Sieberock Big Bore kit

[mkj]

Mt ex Police 1979 R80/7 had become a bit of a embarrassment due to the enormous clouds of smoke it left behind so a rebore was called for. The pistons are ready available but I could find the anybody to do the work locally and a big bore kit just goes on in an afternoon and who doesn't want more power. I had always wondered about the difference my R80/7 and an R100/7 ( 5 horse power) apparently. So the job went ahead and it did just take an afternoon.
Its been a relative success more power less oil consumption and now I feel justified in being exempt from ULEZ. How ever the main draw back is it seems to use considerably more fuel about 25% more especially at low speeds. Difficult to give totally accurate numbers as my odometer went crazy and I just use the trip. Sticking to 55 mph and doing all those little things to reduce fuel use I got 36 mpg yesterday over 140 miles and at 80 mph I get around 42mpg. This I just cannot understand. Any Ideas. On reflection I doesn't seem worth the extra fuel as I don't go any faster. I no wonder if Siebenrock do an 800cc kit as I would save the cost on fuel in around 10,000 miles or so.

[SteveD]

20% bigger engine than it was! Have you done anything to the carbs to match the new engine size? Have you replaced the needles and needle jets too? New ones usually help with improving consumption.

Does it still blow smoke? Do you exclusively use the side stand?

Blowing smoke when accelerating after idling for a while is usually down to worn valve guides.

[mkj]

There is you can say about the Siebenrock kit is that it burns virtually no oil, my motor is smoke free, I very rarely use the side stand as Its probably ruined the original righthand barrel which had corroded at the bottom of the stroke. Just found to my great relief a broken jubilee clip on the right air tube, it had the appearance of a working jubilee clip but had let go on the inside and just came loose all the time. 44 years old though. I have repaired said clip and lined the joint with a bit of inner tube and see how it goes, whilst the new bits turn up.

[mkj]

Couldn't resist so went for a slow run and managed a little, improvement 42.6mpg. Running much sweeter too, now of for a faster run, will even try the super slab ( dual carriage way) , the bit without average speed cameras. Every time I go down the A90 which is average speed cameras for about 100 miles I just wish I new how fast I was actually going, can you believe the little signs on entering .small towns which tell you your speed but never seem to smile at me.

[mkj]

Just been out on what may well be my last run of the year, certainly will not be out for a couple of moths. I have to say the siebenrock is really going well. I went over Cairn O Mount a very popular run in this area, N.E. Scotland, 90 mile round trip very enjoyable . So now its parked up in the garage, float bowls emptied and battery disconnected. On the down side I did notice that both front disc cracked in a couple of places, shame as the discs show now other signs of wear, guess I'll have to change them £300 from motobins