OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters McCulloch Trimmers & Brushcutters Adjusting Carby on a Pro Mac I Super

Hi,

I recently made a post about fuel lines on a Pro Mac I Super.
Got that all sorted, got it running but isnt idling right and stalling when trying to rev it. It's got a walbro carby with high and low idle adjuster screws.

Can anyone help me with the best way to get it going properly?
As mentioned previously I've been given the task to get it going and I heard that the adjustment screws have been played with by someone previous.

Any help appreciated.

You haven't said what model Walbro carburetor it is. If it is a WA, the nominal starting point for tuning is with both jets open one and a quarter turns. From there, set a sensible idle speed with the throttle stop screw. Tune the idle jet by turning it clockwise (leaner) which will make it speed up, then reduce the idle speed setting, then turn the mixture clockwise again and slow down the idle speed with the stop screw. Keep repeating until you find that any more clockwise movement of the mixture screw causes it to slow down instead of speed up. Go just a smidgen richer from there, and you're all set. For the high speed jet, run it flat out and listen to the exhaust. It will be four-stroking, so it will sound rough. Turn the high speed screw clockwise until it speeds up and smooths out. Then turn it anticlockwise just a little until it is on the verge of four stroking. That should be it.

I saw it had WT312 stamped on it.
Anyway thanks for the advice, I might attack it later today.

al7999,

If you follow what grumpy has said you should be ok but if that does not work we have more detailed information in the parts list and manuals area of the forum.

Thanks Bruce, if I have problems I will check out the manuals.

Anthony.

Managed to get the carby tuned, was a real pain, idled good but would stall when increasing throttle. When I got max throttle good it would stall at idle. Anyway did the best I can.

Anothre question for anyone with knowledge on this model which relates to the head. What thickness line does it take and is it a bump head like ryobi's? Looks like a bump head but cannot get it to extend line. Thanks.

The problems you had with the tuning are more or less inevitable; chainsaw carburetors do not give car-like drivability. You can overcome the off-idle stumble by making the idle just slightly richer than would be ideal, and you can usually overcome the stall when you suddenly drop the throttle from full-bore by making the high speed jet just slightly richer than ideal. It's a world of compromises - 2 strokes are like that. On the other hand, who wants to carry around the weight of a 4 stroke chainsaw or trimmer?