Johnson 1976 15HP low o/p on one cylinder

[Rad2101]

Hi Guys,

I have this problem and its doing my head in. Low power on bottom cylinder. Have tried everything I know of to try and isolate this problem. Motor starts ok and will run ok. Last couple of times I had it out it will run ok at idle and well at high RPM's then will suddenly drop the bottom cylinder. Bring it back home and run in my tank and find no bottom cylinder. Usually after cleaning the plugs I get the cylinder back but its low o/p compared to top cylinder. When I pull the plugs it runs well on the top cylinder but dies on the bottom cylinder. If I up the RPM's it will run on the bottom cylinder alone but not well.

I have so far

- rebuilt the carb
- used another carb (runs well on other motor)
- swapped all ingnition and electrical from one cylinder to the other.
- swapped entire armature plate plus points, condensors and coil off another motor
- spark test will jump 1/4" on both
- timing is right on and doesn't move.
- compression test both cylinders are ok 70/71

What else could this be? Always low on bottom cylinder. Could this be a reed valve problem? I was going to have a look at this next but it appears the power head needs to come off to get to the bolts. Before I go and pull this thing down again.

Any further ideas

cheers

 

[oldboat1]

Look for black rubber caps on the condensers (newer style). Did you switch the ignition coils to see if the spark problem followed? (coil under the flywheel is a driver coil.) Spark should be a sharp 1/4" using an adjustable open air tester.

Rent or borrow another compression tester and see if the reading comes in a bit higher.

 

[F_R]

Don't know for sure, but 70 psi sounds low to me. May have internal engine problems or getting water into the cylinders. You may have already heard about the plug fouling problem that plagued the '74-76 models. But it sounds like you have an additional problem

 

[Rad2101]

Thanks Guys,

Yeah. I have good spark on both. As I mentioned I have good 1/4" spark on both. Tested with an open air adjustable tester. Coils swapped over from one cylinder to the other. Coils swapped from another motor also. I tried a second driver coil and even tried it with the driver coil upside down (to swap wires over) Everything I try shows bottom cylinder low. Every time. My compression tester does read low. I have another one of these motors that runs well and has similar compression numbers. I was thinking maybe rings or something but read something about reed valves. If one of the reed valves was stuck shut. There's like 2 valves per cylinder and if one of these was stuck it would get less fuel into one cylinder wouldnt it? Just clutching at straws here.

Does anyone know if I can pull the carb and manifold part without pulling the powerhead?

thanks

 

[Tassie 1]

Best find another compression tester,
Which valves does your motor have?

 

[Rad2101]

Reed valves?? Unsure which type. Id say original. its an old 2 stroke so no other valves

 

[OptsyEagle]

What spark plugs are you using. That model of motor hates Champion spark plugs, even though they are specified. Try NGK B6HS spark plugs, if you are not using them.

One of the symptoms of the Champion plugs is an intermittent bottom cylinder. The other is fouling the plugs themselves. Usually it is bottom cylinder that fouls first. I suspect all this has to do with a poorly designed cylinder head with respect to where the fuel comes into the chamber. Not sure why the bottom cylinder fails first but it does. The NGK plugs tend to resist this better then the Champions. On a 15HP NGK B6HS are best, NGK B7HS will do in a pinch. This analogy, of the fuel intake effecting the spark, also explains why it tends to be weak as opposed to a complete failure.

 

[racerone]

Lower crankshaft seal or reed valves ?---------Remove plugs and pull it over.------Do both cylinders make the same sound with respect to crankcase compression ( pooff, pooff ) sound.

 

[bwkre]

I don't see anywhere that you tried "new" plugs, just cleaning. Spark in open atmosphere is not the same as spark under compression. Google it. As OptsyEagle said, usually the bottom cylinder, and I agree, my 76 9.9 does the same thing. Motoring along and all of a sudden, boom, one cylinder dead! Change the plugs and carry on like nothing happened. I'm a weekend user and it happens once or twice a season. I carry a spare set sealed and ready to go. Mine runs the B7HS

 

[OptsyEagle]

The B7HS will work, especially for the 9.9HP but the 15Hp prefers the hotter B6HS. Both may fix this persons intermittent failure but the B6HS will last longer before fouling then the B7HS, when used on a 15Hp motor. I think it is because the 15Hp puts out more fuel. As I said, I suspect it is the way the fuel comes into the cylinder and the NGKs seem to have a better ability to resist it and keep working then the champions.

 

[racerone]

A properly tuned motor does not know what sparkplugs are in there.-----Trouble shooting would find the real problem on this motor !

 

[OptsyEagle]

You would think.

 

[Rad2101]

Hi guys and thanks for all the help. Sorry I failed to mention I have tried a few different sets of plugs. I have been mainly running NGK BH7S gapped at 0.30 and then 0.40 but also had some BH9S as well and tried running these too. I'll grab some BH6S and give these a go. I thought I had read that the BH9S were the right ones. This may help it run better but I'm thinking there is an issue with it still unless there is a known problem with these running low on bottom cylinder. Still I'm sure there's a fix for it. I'll try to listen for the lower crankshaft seal as racerone has suggested. but I'm suspecting something like that or the leaf valves are causing the main problem.

Does anyone know if the manifold and reeds can be removed without pulling the power head? If I need to tear it down I'll go over a few things. Hell maybe even a full rebuild. However if I can get it running ok with a new set of plugs then it might get me through for a bit longer. It looks like I've been running the wrong plugs so will see what difference that makes first. As I said it does run ok on the water even though bottom cylinder is down on power. If I can stop it fouling up and dropping the cylinder I'll be happy for a while.

cheers

 

[oldboat1]

Might run a phillips screwdriver up into the spring connector in the boot(s) to clean out rust or corrosion -- also might disassemble the lower boot/clip to make sure the spike is in solid contact with the wire.

Might run with a timing gun off the bottom plug wire to see if and when the spark drops out. May be a two-person job, or just arrange the gun so you can see the flash. It's possible the lower coil stops firing when it heats up. (I would test both cylinders, btw, to see if firing is interrupted on either.)

 

[Rad2101]

I've swapped coils over from one cylinder to the other. I've also swapped coils from another motor. Still the same. I've swapped all electricals from another motor. Coils, plugs, points, condensers, driver coil. even the plate they all sit on under the flywheel. I don't think its any of this. I was thinking it had to be a spark problem myself but Ive swapped it all out or from top to bottom. I dont think its spark anymore. I can run my timing light next time I take it out and see what that shows but Im guessing when the plug on the bottom cylinder fouls then it will drop out.

Im still thinking crankcase pressure or reed valve. But this is a bit harder to diagnose.

Might be a couple of weeks till I get it out again but Im hoping it will be sooner. I'll be trying the B6HS-10 plugs and hope that keeps it going longer.

thanks again

 

[OptsyEagle]

If you tried NGK B7HS and you still had your problem, NGK B6HS will not resolve it. That motor has two inherent problems that they redesigned the cylinder head for in 1977. As said, with Champion spark plugs the motor will either bog down like you describe and/or the spark plugs will foul within a few hours if you do any serious trolling. The NGK plugs tend to deal with the bogging down issue quite well and the B6HS tend to fight off the fouling longer then any other spark plug. The B7HS will work OK for quite a long while if you don't troll very often. Do NOT USE B9HS. The higher the number, with NGK, the colder the spark plug. I doubt that motor will run for an hour on those B9HS plugs before you will be pulling out the oars.

All that said. It might be time to take a look under the hood. Pop off the flywheel and make sure your points are set right (gap = 0.020") and are immaculately clean. That motor also tends to get dirty points if it sits for a few years in a row without use.

All this said, I have your same motor and once you get the right spark plugs in it and you tune it properly (set points) she is very reliable and very powerful. It will pin you to your seat when you open her up on the water.

 

[racerone]

???------Owner says that he has done all kinds of work / replacement / guessing on the ignition system already.------Remove the powerhead.-----Look at the reed valves.-----The bottom seal on the crankshaft.-----Open the plugged ( with carbon ) holes in the exhaust tuner..

 

[racerone]

???------Owner says that he has done all kinds of work / replacement / guessing on the ignition system already.------Remove the powerhead.-----Look at the reed valves.---Inspect / replace thermostat.------Look at the bottom seal on the crankshaft.-----Open the plugged ( with carbon ) holes in the exhaust tuner..

 

[Rad2101]

Thanks again guys. Yeah I have cleaned, set, swapped, timed, polished points a few times. I'm thinking I have two issues here really. 1. is running with the wrong plugs (B9HS) which is causing the fouling up quickly. And 2 is an actual problem that may be reed valves or primary/secondary compression, crankshaft seals or something. Its probably time for a rebuild on this old motor. I'll run it with he hotter plugs (B6HS-10) and see if it runs longer without fouling. In the off season I'll tear it down and go through it all. Might as well while its open. New rings, seals, bearings, thermo, carb clean etc.

I'll post an update when I know more.

cheers