Johnson tn-28 power issues

[crosbyman]

I had a question…but it was to easy 🙂

Joining AOMCI has priviledges 🙂

[Flambeau5]

I’ll give that a shot today. Thanks for the advice

[fleetwin]

I haven’t had any experience with this model, but it sounded like it was running on two cylinders…You had the powerhead all apart, I’m assuming the pistons looked OK and weren’t scored and that the cylinders were in relatively good shape. It is a shame you didn’t find new rings while it was all apart, but I’m guessing the compression might come up a bit after some running. I agree with JH, I wouldn’t replace that powerhead, you would have noticed any big problems while you had it apart. I would certainly look into that carb and recheck spark at lower speeds. Did you have the carb apart, was it mucked up inside?

[aquasonic]

Just a few more thoughts for what it’s worth.

The compression on the top cylinder at 60 psi is a problem. An even 70 psi or higher works much better for these rotary valve motors. They don’t idle well with low compression.

The entire fuel system must be disassembled and cleaned, including the intake screen at the tank, the fuel lines, the carburetor, and especially the check valve and associated screen. A stuck check valve or clogged screen won’t allow the low speed circuit to work. A partially clogged screen at the tank could result in fuel deprivation.

One of the pictures shows some serious carbon/oil build up in the exhaust. Hopefully that’s been cleaned out.

[lloyd]

60 and 68 psi is very low. Very Low. Need at least 70 -75 psi for good starting, idle, and high speed power. Best I have seen for that motor is 77-78 psi. Leather primer seals are for initial starting. And be sure to clean out the one way ball valve and stainless steel screen as mentioned above.

[Flambeau5]

Ok, so I think I found the issue. When I re-did the ignition I used part numbers for coils points and condensers I found on a different forum (wasn’t a member here) and it seems that the original points are about an eighth of an inch shorter from the pivot to where they ride the lobe than the new ones. I swapped the points to the old ones and that seemed to make it run much better at the top end. It is now pushing about 8mph instead of 4mph on my 14′ flat bottom. It is still hard to start and won’t idle but I think that is down to compression as the primer works without leaks. When I had the motor apart I did do a thorough cleaning of the fuel system. I still might look into a used but good powerhead if I can get it on the cheap the cylinders were pretty rough even after a light hone.

Thanks to all of you for your help and information.

[fleetwin]

OK, so you found some more top end power…Again, not at all familiar with this engine series, but it looked like the standard OMC universal magneto set up, I always thought there was only one set of points for this set up…The rubbing blocks are probably worn down on your original points accounting for the difference. I would appreciate it if Lloyd or someone else could chime in on this…
Seems like your powerhead is in rough shape for sure, but I’m betting you can get it idling better than it was in the video. Maybe someone can post some pictures of a carb showing the areas that need to be checked out….Sure seems like the low speed circuit in your carb is blocked somewhere. I would hate to see you replace that powerhead only to have the carb problem carry over to the new powerhead….I realize your engine will never idle “great” with the worn powerhead, but I’m betting it is capable more than your video shows…
These engines were great runners, hope to get some experience with them someday…

[aquasonic]

Crosbyman posted this link early in this thread, but it’s worth posting again.

http://pochefamily.org/outboard/

Lloyd was a key player in this website, and there is a wealth of information, complete with pictures. This website is very helpful for anyone that works on “T” or “H” series Johnson motors.

[frankr]

Tell us more about those short points you found. Like fleetwin, I’ve never heard of such a thing. 1/8″ would be a major issue, and would throw the points timing way out of whack. I have doubts in my mind that it would even run. Waiting for more info on this important subject.
Frank

[lloyd]

Pochefamily website is very useful. I helped him with info for the site years ago. But please note that the price for Leather primer seals and instructions was increased to $10.00 per set several years ago. Proper leather to make more has become very difficult to obtain.

[Author]

Posts