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2-stroke backfiring

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gwernybwch
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PostPosted: 21:44 - 20 Mar 2010    Post subject: 2-stroke backfiring Reply with quote

Bought myself a non-running Honda MTX200, stripped down the engine (mainly to fit a new gear selector fork) but also took the opportunity to fit new main bearings and seals.
Rebuilt the engine and it started and all seemed well apart from the engine smoking a little (which I put down to it burning off the oil used in the cylinder / crank during the rebuild). After about 4 miles, the engine was starting to ‘bog’ and then mis-fire.

Checked the plug, a bit of carbon on it, nothing special. Checked the fuel system - cleaned to the petrol tap filter (and to be honest that it was blocked), checked the carb – no noticeable dirt or water in it.
Started it again (starts after three or four kicks) and then almost immediately back-fires).

The woodruff key on the crank / flywheel hasn’t sheared off. I’ve tried a different stator plate but no difference.

No noticeable breaches of gasket around the reed block.
I don’t have a strobe, therefore haven’t been able to check the timing, but I don’t think that I’m able to adjust the timing.

Any suggestions as to why my baby is backfiring?
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Vincent
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PostPosted: 00:44 - 21 Mar 2010    Post subject: Reply with quote

I'd try a new plug and cap before going any further.
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The Artist
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PostPosted: 00:52 - 21 Mar 2010    Post subject: Reply with quote

Check the timing. Adjusting the position of the stator plate can even make the engine run backwards.
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steven_191
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PostPosted: 02:32 - 21 Mar 2010    Post subject: Reply with quote

allymoss wrote:
Check the timing. Adjusting the position of the stator plate can even make the engine run backwards.


+1

first thought is the timing.
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gwernybwch
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PostPosted: 09:41 - 21 Mar 2010    Post subject: Reply with quote

allymoss wrote:
Check the timing. Adjusting the position of the stator plate can even make the engine run backwards.


How can the timing be adjusted?
The stator plate is held on by three screws, which doesn't allow it to be adjusted at all.
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stirlinggaz
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PostPosted: 10:04 - 21 Mar 2010    Post subject: Reply with quote

Hi,
afaik, the timing cant be adjusted.
only problem i ever had with mis-firing/backfiring (& even once the engine running backwards! Shocked ) was due to the woodruff key slot eing enlarged/worn, making the key loose & flywheel was spinning.
cheers,
GAZ
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The Artist
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PostPosted: 12:05 - 21 Mar 2010    Post subject: Reply with quote

On my Senda which was 2 stroke, the stator plate was held with 3 screws which you could loosen and adjust the plate along grooves. I suggest the woodruff key. Make sure it is in the correct place and not slipping at all.
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gwernybwch
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PostPosted: 09:13 - 22 Mar 2010    Post subject: Reply with quote

OK, this is what I have done so far –

Fuel -
Checked / cleaned the carburettor and the petrol tap filter (and are using fresh fuel)
Cleaned the spark plug

Timing -
Fitted a different stator plate (which is non-adjustable) and checked that the flywheel is on correctly and is tight on the woodruff key.
Double checked the timing of the cogs during the rebuild

Electrical –
Tried a different CDI unit (giving a ‘fat’ blue spark)

Tried all this, it even gave a backfire when tried to kickstart it, now won’t even start at all.

Any other suggests that I should be looking at?
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cloric_tzr
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PostPosted: 19:41 - 22 Mar 2010    Post subject: Reply with quote

gwernybwch wrote:
OK, this is what I have done so far –

Fuel -
Checked / cleaned the carburettor and the petrol tap filter (and are using fresh fuel)
Cleaned the spark plug

Timing -
Fitted a different stator plate (which is non-adjustable) and checked that the flywheel is on correctly and is tight on the woodruff key.
Double checked the timing of the cogs during the rebuild

Electrical –
Tried a different CDI unit (giving a ‘fat’ blue spark)

Tried all this, it even gave a backfire when tried to kickstart it, now won’t even start at all.

Any other suggests that I should be looking at?


dont hold me to this but im sure you can change the timing by looening the fly wheel and adjusting on certain bike the can be known to 'slip' minutely adjusting timing enough to cause splutter mis-fires lack of power and starting problems. especially if the timing sliped and then you put a fixed stator plate in thus permanantly puting timing out.
i only had a convo last nite with some1 and thats what he told me cause i gt a tzr tht now wont fire. so justy thought id pass it on.
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gwernybwch
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PostPosted: 22:28 - 22 Mar 2010    Post subject: Reply with quote

Thanks for the replies so far.

One other question -
Would a minor fault in the head or base gasket also make a 2-stroke back fire?

(When I saw minor fault, I did drain the engine oil and there was no sign of it being mixed with water)
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EnergyInducti...
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PostPosted: 09:44 - 23 Mar 2010    Post subject: Reply with quote

Is it Backfire or Afterfire? Coming from the exhaust? or the intake system?
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EnergyInducti...
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PostPosted: 09:54 - 23 Mar 2010    Post subject: Reply with quote

Have you checked float height in the carb?
An incorrect mixture wont burn properely...
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Vincent
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PostPosted: 11:45 - 23 Mar 2010    Post subject: Reply with quote

snaphappyalex wrote:
Have you checked float height in the carb?
An incorrect mixture wont burn properely...


The float height doesn't really have any effect on the mixture, it's just a valve to allow fuel in when required, works like a toilet systern.
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EnergyInducti...
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PostPosted: 11:47 - 23 Mar 2010    Post subject: Reply with quote

I'm just going by what people suggested me when my bike was afterfiring....
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gwernybwch
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PostPosted: 19:33 - 25 Mar 2010    Post subject: Reply with quote

Quote:
One other question -
Would a minor fault in the head or base gasket also make a 2-stroke back fire?
(I have drained the engine oil and there was no sign of it being mixed with water)

Anyone?

Quote:
Is it Backfire or Afterfire? Coming from the exhaust? or the intake system?

Not sure, it happens so quickly, it is hard to listen.
Is there an easy way to tell?

Thanks for the replies so far.

Any other suggestions gratefully received!
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badas
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PostPosted: 21:35 - 25 Mar 2010    Post subject: Reply with quote

hello i have an mtx 125 and had simular issues but mainly more bogging than miss firing and i did the following to resolve:

change spark plug (yes i did clean the old one but new one fixed it)
cleaned air filter
fully charged battery

sorted Smile
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gwernybwch
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PostPosted: 20:12 - 02 Apr 2010    Post subject: Reply with quote

OK for an update, I have a running (and not backfiring) bike, but it now cuts out after about five minutes.

Fitted a brand new spark plug which seemed to fix the backfiring - it started after a couple of kicks, the engine didn’t backfire but it didn’t run crisp either and was bogging down (which I put down to using too much oil in the cylinder when rebuilding the engine) took if for a spin after about two minutes it cuts clean out.

Checked the spark and there was still a spark there (the plug was bone dry), fuel still coming out of the tank and removed the carb drain screw and fuel came out so there was fuel in the carb when it cut out.

Have tried fitting a different carb (again stripped down to ensure that it was clean) and a different CDI and same again – starts and cuts out after a couple of minutes.

Anyone care to offer a solution?
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steven_191
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PostPosted: 14:53 - 03 Apr 2010    Post subject: Reply with quote

does it cut out if you just let it idle for 5 minutes?

maybe check your air filter is clean.

you could try testing that the spark is consistent. or make sure the plug is gapped properly.

the engine isnt siezing is it? just cutting out?
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eddie_46
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PostPosted: 18:19 - 03 Apr 2010    Post subject: Reply with quote

Try setting the carb up proper, adjusting air and fuel, untill it idels nicely, then take her for a spin and see.
A poorly adjusted carb can cause a lot of problems
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gwernybwch
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PostPosted: 19:04 - 03 Apr 2010    Post subject: Reply with quote

Thanks for the replies.

Quote:
does it cut out if you just let it idle for 5 minutes?


It won’t idle on it’s own, but yep, pretty much the same after a couple of minutes regardless if I try to take it easy or immediately tear up the road.

I forgot to say that that I have cleaned the air filter as well and this morning changed over the fibre reeds – but still the same problem.

Quote:
Try setting the carb up proper, adjusting air and fuel, until it idles nicely, then take her for a spin and see.


I’ve set the pilot air screw and the oil pump to the correct settings as per the manual.
I’m assuming that I should try to adjust the carb when the engine is warm?
Trouble is it won’t run long enough to warm up and even in those five minutes it isn’t running smoothly!

One other thing that I have found that it does run better with the choke on.
Based on that, I guess that it could be running either too rich or too lean, which way should I be turning the pilot air screw – in or out? And by how much? A quarter of a turn a time?

Quote:
you could try testing that the spark is consistent

How do I do that?

The one electrical piece that I haven’t changed is the regulator / rectifier. Would this cause the problems?

Thanks for the replies, this really being to get to me now!
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sumpgard
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PostPosted: 21:23 - 04 Apr 2010    Post subject: Reply with quote

try raising the needle one notch at a time this should cure it mine did the same ran rough with the choke on and cut out with the choke off check the colour of the plug once you get it running properly .
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badas
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PostPosted: 22:39 - 04 Apr 2010    Post subject: Reply with quote

as far as i am aware you can setup your carb (according to the manual settings) when the engines cold, just take note somehow how the carb is for it to get going if there is no responce making changes back to manual settings.

is the battery fully charged?

if the choke is making no difference what so ever i would have an incling that something may be wrong with the carb somewhere :S

what about the engine timings too are they right according to a manual?

all advice i am giving is from my own issues i had when i first had my mtx and an old tomos 50cc

but worth checking the user manual etc

p.s. is your fuel clean in the tank there is no water in there?
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gwernybwch
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PostPosted: 22:26 - 11 Apr 2010    Post subject: Reply with quote

I left the bike a week after last weekend when it had cut-out and refused to start.
Today took the plug out gave it a wire brush down, put it back in, turned the pilot air screw ¼ a turn OUT from the recommended, and hey presto fired up first time and sounded as crisp as it has been and spend two miles absolutely gunning it.
Before it cut out again……….

Each time that I take the spark plug out, it is bone dry.
Is this right? Shouldn’t the plug from a bike that has just been running be a little damp from un-burnt fuel?

Spent the rest of the afternoon with the same story – bike cuts out, clean plug / engine cools down - try a different carb setting, bike runs for a few minutes before cutting out or back firing (and then cutting out)

Any other possible solutions?

Can anyone explain what the different carb settings actually do?
I.e. if you turn the pilot air screw out, does that give more or less fuel? Similarly what does dropping or raising the needle height do?
How much should I be turning the pilot air screw when I’m trying different settings – 1/8, ¼ or a ½ a turn?
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Kaph
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PostPosted: 11:30 - 12 Apr 2010    Post subject: Reply with quote

gwernybwch wrote:
I left the bike a week after last weekend when it had cut-out and refused to start.
Today took the plug out gave it a wire brush down, put it back in, turned the pilot air screw ¼ a turn OUT from the recommended, and hey presto fired up first time and sounded as crisp as it has been and spend two miles absolutely gunning it.
Before it cut out again……….

Each time that I take the spark plug out, it is bone dry.
Is this right? Shouldn’t the plug from a bike that has just been running be a little damp from un-burnt fuel?

Spent the rest of the afternoon with the same story – bike cuts out, clean plug / engine cools down - try a different carb setting, bike runs for a few minutes before cutting out or back firing (and then cutting out)

Any other possible solutions?

Can anyone explain what the different carb settings actually do?
I.e. if you turn the pilot air screw out, does that give more or less fuel? Similarly what does dropping or raising the needle height do?
How much should I be turning the pilot air screw when I’m trying different settings – 1/8, ¼ or a ½ a turn?


Could be running a little lean, hence why throwing the choke on makes it run better. Would also explain why the spark plug was a little dry.

No idea about the carb settings I'm afraid.
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Old Thread Alert!

There is a gap of 343 days between these two posts...

gwernybwch
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PostPosted: 20:49 - 21 Mar 2011    Post subject: Reply with quote

Hey all. Thanks for replying and giving your suggestions.
I thought that I would report back.

After spending several days trying all kinds of variations to the carb set-up - well it had to be something to do with the carb right, as I had tried a different stator plate and a different CDI unit but with the same results - I put the bike back in the shed and that is where it stayed for the winter.

I got it back out last week and decided to have another look at the electrics. I decided to send the stator plate off to West Country Windings for a test. The test came back as a slight drop in output, but nothing significant. I was told that the wiring gets old it sometimes fails when it gets hot. To be fair the wiring did look a bit ripe, so agreed for it to be rewound. The plate was sent back next day, I fitted it - and - still backfiring Sad

The last thing to try was the CDI unit - but reading about them is that they rarely fail and mostly never work incorrectly - and I had already tried another but to no avail.
Fitted a new CDI unit and hey presto, one running motorbike!!!!!

When I've got a minute I write a 'show and tell' on the rebuild of the bike.

Cheers.
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Old Thread Alert!

The last post was made 3 years, 179 days ago. Instead of replying here, would creating a new thread be more useful?
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