Modern Vespa

The 22/68 + 36t 4th is still on the table. Question is what would be the max revs with that setup?

Final drive for 22/68 + 35t 4th is 5.16:1 and max revs are 7,600 rpm. Final drive for 21/68 + 35t 4th is 5.41:1 and max revs are 8,000 rpm. Final drive for 22/68 + 36t 4th is 5.28:1 so, at first glance, it would appear max revs would fall in between the first two. Based on the 20t clutch cog experiment, however, it is clear that, at some point, the 8,000 rpm max becomes a function of the engine's limitations, not the gearing. In other words, it is conceivable that a 22/68 + 36t 4th setup could pull 8,000 rpm too.

That would be pretty fast.

Maximum speed is more to do with how the power is tailing off rather than absolute rpm.

I like the comparison between 3rd and 4th gears. For general scooter use, 4th gear should rev out to 500rpm less than 3rd.

Set up like this will make near the highest speed.

I've had that 500 rpm rev drop target in mind all along, but it just isn't happening with my motor. Current situation is max rpms in 3rd are 9,000. I've tried three different clutch cogs and the most I can get out of 4th is 8,000. It is pretty clear my motor simply does not have the cojones to pull more than that against a 70+ mph headwind. Goal now is just to take that 8,000 rpm and make the most of it.

Most likely your main jet is too small. If you're getting 9000rpm and lacks pull, it's likely due to lean jetting. As it still runs, probably not more than a few jets under.

Already getting a hint of splutter in 2nd but revving out in 3rd with AC160/MJ130 combo. Easy enough to try AC140/MJ125 and walk it down from there.

Sometimes a hint of splutter, is actually a hint of stutter. Increase the AC/MJ richness until the wot rpm drops considerably, to be sure it's actually rich. Changing to AC140 or even AC120 would be more appropriate for your kit.

Spent some time playing with jetting yesterday. Exercise confirmed the AC160/MJ130 was not too lean. Going richer only reduced max rpms and max speeds in 4th gear.

Everything points to 8,000 rpm being the max in 4th gear with either a 5.41:1 or 5.68:1 final drive ratio. We also know that max revs with a 5.16:1 go down. The question is where is the tipping point between 5.41:1 and 5.16:1 where max revs stop being a function of the engine and start being a function of the gearing?

Again, the final drive ratio for a 22/68 primary with a 36t 4th is 5.28:1. Can that pull the 8,000 rpm? Likely will be waiting until after the fall riding season ends to find out.

I did a 20, a 21 and a 22 Cog clutch on my PX150. For years I suspected that the scooterhelp gearing chart was wrong, thanks god I trust myself and I prove the chart was indeed wrong. My PX150 us spec 2005' model had a 20 Clutch cog which I suspected for years and I also compained for years. It was unbearable to reach 8000 tucked down and barely 65 mph.

I had a spare 22 which I put and it complety change the game. Unfortuantely the clutch packs were a bit worn but it proved to be a more suited gearing for hwy with absolutely no draws on acceleration, in fact , it accelerated better on the 22 Cog clutch because I was able to rev my cylinder kit. On the 20 Clutch cog I would run out of gear in the middle of the acceleration. The power output was not match to the gearing. Why do I know this? testing, not only with my acceleracion instruments but also because my son would leave me in the dust on his 21 Clutch cog gear when we raced.

Finally , I ordered a 21 Teeth Clutch Cog which is gave me the best of every performance measure. Proper acceleration and proper top speed.

In Summary:
20 Cog terrible all around. Poor top speed, poor acceleration specially with a cylinder kit because you will run out of gear.
21 Best of all combos. best acceleration, power delivery
22 very good acceleration. Almost as good as the 21 cog if you have a decent cylinder kit. Exceptionally good for hwy top speed if you have a good cylinder kit. Too long of a gear if you are riding the stock cylinder.

If I have to do it all over again Like I did I would pick the 21 Cog. Why I did the test? because I have a ton of free time and I am a data nerd, I love testing on my own.

Update.

Removed the 20 tooth clutch cog. It never gave me the rpms I was looking for in 4th and it made things too frenetic in the lower gears. Still was an inexpensive and worthwhile experiment because it taught me my hp vs. aero situation is capping max revs in 4th at about 7,750 rpm regardless of gearing (see below re. calculated rpms). Replaced it with the 22 tooth cog in anticipation of swapping to a 36 tooth 4th gear over the winter.

I believe I am calculating rpms more accurately now that it has come to my attention that not all 3.5x10" tires have the same circumference. I have been relying more on the lofstrom.net gearing calculator, because that lets you input a specific circumference. Based on that, the 22/68 primary is doing about 45 mph at 9,000 rpm in 2nd, about 62 mph at 8,600 rpm in 3rd, and about 72 mph at 7,450 rpm with a 35 tooth 4th. Hope is that a 36 tooth 4th will turn the above-mentioned 7,750 rpm max, which would bring the rev drop under 1,000 and provide a very slight increase in top speed.

Update. Put a 36 tooth 4th in last month. With 22 tooth clutch gear, final drive in 4th is now 5.28:1. Have had it out a couple of times for testing.

Let me start by saying earlier results may have been influenced by some clutch slip. I have a VMC 10 Molle Ergal clutch and was running 10 "soft" springs. Those are supposed to be good to 20 hp. I don't think there is any way I am making more than 17 hp, maybe 18 hp, but I was getting some clutch slip in 4th nonetheless. Changed to 5 "softs" and 5 "hards" (not to be confused with "extra hards"). So far so good.

On the plus side, top speed is up slightly from about 71 to about 72 mph on the flats (gps read higher at times but trying to be conservative). Also carries speed uphill slightly better than before, but still can't carry 70 mph up anything more than a slight grade. Otherwise, with no clutch slip, max revs in 4th are about 7900 rpm, compared to about 9000 rpm in 3rd. So, rev drop is still pretty bad on paper. In real life, though, this only affects top speed in 4th and normal upshifts from 3rd to 4th feel just fine.

With the arrival of warmer weather, main jet is one step too rich, but I don't think that will change anything in 4th gear. Indeed, at this point, I feel like I have squeezed all the juice there is out of a 177 with an SI carb and an LML reed valve. Time to turn my attention to things like tubeless rims, tires with higher speed ratings, and shocks with adjustable rebound damping.