Weather change, stumble at idle and low end jetting issue?

[Terrible1]

Currently I have my bike jetted for the 90 degree weather but lately its reaching highs of 50's and maybe 60's. Well my bike has developed idle issues as well as 1/8-1/4 throttle flat spot.

Here's a rundown of what I've got

74 TX 650 motor
Pamco Ignition (original)
Dynatek Coil
NGK plugs w/resistor ends
7mm wires
UNI Pod filters
Straight pipe exhaust
Ignition timing is set to HBB specs
HHB PMA

Jetting is as follows

Main jet 137.5
Pilot is 47.5
Needle is stock
Air screws are turned 3 1/2 turns out from base.

When the weather is warm (80 plus) out the bike runs perfect. No stumbling issue or anything. Plugs look perfect light chocolate brown. As soon as its gone to winter the bike stumbles and hesitates ONLY at idle or off IDLE. As soon as the bikes 1/2 throttle it clears up and takes off. When I pull the choke out while riding the bike the stumble somewhat goes away but does not clear up completely. The bike will not warm up at any point if I don't have the choke out and my hand on the throttle flipping it back and forth.

From everything I've read in the carb tuning guide Im thinking that I need to go larger on my pilot jets. Thing is I am already at 47.5 and I do see that they go up to 60. I currently have some adjustable needles that I have not installed but am out of larger pilots.

Would the consensus be to go larger on the pilot size 52.5~55 and throw the adjustable needles in at the same time?

Thanks in advance, and if Im forgetting something ask away!

 

[5twins]

It does sound like you need to go richer but I can't say I've ever heard of anyone having to use pilots in the 50s. But you could try it I guess. I don't think I'd jump up a few sizes though, only one to a #50. When you're close on jetting, which you seem to be, you don't jump around several sizes at a time, just one size.

Another option would be to reduce the air jet size. That will also richen the idle circuit for you. Stock is a #135 and a small round main jet is used. You might try a 132.5 or 130.

Yes, you could try the adjustable needle as well, set slightly richer than your stock one. Your stock 5HX12 measures about 50mm from the bottom tip to the e-clip. You would want to set your adjustable needle so it measures 49 or 48mm. If those settings prove too rich, you could shim your original by placing a washer under the e-clip. That would make it 49.5mm long to the tip.

 

[Terrible1]

Thanks for the reply 5twins. I know that you're one of the guru's around here and Im glad you were able to reply.

I'd jump up a few sizes though, only one to a #50

Noted. I was thinking about buying 50, 52.5 after I made the post, but since you said that you've seen anyone need to do that I may just go with the air bleed option to start.

Another option would be to reduce the air jet size. You might try a 132.5 or 130

I had thought about this but never looked further into it. I will read up and see what my options are.

You would want to set your adjustable needle so it measures 49 or 48mm.

This is some great info. I will check and see what mine is at and go from there. I take it I probably should only change one thing at a time correct?

As always thank you,

 

[5twins]

Yes, changing one item at a time is best. It's easier to tell if it's helping (or hurting) things that way. Changing the air jet would be the easiest to access. It is mounted around the outer edge of the intake bell so you could get at it by just removing the pod.

 

[gggGary]

Might want to put a timing light on it, see if the advance might be getting sticky when it's cold. Or open the rh cam cover and check work the advance a few times.
Have never messed with air jets but they are EASY to get at!
Winter fuel blends are typically a bit different also. just for grins might try a hair dryer on the carbs while it's running, see if idle improves. Most newer carbs (and fuel injection) use some sort of a heater near the idle circuit. Early BS38's were plagued by an unatomized fuel drop forming on the idle fuel port, every so often it falls off and causes a rich cylinder.

 

[5twins]

Good idea about the advance. If you've never serviced the advance rod (greased it), that could very well be it.

 

[gggGary]

Might be venturi, throttle plate icing, way more common than most know.

 

[Terrible1]

Might want to put a timing light on it, see if the advance might be getting sticky when it's cold. Or open the rh cam cover and check work the advance a few times.
Have never messed with air jets but they are EASY to get at!
Winter fuel blends are typically a bit different also. just for grins might try a hair dryer on the carbs while it's running, see if idle improves. Most newer carbs (and fuel injection) use some sort of a heater near the idle circuit. Early BS38's were plagued by an unatomized fuel drop forming on the idle fuel port, every so often it falls off and causes a rich cylinder.

I will check this out. I have recently serviced the advance rod and all was perfect but I will take a look at it. Very easy to check off the list.

Good idea about the advance. If you've never serviced the advance rod (greased it), that could very well be it.

Im confident its freely moving but will check it out either way because if I can check it off the list.

Might be venturi, throttle plate icing, way more common than most know.

As in actual icing? It's cold here, but its still Las Vegas so not that cold 50-65ish lol.

 

[gggGary]

Yes actual ice, icing can happen even at 70-80 degrees. Why cars have a preheat tube to supply warm air.
A lot of GA aircraft have hard landed or crashed because after a prolonged idle on decent the carb iced up and when throttle was needed, there was none to be had. That's why apply carb heat is on many a before landing checklist.
The strip is at 2000 AGL, many areas around vegas are much higher. Us flatlanders tend to forget the need to adjust jetting for altitude compensation.
Quote from the kawtriple site
"At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down 1 main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 260 mainjet at sea level you would drop down to a 240 at 4000 feet. Something else goes down as you go up in elevation is horsepower.You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted! For our second example let's say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there's a foot of snow on the ground the skies are clear and it's COLD!. Aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 240 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder than it was the day before you can actually go back to your sea level jetting, a 260 mainjet! If you don't rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference!"

short hand;
higher=leaner, colder=richer
The good news is that CV carbs somewhat self compensate for air density.

 

[Terrible1]

Yes actual ice, icing can happen even at 70-80 degrees. Why cars have a preheat tube to supply warm air.
A lot of GA aircraft have hard landed or crashed because after a prolonged idle on decent the carb iced up and when throttle was needed, there was none to be had. That's why apply carb heat is on many a before landing checklist.
The strip is at 2000 AGL, many areas around vegas are much higher. Us flatlanders tend to forget the need to adjust jetting for altitude compensation.
Quote from the kawtriple site
"At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down 1 main jet Air temperature makes that much difference!"

short hand;
higher=leaner, colder=richer
The good news is that CV carbs somewhat self compensate for air density.

This is very interesting. Thank you for the information.

 

[retiredgentleman]

My BS38 carbs are tuned up for 3400 ft ASL where I live. I sometimes ride my bike up to Highwood Pass which is about 7200 Ft ASL.
I cannot detect any difference at all with respect to how the engine runs. Sure there is a loss of power, but not that I can feel it. Also on the descent from 7200 ' to 3400 ' , I've never experienced any carb icing.
BS38 carbs handle elevation changes with no problems at all. I'm guessing BS34 carbs do equally as well.

That Kawtriple site sounds like just a lot of theory, about swapping jets when they change elevation. I'd like to hear about actual real world events where someone had to change the carb jets while out riding.

 

[weekendrider]

Maybe the article wasn't about CV carbs.

 

[grizld1]

Theoretical? RG, anybody with a little practical experience from the days before vacuum carburetors could tell you about the effects of altitude on carburetion. But since you want specifics, here ya go. I rode a new '68 Trunch Bonneville from Champaign, IL to Boulder, CO back in the summer of '68. Above 5000' things got nasty until I installed smaller mains and opened the air screws a touch. In fact I got lazy and only swapped jets after I'd dropped the bike on a fire road in the Roosevelt National Forest. I needed throttle coming out of a slide and got a fat rich sputter when I asked for it. And yes, the Kaw triples had nonvacuum carburetors.

 

[retiredgentleman]

grizld1.......................that's exactly the kind of real life story I like to here. It's not surprising that non vacuum carbs used on Triumps and Kawasakis need to have jets swapped out, when you change altitude by 4000 or 5000 ft.

The original poster has BS34 carbs (vacuum carbs) , and the thread somehow wandered into talk about Kawatriple carbs that are
non vacuum.
Its good to know that Mikuni vacuum carbs, in my experience, have no difficulty with elevation changes of 4000 ft.

 

[gggGary]

Those kaw triples are two strokes, yes they are fussier. The point was go high go lean, get cold get rich. Perhaps his set up was on the lean side when warm, and cold temps are enough to make it stumble lean?????????? If you managed to read to the end of the post I even mention that CV carbs are more forgiving. not everyone is a jetting pro, grabbed that as an example of a few variables in getting jetting right for where you live. Overhauled a rack of 3-2 barrel BS32's for a guy, even test ran em on my bike, they were running great, he bitched it didn't work for him. Finally the light came on, checked the altitude for his zip code, 5K' plus, minor details, grr.
PS The BS34's were a reaction to the "new" tighter EPA regs and were set raggity lean, even at my local 800 to 1200 AGL a pretty big change from the near blubbery rich BS38s
To the OP, remember him? I add a thin washer under the needle clip on every set of USA spec BS34's I open, for me it gets rid of that flat spot just off idle every time. This is a real world I did it to my bike(s) example. There is room for only one thin washer before the needle binds in the holder causing a hanging idle, that took a bit to find, LOL The adjustable needles are sposed to be good (better?) but I haven't tried them.
Edit; the needle shim is primarily a "Stock jetting kludge" not a no matter what main jet is in there, fix. As always fine tuning is a dance.

 

[retiredgentleman]

Gary................that is some excellent real world experience on carb tuning, and I hope the OP is listening and learning. Certainly large elevation changes call for extra attention with regard to jeting.
For the lads that make trips up into the mountains on a regular basis, as I do, should stick with using vacuum carbs. For the guys using non vacuum carbs, its likely not a big problem, because they seldom change elevation by more than a few thousand feet.
The non vacuum guys could always follow the Cessnas and Pipers and add a rich/lean control on their carbs.

 

[Terrible1]

To the OP, remember him? I add a thin washer under the needle clip on every set of USA spec BS34's I open, for me it gets rid of that flat spot just off idle every time. This is a real world I did it to my bike(s) example. There is room for only one thin washer before the needle binds in the holder causing a hanging idle, that took a bit to find, LOL The adjustable needles are sposed to be good (better?) but I haven't tried them.
Edit; the needle shim is primarily a "Stock jetting kludge" not a no matter what main jet is in there, fix. As always fine tuning is a dance.

This is actually what I am going to try first, but with the adjustable needles I have on the middle 3 position first. I have not had a chance as work is busy but will be doing this by the weekend for sure. Thanks again everyone!

 

[TwoManyXS1Bs]

Three videos showing carburetor icing: