bottom cylinder fouling Big Twin 25hp

[The red boat]

it was running well all summer but it was getting harder and harder to start.
per my first post on this thread I went to change out the spark plugs and somehow managed to strip the bottom hole.
So I put on a NOS head and it hasn’t run worth a darn since then and is fouling the bottom cylinder and plug.

I’ve removed the NOS head and am now trying to replace/clean everything I can while avoiding cracking the case open as a last resort.
I’m going to put on the original head and use a helicoil in the bottom hole since I know it ran fine with the original head.

I was surprised to see that I had parts missing from my upper seal and that there was quite a bit of oil on the mag plate. My first thought was that the excess oil was messing with my ignition system and causing weak spark on the bottom cylinder or that the ignition module for that cylinder was not working perfectly.
then i saw there was oil inside the cur-out vacuum tubing and thought it may have had an effect on the cut-out switch too. That could just be from excess unburned fuel tho.

[fleetwin]

Am assuming the new head is back on now….If not, did you compare the new and original heads? Perhaps there is some difference or flaw in the new head….
It just seems unlikely that with all this stuff going on the engine runs “OK”, even though it is hard to start. Normally, all the oil on the mag plate would foul the points, but your engine has the modules so I wouldn’t think the ignition would be affected by the oil.
I’m pretty sure this engine uses the strange upper seal system that does not have an oring around the upper bearing, relying on intake vacuum to pull oil away from the mag plate. Others will know for sure whether or not this engine has this strange system. Does it look like the powerhead has been split apart before? If so, perhaps someone mistakenly installed the oring on the upper bearing which is a big mistake. I suppose it is possible that all this excess oil is being pulled down into the intake manifold and into one of the cylinders. Please don’t rip this engine apart based on my theories…
You have got a lot going on here, so many possibilities for confusion here. I don’t know much about those atom modules, so that is another source of confusion. You mentioned checking the overboard drain system, are you sure the drain in the bottom of the crankcase is open?
Again, you have got alot going on here, many sources for confusion. Might be best to start over from the beginning… Perhaps figuring out why the engine is hard starting….

[Mumbles]

With parts #’s 91 and 92 missing, the top seal would be inoperative and this is likely where the oil is coming from. Since the oil can get out, that means air can also get in, causing a lean running cylinder.

Maybe the bottom cylinder is burning properly, and the top one is running lean and leaving little deposits on the piston. It might be time to rethink this.

[RICHARD A. WHITE]

With the availability of good quality new coils, not sure why you would go to the Atom modules?

http://www.richardsoutboardtools.com
classicomctools@gmail.com

[fleetwin]

Mumbles wrote:

With parts #’s 91 and 92 missing, the top seal would be inoperative and this is likely where the oil is coming from. Since the oil can get out, that means air can also get in, causing a lean running cylinder.

Maybe the bottom cylinder is burning properly, and the top one is running lean and leaving little deposits on the piston. It might be time to rethink this.

Good point….

[fleetwin]

RICHARD A. WHITE wrote:

With the availability of good quality new coils, not sure why you would go to the Atom modules?

Maybe he switched to the modules because the points kept getting oil fouled….

[The red boat]

I’ve had the Atoms on the motor for about 3 years now and they have been great.
I didn’t have any problems with the points but I had a hard time finding condensers for some reason.
I finally came across some and installed points and condensers and it was all fine. but a couple of these modules fell into my lap so I tried them out.
Like I said – 3 years on and they have not given me any issues. I really liked them. I also like removing the points in my old cars and substituting electronic ignition in them too.

When working they are really maintenance-free and the motor will put along at 400 – 500 rpm without a problem.

to answer fleetwin – It doesn’t appear that the crankcase has ever ben opened, it is a very clean motor with good paint and I don’t see much evidence of previous work. It came to me as just a powerhead and without a mag plate so perhaps the missing parts just got lost in the transition and I wasn’t knowledgeable enough to know they were supposed to be there.

I wont have time to test out the spark until next week but I’ll update everyone when I do. I’m anxious to see the effect of a complete top seal now that I have one installed.
thanks for all the replies.

[The red boat]

Used a spark tester to check the spark and I have a nice bright white spark on both cylinders on every pull of the rope. Jumps about 3/16” or so.

Hope to put the motor in a tank after thanksgiving and fire it up.

[The red boat]

I can’t believe my last post on this subject was back in November! I finally put the motor in a tank this weekend – FINALLY

It took a while to start but when it did, it only ran on the top cylinder. Ran well in fact but no fire on the bottom cylinder.
After the spark test in my previous post this was surprising.

What I discovered was that the Atom module for the bottom cylinder was not working consistently. Maybe my wiring wasn’t secure or it was somehow grounding out but I could not get it to fire consistently.
I swapped out another Atom in its place and the motor fired off an idled perfectly in the tank for as long as I wanted it to.

I let it warm up so I could make another pass and do a final torqueing of the head bolts .
so, for now, all is well. Looking to get on the lake in a couple of weeks to fine-tune my carb needle adjustment.

thanks for all the replies and advice as always.

[Mumbles]

It’s good to know someone else likes the Atom modules to!

• This reply was modified 3 years ago by Mumbles.
• This reply was modified 3 years ago by Mumbles.

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