Author Topic: Dragonfly carburation  (Read 346 times)

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Offline Bynorthsea

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Dragonfly carburation
« on: 28 Dec 2018 at 13:36 »
Just recently got the bike running and now trying to set up the tickover. The bike starts well but the pilot jet adjuster will not lean off the mixture, can be screwed all the way in with no effect. Initially I thought it must be float level and it was too high, combination of new viton needle and washer under the valve body, it is now smack in the centre of the float chamber and whilst the tickover increased the pilot will still not lean off the mixture (the pilot jet is screwed home)
I am now thinking whether the slide cutaway has been increased and it's essentially running on the needle jet.
So some questions you may be able to help with. Can the pilot jet arrangement be butchered in a way that renders it inoperative, any way of testing it? Has any one got a 31/2 slide that you can measure the cut away so I can compare it with what I have? OR are there any more issues that I could address? Thanks.

Offline eddie

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Re: Dragonfly carburation
« Reply #1 on: 28 Dec 2018 at 16:39 »
If the slow running mixture wont 'lean off' as the air screw is wound out, it would suggest that the pilot air drilling in the main body is blocked. If this is the case, try completely unscrewing the air screw. As the screw comes out, it will provide a 2nd supply of air to the jet, leaning off the mixture, causing the engine speed to momentarily pick up, and then die as the mixture becomes excessively weak. If you get these symptoms, you will need to check that the pilot air drillings are clear of any obstructions - use small drills carefully to remove any blockage - do not rely on just poking out any debris with a pin - if the blockage is caused by the sticky goo left behind by modern fuel, the pin will push it out and probably draw it back in as you remove the pin! Another thing to check is that you have the correct hand choke block fitted - although they are rare, Monoblocs were available as LH units - in which case the drillings are reversed in the choke block.
  You say you have fitted a new Viton needle and washer under the valve body - there should NOT be a washer under the valve body - it screws down direct onto the top of the float chamber! Because of the float geometry, the washer will raise the fuel level considerably. Also, the slow running adjustment is an air screw, so winding it in will richen the mixture - the standard way of getting an initial setting is to wind the screw fully in, then back it off 1 - 2 turns. One further point - the slide cutaway should not affect the actual slow running - the cutaway should only affect the pickup as you open the throttle.

  Hope some of this helps,
                                         Regards,
                                                         Eddie.

Offline Hampshirebiker

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Re: Dragonfly carburation
« Reply #2 on: 28 Dec 2018 at 17:47 »
Yes an obstruction in one of the jet block drillings would be my guess. Remove the block completely and soak it in thinners for a few days. Finally blow all the drillings through with a high pressure C/A nozzle. It's usually worked for me.

Offline Bynorthsea

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Re: Dragonfly carburation
« Reply #3 on: 29 Dec 2018 at 14:47 »
Thanks guys, fresh from the garage with that clinging petrol smell but not really any further on. Stripped and rebuilt carb checking all the jets and galleries, confident that nothing is blocked, fuel level dead centre of end plate. Starts ok and can adjust a half decent tickover on the throttle stop but it seems lumpy and rich. Air adjusting screw has no impact over full range of adjustment, when removed fully revs did rise momentarily. With petrol off revs rise as fuel level drops. Any further advice would be really appreciated. Thanks

Offline eddie

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Re: Dragonfly carburation
« Reply #4 on: 29 Dec 2018 at 16:12 »
You still have a blockage in one of the air passages if adjusting the air screw has no effect on the engine speed. Look very carefully at the choke block - on the side toward the engine, there is a shallow drilling. In the top of that drilling there is a very small hole going up at an angle into the bore of the choke block - there is also another drilling to one side, and a third drilling (very small) in the centre of it (often overlooked as it appears to be the flat left by the point of the drill). In the carb body there is another very small hole at an angle into the bore of the carb body. A blockage in any of these will render the slow running adjustment ineffective.
  Lastly, you should be able to adjust the slow running to get a steady tickover with the throttle stop wound right down - if not, you have not completely cured the problem.

   Regards,
                  Eddie.

  P.S.  I am assuming that you still have the original Amal 275 Monobloc carb fitted, and not a later Amal Concentric. Since the demise of Amals, Concentrics have been produced in several locations around the world, and they all seem to have their own different problems!!
« Last Edit: 29 Dec 2018 at 16:19 by eddie »

Offline Bynorthsea

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Re: Dragonfly carburation
« Reply #5 on: 31 Dec 2018 at 11:18 »
Back again. Carb is the 375/7 monoblock, standard I think?  Looked closely at choke block, shallow drilling and angle jet into bore is ok, drilling to the side is clear and angled drilling at the choke side is clear, where is the third very small drilling "easily overlooked", can't find it?

 

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