1977 GS750B Speedo drive

[GS750]

Hi, i've managed to get myself into a load of trouble......

My speedo drive box that sits on the front wheel hub was stuffed and I stripped it down only to discover a load of metal shavings etc.

I managed to get 2 spares from my local bike wrecker and had to disect one to make a new unit from all the parts.

In the process of stripping my original speedo drive box and managed to kick my parts tray that I had sitting in the sun to dry the parts I had just cleaned and managed to get the parts all mixed together, and have just discovered that one has a 25 tooth drive cog and one has 24 tooth, and Im not sure which was what :O

I have 1 factory manual, 3 other brand manuals and a parts manual but they all only show it as the speedo drive gearbox unit, with no other description of the internal parts.

Does anyone know the specs for the 1977 GS750B with spoked wheels and a 100.90.19 front tyre??

I really dont want to get it all back together to discover that I have to strip it all again and change the internal cog thingy.

Thanks for your help.

[Suzuki_Don]

Andrew you should already know this information.
As you are apparently monitoring my computer.
But just in case your search did not come up with anything the answer is 26 on the large gear and 9 on the small one.

[Suzuki_Don]

Andrew just rechecked you post and you say the two that you have are 24 and 25 teeth respectively. I have a '77 gs550B which has the same speedo gearbox as the 750B with identical part numbers. Maybe the ones you picked up from the wrecker are off a different model. Check how many teeth on the small pinion of the original gearbox and also the one you intend to use now. If they are not in matching pairs they will not work. Probably jam. Get back to me with some results.
P.S. Did you put a new oil seal in the speedo gearbox.

[GS750]

Andrew just rechecked you post and you say the two that you have are 24 and 25 teeth respectively. I have a '77 gs550B which has the same speedo gearbox as the 750B with identical part numbers. Maybe the ones you picked up from the wrecker are off a different model. Check how many teeth on the small pinion of the original gearbox and also the one you intend to use now. If they are not in matching pairs they will not work. Probably jam. Get back to me with some results.
P.S. Did you put a new oil seal in the speedo gearbox.

Gday Don, thanks for the reply.

My original had either 24 or 25 teeth lol. I stripped the one which had the other amount but got them mixed together and cant remember which was what :O

The small pinion which drives the cable both had 9 teeth

Is the oil seal the one which goes to the wheel side or the cable side? I havent replaced either but my original housing seal which was toward the wheel was shagged, thats why I got the spare parts, and then discovered that my 9 tooth cable driving pinion was shagged as well (its got large nicks out of it)

[Suzuki_Don]

Not sure what to advise Andrew. The seal I was talking about was the large one that faces the wheel and mine has about 3/4" of the rubber lip missing. Don't know if it will let much grease out when the gearbox is working.
But as I said before my speedo gearbox is definitely original and the two respective gears are 26 teeth and 9 teeth. Maybe someone else will chime in that has one off the bike they can pull apart and check for you.

[Suzuki_Don]

Andrew if you can't find out any further information then I would go with the gear with the most number of teeth as that is closest to the one that I have.

[GS750]

Andrew if you can't find out any further information then I would go with the gear with the most number of teeth as that is closest to the one that I have.

Thanks Don, what size is your front tyre though? if its smaller than mine it will have more teeeth to compensate.

> My front tyre is the 100/90/19

[Suzuki_Don]

Andrew tire size doesn't matter. We have the same front wheels (1.85 x 19) so the gearboxes should be the same. People do not change their speedo gearboxes when they do a tire change. Probably one tooth difference would not make that much difference anyway in the scheme of things.

[GS750]

Hmm, most manufacturers allocate a range of tyres that are acceptable to run on a given vehicle configuration.

In the past 30yrs experiences with cars and bikes I have had instances where I changed a vehicles tyres to lower or higher profile tyres and had to have my speedo calibrated as they read as much as 20-30km out.


Metric tyre sizes are usually shown like 100/90-19 which says the width (100mm), the %of width as the profile (in this case its 90% of 100mm=90mm) and the rim size in inches (19 inch)


Tyre size certainly will change the speedo reading as the rolling circumference is different So im not too confident that it would only be a tooth or so different for the different tyres.


The data below shows differences in the following tyre sizes:

Tire Size Comparison (the 10/90-19 is my tyre size).

Specification__Sidewall__Radius__Diameter_Circumfe rence__Revs/km_Difference
_90/90-19_____81mm__322mm__645mm____2025mm____494____-2.7%
100/90-19_____90mm__331mm__663mm____2082mm____480____ 0.0%
110/90-19_____99mm__340mm__681mm____2138mm____468____ 2.7%
_90/80-19_____72mm__313mm__627mm____1969mm____508____-5.4%
110/80-19_____88mm__329mm__659mm____2069mm____483____-0.6%

[GS750]

Bumping this as I REALLY neeed to get my bike back together and I dont want to have to strip the front wheel again to change the speedo gear.

I have 2 choices here, the 25 tooth or the 24 tooth.

[Steve]

According to your table above, the greatest innacuracy is only about 5%. The difference between a 26-tooth and a 25-tooth gear is just a tad under 4%. Most of the speedos are off by that much anyway, so use the 25-tooth gear.

And ... you are bumping this a week later because you don't want to do about an hour of work? Doesn't make sense to me.
Does not take very long to remove the front wheel. Also does not take very long to take the unit apart. Putting it all back together should fill out that hour. No sense in waiting a week for an answer.

.

[GS750]

According to your table above, the greatest innacuracy is only about 5%. The difference between a 26-tooth and a 25-tooth gear is just a tad under 4%. Most of the speedos are off by that much anyway, so use the 25-tooth gear.

And ... you are bumping this a week later because you don't want to do about an hour of work? Doesn't make sense to me.
Does not take very long to remove the front wheel. Also does not take very long to take the unit apart. Putting it all back together should fill out that hour. No sense in waiting a week for an answer.

.

Gday Steve, the above examples were exactly that, just some that I googled for the quick specs.

It is getting into mid winter here in Australia, Ive been of work 4mths with some serious DVT blood clots and the last thing that I need is any extra physical activity like ripping the front wheel off, because the risk of a bump or scratch that will be seriously bleeding for ages isnt worth it.

I just want to get my bike running again and figured with teh amount of them out there, especially in the USA, that somebody would have the information at hand to save me an added risk to my health and physical abilitys.


I also hate and despise having to put things together and strip them out again, also with the speedo gearbox it is quite old and I dont want to risk damaging the seal on it as they appear to be non replaceable.

It may only seem like a 1hr job to you but to me it is about a 1-2 day job because my energy levels are low and I can bleed really easily etc etc.