Saving a 1980 750 twin.

Your bike looks great! Ed

the seat turned out very nice !
really changes the looks of the bike for the better!

leon

All this prep was to take the bike on a road trip with a good friend. Him on his 1978 650 B we rebuilt together a couple years ago. The bikes performed great. The only issue being my exhaust hangar bolt vibrating loose at one point, but a field fix was easily had. We did 900 miles in 5 days through the Cascade mountains of Washington state. Beautiful riding crisscrossing through the mountains. We strictly stayed on secondary highways the whole way and even got a good amount of forest service gravel road riding in. We camped out in the national forests every night, went to a different brewery every evening to fill growlers up to take with us and cooked out over a fire every night. Washington state has an incredible amount of ecosystem diversity, the riding was awesome. Good weather most of the way, We did ride through a small squall but other than that it was lovely.

Very cool Doc! thanks for posting the pics it looks like you guys had some great riding

Looks beautiful Doc! That's my kind of road trip! Even though, I am a bit nervous about camping in bear country. Beautiful bikes!

SPECTACULOUS!!!

So now we get to the bad news. Just before I left on the trip I noticed a sound coming from the top end. It was a ticking noise that had a ring to it, almost like a rattle. It sounded like it was coming form the valve cover and only on the right side. I checked all the valve clearances, and and checked the cam journals with plastiguage and everything seemed to be fine, cam chain looked ok. I thought it might be the cam sprockets making extra noise since i removed the rubber dampers from them in order to slot the sprockets to time the cams. The sound got worse over the course of the trip and I noticed the oil level was low, and dark. Indicating some blow by. Compression was still good though - 220 psi on both cylinders. Blow by got so bad that riding around in LA after the trip I burned through 1.5L in about 50 miles, which ended up running low on oil without realizing it. :sick: I tore the top end off to find the wrist pins galling and nearly seized in the pistons again this may have been from the low oil levels, but I cannot be sure. Also an incredible amount of carbon on the piston domes and in the chambers. I guess the cheap piston rings weren't a good bet. either that or the 1 piece oil ring was doomed from the start. The wrist pin bore on the pistons are trashed so I have been exploring other piston options. I dunno if it was my poor assembly, the one accidental 11k over rev I had on the fresh build or something else that caused the problem but either way it will not be a cheap mistake to remedy.

I have been doing a bunch of research on piston options to move forward with. As far as I can tell there are 4 choices to move forward.

1) find some OEM overbore pistons - about $350-400 with rings and wrist pins depending on what deals I can find. I'm not really considering this because I want MORE POWER!!

2) Custom JE pistons at 80mm bore - 783cc . custom pistons w/ pins and rings - $400 for two - min order of 4 pistons, I could probably sell the other set. This allows me to use the stock sleeves and stock head gasket.

Anything bigger than 80mm bore will require new sleeves. the cost of new sleeves is 65-150 each depending on what needs to be done, plus an additional $65 to bore the cylinder for new sleeves. It will also require the cases bored for the bigger sleeves - Something I can safely do myself either on a friends mill or with a die grinder. Cometic already has a 85mm gasket for this bike for $55 each. That is about what I was paying for the OEM green gaskets.

3) Other platform piston -ie. Raptor 350 84mm piston - 864cc $260 for two. May or may not work with what i have, probably less than ideal for the combustion chamber design I have.

4) Custom JE piston at 85cc - 884cc again I end up with a second set


The total price breakdown with boring and a gasket will be

1)OEM $570-620

2)custom 80mm $620 + $400 in pistons that i may or may not be able to sell

3)Raptor 84mm $735-915

4)Custom 85mm $855-1055 + $400 in pistons that i may or may not be able to sell

The benefits of getting custom pistons are they will be optimized for the combustion chamber design, and I can have custom features included to help with wrist pin galling, as this is the second time its happend on this bike. for example - forced pin oiling - where a hole is machined from the oil ring to the wrist pin so as the piston drops in the cylinder it forces oil directly to the wrist pin bore.

I'm going to drop my cylinders off at LA Sleeve which is like 1/4 mile from my work. When I get a hard number on the cost of the sleeves, this will give me a concrete budget to look at.

Any thoughts or suggestions?

That really bites. I think I'd go with he JE 80mm option, but none of those are really my favorite choice. Like you, my first choice would be that the original setup worked just fine.
Sigh....

Looks like a rod was seizing on the wrist pin.
Steve

SWest wrote: Looks like a rod was seizing on the wrist pin.
Steve

Absolutely. also seizing on the pistons as well.
I think I’m going to get DLC coated wrist pins and I will have the small ends of the connecting rods bushed with a bronze bushing. That will lower the fiction coefficient quite a bit, but I need to come up with a solution to increase lubrication at the wrist pin

I like your oil ring idea. Maybe a couple pointing to the wrist pin area?
Steve

That bike is absolutely beautiful, Doc. Sorry to hear about the troubles you ran into but I am looking forward to read about the path you take for the solution.