Jubilee idling too fast.

Peter and George thank you for your suggestions.

I think I have found the problem and it is a curious one! After another frustrating session today trying to stop an ever increasing idle speed I took off the float bowl cover to find that virtually no petrol came out. Looking closely, the brass float was resting on the bottom of the float chamber with the float needle resting at a slant and off to one side of the flat on the float that is supposed to bear on the end of the needle.

My working theory is that the when the float was at its lowest point the needle slipped sideways and trapped the float in the low position. With the needle wedged not enough fuel was getting into the carb resulting in an increasingly weak mixture.

the brass float has a narrow flat allowing the needle to slip to one side. The needle is nylon and old school and has a small pip on the end making it easier to slip off the float. 

I have a new float with a wider flat area and I am waiting on a new float needle.

I wish I had taken a photograph of the slanting needle.

I will post an update next week.

Dennis

Hopefully, you can see that the flat surface on the brass float is much narrower than that on the plastic one. Also, if you look carefully, you can see a 'witness' mark on the brass float where the needle has contacted with the float; at least until now!

Dennis

Thank you both for the advice. There are no obvious problems with the slide but it was worth checking. I am still waiting for a new float needle. I'll let you know how I get on.

Dennis

Ian,

No fibre washer under the float needle housing, just the nylon fuel filter in the banjo.

Good point though.

Dennis

The new float needle arrived today. The combination of a new float and needle seems to have solved the problem of a fast idle that could not be slowed down. I managed to set an acceptable idle speed but I have not road tested the Jubilee yet.

I did check for leaks around the carb and manifold using WD40 and I did not detect any increase in engine speed by this method so I am pretty sure that there are no air leaks.

The engine takes a short while to settle back to idle speed when I blip the throttle but the throttle cable and slide move freely so that is not the cause.  The carb might just need a fine adjustment or settling in so for now I think it is problem solved.

Thanks again for all the suggestions.

Dennis

I fixed the needle and float issue with new items but I was still not happy with the idle speed. It was OK but not slow enough for my taste. I happen to have a collection of old Monoblocs so I took the slide out of two of them and compared them to the one on the Jubilee.

I measured the top of the cutaway at its highest point to the top of the slide on all three slides and found that the one on the Jubilee has a slightly larger cutaway, not much but measurable using a vernier caliper.

To satisfy myself that this would make no difference to the idle speed, I chose the best slide with the smallest cutaway (all are standard 3 1/2 size) and put that on the Jubilee. Low and behold I managed to get a lower idle speed (tick over) that I was searching for. It could be coincidence but it achieved the right end result. Perhaps a previous owner had filed the cutaway to enlarge it?

Lastly, in case anyone has this problem in future, I had to replace the float chamber gasket as it broke. I fitted a new one but the chamber leaked and I could not stop it. In desperation I used TWO gaskets, one on top of the other, and that worked. I wonder if the new gaskets are thinner than before? They are supposed to be genuine AMAL gaskets. I checked that the screws did not bottom out before they nip up the gasket and that the float chamber cover is not warped or corroded.

Dennis