Got a question about my fcr 39s that something doesn't seem right. Plugs look rich. Mixture screw wouldn't kill engine with a 60 pilot if I screwed all the way in. I've dropped to a 58 and now 55 to try to get the right mixture screw now if turne in it will slow it down after a few seconds almost killing it. All other mixture settings don't seem to have that great of an impact. My slow air screw needs to be turned in less than 3/4 turn or it will cough spit and die if farther out. All the way in will kill the bike. The stock setting is 1.5 turns out so this seems like it's needing to be turned in too far. IM missing something here I think. Plugs look rich but can't use the air screw to lean it out anymore or it will die. Mixture screw is just not making much of an impact. I checked my intake vacuum and both cylinders are close but I did notice something interesting and have no idea if it's normal. When revving the horizontal vacuum won't drop as much as the vertical and the vertical takes a longer time to recover to idle vacuum. Is this normal or should both cylinders Have same vac at all times?
Any ideas?
Thanks
Quote from: svr on April 02, 2010, 09:31:08 PM
Got a question about my fcr 39s that something doesn't seem right. Plugs look rich. Mixture screw wouldn't kill engine with a 60 pilot if I screwed all the way in. I've dropped to a 58 and now 55 to try to get the right mixture screw now if turne in it will slow it down after a few seconds almost killing it. All other mixture settings don't seem to have that great of an impact. My slow air screw needs to be turned in less than 3/4 turn or it will cough spit and die if farther out. All the way in will kill the bike. The stock setting is 1.5 turns out so this seems like it's needing to be turned in too far. IM missing something here I think. Plugs look rich but can't use the air screw to lean it out anymore or it will die. Mixture screw is just not making much of an impact. I checked my intake vacuum and both cylinders are close but I did notice something interesting and have no idea if it's normal. When revving the horizontal vacuum won't drop as much as the vertical and the vertical takes a longer time to recover to idle vacuum. Is this normal or should both cylinders Have same vac at all times?
Any ideas?
Thanks
Firstly, by one turn out do you mean 180 degrees or 360 degrees? One turn to me is 360 degrees. (Just thought I'd check).
If the idle mixture screw is now having an effect you are on the right track with selecting a slow fuel jet. You may try a smaller slow fuel jet if best/strongest/fastest idle occurs at less than 1 turn out. That is unless you want to err on the side of richness for Winters sake. You must at least be getting close because you can now nearly kill the motor by turning the screw all the way in (bottom it lightly only). You should be able to kill the motor with the IMS turned all the way in or else the slow fuel jet is too big/rich.
Sometimes the strongest idle is a little slower than the fastest with a bigger 'BOOM' from the exhaust without it getting 'fluffy' and 'farty' which would be getting too rich again. The very fastest idle can sometimes be a little weak and a smidgen lean. You really gotta listen for it and us older guys who have tuned all sorts of carbs over the years know what I'm talking about. I'm no expert just over 40!
Note that the vacuum can vary between cylinders at idle and even out at 2000rpmish because the long manifolds aren't quite symmetrical. Mine have been cleaned up and idle very closely and I made sure that the vacuum was the same at 2000-3000rpm. Watch for air leaks on the vacuum hose fittings which will throw your readings right out, driving you nuts. You don't want perfect vacuum readings at idle and shithouse ones at 2000rpm because you'll hate the off idle response.
The asymmetrical manifolds often like a different screw setting on the idle mixture at idle speed. Check whether yours do. It can (believe it or not) be as much as a turn or so difference. (Brad black demonstrated this to me with his EGA readings).
Mine are close enough because the manifolds are ported and rubber seals trimmed to match the manifolds and carb spigots (if you wanna be pedantic). Mine required 1 1/2 turns difference before I trimmed the manifold rubber on one cylinder which had protuded into the passage. (DIY budget blue printing stuff if you are obsessed with it being 'just so').
A manifold air leak or one carb needing more gas (idle mixture) can make that cylinder 'hang up' on idle.
I wouldn't freak if the slow air screws need to be in 3/4 turn from gently bottomed. Get that slow fuel jet sorted first and then play around with the slow air screws to adjust your off idle response to your liking. I found that if the slow air screw was too far in or out, there would be a pause/hesitation when I opened the throttle from closed. Too far in made it jerk like drive lash or jerky fuel injection and too far out made it lag in a softer/too smooth/too lean way.
Welcome to the black art of carb tuning. (Not really, you just gotta be stubborn and try adjustments in one direction or another untill it works). [bang]
Keep us posted.
Cheers
If you still don't make any progress with sorting those carbs you could consult a real expert.
I was looking at Patrick Burns "Keihin Flatslide Tuning Manual 3.3". There is a link to it on the Cal Cycleworks website. Its worth it's weight in gold. He explains how to tune each circuit and has verified his approach with dyno time. Mr Burns has been very generous with his knowledge in that manual. It's a real tuning bible for FCRs.
You can look it up and get detailed tuning tips from one of the States' eminent tuners. Despite his expertise, he displays great humility and candour as to how he arrived at his findings.