Engine turns only 90 degrees

[lakeview]

This el nino is really something! 22 December and I am working in my unheated garage in Canada in a jean jacket and no gloves! Tomorrow is predicted to be more of the same.

I was able to combine good and better over the weekend by picking up a spare motor 2 hours away on my way home from inlaw xmas. I was told it was seized and there was a glob of JB weld on the oil gland nut, missing the carbs and the starter, but it was otherwise complete and not that dirty. Into the stand and clamped stand onto a dirt bike lift, started dismantle it to freshen up. Once the rocker box and clutch side off, put a wrench on it and it turns a bit. Find neutral and try again and I get 90 degrees on it, but there is a mechanical stop somewhere in the lower end. There was no debris on either drain plug. I intend to take the clutch basket off tomorrow and look some more, as well as take off the bottom inspection plate, but at this point, is having a motor that only goes a quarter turn something that any of you experienced.

I am guessing there may be something going on in the gearbox or behind the clutch.

 

[fredintoon]

Hi lakeview,
hopefully you'll find it's something that's an easy fix.
And you know those temperatures ain't gonna last, eh?
FWIW Princess Auto's boxing day flyer has bargains on garage heaters.

 

[gggGary]

Push the bearings off the end of the cam, slide the cam out from under the sprocket and remove the head. I'll bet you will see your answer.

 

[angus67]

I give mis-information

 

[fredintoon]

Hi lakeview,
even if you do find an easy fix, you dunno anything about that orphan engine's history, eh?
I'm sure you'd hate doing a top end rebuild and then have the bottom end frag itself a week later.
I'd say tear it all down and carefully check everything before putting money into it.
And make sure it's squeaky clean in there before you reassemble.

 

[Richardg]

First this I would suspect would be a hung valve, I would remove the spark plugs and look in the hole to see if all the valves were moving up and down when the crank is rotated. Pistons should go up and down and the valves should open and close. I would also check for loose rockers. The cam could also have jumped time which could make a valve hit the pistons. A really hot bike could hang a valve. Whatever you do don't force anything. If you couldn't find anything wrong with the crank or cylinders I would remove the clutch basket to separate the crank from the transmission and eliminate it as a problem. Just start checking everything out and start with the simple stuff first. The cranks are pressed together on those bike and they can separate under certain conditions but you would see lots of aluminum shavings and evidence of catastrophic failure, so don't even consider that until you ran out of all other options.

 

[NONclow]

......is having a motor that only goes a quarter turn something that any of you experienced.

First this I would suspect would be a hung valve.....

This is what's left of my original '71 engine. Last pic is the adjacent chamber.

 

[lakeview]

Head's off, no hung valve, cylinders propped up, one piston does not want to slide free, so a little clean up of old magic fluid, the application of some fresh, let it sit for a bit while I reread the seized motor threads. I tapped a bit with a small hammer and a piece of oak, didn't want to move so will let the mechanic in a can work for a bit.

 

[lakeview]

All is Revealed:

By blocking up the barrels and adding more spacers as the pistons moved down, the dismantling was completed without resorting to brute force. All is well.
However, the cam chain guide had no plastic in it at all, the adjuster had grooves worn in it and I think the cam chain is toast given how much it deflects when I pull it to the side. No force was needed to bend it that way.
The bottom oil strainer was holed of course, but no obvious metal shavings, just crud build up.
I thought that the cam would have had 447 marked on it, like the rods do, but it only has a " 1" stamped on it, as shown. The pistons were original, marked 963, haven't measured them or the barrels yet to see if a rebore is needed, I am guessing probably.
Now that the pistons are out, the motor has no problem rotating.

 

[lakeview]

Hello Fred: Canadian Tire and Princess Auto are two of my favourite stores, other one I guess is Loblaws.

 

[lakeview]

Just found the plastic slider for the guide all in one piece down in the base. The piston tops and the combustion chambers are really free of carbon, I think I even see the sandpaper scratches on the head of the pistons. I am thinking someone did a quick cleanup on this motor, bolted on a bicycle pedal kicker, stuck it in his hard tail and then left it on the sidestand for a winter. By spring the damage was done and it was parted out.
Cheers and Merry Christmas/Holidays/Kwanza/Festivus!

 

[Richardg]

So you'll need to replace the cam chain and guides. I would clean the pistons and hone cylinders, then see if you can cram a .004 thousandths feeler gage between the pistons and the cylinder wall. The reason I say the is that .004 is too much. A .003 should slide between them and .004 shouldn't. It kind of gives you starting point on whether it needs to be bored, or not. I would also want new valve guide seals just to be safe. You might be able to just ring it and run it . Kind of depends on how much you want to spend. If you're not going to race it you can be a lot less picky. Having the valve train and timing right make the biggest difference on how smooth they run at speed, because of that it is worth spending money on, in my opinion.

 

[gggGary]

Well that looks like enough rust I'd be doing a rebore, then again I stock oversized pistons and bored and honed sets of cylinders to match. It would be nice to see a front shot of the pistons and maybe a shot of the top of the rods after you remove the pistons....

 

[lakeview]

OK, here are the photos.
The rod ends, one has a bit of silver showing, the other looks to be all copper; this of the head shows how it has hardly any carbon build up, but is it normal for the valves to seem to be inset?; the pistons with the brown ring around the pins, (?), on the piston that was tight, the rings are stuck in the lands, waiting for the "special sauce" to work; finally, the screw heads in the clutch look clean and a JIS bit must have been used if it has been dismantled, the springs are showing a blue colour still, don't know if that is the colour of new ones or after market.

Any opinions or comments welcomed

 

[5twins]

Things look pretty normal and not too badly worn. Slide each piston pin into it's respective rod small end to check the fit. They should be a sliding, slip fit with no side to side wiggle. The brown bands on the pins are probably just hot oil stains. Those areas are exposed when everything is assembled.

The scrape marks on the piston skirt are normal as well. You may find some in the bores as well. See if you can feel them with your finger or finger nail. If not, then the wear isn't too great. That "963" on top of the pistons was their size when new, as in 74.963mm. There should be another number stenciled on the side of the cylinder sleeves where they hang out the bottom of the block. That is the fractional part of their size when they were new, as in 75.XXXmm. The factory clearance spec was .050-.055mm so you should find something between "013" and "018" stenciled there.

You can measure the piston skirts now to see how much they've changed (worn) from new. The aluminum alloy pistons are softer than the steel bores so usually wear first (and more). Sometimes, just new stock pistons will return you to "in spec" measurements.

 

[lakeview]

Good to hear, thank you.

 

[Edgewood RI]

"and then left it on the sidestand for a winter. By spring the damage was done and it was parted out." Is this bad? Side stand all winter?

 

[gggGary]

The frozen engines I find usually were sitting exposed to weather with the airboxes or carbs removed, water gets in the intake, game over. For whatever reason the old Hondas seem more likely to stick from just sitting than these Yamahas. The 305-350 Hondas were kind of notorious for sticking although I got several "unstuck" and rode them with no obvious ill effects.

 

[lakeview]

"and then left it on the sidestand for a winter. By spring the damage was done and it was parted out." Is this bad? Side stand all winter?

That was more a guess than known reality, however, storage is always better inside and dry.