FCR Tuning help needed

[Howley]

Hi All,

I just put some second hand FCR 41's on my '94 900 with open airbox lid and Open Exhaust.
It idles fine but when I 'blip' the throttle it stumbles.

I have:

Fuel mixture screw  3/4 turn
Float height  9mm
Main Jet  155
Main Air Jet  200
Slow Jet  60.

What do you think it needs? It seems the accel. pump doesn't really 'pump' much, probably only moves a couple of mm in total. Is that normal?

[koko64]

How many turns out are the slow air jet screws? They will influence throttle response from a closed throttle. The IMScrew can also affect this if its out.
The default setting is 1 1/2 turns out. I use a long, turned down screw driver to adjust them. The left hand jet facing the carbs.
Some people have found the 60 slow fuel jets a bit rich and boggy with plug fouling, but if your plugs are good color at idle they should be fine.

Check the accelerator pump to make sure no parts are seized and the linkages are freely operating. You can check for the water pistol squirt when you crank the throttle with the filter out.

Patrick Burns tuning guide is brilliant and can be accessed via the Cal Cycleworks site on FCRs.

[Howley]

Thanks Koko. Couple of things.
I'm not sure about the slow air jets, but looking at them, they just look like jets. If I look in both holes they look the same, any way to tell if they are the adjustable ones? I don't have a tool to adjust them.
Are you able to measure how much the AP moves in total?
Should I check for the 'squirt' when it's running or off?

[koko64]

Facing the carbs, it is each left hand hole. See if they are on the default 1 1/2 turn. You will possibly be able to hear the spring as you screw it down, there is little resistance just spring pressure. Did the previous owner get the carbs from Chris Kelley? Because it should have the adjustable screw jet. If it is a fixed, "solid" jet then buy the adjustable ones from Sudco.

You can have the bike running and make adjustments to the slow air screws which lets you hear the revs pick up or faulter. It will take some testing to get it to your liking, thats why the adjustable jets are worth it. So you will notice the idle speed change when you adjust these screws, but the main aim is to get that crisp transition when opening the throttle and smooth running below 1/4 throttle.

Read and re-read Patrick Burns tuning guide. The man deserves huge credit for sharing his knowledge with us. I owe him a few beers, thats for sure.

If the motor is cold and you want to prime the carbs in lieu of no choke to start it then give the AP a good squirt. If its a warmed engine then have the bike running to avoid flooding and popping/backfires.

Let us know how you go.

[Howley]

Thanks again mate.
A question re: the AP. From what I can see when you turn the throttle, you are not physically moving the pump, you are just moving the stop and letting the spring tension turn it. When I turn my throttle, it only 'pumps' for a few mm then the stop moves away without the pump moving. It seems like there is no way it could pump anything if it's only moving a couple of mm like that. I'll have to look into it, looks like I'm taking the carbs off AGAIN...

[greenmonster]

To avoid too many variables, start tuning w/o AP.
It works very well w/o it, in fact I haven`t connected mine still after many years.
Several friends run w/o it, too.
You`ll get a squirt f start w/o it, too, just twist throttle.

The AP shaft just pressurize a diaphragm a few mm`s f function.

+1 on advice of adj low speed air jet & 1.5 turns out on that.

Your jetting is fine, although idle may take some fiddling.


What needles do you have & what position are their clips?

[koko64]

The pump tod moves only s few mms under spring pressure. Look for the squirt with the filter off. You would be surprised to see how much fuel squirts from such a small movement. Give the spring area good spray of WD40.

Try and deal with off idle stumble via the slow air jet and idle mixture screw.

[koko64]

Gday GM
I hit send before I read your post! Damn smartphone.

[Howley]

Is there any way to visually check that I have an adjustable slow air jet (before I try to adjust it)?

[koko64]

It should turn with finger pressure if its adjustable. To try and see a difference would mean taking off an alloy air box adaptor. Give it a clockwise tweak and you'll hear a spring compress or feel it through the screw driver.
Set them at 11/2 turns if they aren't. Gently bottom the screw and then turn it out.

[Howley]

Ok, so I set the slow air jets at 1.5 out. Stumble is still there, and also I can't see any AP squirt.

[greenmonster]

What needles do you have & what position are their clips?

Some info on what part influence performance:
http://www.thumperfaq.com/jetting.htm

[Howley]

I never checked. I have to pull the airbox off again to look. Anything I could try before that?

[Howley]

Ok so I checked again and I am getting a squirt from the AP. It sprays straight down the throat, is that right? If anything a little more on the side furthest from the pump itself.

[koko64]

Sounds ok. I wouldn't worry about that now.

You can fine tune the idle mixture following Patrick Burns' guidelines and this is one way to deal with the stumble. Go for the strongest, fastest, most even sounding idle.
After that, small adjustments to the slow air jet may resolve it. 1 1/2 turns is a good default setting however.
After that, a carb synchronization may hold the solution, if it is out. Checking is pretty easy, adjusting more involved.

So basically you have fitted the best carburetor technology available and now have to give them a comprehensive tune. Have a good read of Burns' tuning procedures, it's gold. If it's too much mucking around and you don't have time, don't discount a known carb specialist with a dyno doing it for you.