XR70 bottom end with CT70H transmission problem

[916mestizo]

So I have read a lot of helpful threads on here for a while and decided to join and see if anyone could help with the issue I am having. Hopefully this is in the right section.

I have been helping my dad with his CT70 build. At the moment we are using a 1997 XR70 bottom end cases, crank and rod with a CT70H 4 speed trans and shift drum, TBparts 88cc upper end and HD semi auto clutch.

The CT70H trans looked to be in good order when we removed it from its original case and seemed to shift fine while on the bench, but we could not verify it while running since the top end had seen some damage prior to us getting it.

We put the trans and shiftdrum into the XR case, again everything seemed to work fine on the bench, so we put the whole motor together and ran it. While running it shifts from Neutral to first and second gear just fine but when it goes into the third it makes a clunk and a slight semi grinding kind of sound and won't accelerate almost as if it's back in neutral, it will shift from third into forth no problem.

Not sure if it's a gear related issue or a shift fork issue or a combo of both? Any help would be appreciated!

 

[cjpayne]

Was the H motor shifting properly when it was running?

 

[OLD CT]

Worn shift forks. Take it back apart.... Post some pics of each side of both. It makes sense to change both even if only one is totally shot.

 

[916mestizo]

Was the H motor shifting properly when it was running?

Condition was unknown to the seller we bought it from other than it wasn't siezed and it did go into all the gears while rolling the sprocket. We pulled the plug when we got it and could see heavy rust on the cylinder walls, so we tore it down instead of even trying to make it run to check the trans.

Worn shift forks. Take it back apart.... Post some pics of each side of both. It makes sense to change both even if only one is totally shot.

Ok when we pull it back apart I'll get some pics and post them.

 

[69ST]

Worn, or bent, shift forks and/or worn out shift pins (the ones that hold the forks in place) could be the problem. It's even possible that you have a damaged gear. A damaged tooth, or three, is a lot easier to overlook than you might think. Worn/damaged drive dogs are even easier to overlook...and, fortunately, pretty rare.

Even if fork thickness measures within wear limits, contact should only occur at the tips. Look for "smiley faces" on the shift forks, the dead giveaway. The heat-treating only goes so deep; smiley faces mean that you're into the softer "layer" and the wear will accelerate rapidly...if it hasn't already resulted in your gear change issue.

Do yourself a favor, if you're replacing shift forks (which is very likely) replace the pins, too. You'll likely see a stepped outside diameter on them...the equivalent of smiley faces on the forks. Shift fork pins should have a uniform OD, top-to-bottom.

Inspect the gears, under magnification...while it's apart & on the bench. Better safe than sorry. Lastly, verify that all of the thrust washers are present & in the proper locations.

 

[916mestizo]

Worn, or bent, shift forks and/or worn out shift pins (the ones that hold the forks in place) could be the problem. It's even possible that you have a damaged gear. A damaged tooth, or three, is a lot easier to overlook than you might think. Worn/damaged drive dogs are even easier to overlook...and, fortunately, pretty rare.

Even if fork thickness measures within wear limits, contact should only occur at the tips. Look for "smiley faces" on the shift forks, the dead giveaway. The heat-treating only goes so deep; smiley faces mean that you're into the softer "layer" and the wear will accelerate rapidly...if it hasn't already resulted in your gear change issue.

Do yourself a favor, if you're replacing shift forks (which is very likely) replace the pins, too. You'll likely see a stepped outside diameter on them...the equivalent of smiley faces on the forks. Shift fork pins should have a uniform OD, top-to-bottom.

Inspect the gears, under magnification...while it's apart & on the bench. Better safe than sorry. Lastly, verify that all of the thrust washers are present & in the proper locations.

Gotcha! Ok well when we tear it back down we will take a look at all of that up close. At the moment he just double shifts it into 4th gear, lol sucks cuz he is super excited about how fast it is and how much better first and forth gear are for riding on the street. Thanks for the replies and once we get it torn back down I will let know what we find.

 

[916mestizo]

So just a quick update, since we have not torn the motor back apart yet my dad has been riding it the past couple days.

He figured out yesterday that the trans will downshift into 3rd and slow the bike down with no issues. lol so he has no 3rd for acceleration just for decel

 

[cjpayne]

Better than nothing.lol

 

[69ST]

Well...I suspoze that the tranny could be skip-shifted into 4th, then back down into 3rd (assuming that 3-4 upshift is possible) wound-out, then shifted into 4th. Aside from being quasi-comical...not to mention awkward...keep in mind that the longer the engine is run, to further along it gets in the break-in process. Beyond a point, you'll have to deglaze the cylinder and install new rings, if you split the cases...if that point hasn't been reached & crossed already.

FYI...based on your description, OLD CT is right, worn shift forks. IMO, severely worn shift forks, very likely the fork pins too. I'd take care of this ASAP, this much wear is brutal on the drive dogs. And you don't want to know what it'll take to replace worn, OEM, 4-speed tranny gears.

 

[Gary]

If your careful you can pull the piston out of the cylinder just enough to pull the wristpin out. Then you can just leave the piston and rings in place.

 

[OLD CT]

I would buy new tb 88cc rings, head, + a cyl base gasket, center case gasket. Just in case! It would suck to have to order one later. Also a light dingle hone, if you have one that size. It's good to have one in the tool box, for these bikes. After all you are going to have 100 bucks worth of new shift forks,no point in cutting corners. Just my 2c.

 

[916mestizo]

Thanks for all the responses!

When it comes to the rings I just did what Gary said and pulled the piston far enough to get the cir-clip and wrist-pin out while leaving the piston in the cylinder itself, I have done this with other motors in the past and never had an issue. I did buy all new gaskets tho for everything else.

We tore the motor back down today and for sure the shift fork guide pins and the shift forks themselves are gone. I should have just torn the trans all apart the first time instead of just keeping it all together and transplanting it. When we looked at it all the first time, we were paying attention to the outer teeth of the gears and not much else, so to us it looked fine. lol obviously not. Now that I know how the trans comes apart I won't be so hesitant to pull them apart and check everything in the future.

I need to upload the pics of 2nd and 3rd gears when I get a chance, however I have a bad feeling that both sets of gears will need to be replaced based on what I saw. Also I believe they are called "shift dogs", which are on the side of the gears perpendicular to the gears themselves look pretty mangled up to me as well.

I will report back with some pics soon.

 

[69ST]

IF you can keep the piston far enough inside the cylinder that the rings never lose contact, then you're good-to-go. That requires a squeaky-clean engine and, with these, a facile touch...exposing the wrist pin is near the ragged edge of having the piston out too far.

I'll look for your next batch of photos.

 

[916mestizo]

Here are the pics of the 2nd and 3rd gear... seems to me that the only gear that is possibly usable is 2nd on the mainshaft.

 

[916mestizo]

Also do the shift drums go bad? This one seems to have no wear that I can tell, I can upload some pics of it as well if need be

 

[69ST]

I imagine seeing those battle-scarred gears didn't help your memorial day festivities...

Obviously, this tranny has been beaten like the proverbial rented mule. I dunno that those gears are as bad, mechanically, as they look. You made no mention of problems with the trans jumping out of gear and that's the telltale sign of dead drive dogs. That said, if it were mine, I'd be looking for new gears...just as you are. These things look like

I suspoze the shift drum could wear out, I've just never seen one firsthand. The pins should be the wear points. And, most of the damage I see here is from bang-shifting, as opposed to interstellar mileage.

At this point, you may as well source another transmission assembly. This one, imho, is tantamount to "place horn on lift, remove fixing bolt, then install new vehicle assembly". There ain't enough Dr. Guff's in the warehouse to polish this turd. That may come across as brutal but it's the truth and I'd be doing a disservice if I tried to sugar-coat it.

 

[916mestizo]

You made no mention of problems with the trans jumping out of gear and that's the telltale sign of dead drive dogs.

It had no problems with staying in any gear and never jumped out lol weird issue's for sure!

To me they all look suspect but i wanted to see if maybe something could be salvaged.

 

[OLD CT]

The affordable AHP 4 speed trans looks good right about now.

 

[69ST]

The affordable AHP 4 speed trans looks good right about now.

I was thinking that, as well...however...that call depends upon your requirements and budget. Personally, I prefer OEM but, as Pat implied those no longer come cheap.

 

[916mestizo]

Again thanks for responses!

Well he bought new honda forks and fork pins, but decided to get the DRATV trans and just use the 2nd/3rd gears from it with the CT70H 1st/4th.

lol even more of a Frankenstein setup than before, I will let you know how it all turns out.