No top end speed

Hi all. To start with. Sorry if I have this topic in the wrong place. I have got a 750 ltd H3 ( 1983 ) about 3 weeks ago. First week out on it and have noticed it pulls away and up through the gears Very quickly. I mean hitting 4200 rpm in top gear at 45- 50 mph.Thrashing the engine at 55 mph. My first thought is the gearing. Factory fitted is 13 t front and 32 t rear giving a 2.46 ratio. I have got fitted a 13 t front and 33 t rear. Giving a 2.54 ratio. I cannot believe that 1 tooth difference on the rear would make that much effect in the gearing. I plan to change it to a 14 t front and 34 t rear giving a 2.43 ratio. A little more top end from standard. But does any one know if a close ratio gearbox was ever fitted to this model because it does whip up the gears Very quickly like a scrambler bike... Many thanks for reading. Baz



All the work done so far is by me. Respray, rear light, front indicators off my old 440 ltd. rear frame sprayed up after sand down, Battery box bent back into shape and resprayed red. electrics sorted after blowing 2 out of the 3 fuses when i first took it out. And fitted a replacement front fork name badge. Still loads to do but It's a custom. I can have it how I like.

"Thrashing the engine at 55 mph. "

I'm not sure what you mean by thrashing the engine. Are you saying the engine runs poorly above 4200 rpm, or is it that you are concerned that the rpm are too high at 55 mph? If it's running poorly you need to clean the carbs and perform a full tuneup. These engines are engineered to run perfectly fine at high rpm and are perfectly happy to run hour after hour at rpm higher than 4200. Notice in the chart below that they make less than half their peak hp at 4200 rpm. Ed

Thanks for the reply. The engine is at high rev's when in top gear doing only 45-50 mph. It runs up the gears very quickly like a scrambler does. (From standing start and pull away, you find yourself going the the gears straight up to 5th in a few second's.) It just seems the gearing is all wrong. I did not think a 1 tooth difference on the back sprocket would make all that difference and that is why I ask if there was a close ratio gearbox fitted to any. More than likely not but just to rule that out before trying a 14/34 tooth set which would make a 2,43 ratio. And would that be to much or should I stick to the 13/32 factory setting.

I had similar issues with top end speed on my GPz. If any mods have been done to your intake and/or exhaust it will affect your power-band. Most of these mods cause a lean condition and you loose power once you hit a certain rev level and speed.

My exhaust was beat up and leaky so I put on a 4-1 system and it actually made it worse. I ended up having to rejet, fiddle with the timing, sync and resync the carbs cut off the stem on my intake cap.......

It's going to take some fiddling.

Another issue is that gas isn't the same as it was when these bikes were built so sometimes the recommended factory carb and timing settings are no longer the best setup. You can try non ethanol gas if you have somewhere local that sells it.

Edit: Just realized that you are over the pond so I don't know if you even have the stupid ethanol gas.

You might want to consider switching to a 530 chain & sprockets, assuming your bike has the stock 630 stuff.

630 sprockets.
33/13 = 2.538
32/13 = 2.462
34/14 = 2.429
33/14 = 2.357
30/13 = 2.308 <<>> this is probably about as low as you'd want to go, but it might be a tad too low.
32/14 = 2.286

Thank you. I have not even checked what chain is fitted. I believe it's a 630, but thanks for the advice. Will look at changing it all this coming week.

Try a 14 tooth front ..... it would be cheap enough to try out first. A lot of us have always thought these KZ's are geared a little low and are always looking for that 6th gear that's not there... (for most anyway) Thinking I'm around 4,600 at 65.... a good 5,000 with a smaller 15T front. Like Ed says these like to rev and if you gear too high your gonna lose that outta the hole fun. A 17T lowered the revs by a good 400 plus rpm's but killed the acceleration for me.

Mine also has a 4 into 1 system on it. Trying to even get the front down pipes so I could put some nice slash cuts on the end's are like looking for a needle in a haystack over here. Thanks for the input. Once I get the sprockets sorted, I plan on getting a pro to do the carb's and cam chain.

A bit hard to understand the complaint here as the terminology is a bit vague, and gearing vs RPM vs sprockets and symptoms doesn't make a lot of sense to me.
However, I wonder if the OP is actually experiencing clutch slippage?

missionkz wrote: A bit hard to understand the complaint here as the terminology is a bit vague, and gearing vs RPM vs sprockets and symptoms doesn't make a lot of sense to me.
However, I wonder if the OP is actually experiencing clutch slippage?

I took his problem to be that he would get to a certain speed when no matter how much throttle he would loose power with corresponding lack of rpm and speed.

Not that the revs go up but not speed.

I think he's having air/fuel mixture issues not final drive ratio or clutch issues.

Thanks for all the reply's. The clutch is not slipping as far as I know. And although the rev's in 5th gear are at around 4200 at 45-50 mph. It will pull away even more from 50 up to the max that I have took it to which was 60. But at that speed the engine sounds like it's thrashing it's guts out. Like I have said in past post's. It goes up the gears so quick just like a scrambler motorbike. ( Within seconds from a standing start. ) I will order some new sprockets and a chain to see if that helps. It's ratio at the moment is 2.54. and will change that to 2.43. Will let you all know if it sorts the problem out. Thanks

kwak750ltd wrote: Thanks for all the reply's. The clutch is not slipping as far as I know. And although the rev's in 5th gear are at around 4200 at 45-50 mph. It will pull away even more from 50 up to the max that I have took it to which was 60. But at that speed the engine sounds like it's thrashing it's guts out. Like I have said in past post's. It goes up the gears so quick just like a scrambler motorbike. ( Within seconds from a standing start. ) I will order some new sprockets and a chain to see if that helps. It's ratio at the moment is 2.54. and will change that to 2.43. Will let you all know if it sorts the problem out. Thanks

There is no speed below the engine's redline at which the engine should sound like it's thrashing it's guts out. The engine should rev freely and easily all the way up to redline which in top gear on your bike is in excess of 120 mph. I suggest you have an experienced rider test ride the bike to see if they also detect the engine "thrashing" you mentioned. If they do you need to correct the problem, and changing the sprockets is not addressing an engine problem. Ed