velocity stack - carb settings
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Hey Fast Eddie. Hope you're viewing posts. You are the first guy I've found experiencing the same tuning problem I can't fix. Not running a Norton, but having the same problem with FCR35s on a KZ1000. Your comment about the off idle stutter from the longer stacks may be very relevant. My FCR set was factory tuned for my engine using 1 3/8" velocity stacks, but I'm using adapters with K&N filters. The adapters are the same length as the OEM stacks, but straight. I'm wondering if the lack of taper of the stacks is causing an air flow problem killing fuel drawn from the pilot circuit just off idle. Could it be that the long stacks you had the same problem with were straight for the first 1 3/8" from the carbs, causing the same issue?
I've already discovered I had to go from 138 to 128 mains due to the filters compared to the stacks, but didn't think it would also have an effect just off idle with lower airflow.
You reduced the main jet size to compensate for the increased vacuum signal induced from the K&Ns (choke effect), The first thing I suggest is to turn in the idle air trim screws to richen the idle mixture, if it turns out the your FCRs have mixture screws instead (not real familiar with FCRs) then turn them out and lower the sides to regain proper idle RPM. You may need smaller pilot jets, if either of these methods continue to produce a stumble off idle.
If you are in the habit of grabbing a fist full of throttle on take-off you may have to lower your needles or seek a different profile.
Best wishes
Fast Eddie
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Well my issues were with simple old Concentrics, and the velocity stacks were not parallel anywhere, they were tapered. However, FCRs are a different ball game!
But I’d go with the advice above and look at the idle screw setting, but my gut feel would be to weaken it as stutter = richness in my mind.
Also remember that the needle has a very wide impact on these carbs and could easily be part of the issue.
My main thoughts however would be this: FCRs have accelerator pumps, so you can fine tune Jets all you like, but if the pump is chucking to much fuel it’s gonna override everything you do. I’d certainly have a look at that. If in doubt, disconnect it whilst trouble shooting and fine tuning jets. Then reconnect and adjust it afterwards.
All only IMHO of course.
But I’d go with the advice above and look at the idle screw setting, but my gut feel would be to weaken it as stutter = richness in my mind.
Also remember that the needle has a very wide impact on these carbs and could easily be part of the issue.
My main thoughts however would be this: FCRs have accelerator pumps, so you can fine tune Jets all you like, but if the pump is chucking to much fuel it’s gonna override everything you do. I’d certainly have a look at that. If in doubt, disconnect it whilst trouble shooting and fine tuning jets. Then reconnect and adjust it afterwards.
All only IMHO of course.
If a stutter = richness, a cough = lean-ness. If I had a stutter, I would change the exhaust system rather than the inlet tract. Actually, on a race bike, it is very important not to have a stutter. At 70 MPH, in the middle of a high speed bend, a stutter can make you go backwards.To my mind a pumper carb might remove the need to feed the throttle on, when the jetting is correct. In short, you can whack the throttle open quicker without having the leaning-off effect of loss of vacuum. The advantage might be more theoretical than actual. If you have ever ridden a two-stroke racer, you would never whack a throttle open and expect the bike to accelerate. Commandos are no different when they are correctly tuned. With a road bike - you don't usually have them in that state of tune.
It's a lean spot -- better hot. I've tried every combination of Pilot air and fuel. I can kill it going rich on needle diameter, but then I'm too rich at cruise which is the same throttle position (1/8), but higher rpm. Idle is strong. I'm going to install the stacks tomorrow to test. Another possibility is the filter adapters cover pilot and main air jets, so the airflow may be having an effect, possibly even creating a vacuum at the jets.
It's not the pump. Breaks up/surges at steady maintained 1/8 throttle, 1500-2000rpm, slow acceleration from a stop. Idle and above that spot is all good.
Cruise is 5th gear, 4500rpm.
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Filters off, stacks on, made no difference with the problem. Someone suggested I could be too rich on pfj affecting vacuum off idle. The set came with 45 pfj, and 1 1/2 turns paj. Right now I'm at 45 pfj and 1 1/4 paj. Should I go several jets leaner on pfj, the tune paj to get the right mixture at idle and see what happens there? Wish there was a set ratio of pfj to paj to work with.
Fast Eddie
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It was me that suggested it sounded rich, other said weak.
FWIW Norton’s normally run a 58 pilot jet. Maybe you are weak?
But, do you mean that you have only tried 1/4 turn on the air screw?
I’d try a lot more difference than that.
FWIW Norton’s normally run a 58 pilot jet. Maybe you are weak?
But, do you mean that you have only tried 1/4 turn on the air screw?
I’d try a lot more difference than that.
I've gone from 42pfj at 3/4 turns paj, to 52pfj at 2 turns paj. Maybe I'm missing the sweet spot but haven't found it. Which way do you think I should go at this point?
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