TOPIC: cylinder #2 running hot

Hello,

I am still working through issues on my 1964 500 50 HP. Seems like I make one step forward and surprise, something else pops up. Over the winter, I replaced the spark plug wire/boots and plugs (Champions J6C). No telling how old the plugs were. While checking some of the many other issues, I noticed the 2nd cylinder was running hotter than the other three cylinders, the coldest heat reading was on the lowest cylinder. I have a small infrared heat meter and took readings at the base of the spark plug. After running the motor, the #2 cylinder plug began to change colors. I had checked compression and spark on all four cylinders. The bottom three cylinders were all 120 and the top was less. Like I said some issues to work through. I thought the reason #2 may be hotter was a bad spark plug or one not gaped properly. I kept those Champion plugs in while I ran through lots of Seafoam looking to loosen up the rings on #1. No sense fouling new plugs. I did gain some compression in #1. So I changed out all the plugs with NKG. Ran it through a full 6 gallons @ 2 hours of running. Looking at the plugs today, I see #2 plug is dull again. So #2 is still running hot.

What would cause this?

Thanks
Joe

It is a strange one indeed. I have the same issue with my 1960 Merc 400 only the two center cylinders are running hot like yours. I know my block needs to be replaced as I have a couple of cylinders which are worn out of spec, but it should not cause the plugs to get hot. I will be tearing mine down shortly to rebuild it with a better block, I will let you know if I find anything related. Did you get any more RPM out of it Joe?

Hi George,

Thank you for the private emails. They were much appreciated. I have not done any more. I was working up my plans of what to look at and try, when I noticed this discolored plug. So I wanted to get an idea of what this means, before I looked at my list of other issues to pursue. I thought it was just a bad Champion plug, but it is not looking that way.

Joe

I wonder if it is the plug getting hot and not actually the cylinder. If the cylinder was get that much hotter I would expect detonation or something. Move the plug AND the wire to another cylinder and see if the problem moves. By move the wire of course both ends

Is there a thread repair insert in that cylinder ?

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A steel insert would not transfer heat away the same as an aluminum block.

In your photo - the #2 plug looks to be cocked upward. It might be distortion of the image. But you need to see that the plug is at a perfectly right angle to the block. If the plug does not fit squarely into the plug hole, it will leak combustion gases, creating a tremendous amount of heat. Professor's question about the thread repair is also a very good one - has the thread been repaired? It it has, was it installed correctly?

Also - if these things are not a factor - how does the electrode of the plug look? I don't think you mentioned that. If the cylinder (not just the plug) is running hot, your crankcase flange seal may have failed and is causing a lean condition on that cylinder. A leaking transfer port cover could do the same. There is also a series of "weep" check valves on the crank case - these have a little tube that drains into the exhaust tower. The valves can become stuck & cause problems.

I just took my flashlight out to look at the cylinder head. The Professor was correct –nice catch !! There is indeed an extra ring on that cylinder. I think it is called a helio-coil???

I never noticed it until I looked for it tonight. Amazing that you could tell that from the extra heat build up at the plug.

So what does that mean for this motor? Is it a non-issue? Should I use a different plug in that cylinder? Do I use a different plug in ALL of the cylinders?

I was considering a rebuild for this motor. Is that a bad plan given the helio-coil?

Joe

It's Heli-coil as in a helix, (a spiral) A helicoil looks like a spring threaded into the aluminum. They are usually Stainless Steel. If what you see looks like a ring of metal that is flush with the casting - it is probably a "Thread-Sert" which is a spark plug hole repair device. It essentially a hardened steel collar with the correct spark plug threads on the inside, and oversize threads on the outside. A special tap is inserted into the stripped spark plug hole and threaded into the block. This cuts new threads for the Thread-sert. The tap is removed & the Shread-Sert is coated with high strength locktite and threaded into the block. When the insert is installed at the correct height, a special expander is inserted into the repair collar & peened with a hammer. This is supposed to expand the collar into the block to lock it into position. If done carefully and correctly, they can work pretty good. But people often put them in crooked, and I think the one in your engine is crooked. If you are going to restore the engine, it is best to have the cylinder tig welded and new threads cut. This should be done in a machine shop to be certain the threads are cut correctly. You could look for another block, but these old engines often have buggered up spark plug holes.

If the coils are done right there is usually no issue with them,I have done lots of them. But if yours was installed crooked they will indeed overheat. Blocks are common enough however, you may be able to get a replacement.

some body use to make a deal that you could face the cylinder head off when you put a compression relish on a 2 cycle motor cycle
a good machine shop could resurface the plug seat strait with the plug threads and save the block so you would have good heat transfer to the plug
Charles

Time-Sert makes some of the best spark plug hole thread repair kits around, they are not cheap though!

One such kit is available to "repair the repair", it'll take a damaged/oversized spark plug hole and thread it back down to standard size.

Here's one retailer I found with the "Big Sert" repair kit:

www.denlorstools.com/home/dt1/page_54100/time_sert_5141s_m14_x_1.25_big_sert_oversize_spark.html

Here's a used-once kit on eBay for less:



You could probably do the same thing and sell it again on eBay, I'm sure there's other folks that would appreciate the discount on an almost-new kit.

Although a few repairs would probably pay for themselves, considering the cost of replacing a block.

I've used Time-Serts before on outboards and motorcycles, and their tools are a pleasure to use. The insert itself is a wonder to behold, and the repaired hole is far better than how it came from the factory.

If I did my calculations correctly, these 9.4mm aluminum inserts are the correct ones for a 3/8" reach spark plug:



HTH & G'luck with the repairs.......ed

Remember how I said my 400 was doing the same thing? Well I just checked and all the cylinders that have overheated plugs and they have the helicoil insets in them. No signs of damage, but I guess the inserts just don't transfer the heat away as well. Mystery solved.