Mikuni Carb jerkiness

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MartinT

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Sep 28, 2021
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I`ve recently fitted a mikuni 34mm carb to my 73, 850, i find when i ease off on the throttle the bike will jerk quite badly. The carb has been jetted to suit my bike, with a 40 pilot and a 240 main jet, i`m not expecting modern bike smoothness but this seems to be a lot more than usual, Anyone else had a similar experience ? did anyone find a solution? Is there something i should have done ?
Thanks in advance.
 
MartinT said:
I`ve recently fitted a mikuni 34mm carb to my 73, 850, i find when i ease off on the throttle the bike will jerk quite badly. The carb has been jetted to suit my bike, with a 40 pilot and a 240 main jet, i`m not expecting modern bike smoothness but this seems to be a lot more than usual, Anyone else had a similar experience ? did anyone find a solution? Is there something i should have done ?
Thanks in advance.
Click to expand...
Have you tried adjusting the needle height?
Could you be more specific
IE if you hold the throttle in any one position does it give the same jerkiness
Marking the twistgrip can help greatly here
Then you will know what circuit of the carb will need attention
 
baz said:
Have you tried adjusting the needle height?
Could you be more specific
IE if you hold the throttle in any one position does it give the same jerkiness
Marking the twistgrip can help greatly here
Then you will know what circuit of the carb will need attention
Click to expand...
Could a jerky feeling be a studder ?
If seen similar issues motor is slightly bogging and then when the throttle is backed off you get a slight studder effect ...(from being to rich)
 
If you bought this set up from Sudco (great people) they tend to recommend larger jets then needed. Check other posts for member recommendation.
 
MartinT said:
I`ve recently fitted a mikuni 34mm carb to my 73, 850, i find when i ease off on the throttle the bike will jerk quite badly. The carb has been jetted to suit my bike, with a 40 pilot and a 240 main jet, i`m not expecting modern bike smoothness but this seems to be a lot more than usual, Anyone else had a similar experience ? did anyone find a solution? Is there something i should have done ?
Thanks in advance.
Click to expand...
The problem you describe is not when the primary jet in use is the pilot jet or the main jet!

You need to identify needle, needle location and needle jet. Analyse the throttle opening affected (as mentioned).

First action: confirm there is no leak at the manifold and adapter you installed, have you been back to tighten the clips?, could there be a crack?

However, that would cause weakness, often identified by popping and banging as you close the throttle, and the symptom you describe 'sounds' more like being rich, check plug for black deposits, then lower the needle!

The best a carb supplier can do is supply a base starting point, and yes, they will likely err on the side of rich for safety.

Now it's up to you to dial it in for the specific installation.
 
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mean gene said:
If you bought this set up from Sudco (great people) they tend to recommend larger jets then needed. Check other posts for member recommendation.
Click to expand...
Gene, as a diversionary tactic, why do you guys use the word 'then' in place of 'than' ?

Sudco have a good reputation, but they do not change the Mikuni Air Jet! If Sudco did supply it that will be a 2.0, where a jet around 1.0 will be nearer the requirement, but I don't think that is the symptom he is describing, so yes, most likely rich on the needle.
 
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SteveA said:
Gene, as a diversionary tactic, why do you guys use the word 'then' in place of 'than' ?
Click to expand...
You mean people in the USA use "then" where "than" should be used? They don't on the West coast where I am. Could be a PA thing or a spell checker error. ;)
 
I'm not sure what jerkiness is supposed to mean. Misfiring, or repeated hesitation off throttle could be a lack of fuel, bad fuel, or a bad plug. Backfiring out the exhaust could be excess fuel off throttle. A really loose rear chain will act kind of jerky off throttle. Troubleshooting in a message box is a lot of guess work.

Most stock Nortons run with the same jetting in a single Mikuni setup. That is what I have to assume though, because it is true for dual Mikuni carburetion. I have never used a single Mikuni on a Norton. Try the search function as mentioned. Jetting has probably been listed many times.
 
There is a simple way to get the needles and needle jets correct. You simply lower the needles until you get the miss when you ride the bike, then you raise them one notch. If you cannot cause the miss, you need smaller needle jets. But you cannot use this technique for tuning a two-stroke - you might cough-out a crank.
 
SteveA said:
Gene, as a diversionary tactic, why do you guys use the word 'then' in place of 'than' ?

Sudco have a good reputation, but they do not change the Mikuni Air Jet! If Sudco did supply it that will be a 2.0, where a jet around 10 will be nearer the requirement, but I don't think that is the symptom he is describing, so yeas, most likely rich on the needle.
Click to expand...
Air jet 1.0 or 1.1 .
 
Good practice before installing a new carb(s) is to check cylinder head torque, valve clearance, compression and timing, the timing can be done after the engine is running.

If the engine is "sick" new carb(s) will only make matters worse.

A 40 idle jet is fine as long as it is clear. If alcohol infused fuel has been used it can take as little as 6 to 8 weeks for it to constrict/obstruct the idle jet; they are not expensive and, if new, will eliminate any question concerning fuel flow through the idle circuit. Keep in mind that as you open the throttle every stage of carburation is "all in".

The rubber adapter that mounts the Mikuni to the intake manifold is pron to cracking due to no rear support which adds stress to the adapter..

What are numbers/letters on all the calibrated parts?

Best.
 
To my mind the pilot jet in a Mikuni is there for two-stroke applications. It supplies a squrt of fuel when you shut the throttle. That is when a lot of seizures occur with two-stroke motors - usually as you approach a corner going really fast and you shut-off. If the pilot jet is too big, too rich might be your problem. The early Amal carbs fid not even have pilot jets, and were used on the first Japanede two-strokes.
 
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MartinT said:
I`ve recently fitted a mikuni 34mm carb to my 73, 850, i find when i ease off on the throttle the bike will jerk quite badly. The carb has been jetted to suit my bike, with a 40 pilot and a 240 main jet, i`m not expecting modern bike smoothness but this seems to be a lot more than usual, Anyone else had a similar experience ? did anyone find a solution? Is there something i should have done ?
Thanks in advance.
Click to expand...
I interpret your description of symptom as a lurch forward as you close the throttle.

First, tell us more about YOUR skillset.
Was the bike running great on the previous carburetor setup BEFORE you installed the single Mickey?
 
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