Flatspot help please!

[Ade067]

I've got a slight problem with my Comet 125,I seem to get a bad flatspot when slowing down for a roundabout or making a turn etc,the bike takes a good couple of seconds to pick back up again after trying to accelerate away,usually with a nasty snatch to the transmission which is a bit worrying to me.When the engine is warmed up it isn't as bad but it is still there and pulling the choke out seems to help a bit but I shouldn't have to still use the choke after riding the bike for a good 20 miles or so surely!

It has done it ever since I bought the bike and when I took the bike back for its 1st service the mechanic there said its because I'm not keeping the revs up enough but that just seems like bullshit to me to be honest and in any case I have tried to keep the revs higher by using a lower gear and it doesn't really make much difference.

I've had this problem with cars before and have usually cured it by spraying some carb cleaner into the carbs and then giving it a good blast through with an airline but thats with knackered old bangers that have been around the clock god knows how many times and not a new vehicle with only 3500kms on the clock!

My bike is still under warranty so I guess they should fix it when it goes in for its next service if I complain about it but being fairly mechanically minded I was just wondering if there was anything I could do myself to try and fix it,plus it would give me more satisfaction if I done it meself!

Other than this hopefully minor problem the bike has been brilliant so far,I'm well chuffed with it.

Oh the bike has twin Mikuni carbs if thats any help.

Any help is appreciated please...

[Kickstart]

Hi

Chances are that it is running lean for some reason, and when you open the throttle it is leaning out even more.

There is a possiblity that there is an air leak after one / both carbs, but I would hope not on a new bike. Does Malb have the same problem?

I would guess that either the slides in the carbs are sticking or operating slowly (possibly by design to get it to run cleanly for emisions reasons), as I presume it uses CV carbs. Might be worth lifting the needle jets up slightly in the slides to richen the mixture a touch (or at least just check they are in the correctly notch).

All the best

Keith
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[stinkwheel]

I agree with kickstart, you could try raising the throttle needle a notch which should make the main-jet open up more at lower revs.

I suppose another possibility is carb icing, 'tis the season? Is the comet liquid cooled? Does it have a temperature guage? If so, how hot is it running?
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Ade067]

No its not water cooled,its air/oil cooled and has an oil cooler mounted at the front of the frame.

I did consider that it could have an air leak,there are 2 rubber boots below the carbs which are held on by a screw clip (like a jubilee clip) under the carb and a couple allen screws on the engine,I've tightened the clips a little but it hasn't made any difference.

I don't think it can be the carbs icing as its been like it since I've had the bike from new and has done it all through the summer.

I've sent a pm to Malb to ask if his bike has the same problem but I don't think he's been online yet to reply!

How do I check/adjust the needle jets?

[malb]

Yes my bike does do this occasionally, especially in first. When I asked the guy at the garage about it he said that it’s probably due to the fact that its a 4 stroke v-twin and they need a little more "winding up" before they go. I'm not really sure about that but that’s what I was told.

I'll try and keep track of when it happens on my bike, and I'll let you know! I think it mainly happens when I'm in to higher gear, but not always.
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[mr jamez]

Fuel surge? though I think that is more of a characteristic of the carb on my car due to the design, usualy down to a dodgy float. When you are turning round a fairly tight bend or are slowing down quickly there is no power and it slowly pulls itself out of it, like a flatspot.

[malb]

I've spoken to Ade067 quite a bit now, and I think we've kind of come to the conclusion that this is just a quirk of the bike, and hopefully nothing to be too worried about.

As I said I will keep an eye on the situation and report any findings, but so far it hasn't happened since the date this thread was started.

So for future reference for other comet owners, it will occasionally happen and the best thing to do is make sure you let the bike warm up properly before you set off, and make sure you are in the correct gear for the situation.

Cheers Malb.
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[Kickstart]

Hi

Sounds like they have really leaned the mixture off to get it through the emisions tests. I would be tempted to have a play with the needle jet. Either put the locating clip in the next groove down (if it has more than one groove), or use a tiny washer under the clip to lift the needle slightly.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[stinkwheel]

They are probably set up to run in Korea where the air temperature is up to 30 degrees. A colder climate can require a different needle position, this is one of the reasons they are adjustable.

My old Russian 125 had a summer and winter setting in the handbook which involved changing the gearbox oil viscosity, blanking off part of the airbox and moving the throttle needle. I never did it because I realised this was for when you are using them in Siberia.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Project9928]

I must say with my Comet 125 I haven't noticed this but I'm going out in a bit so I'll keep a close eye on it. Mine does need a lot of warming up in the cold weather to get it going in the mornings somaybe it is like stinkwheel says and might be that there not really good in the cold. I rember reading in What Bike? magazine that they think the Comet won't be fantastic in our winters. So it might just come down to the cold?

[Ade067]

I dont think its down to coldness to be honest,mine has done this all through the summer and even after a good thrashing for an hour or so,theres no way the engine can be cold then,I reckon its like others have said and is the mixture because if you leave the choke on slighty its absolutely fine and seems to pull much better.

How do I adjust the mixture and/or the throttle needle? I know lots about cars but bugger all about bikes

[stinkwheel]

Does it have a slide or a CV carb?

If the throttle cable goes in through the very top of the carb and the carb has a lid on top of it that looks like it unscrews it is a slide carb.

If the throttle cable attaches to a kind of rocker mechanism and the top of the carb is flat and screws down with three or four small set screws, it is a CV carb. CV carbs often have 'CV' cast into the body too...which is a bit of a giveaway.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Project9928]

Beats me then

The carb looks like a slide one from the picture in the manual its a BDS26 TYPE(DOUBLE) if thats any help.

Kev

[stinkwheel]

Well assuming it is a slide carb (probably is, most 125s are).

Unscrew the cap on the top of the carb by hand. Pull up. The throttle slide (a metal cylinder with a cutaway) should slide out with the throttle needle sticking out of the bottom of it. Be carefull not to get any muck on any of this, also take care not to bend the needle or damage the very tip of it.

Pull up the spring that goes up the middle of the throttle valve so it is flat against the cap and hold it there with your fingers. You should now be able to unhook the throttle cable from the slide (you'll need to have a look and see how it attaches). This can be a bit of a fiddle.

You can now look down into the top of the throttle valve. There will either be a small spring clip or a metal plate held down with tiny screws over the top of the throttle needle. Removing this allows you to push the throttle needle up and out. There are a series of grooves round the top of the needle, one of which has a circlip in it, move this down a notch then put everything back together and go try. This should provide the engine with slightly more fuel at 1/4 to 3/4 of full throttle.

When you put it back together, you will find there is a groove along one side of the throttle valve which slides over a small pin in the carb body to stop rotation. Make sure you slide the throttle valve back in perfectly square and that it slides all the way home. The cutaway should face to the rear. Take care not to damage the throttle needle, it needs to go down a hole in the body of the carb (the main jet). You should never have to force anything on a carb.

If it doesn't help, put it back where it was.

EDIT: Forgot, there will be two, you need to do both. If you can't see a bit to unscrew by hand on the top of them, they are probably CV carbs which I will decline to advise about due to lack of experience.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Ade067]

I havent seen CV stamped on it anywhere but I will take a look tomorrow,the throttle cable connects via a rocker type linkage like you described very similar to a normal car type carb.

If you check out the pics in the links below that I've found at bikepics.com you can just about see the carbs which may give you a better idea.

https://www.bikepics.com/pictures/104770/
https://www.bikepics.com/pictures/104779/

[Kickstart]

Hi

They look like CV carbs.

The bits you can see from those pics are the caps. That those off and you will find a spring (ping.....) and a slide with a rubber diaphragm. Pull the slide out and you will find the needle. Do with the needle as Stinky says then put it back together. Make very sure the diaphragm is correctly positioned.

All the best

Keith
____________________
Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[stinkwheel]

Ok, they look like CV (constant velocity) carbs. The throttle needle is attached to a rubber diaphragm, as the throttle is opened, the suction of the engine pulls the diaphragm up and opens the throttle. There is still a circlip in there somewhere to adjust the needle position but I don't know how to get at it.

That is about the sum total of my knowledge of them. I don't like to mess as I know the diaphragm is easily damaged.

stinkwheel
Knows his limits when messing with the property of others.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Ade067]

Brilliant.

Thanks for the help guys,I'll give it a go when I get some time to tinker.

If you see a thread entitled 'Help...How do I put my carbs back together' you'll know it wasn't a successful mission!

Watch this space

[stinkwheel]

Here we go.

This article just about covers everything and has good pictures.

I think yours are kind of turned on their side though, judging by that picture.

https://www.gadgetjq.com/keihin_carb.htm
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Ade067]

Oh great stuff cheers for that,who needs a Haynes manual eh

[Project9928]

I've been out on the bike all day today and its always before 6000rpm that the bike seems to be under powered and then it suddenly kicks in. Its like turbo lag in a car it doesn't kick in till a certain amount of revs. I think it might just be a power issue?

[Ade067]

Well like I've described mine just takes a couple of seconds to pick up again when accelerating away from low speed and when it does start to pull again it usually does it with a nasty snatch to the transmission which could be dodgy in the wet and it certainly isn't going to do the chain and sprockets any favours,I'll have a play around with the carbs when I get chance which probably wont be until next weekend now as i'm off banger racing tomorrow (saturday) night and I have to pick up another car on sunday.

[pipnet11]

Sort of related question (just) I thought CV on carbs meant constant vacuum?? is this me being thick?

Phill
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[Project9928]

Hummm well we'll see what happens after you've played with the carbs, o and good luck banger racing

[malb]

Yeah it would be interesting to know how both things turn out!
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