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Post by niz76 on Mar 23, 2015 15:39:25 GMT -5
Hi fellas,
I have yet another jetting question;
I am currently using a Dellorto PHVA 17.5 temporarily on a scoot until the little choke thingy comes in from Treatland for the Dell clone PHBG 21mm that will end up on this scoot- this is for a customer btw. The choke thingy came in today but somehow I didn't realize that although the Dell 17.5 uses the typical Mikuni jets- which I have loads of- this 21 PHBG uses different jets- which I have none of... My question is this: I have tuned the 17.5 carb to use a 140 main jet (free flow filter) so where should I start for jets for the 21mm??
My thought process is this- please guide me if I'm incorrect!- since the 21mm is obviously bigger with more air flow I'll need a bigger jet than what was in the 17.5. Correct? So I'm thinking of purchasing 145, 150, 155, 160 and 165. Think this range of jets will cover me? Please Help! I don't want to buy a poop ton of jets here...
EDIT: I should mention I'll be using the exact same filter of course...
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Post by dude on Mar 23, 2015 16:38:14 GMT -5
On theZuma forums most of the guys have the jet sizes in the signature. Most of the time they are pretty close with different carbs. Not very many running open filters.
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Post by niz76 on Mar 23, 2015 20:14:14 GMT -5
Well I guess scooter carbs are just another mystery of the universe because my solution really leaves me with more questions than answers...
Here's what I did:
I took the 88 Main Jet that was in the Dell 21 and drilled and drilled with no success. I started with a 1/16" drill bit which is 1.43mm wide. I took this to mean this jet would be similar to a 143 Mikuni main jet.. no good so I found more bits and drilled larger and larger and never found success. I would've just given up and ordered the correct main jets at this point but I still had no idea where to start size wise.
Not one to be foiled by small details such as no main jets I did something a bit extreme; I drilled and tapped the jet holder(!) from the Dell 21 to accept the Mikuni 6mm jets which I have a lot of. I threw in a 115- no good too rich- just sputtered and 'cut out'. I jumped down to a 110 and a little better... 105 Boom that's the one!
So- WTH? I had the 17.5 tuned with the 140...
The 21 took a 105... same jets, same filter, same scooter. Hmmm totally not what I was expecting. I assumed since the 21 was bigger it would suck more air so I'd need more fuel right?? Apparantly not!
I guess the moral of the story is that nobody can tell u what jet size is the best for your setup. You gotta tune!!
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Post by humanshield on Mar 23, 2015 20:27:11 GMT -5
Assuming nothing changed but the carb size...and assuming the carburetor was not RESTRICTING the flow before....
Dont forget that when you install a larger carb (venturi) for the same engine / displacement you raise the venturi pressure because you're lowering the velocity which will equate to less suction, which will equate to less fuel especially at lower engine speeds.
Putting a bigger carb as the only change can be tricky. Now, increasing bore size, valve size or anything that will actually increase air flow other than the carb itself (assuming again the carb was not restricting flow) WILL require a larger jet.
The larger carburetor will allow more air to flow into the engine but ONLY if the engine can utilize it.
That could (at least in part) explain why you actually needed a smaller main jet. Because the same amount of air is passing the gate, but at a slightly slower speed and pressure. The larger jet may have been allowing fuel to just drool past it somewhat.
If the carb had been restricting air flow, then you probably could have kept the same main jet.
That fuel really needs to get sucked out of the float bowl with some gusto to atomize quickly. Lower pressure = less dense. Less dense is the same as warmer air. We all know that in the winter we need to richen up the mixture and in the summer we have to reduce the amount of fuel because the air is less dense.
Making any sense?
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Post by dude on Mar 28, 2015 22:18:26 GMT -5
niz76 I have to ask why use a open filter? Its not my old age I always like quite. But a open filter is loud for a everyday rider. Just like being low key. Lol
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Post by niz76 on Mar 28, 2015 23:18:30 GMT -5
niz76 I have to ask why use a open filter? Its not my old age I always like quite. But a open filter is loud for a everyday rider. Just like being low key. Lol Dude, it wasn't my scooter- it was a customers, although I do run an open filter on my scooter too. It's simply a personal choice for my setup... ya know- potatoe, pototoe  |
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