Johnson 115 backfiring

[mrsvt99]

I just bought a 99 Javelin with a 115hp Johnson. <br />I changed the plugs and took it out the other day. It ran fine for about 30 min, and then all of a sudden it started backfiring really bad, especially at WOT. Could the carburetor just be dirty? Or maybe water in the fuel? Please give me any suggestions. Thanks.

 

[Joe Reeves]

Re: Johnson 115 backfiring

Is it actually backfiring out the exhaust with a loud explosion, or are you speaking of a spitting back thru the carburetor type sort of thing? Try to explain that problem in a little more detail.

 

[mrsvt99]

Re: Johnson 115 backfiring

Yes it is backfiring out of the exhaust.

 

[Joe Reeves]

Re: Johnson 115 backfiring

I retiured before that engine came into being, but I assume it is a two (2) stroke engine, being timed via the center magnets of the flywheel etc. If so.........<br /><br />That would be (usually) caused by improper timing. Check the flywheel key to see if it's sheared. If so, clean up the flywheel and crankshaft tapers, then when installine a new key, install it perfectly vertical (not in line with the C/S taper), and be sure to torque the flywheel to factory specifications.<br /><br />Not being torqued properly (flywheel nut) results in having the key shear! Let us know what you find.

 

[mrsvt99]

Re: Johnson 115 backfiring

I took it out the other day to see if it was any better, and it was still doing the same thing. To explain in more detail. When you take the boat out it runs fine for about five minutes at WOT, then the engine starts to backfire. When you have the throttle wide open it won't go past 3000 rpm's. It will go to 3000 for a second and the it will jump back down to 2000 rpm's. It will do this for a few minutes and then it will suddenly start to pick up to full throttle for about 10 or 20 seconds, then it will start its cycle all over again. We played with the engine tonight to see if we could figure it out, but it just did the same thing. We did notice that it was shooting at a little bit of oil or something at one point. We also found out that it is fuel injected. Could it be a bad gasket or something? I just can't figure out why it was running fine and then it started doing this?