TX650 Suddenly Running Horribly

[BoatBiker]

Hello all - brand n00b here, looking for ancient wisdom of the 1970s...I have recently acquired a very nice original condition TX650 - all there, great paint, great chrome, and ran great once it had a new battery and fuel. Unfortunately, while riding last week (have put quite a lot of miles on since aquisition) it suddenly began to stutter and cough and miss. It feels like it isn't firing on every revolution, on at least one, or both cylinders. However, the bike starts, idles and revs (in neutral) just fine, but has zero power once you put it in gear and try to pull away.
I cleaned plugs, which had a little carbon but were in pretty good shape. I put in new plugs and new plug wires but not caps [couldn't find any in my little town]. Runs exactly the same. I had a suggestion to check the coils' resistance, and I find that on right I have 3.9 Ω on the primary side and 8600 Ω on the secondary [which I understand is normal], while on the left I also have about 8500 Ω on the secondary, but only 2.5 Ω on the primary. Could this be the cause of the misfiring? I found a lot of info here on what the resistances should be, but nothing about what the symptoms might be of low primary resistance numbers...(All ignition components appear to be stock, points are in great shape.)
To be honest, I am not even sure that this problem is caused by elec rather than fuel issues, but I thought I would check basic elec components before tearing into carbs...Thanks in advance, J

 

[MaxPete]

Howdy BB: well, it sure seems like either an electrical or fuel problem (duhhhh ) and it sounds as though you have eliminated most of the electrical issues. As I understand it, the engine did run well - but now does not rev-up under a load - although it does start and idle satisfactorily.

I have two suggestions for where to go next:

1. Check the mechanical advance mechanism. It lives under the right hand camshaft end cover and is held on by two big JIS screws (looks like Phillips - BUT are NOT Phillips). You will see two little springs which pull two flyweights in toward the centreline of the mechanism. As the mechanisms spins with the camshaft, the flyweights should move out (against the springs) and that causes the ignition points actuating cam which is under the left hand camshaft end cover, to rotate a little bit - which advances the spark (causes it to happen earlier in the engine cycle). It is necessary for the spark to happen earlier (ie. advance) at higher engine speeds to allow combustion to occur properly - and if that advance does NOT happen, the engine will not rev-up. Checking the mechanism involves ensuring that the springs are there and that the various moving parts on the mechanism can move freely and return to their "rest" position when the engine is stopped.
2. I wonder if your fuel tank may have had some debris in it which has come loose and is now clogging the carburetor jets. Others know far more about this than me - but I'd check the high speed jets first - since the engine starts and idles OK.

Anyhow - those would be the next steps as far as I can see.

Cheers and best of luck!

Pete

PS - on the matter of screwdrivers: if you haven't already, I suggest that you go on-line and get yourself a really solid set of JIS screwdrivers. JIS = Japan Industrial Standard - and while they look like Phillips, they are actually significantly different from Phillips drivers, to the point where a Phillips driver will often damage or strip the JIS screws which were used to build our bikes (and still are used on Japanese cars and bikes).

Conversely, a JIS driver works fine on a Phillips screw and so they are very versatile tools. Here is a graphic which illustrates the difference.

You can buy a nice set of Vessel brand JIS drivers on Amazon for about $60 and a firm in London, Ontario also makes a great set for $24.95 CDN shipped in Canada (I think US shipped price is 34.95 USD). They are at: www.gofastinnovations.com - and are well worth the money.

 

[xjwmx]

You can get jis bits on ebay now too. My good screwdriver is a magnetic Craftsman handle that holds those hex drive bits you see in buckets at the checkout.

 

[MaxPete]

You can get jis bits on ebay now too. My good screwdriver is a magnetic Craftsman handle that holds those hex drive bits you see in buckets at the checkout.

Not wishing to be argumentative - BUT - I'll bet they are Phillips and not proper JIS bits - and they simply are not the same. Really, I've never seen JIS drivers or bits for sale in any North American store.

The only place to get them, of which I am aware, is on-line on eBay, Amazon.com and GoFastInnovations.com.

Anyhow, YMMV - but when you get an actual JIS screwdriver on a JIS screw - you'll feel the difference.

Cheers,

Pete

 

[xjwmx]

^I could never find them as bits anywhere until a couple of years ago. These come from Motion Pro. I know a lot of people use them with the T-handle that you can get with them. In general I like bits more than screwdrivers, especially the flat ones, which have a concave face so that the blade edge is square and fits the slot better.

 

[MaxPete]

Cool - have you got a photo?

I'd like to find some of those - they would be much more compact for the onboard toolkit!

 

[xjwmx]

Just search "jis bit". I've never used them -- still in the package. There is a vid of a guy using them on throttle shaft screws (with the T-handlle).
http://www.ebay.com/itm/Motion-Pro-...ash=item4d51eae308:g:PhMAAOSwt5hYc0IX&vxp=mtr

 

[BoatBiker]

Howdy BB: well, it sure seems like either an electrical or fuel problem (duhhhh ) and it sounds as though you have eliminated most of the electrical issues. As I understand it, the engine did run well - but now does not rev-up under a load - although it does start and idle satisfactorily.

I have two suggestions for where to go next:

1. Check the mechanical advance mechanism. It lives under the right hand camshaft end cover and is held on by two big JIS screws (looks like Phillips - BUT are NOT Phillips). You will see two little springs which pull two flyweights in toward the centreline of the mechanism. As the mechanisms spins with the camshaft, the flyweights should move out (against the springs) and that causes the ignition points actuating cam which is under the left hand camshaft end cover, to rotate a little bit - which advances the spark (causes it to happen earlier in the engine cycle). It is necessary for the spark to happen earlier (ie. advance) at higher engine speeds to allow combustion to occur properly - and if that advance does NOT happen, the engine will not rev-up. Checking the mechanism involves ensuring that the springs are there and that the various moving parts on the mechanism can move freely and return to their "rest" position when the engine is stopped.
2. I wonder if your fuel tank may have had some debris in it which has come loose and is now clogging the carburetor jets. Others know far more about this than me - but I'd check the high speed jets first - since the engine starts and idles OK.

Anyhow - those would be the next steps as far as I can see.

Cheers and best of luck!

Pete

Thanks Pete - I have JIS drivers. So the advance mechanism appears to be working fine. I am able to start the engine and watch the flyweights do their thing, moving out as revs increase. I should clarify that in neutral the bike starts, idles and revs up perfectly - freely revving to the red line. It's only when in gear and under load that it chokes...Does that indicate possibly a partially clogged main jet? Enough fuel to rev under no load, but not enough to produce power? Looks like I may be pulling the carbs next..

 

[MaxPete]

Yup - now that you have checked your advance mechanism, it sounds like a fueling problem is the next most likely thing to check. I am not a carb expert but here are the key thing points I have gleaned from those who are:

• make sure that your screwdrivers fit the jets perfectly (they are all straight slot screws as I recall - and many people actually grind their screwdrivers to fit);
• be sure that all of the seals and gaskets are perfect - even the smallest air leak really affects how the engine runs;
• be very gentle with the rubber diaphragms - they are expensive to replace.
• cleanliness is crucial: - be very thorough in eliminating contamination;
• consider substituting socket head cap screws (i.e. Allen screws) for the JIS screws that hold the carbs together;
• If you can avoid physically separating the carbs, you may avoid the need to re-synchronize them.

 

[Signal]

I had this type of scenario a couple of times both turned out to be a carb balancing barb related ( on different XS 's). One was a missing cover the other the barb had come out of the rubber manifold.

 

[MaxPete]

Ah hah! Something very simple to check BoatBiker.

 

[xjwmx]

It feels like it isn't firing on every revolution, on at least one, or both cylinders. However, the bike starts, idles and revs (in neutral) just fine, but has zero power once you put it in gear and try to pull away.

Once the throttle plate opens up it needs more fuel than it does to idle, so I'd guess the carb is clogged, especially since it seems you've eliminated electric. But if it was that lean it would probably die even in neutral if you opened the throttle real fast. That is a good test for super lean. Also, don't know if this is how it's behaving but is the clutch not working, e.g. plates not separating? I said eliminated electric -- but you should replace that one coil since it needs to be done anyway, and then see what you have.

 

[TwoManyXS1Bs]

...I have recently acquired a very nice original condition TX650...

If your carb diaphragms are also 44 years old, they deserve a close look for tears and pinholes.

Do a forum search on "slide drop test"...

http://www.xs650.com/threads/bs38-carb-slides.34557/

 

[BoatBiker]

Thank you all very much. So, I pulled the carbs and removed the float bowls. I was surprised to find that while the right bowl was almost empty - since I had run the motor with petcock off while checking the advance mechanism - the left bowl was almost full. "Interesting," I thought; "the right cylinder is burning lots and lots more fuel than the left...." Sure enough, when I looked in the main jet there was a speck of schmutz mostly blocking the jet, and some more schmutz sitting in the bottom of the float bowl. All in all very clean, though, with no evidence of varnish. A little carb cleaner and compressed air and hey presto - running like a charm. In fact, although I thought it ran well before, it runs GREAT now! Thanks again everyone!

 

[MaxPete]

excellent!