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@brentscheffler avatar
UTC

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'78, '80 P125X, '80 100 Sport, '74 150 Super
Joined: UTC
Posts: 717
Location: Seattle
 
Addicted
@brentscheffler avatar
'78, '80 P125X, '80 100 Sport, '74 150 Super
Joined: UTC
Posts: 717
Location: Seattle
UTC quote
My 1980 P125X runs like a god damn dream, with one exception. It starts on second kick when cold (full choke), idles perfectly, accelerates with lots of torque through 3rd just fine. But when at WOT for extended periods (30/40 seconds) in 3rd or 4th, and you let off the throttle completely, and then give it up to 1/4 throttle, it bogs HARD - to the point where it wants to die. If you go past the 1/4 throttle and into the main jet stack, it's fine. This only happens in 3rd (some of the time) and 4th ALL of the time. I've cleaned and rebuilt the carb multiple times. I put in a new stock idle jet. I've adjusted and played with timing. I drilled out larger holes in the gas cap. I've mucked and remucked with air/fuel mix screw. I installed new points and condenser and HT coil. I've replaced the plug and tried both B8HS and B7HS. I'm about at my wits end.

Here's what I have:

- 1980 P125X (newly rebuilt engine in January. Replaced seals, cruciform, piston, and rings)
- Stock top end with new piston and rings
- Sito+
- 20/20 Si carb
- #102 main jet, the carb had a #96 main when I got it. I've tried a #98 and #100 with the same results.
- New points
- New condenser
- New HT coil

Since this happens in the bottom 1/4 of the throttle, it HAS to be related to the slow running jet, right? Like it's not letting enough fuel through? Should I try upping the slow running jet to a larger size that allows more fuel through?

Or could it be something else entirely?

Any and all input much appreciated.
@jamesjohn avatar
UTC

Ossessionato
1970 Vespa Rally 180
Joined: UTC
Posts: 4145
Location: Denton Tx.
 
Ossessionato
@jamesjohn avatar
1970 Vespa Rally 180
Joined: UTC
Posts: 4145
Location: Denton Tx.
UTC quote
sounds like the exhaust is clogged.
OP
@brentscheffler avatar
UTC

Addicted
'78, '80 P125X, '80 100 Sport, '74 150 Super
Joined: UTC
Posts: 717
Location: Seattle
 
Addicted
@brentscheffler avatar
'78, '80 P125X, '80 100 Sport, '74 150 Super
Joined: UTC
Posts: 717
Location: Seattle
UTC quote
jamesjohn wrote:
sounds like the exhaust is clogged.
I have another Sito+ I can use to test with. Thanks for the input!
@professor_plum avatar
UTC

Addicted
Wrench
Joined: UTC
Posts: 610
Location: Workshop
 
Addicted
@professor_plum avatar
Wrench
Joined: UTC
Posts: 610
Location: Workshop
UTC quote
You mention changing idle jet.. Are you hip to the idle air intake thing? Solid jet vs. air hole jet? Sound like maybe a wacky idle/slow running jet thing to me..

Jet should be solid if carb has air hole(as pictured).. If hole is blocked jet should be xx/xxx type..

iirc you should be running:

50/120 or 50 solid jet
160
BE3
Main jet(yours #'s sound good)


(sorry for big pic, stole from interweb)
External inline image provided by member with no explanatory text
OP
@brentscheffler avatar
UTC

Addicted
'78, '80 P125X, '80 100 Sport, '74 150 Super
Joined: UTC
Posts: 717
Location: Seattle
 
Addicted
@brentscheffler avatar
'78, '80 P125X, '80 100 Sport, '74 150 Super
Joined: UTC
Posts: 717
Location: Seattle
UTC quote
Professor Plum wrote:
You mention changing idle jet.. Are you hip to the idle air intake thing? Solid jet vs. air hole jet? Sound like maybe a wacky idle/slow running jet thing to me..

Jet should be solid if carb has air hole(as pictured).. If hole is blocked jet should be xx/xxx type..

iirc you should be running:

50/120 or 50 solid jet
160
BE3
Main jet(yours #'s sound good)
Yeah, my Dellorto is of the newer variety with a single buck shot bb plugging the air hole. When I replaced the idle jet, it had the air flow hole in it.
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