B31 spluttering and flashing back through carb

[john11668]

Hi All,

Can valve timing be known to move on these.

Not unless something jammed the timing gears and sheared the key in the crank pinion
Remove the tappet cover, wind the engine to TDC, if both tappets are free (the pushrods should be free to rotate at TDC on compression stroke) then turn the crank one more complete turn
As the piston rocks over TDC you should be able to see the exhaust tappet dropping and inlet rising this should happen equal amounts either side of TDC

I now see you have a a concentric, The pilot passage bush within the body gets blocked,  a 16 thou guitar string or a no. 78 drill can be used to poke through the pressed in pilot bush
It has happened that new concentrics are not set up properly or have swarf and so on inside

John

Hi all ,
Sorry i have not been back on this  but other things (and  the weather) have kept us tied up elsewhere and it is not really biking weather anyway.
So runs best on old carb , timing spot on as far as we know ,  yet still spluttering a bit .
I am trying to understand the pilot jet  function in this .  As we look at the Amal catalogue  it shows a pilot jet which can be varied yet looks to be a screw in item. We dont have that! just a threaded hole in the underside of the body corresponding with a gallery which seems to draw from the bottom of the float chamber. Where do we find the so called pilot bush?

[chaterlea25]

Hi

Where do we find the so called pilot bush?

Remove the horizontal pilot air adjusting screw. shine a light into the hole and the bush should be visible
0,016in dia hole in it, suitable guitar string or no78 drill on an extension
Carb aerosol cleaner or similar when squirted in there should also escape up the tiny hole on the engine side of the slide

I cannot believe you have been swopping mags and carbs and all the rest without first of all cleaning the Fu**king carb
 

John

[Rex]

[  As we look at the Amal catalogue  it shows a pilot jet which can be varied yet looks to be a screw in item.

I'm confused. The Amal Premier carb has both a fixed size but  removable pilot jet (screwed in horizontally into the body) and a mixture adjusting screw. Maybe you're looking at a diagram of this type, though I thought your carb was a Wassell.
According to Hitchcocks it's been common with the new Premier carbs for people to get these two items confused. The pilot jet should remain tight in the body.

[john11668]

We  have been confused too! By the information in the parts books
See  https://www.hitchcocksmotorcycles.com/partsbook-pages/3375
Item 20 is clearly a screw fitted into the underside of the carburettor body hence a threaded tapping  in the underside .
Carb has been ultasonically cleaned , washed in brake cleaner , blown through with Hi  pressure air so no reason to suspect it not clean.
It is not a premier carb.
The wassel carb however does have the premier type adjustable pilot jet  but fares no better (worse in fact)  hence it was taken off again and the original refitted.
And remember from the start of this thread that the problems started  when Dynamo worked loose  and we found that the contacts were  breaking at about 1.5" Btdc.  Is that in itself  not sufficient reason to suspect timing over carburation.

And we have studied a number of manuals , none of which tell us that the pilot jet is hidden away behind the pilot air screw.
You guys  may well have lots of experience with big old singles which is why we come here for help. We on the other hand are learning  still.

[L.A.B.]

We  have been confused too! By the information in the parts books

Item 20 is clearly a screw fitted into the underside of the carburettor body hence a threaded tapping in the underside.

And we have studied a number of manuals , none of which tell us that the pilot jet is hidden away behind the pilot air screw.

Unless it's a 2-stroke or very early 4-stroke Mk1 Concentric then it would normally have the pressed in pilot bush which can be seen when the pilot air screw has been removed (RH image, below and which often becomes blocked see Bushman's link) and NOT the item 20 pilot jet.

http://accessories.hitchcocksmotorcycles.com/Amal-Carburettors/Amal-MK1-Concentric-Hints-and-Tips/How-it-Works-and-Part-Names
"On most 4 stroke models no pilot jet is fitted, but a pilot bush is inserted in the continuation of the pilot air adjusting screw passage."

http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html

It is not a premier carb.
The wassel carb however does have the premier type adjustable pilot jet  but fares no better (worse in fact)  hence it was taken off again and the original refitted.

The Amal Premier and Wassell Evolution carbs have removable pilot jets in the same position as the pilot bush which can be adjusted by fitting either a larger or smaller jet.

There is no screw adjustment so they should be screwed fully home and the idle mixture adjusted using the pilot air screw.

[john11668]

Lot of usefull stuff there L.A.B.
Will have a good read tomorrow night

[john11668]

Just taken the screw out this evening and there is no  Brass pilot bush behind it.   The paths are all clear to the tiny holes in the the body and also to the hole  below the intake aperture . so no crud or obstructions and this carb would seem to be as it was when the bike was bought  and bearing in mind it has run perfectly well on this carb for well over a year.

[R]

and this carb would seem to be as it was when the bike was bought  and bearing in mind it has run perfectly well on this carb for well over a year.

And it runs perfectly well on it now - not !!!

Can you tickle the float while this is happening, and the low speed running improves.
Could be your fuel flow or float height might need some attention.

As was suggested earlier in this thread, if the low speed running deteriorates,
then the carb needs cleaning cleaning cleaning cleaning cleaning - until it runs well again.
Ask me (and a million+ other owners) how we know this....

Sounds like you have an earlier version of amal, with fixed pilot jet.
I forget the finer details, but you can't really do anything to it,
just clean clean clean everything until its good again...

[R]

Most concentrics have a filter screen on the fuel inlet thingy under the floatbowl,
have you checked that is sparkling clean ?

Have you drained a sample of fuel through the fuel tap into say a glass jar,
and checked that its sparkling clean.  Any rust particles or water globules
are a disaster for carby cleanliness, and could well contribute to such problems ...

Some amals have a drain plug under the base of the floatbowl. If you are sure the gasket
is in good condition, you can remove that and examine what comes out for rust or water.
Anything other than sparkling clean fuel is a potential problem...
White corroded crud in particular.

[john11668]

Carburettor is  spotlessly clean, and all the filters including the banjo.. But will go through it all again to satisfy ourselves. 
Were there models of this carb specified with no pilot jet at all .  And in such a case am I correct in presuming that the mixture screw is the only regulaion for pilot fuel
When the bike started to play up there was no doubt  that the timing was seriously over-advanced. (points breaking 1.5 " btdc ) . What if anything might be damaged by this.  It is now spot on at 7/16" and can be seen to be with a timing light and we can see the effect of retarding to fire at TDC,  but none of these adjustments seem to produce better running. 

The carb as far as we can see is as it was when it was running well, yet it is running like a chip machine and no adjustments  seem to improve on that.
Tappet clearances are ok and there is no visible damage seen using an endoscope in the cylinder ( although we cant see all corners )

[TGR90B]

Has fitting the correct carb. occurred. It S/B a Monobloc. I haven't read the full saga, so excuse me if this has been covered.

[chaterlea25]

Hi John,
A pilot jet must be fitted either the bush or a screw in one shown in this rebuild guide
http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter

John

[R]

There are versions of this carb with a fixed pilot jet (non-removeable).

If you screw the idle adjustment screw in or out, does it change the idle characteristics ?

The idle adjustment screw (pilot air adjustment screw) alters the AIR supply getting drawn into the cylinder at low speeds (rpms).
If you screw it all the way in, it cuts off the air supply - and the engine should falter and die.
If you screw it out too far, it gives too much air, and the engine will start to falter and even die.
Ihis tells you everything is intact and functioning though, and petrol is being metered through the idle circuits.
Perfect idling is somewhere midway between these 2 extremes.

If there is any (white) corrosion in the idle passages anywhere, things can be blocked off.
And this can be hard to detect and even harder to clean out...
A trace of moisture can cause such corrosion.

[john11668]

So I am now back after having tried what we considered was everything . Cleaned carbs , prodded carbs , blew carbs out with air ( and carb cleaner ). No trace of even a spec of dust in the fuel filters.  Spoke to Amal technical  (who suggested Mag) .
Checked timing again and again,  tried  a bit less advance ,  a bit more advance , simple when you have made a proper tool.
Checked  full timing gear train (all fine ).
Tested mags on the bench but then swopped anyway ,
Swopped carbs,
Spoke to couple of local experts.  One said the only other thing left to check is the valves . The other said it would definitely be the valves  so off off with its  head .  All fine  but ground valves  in anyway and reassembled it all . 
And all still the same . Starts at first push of the button or kick but  wont run without spluttering and flashing  back unless on full choke.

So!  What have we missed?  we ask ourselves.
Could it be fuel ?
Went and got five litres of fresh 95 ron  and put it in the tank
Tried again  and Voila!  Runs like a dream,  after a couple of minutes warming we could ease the choke back to wide open and it still ran sweet and revved freely. No flashing back.

So after an hour or two of self flagellation, we came to the conclusion that in future clean new petrol will be step one .
But we have learned quite a lot in the meantime,  both from you guys and from  practical experience.  So thanks to you all 
Not quite sure how to sum this up really.

Gonna change the carb  anyway cos a 928 is not the correct throat for the machine and there is significant wear .  Amal say 626 /300 with 210 main jet , .106 needle jet , and 3 1/2" slide .
Any comments please??   

[john.k]

All I can say is your fuel must go off mighty quick.....I buy 5 gallon drum of petrol for the mower etc when its cheapest ,about once every six months,dont have the lid tight so the drum doesnt bang with temp changes,and Ive never seen the slightest deterioration of the fuel......Mind ,I never buy alcohol fuel,cause the chainsaw specifically warns about it,and in any case alcohol fuel is actually more expensive than straight unleaded......by about 4 c per litre,even tho the pump price is 2c less /l.(less energy content)......I might add .I can clearly smell the alcohol in 10%,and also the exhaust smell of the mower is obvious with alcohol,so there is no chance of having it by mistake......Not that Im doing a lot of mowing with the drought......with bikes ,I always drain and remove the tank if Im not using the bike regularly,and swill out with 2 stroke mix.This stops the rust forming around the inside near the cap.