ggryder
MEMBER
- Joined
- Aug 2010
- Messages
- 205
I like Amal Concentrics, so I didn't want to believe that they could wear out in as little as 5, 10 or even 20k miles. However, I was having problems getting my 20k Mk1s to deliver a decent idle and it seemed that excessive air leakage past the slide could be the problem. The carbs didn't look badly worn and measurement using micrometers need some interpretation. I thought I should be able to measure the leakage by blowing air through a carb, measuring the pressure drop (using a water manometer) and comparing the result to that obtained from a "control" carb. The control carb had the slide glued into the body, so zero leakage, all of the air going thru it passing under the slide.
All the carbs tested had the idle stop adjusted so that the flow area under the slide was the same (8 mm^2, about 1 mm lift) as the control carb. The air flow thru the carbs was the same for every test so the pressure drop across the slide was dependent on how leaky the particular carb was.
I tested a couple of new-to-low mileage carbs (Mikuni VM and Wassell Mk1 clone) and they flowed about 1.3x what the control carb did (that seemed reasonable).
Then I tested four Mk1 Concentrics all of which were used, but my own 20k examples were the only ones I knew the mileage of. They all came out in the 2x to 2.5x region, so there was as much, or more, air flowing around the sides of the slides as was flowing beneath them.
Finally a 50k mile Dellorto delivered 1.8x the control carb flow.
It seems that if you have as much air flowing around the slide as under it you would have to drop the slide all the way down to reduce the airflow to something like that needed for a proper idle. Even with the slide bottomed the carb would still pull fuel from the idle circuit but that's got to cause problems at idle and off-idle. So, I've convinced myself, I need new carbs.
All the carbs tested had the idle stop adjusted so that the flow area under the slide was the same (8 mm^2, about 1 mm lift) as the control carb. The air flow thru the carbs was the same for every test so the pressure drop across the slide was dependent on how leaky the particular carb was.
I tested a couple of new-to-low mileage carbs (Mikuni VM and Wassell Mk1 clone) and they flowed about 1.3x what the control carb did (that seemed reasonable).
Then I tested four Mk1 Concentrics all of which were used, but my own 20k examples were the only ones I knew the mileage of. They all came out in the 2x to 2.5x region, so there was as much, or more, air flowing around the sides of the slides as was flowing beneath them.
Finally a 50k mile Dellorto delivered 1.8x the control carb flow.
It seems that if you have as much air flowing around the slide as under it you would have to drop the slide all the way down to reduce the airflow to something like that needed for a proper idle. Even with the slide bottomed the carb would still pull fuel from the idle circuit but that's got to cause problems at idle and off-idle. So, I've convinced myself, I need new carbs.
