Is Compression ok on my Motor?

[y2jericho]

Mercury 1970 650cc Thunderbolt... 65hp<br /><br />1. 98psi<br />2. 95psi<br />3. 100psi<br />4. 94psi<br /><br />Or there abouts don't remember but it was within 6 foru sure.. Mechanic told me that the compression on a motor like this one is fine because older motors had much less compression and its fine and i will still have it for many years to come ... <br /><br />Its a straight 4 not a v4 <br /><br />Thanks again

 

[y2jericho]

Re: Is Compression ok on my Motor?

Also leaded is the correct fuel for this motor right? And what 2 stroke oil do you recommend i am currently using MOBIL 2 stroke outboard oil .. It is a greenish/blackish colour<br /><br />Thanks

 

[JB]

Re: Is Compression ok on my Motor?

G'Day Y2Jericho.<br /><br />You don't say what conditions were when you got those numbers, but they seem okay to me. <br /><br />You might try decarbing your engine. Carbon in the ring grooves will decrease compression numbers.<br /><br />Most important, in a 2 stroke engine the numbers that matter are the differences between high and low cylinders, not the psi measured.<br /><br />Next: Unleaded fuel is preferred for 2 strokes if available in proper octane. By the system you use, you want 91 octane unleaded. Some recommend premium for Merc in-line engines, I don't.<br /><br />Good luck.

 

[y2jericho]

Re: Is Compression ok on my Motor?

but what if compression was 60 65 60 65 on the 4 cylinders ... The engine wouldn't work properly right? I read somewhere on here that a normal petrol motor needs at least 70 psi to work ok.<br /><br />Would you recommend Premium Unleaded over Normal Unleaded used in this motor or should i use a petrol such as Shell Optimax which costs about 20 cents more per litre?<br /><br />Is the Mobil Outboard Oil ok to use with this motor or are there better brands? As this one seems to be coming from a good brandin Australia and isn't that expensive about $13 US for 4 litres<br /><br />Thanks heaps iboat members again

 

[y2jericho]

Re: Is Compression ok on my Motor?

Also how would i de carbonize the motor as its been used for 10-20 years without carbonizing or anything really im doubtful its even been serviced apart from my recent services. I read about seafoam and the such etc but i still don't know how it works/would fix compression and how you actually do it. And possibly would it be available in Australia<br /><br />Thanks again

 

[rickdb1boat]

Re: Is Compression ok on my Motor?

Where do I start?<br />If the compression was 60 on all cylinders, then yes, it would run lousy. If it ran at all! A good Motor has compression of at least 100 on all cylinders, but an older one may very well operate properly in the 90's, as long as they are within 10% of each other. Regular unleaded fuel is what you should be running! Any outboard oil that has a TCW-3 rating is fine. You would decarb the engine by spraying seafoam or similar product into the carbs while it's running, which will in turn inject it into the cylinders. This will remove deposits from the pistons and rings that may be hampering full compression. It works! Especially on older engines! It is available at many auto parts stores and Marine suppy houses. I'm sure you can find it somewhere and if not, it can be ordered through many catalogs. Good Luck,

 

[andrew flavin]

Re: Is Compression ok on my Motor?

i just rebuilt one of those thunderbolt motors. theyre hard running machines i wouldnt mind having one. the most important thing with the motor next to the compresstion is your timing. if its been rinning advanced you could have actually worn down the center of your pistonin turn lowering your compression. this sounds like a crock of **** but wiht the older motors and heavier pistons you could do alot of wearing befor you got a hole. anyway all oild with the 3 rating is fine unleaded fuel is prefered but with ythe year of your motor leaded isnt going to hurt anythign but the water doesnt liek leade exhaust relaesed into it its polution if you care abtou that sort of thing you might ster using unleaded fuel. its cheaper too.<br /> on the motor i jsut rebuile i went 10over and y compression ratios were as follows<br />1 107<br />2 102<br />3 100<br />4 105<br />it runs great i rememberd befor i took it apart they were al inthe high 90's like your is and it still ran great the owner just wanted to rebuild it to ensure many more years or performance. home i helped.

 

[y2jericho]

Re: Is Compression ok on my Motor?

I will try the seafoam product even if i gained a few psi on each cylinder it would be awesome.<br /><br />Thanks again guys by the way roughly how much does it cost?

 

[y2jericho]

Re: Is Compression ok on my Motor?

How much did you charge the guy for rebuilding the motor to after everything and also if i were to rebuild my motor would it be cheaper now or later when compression is really worse? Or will the difference be minimal/none<br /><br />Thanks again

 

[andrew flavin]

Re: Is Compression ok on my Motor?

it cost the guy some where arount 500$ for the rebuild i think parts ran him around 300$ for the power head. and i only charged 100$ i work cheap, its fun work. he also put new gers in the lower unit thats what makes up the other 100$.<br /> with your motor as long as its running akay and its jsut age that lowered the compression and not a problem than its not going to cost you anymore to rebuild now or later. unless somethign important breaks its still the same job weather the compression it 100psi or 80psi

 

[y2jericho]

Re: Is Compression ok on my Motor?

When these motors were brand new what PSI where they running at? <br /><br />How many hours would i probably have left in my motor at my compression if i take care of it as i do?<br /><br />Also you charged the guy $500 but did you bore out the cylinders, bigger pistons etc.. Stuff like that? Or did you just replace everything from another motor either so for $500 i'd be your first customer.. over here theyd want at least $1300 US but with boreing i believe..<br /><br />i have already put $450 US into this motor just to get it fixed which included cleaning carbys, replacing the water pump, replacing the distributor because it was skipping on 2 cylidners on low idle at times.. Im dissappointed that the compression is so low and i might have fudge off this motor soon or later when i wan't it for years and years to come because its an awesome but used machine<br /><br /> <br /><br />Thanks

 

[DJ]

Re: Is Compression ok on my Motor?

y2j,<br /><br />Don't fret too much about the compression readings. Decarb. it first and then see what you have.<br /><br />Compression guages can vary a great deal also.