Partially Restored 74 CB550

[McCabe-Thiele (Ted)]

Would the sale of a 750 tacho not cover the cost of a 550 unit?
Might be Kent400 could swap some internals.

[McCabe-Thiele (Ted)]

Two wheel spares have loads listed.

https://www.ebay.co.uk/itm/334629151614?hash=item4de973a37e:g:3bwAAOSwjkRjc8PU

[davidcumbria]

Thanks Ted I have contacted kent400 to see what he thinks. I suppose the overall ratio is a combination of the cam drive ratio and internal gear ratio in the instrument. If he can convert my 750 tacho saves having to do a buy and sell and my tacho is in good external condition.

[McCabe-Thiele (Ted)]

Useless factoid.

BLMC Mini Smiths Speedometers had a number in very small print on the clock face that we used to refer to as the Wey Speed. This digit was about 6 digits long it was some sort of standard for the calibration. If the Wey Speed was the same for two clocks they were interchangable even if the face markings differed in terms of the maximum speed.

Sometimes a silver faced clock from an old Morris Minor matched that of a Mini so you could switch them knowing they were accurate.

[davidcumbria]

So Kent 400 has kindly got  back to me and clarified there is no gearing inside the instrument  just a spinning magnet . This means the 4:1 ratio 750 and 6.7 :1 ratio 550 is established at the camshaft take off. Looking at the parts list the tacho drive gear spindle is completely different between the 750 and 550 as you would expect so it is highly doubtful they are interchangeable. Looks like my only way forward is to get hold of a 550 tacho and have that refurbed which I will likely do at some point.

Fitted a seat trim with the prepared ends yesterday which involved drilling 12 holes in my seat pan. Very fiddly but got there in the end and happy with my seat after a shine up with bike sprits spray cleaner I heard about on here. So cosmetically that’s it for now 😎

Tested my plug caps and got 13k , 14k, 15k, and 35 k ! Got a replacement on order for the outlier and will see what that measures on my multimeter before replacing all of them. Bike fires up on the button and no indication of problems at higher revs. The plugless leads for  both coils measured 15k., running Denso non restrictor plugs. Kind of shows that the system is pretty robust though some think that the high resistance will shorten coil life. Will play safe and have 5 k caps all round if my meter isn’t trying to fool me.

[davidcumbria]

Thanks to Simon on this forum I now have the correct tacho fitted so all good in that department and I have a nice 750 tacho available if anyone is interested.

Bike was running ok  / good as above but we like to fiddle don’t we? Changing the plug caps made no difference to sooty no 1 plug and digital thermometer showed it running a good 20 % cooler than the other 3. My first effort in (removing ) then replacing the carbs on a fully built bike went smoothly. Air box out and plenum minus rubbers in frame, carbs push onto engine intakes, plenum rubbers push into plenum from carb side with plenty of red grease then fit to carbs. Not the struggle I was expecting.

For peace of mind I rechecked the needle position which is the middle ( third ) groove on all so no slip up there. All main jets 100 and pilots 38. Thought there was possibly a small new blockage  in no 1 idle circuit as the carb cleaner didn’t spray through cleanly at first attempt. Blow through with airline and all good. I went back to original needles and left the new Cruz in image needle jet tubes in place. I found  No change to sootiness and bike idling worse ( hanging then  stalling from 1100 rpm ) so probably should have left the Cruz in needles in and the possible pilot circuit blockage clearance didn’t nail it as I’d hoped.

Finally after the carbs were back on bike I managed to change the no 1 idle jet to 35. Seems to be running best it’s been with smooth idle, no hanging and good top end power but no chance to road test yet. Watch this space.

Btw this has been a very useful and informative thread on the subject for me and confirms my experience that the pilot jets and air screw setting are influential very significantly above idle indeed in a lot of the Rev range you might be using during running in.

http://www.sohc.co.uk/index.php/topic,25673.60.html

[McCabe-Thiele (Ted)]

Just read that old post by fogrider  the link you posted from 2021 - it was very interesting stuff David.

[davidcumbria]

Test rode the bike after stripping carbs, putting original needles back and replacing no 1 pilot 38 with a 35. All good, idles fine, doesn’t hang, no stuttering on pick up, revs cleanly to upper levels. No 1 plug still a little black coloured but not sooty and cylinder temperatures are closer. That will do for now - I’ll sync after a few more miles. Delkevich 4 into 1  is a bit louder than I would like……

[Laverda Dave]

The bikes looks fantastic Dave and the colour really pops. It's always the final fettling that takes the time and is fiddly but worth it in the end. Great thread👍

[McCabe-Thiele (Ted)]

The exhaust noise might change with use?

[davidcumbria]

Fitted the front mudguard  flap that Gareth very kindly sourced for me. I think it’s looks good and fitted the width of the guard perfectly - photos below.

Up to 120 miles now and running great  - no issues at all. Wearing earplugs cured the excess noise , a little more economical than buying a new 4 into 4. It is such fun to bimble along B roads at 40 -  60 mph - torquey, comfortable, pleasing to wind up and fairly nimble. Who needs a 150 bhp £20 k super naked when these look better and are so satisfying to ride ?

[Johnny4428]

Mud flap looks the part. I agree you can have a lot of fun on the 550! Makeshift smile at least once every time I go out.😁