Boxerworks.com

Recently purchased a 1978 R80/7 . Needed a little TLC and have cleaned the bike up nicely.
When going for a test ride It seems to have no power after 70mph with throttle wide open. Here's what I done to date.
1. Checked and adjusted timing with lighted probe rotating engine until light comes on when S mark is in viewing hole.
2. Replaced points
3. Adjusted valves
4. Replaced header pipes with new Keihan Stainless
5. replaced throttle cables
6. Synched carbs.
7. Fresh fuel
Starts right up.
Previous owner said he rebuilt engine and put R100 heads on this R80 engine? Would this cause the Top End problem?
I haven't rebuilt the flat top carbs yet they seem to work fine.
Any Ideas would be greatly appreciated.
Thanks,
Mase

Check the carb diaphrams.

Does it miss or puke or just have no power?

You've covered some basic tuning ...

Replacing heads can be a crap shoot –
squish band profiles & piston dome profiles were changed throughout years of production.
Some don't work so well together ...

And, IMO, "rebuilt" will be a meaningless term without documentation.
If the machine was new to me, and an unknown quantity,
I'd want to start with a leak-down to assess ring / valve / guide condition,
and a few throttle-chop runs to read the plugs and see if the carbs are in the ballpark,
just to know what is on hand to work with.

Quite frankly, given the information presented –
it could be anything that runs the gamut from
(as Garnet) a torn diaphragm ... corroded electrical connection ... a bad coil ... to a wore-out engine.

+3 on the diaphragm. While ya got it apart check that nothing is strange with the needles.I have seen one with the needle broken but the clip was still on ,just near the top so it would not fully "retract".

Cracked diaphrams jumped to mind with your post topic.

Start with compression. Then check full advance with a timing light. Checking it at "S" means very little, as it is just the starting point. Put your strobe light on the advance mechanism as it revs up. Make sure that the movement of weights is smooth, not jerky.

When you find it, it will be obvious, as that isn't even close to normal.

Garnet wrote:Check the carb diaphragms.

Does it miss or puke or just have no power?

I'll check the diaphragms but it just doesn't have power no puke
I'll also have to look at doing a leak down and go through the remaining test.
Thanks,
Mase

If it has been standing for some time there could be crud around the top of the needle jet, spray carb cleaner down the 3/16 hole at the bottom of the carb intake and it should shoot freely out from round the needle. Spray down the smaller hole on the other side and it should come out a pinprick right at the intake end of the carb.

The float level might be to low too - Snowbums site has the good oil on setting it by depth.

The fuel taps, screens or inline filters might be partially clogged - a full throttle 1/4 mile drag run will show this up better, or you can dribble it into a pot - Duane is the expert on this

Check that you are getting almost full battery volts, 12.6 or 7 at the coil - you may have a big voltage drop somewhere and consequently a weak spark which just doesnt cut it at high revs .

When I first got my R65, I experienced the same problem. Would get the bike up to 110kph and then it would be like something was just holding it back. And funny enough the previous owner had done similar things to mine as yours.(with the exception of the R100 heads ) So I started with carbs, compression(wet and dry)and charge, but to no evail. Then someone suggested that it may be 1 tooth out on the timing chain. The bike I assume(and Im no expert in this field) was advanced or retarted to pretty much start and run fine. But when wide open, bang a brick wall. I needed a new timing chain anyway, and sure enough, my mechanic informed me it was 1 tooth out. After that she was fine. However it is an R65 and still doesnt do anything before 4000rpm.
You may want to check that.
Cheers
Craig

I'll go with Roy on the float level.
Diaphragms would make themselves known before 70mph and prolly wouldn't stop the bike exceeding 70 if the acceleration was increased slowly.
Could be anything, as suggested.