Mk3 Flatspot

[pmac]

Took my bike out for a spin today, after rebuild and it now has a terrible flatspot.
1st 2nd & 3rd - fine, pulls and revs all the way, but if you are in 5th or 6th at about 50mph and open it up, it starts to pull, then goes all flat and wont pick up (almost starts to splutter) If you slip the clutch it will go past it and rev fine, likewise if you scream it through the gears its fine also. When you are in the flatspot, the only way to get through it is to either change down or slip the clutch, it will not pull through it on its own.
Has anybody else had this problem or similar?
I think its a carb/fuel problem, the carb has standard jets and needle position. Can anybody recommend jet sizes and needle position, or will I have to get several jets and just trail and error?
The bike is standard Mk3 with a full DEP pipe, Aktive reeds, Moto air filter, and a platinum plug. The carb is set up as Honda intended. It is also running 14/45 sprockets.
It did have a slight flatspot before I fitted the new parts, but nowhere near as bad as it is now.

Hope someone can help!

Thanks

pmac.

[fallenmikethebike]

Try checking the end of the ht. lead. U nscrew it of the plug cap look for any white deposits .if possible shorten lead back to clean copper and screw back on the plug with a dab of silicone on the lead only to aid a watertight seal.
You may end up having to buy a joiner from car parts shop and a short length ht lead to obtain a good connection.

good luck
Mike

[Davey Munroe]

It's the air screw on the carb. Turn it in all the way - and then turn it back one full turn. This is a good starting point. Take the bike up the road opening the throttle getting lots of revs, and you should feel the bike stutter. Stop, open the airscew another quarter turn and try again. keep doing this until the bike goes smoothly into the power band. I've just done this on mine. It's great when you get it just right I just fitted dep pipe and new dual reeds.

[pmac]

Thanks guys,
Tried both suggestions, but it didn't seem to make any difference (the mixture screw slightly improved it, but the bike still would not pull through the flatspot on its own).
Ive had all kinds of stories about upping the jet sizes, and raising/lowering the main needle to sort it, ( some kind soul even advised me to scrap it !).
Has anybody had to change jets or alter the main needle settings on a Mk3 when fitting reeds , air filter and an exhaust?.
The bike has a NGK B8EG plug, which LT sold me saying it is the correct one.
Any advice greatly appreciated.

Thanks
pmac.

[knackeredMk1]

With the modifications you have made you might have slightly changed the jetting requirements. Try playing around with the pilot jet and the needle level.

[pmac]

Thanks Knackered

Any ideas where to start, shall I get the next couple of jet sizes up and just trial and error, or shall I start with the main needle first ?
I have checked the power valve and that is moving freely and is set up as per manual.

Thanks
pmac

[Davey Munroe]

I still think it's your airscrew. I'm back week after next if you wanna bring your bike over and check it against mine. This is if I have the clutch fixed by then?

[pmac]

Thanks Davey

If I can't sort it before you get back, I will take you up on that, and pop it over.

Thanks
pmac.

[fallenmikethebike]

The symptoms you describe would indicate an excess of fuel in the mid range[ rich] The engine is getting more fuel than it can combust.You may be able to confirm this by running it up to the problem area ,and flicking the choke on,if its running lean the choke being on should enable the bike to rev a bit more cleanly,however if no appreciable difference is felt or it then runs worse, that would probably confirm your suspicions that its running rich.Its an absolute pain in the proverbial, but you need to alter it one clip position at a time .Try all the positions on the current needle before altering the main jet.

Have you tried doing a plug chop at the problem area ? that may give you a really big clue to what's going on.

[pmac]

Thanks FMTB

I will give it a go when I get a chance.
I read on here how to do a plug chop, but I can't find it now. I understand plenty of throttle for about 5 mins, then kill engine, pull in clutch, pull over and whip the plug out. I am unsure of what the different conditions of the plug means, and what plug condition I should be looking to acheive.

Thanks,
pmac

[SkidMark]

well, you just ride along at near full throttle, hit the kill switch and pull the clutch in, when you've coasted to a stop pull the plug to see how your running. Sounds like I know what I'm talking about don't it... See:
viewtopic.php?f=6&t=3802&p=22562&hilit= ... %2A#p22562

[BANSHEE 60]

Try disconecting the gear position sensor.Their is a plug under the tank with 3wires which
come from the generator housing.Ithink the cdi retards the timing after 3rd gear.Ihad 55 pilot jet 160 main.CR FMF pipe with shorty end can.

[pmac]

Thanks for all the advice guys, all taken on board.

Went out today and attemped a plug chop (christ them chrome Dep's get hot !). The electrode seemed biscuit brown, but the rest of the plug seemed very "sooty". I will try and post some pics up of the plug - Second and third opinions are very valued.
http://i247.photobucket.com/albums/gg14 ... ug1003.jpg
http://i247.photobucket.com/albums/gg14 ... ug1002.jpg
http://i247.photobucket.com/albums/gg14 ... ug1001.jpg

Thanks again for all your help on this one - I will try that plug Banshee - Has anyone else cone across this gear sensor ?

I will keep you updated

pmac.

[NaBz]

your trying to take them pictures to close ure camera cant focus that close try a little further away