Topic: Why do my carbs hate me?

[scottly]

So the PO fitted the 120s and pods? It is the nature of the beast to run rich below the peak torque RPM; all of the pressure waves bouncing back and forth in the intake and exhaust tracts are out of sync, so the combustion chamber is "over scavenged", where fresh fuel gets pulled out of the exhaust valve during the overlap period of the cam. Don't even bother with WFO readings below 3K.

[American Locomotive]

It came with Pods and 110 jets. I kept the 110 jets, and put the stock airbox back in. Mid-throttle was okay, but the bike had a very bad just off-idle stumble that I could never get rid of, and it always ran out of steam on the top end. I could just about get the stumble to go away with the air pilot air screws almost all the way out, but the bike hardly idled. It seemed to run better on the top-end with the real Keihin 120 mains, but the low-end stumble was still there.

That's when I finally broke down and bought the air-fuel gauge. My gut feeling is that the emulsion tube or needles might have wear (or might be incorrect), which is why it seems to run a lot better with the clip in the top groove.

[scottly]

Once again, you adjust the main jet for WFO 7K+ RPM. You are trying to compensate for a rich main by adjusting the needle. Put the needle in the stock location (or center groove), select the main, and then select the needle position for part throttle.

[American Locomotive]

Yeah, we established that already like 6 posts ago. Since I have real-time feedback as to what the engine is doing, I can now analytically make adjustments to the carb, including making multiple adjustments in one shot since I can characterize what's going on all the time. No more guessing and looking at plugs like I was before.

I was extremely lean at idle, extremely rich at part and mid throttle and moderately rich WOT. So much so that I knew stepping down from 120 to 115 mains wouldn't magically fix everything (since it didn't run right with with 110 mains and the needle in the 3rd position either). I then adjusted the air screws, needle position and main jets all in one shot, and now it's a lot closer. Since I'm still mildly rich WOT,  I know need to step down the mains since there is no way to adjust that othewrise. Since part-throttle operation is a tiny bit on the rich side as well, so I'm going to leave the needle in the positions they are now and see if dropping down the main clears everything up.

[American Locomotive]

Installed the 110 mains today, here are the results:

Idle A/F: 13.5-14:1
Cruise/Part Throttle: 13.5:1
Mid-Throttle Acceleration: 12.5:1
WOT: starts out around 11.5:1 down low, but leans out to 12.5:1 by 7-8,000 RPM.

I really don't think I could ask for much better than that. The engine is running so crisp right now. So, I still don't know why the bike wants the needle in the top groove, or why it didn't run well WOT previously with 110 mains. Maybe just some weird interaction with the needle even though it shouldn't really have an effect at WOT. Either that or the 20 times I've had my bowls and jets out in the past 2 weeks has dislodged something in the carbs.

Thanks for the help from everyone. I know it's been a little hectic, but clearly something's up with these carbs for it to want the needle clip in the top groove. The joys of buying someone else's project. The air-fuel meter was definitely a great investment though.

[PeWe]

Maybe a PO has drilled jets?

I should double check with 1 size smaller main.

If it will give less power you'll notice by butt dyno when giving throttle from 60Mph and up on 5th gear.

[American Locomotive]

Maybe some day when I get a second bike and can deal with not having a bike to ride, I'll tear the carbs completely down and really investigate what I have going on.

I just know now that the bike is running great, my air-fuel ratios are spot-on, and for the first time I've had the bike I can say it actually feels "quick". It's still slower than my CRF450 dirtbike, but not a slug anymore

[scottly]

I know it's been a little hectic, but clearly something's up with these carbs for it to want the needle clip in the top groove.

That's because you are still using the needle to compensate for a too rich main jet. No wonder your carbs hate you, you don't listen.
12.5 at 7000-8000 RPM is NOT spot-on; spot-on would be 13:1. Read reply # 24, where ekpent told you the stock main for your bike is a 105.

[American Locomotive]

Many other people in this thread have said they found the most power with 12.5:1 AFR - which is what I have at WOT. Considering today was a very hot humid day, I'd consider that acceptable. A cool summer night could easily increase the air density enough to pull that up to more than 13.0:1 AFR. Never mind spring/fall where I like to ride a lot - when it's 55-60 degrees out. Also consider that I'm running E10 fuel. 13:1 AFR on "straight" gasoline is 0.88λ. On E10, 0.88λ corresponds to an AFR of 12.4:1 ( since E10 stoichiometric is 14.1)

My kit came with 105 jets that I've tried before last year. The bike still ran bad with those as well with the needle in the middle position. Hence why I went the other direction and went all the way up to a 120 main. Maybe it ran bad last year with 105s because of dirt still stuck in the carbs, or a clogged vent line, or the ignition timing being a little off, who knows. Doesn't matter. The bike is running extremely well now, my AFRs are really decent throughout the rev range at various throttle positions and trying to chase 0.5 AFR on a slide-carb'd bike is pointless when a just 20 degree change in ambient temperature will cause greater swings than that.

Maybe I'll try the 105 mains and play with the needles again in the future if I get really bored. But at this point, the very marginal gain I might possibly get is not worth the ~2 hours of time needed to disassemble the bike, wrestle the carbs out, adjust the needles, and put it all back. Especially if it turns out it runs bad with 105 mains and a middle needle position - meaning I'd have to do the whole thing over again.

I just want to ride.

[scottly]

Many other people in this thread have said they found the most power with 12.5:1 AFR

Do any of those people have any experience with a dyno? And no, the AFR will not change that much with changes in atmospheric conditions, but the power output will be affected.   

[PeWe]

12.5:1 perfect when accelerate in the area the engine has something to deliver. I'm thinking up to 7000 rpm.

Not much more happen after 8000rpm.

I have a thread where I adjust my carbs, Mikuni TMR 32. It started as a question how to remove the very small (short) needle jets. I continued to write about the jets and my changes.

I could not stop until I have found the best working combination. My comments for me and others to.

I had no recommended jet combination.

The problem with those was way too rich needle jets. I could get it to work better by lower the needles. Lost power as a result.

Needles very easy to change.
My goal was to get an even throttle response from idle and up to 3500 rpm on 5th gear that became worse with a better flowing exhaust.

I double (tripple) checked needle jets, needles and main jets.

I synched the carbs again. Must been wrong last year.
I think I adjusted for an even idle only. Not read how carbs reacted on slow opening up to 3500 rpm. They must stay synched.

Last detail for a very smooth engine was correct oriented spark plugs with their gaps facing inlet valve.

After my experiences adjusting these carbs I've learned that needle jets are very important to get right to start with. Pilot jets for clean not sooting idle at red light stops.

I can now finalize the adjustment of my Mikuni VM29 if I had a bike  for them. I was not aware how important needle jets were. I changed pilots, air jets and needles  without getting it right, only more or other wrongs.
http://forums.sohc4.net/index.php/topic,179479.msg2081757.html#msg2081757

[scottly]

Per, 12.5:1 is Not optimum for max power. Why do you think there is a dotted red line at 13:1 on the dyno A/F graph?

[PeWe]

For recommended medium ratio when cruising and still get immediate power when twisting?

12.5 temporary ratio during max acceleration.
I have read it on several sites.
Here is one:
https://www.rbracing-rsr.com/rsrgauge.htm

Scroll down to:
"Widely used 12.5:1 (into the 11.0 range) as a target for temporary acceleration enrichment or a slightly rich full power ratio and or a safe turbo boost target."

I should not want that rich during a steady cruise, not accelerating at all.

Carbs is a compromise, run a little rich to get quick response when open the throttles.

Fuel injection with a data box, throttle sensor and lambda sensor can be adjusted perfect for economic ride and still turn the fuel on during acceleration when throttle sensor show the throttle angle and how fast it change, the program in data box will "understand". Data box adjust the ignition advance too.

Max and min values alarms when exceeded. This is more or less a self adjusting system once dialed in. Std in todays cars
My car got the program adjusted and suddenly the turbo increase torque a lot! Very cheap modification ($300US). (Cat and exhaust 5 times more) The std turbo runs out of air at 5500 rpm during WOT. There are bigger ones, but still OK at lower rpm. Kits with injectors.

Same power increase in an old 1.8L engine with carbs should cost!
 
I'll see how my K6 will look like when running it on the Dyno. I have adjusted all jets to get a good idle, cruising and best acceleration.
Only needle taper not touched. I could not get another needle improving it for how it ran before. I asked a carb dealer here.

I'll see if it will reach 12.5 during a longer acceleration. Acc pump squirt fuel during 1 sec or 2.

I will not change any jets after the dyno even if result recommend it. If I change it runs worse. I have tried all up and down several times.

Maybe pilot to 1 step smaller as I have tried when other things were not OK

[American Locomotive]

Do any of those people have any experience with a dyno? And no, the AFR will not change that much with changes in atmospheric conditions, but the power output will be affected.

Yes, but why is power output affected? It's because the air density goes down as temperature and humidity increases. If the air density goes down, that means you need correspondingly less fuel.

It's the same reason you need to jet-down when going up in altitude. The difference in air density of a 50°F vs 95°F day is almost as big of a change as it is going 1000 feet up in altitude. In the dirt-bike world, they typically recommend going up a main-jet size for every 20-30 degree colder temperature you plan to ride in, or down a jet size for every 1000 feet of elevation you go up. 4-strokes tend to be much more forgiving and will operate acceptably over a wide A/F ratio range than 2-strokes, of course.

But I can guarantee you that if your 836 was running 12.8-13:1 A/F on that 64°F 26% RH day, you will be much closer to 12-12.5:1 on a 90°F 70% RH day.

Per, 12.5:1 is Not optimum for max power. Why do you think there is a dotted red line at 13:1 on the dyno A/F graph?

That's just the default dynojet target air fuel that's displayed on all their graphs as a reference baseline that's "good enough" for most applications. Maybe for your 836 13:1 is the ideal ratio, maybe it is for my bike too. However the gains between 12.5:1 and 13:1 will be extremely minimal, and due to the nature of carbs, my AF ratio will be at the point by the fall where I do a lot of riding, so I'm not worried about it. Not to mention as PeWe pointed out, there are many many sources that indicate 12.5:1 is a good ideal power-making ratio, with some engines even wanting more fuel than that.

[PeWe]

This thread is very informative. I hope the carbs can be fine tuned.
I have a Daytona Wego III not yet installed.
The sensor is a little bit fat. Need a slimmer version.

[scottly]

If the air density goes down, that means you need correspondingly less fuel.

It's the same reason you need to jet-down when going up in altitude. The difference in air density of a 50°F vs 95°F day is almost as big of a change as it is going 1000 feet up in altitude. In the dirt-bike world, they typically recommend going up a main-jet size for every 20-30 degree colder temperature you plan to ride in, or down a jet size for every 1000 feet of elevation you go up. 4-strokes tend to be much more forgiving and will operate acceptably over a wide A/F ratio range than 2-strokes, of course.

The way a carburetor works is that a mass of air flowing through the venturi creates a pressure difference that forces fuel to flow up into the carburetor throat. The greater the mass of the air flow, the greater the pressure difference. When the air is less dense, such as my elevation of 4500', the less dense air has less mass, while the fuel has mostly the same mass, so less fuel is introduced into the throat. I would have to re-jet 2 or more times within 40 miles, in any direction if the 1000' thing were true
The fact that your needle is set to the leanest position is a clue; I can understand your reluctance (laziness) to change them..    

[PeWe]

Too lean needles, lots of fun acceleration never happen and risk to burn pistons/valves in higher speed cruising.

Are the needles jets adjusted with a drill making them to give too much fuel so needles have to be lowered?

[scottly]

Per, he is using 110 main jets in place of stock 105 jets, and has changed the needle clip position to the leanest setting to compensate at part throttle openings.

[PeWe]

OK, there is an improvement (correction) to be  made.

[American Locomotive]

The way a carburetor works is that a mass of air flowing through the venturi creates a pressure difference that forces fuel to flow up into the carburetor throat. The greater the mass of the air flow, the greater the pressure difference. When the air is less dense, such as my elevation of 4500', the less dense air has less mass, while the fuel has mostly the same mass, so less fuel is introduced into the throat. I would have to re-jet 2 or more times within 40 miles, in any direction if the 1000' thing were true
The fact that your needle is set to the leanest position is a clue; I can understand your reluctance (laziness) to change them..

In theory, the the carburetor should compensate for the changing air density, but it doesn't work out like that. It has to do with the fact that "air" is not a single thing. It's many gasses mixed together. As temperature, density and humidity change, the overall composition is going to shift around. It's why carb'd planes have mixture control knobs and altitude compensation devices. It's why there have been countless formulas, papers and articles written about adjusting carburetor jetting for different altitude and temperatures. It's why the old Honda 2-stroke dirtbike manuals had guides in them how to adjust jetting for temperature and altitude.

https://www.hotrod.com/articles/hrdp-0406-density-altitude-tuning/
https://www.sstsensing.com/o2-sensor-working-principle-partial-pressure/
https://mergeracing.com/2016/11/03/using-air-density-guage/
http://dtec.net.au/Air%20Density%20&%20Tuning.htm
http://northwestvets.com/airdensity.html
https://www.elji.se/kund/apnote_14.pdf

4-stroke engines are just amazingly tolerant, and will run acceptably on a huge range of air/fuel ratios. They'll basically run acceptably on anything from 15:1 to 11:1. I can guarantee you that your 13:1 jetting at 4000 feet and 66*F will not be anywhere close to that at 0 feet, 90*F and 70% RH. You disregard other's statements because they may not have dyno'd their bikes, but do you have a wideband strapped to your bike monitoring it at all times?

Like I said, I've run 105 jets in it with the needle in the middle position. It still ran extremely poor with the massive stutter from the idle>throttle transition. I've had these 110 jets run perfectly fine, and then very poor (which is what I mentioned in the first post in this thread). Right now with the needle in the very lowest position it's running very well with good A/F ratios throughout the entire range. ~14:1 idle, 13.5:1 cruise/mild acceleration and 12.5:1 WOT.

[scottly]

I've had my CR 480 that I've owned since brand new at elevations from sea level to 9000', in temperatures ranging from 38*F to 100*F, all with the stock jetting. Yes, the CR manual did come with jetting instructions, but no amount of jetting changes will replace the low oxygen levels at higher altitudes; the low barometric pressure on the graph is the major reason for the correction factor of 1.16.
I'm not saying that AFR isn't affected by altitude and temperature, I'm just saying it may not be as much as you think. I don't know where in the US you are, but I would like to see your AFR readings at different elevations, if possible.
PS Pilots use EGT to fine tune the mixture, not AFR.

[scottly]

I can guarantee you that your 13:1 jetting at 4000 feet and 66*F will not be anywhere close to that at 0 feet, 90*F and 70% RH.

The Honda CR jetting info had nothing about RH, but based on temp and elevation suggests the same jet for both conditions. Also, the manual stated to select the main jet before adjusting the needle from the stock setting, and even then it was only in the most extreme situations, and then only by +/- one clip position.

[American Locomotive]

So this morning we had reprieve from the merciless heat. It was 72°F - still 80% RH though. So I geared up and took my bike to a road where I could do a nice WOT pull. The result: 12.92:1 A/F ratio at 7,500 RPM. So by your very own admission - my fueling is now spot-on, and I did not change a single thing on the bike. Yeah, the 110 jet is slightly larger than the stock 105, but based on objective feedback from my gauge - it's what my bike wants. Same thing with the needle position - my low-end and mid-range fueling is just about perfect. It's quite clear that something is just "wrong" with my carbs for it to run best with this configuration. And anyways, I'd rather my jetting be very slightly rich on the hottest days than risk going lean on a cool autumn day.

A/F ratios swing around on carbs based on temperature, humidity, and elevation. The delivered AF ratio changes A LOT. The above test is proof enough. Some bikes (like your CR480, or a CB750) may be very tolerant of a wide range of A/F ratios before they start running poor.

Look below at these excerpts from the '92-97 CR125 manual. Their chart compensates for altitude and temperature, and on the next page they even mention that you need to account for humidity as well. Follow the "0 ft" line from 32°F to 95°F. That's a correction factor span of 1.04 to 0.97 (which would be typical for New England). With the stock 172 main jet on a '92 CR125, that's a range spanning from 179 to 167. That's 12 jet sizes from winter to summer in the same geographic area! If you ride in Alma, Colorado in the summer, and Bangor, Maine in the winter, you're looking at a 184 vs a 151 main jet. That's 33 jet sizes!

This thread is very informative. I hope the carbs can be fine tuned.
I have a Daytona Wego III not yet installed.
The sensor is a little bit fat. Need a slimmer version.

I think the NTK wideband sensors are a little smaller, might be worth looking into.

[PeWe]

If both your AFR gauge and butt are happy, it must be right!
When I tuned my TMRs I had no gauge, only butt and plug reading. Eyes and nose at idle when parked increasing throttle without smoke or stinking rich.

When I got all right I had to test needles leaner and richer to verify correct height.

Same with main jets.

I had earlier tested different neede jets and different needle heights to really ensure I use right  size.

Have you checked the orientation of all 4 spark plugs?
Remove, mark with pen where the gap face and tighten again. You'll see.
It is a big difference when right, facing the inlet valve. I have photos in my K6 project thread.