OP
@az_slynch avatar
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Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '74 VBC1, '64 V90 and 3 Ciaos
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@az_slynch avatar
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UTC quote
I've recently wrenched on a GT200 for a friend, which was built from an assortment of damaged and/or incomplete GT200s. It's running well enough, but I'm still a bit bothered by the performance of it.

When I rebuilt the Keihin carburetor, I noticed the foam "filter" that fits into the right side of the carburetor (as-installed in the scooter) was pretty desiccated. A second carburetor had the cap, but no foam at all. When we put the bike together, I re-used the foam.

The bike was very boggy and lethargic. I suspected it was because the foam "filter" was pretty worn and that the bike was losing too much vacuum on the diaphragm and not lifting the throttle piston sufficiently. I made a replacement insert out of a new-spare moped filter that was a bit more open-cell than the original filter foam, but used a larger piece in order compress it a bit it increase the filter density. It seems to work, though the bike still seems a touch boggy when revving up.

Is there a recommended part for this? Keihin doesn't mention it anywhere and to be honest, this is the first CV carb I've come across where it deals with air that isn't in a vacuum line or manifold. I've even looked at OKO carbs for this and struck out thus far.

Any guidance is appreciated.
@greasy125 avatar
UTC

Sergeant at Arms
Weird 80's Vespas & Cool Vintage Lambrettas
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@greasy125 avatar
Weird 80's Vespas & Cool Vintage Lambrettas
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UTC quote
did you have the diaphragm cover off to inspect the diaphragm? my guess is that it is dirty or stiff or possibly slightly torn. i'd start there and inspect the slide and needle while you've got it all apart.

best,
-greasy
OP
@az_slynch avatar
UTC

Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '74 VBC1, '64 V90 and 3 Ciaos
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@az_slynch avatar
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UTC quote
Greasy,

Thank you for the follow-up. The carburetor was fully dismantled and cleaned nine ways to Sunday. I even had my buddy buy the D-tool from ScooterWest for adjusting the mix screw so I could figure out how many turns out it was, then pull it out to clean those areas. The throttle piston was de-shellac-ed and carefully polished. Same for the fuel needle. The vacuum diaphragm was cleaned and inspected for pinholes and tears and treated with a bit of Gummi Pflege prior to re-installation. The diaphragm seat fit easily into the groove in the carb body with no distortion.

The only alteration I made to the carb was to slightly re-shape the actuator arm for the accelerator pump. I adjusted it so that it wouldn't hit the accelerator pump plunger until just after the throttle butterfly was cracked and allowed the pump to plunge a full stroke just prior to the butterfly opening to WOT.
@sc_frontier avatar
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UTC quote
Here is the sketchy way to tune it. Take off the seat, pet carrier, intake/air-filter. During boggy acceleration place your hand over the opening of the carburetor for a split second. If it takes off then you need more gas. If it gets worse you need less gas. Set idle mixture to where the idle is fastest at a very low rpm/throttle stop setting, then set throttle stop to correct rpm.

Also, I think the accelerator pump needs to be back where it was, but I am not 100%.
@modnrod avatar
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Addicted
Lambretta child
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@modnrod avatar
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UTC quote
The accel pump needs to squirt fuel as soon as the throttle plate moves, because that's when the greatest lean change comes in (a sudden rush of air as the throttle is cracked).

CVKs are very sensitive to vacuum differences. Quite often just drilling two small 1/2" holes in a standard airbox will net a needle adjustment to compensate, so using a foam pod filter will throw it right out the window. The best way to "re-jet" is to trial fit an air compensator jet fitted into the main air feed, this then will allow the ratios to follow the throttle lift curve the same as a stock airbox. Sometimes you also need to vary the compensating vacuum hole in the plastic slide to make the slide lift at the correct rate, bigger is OK, but making it smaller is difficult! If you are using a pod filter though on a vacuum carb, then just oiling the filter will be enough to throw the jetting out again. Very frustrating. VERY. Laughing emoticon

Or you could just buy a flat slide. EASY tuning, 10% more power, INSTANT throttle response, and when cruising the same economy. Get one 10% smaller than your vacuum slide, it will work better in the real world than a big one (eg 34mm CVK, get a 30mm or 32mm flat slide). Run any filter you want.

OKO's are great, excellent casting density (better than genuine Keihin).
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Grumpy Biker
1980 Vespa P200e (sold), 2002 Vespa ET4 (sold), 1949 Harley-Davidson FL
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@caschnd1 avatar
1980 Vespa P200e (sold), 2002 Vespa ET4 (sold), 1949 Harley-Davidson FL
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Location: Sparks, Nevada, USA
UTC quote
az_slynch wrote:
Greasy,
...

The only alteration I made to the carb was to slightly re-shape the actuator arm for the accelerator pump. I adjusted it so that it wouldn't hit the accelerator pump plunger until just after the throttle butterfly was cracked and allowed the pump to plunge a full stroke just prior to the butterfly opening to WOT.
I think this might be your problem. You need that squirt of fuel right off the bat or you will get a lean "bog" which it sounds like your problem. The squirt of fuel from the fuel pump is much more important in the low to mid range than in the WOT position.

-Craig
@greasy125 avatar
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Sergeant at Arms
Weird 80's Vespas & Cool Vintage Lambrettas
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@greasy125 avatar
Weird 80's Vespas & Cool Vintage Lambrettas
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Location: The state of insanity, SoCal
UTC quote
sc_frontier wrote:
Here is the sketchy way to tune it. Take off the seat, pet carrier, intake/air-filter. During boggy acceleration place your hand over the opening of the carburetor for a split second. If it takes off then you need more gas. If it gets worse you need less gas. Set idle mixture to where the idle is fastest at a very low rpm/throttle stop setting, then set throttle stop to correct rpm.
besides this being extremely dangerous, it's also flat out wrong.

because the CV is vacuum operated carb it operates on a tuned intake tract. removing the intake/air filter ass'y will not only make it nearly unable to run; any tuning you do would be instantly negated once you reattach the components.

-g
OP
@az_slynch avatar
UTC

Molto Verboso
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '74 VBC1, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1408
Location: Tucson, AZ
 
Molto Verboso
@az_slynch avatar
'07 GTS250, '07 LX150, '81 P200E, '78 P200E, '74 VBC1, '64 V90 and 3 Ciaos
Joined: UTC
Posts: 1408
Location: Tucson, AZ
UTC quote
Thank you for the feedback; I'll tweak the arm to open it a touch earlier. It currently starts actuating the pump as soon as the ports for the pilot circuit are exposed by the butterfly.

I had two carbs between the three bikes and both accelerator pump actuator arms were bent differently. One came on at 1/3 throttle and actuated the pump fully. The other actually partially depressed the accelerator pump with the throttle closed and only did about a half-stroke on the pump.

I need to find a non-parts bike with a non-molested Keihin to see how it was set up from the factory. Until then, I'll tweak the actuator arm to actuate the pump a bit sooner.
@sc_frontier avatar
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Location: Texoma area
 
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Location: Texoma area
UTC quote
greasy125 wrote:
sc_frontier wrote:
Here is the sketchy way to tune it. Take off the seat, pet carrier, intake/air-filter. During boggy acceleration place your hand over the opening of the carburetor for a split second. If it takes off then you need more gas. If it gets worse you need less gas. Set idle mixture to where the idle is fastest at a very low rpm/throttle stop setting, then set throttle stop to correct rpm.
besides this being extremely dangerous, it's also flat out wrong.

because the CV is vacuum operated carb it operates on a tuned intake tract. removing the intake/air filter ass'y will not only make it nearly unable to run; any tuning you do would be instantly negated once you reattach the components.

-g
Yes it is dangerous, but i have tuned 20 plus cv carbs this way (not on public roads) in order to get a baseline tune; never a disappointed customer. Maybe some day I can get a dyno.
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