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stumbling/hesitation upon acceleration #742255
07/17/18 7:38 pm
07/17/18 7:38 pm
Joined: Jul 2017
Posts: 40
Hangtown, CA>
B
barncobob Offline OP
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barncobob  Offline OP
BritBike Forum member
B

Joined: Jul 2017
Posts: 40
Hangtown, CA>
new to Nortons, 74 850, mikuni carb..bike starts great and idles good,runs s trong thru about 2500rpm...starts to stumble until about 4000 and then runs strong again to 6000+

not sure if its a timing(paxon) or the carb..

there is a shop in town but it takes weeks to get it back once there..if its the carb what needles,screws do i start fiddling with to see if it runs better,,i can mark the setting of all the screws for a reference point..
thanx in advance

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Re: stumbling/hesitation upon acceleration [Re: barncobob] #742258
07/17/18 8:11 pm
07/17/18 8:11 pm
Joined: Mar 2005
Posts: 10,251
Boston, Massachusetts
J
John Healy Offline
BritBike Forum member
John Healy  Offline
BritBike Forum member
J

Joined: Mar 2005
Posts: 10,251
Boston, Massachusetts
As a rule of thumb if it stumbles at the same throttle opening it is carburation. If it stumbles at the same RPM it is ignition.

If it appears to be happening at the same throttle opening mark the throttle so that you can see which fuels circuit you need to change.
It will only get better if you make your changes to the fuel circuit that is causing the problem.

If you roll the throttle back slowly and it seems to get worse it is too rich.

If you roll the throttle back slowly and it seems to get better it is too lean.

If you have an air cleaner on the carb and you remove it and it seems better it is too rich.
If it gets worse it is too lean.


Re: stumbling/hesitation upon acceleration [Re: barncobob] #742266
07/17/18 9:57 pm
07/17/18 9:57 pm
Joined: Nov 2011
Posts: 201
Nova Scotia, Canada
K
koncretekid Offline
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koncretekid  Offline
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K

Joined: Nov 2011
Posts: 201
Nova Scotia, Canada
"If you roll the throttle back slowly and it seems to get better it is too lean."

Why is this? I always thought that if it stumbles at full throttle and I back off and it picks up, it is too rich?

Tom


Life's uncertain - go fast now!
Re: stumbling/hesitation upon acceleration [Re: barncobob] #742275
07/17/18 11:18 pm
07/17/18 11:18 pm
Joined: Mar 2005
Posts: 10,251
Boston, Massachusetts
J
John Healy Offline
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John Healy  Offline
BritBike Forum member
J

Joined: Mar 2005
Posts: 10,251
Boston, Massachusetts
When you roll the throttle back you cut back on the amount of air entering the venturi.

The fuel entering the venturi will not respond as quickly and for a brief time you will have richened the mixture.

You will find references to the "Roll Off" tuning on many web sites. I grabbed this one just to show you I am not making this up.
The following "Roll-Off Method" is an easy method to check the jetting of the main circuit:

With the engine fully warmed up, cruise at 3500 RPM in third or fourth (allows a little more time to evaluate the test response). Apply full throttle and allow engine to reach 5000 RPM (time enough to settle-in and for accelerator squirt to dissipate). Quickly Roll-Off the throttle to a 80% throttle open position. This will momentarily richen the fuel mixture. If the engine responds by gaining power and accelerating then this indicates a lean jetting configuration. If the engine goes flat, stutters or runs rough then the mixture is rich. Remember that the fuel delivery on the main circuit is the summation of fuel supplied by the main jet, the emulsion tube and the main air correction jet. The Roll-Off test provides an indication of jetting during a particular region of operation.

Quote
The Roll-Off test provides an indication of jetting during a particular region of operation.


This is the most important thing to come to grips with tuning any variable venturi carburetor. The effect reflects ONLY the fuel circuit the carburetor is using at the time of the test. Basically there are 4 combinations of fuel delivery circuits and all must be tuned separately. Idle, 0-1/4 throttle, 1/4 to 3/4 throttle and 3/4 to Full throttle.
Idle = pilot air screw and pilot jet; 0-1/4 tapering off pilot circuit, and on to the slide cutaway and needle jet size; 1/4 to 3/4 tapering off needle jet and slide cutaway, and then onto needle taper position; 3/4 to Full tapering off the tapered portion of the needle and on to the main jet.



Moderated by  Dave Comeau 


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