KG7 Lower Cylinder not firing

[Mark Quint]

Hello, I am working to improve operation of my KG7H that I run on a hydroplane. This summer, the lower cylinder was not firing well/at all as I was getting up to 30mph but not more! Could not get higher RPMs after tuning in the high speed jet. So, first I checked spark and it seemed like the lower plug spark was weak. It has been 20 years since I replaced the ignition wires and coils, so I thought I would do some work. Points open and close fine, Coils seem good (measured with ohmmeter). Condensers… might as well replace. Where can I get new condensers? What else would cause lower cylinder to not fire?  Bad compression resulting from lower seal leak?  I have not replaced the upper or lower seals …. since owning the motor for 40 years and running it a few times every few years. As I type that, maybe replacing seals would be a good idea. What do you think of this rant so far?  I do have the service manual, I guess I should read it.

[Mark Quint]

Using a video scope I see some rust at the top of the piston travel in the cylinder. Is that concerning?

[Sam M.]

Mark Quint wrote:

I have not replaced the upper or lower seals …. since owning the motor for 40 years and running it a few times every few years. As I type that, maybe replacing seals would be a good idea. What do you think of this rant so far?  I do have the service manual, I guess I should read it.

I like the way you’re thinking! 😁

I see some surface rust, so starting with something simple, are the points clean?  Also, I don’t think you’ll want to let the rust stay on the cam either, as the part of the points that touch the cam would probably prefer a smooth surface.  That way, the rust won’t act like sandpaper.

1 user thanked author for this post.

Mark Quint

[Sam M.]

Mark Quint wrote:

Using a video scope I see some rust at the top of the piston travel in the cylinder. Is that concerning?

If it were my motor, I would remove that rust, especially if a piston can touch it.

1 user thanked author for this post.

Mark Quint

[Mark Quint]

To remove rust, would you recommend removing the cylinder block cover and polishing (with pistons still in and protected) or would I be completely disassembling?

Also, can I replace the top and bottom oil seals without complete disassembly of crankcase?  Seems like yes if I read the service manual and replace the oil seals carefully.

Also I see that the points on the bottom cylinder open only to 0.012 in… not 0.018 in as recommended… need adjustment (and cleaning). That could cause weak spark. I do not see pitting or wear on the points, I think they are OK.

[Tubs]

Is it really rust or varnish from dried up oil and fuel. Anyway its above the rings. I personally wouldn’t mess with it. You decide. Point can however needs to have the rust removed and made as smooth as possible. I refreshed a KF-7 with new coils, condensers, plugs, wires, impeller prop shaft seal, drive shaft seal, upper and lower crankshaft seals, and a bunch of gaskets. After 4 years of running it quite a bit, the bottom cylinder started dropping out. After 2 summers of getting nowhere I decided the Bendix mag was bad. Spent over $200.00 rounding up the pieces to a Phelon magneto. The problem was actually the new lower crankshaft seal. That may not be your problem but I recommend you change both crankshaft seals first. Here is a tip. When you put it back together, but the bearing in the freezer, and use your heat gun (1500 watt), to heat up the bearing holder and the bearing will drop right in.   

A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.

[Sam M.]

Mark Quint wrote:

To remove rust, would you recommend removing the cylinder block cover and polishing (with pistons still in and protected) or would I be completely disassembling?

Also, can I replace the top and bottom oil seals without complete disassembly of crankcase?  Seems like yes if I read the service manual and replace the oil seals carefully.

Also I see that the points on the bottom cylinder open only to 0.012 in… not 0.018 in as recommended… need adjustment (and cleaning). That could cause weak spark. I do not see pitting or wear on the points, I think they are OK.

I would put Evaporust in through the sparkplug holes, put the sparkplugs back in to reduce evaporation, and let it sit for several hours, while making sure the rust is not covered by the pistons and that the pistons are covering the ports so the Evaporust doesn’t leak out.  Then, drain the Evaporust out and check with your video scope if the rust is gone.  Repeat if needed.  I would then blast out the dislodged rust particles with carburetor cleaner, which would probably be more effective if you took the cylinder head off.  Maybe you could try to shoot it out the exhaust port(s) if you kept the cylinder head on, but be careful not to wedge it between the pistons and cylinder wall as it could cause damage there when your motor is running.  I bet official published procedures would recommend removing the pistons for an even more thorough cleaning, however for that little amount of rust I think it will be OK not to remove the pistons so long as you don’t let the dislodged rust particles go where you don’t want them.

I don’t know about your oil seals.

In that case I would just reset the points.  No need to wear them away by resurfacing unnecessarily.

[Sam M.]

Tubs wrote:

Is it really rust or varnish from dried up oil and fuel.

Mark, if this is the case then carburetor cleaner should take care of it.

[Author]

Posts