compression on low HP motors

[txbandit44]

I am needing to know what the compression should be on the 25hp and lower hP motors. Electric start vs pull start. Winter is comming an I need some projects. So I need to start shopping and the only thing I cant fix myself is low compression.

Thanks

 

[JB]

Re: compression on low HP motors

Out of my 36,000 posts, it could be as many as 30,000 times I have typed:

There is no spec on compression except that all cylinders should fall in a 10-15% span. The chances that several cylinders would all go bad by the same amount at the same time is just about the reciprocal of infinity.

Depending on the guage, conditions, engine temp, when it was last run and how fast it has been turned over how many times a rope start engine usually compresses over 50psi and an electric start over about 70psi from healthy cylinders and all cylinders are within 10%.

A search on the topic should produce over 10,000 threads.

 

[txbandit44]

Re: compression on low HP motors

WOW 30000 times well I can see why that would be irritating. I have read several of your posts (very informative by the way). I also did a search. I know that there are no specified compression ratios. You are correct they must be with in 10% of each other. But I would assume that there should be some number that would indicate low compression. I mean if you have only 25lbs of pressure in all cylinders would that not indicate that there is problem?

What spawned my question:

Since I have never owned anything less than a 50hp motor or messed with anything lower than 18 hp I was thinking that the lower HP motors 9 hp and under would be a single cylinder engine. If that was true then the 10% rule would not apply. You would not have anything to compare your readings too. Your reply prompted me to look up the exploded diagrams and now your answer makes sence.

So thanks for the help, now a project hunting I go.

 

[Marvxyz2]

Re: compression on low HP motors

6 HP Johnsons are 2 cylinder

 

[crxess]

Re: compression on low HP motors

I have a '73 6h.p. and a '67 9.5h.p. Both show about 70psi Dry reading on Rope start pull with 4 pulls.

The numbers will vary slightly with Temperature, Pull strength, cylinder dampness and gauge quality.

Cylinder balance is most important but compression should be above 65 for any decent performance.

 

[samo_ott]

Re: compression on low HP motors

I have a several 2.5hp's and 3hp's that are all 2 cylinders. I think the 2's are 1 cylinder. That's why J/E's are so smooth

On a 25hp, I'd want 75 psi or more per cylinder... But that's me... It will run on lower... But not a lot... And not as well...

 

[bassboattech]

Re: compression on low HP motors

better ck that there are alot of boat motor manufacturers that will and still do have actual reading that it should fall into a high and a low, a general rule is yes ten percent within each other (like on a v8) but on other motors if can and will vary from model to model. a 1967 criscraft 427 has in plainly to view in there manuals. so i guess some do some dont

 

[bassboattech]

Re: compression on low HP motors

2003 yamaha hpdi250 has a min compression ratio...i could go but i wont, and if your compression gauge does not read accurate....you dropped it...its a gauge its suppose to be accurate.

 

[Whoopbass]

Re: compression on low HP motors

I have had 4-5 25 hp Johnnyrudes and they have all had 110+ lbs psi.

I've had a bunch of 9.9/15's and they usually have 75-100 psi.

If I remember right the difference in psi with the same motor while using rope start versus the electric start was 10-15 psi less with the rope.

I've had some smaller motors and the psi has been anywhere from 60lbs to 90lbs and they all ran well but i'm not sure if the higher psi motors were faster at WOT. I did read where one guy said he had two identical motors and the one with the higher readings ran faster.

If your looking to make money then a mid 70's and later 15 hp Johnnyrude is the way to go. Even if you can't get them running you can part them out on ebay and make an easy $400+ as long as the parts are good. The carb alone will bring $100-$175 depending on the time of year.
The hard part is finding a 15hp for under $250 and then being the first guy to respond to the ad.