Topic: Further Adventures of the Missing 6th Gear

[breezy]

My 74 CB550 is having a gear ratio problem.  The bike was worked on 4 years ago to replace a broken 2nd gear and the mechanic was not Honda trained and was using a spare parts motor for parts that may have been a different year or different model.  When I got it running, all the gears were lower ratio'd than stock as the bike topped out at 65mph in 5th gear. 

Looking at both front and rear final drive sprockets, the front has 17 teeth and the rear has 34 teeth.  The rear sprocket is in excellent shape and I'd like to keep it even tho it's got fewer teeth than what you guys say is standard (37 teeth).  The front sprocket is totally thrashed, teeth worn to points and some points broken off, so I'd rather replace that.  Are taller 19 or 20 tooth front sprockets available?  I've noticed that other engine sizes use a similar looking front sprocket that might provide a taller ratio.  Is there any standardization on that sprocket size or will I need an after market part?

[Tews19]

Can you post some pics of the sprockets?

[Don R]

When you say topped out, is it at redline or the engine just won't pull it any faster?  I'm thinking raising the front gear tooth count will be going in the wrong direction if the engine is lugging and just can't pull it any faster. It the tach is at redline it may not be correct.
 I'm having a problem believing there was an internal ratio that different.

[Lostboy Steve]

I'm thinking someone screwed with the primary ratio? Better make it a hill climber. Go get yourself a 9 foot rear swing-arm and a paddle tire. The hard work was already done (Gearing). Seriously, this sounds like someone made a significant miscalculation during the rebuild. Is it possible your clutch is destroyed? That will cause a similar situation to the one you describe.

[fmctm1sw]

I didn't want to pile on but that other "6th gear thread" was hilarious.  I don't know, if the only thing he did was replace second gear, I'm not sure that would make you top out at 65.  Perhaps the gear ratio could be mathematically deduced?  Could someone with a stock 550 possibly count the number of times the rear wheel turns in relation to how many times the front sprocket turns?  In second gear?  Top gear?  Just trying to throw out ideas...

[Cheffish]

Why don't you tell us your speeds and revs in other gears so they can be compared with others' bikes as a baseline?  I don't know much about anything but I have to think that if someone used parts from another model they would probably not have been able to work properly in any gear?  I also think that by putting too many teeth on your front sprocket you are risking your chain binding up in the cover and doing even more damage. Also even if the tach is screwed up, your bike topping out at 60 mph may not even be a transmission problem at all. If all else fails, maybe it's time to have another set of eyes try to work it out for you?

[dave500]

if the primary ratio is still as per 550 having a 34 rear should in fact make all gears taller/longer legs,missing front sprocket teeth ?the chain isnt slipping over it is it?and or the clutch slipping?youve counted the sprockets accuratley?and you have an 18 inch rear wheel?

[breezy]

When you say topped out, is it at redline or the engine just won't pull it any faster?  I'm thinking raising the front gear tooth count will be going in the wrong direction if the engine is lugging and just can't pull it any faster. It the tach is at redline it may not be correct.
 I'm having a problem believing there was an internal ratio that different.

I'm having a problem believing it too.  I think the 34 tooth rear sprocket is the culprit.  The bike is at full throttle in 5th and going 65mph.  Raising the tooth count on the front sprocket will raise the gear ratio and give me a higher top end but since the rear sprocket is in excellent shape, I'd rather change the front sprocket if a 19 or 20 tooth sprocket is available.

[breezy]

Can you post some pics of the sprockets?

Yes, but I'm not sure why that would be necessary.

[breezy]

I'm thinking someone screwed with the primary ratio? Better make it a hill climber. Go get yourself a 9 foot rear swing-arm and a paddle tire. The hard work was already done (Gearing). Seriously, this sounds like someone made a significant miscalculation during the rebuild. Is it possible your clutch is destroyed? That will cause a similar situation to the one you describe.

How will a destroyed clutch do that?  It's not slipping.  Did the CB550's come with different primary drive gearing in different years?

[breezy]

I didn't want to pile on but that other "6th gear thread" was hilarious.  I don't know, if the only thing he did was replace second gear, I'm not sure that would make you top out at 65.  Perhaps the gear ratio could be mathematically deduced?  Could someone with a stock 550 possibly count the number of times the rear wheel turns in relation to how many times the front sprocket turns?  In second gear?  Top gear?  Just trying to throw out ideas...

In trying to Sherlock this mystery, I can't believe that Honda would change the gears in the gearbox on the CB550 over the years, but they may have had different primary drive sprockets on the triplex chain matched to different final drive sprockets.  If a mismatch occurred there due to the parts 550 being a different primary drive and final drive sprocket set, that would account for it.

[breezy]

I didn't want to pile on but that other "6th gear thread" was hilarious.  I don't know, if the only thing he did was replace second gear, I'm not sure that would make you top out at 65.  Perhaps the gear ratio could be mathematically deduced?  Could someone with a stock 550 possibly count the number of times the rear wheel turns in relation to how many times the front sprocket turns?  In second gear?  Top gear?  Just trying to throw out ideas...

I originally thought that the 550 might have 6 speeds cuz I glanced at the Haynes manual specs and read "6 speeds" and neglected to note that it was in the CB400 column.  The shade tree mechanic replaced 2nd gear and left a totally thrashed final drive front sprocket on the bike, so I'm not sure he didn't take a bong break too many.

[breezy]

if the primary ratio is still as per 550 having a 34 rear should in fact make all gears taller/longer legs,missing front sprocket teeth ?the chain isnt slipping over it is it?and or the clutch slipping?youve counted the sprockets accuratley?and you have an 18 inch rear wheel?

I counted the sprocket teeth several times.  The chain wasn't slipping over the thrashed front sprocket final drive teeth.  The bike pulls smooth, no clunking sound, no jerks or twitches.  The clutch isn't slipping.  The rear wheel is 18 inches.  Is a taller front final drive sprocket available from after market suppliers?

[HondaMan]

if the primary ratio is still as per 550 having a 34 rear should in fact make all gears taller/longer legs,missing front sprocket teeth ?the chain isnt slipping over it is it?and or the clutch slipping?youve counted the sprockets accuratley?and you have an 18 inch rear wheel?

I counted the sprocket teeth several times.  The chain wasn't slipping over the thrashed front sprocket final drive teeth.  The bike pulls smooth, no clunking sound, no jerks or twitches.  The clutch isn't slipping.  The rear wheel is 18 inches.  Is a taller front final drive sprocket available from after market suppliers?

The stock 750 front sprocket (18T) usually fits fine, and was the base sprocket on many 550 bikes.

What RPM is the engine running at 65 MPH? If it is at redline, I'd suspect clutch slip long before anything else. Top speed in 5th should be a little over 105 MPH.

[bryanj]

You are going about this totaly the wrong way, 17/34 was standard on the 500 and just changing internal gears would not alter the top speed.

It is far more likely that your tacho or speedo are wrong

[TwoTired]

You are going about this totaly the wrong way, 17/34 was standard on the 500 and just changing internal gears would not alter the top speed.

It is far more likely that your tacho or speedo are wrong

+1

All the internal gear ratios are listed in the Honda CB500/550 shop manual. (Which, anyone working on these bikes should have.)
They are different internally only between the 500 and 550 models.  Ratios within models were the same for all years.  Only the primary reduction was different between the CB500 and CB550 (2.000 vs, 3.063), which is why the rear sprocket was 37T for the 550 and 34T for the 500.

You have something wrong with the bike that a larger front sprocket is not going to fix.  At best, it will just mask and/or evade the real problem.  I would investigate/prove that the tach and speedo aren't lying to you.  A measured mile takes 60 seconds to traverse at 60MPH.  Measure the mile with a different vehicle. (Or, use state mile markers).

[trueblue]

Stock gearing on my 650 is 16/40, when I got it, it had 16/36 on it with a very poorly tuned engine and poorly fitted cam chain as I found out later.  Anyway, in the processes of trying to find out why it had no get up and go I replaced the drive chain and sprokets to standard size.  Taking into account that the bike wouldn't have been able to pull a skin off a rice pudding at that point in time, before I replaced the gears it would barely reach 120kph, then without doing anything else other than putting shorter gearing on it, it would do 140kph.  What I'm trying to say is a taller gear isn't always the answer, sure it will give you more speed for any given RPM but it will also take more power to turn and if your engine isn't quite up to par in its tuning then the effects will be more noticeable. Honda engineers knew what they were doing when they set up the gear ratios on these bikes, don't go assuming that you know better.  Go with what has worked for the last 30 odd years, if your bike isn't performing the way it should then move it back towards stock condition.

[bwaller]

I'm sure you would know the difference between 65 & 95mph. But what is the tach showing? You claim the clutch isn't slipping, but are you sure. If someone did transmission work and it now shifts ok through all five gears then stop blaming him and figure this out.

Pull the clutch and have a look. If the steel plates aren't blue and look fine then maybe replace just replace the discs. If they are blued or burnt looking replace them too and eliminate one potential issue.

If the front sprocket is junk and you've been running it that way, the chain likely is as well. When you solve your issues that 34T rear won't be much use either so consider replacing all as a set. 

[Tews19]

Can you post some pics of the sprockets?

Yes, but I'm not sure why that would be necessary.

It's not necessary, but it is advantageous because most members know what is going on by a visual. Visuals help those experts notice even the smallest things that are out of line.

[breezy]

if the primary ratio is still as per 550 having a 34 rear should in fact make all gears taller/longer legs,missing front sprocket teeth ?the chain isnt slipping over it is it?and or the clutch slipping?youve counted the sprockets accuratley?and you have an 18 inch rear wheel?

I counted the sprocket teeth several times.  The chain wasn't slipping over the thrashed front sprocket final drive teeth.  The bike pulls smooth, no clunking sound, no jerks or twitches.  The clutch isn't slipping.  The rear wheel is 18 inches.  Is a taller front final drive sprocket available from after market suppliers?

The stock 750 front sprocket (18T) usually fits fine, and was the base sprocket on many 550 bikes.

What RPM is the engine running at 65 MPH? If it is at redline, I'd suspect clutch slip long before anything else. Top speed in 5th should be a little over 105 MPH.

The only other variable is the tire I put on it.  It was an 18" tire, but I'm going to measure its outside diameter and see if it is near stock dimensions.  The 34 tooth rear sprocket was stock on a 500.  550's had 37 teeth. 

[breezy]

I'm sure you would know the difference between 65 & 95mph. But what is the tach showing? You claim the clutch isn't slipping, but are you sure. If someone did transmission work and it now shifts ok through all five gears then stop blaming him and figure this out

Pull the clutch and have a look. If the steel plates aren't blue and look fine then maybe replace just replace the discs. If they are blued or burnt looking replace them too and eliminate one potential issue.

If the front sprocket is junk and you've been running it that way, the chain likely is as well. When you solve your issues that 34T rear won't be much use either so consider replacing all as a set.

The only other variable would be the new rear tire.   It's 18" but it may be a thinner profile than stock.  The clutch isn't slipping. 

[bryanj]

It Aint the tyre if its 18 inch---its not possible to get enough difference to make the problems you have, i still think one or both of your clocks are wrong

[Duanob]

You may not want to change out the 34T rear but that's your answer. Putting anything bigger than an 18T in front you will start to grind away at your cases in the front sprocket area. There isn't a lot of optional room in the 550. I just bought a chain and two sprockets 17T front/36T rear) from powersportparts.net for <$100 delivered.

[Don R]

My K0 750 right now has a tach that reads about double the auctual RPM, I have ridden it enough to know it is incorrect.
Re-read what trueblue said, I think he is right. If you are down on power adding teeth to the front sprocket will make it worse. Like was posted if your front sprocket is that bad you will need to replace both sprockets and the chain eventually, because they are usually toast and will continue to wear each other out.
 Forgive me if I missed it, but at 65mph is the throttle all the way open or is the tach at redline?

[breezy]

It Aint the tyre if its 18 inch---its not possible to get enough difference to make the problems you have, i still think one or both of your clocks are wrong

Gauges are fine.  Regardless of the tach or speedo reading, the engine is screaming at 9/10 throttle and doing 60-65 mph.  It's gearing.  Does anyone know if there were variations in the primary drive triplex chain sprockets during the 550 production run?