POST 13: Rebuilding the orange bike's motor

Well, I was hoping that this would be a quick rebuild since the orange bike looked pretty good- and as it had been taken off the road because it had siezed, I'd assumed that it had been running at that time!

Well best laid plans etc etc! The general picture here was  that having replaced the cylinder and piston, the motor would run once I'd sorted out the carb float level by bending the tab to  raise float height...  BUT as described, the motor simply died whenever the throttle was opened. Further, the air screw had little effect on  idle. Both of these observations suggested an air leak. Id found one around the reed valve but sadly this hadn't cured the problem. So I looked further. I found that squirting carb cleaner on to the case junction in front of the inlet manifold led to an increase in rpm. Also squirting it behind the flywheel led to a similar increase- looks like at least 2 leaks.

I removed the flywheel and stator and found that a puff of air could be clearly felt (puffing from behind the crankshaft seal) when the motor was kicked over. This seal is gone - Its possible I could simply replace this seal from the outside using a seal puller but there also seems to be a leak from the crankcase join as carb cleaner here increased rpm. When I'd replaced the barrel I had noticed that there seemed to be no gasket between the cases. I'd assumed that a PO had joined them using  Hondabond or equivalent. Well if so, then this has failed as well.

Consequently it seems pretty unlikely that just changing the crank seal will solve this problem and I'm now looking at a full strip down, new gasket and replacement of the crankshaft seals. Obviously I'll change the LHS seal beneath the clutch as well. This seal runs pretty much immersed in oil so may well have survived but I'm not going to chance it- its a case of the job snowballing under the "might as well" heading - and yes, its probably worth changing the main bearings at the same time! I'm not happy at this moment since it really DOES mean undoing most of what Ive done whilst I was hoping for the quick fix!- Yes, the top end, intake manifold and LHS case cover have to come off once more.

Quick perusal of the parts list shows that whilst most parts are the same as the blue bike, the main bearings may have been modified from 6203uu to type 6004uu so I ordered  2 in case- I wont really know until I get into the cases.

First step is to remove the motor. Ive learned a  little from  the last time but Im sticking to the "leave carb and oil  pump in the frame" approach. So I loosened the oil pump clamp collar...

... at the crosshead screw in the clamp, and disconnected the pump cable.

The pump could then be pulled up and out of the motor.

Leaving the clamp on the case, although this does now lift off.

The oil pump can then hang by the hose connnections.

Remove the exhaust and then the rear wheel by removing the split  pin and nut and pulling it forward off the brake shoes.

Protect the brake shoes with masking tape, Ill probably remove them later. Disconnect the rear brake cable.

... and pull  it clear.

Loosen the carb bolts and pull it forwards to detach from the manifold and airbox mounting plate. Pull the rear brake cable out of its mounting on the engine case.

Disconnect the rear mudguard mounting. The mudguard will then come free.

With the motor now disconnected from the ancilliaries, loosen the mounting bolt- here at the rear shock absorber...

... and at the forward mounting bolt.

I built a wooden support to fit under the footrest bar.

Tap out the forward mounting bolt and allow the  motor to pivot down around the rear mounting (check that the plug will clear  the wooden support below or remove it). The bike will tip forwards to rest the footrest on the wooden blocks. The motor can swing down to the floor and the bike remains supported on the wooden blocks. I used tie downs to hold the front of the bike down. Remove the rear mounting bolt  and then lift out the motor.

And transport the motor for stripping.

The centre stand needs to come off the cases for ease of handling. This is made of two sections - the folding stand itself and a support frame bolted to the motor. The folding section is sprung to the frame and retained by a hinge pin. Here you can see the head of the hinge pin and also the hex head of one of the stand frame mounting bolts.

I couldnt remove the spring so opted to remove the stand and detach the spring at the same time.

Remove the pivot pin by removing the split pin and washer before tapping the pin out to detach the stand and spring.

Pin pulled clear.

The stand frame is held to the cases by 2 M8 bolts 45 and 80 mm in length. I noted the routing of the drain hose through the frame before removing it.

In my case the two mounting bolts appeared to be made of cheese and rounded off when I tried to unscrew them. I"ll replace these on rebuild. M8 x45 and M8x 80mm.

The stand and motor cases then needed a good clean before repainting and reassembly.

Stripping the motor- started by removing the structures that bridge the cases- these are the cylinder barrel and the inlet manifold.

Detach the auto choke tube from the manifold

7

And remove the 4 nuts. I found this one ( front left) was shorter than the other 3.

Loosen the head bolts with a 10mm socket. This bike has bolts but I think Ill change them for studs to make head removal easier.

Head and barrel then just pull off.

This releases the spacer tube- photo'd here so I dont forget to pit it back on reassembly.

Undo all.the case bolts on the lhs  engine case.

This time I'm modifying the approach. I'm leaving the kickstart in place so that the spring is not disturbed. It shoukdnt unwind as the sring will be held by the ratchet. However, Ive not done this before and it could go wrong so I marked the position of the kickstart so that if it did unwind, it would be easier to refit and retension

Then I tapped the cases apart. Since I'd only used compound on one side, the cases came apart easily

Kickstart remained successfully in  position and under tension. 

I lifted the final drive out. The idler shaft and chain will come off with the clutch. Noted the 2 shi.s an 1 roller bearing on the end of the shaft.

Remove the clutch centre bolt using an impact ratchet.

I used my home- made puller to remove the clutch drum, chain and kickstart idler shaft. As in the case of the blue bike, the shim and wavy washer that should be behind the idler shaft to preload the roller bearing on the front of the shaft, were both missing. Does this mean Honda just didnt fit them - or do all owners just lose them when they rebuild? As before; I'll mill out the 17mm recess to 18mm in order to fit larger washers as the original 17mm honda items are no longer available.

The plastic chain tensioner at the bottom just lifts out and the upper one was removed by removing the spring tensioner which is retained by a bolt   and washer.

I could then try and separate the cases. There are 4 crosshead screws in the alterntor housing and 2 longer screws that cross the cases either side of the piston. You'll need an impact wrench but even so it was very hard because a PO had used retaining compound on these screws. The long 80mm machine screw that crosses the cases below the barrel.was very tight indeed and needed heat to shift it.  Ill need to change all of these screws as the heads were distorted by the removal process. I ordered new Allen cap screws in M6 size as 4x45mm and 1x each of 60mm and 80mm. This shoukd give an improvement in grip when tightening and releasing.

FIortunately all except one did eventually inscrew although I had to drill that one out.

This is the leaking crank seal- the shaft looks quite grubby with rust and scratches. I will polish it with a brass wheel once the shaft is out. You can also see where I had to drill the head of one of the M6 case screws prior to separating the cases.

I could then use a case separator to open the motor.

The cases were quite hard to separate. I was worrried Id missed a screw but in fact it was jointing compound on the motor dowels which proved very hard to detach. The crank then tapped out of the other case. This left the lhs main bearing in the lhs case, but the rhs bearing came out still firmly attached to the crankshaft, close to the crank web and below the drive gear for the oil pump. In fact this bearing seems to be in good condition, but the lhs bearing left in the case was very rough and definitely  needs replacing.To be safe I'd like to replace both, but removing the rhs from the crank proved very difficult. Its one of those procedures you wish you'd not started, bit having started, I cannot now be sure that the bearing wasnt damaged by my attempts. Consequently, I have no choice but to press on. I will need a bearing separator to get this off the crankshaft since its pressed on very close to the crank web and I cant get a conventional puller underneath it.  I think the 30-50mm size should do as its a 42mm bearing. I fact this worked perfectly!!

Both bearings are the same, type 6004 unshielded.

Separator slipped under the bearing as it was tightened. Went with a bit of a frightening clunk but worked  ok.

The threaded puller coukd  then be fitted and drew the bearing off easily.

Turning to the lhs case, I levered out the oil seal...

 And checked that this is type 15x25.5x7

I tapped the remaining bearing out using a socket to pass through the case and  bare on the inner race.

The bearing is type 6004 unshielded.

I cleaned up the cases- the honda dowels were quite dirty and I needed to remove them to clean up the case around them.  They are quite tight, but to remove them you need a 21/64" drill bit to slip inside to prevent them from being crushed.

Wrap in emery and pull out with pliers or mole gri

Even so one was so tightly fixed (see case separaion above) that I had to destroy it to remove it. Ive cleaned up the sockets with carb cleaner and Emery paper so hopefully reassembly will be straight forward.

I removed the oilseals remaiining in the cases by tapping them through with an appropriately-sized socket.

To start assembly I chilled bearings in the freezer and warmed the smzer crankcase with a  hot air gun for 5min. To my surrise this aowed the bearing to dimply drop in.

The same approach was less successful with the karger crankcase- possibly because I couldnt heat it evenly. Consequently, I had to use the press to insert the bearing.

I let the cases cool before inserting fresh oil seals- tthese go farter sorings towards the big end in both cases.

I lubricated the bearings, oil seal and crank shaft with assembly  lube and fitted the crank by pulling it into the bearing now fitted in the rhs crankcase (the smaller one). I used a Tusk style puller to do this although I did need to use a large repair washer to provide a flat, firm base for the puller to seat on. The parts are shown below.

Place the washer over the crank...

And assemble the puller onto the crank thread using the appropriate  adaptor. This is a n M10 1.25 thread.

Tighten the puller to draw the crank into the bearing. Its not clear how far the crank should to drawn in so I pulled it until the resistance became considerable.- once more Id not pulled it in far enough.

I assembled the central crankcase cover to the smaller one incorporating a gasket between the halves. I stuck this to one side only using permatex as I wasnt confident that this wouldnt need disassembly and adjustment. 

I drew the crankshaft through the central case using the puller but found once again that I couldnt close the cases unless I also drew the rhs through a little more. However eventually I got a good seal and could insert the replacement Allen cap screws I'd bought to replace the Phillips screws that were damaged on disassembly.

I photgraphed the position of the crankshaft oil pump drive worm through the oil pump housing. This gives an indication of how far the crank should be drawn into the rhs case.

The drive worm should cover almost the whole side adjacent to the crankshaft. Tricky to photograph but visible below.

I assembled the transmission, incorporating the shim and wave washers beneath the kickstart idle shaft and the thrust washers and roller bearing on the outer side.

Reassembly was surprisingly complication-free. The alternator was refitted easily although I will need a new nut (m10 1.25) to secure. The kickstart side of the motor was never disassembled and refitted easily restoring kickstart function and avoiding the tedious finger-shredding tensioning of the spring!  I refitted the cylinder and head, reed valve and intake manifold. I used RTV gasket on the last and permatex around the cylinder base as I was reusing the gasket. I fitted the stand using new bolts and then positioned the forward mounting bolt sheath/hose guide in place and rethreaded the hoses through their hooks before trying to refit the motor. This is much easier than forgetting them and struggling later on. However it was still quite complex to route the hoses and cables the most ergonomic manner and it took several attempts - and modification after refitting the oil pump and carb,  however I eventually arrived at a system which I think is an improvement on that achieved by the PO - which had probably diverged frm stock over the years.

I then refitted the wheel and mudguard and adjusted the rear brake and oil pump cables before fitting the exhaust. I still lack a heat shield for this and Im awaiting a new nut to secure the alternator, however Im almost up to attempting a motor-start!

Well I started  the bike but its still behaving as  if it has an air  leak  and will only idle it dies immediately I open the throttle.

I have checked the hosesxare vonnected properly from the autochoke to the carb. I have conne ted 

Hose 1- intake manifold

Hose 2- (plastic coated) Carburettor adjacent to adjustment screws

Hose 3- To bystarter  chamber with a short...

Hose 4- From bystarter chamber to carb to (adjacent to thtottle cable entry)

However examining this connection revealed that some vomponents of the bystarter chamber are missing. There shoul be a filter support plate and a filter element; the whole assembly fitting onto the rear of the airbox.

However- I I blocked the bystarter chamber using a bung (which effectively connects hoses 3 and 4) restored throttle function nd improved running.

This is a mystery to me since in normal function the bystarter chamber would be open to the air- albeit through a filter.

I suspect the auto choke or bystarter chamber. The auto choke is difficult to adjust and fiddling with it isnt recommended but if you must...

https://www.mopedarmy.com/wiki/Honda_Express_auto_choke

In fact I didnt need to tackle the autochoke. It took me a long time to sort out how it works but its simple when you do. Its not a choke- it doesnt restrict air it supplies extra fuel to the same amount of air. I think I have my head around the function of the autochoke/enrichment system and bystarter chamber.  Vacuum in the inlet manifold communicates with the auto choke via hose [1]. This draws a mixture of fuel and air down the plastic coated pipe [2]. The fuel comes from the carb choke circuit. When the motor is cold hose [3] is shut off so all the air for this suction is drawn from  the bystarter chamber via hose [4] and through the carb where it picks up the fuel.  As the motor warms hose [3] is progressively opened which supplies air from the bystarter chamber directly to the auto choke unit. This decreases airflow down hose [4] into the carb, and thus reduces the amount of fuel sucked out with this air via hose [2]. In other words, this leans the mixture that eventually passes into the motor via hose [1]. When the motor is fully warm, both air hoses are closed and the motor runs entirely on air drawn though the carb throat.

Putting a bung in the bystarter blocks air into both hoses [3] and [4] and so mimics a warm motor, so Im assuming there is something wrong (air leak?) in the choke circuit.

Looks like Im going back into the carb!!!

I replaced the carb with one with the idle/choke circuit jet nearer the bottom of the reservoir tube.

The bike then ran- albeit poorly. I discovered that the oil pump inlet elbow was loose in this carburettor - which would permit a large amount of extra air into the carb.
 I refitted this brass elbow, securing and sealing it with loctite and this improved running greatly.

However, after the rebuild it was also clear that the exhaust was blowing from the barrel/pipe junction. The pipe could not be secured tightly because the stud thread in the barrel was stripped. I was therefore forced to strip the top end again so that I can drill and fit a helicoil to secure the loose barrel stud. The other stud has already been helicoiled although not particularly well. I think a PO has persevered with these worn, and failing stud drillings for too long and the drillingsxare enlarged further than ideal for an M6 helicoil. Helicoils were probably needed a while ago