M74 Question

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Johnno
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M74 Question

#1

Post by Johnno »

I have a M74 on a 53FL & according to all info on how to tune low speed mixture screw should be 5 turns out as a starting point & adjust from there. To get it to idle properly I have it just over 2 turns out. Is this normal? Is it getting fuel through another port/casting fault, should I dismantle & blow out all the ports? I have bubble tested manifold & set float level 1/4". High speed mixture 1 1/4 turns. Points timing etc spot on. Bike runs well on road & will accelerate away in 4th gear from a low speed with no hesitation or miss. Just seems odd to have L/S screw so far in (anywhere near 5 creates black smoke & very rough idle). Thanks in advance. Regards Johnno
Cotten
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Re: M74 Question

#2

Post by Cotten »

Johnno!

I send all M74 and later B carbs out at 3½ turns on the lowspeed, and 3/4 of a turn on the high.
The manual's suggestion of five to start was probably to accomodate vacuum leaks.

Does your M74 have a stepped highspeed needle like the one in the middle?
STEPTIP.jpg
.....Cotten
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Johnno
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Re: M74 Question

#3

Post by Johnno »

Hi Cotton
Mine just has a taper to a point, no step. Cheers Johnno
Cotten
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Re: M74 Question

#4

Post by Cotten »

Johnno!

I haven't one handy to inspect, but I seem to think the seat orifice was smaller for the stepped needle. There may have been a mid-production change, but certainly the wider tip should still adjust.

But it also may account for a wider setting.
The HS needles are notoriously insensitive, so if you get any tuning responses while slowly adjusting at a constant highway speed, you are ahead of the game!

(Please note that the LS reverses itself at highway speeds, to meter air instead of fuel, so some back-and-forth adjustments between the two may be necessary.)

....Cotten
Johnno
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Posts: 61
Joined: Wed Jul 07, 2010 11:48 am
Bikes: 1924 F
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Location: Australia
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Re: M74 Question

#5

Post by Johnno »

Thanks Cotton. If it ever stops raining I will fine tune. Cheers Johnno
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