Phase 3 : Results & Review
Prices may vary depending on the dealer and shipping charges. The MPH shown is inconsistent in this phase due to CVT issues (explained earlier in the phase in detail). I'm simply including it to have all the data from this phase in one spot, but it does not reflect any power differences between carburetors.
Carburetor | Max Speed | Peak RPM | Price |
14mm Stock | 59MPH | 8,500RPM | N/A |
19mm Arreche | 58MPH | 8,500RPM | $130 |
19mm DellOrto PHBG | 59MPH | 8,500RPM | $100 |
21mm Dellorto PHBG Racing Edition | 58MPH | 8,500RPM | $120 |
25mm DellOrto PHBL | 59MPH | 8,500RPM | $150 |
28mm DellOrto PHBH | 57MPH | 8,500RPM | $150 |
I kept all rides as consistent as possible. I rode to the same town each time, maintaining 40MPH cruising most of the way there and back, with short bursts of WOT cruising. I took a very similar course through the town each time, and all rides were at night when traffic is not a factor. I then headed back toward home and got fuel at a station near my house. Any additional mileage (some rides were longer) was after the fill-ups.
Changes in carburetor tuning would likely change these numbers to some degree, but I think they would stay reasonably close. I made sure none of them had bad rich or lean spots anywhere.
You will notice there are two DellOrto 21mm entries on the table below. The first is the original test with the stock connection to carb. The one marked "2" is the follow-up with the larger tubing to connect the airbox to the carb.
Carburetor | Main Jet | Fuel Economy |
14mm Stock | 90 | 56.95MPG |
19mm Arreche | 90 | 38.18MPG |
19mm DellOrto PHBG | 95 | 49.38MPG |
21mm Dellorto PHBG Racing Edition | 98 | 62.86MPG |
21mm Dellorto PHBG Racing Edition 2 | 106 | 62.35MPG |
25mm DellOrto PHBL | 108 | 46.91MPG |
28mm DellOrto PHBH | 125 | 49.80MPG |
I believe the most important numbers here are the wide-open throttle temperatures. 40MPH cruising can be swayed more by other settings. The carbs required different throttle openings to cruise at 40, stock is closer to half throttle while the largest carb is probably 1/8 - 1/4 throttle at the same speed. I spent the most time to ensure that the main jet circuit was correct at WOT. The rest of the tuning was done to ensure there were no obvious rich or lean conditions throughout the throttle and for good throttle response. From the differences I saw when jetting the carbs, a clip position either way could probably sway some results to be quite similar at part throttle cruising at 40MPH.
Ambient temperature didn't seem to make much difference, judging from my tuning sessions that would normally be when it was warmer in the daytime versus what I saw on my cooler rides. I'm sure it makes some difference, but with the close range of temperatures for testing, I don't feel that it had much influence.
Carburetor | Ambient Temp | 40MPH Cruise | WOT |
14mm Stock | 50°F | 275 - 285°F | 325 - 340°F |
19mm Arreche | 45°F | 225 - 240°F | 285 - 300°F |
19mm DellOrto PHBG | 45°F | 235 - 245°F | 280 - 295°F |
21mm Dellorto PHBG Racing Edition | 50°F | 240 - 255°F | 265 - 280°F |
25mm DellOrto PHBL | 55°F | 240 - 255°F | 275 - 290°F |
28mm DellOrto PHBH | 50°F | 240 - 255°F | 270 - 285°F |
Unfortunately I experienced some CVT difficulties along the way that made speed figures pretty much useless for this part of the project. They really aren't necessary IMO however. I can tell you that there was no major gain in wide open throttle performance with any of the carburetors. I'm not saying you couldn't see numbers on a dyno or feel any difference, but nothing to write home about. I wouldn't spend the money on any of these carbs strictly for performance with the modifications done at this point.
I can't graph it too well, but throttle feel and responsiveness did change as the carburetors got larger. Larger carbs felt very good at part throttle, since the increased venturi size allows more flow at lower throttle settings. The stock carb and up through to the 21mm DellOrto felt pretty good throughout the pull of the throttle. The larger 25 and 28mm carburetors did not.
The 28mm carburetor felt very responsive up to perhaps 1/4 throttle and then reasonably responsive to 1/2 throttle or a little less. Up to 1/4 throttle it felt amazing actually. If performance remained linear, it would have turned the scooter into a monster. The big carbs definitely make you feel as if you are riding something much more powerful... until you get closer to WOT and realize most of the work is done at low throttle positions and the last bit of throttle feels almost unnecessary in the worst cases.
One fairly large negative for the largest of the carbs, especially the 28mm, is that it makes low speed riding a bit more difficult. I found that by the time I was trying the 28mm DellOrto, it was hard to maintain constant speeds in town because such slight movements in the 1/8 - 1/4 throttle range affect speed a good bit. It made the ride a bit less relaxing IMO, because I had to pay close attention to moderation of the throttle at low speed or I'd look down and realize I was 5MPH or more above where I wanted to be cruising.
Fuel economy varied quite a bit with the selection of carburetors testing, varying by over 20MPG from best to worst. The Arreche was the clear loser here at nearly 19MPG worse fuel efficiency than the stock carburetor. The 21mm DellOrto PHBG came in first with almost a 6MPG improvement over stock.
Cylinder head temperature drops were definitely the biggest improvements among the data shown. It appears that 19mm and larger carburetors are much better suited to cooling the big air cooled jug. I suspect that higher RPM and wilder setups may show that cooling doesn't level out until a little larger carburetor is used, but for this nearly 100cc engine revving to 8,500RPM at max speed, 19mm seems to be all that is necessary for a dramatic improvement in cylinder temps.
Ease of installation and use is another factor that you should consider. The 19 and 21mm carburetors tested fit quite easily. They fit into the stock intake and the stock airbox will fit over them as well. The stock throttle cable should be able to work without too much effort. The larger carbs of the group required larger intake manifolds to fit them. If you don't already have an aftermarket throttle and/or cable you may need one to operate them properly. Then you need to adapt your stock airbox to them or find another solution. All of those things add up and make the cost of using the biggest carbs much greater. Clearance issues with the frame and seat could be a real problem on some scooters. This is a fairly large-framed scoot and I have a huge advantage of not having body parts in my way. Small frame scoots and those with full fairings will have even more trouble with the larger carbs than what you've seen in my installation details along the way.
I should note that this applies to the carbs tested, but other brands and models of carburetors could be larger or smaller bodies. Don't assume any 21mm will fit easily and any 22mm won't. Research before you buy.
Looking over all of the details, the choice of carburetors seems pretty simple to me. The 21mm DellOrto PHBG showed the best fuel economy, is capable of keeping cylinder temps as low as any of the carbs, has a good throttle feel throughout it's range, ran as well as any of them, and installs easily in place of the stocker. The 19mm version offered many of the same benefits, but fuel economy was a bit less and I feel the 21mm would leave more room for growth with the setup.
I wish I could explain exactly why things are working as they are. For example, why the 21mm seems to be just right and above or below that the fuel economy drops. I'm not completely sure how much is an effect of the DellOrto's design itself and how much is due to the bore size of the carb. I think 190mech is on the right track with thoughts he shared...
It looks like the Mina 90/100 has a carb size practical limit as you have carefully documented here.I suspect it is due to the engine design,small transfer tunnels,short connecting rod and small reed valve area.All those are hard to over come
I also wanted to comment that I apologize for the lack of diversity in carburetors, with almost all being DellOrto. I bought the Arreche 19mm years ago and was reasonably pleased, but after I purchased my first DellOrto carb I kept buying more, so that's what I had around to test.
Here is another thread with some carb selection info just to add more food for thought...
49ccscoot.proboards.com/index.cgi?board=carb&action=display&thread=1366 
