Compression testing where I cant get good crank speed, 115hp...can it be impeller?

[rothfm]

Hi All,

I have two motors I'm working on. My 88spl is showing 95 even on all 4. following the Sticky on testing so far so good there.

But my 115 I just started working on, shows about 40 on all 4. Same Guage, same teqnique. However on the 88 I have the lower unit off, the 115 L.Unit is in place.

When I crank with no plugs in, its still not quite up to speed. Turning it by hand it has some resistance (unlike the 88 which is very easy to turn by hand). When I turn this 115 by hand with no plugs, I can hear what sounds like the water impeller really "squeeking"- like a rubber sound. I'm guessing this is slowing up my cranking speed, thus my compression test is inaccurate. Havent pulled off the L.Unit yet. But can something down there slow me down significant enough to kill my test?

When I crank with Just the tester in a cyl, the already low crank speed slows even further...And If I put another plug in, the crank gets more slower. Again, same equipment works AOK on 88SPL.. I think something with the impeller is slowing me down--Will have to pull it.

 

[emdsapmgr]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

You are correct to remove all the plugs before you start you compression tests. Usually that permits quick cranking and should yield valid compression numbers. Slow cranking will definitely reduce your compression numbers, to you are correct to be concerned. It's pretty normal to get some drag on the impeller, even hear some noise. The slow cranking speeds won't hurt the impeller during the brief test. I doubt the lower unit is causing excessive drag. It is possible that you have insufficient current to spin the starter. 1. Make sure the battery is charged fully (even try a second battery). 2. Make sure all the battery connections are shiny clean and clamped hard to the battery. 3. It is possible that the contacts inside the starter solenoid could be burned and not transfer current fully. You can test around all these issues by connecting a heavy-duty set of jumper cables direct to the starter itself. If it cranks normally while jumped, the slow cranking is due to something in the electrical system. If it still cranks slowly, I'd look at the starter. If the 115 is a crossflow, why not just swap the 88 starter onto the 115 to do your tests? See what happens after you swap the starters.

 

[rothfm]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

emdsapmg--Thanks.

Was using same battery, and direct jump to the Starter also....as the test on the 88spl. Good Idea about swapping starters before I pull the lower (but which I have to do anyway). The one on the 88 is pretty new. I'll try it on this 115.

Else I'll pull the lower and see whats going on down there, and retest.

 

[adoseth]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

drop the lower end and repeat the compression test, and mist some fuel/oil mixture in there,crank it over,then do comp test in case the cylinders are very dry as a precaution against scoring

 

[ScorpionRage]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

How did the starter swap work? To share a success that I had in the same situation with my boston whaler rage with the 115 OMC Turbo jet. I removed the starter and opened it up, very easy to do so only had 2 bolts, to find that the copper coils going to one of the leads was cooked. I had a second busted starter sitting in the garage and opened it as well to find that 2 of the 4 brushes were shattered. So I made a frankenstein starter. I experienced accurate compression tests and a great day on the river.

 

[emdsapmgr]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

When starters get old, typcially two issues arise. The grease for the armature dries out and the starter squeals rather loudly when cranking. Secondly, the brushes wear out-won't transfer current for armature rotation. Both are easily remedied on a workbench. You can get inexpensive replacement brush sets for your starter. (You will also find a lot of carbon black residue inside.) After a good cleaning, and some new white lithium grease , you are ready to go boating.

 

[rothfm]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

Thanks All, Problem Solved (I believe). That impeller was slowing down my crank RPM. I removed the lower and it cranks with the same speed (and feels fine by hand) as my other motor.

Its the 1984 115 Johnson, that has been sitting since this past June. Repeating my test now I got around 100. I put in a little oil and repeated...One head is at 118 & 115, The other head is at 111 and 106. I'm using an inexpensive tester from Harbor Freight. Do you guys have any input on these numbers????

I'm hoping later on when I can test warmed up, and after a SeaFoam treatment--They will improve. Also my 1985 88spl has a solid 96 on all---no seafoam yet either..IF they remain at 96 is that ok?

 

[adoseth]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

all sounds good to me, i would go with it !!!

 

[rothfm]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

Thanks for the input.

 

[emdsapmgr]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

Putting a small amount oil in the cylinder is good on an engine that has not been turned over in a long time. Too much oil in the cylindeer will tend to coat the rings and give erroneously high compression readings till that excess oil works it's way past the rings. I would not worry too much about the compression variances between heads. Due to the way in which the factory originally bored the blocks, it is fairly common to see some compression variance between heads. 5+ lbs variance is fairly normal. The compression variances between cylinders on each head seem pretty normal.

 

[rothfm]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

Thanks emdsapmgr, Appreciate the input. So far it looks like both motors are aok. Doing lower seals, tstats, coils/wires--Then will fire them up for testing before paint and decals. Whichever one (88, 1150) comes out better will go on my 17.5 CC. Will sell the other.

 

[toddschubert]

Re: Compression testing where I cant get good crank speed, 115hp...can it be impeller

Just a note. Never crank muffs on. Dry impeller cAn fail instantly