Honda GXH50 apparent seizure

[Irish John]

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.

 

[Irish John]

Actually I now think the gearbox meltdown was simply a case of disintegration of gear teeth on the bell housing gear.

 

[zean]

Hi Irish John. I've been interested in the G-stageIII gear box, and I've followed your insightful and informative threads I think for quite some time. Irish John, if you were cruising at @ 30km/hr could that seizure lock up the rear wheel if running the gear box with solid output sprocket? Would the centrifugal clutch disengage and prevent rear wheel lockup? Could a piece of chipped gear tooth lodge itself between two gears and then lock up the gear box? Does that type of engine or the clone always give notice by noise or vibration that a seizure is going to happen? I'm sorry for the twenty questions, these things are on my mind. Thankyou Irish John.

 

[Irish John]

Hi Irish John. I've been interested in the G-stageIII gear box, and I've followed your insightful and informative threads I think for quite some time. Irish John, if you were cruising at @ 30km/hr could that seizure lock up the rear wheel if running the gear box with solid output sprocket? Would the centrifugal clutch disengage and prevent rear wheel lockup? Could a piece of chipped gear tooth lodge itself between two gears and then lock up the gear box? Does that type of engine or the clone always give notice by noise or vibration that a seizure is going to happen? I'm sorry for the twenty questions, these things are on my mind. Thankyou Irish John.

Good question Zean. I've got a Grubee Mark 3 GB as well waiting to be used and I've been thinking that if I experienced that sort of GB seizure on a Mark 3 I'd have gone over the handlebars. With the Mark 3 the only freewheeling is from the clutch bell turning free of the clutch but with the gears locked up the clutch bell can't turn forwards or backwards so the rear wheel would have locked up.
I've had that happen to me on the 2-strokes and it is instantaneous. You are lying in a ditch before you even know what happened. If it happened on a highway it could be fatal (mine happened on a lonely road a few minutes after I'd turned off a busy motorway).
There is a Grubee freewheel rear hub you could probably use that has the rear sprocket on a freewheel (cheap and probably poor quality chinese hub it is) but when you are talking 4-stroke frame mounted motors you are almost certainly talking beach cruiser bike and that probably has a 3 speed rear hub that you would have to kiss goodbye to. To fit a 7 speed derailleur onto a bike that wasn't meant to have one is not that easy because there won't be cable guides where you need them.
Personally I wouldn't feel safe without a freewheel between the rear wheel and the gearbox or motor and there is a very good reason for saying that. Your life could well depend on it.
The Mark 3 was built like that because the quality of the 11T freewheels was causing Grubee problems. They were coming undone all the time and the bronze coloured metal that the teeth are made from was rather soft. the older silver ones were better apparently. To fix the problem Don Grube used the 10T rigid sprocket (no thought was given to the fact that it was sold with a rear sprocket that made the gearing all wrong). He's done the same thing with the 4G - when he realised that the original was completely wrongly geared he made the rear pulley wheel smaller to get the ratio right but he then put a 10T rigid sprocket on the drive shaft because the 11Ts were so problematic. The gain from the smaller pulley wheel was largely lost on the smaller drive sprocket. The shaft is a smaller size so you can't substitute your own sprocket to get everything working properly. What is needed is a stronger well-made 11T freewheel sprocket. The Mark 3 would be harder to cycle around town with the engine off because you'd be turning over the gears.
There are all sorts of pros and cons and if Grubee ever rode their bike kits they'd know of these factors.
I still use the Grubee Mark 2 because the Mark 3 would cause me problems (I'd need to use a smaller 44T sprocket or thereabouts and that could cause me more broken spokes than I already get on the heavy steel Schwinn cruiser) so I have my own method of "improving" the 11T by dismantling it and rebuilding it with grease and loctite and leaving it to sit for 48 hours until the loctite sets. The last bit is very important and that means that the 11T can run its full 3000 - 4000kms until the teeth wear out without coming undone.

 

[zean]

Thankyou Irish John for your prompt response. You always explain the details very well. Fixing all those problems must have given you a lot of satisfaction inside, and kept your mechanical skills at a good level. I'm curious. Would you consider an EZ Motorbike transmission? Mine looks very well built. I haven't attatched the kit to my bicycle yet. I think most of the positive stuff I've heard about it is true, and it's built to be repaired. I have a GEBE kit now running good. The whole deal is fun, riding and adjusting the kit. The Grubee dealer here has a lot of information on their website, I guess they want to promote people to do their own work, start them on a journey of working and riding on motorized bicycles. I have very limited mechanical experience, and was encouraged to start by reading their website and looking at the pictures and your good pictures too. Thankyou Irish John.

 

[Irish John]

Thankyou Irish John for your prompt response. You always explain the details very well. Fixing all those problems must have given you a lot of satisfaction inside, and kept your mechanical skills at a good level. I'm curious. Would you consider an EZ Motorbike transmission? Mine looks very well built. I haven't attatched the kit to my bicycle yet. I think most of the positive stuff I've heard about it is true, and it's built to be repaired. I have a GEBE kit now running good. The whole deal is fun, riding and adjusting the kit. The Grubee dealer here has a lot of information on their website, I guess they want to promote people to do their own work, start them on a journey of working and riding on motorized bicycles. I have very limited mechanical experience, and was encouraged to start by reading their website and looking at the pictures and your good pictures too. Thankyou Irish John.

Thanks Zean. I am moving over to EZ gearboxes because I am forced to. It also seems like a good product but I haven't tried one yet. I had very little mechanical knowledge when I first joined this Forum and what I learnt here is beyond words. The people I remember from 2008 seem to have moved on and I miss them. I don't post as much these days because many of the users are actually suppliers and like to invalidate everything I say if it concerns their product. That began happening in 2009 and has got really noticeable now.