I've got an M20 in for some remedial work at the moment, part of which was fitting and setting up a reconditioned carb. When it came to running the bike it would not work at anything over tick over without displaying signs of extreme weakness. With the choke fully closed it would run however....
After checking other areas which might have caused weakness such as the valve lifter setting, manifold spacer and gaskets etc.etc. it appeared the problem was in fact the carb.
Checking through the petrol supply and float operation was the first thing..no result there...I then completely stripped the carb to remove the jet block..blockages in the small drillings of the block can cause this problem..They seemed clear but as a test I changed the jet block anyway..No difference at all...Comparison of the needle with an original one didn't indicate any real differences dimensionally, though this was just a visual check. Again I substituted the pattern needle for an original..no improvement.
I then changed the needle jet and main jet..these were both new but pattern items...bingo! I could get the bike to run without choke. I then fitted the pattern needle jet with the original main jet and the problem immediately returned..So I swapped them back to an original needle jet but with the pattern main jet...it ran OK again. I have yet to measure either the pattern needle or the pattern needle jet but one or both of these were causing the problem..most likely the needle jet.
When the job is finished I will get down to examining these parts more closely...
Apart from wasting over 5 hours of labour time in total, experimenting and sorting out replacement parts, this is now the third time that I have had serious problems with a 'reconditioned' carburettor...
Bear that in mind if you are considering one...Personally I would rather pay the extra and have a new carb after these experiences and if you don't have a lot of carb spares to hand you are going to have trouble identifying any fault...Ian
email (option): ian@wright52.plus.com
Hi Ian, I have had the run-around with pattern parts in the past and its bad enough when its your own bike and the time spent is your own,but if your trying to make a living doing the work and it's 5 hours wasted that can't be charged on to the customer,it's no joke.
We'll chat about that at the weekend Ron...but I can't see him covering the labour time..I'm not even the purchaser so I guess it would be him that would have to seek redress on his/my behalf...Ian
email (option): ian@wright52.plus.com
Mmmm yes I remember having starting/running probs with my 16h solved them in one stroke by fitting a NOS. carb runs lovely now for a 16H
Jez.
Hi Peter, I agree, I used to be quite keen on the reconditioned carbs and in principle there is no reason why they shouldn't work. However, the 'school of hard knocks' has changed that. The fact is these units are not being reconditioned to a high enough standard, and I really don't feel happy about spending the money and then getting a carb that doesn't work..
My bikes now have new carbs on them and I have found they are a 'fit and forget' component...Ian
email (option): ian@wright52.plus.com
Hi Leon..This one had a problem with the needle jet..The others had different problems. Only one of the problems could not be sorted but they all took up a lot of time to identify and correct and required the fitting of alternative parts...not quite the idea when you purchase a 'reconditioned' unit.
One carb was damaged in the pilot air bleed drilling in the body..that one could not be rectified. This drilling is so small that a slight increase in size is barely noticeable but represents a large increase in the area of the hole...That one took me the best part of 2 days to find and at the end of it the carb body was scrap. That also scrapped the slide as both slide and body had been reworked to 'one off' dimensions...
As if all those problems weren't bad enough I commissioned one M20 carb for a customer (for a WD model) and when it arrived it had a pre war brass topped float chamber on it...that cost me a good one from my own stock...Ian ..
email (option): ian@wright52.plus.com
Hi Moto..I've used a few of the new Amal carbs and haven't experienced any problems yet...One feature of them however is that the fittings are finished in what appears to be bright nickel or the newer type of chrome that has a similar finish. So, for 'military' bike applications if that bothers you the parts have to be refinished in the correct dull or satin chrome finish...Ian
email (option): ian@wright52.plus.com