spit and kick back - timing or weak mix?

[hoogerbooger]

I've put a new 626 Concentric carb on my B40F Star. It started very easily and was idling better then a with the old carb, which I also attribute to a now better sealed exhaust.

Have done a couple of short runs on it. I reset the tick over to the lowest stable(well almost) level after the first run. On the second run it cut out a couple of times when I held the clutch in, coasting into a layby/stop. On trying to restart it spat back through the carb and kicked a couple of times before I got it going again. When I got home I reset the idle a little higher. Starting from cold yesterday it still spat back and kicked a few times.

Now it wasn't kicking-back and spitting when I first set up ( might have spat back once when the carb was flooded after a first kick failed)

So it seems to me something has changed....which seems more likely to be an air leak (due to vibration ?) than the timing changing ?

Any views welcome on most likely cause/tweek to consider.

[rhyatt]

Do you have the Tufnell spacer between the new carb and the head?
Aim for a steady reliable tickover , not necessarily slow.
The kicking back as you ease it over tdc is just the points opening as you ease it over.

[hoogerbooger]

Yes, it is mounted onto a tufnell spacer.

Just to check. When I say spit and kick back: there is some partial ignition with a small puff of smoke/or un-ignited fuel from the air filter and the kickstart tries to launch my leg upwards.

(I don't try to kick it near TDC. If I get that sort of resistance I stop and use the valve lifter to get it passed TDC. the try again) 

[Steve.S]

I agree with you. Either timing or mixture.
I think I would check the ignition timing, just to eliminate that. Is the A/R mechanism stuck on full advance?
You can check for air leaks by spraying some oil around the flange with the engine running, similarly with the inlet valve guide with spray grease.
Perhaps you could set your idle mixture a little on the rich side or give the carburettor a bit more of a tickle?
You might want to try a slightly richer slide. Finally, but unlikely, but you may want to try raising the jet needle by one notch. Who knows?
In the meantime, I would only try to start it using the valve lifter to ease it over compression.
Let us know how you get on.

[hoogerbooger]

Have reminded myself of how to check the timing...and just done/reset. I market the points plate before tinkering

As I have a B25 chaincase cover I had marked the rotor previously to 33 1/3 BTDC. I rechecked this a few times and decided the mark was as good as I could get.

Last time I think I used the fag paper method. This time I used a multi-meter between the case and the top point. I was surprised at how much I ended up moving the plate....to where current on the multi-meter cut out.

My head is a bit muddled now, but as you can see in resetting I ended up rotating the plate clockwise. Is that advancing? or retarding?

(AAU is not jammed in full advance. can turn cam and see bob weights move)

[hoogerbooger]

Oops. Ignore last post. (forgot to advance cam!! will try again tmorro)

[Steve.S]

Very easy to get a muddled head. Happens to me all the time.
I wonder if the following might be helpful?
Viewed from the timing side, the engine rotates clockwise.
As far as I can see, the cam rotates anticlockwise.
So, moving the base plate clockwise will advance the ignition.
Moving the plate anticlockwise retards the ignition.
When everything is tightened up, you can easily confirm this is correct by trying to rotate the base plate anticlockwise, but it will not move. If you rotate the plate clockwise, it will move against the action of the return springs, i.e advancing the ignition.
Always rotate the engine clockwise when moving up to the timing mark.
If you use a wedge to keep the points advanced, don't forget to remove it after you have set the ignition.
If you find that you need to advance the timing from the original position, this may make the kick back even worse, and we will have to have a rethink.
Finally, bearing in mind you will be trying to start the engine on full retard (and this is when it kicks back), it might be worth checking where the timing is on full retard. It might give us a pointer. Or, indeed, is  the timing advancing when kick starting, perhaps due to worn springs. Just a thought.

[Steve.S]

Correction. For Base Plate, substitute Cam.

For clarification, when you turn the cam by hand, it will only turn one way against the springs. That is advancing the timing.

[rhyatt]

I use a pair of long nose pliers to grip the cam and rotate it every now and then whilst checking the timing , multimeter on beep , get the engine in the right position and adjust the backplate whilst rotating the cam with pliers .
Gets it pretty accurate.

[hoogerbooger]

OK I have reset with cam fully advanced (turned anti-clockwise). As picture shows points plate had to move anti-clockwise a tad. (I am pretty happy I have been as careful as I can this time.  Timing disk and multi- meter used. each stage triple checked)

So have I fractionally retarded the ignition? (anticlockwise cam. anticlockwise turn of the points plate)

Anyway 2nd kick from cold (after tickling) it almost started. 3rd kick: a small spit through the carb and a bit of kickback (perhaps lighter than before).  Went away for a few minutes, re-tickled. Started 1st kick. got it almost warm, and not too lumpy at tick over. have not ridden yet

I used a washer just bigger than cam spindle to lock the cam. Couldn't hold the cam in fully advanced  without it slipping by the time I'd tigtened the cam bolt/washer.....so as I could see an AAU bob weight pivot through the cut out I used a flat blade screw driver between the pivot and edge of cam plate to gently prize to fully advanced. ( I have a spare AAU and this seems to work fine without much pressure needed)

By the way I'm using E10. So more oxygen in mix. But, as I understand it, below 1/8 turn on the throttle, the mix is controlled by  the idle screw on the carb.......Which I can't try re-setting til I've taken it for a run to get it fully warm.....which may not happed until later this week.

[rhyatt]

E5 or e10 , I have no preference in my ss80 or bantams.
The both run well on either ,

[Steve.S]

Yes, I would say you have retarded the ignition. You make no mention of using the choke. Does it have one?
It's good to give it a run, but you could easily warm it up in the garage.....depending on the neighbours.
Bear in mind the mixture screw is an air screw, so you might want to screw it in a tad more to richen the mixture if it still spits back.
If you're sure you have no air leaks, you might consider a richer slide.

[hoogerbooger]

(Ans:  no choke currently fitted...have all the parts in a box though)( and yes the neighbours tolerances have limits)


So I checked the points and spark gaps ( as I hadn't after re-doing the ignition timing).  spark gap fine. Points a tad too open so reset.

Two short runs since then. Checked the idle after the first run and it was at the highest revs for the stop & idle speed as low as seemed sensible. So seemed right & left it there. 2nd short run today. 

Of the 6 times of starting it up for warm-up and runs ( so circa 20 kicks):

No kick-back.......but 3 failed kicks with spit back and puff of smoke through the carb/air filter.

So better, but presumably still some issue to address/adjust for.


The spit back seems to be more likely on 2nd to third kick. I though this may be as I hadn't tickled again after a failed kick. But today I tickled each time I tried starting and still got one spit back. Going away for a couple of minutes...then re-tickling before another attempt seems to increase likelihood of it firing up.....or may be co-incidence!

RE: comment on try turning in the pilot air screw:
I can see it'll richen the mixture for up 1/8 throttle.....so at start up. It's not what the Amal set up guide advises, so is the suggestion basically a work-around if I have no choke fitted... and my issue is it's just a tad cooler currently and spit/kickback is co-related to starting temperature??

My head is thinking if you find the highest revs on tick-over...the air/fuel mix for start-up must be as good as it can be ??

Another point I've seen elsewhere on ignition timing for E5/E10: suggest that the ignition should be "backed off by 2-3 degrees" which I presume means retarding ? , which again if my head understands means moving from 33.5BTDC to 31.5 to 30.5 degrees BTDC ?? Have others tried this? and found it helps?

[rhyatt]

When all our classic cars and bikes were new , petrol was very different .
Nowadays , any carb and ignition setting are a starting point only and not set in stone.
Get the vehicle running and tweak both ignition and carb until you find a happy spot.
Could be either side of what the book says and be prepared to keep fiddling.

[Steve.S]

You definitely seem to be getting there. Keep at it.
I strongly advise fitting the choke, it may well solve your problem. I wouldn't spend any more time trying to diagnose the spit back until you have done this.
Closing the points will retard the ignition a tad, which is the direction you need to go, although, as far as I can gather, it no longer kicks back?
Yes indeed, the suggestion is a work-around, although it may be necessary even when you have fitted the choke.
Your head is correct in thinking that the highest revs adjusting the mixture screw, gives the correct mixture on tickover. To fine tune, lower the idle screw to get the lowest tickover, and adjust the air screw again.
It's just a question of fiddling about until you're satisfied.
Regarding ambient temperature, I've found this normally affects the older water cooled motor car engines, rather than ancient air cooled motorcycle engines.
In my view, the issue of ignition timing with E5 or E10 is more to do with Octane rating than burn characteristics. But what do I know?
Personally, I have so far never had a problem with E5, or found the need to retard the ignition on many different British motorcycles, but then I use an Octane booster, and can only speak for myself.
If the engine doesn't 'Pink' on E5, then the ignition doesn't need retarding.
I hope this is helpful.