Odd running on a closed throttle

[BSA500]

Strange thing happened on the way to work this morning. Bike running crappy at the lower rev range, then the main part of the tank ran out so switched to reserve. And the engine picked up and ran near perfect. A little rough as I have been blindly fiddling with the air screw etc. looks like the main tap is not allowing enough fuel to pass at the lower rev range.
 For weeks I have noticed it takes awhile to tickle the carb thought it was air lock/heat etc.
Might be the answer??

[Greybeard]

Strange thing happened on the way to work this morning. Bike running crappy at the lower rev range, then the main part of the tank ran out so switched to reserve. And the engine picked up and ran near perfect. A little rough as I have been blindly fiddling with the air screw etc. looks like the main tap is not allowing enough fuel to pass at the lower rev range.
 For weeks I have noticed it takes awhile to tickle the carb thought it was air lock/heat etc.
Might be the answer??

Sounds like you may have found the cause. 👍

[mikeb]

Not sure about the starvation theory as that would splutter when adding the gas / up a hill, not when rolling off (tho maybe it would a bit if thereby becoming lean).

Having a bent needle would be a bit unsettling for me. Has it thereby worn the main jet? I think I’d want to strip the carb, fix that and put everything back to the standard as per swarfy and start from there.

So it pops and bangs just on the overrun? As well as the comment it may be running lean, it could also be an air leak into the exhaust (try sealing around the exhaust headers with some hi temp RTV).  FWIW I have fond memories of a ducati with exhaust leaks that did this – it make slowing down thoroughly enjoyable. A feature. As Angus suggests, enjoy it!

[BSA500]

I did put the carb back to more or less standard(straight needle  ) and it ran terrible until I had to switch to reserve and it picked up straight away. The carb bar the bent needle(which ran really well like that) was ultrasonically cleaned last winter with new gaskets and resleeved a while back. This last week the exhaust of course has been leaking just to p**s me off. So for now repair the taps,exhaust and primary is too tight

[BSA500]

Right found out the main tap has a pipe sticking out of it(ie it's a universal reserve tap) so I could see there was fuel in that side but it wasn't coming out because it had dropped below that pipe. I have been running without really filling the tank so this seemed to be happening more often. So we will see if that improves things

[Greybeard]

Right found out the main tap has a pipe sticking out of it(ie it's a universal reserve tap) so I could see there was fuel in that side but it wasn't coming out because it had dropped below that pipe. I have been running without really filling the tank so this seemed to be happening more often. So we will see if that improves things

A tap with a pipe is the main; one without a pipe is the reserve.

[BSA500]

The A7 tank has a lower section for the reserve so doesn't really need a tap with the pipe

[BSA500]

It was just strange that when switched to reserve side it improved alot, and now with the pipe removed the main tap floods the carb nicely. We will see also a little light on cash at the mo so buying bits to faff around is frowned on

[Greybeard]

The A7 tank has a lower section for the reserve so doesn't really need a tap with the pipe

I didn't know that

[BagONails]

Glad you got a result BSA500.

Just a general query, which side of the tank usually has the reserve tap fitted. My tank is the smaller west coast job but I doubt it makes any odds and of course someone could have swapped em at any time but I am totally in the dark currently and it would be good to know this.

Thanks in advance
Ian

[mikeb]

BagONails - i believe the reserve tap should be on the left so you can operate it without taking your hand off the throttle.

when i measured (guestimated) the capacity of the lower left hand section of my A10's tank I calculated i'd get about 2-3 miles if the RHS also had a 'non-pipe' tap. not much of a reserve unless you are very close to home.

[Slymo]

Had exactly the same experience with my Super Rocket. Perfect at all rev ranges and a willing and even idler but not happy tootling along at 15-20mph in roadworks zones. Fussy and uneven. I initially thought slide cutaway (my carb is a 1 1/16" 276 with a 106 needle on position 4 with a 30 idle jet and a 240 main and a 3 1/2 slide) and changed it from a 3 1/2 to a 4 which did cure the low speed issue but made it pink on takeoff after a fast run. Then I checked the 106 needle jet which rather than being .0106" was more like .01075 or .0108 (crudely measured with a digi caliper). Changed it for a new one and returned the slide to 3 1/2. Once I'd richened up the idle screw the low speed issue was solved and no pinking on take off. The fuel economy is substantially better and rather than running a largely black plug with a mushroom electrode it now runs a more generally mushroom plug.

[BSA500]

Yes left side is the lower portion. Not sure why I am having these issues I have run this setup for years with no issues and you would have thought a stand pipe shouldn't really be a problem other giving you more reserve. I am wading through the issues. As I said it had been running really well apart from the rough/poppy/etc on a closed throttle(should have put up with it).
I reset the valve gap on one cylinder and swooped over to a straight needle and removed washer from under the carb feed bit.
Then my problems started put it all back, set valves to 'power, setting 10/12 thou and removed stand pipe. Now starts better and easier to flood, a little spitty on starting.
Carb settings 376 270 main 1065 needle jet, 25 pilot

[BSA500]

Okay its being a bitch again. After working out why the clutch was dragging and slipping(adjuster and spring has fallen out ) I rode it today spitting from pulling away open it up and seems fine. 6 miles later now lumpy reluctant to tick over and will die if the revs drop too low but always restarts easily. Still revs out well but not so happy. Of course has been said when you have a carb problem its usually the mag.

[BSA500]

Was really rough on the way home for the first 5 miles, then settled down and ran fine ticked over etc etc. So stripped the carb pilot jet was covered with black gunk. Then I remember I replaced the pipes when one swelled up and split. Replaced them with ethanol proof ones but over time it's settled in the carb and seems to pop up now and then.
 Full carb clean on going