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Larger acceleration pump jets and airbox design
by Anders Dinsen ... November 05, 2008, 09:04:42 am
Progress is still slow due to lack of time, but this time the mail man was quick: Three or four days delivery time from Wales to Copenhagen is not bad at all.

I changed two things in the jetting last weekend: Acceleration pump jets up from 35 to 45 (bleeds still at 40), and main jets down from 150 to 145. Accel pump jets affect the top end of the main system, so with increased size, the main system must be leaned off. Since I thought that the main system was too rich anyway, I reduced that overall instead of just increasing the air bleed (which affect mostly the top end).

If you look carefully on the right side of the diagram, you can see that the acceleration pump jet feeds fuel into the engine during full load: The accel jet simply becomes an additional main jet. (The other two diagrams shows idle and progression phases respectively.)

The effect of the accel pump jet change was very noticable: Throttle response has improved a lot. The larger jet means a larger amount of fuel (about 20% more) being sprayed faster (35% shorter period) into the engine when the throttle is depressed.

On the high end, I still think it's too rich, so I'm going to try a larger air bleed (going from 180 to 190).

Cold starting was quite problematic with spitbacks and hesitation, but I beleive it is due to the bad alignment of the manifold on the engine. This will cause fuel condensed on the walls of the cold manifold to stay in the manifold instead of being drawn into the engine.

I have tried two different airbox designs and is now ready to make the final drawings. The airbox will be welded up in steel. Here's a photo of my last iteration prototype fitted on the carbs:

The relays will be moved away from the current location and the airbox will be shortened a bit on the bottom edge so it will not be rubbing against the cables and pipes coming up behind it. Not visible in the photo is the beautiful 39 mm long ram pipe I test fitted in the box.

The most difficult part of the airbox is getting the flange against the carbs right. I have spent quite a bit of time in front of the computer getting the drawing right.

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