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Yet another jetting question


Wombat
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I have removed the cats, installed a K+N filter and rejetted to 42/127.5, mixture screws 1.5 turns out.

 

It feels a bit 'flat' on take off.

 

Should I wind the mixture screws out a tad more, or would lifting the needle help?

 

 

Are your throttle cables synched? That will cause a hesitation on takeoff.

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Yep

Throttles synched.

 

It is not so much a hesitation as it feels like a flat spot.

 

Is 1.5 turns out about right for the mixture screws?

 

When I rejetted the Trumpy, I lifted the needle a tad to get rid if the flat spot that it had.

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Well that flat spot ain't your carbs (if they are sync'd correctly). And there is no way you need to shim the needle. If you had the stock lean jetting and no flat spot...you sure aren't going to get one with richer jetting...unless you where too rich...but that wouldn't make sense.

 

I mean think about it, why would you be different? All the folks that have re-jetted and 99.99% have not needed to shim the needle (as I'm sure someone out there has just HAD to re-shim theirs).

 

If the flat spot is just off idle, I'd venture to guess that your throttle sync is off. What do you use to sync them and how?

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.

 

If the flat spot is just off idle, I'd venture to guess that your throttle sync is off. What do you use to sync them and how?

 

I use a Morgan Carbtune and sych procedure is as per the manual and synch is spot on.

 

When rejetting my Triumph (which is the only experience that I have in this area), it is common practice to shim the needle or use a slightly different tapered needle to get a bit more fuel a bit earlier as the needle lifts.

 

The reason for asking is not that I feel that I am any different, but seeking advise from those that have more knowledge and experience than myself of things Ural .

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.

 

If the flat spot is just off idle, I'd venture to guess that your throttle sync is off. What do you use to sync them and how?

 

I use a Morgan Carbtune and sych procedure is as per the manual and synch is spot on.

 

When rejetting my Triumph (which is the only experience that I have in this area), it is common practice to shim the needle or use a slightly different tapered needle to get a bit more fuel a bit earlier as the needle lifts.

 

The reason for asking is not that I feel that I am any different, but seeking advise from those that have more knowledge and experience than myself of things Ural .

 

Shimming a needle, is no different than changing the needle clip position on a Mikuni, an Amal, etc... I've been doing it for about 8 years with keihin style carbs, and about 40 years on various other carbs with their needle clips. It is simply another variable that allows one to maximize their tuning eforts, for any given main jet used. Many is the time, a simple needle height adjustment meant the difference betwen qualifying for a race, or going home. And even though we aren't racing, there is no need to short change yourself with every tuning aid at your resources.

 

I would also suspect "synch" as your problem, for the lat spot though. I'm running the stock air cleaner, and cats, with a 42/125/single .020" shim combo, and "approximately" 2 turns out. With the engine hot, I back the screws out till the engine falters, or rpms drop. Then slowly screw in, counting the turns, until it falters again...then back it out 1/2 that difference. Each engine is different. Doing it that way, they are adjusted to what YOUR engine needs....not someone elses.

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I'm not trying to be a smart arse (for once), the reason I ask "how" you performed the sync'ing is because I've come across many folks who swear they know what they're doing, have "done it by the book" ...and still don't have the engine sync'd correctly because they don't understand what they are doing. Most folks get it close enough, but if (especially if) the throttle cables are not done correctly you will have a hesitation or flat spot until the two carbs catch up with each other-just off idle (first 1/4 of throttle).

 

Shimming the needle does little if anything for just off throttle. Shimming a needle is more for mid-range mixture (yes, yes...if you raise it high enough...). If the carb. is properly jetted, there is absolutely no need for shimming. Shimming is poor man's jetting. It is for folks too cheap and too lazy to take the time to jet a carb properly (not that there's aaaaanything wrong with that). Should I throw in drilling out the jets (looking over my shoulder for items tossed by SB). Now, if you have an inadequate carb. (undersized) for your application...shimming may be the only way to get it to work. If you hop up your OEM engine to race using the OEM carb, shimming may be the only way to get it to work...but neither applies with a Ural...especially the hopped up part! The Keihin 32CVK is more than adequate. Even with stock jetting the bike is fine. Very cold natured until warmed up...but fine. Popping in .45 idles and 125 mains is just about perfect below 5000' MSL with 1.5 turns out in the summer and 2 turns out in winter at sea level. Your results may vary.

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OK, fellow Foilheads- it's time for some unsolicited advice based on my half-vast knowlege of carburetor idle circuitry in general. First, clear your minds of all thoughts.... and then picture an idle circuit. Why they call it a "circuit" is beyond me- it has a beginning and an end, and the two never meet....go figure. Ok- back to picturing an idle circuit: think of it as a tube or passage that extends from the float bowl to the carb throat. At the bowl end, add a restricting orifice (AKA: the idle jet). At the carb throat end, split it into two or more openings into the throat.

 

Fuel moving from the bowl to the throat must pass through the idle jet and then through one or more of the openings in the throat. At idle, only the first opening in the throat should be downstream of the butterfly valve. Since the float bowl is vented to the air upstream of the butterfly, the pressure in the bowl is slightly higher than at the opening downstream of the butterfly. This differential in pressure causes fuel to flow from the bowl (the higher pressure), through the idle jet, and onward through the downstream throat opening (the lower pressure). No fuel is flowing through the hole (or holes) which are upstream of the butterfly because the pressure in the bowl and carb throat at that point are equal.

 

Now, let us consider the relative sizes of the various orifices. The openings into the throat are each the size of small pinholes. The idle jet orifice is somewhat larger, and therefore capable of passing enough fuel to supply all the individual throat openings. The purpose of the idle circuit is to add the proper amount of fuel to the volume of air being passed by the butterfly valve. The idle jet and idle needle valve limit the "maximum" amount of fuel that can be delivered through the first throat opening. The first throat opening is purposely designed to pass a little more fuel than necessary for a proper mixture at idle, and the idle mixture needle is added to adjust the flow through this first opening into the throat, allowing for fine-adjusting the mixture at idle. Likewise, the other throat openings are designed to provied somewhat more fuel than will be needed- fuel flow through those will be limited by the idle jet.

 

So... at this point, we have the fuel passing from the bowl, through the idle jet, past the idle mixture needle and out through the first opening in the throat- propelled by the difference in pressure between the bowl (which is vented to the air cleaner side of the carb), and the downstream side of the butterfly valve. This results in an amount of fuel, set mainly by the needle valve, being added to the volume of air being passed by the butterfly valve. And all was well.... until the butterfly valve begins to open further...........

 

Just for the sake of illustration- let's say that at idle, the idle needle is allowing 40% of the maximum amount of fuel that can be passed by the idle jet. Let us also say that our carb has two idle openings into the throat. As the butterfly opens further, the second opening is oncovered and finds itself on the downstream side of the butterfly. The pressure at the second opening now becomes lower than the pressure in the bowl, and fuel begins to flow through it as well. How much fuel is determined by the idle jet. If the first opening will pass 40% of the capacity of the idle jet, the remaining 60% can now be drawn through the second opening.

 

Enter the main circuit. Keep in mind Bournulli's Principal- as the velocity of a fluid (in this case, air) increases, it's pressure drops. At about 20-25% of throttle opening, the air flow through the carb becomes great enough for the main circuit to begin working. At this same point, the butterfly has moved far enough beyond the two idle openings that the speed of the air passing over the throat openings begins to become slower, and the amount of pressure differential between the bowl and idle openings begins to decrease, causing fuel flow through the idle circuit to be diminished accordingly. If all is well, there will be some overlap where the idle circuit is still providing fuel as the main circuit begins to take over. On the Ural with CVK carbs, this is at about 20-25% throttle opening. Make some marks on your twist grip so that you can see where this point is, and look for hesitation or any signs of a flat spot here. If you do- it's telling you that the idle jet can't pass enough fuel to meet requirements at that end of the low speed area- time for a bigger jet.

 

Another tell-tale sign of a too-small idle jet: When I first got my '05 GearUp, it came with a #38 idle jet. Even with the idle mixture needles turned out four full turns it was still too lean to cause black smoke or a reduction in idle speed. By changing to #48's, I can now actually get a rich mixture by screwing out the idle mix needles. My setting is now about 1 1/2 turns out but remember; bikes are like snowflakes- every one is slightly different. Your milage may vary.

 

This information is only valid for carbs with fuel mixture needles such as the Keihin CVK. The K-65 and K-68 idle mixture adjustments control air instead of fuel, as I understand it.

 

Sorry for the long-winded post.... and as my friend Forrest would say: "That's all I got to say about that."

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