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Discussion Starter #1 (Edited)
Subject says it. Is it the cv slide slow to drop back down that keeps the revs from dropping like I am used to on every other gasoline powered engine in the world. Just one of a few quirky things I do not like about this carb. Also the Racing high idle on full choke on a cold start seems excessive too. Is this normal. I should have my other bike running today and begin to compare.

EDIT: Sorry for the typos, I'm not worth a darn on my little screen phone and can't edit a subject.
 

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Definitely not normal. Also check that the throttle quadrant is returning all the way closed and that the return throttle cable isn't too loose or that the main cable isn't too tight. Main cable should have about 1/16 " of slack when throttle closed and return cable the same when throttle open. You can see the cables bow slightly in their respective positions.
 

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Yes I’ve had interference with the grip on the throttle too. My bikes have pretty high idle right away on any choke with a 34 pilot jet. With that pilot jet here in SD we don’t need to choke at all. I have little to no lag but I have tuned my bikes for the DG exhausts. Some guys have enlarged the hole in the slide but I haven’t found that necessary. Getting the right needle height using washers for the mid range really made my bikes snappy.
 

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Discussion Starter #5
Yes I’ve had interference with the grip on the throttle too. My bikes have pretty high idle right away on any choke with a 34 pilot jet. With that pilot jet here in SD we don’t need to choke at all. I have little to no lag but I have tuned my bikes for the DG exhausts. Some guys have enlarged the hole in the slide but I haven’t found that necessary. Getting the right needle height using washers for the mid range really made my bikes snappy.
I put a 34 pilot in my 03 as I went through that carb and it will come to life this afternoon so I will have something to compare to. I did loosen the return side cable and it maybe seemed a little better even though i think it was making it all the way back to the stop before, but maybe not quite as snappy and or decisive, maybe a bit a drag with it before. I amde a short video if I can post it, not sure how on this site.

 

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Before you do anything, adjust your valves! A leaky/tight exhaust valve creates this very symptom!

Jim
Would a leaky header to muffler exhaust leak cause this as well? I noticed my XT200 (same engine as the TW) seems to sometimes have the same symptom as what's being questioned here by hanging. On my ride a couple weeks ago I forgot to pay attention as my focus was on the trail.
 

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Would a leaky header to muffler exhaust leak cause this as well? I noticed my XT200 (same engine as the TW) seems to sometimes have the same symptom as what's being questioned here by hanging. On my ride a couple weeks ago I forgot to pay attention as my focus was on the trail.
Wouldn't think so but Old air cooled VW's sure do not deal with exhaust leaks at the head to header... Causes flat spots.... along with the exhaust popping of deceleration.

jt
 

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Discussion Starter #9
Does the video seem normal to you guys, It just seems to be slow in getting back down to idle on a throttle chop. Not just the chop but it seems to be pretty obvious when changing gears and you are wanting the rpm to fall off a little.
 

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Does the video seem normal to you guys, It just seems to be slow in getting back down to idle on a throttle chop. Not just the chop but it seems to be pretty obvious when changing gears and you are wanting the rpm to fall off a little.
Seems a little longer. I'll listen to the 3 TW's in my shop tonight and see what they sound like after they warm up.

My XT200 has hung up a lot longer than the TW in your video until recently when I reduced my idle speed in the hopes something sticking was the problem. This did help but not illuminated the hang. So I'm not sure if it's a linkage issue or vacuum issue or valve issue or exhaust issue. I suspect or would hope with a newer TW you wouldn't have one of these issues. I'm curious now about the valves being off and I don't remember a hanging idle before making any valve adjustments before.
 

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Discussion Starter #11 (Edited)
I have went for about a 20 mile ride out to a small town east of here this afternoon and it has gotten a lot worse and I have confirmed is it is not related to idle speed setting or butterfly not closing against the stop, it is intermittent and I can get it to idle back down with another blip and quick release after realizing it is hung up. I would say if idle is 1400 this is more than 2000 or 2500 rpm and is not exactly consistent.Going to head back to Home and look at it there some more.
 

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I’ve had the small spring under the small white cap on the needle fall off into the slide and that’s caused problems. Thought I had it on right.

I have went for about a 40 mile ride out to a small town east of here this afternoon and it has gotten a lot worse and I have confirmed is it is not related to idle speed setting or butterfly not closing against the stop, it is intermittent and I can get it to idle back down with another blip and quick release after realizing it is hung up. I would say if idle is 1400 this is more than 2000 or 2500 rpm and is not exactly consistent.Going to head back to Home and look at it there some more.
 

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Discussion Starter #13 (Edited)
Just now got back and caught it doing this as I stopped in from of the shop, here is a video of it on high idle and the throttle blades ae for sure seated on the idle speed stop, I blip the throttle and it drops down. Does this all the time when riding and has been making it feel crappy going through the gears but now I am closing in on a problem. This is still from the factory other than the original owner said he had pulled the pilot screw cap and tried tweaking it ?? I will learn more as I work on it. Motorcycles that won't idle back down make me nervous. I had a friend now long gone that his son crashed and was paralyzed by a fresh carb clean and his throttle stuck open on a pass and he panicked, and did not get to the kill switch.

 

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Seems a little longer. I'll listen to the 3 TW's in my shop tonight and see what they sound like after they warm up.

My XT200 has hung up a lot longer than the TW in your video until recently when I reduced my idle speed in the hopes something sticking was the problem. This did help but not illuminated the hang. So I'm not sure if it's a linkage issue or vacuum issue or valve issue or exhaust issue. I suspect or would hope with a newer TW you wouldn't have one of these issues. I'm curious now about the valves being off and I don't remember a hanging idle before making any valve adjustments before.
Most have not had the valves hang the Idle.. There have been 2 in the past year or two. Both of which my suggestion cured the problem.
When I got my tw it had this problem. It was a major mess and in my opinion never got any maintenance. It was used solely to run MJ out of the mountains on miles of RED DIRT road.

I rebuilt the carb, replaced the intake boot and it still had a problem.... I checked valve adjustments and my exhaust valve was way tight. I adjusted it and my throttle hang was cured. I was worried that something happened to the valve and seat but the problem never re-occured. I recently checked the valves after <>3000 miles and neither valve has drifted.

When you think about it it makes sense. If you do not have enough vacuum to pull the piston down properly the rpm will hang.. If you blip the clutch I found that the engine would return to idle.

Checking the valves before tearing into the carb and other things checking the valves is an easy thing to do and if the bike is new to you it would be a necessity just for a benchmark.

Jim
 

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Discussion Starter #15
This one is a 2016 with 490 miles on it now, so it seems unlikely but I will do it in the morning reguardless, it really is too simple to not do and I did buy it used so !!
 

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My Radian did this. It was a vacuum leak on the intake side which caused mine. Occasionally I had a stuck float needle which gave too much gas and caused this also. If the carb is adjusted too lean it can also cause the high idle slow throttle deceleration.
 

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^^^This^^^ I think it's still too lean to.
My Radian did this. It was a vacuum leak on the intake side which caused mine. Occasionally I had a stuck float needle which gave too much gas and caused this also. If the carb is adjusted too lean it can also cause the high idle slow throttle deceleration.
 

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Discussion Starter #18 (Edited)
Yes, this is my 2016 and is still stock jetting. I have bought the next larger main and pilot jet but not cracked it open yet. The PO says he tried adjusting the pilot air screw but I have not yet confirmed that yet. I will check valves, change oil and rotate the carb tomorrow to gain access and make sure things are at least looking clean. I have only drained a bit of fuel off the bottom to sample it for now. For some unexplained reason I am picturing the diaphragm is not dropping back down due to lack of vacuum so I will add a flammable spray vac leak check to the top of my list for tomorrow. I am having trouble thinking through how lean jetting causes the hanging high idle ??? Does lean idle jetting cause lower vacuum ?? If someone can explain, I will happily digest your every word, I am generally good on carbs and weak on these cv type carbs. That whole extra CV design just screws up 37 years of thought process tuning holleys on my racecars.
 

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The TK28 has a lot in common with the Harley-Davidson version of the Keihin CV40.

This troubleshooting guide may be of some help: Carburetor Troubleshooting Guide

My 2017 TW200 was very lean from the factory. However, all it needed to run well was to open the mixture screen (adds fuel, not air) to about 2 3/4 turns out from gentle seat. Good performance from 2,000 feet elevation to 8,000 feet elevation.

The CV40s on both Sportsters required
a. larger pilot jet
b. Mixture turned out to ~ 2 3/4
c. Richer (skinnier) needle
d. Larger Main (only for oil temperature control running sustained WOT up a long grade)

Back to the original question about returning to normal idle speed: The coaster circuit keeps the idle from dropping too fast to reduce hydrocarbons
See photos in this link: http://tw200forum.com/forum/technical-write-ups/881-tk-carb-photos-parts-identification.html

So, even if no vacuum leak, throttle plate closes as much as it should and proper slack in throttle cable, that diaphragm provides a damper.
 

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Discussion Starter #20
The TK28 has a lot in common with the Harley-Davidson version of the Keihin CV40.

This troubleshooting guide may be of some help: Carburetor Troubleshooting Guide

My 2017 TW200 was very lean from the factory. However, all it needed to run well was to open the mixture screen (adds fuel, not air) to about 2 3/4 turns out from gentle seat. Good performance from 2,000 feet elevation to 8,000 feet elevation.

The CV40s on both Sportsters required
a. larger pilot jet
b. Mixture turned out to ~ 2 3/4
c. Richer (skinnier) needle
d. Larger Main (only for oil temperature control running sustained WOT up a long grade)
Thanks, good tip on the harleys CV's and study guide for theory

Good to hear on the stock jetting and the pilot screw setting as I am at 800 to 2500 feet myself. I am committing the cardinal sin of working on both bikes at once, 2016 with all stock jetting and no mods other than pilot needle cap allegedly removed by PO but I have not checked yet. it seems lean on the idle end for sure, takes excessive choke to get and keep running until hot !! Also working on my 03 I just got bacislly not running and only 331 miles on the clock, it had to have a real deal carb cleaning and soak/dipping to recover it. looks new again now. I added the popular fatter 128 main in place of the 126 and the 34 pilot jet in place of the 31 with plans that both would at least run, one on the lean side and the other maybe on the rich or just fine and I could compare and contrast. The 03 is getting a DG pipe right away so that partially plays into fattening it up. I maybe even put my wide band setup into a spare head pipe for some date collecting as it is always a good telling chunk of information on fuel delivery.

Still my main focus in all this is the failure to return to proper idle idle sometimes on the all stock 2016 here at first. It seems to mostly do ok on quick throttle chops but hangs at a high idle when coming down slowly and gently with the throttle. it is absolutely confirmed it is unrelated to the resting on the idle setting stop screw. Tomorrow is another day. Good to be semi retired on these projects.
 
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