Intake manifold spinner plate yx140

[4T_Goblin]

I have a spinner plate 28mm id with poor milling. Its the one that comes with the o-ring. the o-ring is awful and goes bad quick. I believe it to be the source of a tiny, yet permanent air leak. I purchased a plate with no o-ring or channel with an id of 26mm to replace it.

My yx140 head has an intake inlet of 27mm, and I'm using a manifold with 28mm, and a pe24fl Nibbi.

If I use the 26mm plate between the 28mm manifold and 27mm inlet, will there be lots of turbulence at the adapter and cause issues with the engine running properly?

I know some folks who use the 26mm adapter on their zs190, which has a 30mm intake inlet.

What are your thoughts on this? Is the size enough to cause trouble or would it be negligible one a motor this small?

Thanks

[Mudflap]

It can indeed cause a problem. The intake tube that was supplied with my Lifan 140 measured 26mm on one end and 24mm on the other. Both the carb and head were 26mm. It would idle fine and run OK at low RPM but crappy at high RPM. It ran fine after I replaced it with one that was 26mm all the way through.

[4T_Goblin]

That is odd that you would receive an intake manifold with different diameter at each end.

How would you describe the poor running at top end?

[Mudflap]

I assume that intake was made for putting a 24mm carb on a 26mm head or vise versa. I would describe the way it ran as ragged, almost like a 2 stroke with a spark plug whisker. That's probably an obscure reference for anyone who isn't an old 2 stroke rider.

[4T_Goblin]

Ragged like running lean and hot at WOT high RPM?
I'm not familiar with 2-strokes like that, the reference is lost on me lol

[Mudflap]

More like you can feel and hear the uneven power pulses, not smooth.

[4T_Goblin]

I installed the 26mm adapter. Its really not that much smaller. It appears to be having an affect as my engine revs rich now. It could be a sign that its stopped the leak. I have yet to test ride.

[4T_Goblin]

I believe that I am experiencing the power pulses you mentioned. I can only get 63mph now instead of the obtainable 66mph with the 28mm adapter and air leak.

With the 26mm adapter, the hanging idle issue appears to be fixed. With a 128 main jet I could reach 60mph (only 8500 rpm in 4th WOT), a 130 got me 63 (~9000rpm), a 132 got me 62, and a 135 got me 63. Spark plugs were lean but not too lean, which is why I jetted up even further.

I haven't tried smaller at 125 or the final larger jet, 140.

The choke adaption does not seem to be causing issue.