'88 90HP Johnson its not the spark (Ouch!)

[waynehol]

Heres a recap of my problem from another post:

"I was running WOT after a day of fishing. Ran great as usual. All of a sudden, it dropped about 20% or so RPM. It was sudden and smooth, in fact I looked to see if my dog had knocked the throttle control back. It had not moved. It ran very smooth, no sputtering, no odd sound, just lower RPM and slower speed. I backed it down some, then put it back up, still maxed at the lower speed"

So I checked the spark on each plug by holding each one to the block and hitting the starter. They all sparked nicely,

So, I am guessing fuel?

Another "sympton" Today I had to drive the boat to the ramp. I did not run WOT, prob about 50%. A couple of times, the boat revved fast, like someone shoved the throttle to full. Then it came right back down. The whole time it was running smooth.

So, where do I go nex? I have a plastic 10 gal fuel tank with vent cap on the filler cap.It was verified open about halfway. The bulb works, though it rarely gets real firm. Sometimes it does, sometimes not. Always starts easy.

Looking for guidance on trouble shooting. thanks!

 

[petryshyn]

Re: '88 90HP Johnson its not the spark (Ouch!)

Check compression.
Try new plugs.

If you are convinced that the spark is fine, try assisting with the primer ball while its acting up. If it helps, look to the pump, filter, lines and vent. If it doesn't make a difference, it likely time to go through the carbs

cheers...

 

[F_R]

Re: '88 90HP Johnson its not the spark (Ouch!)

Check compression.
Try new plugs.

If you are convinced that the spark is fine, try assisting with the primer ball while its acting up. If it helps, look to the pump, filter, lines and vent. If it doesn't make a difference, it likely time to go through the carbs

cheers...

Your motor certainly is not running on all cylinders. The only question is why not. Since your multiple carburetors each feed the cylinders they are associated with, one clogged carburetor can knock out just those associated cylinders.

But your description of intermittant and sudden change strongly suggests a spark problem. As already suggested, it may be as simple as a bad spark plug. I sure would try a new set of plugs before going any further. Your method of testing for spark is ok for emergency troubleshooting, but is very unreliable and misleading. That motor should be able to produce a 7/16" long spark in open air. If you subject yourself to full spark voltage, it will knock you right out of your BVDs.

 

[tashasdaddy]

Re: '88 90HP Johnson its not the spark (Ouch!)

with the motor idling on muffs, pull a spark plug wire one at a time then replace it. when you remove the wire the rpms should drop slightly, and pick back up, when wire is replaced. if this does not happen, you have a problem with that cylinder. the top carb services the top 2 cylinders, the bottom carb the lower 2.

the only time the bulb gets hard is when priming to start, the bulb has an arrow on it. point the arrow to the sky when pumping, makes it easier to prime. if the bulb does not get hard, you have a leak somewhere, between bulb and intake, could be stuck float needle valve in one of the carbs.

once the engine starts to draw fuel, the bulb will soften.

 

[waynehol]

Re: '88 90HP Johnson its not the spark (Ouch!)

with the motor idling on muffs, pull a spark plug wire one at a time then replace it. when you remove the wire the rpms should drop slightly, and pick back up, when wire is replaced. .

I started to try this method and when I grabbed the wire boot with rubber coated pliers, it "knocked me out of my BVDS" lol. So I abandoned that method and removed each plug one at a time, reconnected wire, and held a short distance from the block, etc. So, I know it sparks but I guess I really dont know if the spark is the correct amount. How do I check that? I will change the plugs, but they look pretty good and I put new plugs in back in May. (We do use the boat a lot however. 2 times a week min)

When I replace plugs, how do I check if the spark is proper? (My manual has gap setting so I can handle that)

 

[F_R]

Re: '88 90HP Johnson its not the spark (Ouch!)

I started to try this method and when I grabbed the wire boot with rubber coated pliers, it "knocked me out of my BVDS" lol. So I abandoned that method and removed each plug one at a time, reconnected wire, and held a short distance from the block, etc. So, I know it sparks but I guess I really dont know if the spark is the correct amount. How do I check that? I will change the plugs, but they look pretty good and I put new plugs in back in May. (We do use the boat a lot however. 2 times a week min)

When I replace plugs, how do I check if the spark is proper? (My manual has gap setting so I can handle that)

There are a number of spark checkers on the market. Heck you can even make one yourself. You are looking for a 7/16" spark gap in open air.

 

[waynehol]

Re: '88 90HP Johnson its not the spark (Ouch!)

OK I just purchased a spark checker. Before I go looking at fuel, I am going to test all plugs and make sure I am getting 7/16" spark on all 4.

I'll keep you posted. Please check back!

 

[jonesg]

Re: '88 90HP Johnson its not the spark (Ouch!)

I'll bet you a pint of seafoam you can't get them to spark at 7/16th inch gap.

 

[waynehol]

Re: '88 90HP Johnson its not the spark (Ouch!)

OK I checked the spark with a tester. NONE of them sparked 7/16th inch. I had to close the gap to closer to 1/4 or 3/8 to see a spark.

So, this test was sparking the wire from the coil, not on the plug itself. I connected the wire from the coil to the tester, then tester to ground. All 4 wire were unplugged when I did the test.

So, it looks like a spark problem. Any ideas? Is it the coil? What would you recommend I try next in my diagnosis?

Thanks,