650 to 750 conversion question

[Wharfcreek]

I've been away from this site for a while....but I'm determine to get back involved; both here, as well as on my small fleet of XS650s. I think I've got 6 or 7 if you include 'potential' complete bikes. I know there are 5 out there that are together and candidates to get on the road quickly. And, 1 or 2 more in varying stages of being stripped and either parted out of to be restored.

Anyway......one of these bikes was an old 'flat-track' beast that I picked up a few years ago. This bike has the full Omar's kit, as well as a lot of other additions as well.....including the 750 kit. As a bit of a side-bar, I was selling one of my Harleys a few years back, and the buyer, who was from another State, happened to look in my garage and saw the bike......and recognized it. He claimed that he had it in his shop at one point and determined that the pistons were in it backwards. Evidently the 'arrows' on the piston tops were facing forward, and he claimed they should be facing rearward. Supposedly he did reverse them, reassembled the bike, but it still ran poorly. After my purchase of the bike, I managed to get it running, and 'poorly' is a pretty good description. But, back to the claims from this Harley buyer: According to him, he was suspect that the cam was out of time.....not by the chain, but by the cam drive gear as pressed on to the cam. I suppose this is yet another question for another thread. But....as related to the pistons, can anyone give me a definitive answer as to which direction the arrows ought to be pointing? For the record, I think I'm going to simply tear the top end off this bike, pull the jugs and pistons, and fit them to another engine that I know is running OK. But, when I do this, I want to make sure I don't put them in backwards as well.

Many thanks!! WC

 

[Bushyeyed1]

Oh yeah the arrow should be forward. The piston is shaped to fit the exhaust and intake valve. One is bigger than the other. Hopefully it didn’t strike the piston.

 

[Wharfcreek]

That's what I would have guessed. BUT....this guy (the Harley buyer) claimed that the arrows were supposed to be pointing to the intake ports. It may be that I'll be able to just look at the piston top when I pull that engine apart and see which side of the piston would 'better fit' the intake and exhaust valve configuration. I fear it may not actually be too obvious....in which case I'll be going to the responses here for a final decision. And, hopefully you're right, and that no internal contact has been made which might have damaged anything.

Thanks for the response. I hope to get to this engine soon and if nothing else just tear the top end off so as to clean and prep the piston/cylinders for a transplant!

Tom D.

 

[Bushyeyed1]

The intake valves are noticeably different from the exhaust valves size. Maybe on a Harley it’s different but I highly doubt it. I guess you’ll see once you tear it down; good luck and hope to see some pics of the progress.

 

[Ratranger]

Maybe on a harley he's right, but the cylinders on those mirror each other with the intake in the center. XS pistons even without a dot or arrow the big valve pocket goes to the rear.

As for running bad, that could be timing(cam or ignition), low compression, carbs, I've seen pics of cams in backwards, or it could be a bad ground giving weak spark.

 

[gggGary]

Yes aftermarket cams do need to have the sprocket pressed on, position is fully optional, there are no sprocket "timing marks" since there are 36 teeth you are always within 10 degrees. Decking to change compression will also affect cam timing. Full race cams can be just that, not designed for street use or the ability to run well at low RPM.

 

[Wharfcreek]

OK, as to the subject question, I'm going to assume that piston direction will be rather obvious when I remove the head. So, arrows or not, I should clearly see the larger and smaller casting depressions, and simply align those with the intake and exhaust valves. Guess we can consider that subject put to bed for now. I'm going to assume that since the engine did run, albeit poorly, the direction in which the pistons are currently installed is correct. I would guess that a compression test would confirm if any valve contact might have done any damage. Assuming the engine compression is in the acceptable range, I'm just going to start looking at other issues. The engine really was a slug when I did get it started and running. Admittedly, I didn't pursue it. It has the 750 kit, a performance cam, round-slide carbs, an electronic ignition, and some kind of stainless steel exhaust system with some larger tubes. I'm pretty sure that if it was running properly, it would be pretty impressive. But, the most I could get out of it was to crawl along as if running on one cylinder (which it actually wasn't)......and probably not much more than walking speed in first gear. Pretty pathetic! I had hoped that as I rode it up the block, it would clear out and rev up, but it didn't. When I got it back to my garage, I pushed it in to the back, and there it's sat for the last 5+ years. So, carbs, cam, ignition......all the above, something is clearly wrong with the thing....and I just haven't had the inclination to deal with it until now. But, it's time, and I need to either fix it, or get it gone 'as is'. Aside from the added on Street Tracker kit (which I believe to be the Omar's version), the bike has some impressive wheels and tires, fork braces, aftermarket lever assemblies and ft brake master cylinder, ft and rr disc brakes, and is pretty well 'race-ified'. I'd like to get it running right.....but given the variables of possible carb problems, ignition problems, cam timing error, and even this 'piston direction' issue...... it will likely be no small challenge! No doubt you'll be seeing more of me here.......... Thanks guys! Tom D.

 

[gggGary]

One thing at a time. You can do it!
Before you tear down do a compression test.

 

[Wharfcreek]

Roger that! Thx!