With the tank just about done I wanted to check the engine was still running ok - it wasn't. It just did not want to start at all so the carbs came off for a strip down and clean. I saw nothing in there to worry me but it was interesting to note that the main jets had been changed from 105s to 125's and the idles were 38's, the idle screws were out 3 full turns from the usual 1.5. They had obviously been cleaned recently and were in very good order. I it them back on but the bike still would not start, even with starter fluid and heating the plugs, it would pop very now and again but it simply would not start. My immediate thought was ignition but everything seemed to be in order, the extortionately expensive NGK plugs were new, it was driving me mad.
In the end I blocked off a large port that went from the carbs to the original air box and she spluttered in to life before settling down to a smooth but high idle. Why it had run before like that I have no idea, it's all very odd. Anyway I let it warm up and the measured the exhaust temperatures, the carbs are definitely in need of a balance, an utter nightmare on this engine as it's nigh on impossible to get at the adjuster screws with the carbs in situ. I will worry about that later, I'm just glad it's running I was beginning to think I had blown the engine or something. I did resort to a compression test, it's fine.
I drained the coolant while I was at it, I suspect the stuff in there is 20 or 30 years old, nasty. New stuff is on order.
Next job is most likely the fork oil seals, I have no idea what the forks are from, I think I was told CB400 but they are definitely a different size. I will measure them once they are out and if I don't have any will order some by size rather than by application.
Hopefully by then the decals will have arrived and I can put them on and apply the final coat of lacquer.
UPDATE- I took the bike to a bike night and when I tried to start it the starter had lost interest, it was just worn out after being in existence for 40years, the brushes were worn down to nothing. I have a new one on order, while it as out of service I decided to look at the carb balance - it was - an absolute mile out. I realised something was wrong when I compared the temperature of the exhaust headers, 1 was at 65 degrees, 2 was at 130, 3 was at 220 and number 4 about 140, they should be within a few degrees of each other so something was clearly wrong. The usual way to balance bike carbs is to connect up vacuum gauges and measure each carb relative to number 3, which is the one the throttle connects directly to. Adjuster screws allow you to then set the other 3 carbs to pull exactly the same vacuum and so therefore draw the same amount of mixture and be in balance with it's neighbours. The screws on the cbr are so inaccessible in the CB72 frame that such an exercise is impractical so I took them off and set them up on the bench instead. You do that with a feeler gauge and a torch, you want the throttle butterflies to all open exactly the same amount, using feeler gauges showed mine to be out by significant amounts. The torch gives you a visual clue , you shine it through the venturi and check to see how much light shines through. it won't get you an absolutely perfect set up but it will get you very close, final tweaking can be done on the bike safe in the knowledge it will be a lot less messing about. I am still waiting for the new starter to arrive, I can then see how close it is and get it perfect, i should start and run a whole lot better then.
Add comment
Comments