G9 running rich

Information relating to the Matchless G9 or AJS Model 20 500cc twin
hdawson
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G9 running rich

Post by hdawson »

Any advice would be appreciated on why the matchie is fouling it's plugs.
It was running well but started running so rich that the plugs are sooting up unbelievably.
Took the Carb off for an inspection but was unable to remove the float cap.
It is so tight that all the usual methods to free it were thwarted.
So I am unsure if the float may be stuck.
Should I replace all jets as a matter of course?
All subsequent adjustments have not improved the situation.
Valve clearances have been checked/adjusted.
Timing has been previously adjusted.
I feel that I am missing something simple.
What sequence of tuning should I follow.
Cheers,
Hadrian.
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Rob Harknett
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Re: G9 running rich

Post by Rob Harknett »

Has the bike been standing and sumped oil, if it smoked a lot on starting, this is probably the problem. Drain sump if oil level has dropped when standing. Allow a good 5 mins running on fast tick over on starting, to refill sump to correct level, before oil starts to return to the tank.
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clive
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Re: G9 running rich

Post by clive »

Have the same problem on a G11CS. Have an air filter fitted and have reduced the pilot jet from 30 to 20. Some improvement. Works well with correct plug colour at other than at idle speeds. ( main jet down from 280 to 270)
So with regards to your bike. Did you make any changes before it started running rich. Eg fit or change air filter, change make or grade of plugs. If so suspect the last change you made.
Is the problem throughout the range or only at slower speeds? I would probably suspect at this stage that the float is either sticking or punctured. You don't say what carb but I assume pre monobloc as you cannot get the float cap off. If it's a metal float and it is leaking you can usually see the vapour coming out of the crack and once it is liquid free you can usually fix it with solder.
clive
if it ain't broke don't fix
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Samuel
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Re: G9 running rich

Post by Samuel »

clive wrote:once it is liquid free you can usually fix it with solder.
or else B A N G !
Sam
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Re: G9 running rich

Post by SPRIDDLER »

Has it started running rich over time or immediately after the last time you ran it?
Is the zorst smoky or the silencer outlet oily/sooty?
If the float is stuck I would expect the carb to flood/dribble continuously until after the tank tap is turned off.
If it has been running o.k. and the jets are correct (and have not vibrated loose) I don't think there's much to be gained in replacing them.
One way or another you really need to remove the float cap (as you obviously realise).
As Rob says, if it's been standing a while it could be oiling of the plugs due to a build-up of oil in the c/cases (i.e. 'wet sumping').
Or as Clive asked, have you changed anything since it last ran o.k?
Here's a tuning procedure. It relates to a Monobloc carb but the process is the same for your (assumed earlier) carb with the separate float chamber:
http://www.hitchcocksmotorcycles.com/mo ... to-tune-up
'There is a tide in the affairs of men
Which taken at the flood............'
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clive
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Re: G9 running rich

Post by clive »

Try removing the float bowl whilst the carb is mounted on the bike, tightening very slightly first, then trying to undo. If I remember from when I owned Sam's 49 G80 the float top has a weird sized nut on it and also has a small bolt to one side which nips up to stop the top vibrating off. It's occurred to me that the spring clips that hold the float to the needle can go weak and perhaps slip up to the next needle slot. As the float needle works by pulling up a cone to seal the fuel supply off, if it has slipped to the next groove it would make it run rich by overfilling the float bowl. The springs can fixed by just pulling them outwards again.
clive
if it ain't broke don't fix
hdawson
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Joined: Wed Nov 05, 2014 11:40 am
Location: Victoria AUSTRALIA

Re: G9 running rich

Post by hdawson »

Thanks for all the replies.
Yes it is a pre monoblock and yes the headers are covered in soot.
I always drain the sump pre ride.
Before I started messing around, it started easily and idled nicely but when accelerated it misfired and lost all power.
I had lately changed the plugs to NGK BP6ES as the old Champion plugs were looking a bit worn.
I will remove the Carb again and carefully ease off the cap.
Cheers.
hdawson
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Re: G9 running rich

Post by hdawson »

Hi all again. Just to update, I managed to undo the cap on the float bowl.
In the end, after all suggested techniques, I had to use brutal tactics involving the vise and hammer but no harm done.
After many assemble/disassembles and plenty of fuel pouring from the carb I have come to the conclusion that the fault is with the needle/seat.
However the seat does not seem to be available without purchasing the whole float bowl assembly.
Is the brass needle seat removable and repairable?
If not has anyone used grinding paste to correct the seat using the needle?
Cheers,
Hadrian.
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Re: G9 running rich

Post by SPRIDDLER »

Well done, Hadrian. Another wall climbed, eh? ;)

I think grinding paste will be far too coarse.
I've read that some use Brasso but never tried it myself.
It may help if you replace the needle.
'There is a tide in the affairs of men
Which taken at the flood............'
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ajscomboman
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Re: G9 running rich

Post by ajscomboman »

Solvol Autosol is a good compound to lap the needle into the seat. As Nev has mentioned grinding paste is too coarse.
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