GEARBOX JUMPING in 2nd Gear ONLY

[terrylangley]

I have said jumping in the subject as I cant think of a better way to describe it. After a rebuild all was well for about 150 miles. The engine is rebuilt with a gearbox from a donor engine which looked good when assembled and was changing gear nicely. Now in second gear only the engine appears to misfire on both cylinders giving a little jerk but only in second gear. I have conducted several trials on the road and it is definitely only in second gear. I have ruled out an electrical fault as it is only in second gear. When the jerk happens there is no mechanical noise from the engine or gearbox. If I hold second gear it jerks every few seconds which makes 2nd gear almost unusable. I have now stripped the gearbox and clutch and have found nothing seriously worn, damaged or incorrectly assembled. Something is wrong somewhere and I am reluctant to reassemble without finding the cause of the problem. I have been reliably informed that jerking in 3rd gear has been known and the selector/ pins may have been the cause but mine look OK.
Thanks for any help for this problem.

Regards, Terry Langley

[Eero.Korhonen]

What gearbox do you have, Terry?
Br, Eero

[terrylangley]

Hi Eero, the numbers on the top of the kick starter case are: GB55 C 60 L517. My engine is a Golden Arrow and the donor gearbox came from a standard 250cc Arrow.
Thanks, Terry.

[bh.sprint]

Hello Terry,

This is not something I have come across but does the engine vibrate differently in 2nd gear or does the clutch slip when the torque in the lower gear is applied and cause a resonance which affects either the carb or electrical system making the engine misfire.

Thats the only thing I can think of. Is the jumping at all rev ranges in 2nd or just in one range ?

Brian H.

[terrylangley]

Hi Brian. There is no noticeable difference in the engine or gearbox in 2nd gear. The jerking happens at all revs in second gear. I held 2nd gear and accelerated up the rev range, the jerking was there until I changed up to 3rd. A friend who also rides Arrows had a thorough check of all the gears and shafts today and he found nothing wrong either. The only possibility we came up with was perhaps the gearchange cam indent for 2nd gear was not seating correctly. The cause must be something unique to 2nd gear. I have Pazon ignition fitted with a twin spark coil but struggling to see how a misfire could only be in 2nd gear.
Thanks for your interest, Terry.

[Ivor Collins]

I’m wondering if it’s a mechanical feeling to the jerking, like it’s slipping out of gear by a tooth or two before re engaging or it’s more of an engine misfire?
If mechanical, is there any mileage in trying to recreate the load to the back wheel in second gear by applying the brake and running the engine with the back wheel off the ground or manually with a socket somewhere in the drive train?

Regards
Ivor

[terrylangley]

I have now stripped the gearbox. The only wear found is on the 2nd and 3rd layshaft gears T818 & T821. The engaging teeth and dogs have the edges worn away. When assembled on the bench it can be seen that the selector fork was not pushing the gears fully together. As an experiment I put a 1.5mm shim behind the selector fork which pushed the gears further together, reassembled and road tested. The jerking was gone but 1st gear selection was noisy. I will now fit two new gears and replace the slightly worn layshaft bush. Hope to have the new parts fitted by the weekend. After hopefully a good road test I will post another reply.
Regards, Terry.

[terrylangley]

Pleased to report the "jerking" has been fixed. I replaced the two gears mentioned above, the layshaft bush which when compared to a new one was worn and the layshaft selector fork which had some wear as well. Only got about five miles road test today but very pleased with the result. I also dropped the carburettor needle down to the middle position and she revved much more freely and pulled well. Good result
Regards, Terry

[Eero.Korhonen]

Good to hear!
Br, Eero