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?
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.
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?
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.
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...
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?
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.
Gday GM
I hit send before I read your post! Damn smartphone.
Is there any way to visually check that I have an adjustable slow air jet (before I try to adjust it)?
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.
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.
What needles do you have & what position are their clips?
Some info on what part influence performance:
http://www.thumperfaq.com/jetting.htm (http://www.thumperfaq.com/jetting.htm)
I never checked. I have to pull the airbox off again to look. Anything I could try before that?
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.
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.
It wouldn't idle initially but now idles fine after I move the idle speed knob. Should I have done that with the ims?
Thats fine, you got it running, so now do IMS adjustment and see if the idle rpm increases. As fast and strong as it will go then reset the idle speed knob to 1200-1300 rpm if it gets too high. Too rich or lean an idle mixture can easily cause the stumble.
I'm trying to get you to do the basic tuning before even having to consider friggin' around with accelerator pumps! APs are best left alone if you can get away with it. There's a world of pain in tuning those things. Fortunately, adjustable APs can be sourced from the MX scene. Don't worry about that.
Right, that makes perfect sense now. Thanks both you guys, I'll let you know how I go!
Throttle sync is cruisal, 1mm off is quite noticeable in response, FHE.
Second hand carbs could be dirty/worn, i e inadequate working.
Good luck, keep us updated!
Carbs are from jv, and they are CLEAN. I'd say they were new if I didn't know better. Do I check the sync at the carb throat? Between the slide and the throat?
I took it for a ride and HOOOOOLY CRAP! The difference is insane. They ride real nice.
Do you have a manometer? If you don't you can use the drill bit method. Light drag between the slide and carb mouth. Is that stumble still there?
I do but I think I don't have the right fittings to fit the manifolds. It it just me or are they different thread sizes on each manifold. There is still a stumble, but only if I crank the throttle really fast. Is that normal?
Should be the same. They are little flat head screws on the outside of each manifold.
With very fine tuning you can eliminate any stumble, but it is necessary to experiment with small changes to the slow air jet and IMS. Focus on any stumble under load when riding and asking for power which is more important than blipping while the bike's sitting in neutral.
Well synched carbs certainly help with crisp throttle response.
Drill bit sync works good enough.
Now, w 1.5 turns on air screw, how many turns on idle mix screws?
Can I drill bit synch from the outside? I don't really get what I'm trying to achieve. Is it just testing that the edge of the slide to the inside of the barrel is the same on each.
I think I ended up at 2 turns out on the fuel screws.
Yeah, you want the same gap or clearance so the slides open simultaneously from the
same start point.
In regards to the idle mixture, it idled fastest at 2 turns out?
The idle speed wasn't changing much, but that's where I felt I was getting the best response, and where the revs were dropping back to idle speed the fastest. This stumble is really only if I suddenly jam the throttle more than half open. I guess it's just getting way too much air. I still don't think it's 100% right.
Sounds like you've got it close. Might just be the synch in it.
Yeah I'm certainly happy with it for now. I might ride it for a bit and make some notes, then sort the sync next time I have time to pull it apart. Thanks for your help, I had a dumb grin on my face for the whole ride.