A7 first ride.. (problem at open throttle)

[Rex]

...Wasn't entirely successful. The bike ticks over like a Swiss watch, accelerates cleanly up the rev-range when stationary, but pops and farts when the throttle is opened when actually under way and under load.

The carb is a new Amal premier, the mag looks (no paperwork) to have been recently rebuilt and the timing has been checked and rechecked and there are no manifold air leaks. The mag pick-ups, end-cap, leads, plugs etc are all new, points clean and adjusted.
The carb needle clip is in the bottom position.
Before I start chucking money at additional carb jets, pro mag checks and rebuilds, etc anything obvious I might have missed?
NB I noticed in another thread that someone recommends a 160 or higher main jet.
 My bike is running filter-less and has a 140 fitted, but would a too-small main jet to  affect the "acceleration" in this way, or for the cheap price of a new larger jet would it be worthwhile to eliminate the jet by buying another?

The test run was too short and over urban roads to get up to more than about 30MPH anyway.
Any thoughts?

[muskrat]

G'day Rex.
What model A7 and type of carb?
Yes 140 sounds too small especially when no air filters are fitted.
Cheers

[ellis]

Hi Rex.
First thing is to check the float height is correct as it sounds to be running a bit lean. Secondly you could try a No 19 primary jet as it has probably got a No 17 jet in as standard so could be a little lean.

ELLIS

[a10 gf]

Try to 'strangle' some of the air intake and take a ride, it could (indirectly) be telling something about jets... ?

[Triton Thrasher]

Secondly you could try a No 19 primary jet as it has probably got a No 17 jet in as standard so could be a little lean.

ELLIS

I wouldn’t advise spending money on parts to “improve” the low speed performance that he has no complaints about.

There is a tuning procedure for Amal carburettors.

[Swarfcut]

Rex   Mis firing under load... I would suspect an ignition fault. Starts and runs OK means timing is fine, so do your usual check of substituting plugs, leads, pick ups. That they are new cannot be relied upon that they are up to spec.  Worth checking free movement of all carbon brushes and their conductivity, especially the earth brush at the drive end,  plus the  correct hardness of the brushes.  Check the Slip ring on the maggy,  for signs of contamination, arcing or burning.

 Depending on your contact breaker points, brass backing or later steel, the points return spring touching the cam ring will give erratic running with the steel points  set up. Worth checking the points plate is correctly seated in the maggy shaft taper and the points rotate without wobble.

  Other possible easy checks are  that the pushrods are correctly seated and that the valve gear is set up and working correctly.

 Swarfy.