Author Topic: Tuning carburettors  (Read 580 times)

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Offline Pallijen15

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Tuning carburettors
« on: 27 Aug 2018 at 11:50 »
Hi, just put carburettors back on my Douglas MkV. Nothing in the manual about how to tune. Do I need to start at high tickover and screw idlers down? Also, advance/retard lever. Slip ring appeared to be jammed at first. Took the points cover off and eventually got the slip ring to move. The part that holds the adjuster which screws into the points housing. This was only screwed in half way so I've screwed in all the way and took up the play on the cable adjuster. The advance/retard only moves about the width of the handlebar. Is this sufficient to get the  full range of the cam ring. Thanks for any help

Offline Hampshirebiker

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Re: Tuning carburettors
« Reply #1 on: 27 Aug 2018 at 13:32 »
With the cable and lever tight the ignition is probably at full advance. This is the right setting for normal running. The slip ring is probably still sticking and not returning to the fully retarded position. From memory the lever should move about 30 degrees.
With the carbs, set the throttle stop screws (the ones inclined upwards) to give a fast idle and adjust the mixture screws (the horizontal ones) to give the fastest idle. Finally back off the stop screws to give a good idling speed. If the mixture screws don't have any affect you still have an obstruction in the jet block.
All from memory as I no longer have the bike. Those more informed please feel free to correct me.

Offline eddie

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Re: Tuning carburettors
« Reply #2 on: 27 Aug 2018 at 14:25 »
Start by screwing the mixture screws in until they bottom out on the seats (don't force them!), then unscrew both of them the same amount (about 1 - 2 turns). Wind the throttle stop screws up until they lift the slides about 1/16". Set the ignition to about 1/2 advance and start the engine. Adjust the throttle stops to get both cylinders pulling equally, then fine tune the mixture screws to get the smoothest running. When happy with the running, stop the engine and adjust the throttle cables so that both slides lift simultaneously. Restart the engine and check that both cylinders pull equally as the throttles are opened. When road testing the bike, you will need to advance the ignition further to get the best performance, but running at full advance will probably result in an erratic and fast tickover.

  Regards,
                Eddie.

Offline Hampshirebiker

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Re: Tuning carburettors
« Reply #3 on: 27 Aug 2018 at 14:25 »
Just going back over your previous posts I'm wondering if the slip ring is completely free. If it's still  retarded the performance would certainly be flat.
P.S. Posted at the same time as Eddie above.
« Last Edit: 27 Aug 2018 at 14:33 by Hampshirebiker »

Offline eddie

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Re: Tuning carburettors
« Reply #4 on: 27 Aug 2018 at 14:48 »
Check out the advance/retard. The fitting on the end of the cable has a shallow head on it that engages with a slot in the side of the cam ring - if the cable is over-long, when the cam ring is removed, the fitting can drop below the cam ring and then cant be lifted again with the cam ring replaced. To check it out, remove the points assembly and the cam ring. In the side of the housing, you will see a 'window' below the cable fitting. Operate the advance/retard lever and watch for the head on the cable fitting passing the window - with it in the middle of the window, refit the cam ring so that the fitting engages with the slot in the O.D. of the cam ring. With the cam ring pushed fully home, refit the points assembly - you should now get full travel of the advance /retard mechanism.

  Regards,
                Eddie.

Offline Pallijen15

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Re: Tuning carburettors
« Reply #5 on: 27 Aug 2018 at 15:00 »
Ok thanks, just been for a 15 mile run. Mixture screws were about 2 turns out, certainly flat on the hills but seems to run ok otherwise. Didn't go above 40mph, no coughs or splutters this time, carbs seem to be clean now.  New petrol tap leaking when opened fully, grrr, seems ok when only pulled out half way. Got back from run, not ticking over as well as when first starting from cold, had to keep hand on throttle. One question, is it better to tune when engine warmed up or doesn't it matter? The cam ring only moves about 1cm when lever operated. Thanks for replies PS. Eddie, removing the contacts(by the screw in centre of points)will this upset the timing?

Offline eddie

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Re: Tuning carburettors
« Reply #6 on: 27 Aug 2018 at 15:51 »
It's always better to warm the engine up before tuning the carbs. If the tickover has deteriorated after the run, the engine is probably running a little rich, so may require the air screws to be opened a little further.
The points assembly mounts on a taper with a keyway, so removing the points wont cause any problems, but when you replace them, be careful that you get the points assembly located on the key before tightening the screw. The key is a bit flimsy as it is only formed from the brass of the points plate.

    Eddie.

Offline Pallijen15

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Re: Tuning carburettors
« Reply #7 on: 27 Aug 2018 at 17:47 »
Ok thanks, the r.h. plug was a bit sooty after the run.

Offline Pallijen15

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Re: Tuning carburettors
« Reply #8 on: 03 Sep 2018 at 15:14 »
Hi, removed the centre bolt holding the points but unable to remove the points assembly. Stuck fast. Put the centre bolt back in and tested the compression on each cylinder and got 45psi on each(Sealey compression tester). Going to try again tomorrow but need to be careful not to damage points. Was thinking of removing points housing if necessary. Any advice please. Will also check slides with a digital caliper to see if any modifications have been made and check throttle needles are in same groove then adjust as have been advised. Warmed engine up to test compression and adjusted carbs a little. Has made a bit of difference but will get a full day in tomorrow

Offline eddie

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Re: Tuning carburettors
« Reply #9 on: 03 Sep 2018 at 15:41 »
 The brass base plate for the points is held in by the centre screw, but is located on a taper. Undo the screw until it can be withdrawn, then put it back without screwing it in, then just wriggle it until the points base plate frees from the taper. Removing the housing will not provide any more access. With the points plate removed, you will be able to get a small screwdriver behind the camring to prise it out.

  Regards,
               Eddie.

Offline Hampshirebiker

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Re: Tuning carburettors
« Reply #10 on: 03 Sep 2018 at 18:12 »
If you're doing it on the bike you will find a crash course in gynaecology quite useful.

Offline Pallijen15

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Re: Tuning carburettors
« Reply #11 on: 04 Sep 2018 at 11:56 »
Hi Eddie and Hamshire biker, removed points and camring. Now the lever can be operated for the camring to advance and retard to the full extent of the aperture of the camring housing. The lever is moving nearly twice as far. The oil hole in the camring was blocked. Should there be a felt strip in the machined out inner part of the cam housing? I had motorcycles about 35 years ago and seem to recall that this was the case. Thanks, will remove slides and measure while I'm waiting for advice

Offline eddie

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Re: Tuning carburettors
« Reply #12 on: 04 Sep 2018 at 12:19 »
Yes, there would originally have been a felt strip in the cam housing, and a small felt slug in the oil hole. What you assumed was a blockage, was probably the old felt slug that was filled with solidified oil.

  Eddie.

Offline Pallijen15

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Re: Tuning carburettors
« Reply #13 on: 04 Sep 2018 at 12:53 »
I've taken off the housing to clean as it was a little dirty inside. Can I put a little grease on the bearing before putting the end housing back on? Also, The camring that is fitted, the aperture which ascertains how far the ring can travel, is 7/16" wide so if this is the correct ring I think the amount of advance/retard that I can achieve will probably not be very much different when I assemble it again. Are camrings a standard size?

Offline Pallijen15

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Re: Tuning carburettors
« Reply #14 on: 04 Sep 2018 at 14:13 »
Lightly greased bearing, replaced housing with oiled felt strip in. Normally has been starting when lever fully retarded. Had to start with lever midway this time. Camring now  moves freely back and forth to full extent of its limit.

 

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