I
Irish John
Guest
Today I was going uphill on my trusty "Indian Pacific" with what I thought was a really noisy crank or gudgeon pin bearing when I thought the motor had suddenly seized. The engine stopped dead and it was locked solid and wouldn't turn over. I had been expecting this seizure because I had noticed a metallic knocking noise that I couldn't seem able to fix for 2 weeks now.
First I checked the gearbox about 2 weeks ago to see if the bell housing was loose or the clutch spring was broken because those are usually the reasons for all nasty noises. All seemed OK and the bell housing was tight on its bushing with no lateral play.
Then last week I changed the full length exhaust for a new short one because the join between the downpipe and silencer was worn and that sometimes makes a big noise if exhaust is escaping. Still no improvement in the metallic knocking noise.
2 days ago I adjusted the valve clearances and still the knocking noise continued. I began to suspect the worst - that my trusty Honda was showing signs of wear with main bearings worn so I let a Honda small engine mechanic listen to it. He agreed that it could be something inside the engine but splitting the motor costs $130 so we reckoned it best to wait for the engine to break and then split it open to see what the problem was - probably big end or little end bearings we both surmised.
The noise grew worse very quickly and I was expecting this engine seizure when it happened under load on a steep uphill. This motor has done 12000 kms on a bike that has done 20,000 kms but it has always been diligently serviced and cared for even if I do ride the bike hard and fast most of the time.
Those of you who know me will know I've built a lot of these four strokes using mostly the real Honda GXH50 and always the Grubee Mark 2 gearbox. I've never had serious issues with any Honda motor and the Grubee Mark 2 is OK as long as I keep replacing the clutch, bell housing, bushing and 11T freewheel sprocket on a fairly frequent basis. It's a noisy gearbox but it works better than any other.
I rode the bike manually home to my workshop and took the gearbox cover off. I was shocked to see the main bell housing gear all mangled and jammed up with a third of its teeth gone but I was really glad it wasn't the engine itself. I took the gearbox right off the motor and was overjoyed that the motor ran like new with a slight wheeze and the lovely snickety-snick of healthy valve springs. What a lovely sound the Honda makes without the Grubee gearbox attached to it.
I attach pictures of the damage. What I now think happened is that the centrifugal clutch wasn't mounted absolutely centrally on the main engine shaft - you can actually see it on the photo - and I reckon this was banging against the clutch bell. The metallic knocking noise didn't really diminish when the gears were engaged and in the end it was constant when running at speed. I think the clutch being off centre must have been throwing the bell housing about and that is what caused it to throw itself away from the second gear wheel enough for the teeth break out of their mesh.
I think I can fix the GB with new parts I keep for spares (the Sydney importer keeps no spares for the old gearboxes because he is that sort of guy - never mind how many hundreds of old gearboxes he sold). I've replaced the clutch, bell housing, bushing and 11T freewheel drive sprocket and I'm hoping when I start it all up it will be as quiet as it once was. I can't try it out until 2 days when the red threadloc I have to use in the 11T freewheel sprocket sets. If you don't rebuild and seal up those freewheels they come apart.
Anyway I was shocked to see what happened to my GB but it could have been worse. The Honda motor really is a great piece of engineering and I feel bad that I doubted it.
The moral of this story is it IS NEARLY ALWAYS THE CLUTCH, BELL HOUSING AND BUSHING THAT IS THE PROBLEM.
First I checked the gearbox about 2 weeks ago to see if the bell housing was loose or the clutch spring was broken because those are usually the reasons for all nasty noises. All seemed OK and the bell housing was tight on its bushing with no lateral play.
Then last week I changed the full length exhaust for a new short one because the join between the downpipe and silencer was worn and that sometimes makes a big noise if exhaust is escaping. Still no improvement in the metallic knocking noise.
2 days ago I adjusted the valve clearances and still the knocking noise continued. I began to suspect the worst - that my trusty Honda was showing signs of wear with main bearings worn so I let a Honda small engine mechanic listen to it. He agreed that it could be something inside the engine but splitting the motor costs $130 so we reckoned it best to wait for the engine to break and then split it open to see what the problem was - probably big end or little end bearings we both surmised.
The noise grew worse very quickly and I was expecting this engine seizure when it happened under load on a steep uphill. This motor has done 12000 kms on a bike that has done 20,000 kms but it has always been diligently serviced and cared for even if I do ride the bike hard and fast most of the time.
Those of you who know me will know I've built a lot of these four strokes using mostly the real Honda GXH50 and always the Grubee Mark 2 gearbox. I've never had serious issues with any Honda motor and the Grubee Mark 2 is OK as long as I keep replacing the clutch, bell housing, bushing and 11T freewheel sprocket on a fairly frequent basis. It's a noisy gearbox but it works better than any other.
I rode the bike manually home to my workshop and took the gearbox cover off. I was shocked to see the main bell housing gear all mangled and jammed up with a third of its teeth gone but I was really glad it wasn't the engine itself. I took the gearbox right off the motor and was overjoyed that the motor ran like new with a slight wheeze and the lovely snickety-snick of healthy valve springs. What a lovely sound the Honda makes without the Grubee gearbox attached to it.
I attach pictures of the damage. What I now think happened is that the centrifugal clutch wasn't mounted absolutely centrally on the main engine shaft - you can actually see it on the photo - and I reckon this was banging against the clutch bell. The metallic knocking noise didn't really diminish when the gears were engaged and in the end it was constant when running at speed. I think the clutch being off centre must have been throwing the bell housing about and that is what caused it to throw itself away from the second gear wheel enough for the teeth break out of their mesh.
I think I can fix the GB with new parts I keep for spares (the Sydney importer keeps no spares for the old gearboxes because he is that sort of guy - never mind how many hundreds of old gearboxes he sold). I've replaced the clutch, bell housing, bushing and 11T freewheel drive sprocket and I'm hoping when I start it all up it will be as quiet as it once was. I can't try it out until 2 days when the red threadloc I have to use in the 11T freewheel sprocket sets. If you don't rebuild and seal up those freewheels they come apart.
Anyway I was shocked to see what happened to my GB but it could have been worse. The Honda motor really is a great piece of engineering and I feel bad that I doubted it.
The moral of this story is it IS NEARLY ALWAYS THE CLUTCH, BELL HOUSING AND BUSHING THAT IS THE PROBLEM.
Attachments
Last edited by a moderator: