I purchased a 1978 Yamaha 650cc vertical twin for $100. I didn’t know it at the time of purchase, but the previous owner rebuilt the engine. When he rebuilt it, he set the cam chain 180 degrees off. Timing is spot on but it fires on the wrong stroke.
So after much hair pulling, I figured out the problem. My solution? Switch the left coil to the right plug and the right coil to the left plug.
It does run but not well. It fouls plugs and hesitates badly. Compression is great. I suspect that the mechanical advance is, well ass backwards because the engine is 180 degrees off. Though, when I check the timing with a light the advance is spot on.
The mechanical advance runs off of the same shaft that turns the cam for the points. I have checked everything else I can think of. The carbs seem fine.
I am prolly lucky that it runs at all.
My suspects for the fouled plugs and poor performance. ----
Mechanical advance is now 180 degrees off, causing poor performance.
Previous owner did not install valve guides correctly (would not be a surprise), causing plugs to foul.
Anyone familiar with vertical twins?
P.S. Yamaha dealers won’t even touch it - it’s too old. I may have a pit thread about that some day.
I’m not familiar with this particular engine, but I think you may be OK here. It sounds analogous to me installing a distributor 180deg off, and that worked out fine (once I moved all the plug wires).
It all comes down to the relationship between cylinder layout and crankshaft shape. If the cylinders are supposed to fire 360deg (crank) apart, you’re fine. Anything else, and it needs to come apart.
Have you checked both cylinders with the light? The timing mark should be in the same place for both.
The fact that it runs at all is encouraging, and if the compression is good, you’ve got to be close. I’d try advancing the spark a few more degrees or installing hotter plugs. You might also want to check the carb mixtures.
I wouldn’t worry about the advance. It either works, or doesn’t. It shouldn’t care about phasing.
I may pull the engine out and start working on it this winter. But I have a lot of projects that are more pressing (house stuff). If I had a garage, It would be much easier.
I am just fishing for ideas. I have the Clymer book for the bike. The rebuild does not look easy. On the other hand, it was tough getting to the point I’m at, but it sure was fun.
I had the bike on a trailer all summer to work on it. I built a shelter above it. The trailer is a car hauler, 2 x 8 pine for a floor. I was pulling splinters from that floor out of my knees all summer.
Yep both cylinders are timed correctly. 360 degrees crank.
Would a hotter plug help for high altitude? I live a 11,200ft.
I have run lots of 2 stroke bikes up here with no problem. They have never fouled plugs. I don’t have as much experience with old 4 strokes at altitude.
A fellow told me this engine fires on every stroke. Having it 180 out doesn’t matter. If so, you’re problem is somewhere else. I’m not the authority, but this might encourage you to ask more questions before tearing it down.
There should be marks on the camshaft ends that you line up the sprockets with, or the camshaft end may be half round so that it can only be fitted one way.
Bring the engine to top dead centre and line up the camshaft with the marks on the casing.
If it has a distributor rather than CDI then that may need lining up too, try something like 5[sup]o[/sup] before TDC and move closer.
You are fortunate that you haven’t hit the pistons against the valves yet so you cannot be far out in your timing.
That’s just it, the timing is spot on but for the wrong cylinder. So I switched the spark plug wires from left to right.
My biggest concern was whether or not the ignition advance would work properly for an engine 180 degrees off. I don’t see why it wouldn’t. It only advances the ignition. It does not affect the valves. So, why would it matter as long as the piston it sends the spark to has the correct relationship with its intake and exhaust valves?
I don’t want to rebuild the engine unless I have to.
Me thinks the problem is elsewhere and probably related to the mixture.
Jsleek-
I don’t think I would let that fellow work on your car ;). Most engines are 4 strokes. Or they fire every fourth time the piston reaches the top of the cylinder. The other common type is two strokes. Motorcycles, chainsaws and such.
Steam engines can be one strokes, the steam pushing the piston in each direction. I have heard of a one stroke diesel, but the source was somewhat questionable (and an insufferable know-it-all).
To further clarify, and to keep me from eating crow.
A lot of older Japanese bikes where 2 strokes, so the engine would fire every time the piston came up.
There are still a lot of new bikes that are two strokes, but would never encounter the type of problem that I described in the OP. Two strokes don’t have valves or timing chains, they have ports (holes) in the piston and cylinder to control intake and exhaust gases.
Your friend must have assumed we where talking about a two stroke. Did he read the OP or did you just explain it to them?
In any case, I didn’t mean to slam your friend. But there is a big difference between two and four strokes.
Thanks for posting, and welcome to the Straight Dope.
I don’t see why the engine wouldn’t run ok if the timing and cam were both 180[sup]o[/sup] out. Could the cam be a tooth or two off? Beyond that, I would check jetting or maybe timing (you’re not running aftermarket air filters, are you?)
Sorry I can’t help more. The only manual I have for this bike is a green “all motorcycles through '76” Chilton’s and these bikes rarely came into the shop. FWIW, you might be able to just take the head cover and adjuster off and slip the chain off the cam sprocket to time the cam.
Oh yeah, the cylinder head studs go through the head cover. You probably will have to pull the engine to adjust the cam timing (I take it the timing mark is behind the points plate, right?)
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I don’t see why the engine wouldn’t run ok if the timing and cam were both 180o out. Could the cam be a tooth or two off? Beyond that, I would check jetting or maybe timing (you’re not running aftermarket air filters, are you?)
That worries me too. The cam may be off 179 or 181 degrees. It’s close but not perfect. The guy I bought it from screwed up, and maybe screwed up an extra tooth on either sprocket.
But, I checked the valve timing on the motor as well. They where very, very, good. I made some very minimal adjustments. That tells me that it’s at a perfect 180 degrees off. Not an extra tooth or two on the cam chain.
At least it was only $100 dollars, she is a beautiful machine and she deserves to be back on the road.
I have the original air filters, and am using them. I get the same response from the engine whether I have filters on or off.
It seems to do better with them off. But not much. I wonder if it’s the altitude (lots of people say the same thing about me :D)
And yep, I will have to pull the engine to adjust the cam chain. I probably should just get over it and start from scratch.
I will have to put it on my list of stuff to do <sigh>.
The more I think about this, The more I think the carbs are just too rich.
This:
makes me think so even more.
Are you sure the choke is correctly adjusted? At 2 miles up, you are almost certainly going to need different jetting than stock, and who knows what the previous owner did with them.
As the owner of a 1975 XS650 who has been on a mailing list devoted to that particular motorcycle for a few years, I feel unusually qualified to answer your question. I suggest you try the following, in order:
subscribe to the mailing list. The signal to noise ratio is a little poor at times, but the amount of useful information is amazing. To subscribe, email listproc@micapeak.com with no subject line and a message body consisting of:
SUBSCRIBE YAM650 My Real Name
Where “My Real Name” is, of course, your name. Or nickname. Or your dog’s name. Whatever.
Clean the points contacts with emery paper.
Get a manual and a multimeter. Check the coils and the charging system, both of which are notoriously weak on the XS650.
Adjust the cam chain tensioner (between and below the carb intakes)
4a) Drain the oil, look for little pieces of black plastic or aluminum shavings. Plastic means the tensioner surface is wearing, aluminum means it’s gone and needs to be replaced.
Be absolutely sure the carbs are CLEAN.
Go to the list and ask for advice. I guarantee you’ll find your solution, most likely with URLs of a few pages with information and pictures of whatever process you may need to perform, as well as people offering to sell or give you parts, or a heads-up on the part you need on Ebay. They’re a good bunch.
If you need to open up the engine, It’s really not too bad. Undo all the connectors and whatnot, remove all the engine mounting bolts but the very bottom one, put down some carpet or cardboard or something, tip the bike over (gently, of course!) onto whichever side will allow you to pull the last mounting bolt out (ie make sure the bolt head is facing up). Pull that bolt out, give the frame a little shimmy, and tip it back up, with the engine still on the ground. If the timing is off, it’s not hard to fix: there’s a clearly visible mark on the camshaft gear that should be pointing straight up when the timing mark is at TDC.
Unfortunately I don’t have my manuals close at hand, so I can’t give you the exact specs you may need, but I’ll try to remember to fetch them later, so if you have any specific questions, please ask. I urge you again, however, to get on the mailing list. I think you’ll find it’s well worth it, and the folks there will provide you with (literally) more information than you’ll need. However, if you’d rather not, I’ll answer your questions as best I can, and run to them when I need help.
Oh, and the advance mechanism can’t be installed backwards, so don’t worry about that. It can, however, have weak springs, which can be replaced with Briggs and Stratton lawnmower springs for cheap. If you suspect the advance is fubar, try wedging something in there to keep the weights either open or closed and see what happens.
There’s plenty more that it could be, but electrical or carburetion problems sound most likely to me, in that order. Good luck.
Wups, I didn’t notice your altitude. Could very well be that the carbs need to be rejetted, too, as pmh just mentioned.
And a tip I forgot to mention: If you need to fiddle with the camshaft, you don’t need to break the camchain to do it. After removing the head, slide the bearings off one end, then the chain can be lifted off the gear and the shaft can be pulled right out. Just make sure you don’t let the chain fall into the engine.
Also, if you suspect the timing is a tooth off, which sounds unlikely, you can take out a plug and put a dowel in the hole. Turn the engine over by hand (using a 17mm socket on the rotor nut, or the kickstarter) and see if the dowel reaches its peak at top dead center, then starts coming back down.
If those springs are shot, the motor will be running with full mech advance all the time, so there won’t be any left when it should come in. That would definitely cause your symptoms.
This is why I always check ign timing at 4k rpm or so. Then you can be sure you are reading the total advance and not somewhere on the “slope”. Of course, the total advance (~30-40deg) will probably not be specified in the manual(s), and you’ll need to play around with it to see where the motor is happy.
