High RPM miss

[psycoreefer]

I've been playing a lot with my carbs trying to get this thing running right and It seems like I'm stuck between two different main jets.
The bike has aftermarket 4 into 2 exhaust and stock air box. I'm running ngk iridium plugs, when I got it the pilot circut was screwed and iridium's kept it from fouling out all the time.
I should also mention that this bike may have other internal mods, I got it used of course and it has about 165lbs compression on all four cylinders with the carbs off. That seems high for a bike with 40,000 on it, especially when clymer says it should be under 150.

I have both 137.5's and 140's. With the 140's in the bike seems to pull better WOT from about 2500 - 5000 RPM but then not as hard from 5000 up.
According to what I've read this doesn't make much sense as the needle should be the primary influence in that range. The plugs look a little dark, not nice tan but a touch gray/black.

With the 137.5's the bike seems to pull harder from 4000 - all the way to red line then it does with the 140's but at 7000 - 8500 I can hear a miss, also the plugs are very light, not bleach white but whiter then I'd like.

I've read the factory pro tunning site and it seems like our xs are in between the directions for High/low RPM bikes. Does anyone know what RPM ranges are effected most by what settings. The site seems to suggest Main jet selection should be based on 7.5k - 8.5k on our bikes. Based on that I should pick the 140's as they get rid of the miss but I really think they are to rich. Maybe go with the 140's and drop the needle one position?
Anyone know what RPM the needle stops effecting WOT operation? Its easy to tell that we go off the idle circut at about 3-3.5k. Should I assume that we transition from needles to pure main jets around 6.5 - 7k because thats where I get the miss with the 130's?

Maybe something like this:
WOT idle - 3k float hight (from factory pro)
WOT 3k - 6.5k needle position (my guess)
WOT 6.5k - up main jet (my guess)

Any input or suggestions? Its really hard to tune by seat of the pants feel when I don't really know what a good running xs feels like.

[psycoreefer]

another thought

If the 137.5 are to lean and thats causing my mis, would opening the choke add enough gas to effect anything? Could be an easy way to help diagnose a lean vs rich condition. I think maybe I'll give it a try on my way to work in the morning.

[mro]

Float height affects all rpm ranges.
Stock = 25.7mm + - 1 mm
Might try raising the needle with the 137's.
Enricher/choke should have no effect above 2000/2500.

Can raise or lower float height (lower = richer) after checking all else. Would change it no more than a 1/2 mm at a time. More than 1 mm float height change from stock can cause float(s) to hang up. (learned that the hard way, but YMYV)


mro

[psycoreefer]

ha, I included tons of info about the bike and forgot to include that I have the floats set at 25mm right now. I had it set at 26 when I fist put the carbs back on and had a bad stumble at around 2.5k. Lowering it to 25 seems to have helped that a lot. I was hoping that going to 25 might help the top end to but it didn't seem to have much effect. I suppose I could try 24.5mm that just a touch out of the stock range. Although having to go outside the stock range seems to suggest there may be another problem I haven't found.

MRO, you think rasing the needle would help at the extreme top end? I always thought that the needle only played a role lower down in the RPM range. I guess maybe half the problem is I don't have a good understanding of how a CV carb really works.

Didn't ride the bike so I didn't get the try the choke thing this morning.

[mro]

originaly post by mro
Enricher/choke should have no effect above 2000/2500.

Please pass the salt!
I be eating those words Enricher circut does have some effect above 2,000 rpm, just how much don't know. Something I will check into when I figure out how to test it.

The slide needle slides thru the needle jet limiting fuel thru the needle jet, higher = more fuel.
Fuel from the bowl goes thru the main jet to the needle jet.
Main jet limits how much fuel is available to the needle jet, so changing the main jet will affect all higher rpm.

When engine at low rpm and you crank the throttle, butterfly opens and high vacuum raises the slide/needle. As rpm's increase vacuum drops off and slide/needle come down alto not all the way.

Float height deturmins how close the fuel is to the main jet and higher fuel level in the bowl requiers less vacuum to draw it thru the main jet, then thru the needle jet. Opposite, lower fuel height in bowl leans it out as it will require more vacuum to draw fuel.

TC had a video of his carbs showing what was happening to the slides at idle and when cranking the throttle. When the slides rise up WOT they don't stay there.

Main jet, needle jet, slide needle and float height all effect higher rpm.
With a problem only at higher rpm with the BS34 II carbs would try changing the needle height first before changing float height, main jet, needle or needle jet.


mro
BTW, don't remember if you have stock exhaust/air box ???

[psycoreefer]

Thanks for the description mro. I've been doing a bit of research (that maybe I should have done first) and reading a lot about how CV carbs work. Not really that hard to understand but a lot of inter-related things to keep track of. I did find this little diagram that helps a little bit, although it would be better if it also showed RPM's on the y-axis and was labeled as vacuum at the bottom because vacuum and throttle position aren't always the same thing.



I have stock airbox/filter with 1 hole drilled in the bottom by PO and what I'm guessing is a MAC 4/2 exhaust (no crossover).

I haven't had time to play with the carbs yet but SWMBO is going to a baby shower on Saturday so I'm hoping to get it all taken care of then. I'm definitely going to try adjusting the needle hight as thats about the only thing I haven't tried yet, it gives me hope that you say it would have been the first thing you tried.

Anyways... off to look for that video

[psycoreefer]

OK I had some time to work on the bike yesterday night and this is what I found.

I started out by raising the needle one positions = richer. Helped but still not right. Removed carbs again and got out the old needles for comparison (the ones I was using are new from a rebuild kit). On closer inspection the taper on the new needles is quite a bit less then the taper on the original needles. In addition the new needles are about 1/2 of a clip position longer then the originals. So I put the original needles back in and wow what a difference. The top end from 3500 up is much more powerful now. However... I'm rich now at cruise and low speed because I adjusted the floats down to compensate for the other needle so I'll have to pull the carbs one more time at least and adjust the floats up a bit. It also seems that the number four float is hung up on something because that cylinder is very very rich. Strange because I didn't remove the bowls, only the tops.

FYI, I tried the choke at high speed to see if there was an improvement and I can say that it had a noticeable effect. I couldn't see the difference when I pulled the plugs but the miss I had was nearly gone with the choke one click out.

Two clicks vs one click didn't seem to make any difference though.
From what I've been reading this might make sense. Correct me if I'm wrong but the enrichener circuit feeds extra gas and air at two clicks out so that would almost be like opening the throttle a little bit. At one click out it only feeds extra gas and cuts the extra air out of the mix, because a cold engine/intake has a hard time atomizing gas. So at WOT the engine already has as much air as it can get so all we are using the circuit for is the extra fuel which we can get at one click out.

[boyat68]

psycoreefer:

From The Yamaha Manual
Page 4-2 Carburetor:

b of paragraph2

b. Operation of two-position starter jet

"Full-open
To start a cold engine, a rich mixture is required. To supply a rich mixture, pull the starter lever all the way out so that the needle regulating the fuel flow is set free and the flow rate of the incoming fuel is increased to a maximum. The fuel is mixed with the air supplied from the diaphragm lower chamber, and thus a rich mixture is produced.

Half-open
After starting, that is during warm-up, a slightly rich mixture is required. Push back the starter lever half-way so that the fuel flow is reduced by the needle. The fuel is mixed with air from the diaphragm lower chamber, and thus a slightly rich mixture is produced."



Note that the diagrahm shows two ports open to the air coming from the lower chamber of the diaphragm mixing with fuel supplied for full. and the other shows that one of the ports is closed off from fuel and air flow for half-open. I would say that this action restricts both the air and fuel allowed into the throttle bore on the half-open position

[John]

The choke circuit has an air bleed circuit that mixes air with the fuel before the fuel reaches the choke valves, so by setting the choke at one click, your are just restricting the fuel to the engine, but it already has it's air mixed in.