Bike running rich & re-jetting questions

MarkJ · World Chat Champion Joined: 11 Apr 2006 Karma :

Took my bike to have its electrics fixed yesterday, which is now all sorted. But when I went to start it up it refused. The starter motor was spinning fine, but the engine just refused to turn over (at best, it would sound like it was turning over for half a second then stop). So the guys came out and we bump started it, which got it going, so they went back inside and all was fine. Then as it was idling away on the stand whilst I got my kit on, it just died. No spluttering or anything, it was just like it went "Sod this, I can't be arsed!" and turned the engine off. One bump start later and it did the same again - idles for 20 secs then dies.

So the mechanic comes out and tells me that he thinks it's running rich. Him saying this reminds me that when I bought the bike it hade a Scorpion race can on it, which probably means it was re-jetted for the can. So he reckons I need to get the bike re-jetted.

So my question is: How much would it cost for someone to re-jet my bike? I've had a look at an explanation on the internet and it looks like a job that wouldn't be cheap. I'm inclined to do it myself but I have a basic knowledge of how a bike works at best.

Or is the bike not running rich at all, and it's something different all-together (although someone else has said it sounds like it's running rich to me)?

code guru · Brolly Dolly Joined: 25 Jan 2006 Karma :

if u know wot carb your bike had and wot jets u need its only a 5 min job 2 change the jets and jets r cheap anyway. have u checked the color off your spark plug 2 make sure its rich?

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

It is possible, but I would doubt it would make that much difference.

How long has the bike been used since the can was taken off? Doubt it would have needed the jetting playing with that much for just a can.

I would be inclined to look elsewhere for the problem first.

As to what is required when rejetting, there are several fueling circuits in the carb (following assumed a CV carb). Basically a circuit to provide fuel at idle speed, and this can typically be adjusted with a screw somewhere. Through most of the rev range fueling is controlled by the needle jet and the carb slide. Air pressure in the carb moves the carb slide, and the needle attached to that is tapered. As it is lifted higher then narrower parts of the needle are used, allowing more fuel to flow. These are adjusted by moving the needle so it sits higher or lower, or swapping the needle for one with a different profile, and by changing the spring above the slide so it moves differently with changes in air pressure. Lastly the main jet provides the ultimate limit on fuel flow, and this is adjusted by substituting it for a different one.

All the best

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

MarkJ · World Chat Champion Joined: 11 Apr 2006 Karma :

The bike's had the can off for a month, which is the length of time I've had it (the original can was put back on when I bought it). Also, The guy at the shop turned the idle speed up to stop it stalling, and now sits at just under 2k, yet it still stalls. Do you think it could be anything to do with the battery?

Fadel · Trackday Trickster Joined: 06 Feb 2006 Karma :

I have recently had the exact same prob. Kickstart fine but wouldn't electric start from cold.

Then I had to take my engine out and since I have put it back in it seems to work fine on electric.
I have found my bike cuts out while idling only when its really cold and you don't give it abit of rev before you go to put your kit on.

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Well if it had been OK for a month and then suddenly started stalling it is unlikely to be jetting for the can.

What was the electrical problem they fixed?

All the best

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

MarkJ · World Chat Champion Joined: 11 Apr 2006 Karma :

MarkJ · World Chat Champion Joined: 11 Apr 2006 Karma :

I've just done a video of the problem here: https://media.putfile.com/Bike-starting-problem

The bike starts up first time with no choke and no throttle (my dad's told me to start the bike up with throttle) which works fine. as I rev it the second time it cuts out. The fuel tap is on and there's plenty of fuel in. I try it after that and nothing happens with choke or anything.

Mum came to see what was happening Laughing

Any ideas?

Edit: The video is 10mb, so give it a few minutes to download. It says "Connecting" at the bottom of the video, but it's actually downloading the video.

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Strange. I would not expect the mixture being a bit out to cause it to do that, and from what you say it had been running fine for a few weeks with the jetting like that.

Be tempted to whip a plug out and check it it sparking. I notice that when it isn't starting the rev counter isn't moving at all. I would expect it to move a bit (unless the stop position on the rev counter is ~1000rpm).

All the best

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

Fadel · Trackday Trickster Joined: 06 Feb 2006 Karma :

Yep, that's EXACTLY what mine did.

I really don't know what I did though, it must have just been something I did when I put the carb back in last time as now I took it out again it works fine.

MarkJ · World Chat Champion Joined: 11 Apr 2006 Karma :

Hmm, strange. I tried to have a look at the spark plugs today, but I couldn't get the tank off because the fuel line from the bottom of the tap kept catching on something which wouldn't let me swing the tank up very high. And I didn't want to have fuel going everywhere.

The fuel line didn't have one of those small metal tie things round it, will it be OK? Shall I just pull this fuel line off and stop being a big girl about it?

Thanks for your help guys Thumbs Up

Edit: Do you think it might be something to do with the fuel filter? A while back I ran out of petrol and it still wouldn't start when I put it onto reserve, could this be related in some way?

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :

Hi

Should be a clip, but not essential for it to run.

Presume it is a vacuum fuel tap, and if so it will not allow fuel to flow unless the engine is running (unless it is on Prime). That is probably why it took ages to start from running out and going to reserve.

All the best

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