'94 Johnson 120 hard miss

[JasonB]

Replaced the Power Pack and plug wires over the winter and finally got it out for a run today. At 3500-3800 roughly, there seems to be a hard miss. I can back the throttle down just below it and it ran perfect up to 3500 and cruised a couple miles. Throttling up, As it started to push over in plane, it started missing. I throttled up a bit and it clears and takes off. I briefly ran it up to 5500rpm for a few second and it seemed to have normal power, so i cruised at 4000-4400 for a mile or so. Played with it a bit and it doesn't always do it, seemed to be a bit better when warm. After you clear past it once, it seems ok for a bit. I went through that rpm+load range a dozen times and it only seems like its the first time or two after idle. I replaced the original due to a short in the kill wire. This is the 2nd power pack I've had from this vendor. The first one would only fire erratically under load so I put the original back on to finish last season. Having trouble thinking 2 bad power packs, but original seems to work ok? Any thoughts on the best way to troubleshoot this besides putting the original back on and see if it happens again?

Also time to rebuild the carbs. The boat only gets a 4-6 trips a year. Last built 4-5 years ago. When I test fired it yesterday before today's trip, one of the carbs was pouring fuel out the brass hole in the middle of the carb under the throat. I pulled it off, removed the bowl, and made sure the float was free and the needle/seat were ok. Put it back together and it seems ok, but maybe there is a piece of trash in there somewhere? I don't think that would cause the miss, but seems like time to make sure carbs are 100%.

 

[oldboat1]

Sounds electrical, offhand. Make sure there is metal-to-metal contact in the coil to plug boots. Hook up a timing gun to each wire when running and see if there is a change in the flashing, particularly at the rpm where problems seem to be showing up.

 

[JasonB]

Sounds electrical, offhand. Make sure there is metal-to-metal contact in the coil to plug boots. Hook up a timing gun to each wire when running and see if there is a change in the flashing, particularly at the rpm where problems seem to be showing up.

I did connect a timing gun to each lead to confirm spark, but only at idle. If I can get this thing in a barrel of some form to keep enough water for the cooling system, is it safe to take the prop off and wind it up to that speed for a moment unloaded?

 

[oldboat1]

Do it on the boat -- 2 person job. Check the contacts in the boots, making sure the spring connector makes contact with the wire core. Check for corrosion. Simple stuff first.

 

[JasonB]

In the interim, I'll pull the plugs and see if one of these things is not like the others, but I don't expect to see anything, but on the off chance one shows different, it may narrow the search.

 

[oldboat1]

I would be inclined to put in fresh Champion plugs, gapped to .030. Believe you would use J4C.

 

[JasonB]

I would be inclined to put in fresh Champion plugs, gapped to .030. Believe you would use J4C.

Currently has a set of QL78YC's with maybe 5 hours on them. I installed new plugs with the first power pack.

 

[racerone]

No do not use J4C in this motor !

 

[oldboat1]

^^R. has it right. I'm looking at '57/'58 issues in another thread -- not your motor!

 

[racerone]

Still the wrong plugs ( J6C ) for this motor !!----J4C and J6C are 3/8" reach !!---Gas oil mix is 50:1 if VRO is bypassed.

 

[JasonB]

Still the wrong plugs ( J6C ) for this motor !!----J4C and J6C are 3/8" reach !!---Gas oil mix is 50:1 if VRO is bypassed.

VRO is still on it. Most places I look list the QL78YC for this motor, including BRP parts, but just noticed the Clymer I have on the shelf lists the QL77JC4.

 

[JasonB]

Interesting side note that probably means nothing. Noticed that the new power pack doesn't have the brass spacers in the bolt holes that the original had. When I put the ground wire on, I put it under the same nut and im not convinced it tightens against the housing. I would assume there is still a good connection, but I think I'll at least move it to one of the back bolts that pulls down tight against the housing and I can pinch the ring terminal between a washer and bolt.