And the impossible looks more likely… (quote from a facebook conversation with Jon, a friend and fellow Drascomber.
I posted on the Drascombe owners facebook group, with the details of what I had been suffering with the engine. From the broad range of experience there, I gleaned that carb problems are all too common, especially with modern fuels.
The advice was very good, on the whole, with a wide range of points of view. I was left expecting a worst case of needing a new carburettor (or one suggestion to trade in the whole engine against a new one being the preferred choice). I also had a range of suggestions, such as clean the carb (solvent, compressed air, thin wire, ultrasonic cleaner), change the fuel lines, change the filter, check the piston rings. One gent from the states even offered a fuel additive which is, apparently, in common use over there…
But they were all in agreement, don’t touch the setting screws!
One member of the group even provided me with a scanned copy of his manual, but it proved to be not quite a match.
So, yesterday, I removed the carb, and gave it a good look over and clean – even though it looked immaculate inside. I ran a thin piece of wire (a broken needle threader) through every hole I could, and then reassembled it.
I could not see any issue, nor could I quite understand how the thing worked, as the various parts did not seem to be in an order that made sense to me, but I kept looking until I think I got it.
The thing that threw me was the position of the fuel pump, which seemed to draw directly of the bowl. However, after much inspection, particularly reassembling (repeatedly) the remarkably fiddly controls for cold start and idle speed, I worked out that the engine does not, in fact, have a choke at all in the traditional sense. What it appears to have instead is a tiny diaphragm pump, which sucks in from the bowl, and squirts directly into the plenum chamber.
Once the thing was re-built, I filled my big bin with water, after George (assisting today, not to mention providing moral support!) had bunged up the hole in it, and started her up. All systems were not go. Basically, that couple or three hours work came to nothing, as the symptoms were all still the same.
I was starting to lose the will to live, so threw caution to the wind! first marking the position of the adjustment screws, I then fiddled with them, a little one way, try it, a little the other way, try it.
It didn’t take long to find the one that made the difference! I think possible a mixture setting, it ended up a quarter turn different to its previous position.
(that’s what the opening quote / title of this post refer to!)
So, although it has not been proven in anger, the engine might appear to be fixed, as it ran like that whether hot or cold! The only thing that seems to not work is the idle speed adjustment…
It is worth noting that the reason I have to have the lid off in the video above is a rather cockeyed arrangement of throttle and gear control, all controlled by a single twist grip, which, while convenient, is not intuitive to operate, and means there is no way of revving the engine out of gear.
By having the cover off, i can access the linkages directly on the carb, and having had it to pieces, I now know which bit to move which way. Operating in gear in the bucket at anything above idle just empties the bucket, risking overheat.
Also, note the good strong tell-tale from the coolant system!