Poor running at middle throttle opening

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Johninreading
Posts: 18
Joined: Sun Feb 17, 2019 9:49 pm

Poor running at middle throttle opening

Post by Johninreading »

GT550A carburation: engine idles and pulls away very smoothly, but rapidly begins to run rough until about 4-5,000 revs, when it pulls smoothly again. Any suggestions (I think there's only float height and needle height settings that I can adjust). Plugs are black around seat and dark brown at the electrode. Any tips?
barney01
Posts: 2919
Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Poor running at middle throttle opening

Post by barney01 »

Hi
Did this suddenly start or have you had the carbs off ?
If it is carburation then the answer can be found in the throttle position rather than engine speed.
Just confirming your petrol is fresh and air filter and carbs are clean. How clean is the petrol tank and do you have an in line petrol filter? Do you have genuine Mikuni jets or a cheap aftermarket ones?
NO SMOKE .......NO POKE
Johninreading
Posts: 18
Joined: Sun Feb 17, 2019 9:49 pm

Re: Poor running at middle throttle opening

Post by Johninreading »

I rebuilt the carbs (the throttle body had a crack in it and parts of the choke were missing, so I rebuilt some 2nd hand carbs (with parts bought online - i.e. not genuine Mukini). The air filters are new, as are the plugs and the petrol. It's been running like this since I rebuilt it.
barney01
Posts: 2919
Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Poor running at middle throttle opening

Post by barney01 »

The first concern is the throttle valve. If the cutaway at the bottom is different (they have different numbers stamped on them) they have a lot to do with the performance of the carb in the zone you are talking about. It works together with the needle jet and the Jet needle.
You need to check the number stamped on the underside of the throttle valve is the same.
The numbers all should read 2.5
I'm afraid working with non Mikuni jets causes many carb performance problems and you might want to consider replacing them.
It may also be simply that the carbs need balancing.
NO SMOKE .......NO POKE
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