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CB750 / 750 K3 rebuild
« on: October 28, 2013, 01:37:22 PM »
I'll put this here for those that are interested. I recently bought a K3 off ebay which had been imported from America. It is largely stock but seriously unloved. The odometer reads 12,000 miles but the bike seems to have been left outside for years. The wheels are rusty but only in one section so presumably where the water pooled over all those years. The clock faces have faded and curled at the edges so that they now look like giant green Pringles. The previously beautiful 4into4 system has holes in it so big I can get my hand into them.
When I got it the carbs were seized shut and the tank was 1 in ch deep with milky petrol. The number plate has a sticker on it with the number 94 on it so presumably that was the last time it saw any action. Following some sterling advice from the US forum I bought a heat gun and used that to soften the inlet rubbers until the carbs came off. Then, the same heat gun was useful in melting the varnish that had seized the throttle slide in carb 4.
I took the carbs fully apart and gave them an ultrasonic in hot water and fairy liquid. Then I jury-rigged my own soda blasting kit using a very small compressor and a spotblast gun. This cleared most of the crud off the carbs. A bit of carb cleaner squirted through and some new pilot jets and float bowl gaskets and the carbs are back together. If this seems a bit half-baked then it is. I just wanted to see if I could get the thing to run so I could hear it. I wanted to know if it would start at all and whether there was anything awful going on deep in the engine..anything audible at least.
I fitted a spare Newtronic electronic ignition set I had from a scrapped 400/4 (frame was bent and it had CB250N wheels on it so I don't feel bad about breaking it), cleaned out the fuel tap, connected a spare battery from my Defender and whoa! it fired up. Admittedly it did blow a huge amount of what remained of the silencers onto the deck but it runs. Problem is that pipe 4 is smoking. I can't work out if it is running super rich - i.e. there's an airway clogged on no4 carb or maybe the rings have rusted or who knows what. I can't help noticing that No4 carb was the seized one and now no4 cylinder is suspect. Also, this is the uppermost pot when it's on its side stand.
I intend restoring this to head-turning but useable standard rather than concours. The gold CB in the most recent classic bike is my target. In fact, I'll do it just the same as that.
I've attached most of the pics that I already posted on the US forum.
I've ordered the rebuild manual - My CB750 Book - which was written by a prominent member of the US forum. It is very highly regarded and I am keen to get hold of it just as soon as they've printed the thing for me.
Advice is always welcome.
When I got it the carbs were seized shut and the tank was 1 in ch deep with milky petrol. The number plate has a sticker on it with the number 94 on it so presumably that was the last time it saw any action. Following some sterling advice from the US forum I bought a heat gun and used that to soften the inlet rubbers until the carbs came off. Then, the same heat gun was useful in melting the varnish that had seized the throttle slide in carb 4.
I took the carbs fully apart and gave them an ultrasonic in hot water and fairy liquid. Then I jury-rigged my own soda blasting kit using a very small compressor and a spotblast gun. This cleared most of the crud off the carbs. A bit of carb cleaner squirted through and some new pilot jets and float bowl gaskets and the carbs are back together. If this seems a bit half-baked then it is. I just wanted to see if I could get the thing to run so I could hear it. I wanted to know if it would start at all and whether there was anything awful going on deep in the engine..anything audible at least.
I fitted a spare Newtronic electronic ignition set I had from a scrapped 400/4 (frame was bent and it had CB250N wheels on it so I don't feel bad about breaking it), cleaned out the fuel tap, connected a spare battery from my Defender and whoa! it fired up. Admittedly it did blow a huge amount of what remained of the silencers onto the deck but it runs. Problem is that pipe 4 is smoking. I can't work out if it is running super rich - i.e. there's an airway clogged on no4 carb or maybe the rings have rusted or who knows what. I can't help noticing that No4 carb was the seized one and now no4 cylinder is suspect. Also, this is the uppermost pot when it's on its side stand.
I intend restoring this to head-turning but useable standard rather than concours. The gold CB in the most recent classic bike is my target. In fact, I'll do it just the same as that.
I've attached most of the pics that I already posted on the US forum.
I've ordered the rebuild manual - My CB750 Book - which was written by a prominent member of the US forum. It is very highly regarded and I am keen to get hold of it just as soon as they've printed the thing for me.
Advice is always welcome.