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Discussion Starter #1
Hi all,
I have 1750 that idles roughly when removing the air filter and blocking air from each cylinder with my hand 1,3&4 effect the idle but number 2 has no effect at all. Additionally there seems to be little to no suction through the carb on #2.
Number 2 spark plug fouls up some but the number seems to come on a bit better what the revs are increased
I’m at a loss to what may be causing this.
Any ideas on what I should look for next?

Thanks Phil
 

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Black plug would point me to either too rich or weak spark, check leads and plug by swapping, check rotor cap post
If that's not it
Compression test.
If you have one check carb airflow with an air balancer
Check for air leaks carb to manifold and manifold to engine.
Check idle mixture.

If compression is good check to see the carbs are flowing equal a balancer is best for this but listening to the sound with a short length of hose can work. If compression and balance are good check mixture, does screwing in or out the mixture screw alter the revs? if not get some petrol or engine start and carefully apply to the joints at carb and manifold does this change rpm?

If all of the above are good pull the idle and main jets on cyl 2 carb and clean them clean the connecting galleys to ensure the jets are getting fuel
 

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Could be an air leak from the rubber carb mount on #2 cylinder. With the engine running, spray carb cleaner on the mount an see if there is a change in idle speed. It could be loss of compression on #2 - get a compression test kit. Could #2 butterfly be fully closed when the others are not (twisted shaft). If you slightly open the throttle, do you feel vacuum on #2?
 

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Discussion Starter #4
Hi thanks for your replies , I did a compression test and they all came up pretty even.
I’ll check with the petrol / carb cleaner method looking for vacuum leaks in the manifold.
i have installed CA aluminium carb mounts so that eliminates perished rubber mounts.
both butterflies open and close perfectly with no twist in the shaft (removed and reinstalled carbs to check today)

Thanks Phil
 

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Check the valve clearances.
 

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Discussion Starter #7
Hi All,

I checked the valve gaps today:
Inlet
#1 = 0.52 #2 = 0.52 #3 = 0.50 #4 = 0.50
Exhaust
#1 = 0.42 #2 = 0.42 #3 = 0.42 #4 = 0.40

The all seemed within spec
 

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Some thoughts. Try a different HT lead on #2, try moving the idle jet from #2 to another cylinder. Does the idle adjust screw have any effect. How's the dizzy cap, how's the distributor - are you running points or electronic ignition ? What's changed since last time it was running OK?
 

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Exhaust should be larger valve lash (gap) than intake — due to heat on that side...18 thou intake 21 thou exhaust ( as I recall, in inches)
 
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