Valve clearances ?

 

I’ve checked them and they are fine

 

if its lean it can cause it to pop and bang on deceleration .

drop the clip on the needle or fit 1 size bigger pilot jet and it will most probably cure the issue provided theres no air leaks

 

Agreed

 

Air leak on the inlet from when you put the carb back on?

 

Pilot jet sizing or an air leak on the carb boot, has a tendency to split under the jubilee clips, easy to miss unless you take them off the boot.

 

A guy on an SWM who ride with us on a newly serviced bike yesterday had popping and banging from the exhaust for a while until it finally gave up. The pipe from the throttle to the engine hadn’t been clamped up properly so it was running horrendously lean.

I had a similar thing with a vacuum Scottoiler on an old Triumph that hadn’t been sealed into the manifold properly and some of the overrun explosions were huge.

 

Fuel screw best on these at 2 turns out.

 

Right, had a good check of the inlet rubber and it looks ok. I’ve reseated it and tightened, still the same.
I think the main jet may be a little rich as my spark plug is a bit sooty. Again it’s stock 178 I think. Needle clip position is in the middle.
I can only assume the pilot jet is ok as it ran ok before, it’s a stock 42. Fuel screw is set at about 1.5 - 1.6 turns so I’ll try with 2 and see what it does. Not the easiest to adjust when engine is hot even with a fancy extended screw. What’s the trick my hands are just too big to get in the space.

I did find the silencer to header joint not quite seating square but its still the same even when realigned.

Could it be ignition timing?

Cheers Lark09

 

Buy one of the fuel screws that have a knurled knob on them, you can adjust them easily by hand. Less than a fiver well spent.

 

That’s what I have, I think it’s a JD one, but I can’t get my hands in without burning my self. Really bloody awkward.
Guess I need a gynaecologist.
I’ll have another go tomorrow and try a few more things.

Many thanks

 

So just tried messing with the fuel screw and engine falters at about 1.3 turns out and recovers at about 1.5, I then wound out 2 and there is some improvement in response but still popping a bit. I then went to 2.5 again a little better but was expecting rpm to drop but didn’t then out to 3 turns
No difference. Still popping and response much the same.
So does that mean I’m running on just the jet size with no fuel screw reduction?
My needle is on the middle position which I think is richer than stock.
Do lift the needle up one more or fit a 45 pilot jet?
Unfortunately, the missus was moaning that I was making too much noise so activity curtailed tonight or I would have tried the needle.

 

Aside from jetting, if a plug and plug cap is over 40/50 hrs it can cause running problems.

 

This is very true but if it’s the plug cap it’s usually fine when cold but as the bike gets hot the popping and banging gets much worse and eventually will hardly run at all.
New plug cap is a good idea anyway.

 

I’ve had this problem and it can be difficult to solve, I did eventually do it with carb cleaner but in the end it would probably have been quicker to get the carb ultrasonically cleaned.
Anyway
Check all the circuits in the drawing, it’s possible to have the slow jet (pilot jet) clear but the passageway above it or the air supply to it blocked. Sounds like your pilot screw is working ok but the slow circuit not.

 

This is usually the issue for me... [emoji1360] Carb cleaner and compressed air, do it a couple of times but don’t go too mad or you’ll swell the seals on the slide and it will jam up.

 

Could always try a treble dose of 'petrol magic' first .....

 

try both, also the jets are quite hard to entirely clean so treat yourself to a new 42, 45 pilot and also do a test with the needle in the richer position.

remember to only try 1 thing at a time otherwise you'll be chasing your tail with it

 

Thanks there’s some good stuff here. I replaced the plug cap about 10 hrs back when I had an issue with the previous one so I doubt it will be that but worth a shot.
The carb was cleaned and rebuilt which is where this all started, there was some muck in it and the float valve seat O ring was leaking so I replaced all the seals. It was ultra sonically cleaned so I would hope it was clean however we all know stuff does get in.

So I will try the needle first and see how we go. Fit new plug cap. If that doesn’t work I’ll strip the carb again and re clean. If that doesn’t work then I’ll try a 45 pilot jet. Then I’m screwed.

By lifting the needle where does it richen? 1/8 upto 1/2 throttle?
As a matter of interest the exhaust is an Ackropovic would that have any bearing on the situation.

Cheers Lark09

 

This will give you a better idea...

Just make sure you give this part a good clean, it can take a couple of goes to dislodge and dirt or stale fuel.

Bigger bore and / or more free flowing exhausts can make it lean across the range... Pilot is less sensitive in that regard as long as you get it idling correctly and jetted in the right range for temperature and elevation. Should only ever be a jet size or clip setting from there. Check it again for air leaks when adjusting the clip height (lower - richer).

 

An akra will be a bit more vocal than an oem can. It will also therefore more readily 'show' associated fault/s. Are you sure that the pipe/can junction is good? You will need to use sealant here to prevent air leaks.

 

Thanks for the carb info very useful and explains nicely.
I reseated the can /header pipe joint which did make a slight improvement. I am not using any sealant and wasn’t prior to all of this. The joint is held together with a spring.
I did think about using a 50mm O ring in the can/pipe junction but haven’t tried that as yet. Not too sure if the temp might be a bit high. I guess a viton seal would be the best for temp.
My 200 exc exhaust had O rings at the barrel end and worked ok.
I’m away on hols now so I’ll have another go in a weeks time.

Appreciate the help
Lark09

 

Try an overdose of petrol magic, works wonders. Drain the tank then put a litre back in with 50ml of any fuel system cleaner.
Spray around the inlet rubber with brake cleaner, see if the revs change.
Or just turn up the idle a bit and go ride it

 

You need to be careful with viton o rings, if they melt and the goo then gets on your skin (even when cold) it will eat into your skin, the only way to stop it is to cut the skin away!

Just did a quick google, viton gives off Hydrofluouric acid when burnt.

 

No room for an o ring there anyway ..... high temp sealant territory that join!

 

Will give the petrol stuff a go.
Any particular brand of sealant recommended?

 

Red instant Gasket Maker RTV silicon high temp instant sealant water oil resists | eBay

K2 instant gasket sealant high temp sealant 20ml.

www.ebay.co.uk

That'll do you.

 

As a matter of interest what are the green o rings made of that are used on the 200 exc exhaust I thought they were Viton.

 

They may well be made of viton but the whole power valve assembly on the 2 strokes is water cooled which means that the exhaust pipe temperature is going to be much cooler than a 4 stroke exhaust. I'm pretty sure that the actual exhaust gas temperature on a 2t is cooler anyway than a 4 banger.