Reed Valve: or not ?

The later Jumbo’s were shipped with a modification, where the reed valve block was removed and replaced with a blanking plate. Unlike the reed block on a Yamaha TY or on the later Beta TR240’s, which is directly between the carb and the inlet manifold on the barrel, the TL350 engine has its reed block between the inlet manifold and the lower crankcase chamber.

On the operating table

Not sure what the reason for the modification was, probably to soften the power, but having seen Woody’s (now Paul Cooks) and Chris Dark’s in action they have significant go from a standing start. My Jumbo has always struggled to pull away strongly and smoothly and hance gained a reputation has being a bit controlled when the power kicks in.

Blanking plate and Reed Valve Block

It took a relatively swift 1.5 hours to fit the reed block, as there’s enough room to fit the block without removing the engine from the frame. The main thing is to place a rag over the crankcase mouths as you don’t want to loose the screws from the block into the cases.

First impressions is that the pick-up and response is better and the transition from idle to main jet is much smoother. That said, I’ll need to change the jetting as the replacement carb I obtained has the ones for the plate, not the block.

Both bikes have a PHBL 28 carb (with you can new at a reasonable price from Eurocarb ) and have a range of needles, jets and tubes.

|Carburettor Part |Later Non-Reed Valve|Reed Valve Model|
|Pilot Jet|50|60|
|Needle|X2 1st stop|SX200 2nd stop|
|Valvo Gas|60|70|
|Emulsion Tube|AS272|AS268|

So that’s the next part of project and also to potentially fit it out ready for a trial; there’s a Wye Valley one in a couple of weeks. Here’s to a better performing Jumbo

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