Compression test/low rpm

[Caddpro]

I have a 1975 70HP Johnson that has been running very well. I spun a prop and now fighting with that see Hustler prop restriction in Boat Topics and Questions (not engine topics) section. During this I thought I might check my compression and it was 60 straight across all 3 cyl. I added oil to each cyl. and it went up to 75 all the way across. I do not know how accuate the gauge is as it is borrowed (looks rough though). What is the acceptable compression range? I cannot afford to rering/rebuild it till at least tax time next year. Should I be worried? Thanks for your help.

 

[DHPMARINE]

Re: Compression test/low rpm

Caddpro, <br />How was the engine running before the spun prop?<br />I wouldn't expect 60 PSI to start never mind run well enough to spin a prop,altho just overheating CAN do that.<br /><br />DHP

 

[Caddpro]

Re: Compression test/low rpm

I have had the engine for ~4 years and never had a problem. fires right up and runs like a scalded dog (couldn't resist). I rebuilt the carbs and put a new power pack (motor would cut out after it warmed up- but the PP fixed it) on it recently. It ran and still runs great - had it out this afternoon testing my new Hustler prop. Taking it back tomorrow motor wouldn't turn over 5000rpm... look at my other post for more on that though.

 

[rodbolt]

Re: Compression test/low rpm

try a different gauge<br /> at 60 psi the motor may not run. usually about 60 is the break.<br /> I bet the compression is actuallly higher but if its 60 across the board and runs well note it and go play.<br /> good luck and keep posting

 

[blgrams]

Re: Compression test/low rpm

I would also get (or better yet borrow) another guage It would be unusual indeed to see two cylinders loose compression at exactly the same rate. did you have it at full throttle when you checked the compression? if not try again with wide open throttle.

 

[Caddpro]

Re: Compression test/low rpm

Well today was disappointing. I returned the Hustler props and bought the Michigan Vortex XHS and went out for a test run. It too only turned ~5000 rpm. So I put my old SS prop that used to turn ~5500 and now it only turned 5000. If I went into a turn and got the prop to start blowing out it would turn up to 5700 as soon as the prop bit back down to 5000. What happened? I am going to get another compression gauge tomorrow and retest that. Other than the 5000 rpm limit the motor runs great.

 

[Caddpro]

Re: Compression test/low rpm

Okay we are in business now. I coughed up the money and bought a compression tester and with WOT the pressures are as follows:<br />top cyl- 140<br />middle cyl- 130<br />bottom cyl- 125<br />Does this help in solving my low RPM problem? Any recommendations are greatly welcome and appreciated. Thank you again for all the help.

 

[Caddpro]

Re: Compression test/low rpm

Hopefully this is the next to the final post from me on this topic. I tested my coils to see if any where bad after I had warmed the engine and sure enough the bottom coil was shot. I retested the compression with the engine warm and WOT and got from top to bottom- 145-150-145 (next time I will read my manual more throughly "sp" before I test anything). Will post tomorrow the results when I get back from running it @ the lake. Thanks again.