Home › Forums › All Things 750 Twin › Projects and Progress › Old Chop Starts after over 15 years
Tagged: New Valves and Shims fitted
- This topic has 12 replies, 2 voices, and was last updated 1 year, 8 months ago by Jimi5150.
18th April 2021 at 8:38 pm #27555
The old chop looks a mess and the engine hasn’t been started for at least 15 years. Using a loop and electrics from a B1 and hooked up to auxiliary fuel tank, a few kicks to get the fuel into the chambers and it fired up, I thought it would run for a few seconds then die, but it ticked over ok and didn’t die when given a some revs and very little smoke. After a few mins it had to be switch off as the rocker cover gasket is leaking quite badly in a couple of places. The chop frame will not allow the rocker cover to come off so engine will have to come out. Compression is 90 on both cylinders but may improve my running it for a longer time, but valves gaps etc will be checked once the engine is out. The frame getting in the way of the rocker cover has strengthened the case not to keep it as a chop.19th April 2021 at 6:14 pm #27557
That is indeed good news and great progress.
The compression is obviously low and will need to be addressed (156 standard – 114 min with no more than 14 difference between cylinders). This could be down to several things but most likely will be valve clearances and/or stuck rings. Leaking head gasket could also cause this but I think you would have noticed that.
I can probably help you out with a standard frame and other parts if you need them.
KK23rd April 2021 at 5:18 pm #27565
Cheers KK I will most likely need help with a frame in future, as the parts I have are off 76 model I would be preferably be wanting a Kz750 B frame. No doubt I will need help with other part too.24th April 2021 at 9:49 pm #27570
Got engine out and measured the valve gaps, sort off. I can’t measure the gap on all valve as not even the thinnest gauge will go in, which will make getting the right shims a pain. Order a shim tool for the next stage.26th April 2021 at 8:18 pm #27583
The way to test for the correct shims required is to fit one that is too small (the smaller the better) then measure the gaps. This way you can work out what sizes are required.9th May 2021 at 9:21 pm #27603
Found a shim not a KZ750 shim which was 200 so used this to get the readings I need as it gave gaps for all the valves. I would need shims 180-190, the smallest KZ750 twin shim is 220, balls it looks like the head will have to come off.20th September 2021 at 8:01 pm #27805
Head was taken off with new valves and gasket fitted. Valve gaps checked and new shims fitted. Time for a compression test, cannot turn the engine over with stater motor even with both plugs out (clutch rebuild kit fitted, but gear needs changing), so have to do it with the kick start. No down pipes or airbox/filter fitted and kicking it over the bike is definitely squeezing and blowing out a lot more air then before but could not get the compression more then 60, tried again but putting the spark plug back in the other side, this should make it harder to kick over but it did stop my foot slipping off smacking my leg on the chop frame, now got the readings to be 152 & 155. I was going to try to start it but the carbs are now peeing out fuel out the right drain tap, so carbs will have to come off again to sort this.20th September 2021 at 10:08 pm #27806
152 & 155 are very healthy. In what way is the engine failing to turn? Is the starter motor sluggish or is it spinning quite freely and chattering?21st September 2021 at 7:55 am #27807
I can hear it spinning freely.22nd September 2021 at 11:38 am #27809
That’s surprising. If everything is fitted I would expect to hear chattering. Spinning freely normally happens when the chain has been removed which some people do when the clutch fails. The only other things that come to mind are a bit more serious.
Firstly it might be the large gear that engages with the starter clutch is either a wrong one or the boss has been machined down so it will not engage with the rollers. The diameter of the boss should be 42.2mm. I would guess that it should be no less than 41mm. If that is fine then we need to look deeper.
It may be that the whole of the rotor assembly is rotating on the crank shaft. When fastened up correctly the taper of the rotor mates and secures to the taper of the crankshaft. Obviously the tapers are very important so if one was different to the other then they would not mate and it could slip. The B1 and B2 have different tapers to all other models and are NOT interchangeable. This is because the B1 and B2 are 3 phase charging whereas all other models are single phase so a different taper was used. It is possible that the crankshaft or the rotor have been changed at some point. Maybe this is why the bike got laid up? You should be able to tell that as the rotor would ‘wobble’ before tightening the centre screw, quite an easy check. I would also suggest check all the components are there, in good order and fitted correctly.
I hope this helps.27th September 2021 at 6:46 pm #27815
Made a bit more progress today, the carbs was leaking fuel out of the right overflow, a tiny adjustment to the float bowl height sorted this. Once fuel was in the carbs it only took a couple of kicks to start. Then it was time to look at the electric starter problem. Dropped oil out and took the side cover off, pressed the starter, I can hear the starter motor spinning fast but the chain doesn’t move, took the chain off and pressed start, I can hear the starter motor spin fast but the starter cog doesn’t move, so its a problem with the starter motor, the starter clutch may be OK after all.29th September 2021 at 12:22 am #27816
Well that probably eliminates a lot. The head of the starter motor is a gear box. What you say suggests the gears have probably been removed. I do have a new starter motor head but that doen’t come cheap but should be able to supply you with any missing parts in used condition.29th September 2021 at 8:14 am #27817
Took it apart, the head looks O.K, the gears are there but all the Thrust washers and O rings are missing, managed to get hold of some replacements. Main problem looks like it wasn’t tighten up properly.
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