XR70 top end upgrade

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surveywaters

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I need to replace the rings on my boys 70.

I figure I might as well go on and do the 88cc bore.
Looks like this would require a hi volume oil pump.
Probably should go ahead and get the 20mm carb and intake which may require a new throttle cable.
A lot of these kits come with cams, is this worth the money and will the bike maintain reliability after all this?
Can I continue to use my stock exhaust?

There is an over whelming number of options out there, but I don't want to spend $400.

Who have you guys purchased from that you would or would not deal with again?
 
B-X is so-so at best. No customer service or product support. The manifold is too small, filter junk. TB kit is a couple bucks more, but worth every penny.

On a budget, TB 88 kit and the stock head and 17mm carb is a ton stronger than stock low-compression deal. Add some valve seals and a cam chain for good measure. Shhh, but your 70 has a higher volume oil pump and stouter clutch than the 50's already;) TB 3-disk plates in the stock basket, works great, if yours wears out with the extra torque.
 
B-X is so NO!!!! Complete junk......go TB and you will not be sorry.....go FPM and you will not be sorry as well
 
Shhh, but your 70 has a higher volume oil pump and stouter clutch than the 50's already;


So I don't need the oil pump? I know the $25 isn't much but they add up. I would rather use that for a new DID cam chain.

I appreciate the input, this is exactly what I wanted to know. I will definetly be going TB.
 
Your stock head and 88 kit would be stronger than either crate engine. Adding the race head, it's no contest.

But, the Piranha/YX 90 is a nice option. T-Bag isn't a forum supporter, FWIW ;)
Piranha YX 90cc Engine
 
I apreciate the response and advice GDFP. I think I'll stick with the honda case and put the 88cc jug, Hi comp piston, reprofiled cam, and the 20mm carb on, plus a new cam chain. That will run me around $250 +s/h.

Now I just need to know if my factory exhaust will be okay for a while cause this will leave me broke.
 
You will like the REAL HONDA with a 88 on it....fun and Honda reliable

p.s. support the peeps that support this forum.....please
 
I have already made arrangements with ....he who must not be named... on the 88 kit. However, I have ordered everything else I feel I'll be needing from Fire Power Minis to spread it around a bit.

Now we'll see who ships faster ;)

Thanks for all the help guys.


Jon
 
On the stock pipe, pop it off and look at the intake end. You'll see a messy weld choking it off. Get a grinding stone for your dremel and grind that puppy down. At the other end, you can remove the spark arrestor for a lil more flow but only do it if you don't ride where fire is a concern.

On the clutch, pull it apart and look at the weights. Those wasteful, heavy weights... Replace them with Powroll's TI versions, or carefully drill a hole in the fat portion of each to reduce their weight. Deburr them nicely after drilling. This will raise the engagement RPM and make even a stock 70 'peppier', it's a blast if your motor has some umph to it.
 
thanks for the tips. I have already honed the exhaust and remove the spark arrestor, but I haven't found a working link for the clutch mod. I've done this on my Lt80, but all I had to do was remove weigths. I drill these puppies out asap!
 
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XR 50's and XR 70's are horizontal, 75's and up are vertical.
 
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I got my parts from FPM and everything is there and lookis good!

I discovered why I am having to replace the topend on this bike. I noticed excessive cam chain noise when I bought it a month or two ago. Apparently someone had removed the cam chain tensioner accidentally and then after reinstalling, failed to prime the hydraulic cylinder causing an inefficient tension, resulting in a worn out oil pump drive sprocket. I have heard of this on many other 70's and would say that anyone doing a topend would be wise to go ahead and replace this gear as well as the tensioner spring which seems to fail with out the assistance of the hydraulics.
 
Glad it made it OK. I've seen the wear issue, plenty. I've found that the tensioner will quickly re-fill and work soon after fire-up, even if assembled empty. Unless, the check valve in the bottom is faulty. What I've seen, is a worn cam chain, on about every 70. The OE chain is light and stretches easily. The 70 has a higher-volume oil pump and bigger cam, both adding drag and speeding the chain's demise. The stock crank and cam sprockets are a lot tougher and seem to fare well, but the pump sprocket is dead-soft. I have a bunch of good used ones, if needed. New is about $18 from Honda.
 
I pulled a barely used gear out of a clone 70. I also used the clone's tensioner spring, it was a good bit stiffer.

I hope I didn't make a mistake buying the D.I.D. chain from you, I didn't know anything about the oem being weak.

Do new top ends usually smoke a bit while the rings are seating? I followed the TB install directions which called for a wet install; I typically go dry and have not seen the smoking.
 
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There are 68g and 86g chains. I only supply the 86 gram ones. Many stockers have the light one. Not as strong, and those are the ones I see in the worst shape.

Hopefully, it's just the guide and bore lube burning off. Smoking should stop with a few minutes of running and a light load to seat the rings.
 
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