) a piston.

Or, even if the cylinder is scored, it can be bored out and oversize piston/rings installed. Definitely a motor worth rebuilding if you do the work yourself.

Cheers and have a Happy Teardown!............ed

 

[ebrown999]

Re: low compression

I tried that before I bought it. Used every trick I know other than power punch but power punch could ultimately ruin the other cylinders if all doesn't go right.

try squirting some oil in #3 and see if the numbers come up, that will give you some idea? maybe, J

 

[Kev144]

Re: low compression

As said earlier, a tear down is in order. The other 3 cyl look great! For 40 bucks and maybe just maybe the cost of rings and good to go would be greatly worth it. Can't say for sure, you'll have to let us know!

Great Motor.

 

[ebrown999]

Re: low compression

Do Reed valves get hung??? after doing some research, I guess two strokes do have valves although they do not work the same way as conventional valves in a 4 stroke engine. However, is this a possibility? I fully expect a broken ring and I am beginning my teardown as soon as my digital camera is recharged so I can take pictures of everything as I go.

 

[hkeiner]

Re: low compression

Do Reed valves get hung???

I don't know what "hung" means but reeds can break or bend and become ineffective. However, even if they were to fail, this would not affect compression. It would only affect the ability of the piston to draw sufficient fuel into the cylinder during the intake stroke.

 

[ebrown999]

Re: low compression

yeah, MY camera is recharged and I will begin in ernest on Sat. still wondering about the total rebuild versus fixing the one bad cylinder. Should I worry to much? the compression is very good in the other three cylinders.

 

[Fuzzytbay]

Re: low compression

It gona be apart anyhow, I'd figure out the extra cost, most likely just need rings. So if it doesn't add too much extra money, why not change them.
Just my two bits on the subject.

 

[ebrown999]

Re: low compression-update

Re: low compression-update

Well I started the tear down and discovered I had the worst teardown manual made by Selco(?). It barely tells me anything. It came with the motor and I saw chilton on it but it is not very helpful. I have purchased a 3 part cd on ebay that is supposedly really good-the guy had a good seller rating and everyone raved about how good the cd's were. I also purchased a screw in lifting eye/flywheel pulling combo tool on ebay. Doesn't look like I'll have it apart this weekend but maybe next weekend once I get everything I need to get going. Any other suggestions on tools or products I am likely to need? I have a pretty complete set of tools for working on car engines (about $15,000 worth of snap on tools not including the $5,000 matco tool box) but not so sure for boat motors. I am also considering getting into the boat motor rebuilding business. There doesn't seem to be alot of peeps interested on working on the reliable old timers in the Pacific NW but we have so much water there is a huge market. any suggestions?

 

[ebsworj]

Re: low compression