Minimum compression on 9.9hp 1986.

[cprodave]

Starting a winter project on a J10RCDB that I set aside last year. As I recall it ran OK in a tank after I changed the Impeller. But on the water under a load it kept dying out. Compression I tested back then was 80psi on each cylinder.
Is 80 too low for this engine?

 

[racerone]

Was your compression tester calibrated ?-------Did it start and idle ok ?----What other trouble shooting was done ?----If I saw 80 PSI on my tester then the motor would be coming apart.-----Was it run hot ?----Was it run at 100:1 mix ratio ?

 

[oldboat1]

My '82 and '84 both have compression in the 120s. Use a screw-in tester, and crank until the needle stops moving. Record finding. 50:1 mix on those.

 

[Crosbyman]

Compression may or may not come up depending on the meter used, maybe a good decarb ??? but the question is ... does it run ?

the fact that it is dies out suddenly on the lake may be indicative of another problem... like fuel flow issues , ignition etc....

I've had 60 year old engines run fine with low 70'S so racerone's question What other trouble shooting was done ? needs some clarification before you dig to deep.

was the carb cleaned up (HS orifice) ,filters, tank venting etc....

 

[racerone]

For a short few years they ( who are they ) said you could run these motors at 100:1 mix.-----So these motors use much less fuel than an older motor at 24:1----So 1/2 the fuel with 1/4 of the amount of oil means that these motors ran very lean of oil !!!-----Have I said that oil and lubrication in 2 strokes is not well understood.-----No surprise that these motors suffer from piston / ring / lower compression issues.----At a true 80 PSI the motor needs to come apart !

 

[cprodave]

Racer, the engine seems to start and idle OK. for quite a few years I have been borrowing a Compression Tester from AutoZone (at least 2 different stores in 2 different States) when I need to perform Compression Test. I doubt that Autozone gets these Testers calibrated on any regular basis. How are Compression Testers calibrated (for home/amateur use vs. Industrial--after I retired I lost any possible back door connection to a Calibration Lab traceable to NBS Standards etc). Confidence in my abilities to do a complete teardown in this 9.9hp Powerhead is not so high, therefore my hesitation to dive in. However I have been pretty deep inside 4hp and 6/8hp OMC's. But I suppose if I read LeeRoy's info I can stagger through it on this 9.9 which has quite a bit of saltwater corrosion from previous Owners. Powerhead needs repainted too while I am at it. I do need to repeat an on-the-water test. Reviewing notes I kept on this motor, after the bog-down and loss of power during on-the-water test I did in June 2020 (not 2021!) I did some work on the Water Pump (replaced in April 2020) to eliminate a "squeaky" pump. I then ran this motorSeptember 2020 in a tank and it ran very strong (as I recall I ran it In Gear to get past the Neutral OverRev Interlock. It never overheated/smoked overheated for me. Engine Block and Cylinder Head are factory Black, not sure if I would be able to see (using Head color as an indicator) if Previous Owners oveheated it. I once owned a 6R69M which overheated and the factory gray color on head turned a slight Iron Oxide Primer shade of reddish brown. On this 9.9 I see no discoloration but maybe the factory Black hides it. Likewise I don't know if engine was run on 100:1--I am familiar with this debacle by OMC--I have a BE8BACDE that I bought for $5 as a parts motor--Powerhead is seized and it has the famed 100:1 factory sticker on it. But for $5 I got a complete/good Lower Unit from it.

Oldboat, thanks for the comparison on your '82 and '84. My borrowed gauge would have to be quite far out of Calibration to explain 80's vs. 120's readings.

Crosbyman, re: other troubleshooting--I never decarbed this motor. and I did only a partial carburetor rebuild some time prior to April 2020--I found slight rust and water in the bowl, removed/inspected the Idle Needle but not the High Speed Jet, didn't remove Welch Plugs. I probably need to do complete Carb Rebuild regardless of whether I teardown the Head, etc.

Another piece of evidence I almost forgot--when I bought this motor (around 2015?) the inside of the Cowl was quite oily. At the time I didn't think about possible Head Gasket leakage, but this could be a simpler fix vs. teardown to Rods/Pistons/Inspecting Cylinder Walls, etc. Heck, I might need to just loosen/retighten properly the Head Bolts (many of which have considerable surface rust, some are very difficult to access unless Powerhead is lifted from Lower Cover/Midsection.

Thanks to All for the input.

 

[flyingscott]

Try a different gauge. I have a 1977 15 hp that runs fine with a hair over 90 psi of compression, I have had several that ran with that compression. What happens if you pump the primer ball when it bogs? Does pulling the choke out make a difference? Did you pull until the needle on the tester stopped moving? Minimum of 5 pulls. Nice fast brisk pulls with both plugs out?

 

[kbait]

In my experience, if motor idles down very slow and steady, and is running on both cylinders, compression is fine, or at least adequate to run well.

 

[Crosbyman]

and keep you from rowing all the way back

 

[ct1762@gmail.com]

ehh i've had a few of these in the low 100psi range that were a bear to start on a crisp morning. after a bore job, compression was up around 125-130 and start on half a pull. easy to forget why they sold so many of these new... ran so nicely.

 

[flyingscott]

My 77 15 hp with 90 psi starts fine on my cold duck hunting mornings.

 

[ct1762@gmail.com]

My 77 15 hp with 90 psi starts fine on my cold duck hunting mornings.

interesting. they did change the head a few times. never peeked into one of that vintage. actually doing a 1982 one tomorrow. just got the block back from Jay's. had 105psi and 115psi. even with a clean carb and strong spark she was a ***** to start and noisy

 

[flyingscott]

interesting. they did change the head a few times. never peeked into one of that vintage. actually doing a 1982 one tomorrow. just got the block back from Jay's. had 105psi and 115psi. even with a clean carb and strong spark she was a ***** to start and noisy

Those were always cold blooded motors. I always grind the idle throttle stop back to allow more throttle for starting. Makes a huge difference.

 

[RBoyd1971]

Yea, I don't think compression has anything to do with the cold start on these. My 15 is hard to start with 142 to 145 on both cylinders. Oh and on another note, I have one of the old 9.5's with 85 psi you can crank on first pull most times and once it's warm you can spin the flywheel with your hands and crank it. I share that to say it has more to do with the design and how well the points timing is set/tuned.

Edited to add, my 15 hp is a 74 model but was rebuilt a few years ago.

 

[racerone]

There is no reason for these to be hard to start when cold.

 

[cprodave]

I remeasured Compression today using 2 Gauges different than I used back in August 2019. Good results: 1 gauge (wrench-tightened into Spark Plug Hole) measured 100 to 105psi. Other gauge (finger tightened, i.e round not hex head)--probably leaking slightly gave slightly lower readings 98-100psi. I measured 2 ways--having the opposite (from being measured) Spark Plug in-place and also with the opposite Spark Plug absent. I did not see any difference in these two measurement methods. Each test had 7-12 pulls of the Recoil Starter Rope (which has 3-4 inches of slack and needs to be fixed--I didn't have on-hand a rope suitable for winding around the Flywheel which usually gives a faster spin than the Recoil Starter). I also found in my August 2019 notes that I had only pulled Recoil Starter (or maybe it was rope around flywheel?) 5 times--probably not enough. Anyhow, conclusion is Compression is 100-105psi so is not likely the Root Cause of any poor performance. Weather looks nice this weekend, I hope to re-do an on-the-water test/troubleshooting. Will post update afterwards...thanks All for the input.

 

[cprodave]

I ran on the water yesterday for 30 minutes or so. Motor runs ok to middle RPM'S but above that it bogs down and wants to die unless I back off the throttle. It restarts ok if I do bog it down until it dies. My guess is it needs a full carb rebuild including High Speed jet. Make sense?
I also noticed that after I run for a minute or two at highest-allowable throttle and then go to slow speed (slightly above idle) there seems to be more smoke in the exhaust. I ran with cover off to assure the power head itself is not smoking. Is this smoking normal?
I will also try the remove Thermostat Cover so I can replace Thermostat. I am hoping bolts are not seized...

 

[Crosbyman]

what is your feeling .....

it bog down at HS because it drowns in to much fuel ?
anybody mess with the HS orifice ? (to big or damaged)
it bogs down at HS because is lacks fuel ?
clogged HS orifice ? inlet not doing it's job (sticky) float clip installed ??
it bogs down at HS because it is to HOT ?
to hot to touch ? tried IR reading ? ... if not HOT thermostat should be OK

 

[RBoyd1971]

It could be sucking air into the line somewhere as well, prevent the carb from getting enough full at higher rpms under load.

 

[cprodave]

Crosbyman, I really don't have a hunch re: whether it is getting too much fuel or too little fuel whenever it bogs down. I bought the motor 5 years or more ago from a young guy who bought it from another guy. IIRC the young guy said that the guy he bought it from said it was converted to 15hp. I asked what all the first guy did to it and the younger guy didn't know. I understand that for the 9.9 to 15 conversion modifying the carburetor alone may not be sufficient--the exhaust may need modified as well. But I haven's dissasembled Midsection to see what the Exhaust looks like. As mentioned, I did a partial carb overhaul a while back, IIRC I couldn't get the Main Jet unscrewed. I probably need to take another look at that. I don't have access to an IR Reader but I did notice that the Engine Block is NOT getting too hot to touch. The pee stream was warm but I understand that doesn't always mean much.
RBoyd, good idea on the line possibly sucking air. I know the Fuel Tank, Bulb and Fuel Line seem to be OK as my 8hp runs fine with these. But there could be a problem somewhere between the Fuel Line Quick Disconnect and the Carburetor. A year or two ago I installed clear Fuel Line between the Fuel Pump and Carb, I don't see any bubbles in that line but there would be no visible bubbles if there is a problem where the Line meets the Carb.
One other thing I noticed: the engine seems to smoke a lot after I run as fast as I can (which is only about half throttle--or engine will bog down)then when I back off to a slower speed there is a lot of smokefrom the exhaust (but good news not smoking on the Powerhead--a sign of overheat which I have witnessed before on another engine which showed typical Overheat damage). Does this smoking provide further clues?