Author Topic: Mixture Issue  (Read 1043 times)

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Offline Tazmantic

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Mixture Issue
« on: 27 Feb 2021 at 16:28 »
Hi All took bike for a spin today and all seemed fine,only did a couple of miles and it started running rough when I got home the front plug was sooted up and was sparking inside so i put in 2 new plugs did a couple of miles and seemed to be running fine got home and let it tick over a few seconds then stopped it and removed the plugs, rear cylinder fine front cylinder all sooty, didnt have time to investigate anymore, Ive attached a pic

Offline cardan

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Re: Mixture Issue
« Reply #1 on: 27 Feb 2021 at 23:20 »
Was it running on two cylinders? the back plug looks new!

Leon

Offline Tazmantic

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Re: Mixture Issue
« Reply #2 on: 28 Feb 2021 at 06:41 »
Yep seemed to be running fine and yes both plugs were new thatís how they were after a couple of miles

Offline cardan

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Re: Mixture Issue
« Reply #3 on: 28 Feb 2021 at 09:42 »
Presumably, then, it's oil in the front cylinder, as noted previously.

Leon

Offline Tazmantic

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Re: Mixture Issue
« Reply #4 on: 28 Feb 2021 at 11:10 »
Would this indicate then that my little oil issue before has knackered the rings on the front cylinder ? As Iíve just checked the compression with the rear cylinder and itís the same  :| and the soot was dry not oily
« Last Edit: 28 Feb 2021 at 11:29 by Tazmantic »

Offline cardan

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Re: Mixture Issue
« Reply #5 on: 28 Feb 2021 at 20:56 »
Something is obviously amiss. Does the carb/manifold somehow let raw fuel run down into the front cylinder? Any sign of flooding while the engine is running?

Offline Ian

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Re: Mixture Issue
« Reply #6 on: 01 Mar 2021 at 01:51 »
Maybe check that the inlet manifold is sealing properly on the rear cylinder ?

Offline Tazmantic

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Re: Mixture Issue
« Reply #7 on: 01 Mar 2021 at 10:50 »
Discovered the lead to the front cylinder had a high resistance will sort this and try later (fingers crossed)

Offline Tazmantic

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Re: Mixture Issue
« Reply #8 on: 01 Mar 2021 at 17:48 »
Sorting that lead has made a massive difference, didnít get out on it but ticked over fine and seemed to Rev ok, rear cylinder defiantly got hotter than the front just ticking over and if I pulled a plug lead off the rear cylinder defiantly ran better than the front just needs a road test to see what happens as the front cylinder is running richer.




Image orientation corrected, cropped and enlarged - Dave - 02Mar2021
« Last Edit: 01 Mar 2021 at 20:20 by Dave »

Offline eddie

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Re: Mixture Issue
« Reply #9 on: 01 Mar 2021 at 19:12 »
You shouldn't ever pull an HT lead off a running engine (unless you want to pay for a mag rewind!) - the spark will find the next easiest route to earth, and that is usually across the coils! If you need to kill one cylinder, short it to earth.

   Regards,
                   Eddie.

Offline Tazmantic

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Re: Mixture Issue
« Reply #10 on: 01 Mar 2021 at 19:22 »
Thanks for that Eddie that's the trouble when your a newbie to older technology it usually ends up costing you big time lets hope my stupidity hasn't caused any damage. I presume that also then means you should not spin a mag that has no plugs connected or at least a route to earth from the HT lead?

Am I also correct to presume as I'm sure I've read it somewhere that the front and rear cylinders will have different mixtures due to the design and you have to strike a happy medium?

Offline Tazmantic

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Re: Mixture Issue
« Reply #11 on: 02 Mar 2021 at 15:50 »
Well test ride today and still just the same few miles and front plug black and sooty :cry:

So I am now deff looking to fuel as I reset the plug gaps, checked the points etc etc with mag on bench both plugs sparking a treat, refitted mag started first kick seemed to be running a treat but once again rear cylinder noticeably hotter than front one, front plug starting to soot up.

Removed carb and there was defiantly fuel in the inlet manifold (enough to wet finger) so I am presuming this would of course make for a very rich mixture and soot up a plug but why only the front one ? is it because of the natural flow to the front and less likely to be drawn to the rear ?

So the next question is how will fuel be running into the inlet manifold when the carb does not appear to be flooding? if I turn the fuel on and leave it the float chamber shuts off and doesn't flood, I'm coming close to match and insurance claim because I'm at a loss as to what to try next....
« Last Edit: 02 Mar 2021 at 19:22 by Tazmantic »

Offline cardan

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Re: Mixture Issue
« Reply #12 on: 02 Mar 2021 at 20:45 »
Hi Neil,

Are you still running the large carburettor? https://www.douglasmotorcycles.net/index.php?topic=7884 It's air speed over the jet that does the atomisation of the fuel, and since the 25/32" carb has 42% more area than the correct 21/32" model, the air speed at full throttle is roughly 42% less than designed. It can't help.

The big worry is the difference between front and back cylinders. Clearly you need to lean out the mixture somewhat - maybe drop the needle, or fit a smaller main jet - but keep an eye on the rear cylinder as there's no point leaning things out until that seizes. As Ian suggested above, check you don't have a major air leak somewhere.

I'm pretty sure we can keep the insurance man out of this!!

Leon

Offline Tazmantic

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Re: Mixture Issue
« Reply #13 on: 02 Mar 2021 at 21:27 »
Hi Leon,
Thanks for the reply no this is the bike I bought second and it has the correct 21/32 carb it is even the older type with the 4 external holes round the base. Iíve been looking towards a mag fault as Iíve just changed it but it sparks fine and canít see how one cylinder would be ok but not the other and when I found fuel in the intake today now presume it must be that.

When I got it running last year I know the fuelling wasnít quite right but it did run without fowling up and nothing has changed since then only the mag. Iíve checked the compression itís the same on both cylinders and also used compressed air on the cylinders and inlet and thereís no leaks that I can detect.

It is very odd how different the mixture is between the 2 cylinders I can only think the fuel is running into the front cylinder some how and this is not being sucked into the rear cylinder

Offline eddie

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Re: Mixture Issue
« Reply #14 on: 03 Mar 2021 at 06:50 »
It sounds to me like a mag problem. If the mixture starts off reasonably well balanced between the 2 cylinders, it wont suddenly change as the engine warms up. I would check out the mag bearings (especially at the points end) - they have an insulating paper cage around them to prevent the HT current from etching the bearing tracks. It is not unknown for the bearings to become loose and the cage collapse. As the mag body warms up and expands, it allows enough end float on the armature for it to float. Under these conditions, you will still get a good spark but the timing will vary - one cylinder advancing whilst the other becomes retarded - with one cylinder working harder, the other plug will tend to foul up.

  Regards,
                 Eddie.

Offline Hamwic

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Re: Mixture Issue
« Reply #15 on: 03 Mar 2021 at 08:05 »
It might be interesting to see if the inlet valve clearance on the rear cylinder is still good when the engine is hot and the valve is actually seating properly.  Should the valve be sticking open or floating, then compressed gas will be pushed back into the inlet manifold towards the front cylinder. Weak springs generally cause bad running and mysterious effects, as do worn guides. With the back cylinder toasting up, the valve springs may be giving up the fight?

Offline Tazmantic

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Re: Mixture Issue
« Reply #16 on: 03 Mar 2021 at 10:35 »
Thanks guys it just seems everything is going wrong at the same time so getting very frustrating, I need to step back take a breath and evaluate the situation.

I only changed the mag as the one I was using (spoke about in a different post) was actually an anti clockwise one I got to run clockwise and wanted to change it.

The answer might be to check clearances etc, take the mag and or carb off my other bike that seems to run ok and eliminate items bit by bit.

Not having the time I used to to play with them and also a neighbour thatís reported me to the council for excessive motorcycle noise is also taking its toll  :(

Offline Tazmantic

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Re: Mixture Issue
« Reply #17 on: 03 Mar 2021 at 18:58 »
ok tonight's instalment, put the advance cable and HT pickups and leads on the original mag span it up lovely spark from both plugs and then succeeded to drop it on the floor no problem I thought landed on carpet gear end first..... fitted and timed but bike would not start, nothing, zilch.

Checked and yep no spark, so removed mag stripped it down and checked everything and all was ok resistances etc but I found that when I tightened up the points centre screw the primary windings were shorted out, if I didn't do the screw tight so it was just nipped it was fine and sparked Tighten it up and boom no spark (can anything else go wrong)(and it took a long time to find this out) so out of desperation I tapped the screw once tight with a mallet and it seems to be working ok but there is obviously some damage to it somewhere.

Re fitted mag, refitted carb, kicked, started and yep you guessed it not running quite right and rear cylinder once again much hotter than front...... removed plugs and front plug sooted up again removed carb and fuel in inlet manifold.

Was about to give up again for the night but grabbed a spare carb I had, fitted the jet and fload chamber etc from the original carb started her up and it seemed to be running not to bad, left it running and low and behold both cylinders about the same temp left running for a bit then stopped removed plugs and both looked about the same (front maybe a tad blacker but only just)

After a dance around the garden I removed the carb thinking it would be dry in the inlet manifold but there was still fuel In there so I am completely stumped as to why a carb that worked fine last year has decided to just soot up the front plug....

Lets see what tomorrow and maybe a ride out brings.

Offline Hamwic

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Re: Mixture Issue
« Reply #18 on: 04 Mar 2021 at 08:15 »
It sounds like your mag points centre screw is a tad over-long, and is shorting out the internal condenser - which is now damaged. It could be that the side loading in one cylinder firing position via the points is compromising the spark on that cylinder. It would be instructive to view the points with the engine running  in the dark, or well shaded to see if the points are arcing, but I think the mag failure is more likely an internal issue. Carburation faults are often caused by ignition faults and vice versa, so getting the mag sorted might be what's required.

Offline Tazmantic

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Re: Mixture Issue
« Reply #19 on: 04 Mar 2021 at 18:37 »
It only happened after I dropped it so think that did something  :cry:

Put the other mag back on today (the correct rotation one) with the carb I put on last night it still seemed to run fine (both cylinders hot) just need to test run it now.

So its looking like some how the original carb was flooding the front cylinder???? so seems to be possible to have 2 completely different mixtures off one manifold and can only assume neat fuel was running to the front cylinder.

Offline Tazmantic

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Re: Mixture Issue
« Reply #20 on: 09 Mar 2021 at 19:22 »
Finally got out for a run and all seems ok but did notice that at low revs as I try to accelerate under load it did cough a bit, but this bike has a bell mouth on the intake not the bent tube.

So when I installed the tube off my other bike it seemed to accelerate under load from low revs fine no coughs at all (obviously richens up the mixture a bit) so my question is should I be able to get it to run the same by just installing a bigger jet (this bike is running a 70 at the moment, 60 is standard) or is the bent tube required as I know the jet size should be set for full throttle and the slide is a 4/3 and they dont seem to make a smaller one.

 

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