28mm DellOrto PHBHNow comes the big boy, the 28mm DellOrto PHBH. You can see below that it's quite a bit larger than the stocker, double the size if you go by bore diamter since the stock carb is 14mm. Much more if you go by area. Stock would be roughly 194mm², while the 28mm is about 616mm². Just over three times the area.
Stock.
28mm.
Connections.
The PHBH has the same type of bowl and main jet setup that the 25mm PHBL has. The main jet comes out with the plug in the bowl, and that plug is all that secures the bowl.
Inside of the bowl.
Starter jet, pilot jet, main jet, atomizer, needle, throttle valve. This carb uses the 6mm main and 5mm pilot jet.
The stock specs of this carb are listed below. It's mostly good to go on my setup, but the main jet is way off for what I expected and needed.
Stock SpecsMain Jet : 105
Pilot Jet : 55
Starter Jet : 70
Atomizer : A4264
Needle : X2
Throttle Valve : 40
This one uses the same enricher as the 25mm PHBL, with a manual operated lever.
This big carb requires a much larger intake than stock, and is too large for the Polini intake I used on the 25mm. I chose a Hoca intake (PN 169-124 from
Parts For Scooters ) that could handle the task and came in at only $35.
Here's the difference in the stock intake vs the Hoca.
It's got a very large ID throughout.
The Hoca is a bit longer than the stocker as well.
It includes 2 ports that can be used for a vacuum source or for plumbing in oil injection.
You may have noticed in the initial shot that this intake came with reeds. It's available with fiberglass (as shown) or carbon fiber reeds. I chose not to use them. They don't get great reviews. I have used them long ago, but after using brands like Malossi and Polini I found them to be inferior quality. Here's a look at what it comes with anyway. Still hard to beat for the price if you go with a big carb.
I tried using the carb along with the intake just as it came, but clearance problems with the frame wouldn't allow that.
While this may not look too bad, there are grooves inside of the boot and if it doesn't sit straight it's not in the grooves and will come off pretty easily.
Luckily, I had an adapter from some past setup around that's a bit longer and allowed for a little more flex.
Clearances are still close, but it works at least.
The next issue is that the tubing I use on my modified airbox is the same diameter as this carb's inlet. It's a good match, but won't fit over it.
I used a piece of a rubber pipe coupler and a couple of hose clamps to connect the two.
By this stage, you'll almost certainly need a new throttle assembly and cable setup. This carburetor has the longest travel of the bunch and needs more cable length than the rest. Here's a look at the stock setup beside this one to give you an idea.
With everything else taken care of, it fit's OK. Not great, again all the clearances are tight, but not too bad. I expect this carb would be less than fun to fit into a scooter with all of it's fairings or a small frame though.
Well, until I put the seat on that is. When I went to install the seat, it was pushing down hard on the carb and cable. I added a bunch of washers below each of the front mounts to the seat and it's enough to make it work. Again, not ideal, but it works. If you have fairing, this may not work out so well, since often the fairings tie in with the seat.
With everything finally fitting well enough, I started tuning. I used a 125 main jet, 55 pilot, and set the needle in the richest position.
I went for a 33 mile ride and recorded 49.8MPG. Not bad for such a big carb. WOT cruising speeds remained the same again, 48-52MPH. Max speed was 57MPH at roughly 8,500RPM. It felt as strong as any other carburetor tested, but the CVT is still being inconsistent. Cylinder head temperatures also stayed pretty close to what most of the other larger carburetors have shown. On a 50 degree night, cruising at 40MPH produced 240 - 255°F CHT. WOT cruising showed 270 - 285°F.
The biggest difference with this 28mm carburetor was the responsiveness at low throttle positions. It's very sensitive to slight changes and it actually makes it a bit of a chore to try and ride in town at a steady speed. Then it feels like there's little change after 1/2 throttle. You still need to get to full throttle for max power, but it's pretty unresponsive in the last half of the throttle. Overall it's not a bad carb, but I dislike the feeling the throttle has and the mounting problems. Can't say I'd recommend it unless you have a setup that really needs such a large carb.
