Maxim 400 with dead cylinder

[wirehairs]

Yesterday, I thought I was at the very tail end of restoring a girlfriend's 82 Maxim 400. I was zooming around local streets, and braking hard to break in the new front brake pads, and then I took her for a ride. Halfway through that ride, the bike started to hesitate on acceleration, and once we returned, noticed a little (white?) smoke coming out one of the mufflers. I pulled the plugs and one seemed ashen and the other oily, but I'm not so sure anymore. I started it up a while later, and realized that only one cylinder (the left) was firing. When I pulled out cylinder 1's spark plug and ground it against the cylinder head, it was sparking. So, I'm assuming it's a problem with the fuel flow and thus carb number 1. I had cleaned these out previously. I did a compression test last fall after adjusting the valves and got 130 and 134 I think, which is respectable for a 400.

Anyways, I wanted a little confirmation that pulling the carbs off again and looking inside is the best course of action. Not sure what I should be looking for in there though.... Advice appreciated!

 

[JARichmond]

I'd give it a go with fresh plugs. I had a similar problem where one cylinder would drop for no apparent reason. I cleaned the carbs a bunch, tossed in sea foam whenever I could, but to no avail. I had a look at my plugs and found out I was running Champions rather than NGKs (equivalent, but ). So I replaced them with fresh NGKs and haven't had a problem since.

 

[wirehairs]

Thanks. I was thinking the same thing too. For a few bucks, it's always worth trying the plugs first.

I'll let ya know how it goes!

 

[wirehairs]

Uh oh. I might be in trouble. Here's the deal: I have oil fowling my right spark plug. After 5 minutes running or whatever, it will fowl it to the point where it stops sparking. I'm thus assuming that there are only 2 options: either my piston rings are worn and leaking, or my valve seats are. Correct? I'm testing 134 PSI on the right, 130 on the left. These are within spec, but maybe my compression gauge is reading high? After I put the valve clearances (shims) within spec last year, and got those readings, I didn't expect to have engine problems.

So first things first: am I correct that I need a valve or ring job? If so, how do I diagnose which one?

The bike has 15,000 miles on it.

 

[16VGTIDave]

It could be the intake valve seal that has failed. But I wouldn't expect the plug to foul that quickly. Possibly the head gasket is leaking as the oil for the cams goes up the right head studs.

A leak down test will tell you if the rings, head gasket or valve seats are good or not. You can also remove the carbs and right carb mount and take a look into the intake port and see if there is oil on the right intake valve.

 

[wirehairs]

Thanks for the input Dave. I have ordered a leak down tester, so hopefully that will help me diagnose the problem. BTW, if I see oil residue on the air filter, I still don't know if that's coming from a bad valve, rings, or gasket right? I haven't checked it (the bike's at my girlfriends), but I'm just thinking out loud.

BTW, yesterday I acquired another complete Maxim 400, so at least now I have an engine to swap out if it comes to that!!

Now if the leak down test shows I've got dirty valve seats, can I just pull off the top of the head to hone the valve seats? I don't have to mess with the block, do I? I imagine I'll have to dick with the camchain and such.... This is new territory for me.

 

[Wolfe_11B]

Could you be pulling oil from the breather tube? That would explain oil in the cylinder and on the filter.

 

[wirehairs]

Nah - that would require me overfilling the crankcase, which won't happen.

 

[wirehairs]

So, I've ordered a leak down tester that should be here in a couple of days. However, while waiting, I ended up buying another (almost complete) Maxim 400 for $150! I mostly care about the motor, because if it's blowing good numbers, I might just elect to swap them out. However, the ignition switch is missing and when I tried to bypass the switch using this technique:

I couldn't get the starter to turn the motor over. Any advice there?

If I don't figure the issue, I was wondering: won't a leak down test tell you everything that a compression will plus? If the leaking is below 20%, am I safe to say that the compression of this new motor will be within specs?

Thanks for the input!