AFR testing

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jgra1
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AFR testing

Post by jgra1 »

hi all, I am trying to get an old single cylinder bike to run right.. I feel I want to know it's air fuel ratio and then make changes to jets etc.. (I have tried looking at plug and so on, but I want something a little more certain)

help! so far I have looked at :
Chinese gauges and O2 sensors (no problem to fit these)
Gunson testers
Colourtune (not sure if I can fit one in the plug hole)
Sun garage machines!
buying an 02 probe and trying to measure it's voltage..

as you can tell, I am a bit clueless ! :mrgreen:

John
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Re: AFR testing

Post by Dormouse »

Hi. What size and make is the engine? How old is it?
jgra1
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Re: AFR testing

Post by jgra1 »

hi Dormouse, it's a single 500cc Rotax 4 valve, circa 1984.. It's using the original Amal carb (which most people bin) I have 2 carbs and two sets of jets.. I am trying to make the best of it really.. Plug is sooty and bike is smelly and a bit misfirey.. to my mind..
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NewcastleFalcon
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Re: AFR testing

Post by NewcastleFalcon »

jgra1 wrote:
17 Jun 2021, 10:09
hi Dormouse, it's a single 500cc Rotax 4 valve, circa 1984.. It's using the original Amal carb (which most people bin) I have 2 carbs and two sets of jets.. I am trying to make the best of it really.. Plug is sooty and bike is smelly and a bit misfirey.. to my mind..
Will that be your MT500 John :?:
NewcastleFalcon wrote:
25 Sep 2020, 09:02
Good Luck John curiosity had me looking up the MT500

This bloke resurreccted one after a 10 year gap fairly easily, broken cam belt.



Regards Neil
jgra1
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Re: AFR testing

Post by jgra1 »

thats it Neil ;) I have been using it, and am not expecting refinement just want to know it's reasonably tuned..

think I have just bought a crypton for fun.. not that I have any room left in garage lol
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Re: AFR testing

Post by Dormouse »

OK! Crypton on it's way! which model?
Now. Rotax. A sound engine. I would test cylinder pressure dry and wet to ascertain a basic set of criteria to see if the engine is worn. I actually rate Amals in the same way I love SUs. I know you can get better but you need to maintain / set up better to extract more power / reliability. I can go down that route if you like.
first thing - get the engine in good basic condition ( rings, valves)
next - decide where you want to wander with tuning
the Rotax can produce good specific power - it just needs to be set for your preferences
jgra1
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Re: AFR testing

Post by jgra1 »

thanks DM.. engine rebuilt about 400 miles ago ;) ,, happy with how it went back together but other issues have since surfaced.. the Cripper is a 315 I think with an emissions tester underneath.. will be fun having a mess around with it ;) - arrival prob Sunday
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Re: AFR testing

Post by Dormouse »

Have fun! better you than me.
What issues have surfaced?
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Re: AFR testing

Post by Dormouse »

The Motorscope is old tech but perfectly useable. Which gas tester is it? MoT or 4 gas?
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bobins
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Re: AFR testing

Post by bobins »

I asked a mate who knows about such things. Apparently.....

"........ the carburation was never set up properly on the
MT500 Army bikes as Alan Clews (Mr Armstrong) ran out of time and was
under pressure to supply the finished machines to the MoD."

His MK2 30mm Amal carb was :
Slide 3.5
Main jet 190
Pilot jet 20
Needle Jet 105
Needle circlip 3 from bottom
Needle 2A7
(pilot air screw one and a half turns out)

But despite the fact it would run well it would stall for no reason. He reckons a visit to the MT Riders forum where you'll probably get chapter and verse on it.

Hope that helps.
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Re: AFR testing

Post by Dormouse »

I agree about the MT Riders bit. Well worth reading. I would go there.
The MoD spec was typically vague/contradictory.
The bikes were "conservatively" set up to run on whatever petrol was available. So, modern fuel/jetting and ignition timing will go a long way to improve the engine. Stalling could be lots of things but usually gets blamed on the Amals. It could be air leaks, sticky float, ignition performance. Et al.
Amals. If you can see "lines" ( not even scratches) on the needle near the base it is worn. But what most people forget is that the jet will have worn as well so just changing the needle is not enough. Needles and jets are also matched quite often so just changing to a different Fuelling needle is not enough either.
Sticking cables, throttle handles and throttle bodies don't help, so you just need to go through all the basics first.
Somewhere in the back of my bonce I think I recall someone using a Rotax in a speedway bike. Will have a search.
jgra1
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Re: AFR testing

Post by jgra1 »

thanks both. I am on the MT forum but its a bit quiet these days...

my initial carb was :
Pilot : 20
Cold SJ : 60
Main Jet : 190
Needle Jet : 105
Needle: 2A1 (or 7?) @69mm (5 slots)

the bike was hard to start and the headers were red!
I changed to spare jets and it has been much better but not quite right (it backfires on overrun, soots the plug, and is a bit boggy)

Pilot : 25
Cold SJ : 55
Main Jet : 230
Needle Jet : 106
Needle: 2C3 @ 71mm (3 slots)

Appreciate DM;s comment about worn jet / needle combo.

My feeling as said at the top is I just want to know roughly where my emissions are, and to see if I can make small changes / buy jets - to bring them inline really..

What issues have surfaced?
the clutch plates have some warpage, I am not to bothered about this yet but will get new plates soon.
there is oil seepage from various places, the oil looks to have been contaminated with fuel and carbon (possibly in part due to rich running?) and was very thin and smelt fuely - have changed oil already..

DM, I don't know which emission tester the crypton has yet :O
edit : TI Crypton 64 exhaust gas analyser ?
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moizeau
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Re: AFR testing

Post by moizeau »

I am going through the same thing with my latest toy and put a z1000st back on the road after it had stood for 10 years, 3 years ago.
I haven't played with amals since I was a kid, we fitted them on the aircraft version of the bike engine at Norton.
Anyway, regarding the fueling, firstly make sure the carb(s) are as clean as new. You should have a drain screwe on the float bowl, attach some clear pipe to it and tape it to the side of the carb. Turn the fuel on and then you will see the fuel height inside the carb in the clear pipe. I don't know the exact height for your carb but roughly speaking set it to the same as the mating surface of the float bowl to body.
Colortune is your friend. You can't accuratelt plug chop with used plugs and todays fuel. If you plugs aren't 14mm you can get an adaptor. With the colortune you will also find out if the choke plunger is seating correctly because if it isn't it is very hard to set the pilot circuit.
jgra1
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Re: AFR testing

Post by jgra1 »

thanks Moizeau. carb has been cleaned and rebuilt, air lined etc.. float height doesn't appear to be adjustable. It did flood once though, only takes a bit of grit and I forgot that time to turn fuel tap off! :twisted:

Happy to see if Colortune will work, its just a tight fit for the plug (I had to buy a thin walled socket specially for it)

If the crytpon is a massive overkill, wont work, takes up too much space etc I can cancel it.. I thought the HT trace may have been useful on other vehicles if nothing else? its a fairly long journey to go and get it, although its cheap.

John
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Re: AFR testing

Post by Dormouse »

The 64 is a single gas hot wire guage unit I think. Give it a good clean and run the unit with as short an inlet hose as possible. This will improve the meter response times.
I think you may have gone too rich with the jetting if it is bogging down but a check of AFR with your new toy would confirm this.