- Joined
- Aug 3, 2010
- Messages
- 203
My bike has a Joe Hunt magneto and 36 mm Amal MkII carbs, but this could be of interest to folks with closer to standard set-ups. After the winter lay-up I came to start the bike for the first time this year and it would only start and idle on one cylinder (the left). First I checked for sparks: with the magneto ignition the HT leads and plugs are the only duplicated parts everything else is common. I had sparks on both sides with the plugs out, but I swapped the plugs over anyway and eventually made two new HT leads: still no firing on the right.
Switching attention to the carburetors, I took them off to check the pilot jets. Everything was clean and the jets were all the correct size. I switched the carbs left and right, although they don’t fit too well like that, and here I was thrown a bit of red herring because the bike then ran on both cylinders. I figured I must have just dislodged something allowing everything to work as it should, but switching the carbs back to their proper sides caused the right hand side to refuse to fire again. Carbs off again and this time I found some fibers in the pilot jet; the float bowl gasket had started to break up, liberating fibers. Cleaned and re-assembled with a new gasket the problem persisted.
Most of the time, whenever I pulled the plugs out, the right hand one from the non-firing side did not look wet, reinforcing the idea that the pilot fuel supply was restricted, but now I noticed that, despite it apparently not firing, it was actually looking black if not sooty. Carbs off again and this time I noticed (as others on this list have done) that a jet was missing: the cold start jet that is buried deep in the float bowl, the existence of which doesn’t even appear to be acknowledged on the Amal website. Interesting, but I still didn’t figure this as the cause of my problem since I had the cold start circuit “off” at all times: jet or not, nothing should have been getting past the cold start plunger.
Having gotten tired of taking the carbs on and off I finally arranged a low pressure air supply which I hoped would allow me to measure the airflow through the various jets and circuits. With the carb body in a bucket of water I applied air to various intakes and watched bubbles stream out here and there. As a means of measuring air flow it needs a bit more work, but I did discover that the cold start plunger wasn’t very effective at stopping the cold-start fuel flow. I was getting as many bubbles from the cold start circuit at the outlet on the downstream side of the throttle slide as I was from the pilot circuit and with the cold start jet missing it was likely that the right hand cylinder was pulling a lot of fuel from the cold start circuit.
Not having the correct size #50 jet for the cold start circuit I put in a #20 and a cold start plunger with the elastomeric pad glued over its nose rather than inset which was almost guaranteed to provide a good seal. The bike now runs great on both cylinders.
Switching attention to the carburetors, I took them off to check the pilot jets. Everything was clean and the jets were all the correct size. I switched the carbs left and right, although they don’t fit too well like that, and here I was thrown a bit of red herring because the bike then ran on both cylinders. I figured I must have just dislodged something allowing everything to work as it should, but switching the carbs back to their proper sides caused the right hand side to refuse to fire again. Carbs off again and this time I found some fibers in the pilot jet; the float bowl gasket had started to break up, liberating fibers. Cleaned and re-assembled with a new gasket the problem persisted.
Most of the time, whenever I pulled the plugs out, the right hand one from the non-firing side did not look wet, reinforcing the idea that the pilot fuel supply was restricted, but now I noticed that, despite it apparently not firing, it was actually looking black if not sooty. Carbs off again and this time I noticed (as others on this list have done) that a jet was missing: the cold start jet that is buried deep in the float bowl, the existence of which doesn’t even appear to be acknowledged on the Amal website. Interesting, but I still didn’t figure this as the cause of my problem since I had the cold start circuit “off” at all times: jet or not, nothing should have been getting past the cold start plunger.
Having gotten tired of taking the carbs on and off I finally arranged a low pressure air supply which I hoped would allow me to measure the airflow through the various jets and circuits. With the carb body in a bucket of water I applied air to various intakes and watched bubbles stream out here and there. As a means of measuring air flow it needs a bit more work, but I did discover that the cold start plunger wasn’t very effective at stopping the cold-start fuel flow. I was getting as many bubbles from the cold start circuit at the outlet on the downstream side of the throttle slide as I was from the pilot circuit and with the cold start jet missing it was likely that the right hand cylinder was pulling a lot of fuel from the cold start circuit.
Not having the correct size #50 jet for the cold start circuit I put in a #20 and a cold start plunger with the elastomeric pad glued over its nose rather than inset which was almost guaranteed to provide a good seal. The bike now runs great on both cylinders.
