2002 15hp diagnostic head or carb?


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This 4 stroke J15R4SNF idles roughly and just gurgles when throttle is applied. When I first got it no water was pissing so we Replaced the water pump, fuel pump, plugs. Mikuni carb I cleaned really well cause the rebuild kit is on back order. Air/fuel adjustment screw has little effect.

Compression tested both cylinders and they are both registering 50 psi. Maybe this cheap $30 tester is way off but this seems awfully low. The last owner was a young kid now I wonder if maybe he overheated the motor and blew the head gasket or worse.

Was throttle open on this SUZUKI built 4 stroke during compression test ?------You should be seeing 150 + PSI on that.
Well its a Johnson 15hp and the carb was removed altogether during the compression test.

Although that would be a great test, I dont have access to a leak down tester. When the motor was running on the test bed I did notice the water in the trash can was getting contaminated but I thought that was from the exhaust and was normal.

I did notice the water in the trash can was getting contaminated
What the contamination look like as emissions on 4 stoke are way higher than a 2 stroker.. If fuel the compression is low as suspected due to incomplete combustion
The motor is built by SUZUKI.----I imagine they use japanee built carburetors.------This motor is a SUZUKI with Johnson decals all over it !!!
The last owner said his mechanic replaced the carb. So this is not the original.

Do the symptoms point to a blown head gasket? I never changed one before is that a job a backyard mechanic can handle with a set of sockets and a torque wrench?

You have a Suzuki motor rebadged as a Johnson and Mikuni carb is a original on them,not a aftermarket.
Whats the contamination look like as emissions are way lesss on 4 stoke than a 2 stroker.. If fuel the compression is low as you suspected due to incomplete combustion"
The contamination of the cooling water is cloudy. When you look at the water in the barrel you cannot see bottom. No oil is forming on the surface though.

I got a hold of a Seloc shop manual. Maybe I just missed it but I cannot find the torque specs for the head bolts. How much torque is specified for the cylinder head bolts?

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These are for the Suzuki...same as yours

Thanks thats great. Which of the Yamaha engine models is similar to the Johnson 2002 J15R4SNF?

I found several 2002 Yamaha 15HP outboards like the F15PLRA thinking I could interchange those parts but the carb assembly and cylinder assembly look slightly different.
How embarrassing. Yes the Suzuki DF15 assemblies look identical. Makes you wonder if I have this much trouble with simple reading, what chance could I possibly have repairing this outboard?

I am about to pull the head cover and cylinder cover off so we can inspect the condition of the pistons rings valves and head gasket. Any tips appreciated. I will try not to score the piston walls or valve seats. I plan on using carb cleaner that contains acetone, ketone, and MEK, have also heard about a cleaner called Seafoam which contains naptha I believe.

Before pulling the head Someone also suggested a simple wet test where you put a spot of oil in the head to temporarily seal any piston ring leak to see if the compression goes up.

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The 50 PSI compression needs to be looked at.

I was able to increase the compression from 50 to 70 psi. Still not 100psi but is still a 40% increase. I put the cylinder at top dead center and found the cam was off by 1 notch of the timing belt. Either the timing belt slipped or was installed incorrectly.

Regarding cylinder cleaners I see little value. Watched a video where someone pulled the head off a small motor before and after pouring half a can of Seafoam into the carb intake and letting some soak inside the cylinder head. The white smoke you see is solvent burning off. OMC Engine Tuner 777185 would be similar. Leaving solvent inside the cylinder can also leak into and contaminate the oil or damage the engine with hydro lock.

Next step is to toss the carb back on to see how she runs. Seloc manual outlines how to adjust the rockers and re torque the head bolts.

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I pulled the valve cover and measured the valve clearance against the cam. I measure .003 inches on all 4 valves but the Seloc manual calls for .004” on the intake valve and .006” on the exhaust. Can you advise me as to which two of the four valves are the exhaust valves?
Guess I will just cycle the crank around and watch the order of the cam lobes while observing where the cylinder is at. The exhaust lobe opens the exhaust valve just as the cylinder rises, then The intake lobe should open the intake valve just after the exhaust valve closes and the cylinder travels past the top of its cycle drawing in air and fuel as it travels all the way down. Now the compression cycle happens on the upstroke of the cylinder with both valves still closed. The spark firing just at top dead center. Valves still closed. Cylinder travels down from the fuel and air expansion force then on the cylinder upstroke the exhaust valve opens to let out the gases and the four cycle process starts over again with the intake valve opening just as the cylinder passes through the top of its cycle and on and on we go.

So the exhaust lobe and intake lobe are relatively close to each other with the exhaust lobe first and the intake lobe just following. The remainder 75% of the cycle both valves are closed so the valve clearances should be there for that same amount of cycle time in other words for most of the time. The actual clearance measured of course is between the cam lobe and the rocker arm. The rocker arm transfers the motion of the cam lobe over to the valve stem.
Carb rebuild when kit comes available. Check eBay possibly if back ordered currently. One more carb clean n compressed air. Good too go. Replace plugs if worn. As long as spark n compression was decent
Carb does not need to be installed for a compression test, the test is more accurate without it.

Still would like to hear any opinion on whether the .003 exhaust valve clearance is of concern when the spec calls for double of that.
I pulled the valve cover and measured the valve clearance against the cam. I measure .003 inches on all 4 valves but the Seloc manual calls for .004” on the intake valve and .006” on the exhaust.

These specs are incorrect. The Seloc manual I had was for an earlier model year. I did find a manual for the equivalent 2002 Suzuki DF15. Valve clearances should all be in the range of .007 inches to .009 inches. (Torque specs on the cylinder head cover bolts are 10Nm or 7 lb-ft and on the valve adjusting lock nut 11 Nm or 8 lb-ft. )

I have some good news on this update. Just to recap back to the original subject, the cylinder compression test showed a low 50 PSI on both cylinders. This 4 stroke J15R4SNF idles roughly and just gurgles when throttle is applied.

Yesterday (Nov 28, 2020) I adjusted the valves to .008 inches. To find the correct adjusting position, I just rotated the crank by hand until the feeler gauge slid without resistance between the cam lob and the lifter. Then loosened the adjusting lock nut, backed out the valve adjusting screw, and re-tightened the lock nut. Each was adjusted by about 1/2 a turn. Put it all back together and ran a compression test again. Compression (which was 50 PSI) now measures 75 PSI on #2 cylinder and 90 PSI on #1! Happy to tell you that the motor now revs up smoothly without gurgling and idles properly as well.