Easy Starting but dies when given throttle

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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Re: Easy Starting but dies when given throttle

Postby SPRIDDLER » Tue Aug 07, 2018 3:44 pm

There have been several posts regarding problems with manufacturing swarf blockages found in new Monoblocs when straight out of the box. The Pilot jet and its drillings are very fine. From throttle closed (tickover) to about 1/3rd open only* the Pilot jet is supplying the fuel. At tickover (throttle shut) fuel is delivered via the pilot jet bypass drilling but as soon as the throttle is opened the fuel is drawn from the pilot jet and its associated drillings.

*The slide cutaway also has an effect on the fuel/air ratio (but the slide cutaway can't become blocked).
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Re: Easy Starting but dies when given throttle

Postby CAB » Wed Aug 08, 2018 9:45 pm

Hello
I had a similar problem with a BSA Spitfire - started fine after using the tickler but would then cough and splutter. It would start easily again using the tickler but then the problem would re-occur. It took ages to figure it out but the float was sticking up and it was running out of fuel. Working the tickler slowly would keep enough fuel in the float chamber and allow it to run. We then fitted a clear plastic tube to a bit of brass pipe soldered to an old jet nut and the fuel level after priming could be seen to drop to nothing as it was used up and not replaced. The level could be maintained with the tickler and the engine would pick up and keep running.
Worth checking and quite easy to do.
CAB

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Re: Easy Starting but dies when given throttle

Postby PeterJCorby » Thu Aug 09, 2018 3:19 pm

thanks to all. plemty of options to keep me tinckering through the upcoming weekend. I will advise what worked and what didnt there after.. Appreciate your wisdom.

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Re: Easy Starting but dies when given throttle

Postby PeterJCorby » Fri Aug 10, 2018 4:28 pm

Just a quick update. I have swapped the slide from the old carb to the new carb body. Seems the old is a #4 and the new is a #1, Checked the float level and that seems to be fine too.Needle Jets are both 106 Old and New, Pilot are both 40, Main is 130 Old qand 140 new..

Using the old slide made for an interesting start, Plenty fart through the carb, on retarded timing. Once running and advanced it was a good steady idle. After 3 mins tried blipping the throttle same result as before. i.e. nothing. I tried ticking the carb while opening the throttle and that seemed to help it rev up a tad. But it is still a battle. Given as previously mentioned the plaugs are new, Condensor points and Mag are new/recently refurbed. New HT leads and caps, what is the possibility or the timing affecting the attepted rev up. I am pretty good at fixing motor cars and have a good few years experience on 4 stroke and 2 strokes, but must say this one has me stumped. Anyone know of any good motor mechs in Perth that are have knowledge of old Brit Bikes?

Midnight now so will wait til daylight and swap over old main ket and see if that makes a diffence.Last resort will be using the complete old carb.

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Re: Easy Starting but dies when given throttle

Postby dave16mct » Fri Aug 10, 2018 5:28 pm

Surely the main jet should be 390 on a G12, maybe slightly smaller with air filter.

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Re: Easy Starting but dies when given throttle

Postby ajscomboman » Fri Aug 10, 2018 6:11 pm

dave16mct wrote:Surely the main jet should be 390 on a G12, maybe slightly smaller with air filter.


Spot on Dave and also the pilot jet is reported to be 40 where it should be a 20. Never seen a number 1 slide either.

Without having seen the bike and whether it's fitted with an air filter or not the following spec are what the carb should have;

Main jet 390 or 340 with filter, number 20 pilot jet, 106 needle jet, needle set in 4th notch from top, number 4 throttle slide. The carb should also be 1 1/8th bore.

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Re: Easy Starting but dies when given throttle

Postby SPRIDDLER » Fri Aug 10, 2018 6:14 pm

Strange set-up you seem to have.
In the Owners Manual for the '63 Model 31 (same as G12) here:
http://archives.jampot.dk/book/Owners_m ... Manual.pdf

it gives............

Mod 31 G12 carb settings.JPG


Oops! Reply crossed with Rob's.
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Re: Easy Starting but dies when given throttle

Postby PeterJCorby » Sat Aug 11, 2018 5:00 am

Hello again. I think I must have been more tired than I though re those jets sizes.. new Carb has Main 400 pilot 40,needle .106 Slide is a #3.

I cleaned up the old carb and installed that one. Great news is it started, Idles rough, but revs up (kind of). Still better than it was. Took it for a ride and it is flat as can be.I think a "80s' 50cc Two Stroke has more power!. Having owned a "54 AJS 500 twin ( proper Jampots on it), I know the G12 should run a lot smoother. Guess I have reached my limits and will need to look for someone better versed in Classic bikes to help me get this old girl running properly. Living in Perth Western Aus, I beleive there are a few such folk around, now to find them and make contact. Thanks for all your advice.

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Re: Easy Starting but dies when given throttle

Postby Janet » Sat Aug 11, 2018 8:00 am

Having exhausted all the carb possibilities, I think your ignition timing may be a bit off. Yes, I know nothing but you said "Took it for a ride and it is flat as can be." I have several bikes that are like that when I've reset the ignition/points/whatever they had/ during routine maintenance. Sometimes I even get them right within a day :) . The longest was 3 weeks :( but it was really good when I'd finished. It's probably just me and I don't know why it happens unless it's because the sparky thing isn't at its biggest when it's correct for the burny thing and the squashy thing.
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Re: Easy Starting but dies when given throttle

Postby ChrisTheChippy » Sat Aug 11, 2018 11:07 am

Hi Peter
Just reading been reading your posts .As you know I have the opposite problem my bike revs great but won't tickover .
Don't give up yet I suggest you go back to complete standard factory settings with the carburettor set it correctly be very meticulous with cleanliness and checking things as you go then if still no joy look for the cause elsewhere .
I recently had a problem with a G2 that just refused to run despite me thinking that everything was absolutely spot on . After months of playing around I finally gave myself a good talking too went over everything I had done and carefully checked it all again asking the question is this really correct ? everytime I looked at something.
Finally I found the problem ( ignition timing ) and it was a wonderful feeling when the bike fired up immediately.
What I'm trying to say is be persistent and patient very difficult when you are tearing your hair out .
Keep your chin up I would be interested to know how you get on .
All the best
Chris

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