Compression in once cylinder is 10 psi's higher, 91 60HP Johnson, fix?

[twierz]

Having some intermittent starter problems with my 91 60HP JOhnson. Mechanic stopped and started it 8-10 times today without any issues. Tough to fix a motor that runs I guess. Said the starter looked fine, plugs looked new etc...<br /><br />The only thing he could find was that the middle of the 3 cylinders was 10 psi's higher than the other two. He didnt think this would be the problem. Could it? Is that a big difference. Two were at 125 psi, one was at 135 psi.<br /><br />What should they be at? Please help if you can. Thanks. I already have to pay this guy close to $100 to say the motor looks fine.

 

[Hooty]

Re: Compression in once cylinder is 10 psi's higher, 91 60HP Johnson, fix?

The compression is fine. Have you been trying to start the engine in the water and the wrench started it on the hose? If so, there's a big difference.<br /><br />c/6<br /><br />Hooty

 

[DJ]

Re: Compression in once cylinder is 10 psi's higher, 91 60HP Johnson, fix?

Tim,<br /><br />Typically, the compression is looked at in terms of percentages, not actual psi. If they're within 10% of each other-that's good.<br /><br />The engine, you describe, appears to meet that criteria.<br /><br />A good decarb. may bring them all up to their optimum.

 

[Jack Shellac]

Re: Compression in once cylinder is 10 psi's higher, 91 60HP Johnson, fix?

OMC only specifies that they be within 15PSI of each other, so it sounds like you're in good shape. Agree with decarb if it hasn't been done recently.

 

[twierz]

Re: Compression in once cylinder is 10 psi's higher, 91 60HP Johnson, fix?

How big of a difference is there starting it on a hose instead of in the water?

 

[dkondelik]

Re: Compression in once cylinder is 10 psi's higher, 91 60HP Johnson, fix?

how big?<br /><br />back pressure.<br /><br />in the water there is back-pressure on the exhaust.on the hose, no back pressure. a bit tough to quantify. in the water you get about 1 additioanl atmosphere of pressure for every 32' of depth. so if you exhaust about 1 foot under, then you've picked up about 1/32 additional atmosphere of pressure. sorry i don't remember the numbers. maybe it's about 14.7psi/atmosephere but i really don't rememeber (been to long ago since i knew). if thats correct then you'ed have 14.7/32 or about half a pound/sq in. at 3sq in exhaust (approx) this would be about 1 1/2 lb of additional back pressure<br /><br />can anybody correct this?

 

[G DANE]

Re: Compression in once cylinder is 10 psi's higher, 91 60HP Johnson, fix?

Hi Tim J<br /><br />The biggest difference in running in water and on muffs isnt really the pressure resistance. The water will fill your midsection to same level as the sea, which will reduce the expansion chamber size in exhaust. Remember the expansion chamber in a twostroke acts like kind of exhaust valve when iddling. Some of the fuel that enters cylinder will infact end in exhaust and be returned to cylinder by a shock wave, before the piston clouses exhaust port. Pressure from exhaust will excape mainly through the small exhaust relief holes when iddling in water. You will get less intake when iddling in water as when on muffs. Motor will have to run on a leaner mixture, with more resistance. The reason you cant adjust iddle mixture or mixture at all on muffs.