Swapping carbs and the dark art of spark plug reading

[DAVE BRADY]

Good evening,

I have just fitted a new Amal Conc. Mk1 Prem. to may 650 A65T and noticed straight away a great improvement in just about everything.  I checked the plug after about 40 miles and it looked cleaner than when I put it in.
I had always had a 30 mm carb on this bike.  Originally a 30mm conc. fitted by the PO and then when this was well worn out I fitted a 30mm Mikuni.  Several things I read said that the Thunderbolt went better with a 30mm carb. but....  The Mikuni has done a great job and is still like new after thousands of miles of use but I want to put this on my long stroke 750.
So it looks like the original spec 28mm Premier is a good choice.
The 750 has had a 32mm Mk2 Conc. fitted for its whole life as a big bike and as it is just about the same configuration as a 750 Triumph Tiger I am going to try the Mikuni jetted pretty much as the the Triumph.
Some top end may be lost but I am hoping for more low to mid range and economy.
In the process of surfing for ideal plug colours I found this plug chart.  It may be helpful for any bike.

Dave

[DAVE BRADY]

Hi,

Another new Amal Premier Conc. fitted.  This time to my wife's '51 Flash. The one taken off had been on for a while and was the second one to get a bit worn out on this bike.  So today we went out for ride so that my wife could see if it had been worth the cost and bother.  There was no point me trying it as I have rarely ridden her bike so would not know if it was better or what.
So was it worth it or not.  Definitely.  The bike is sharper in its pick up at all throttle openings, is smoother    to ride and pulls better from lower revs. and seems to develop a bit more power.  A quick twist of the throttle at 55/60 resulted in a bit of clutch slip whereas before it did not do this so some clutch adjustment needed.
I know a 28mm Conc. is not the standard fitment for a '51 Flash but the bike is a bit non standard anyway.

Dave.

[DAVE BRADY]

Hi,

A short run out today with the 30mm Mikuni on the long stroke 750 Thunderbolt.  As it was replacing a 32mm Mk2 Concentric I was expecting there to be a noticeable difference.  This was not the case.  Not exceeding 70mph meant that top end was not really tested but bottom end and mid-range felt a bit better.  As I am looking for better fuel economy I hope this is also the case.  Once I have checked the settings with a 'Color Tune' I will have a better idea of the suitability of the jets I have been using for the 650.  Perhaps no change needed as although the 750 is bigger the extra 'suck' from the longer stroke may well compensate for this.  I may actually need to weaken the mixture a bit.
It was quite cold but bright sunshine reflected off wet roads was a bit dazzling and the roads are filthy.  Almost as much time cleaning the mud off as riding.  I must fit a mud flap to the rear mud guard as the spray had covered the number plate and light with a film of mud and it had even on to the back of my wife's jacket.

Viva la Vaccine

Dave.

[Dean Southall]

Dave
I have never worried about mud covering the rear number plate. Mind you we probably have more speed cameras around here 

[DAVE BRADY]

Hi Dean,

It was particularly crappy today and I actually had to hose the bike down to get the worst off.  One good thing about swapping the carbs. is that with the Mikuni I can fit an air filter so nothing will have been drawn in to the engine.  The Mk2 conc. sits too close to the oil tank. 

Dave.