Discussion in 'The Garage' started by Highside, Oct 20, 2011.
No... you haven't acquired the skill yet.
Gents, tests like the WOT to 7/8 roll-off yield symptoms that indicate possibilities, not hard conclusions. If a momentary surge results from rolling off to 7/8 throttle from WOT, this is a symptom of a lean condition because quickly lowering the slide a bit results in less air being fed to the engine, momentarily richening the A/F mixture.
This test works pretty well on carbs with direct mechanical lift. But vacuum diaphragm carburetors are devious, surly beasts, and the slight response delay of the vacuum operated slide can make things harder to read. Also weak spark can cause WOT power weakness or even breakup that resolves when the throttle is rolled off a little. Want to get the main jet as right as it's going to be? Make sure the ignition is perfect, valve lash and cam chain tension are right, and compression is good in both cylinders (>125 psi, <10% difference). Then find the smallest main that will run clean at WOT and go up 1 step. If you're working from the top down, revisit the mains after everything else is dialed in. You may wind up doing three or four rounds of change and compensation. Expect it.
There's no "perfect" jetting. Carburetors have to constantly respond to changing demand with a small number of simple components, and compromise is the name of the game. There are jetting combinations that work, combinations that work well, and combinations that don't work at all. Tuning takes patience, a high frustration threshold, and willingness to take a deep cleansing breath and walk away from the machine when you run out of the former and hit the latter.
CR, PWK, PJ, & PE carburettors are all flat slide type carburettors ,....not CV constant velocity carbs ...like the Keihin CVK GPZ500 carbs this thread is about
Keihin CVK carbs do not have accelerator pumps and high speed screws or fixed rate flat slides
Interesting subject but the advice is applicable to CR, PWK, PJ, & PE carbs not strictly relevant here to Main Jet selection and tuning of the CVK34 type Keihin carb we are discussing
Just stop it, bbb both of you!
One correction here, peanut. The Keihin CR is a cable lift round slide 4-stroke carburetor. The FCR is a bellcrank lift flat slide 4-stroke pumper carburetor. (A pair of those will cost well into 4 figures for the carbies alone, then you get to fabricate brackets and throttle shaft. Too rich for my blood, but they're beautiful: the throttle shaft turns on needle bearings, and slides run on nylon rollers so that wear is nearly eliminated.) Some Keihin main circuit designs include a secondary main jet--they know how to keep life interesting. Anyway, round slide, flat slide, pumper or not, basic tuning techniques remain the same.
thanks for the correction griz ...I hadn't noticed that when i looked them up.
I think the point I was making is that the slides were all fixed rate governed directly by the throttle whereas the slides on CV carbs are governed primarily by engine vacuum and atmospheric pressure. This means that the slides of the CV carbs are subject to slight changes in venturi pressure and therefore fuel/air delivery at any given throttle position
The engine response of the 2x different types of carburettor will give a different response when applying WOT to 7/8ths rolloff throttle testing .
( he says ......never having tried WOT rolloff at 90mph )
The FCR sound delicious but if I were to go that route I'd fit fuel injection . I was looking at a BMW at my local MOT testing station yesterday and I can't help wondering how well our XS650s would perform with fuel injection.
They respond to EFI pretty well. I used a microsquirt ECU, and did my own set up with it. Cost around $750, read more here http://www.xs650.com/threads/sumotrack.48483/page-5 to keep this thread on carbs.
Well, I haven't put the 142 mains in yet, I have some 140s sat about too now just in case.
My reason is, and I don't know if any of you are great at reading spark plugs, but after a few hundred miles of riding (slow and highway), I noticed my exhaust mufflers are quite coated with soot, and my plugs have a sooty ring on the top couple of threads, I didn't have time to take a photo this morning, but they are pretty much like this:
Slightly sooty ring, golden insulator and grey electrode. With the soot in the pipes and plugs like this, would it not suggest a rich condition?
Slightly contrary to what I first though, considering the foam pods and open pipes!
It may be wise to try a leaner needle, I probably use the midrange more than the mains, so it would make sense that a leaner needle could help a rich condition at mid throttle position.
It might be advisable to make certain at which throttle position the over-rich mixture is occurring first.
Its quite possible that it is the idle circuit that is too rich or even a choke valve not always fully seating
When shes nice and hot try re-adjusting the idle mixture screws and idle speed screws until you have the best idle speed . You may find you need to go down one on the pilot jets if your mixture screws are near the end of adjustment . Don't forget that the idle mixture enriches all throttle positions up to 3/4 throttle so it is still having an effect at mid and 3/4 throttle and especially deceleration
To test the mid range throttle ride the bike in third or forth holding a constant mid throttle position for about 100 yds . Note what the engine does.
Does it buck and lurch with the engine revs going up and down constantly ? which would indicate a lean mixture at mid to 3/4 throttle ....or does it keep bogging and sound like it needs more throttle ...too rich at mid throttle
Each throttle position uses a different range of jets and components so ideally it is best to mark off the throttle into 4x zones and test each zone to identify which jets ,needles, slide etc needs adjusting .
Thanks Peanut, I had seen this done on two strokes, notching the grip and marking the housing at increments to get jetting right.
I will try your suggestion on the way home from work tonight -- I have a feeling that apart from sounding "Woofly" (technical term), it pulls nice and evenly when held at 3/4 throttle. I came to work on the dual carriageway today and was behind a lorry, so would have been hanging back in 4th gear for a good stretch.
Another test I frequently do is to apply a tiny bit of choke and note the engine responses.The chokes feed in a completely separate and independent fuel and air supply at any throttle position Its a useful quick mixture diagnostic as long as you don't fall off the bike when doing it lol
Wish I was riding home from work on my bike .. still... retirement isn't all bad lol
Well, tried out a few things, it's essentially smooth held at any throttle position; so apart from the sooty exhaust/plugs it seems ok.
I think can detect the carb sliders fluttering, and I feel like it's holding back a bit, vague I know.
I couldn't get it past 88 mph today, I recently put a 31T sprocket on so not sure where I should be getting to in 5th?
It might just be how it is, I just think there's more to try!
Yes, gear these too high and you won't pull redline in 5th. We're only dealing with about a 50hp motor here, remember. And at a 31T rear, you're approaching that "geared too high" range, depending on your size.
If you're running K&N style pleated pod filters, they will cause slide flutter. Foam pods work best on these CV carbs.
Thanks, I'm 100kg (220lbs), so no racing snake...
I do have foam pods filters, but I feel like these carbs would do better with a pipe to suck through, so I'm going to stick some silicon reducer tubes on between the carbs and the air filters.
I read a post on the Kawasaki ex500 forum where he found angled entry filters eliminated flat spots on the original bike, as drawing the air round a slight corner helped channel the airflow.
After scouring eBay, I found some 54mm to 51mm, 5 inch long straight reducer pipes, I have concluded that based theses silicon pipes I will end up with 17.5 cu in total of "airbox" (obviously this is far below the volume of the original EX500 box). These are the longest I could fit in without resorting to 45 degree bends.
I have then found some 51mm > 36mm Venturis, reason being that the EX500 has 36mm intake ducts for it's factory airbox, so why not allow the same sized opening for the carbs to pull against?
I am fitting these at the filter end of the silicon tubes, then putting my standard pod filters over the top. I *may* fit a crossover but as it would be inside the tubes it might disrupt any clean airflow I have managed to establish, although it would allow each carb to draw from double volume of "still" air.
The parts are in the post, I'll post pics and results after testing! See if I can eliminate the flat spot.
Parts arrived, will probably need some support, may make one from p-clips or use safety wire or something.
Nice quality silicon, smooth inside and pleasantly thick. I don't think there's any danger of the XS collapsing them!
Well, that all fits, need to get out for a ride now, but it snowed here last night...
Definitely function over form here...
Yeah they kind of look like a pair of, oh never mind.
Seriously, I'll be waiting for your ride report!
Some other large black silicone item?
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