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Post by mochimochi on Sept 10, 2008 22:50:37 GMT -5
Hi Folks, I'm new to the forum after having been reunited with an SR500. Had one many years ago, sold it (stupid), just bought a lovely rebuilt machine. Generally speaking this is a beautiful bike, but I've got a problem with part throttle smoothness and cruising. Here's what happens.
The bike idles perfectly and runs well at larger throttle settings. But after running or a while at more open throttle settings if the throttle is shut the engine dies and is hard to restart. Also at low speeds and part throttle the engine hunts and surges - low speed part throttle cruising is not fun especially on a slight down-grade - lots of pops and odd burbles from the exhaust.
This problem seems worse when the bike is warm, completely warm or has been running for some time - but happens at all temperatures.
Bike is equipped with a mikuni and a supertrap exhaust - K&N air filter.
There is no battery - there is a battery bypass.
The bike was upgraded to a 550 and has a fast cam.
I pulled the spark plug to check it and the mixture. despite the clean freshly rebuilt engine the plug was old and worn out. I figured that might be the source of my problem. I replaced the plug with a new one. the problem did not go away. nothing changed.
After running for a while the plug showed a lot of soot - obviously running quite rich.
Relying on memory I decided to replace the main jet ( I know not the smartest place to start - memory was not fully engaged or accurate). I went from a 260 to a 240. The resultant plug reading was improved but odd. The plug had been uniformly black before the change. Now it was still mostly black and sooty, but one side of the plug was now a nice medium brown, but only on one side.
The symptoms:
- randomly dies coming off idle.
- when running at slow speeds, low rpms, part throttle the bike surges and stumbles, basically runs very rough
- when the bike is at part throttle (engine braking) down an incline there tends to be a lot of unevenness and odd noise from the exhaust (sounds a little like a 2-stroke on deceleration)
- The bike accelerates smoothly and enjoys a lot of throttle, seems to run fine at that kind of condition.
- After riding for a while if I pull up to a stop light, or if I suddenly pull the clutch and close the throttle suddenly, the engine dies.
- In a case of being at idle I need to keep the throttle open slightly and the rpms up, or bip the throttle... or the engine will die.
- once the engine dies, it takes several kicks to get it started. it does not like to restart after one of these events.
This is really different behavior than my old SR500. My old bike started easily and ran smoothly. It had a mikuni much like this one and it never did this die out thing. I'm assuming this is a carb tuning problem because of the soot. It could be some kind of electrical issue, but I tend to suspect the carb.
I am going to check with the previous owner and ask for a few more details about the bike. I don't have all the details and specs for what he did. He did a lot and to my eye it all looks really good. The bike was purchased with the understanding that the carb needed a little tuning. The previous owner is a good guy and really loved this bike as do I. I just need to work out this problem.
Any suggestions on where to start or what might be the problem? Your help will be enormously appreciated.
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Post by solo2racr on Sept 10, 2008 23:03:16 GMT -5
The carb sounds like it is to lean with the pilot jet and a bit rich on the main. I would also check for possible air leaks. A long shot here but the timing may be too advanced.
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Post by mochimochi on Sept 12, 2008 0:46:23 GMT -5
Thanks for the suggestions. I found some good references and procedures for setting the main, needle, and pilot. I'll give it a try this weekend and report back on the results.
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Post by andy on Sept 14, 2008 14:11:56 GMT -5
Hi mooch,read your thread,sounds similar to my set-up.I'm going through the 'experimental stage with settings'.At constant low throttle running i feel a bit of surging and its only when i get into the 4000 r.p.m and above that it really takes of and runs fine,however onthe over run it doesnt sound good and the symptoms are similar to yours.As i see it [unless anybody can tell me different] there are 2 main areas to play with 1.get 2-3 bigger/smaller pilot jets and try these first 2.try different needle clip positions.If you take a few bits of kit with you you can then make the changes and compare the difference.Like solo says i reckon the pilot has to match up with the main as you can get rich/weak on one and opposite on other.Right nowI have just gone down a notch[richer] on the needle and put the pilot up to 22.5 from a 20 [i've also got a 25 pilot]result? its not as good as the previous set-up overall but really picks its feet up 4000 onwards hence i've left the needle alone for the next test.The final solution if you cant crack the problem is find a good dyno centre that will advise you exactly what your mixture is across the whole range.My slight advantage over you is that i can go back to a set-up that works fairly well if all else fails.Keep in touch,let me know your results,cheers.
Regards,Andy
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Post by mochimochi on Sept 15, 2008 3:35:26 GMT -5
Thanks Andy. Also good suggestions. I'll look into this a little deeper. But I have been able to make some headway. I'm breaking the problem down and isolating areas of adjustment. Update the carb in question is a 38mm VM Mikuni round slide. This evening I worked solely with the air screw and idle adjust. The air screw does not seem to have any major effect on idle but it is noticeable. I set the idle slightly high 1200rpm. And dialed the air screw until I found the maximum idle. The brought the idle down via the adjuster screw and tried tuning the air screw again. As I said - no major differences but I found a subtle maximum ( and when the air screw was all the way in or too far out the idle got bad). I think this is a good sign because it indicates that the pilot jet is not completely off. The first thing I addressed was the engine stalling when the throttle was closed after running part throttle. The solution to this was turning out the air screw to make the bike run a little richer. I also very slightly inched the idle up - but not more than a few revs. When I went leaner with the air screw the engine would die when the throttle closed. Given that there were two options, having it die when lean, or not die and continue to run when slightly richer, the latter seemed like the better option. Next was a more detailed understanding of the surging at part throttle. I think my neighbors are pretty tired of my slow runs up and down the road in front of my house. I believe one of them shouted some invective from an over looking balcony. I continued to make minor adjustments to the air screw dialing in more/less richness. This resulted in minor changes in the second problem... At very slight throttle openings when running in the range of about 2250-2750 rpm there was occasional roughness and surging. Above or below things were fine. This surging was most noticeable going up a slight incline. Coming down the same incline at about the same rpm the hesitation was also there. The needle is one notch up from the lowest setting. I don't as yet have a specific percent of total throttle opening which corresponds to the 2500rpm stumbling problem. I know I'm only cracking it open very slightly on the down hill, so that pouts things in the range of the pilot jet and air screw. Going richer did seem to help here. It is possible that I may need to go to a richer pilot jet. Up hill at 2500 may be getting into the range of the needle jet. I think it will make sense to try going up and down one needle position to compare what happens at the 2500rpm mark. This will be my next experiment. I'm looking at Mikuni's manual and suggestions for adjustment. If anyone has any suggestions please let me know. I'll update this post with my results. Perhaps this next time I'll find out what my neighbors are shouting. Thanks again for the thoughts and consideration. I really appreciate the helpful suggestions. manual: www.mikuni.com/pdf/vmmanual.pdf |
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Post by caferacercarl on Sept 15, 2008 3:49:15 GMT -5
its difficult to get a two stroke carb to carburret perfect on a four stroke at just that point, its all in the needle jet, PM curly on this forum for his pilot and needle jet settings.
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Post by solo2racr on Sept 15, 2008 11:36:47 GMT -5
This post may help some..... VM-34, VM-36, VM-38 TM-36, TM-40 (HS-40) Sizes and tuningsGreat tuning manual for Mikunis TM-36, TM-40 and HS-40www.mikuni.com/pdf/hs40_manual.pdfVM34-168Engine side:40mm, Aircleaner side:58mm This from Spindlehead in regards to a VM34-168.: [ If you have:
aftermarket intake (e.g. K&N, Uni, or other free-flowing device) AND
a freer flowing aftermarket exhaust (e.g. supertrapp),
then a 240 main,
25 pilot,
P8 needle jet, and the 6DH4 needle that comes with the carb is a good start.
If you have:
aftermarket intake (e.g. K&N, Uni, or other free-flowing device) AND stock exahust
then a 240 main,
25 pilot,
P7 needle jet, and the 6DH4 needle that comes with the carb is a good start
I suggest buying main jets 220-260; pilot jets 25, 27.5, 30; and the P7 and/or P8 needle jet.
VM-36Most stock VM36's come: Mikuni VM36 round slide carburetor: Jet sizes : Main : 310; Pilot : 35; Needle : Q-5; Throttle Valve : 2.5; Jet Needle : 6FJ6; Air Jet : 2.0. Engine side:43mm, Aircleaner side:62 Curley's setup VM36"
Here's my VM36 specs
Pilot 35
Main 230
Slide 2.5
Jet (needle) P2(188) custom drilling 1or2 thou larger
Needle 6FJ4/7 Top clip with a shim
VM-38Engine side:43mm, Aircleaner side:62 StewRoss VM-38 tuning recommendation: 280 main
30 pilot...possibly too small?
Needle 6F8/2
3.0 slide cutaway
Perhaps a P4 needle jet...
Powerjet is blocked off...
I have several lots of 38 settings, so to totally confuse you here are some more:
240-270 main
30 pilot
Needle 6F8/2
P4 NJ
#2 air jet
1.5 slide cutaway
or
270 main
35 pilot
Needle 6F8/3
P6 NJ
#2 air jet
7.0MM slide cutaway (MEASURED)ColinJays race engine with a VM38:230 Main
6F9 needle on the 2nd clip
P-6 needle jet
#2.0 Slide
20 pilot jetTM -36-68Most stock TM36-68 flatslides come stock like this: 42mm air spigot (engine side of carb) 54mm-air cleaner side Jetting: 130 MJ, 12.5 PJ, P8 needle jet, 2.0 Slide, 1.0 airjet. Engine side:42mm, Aircleaner side:54mm From what I can glean at this sight this is a good starting point for the TM36-68 for stock SR 500. Greymin's tunings for the TM36: pilot 17.5 screw 1.25 turns out
main 150
needle clip second from leanest
factory default needle jet (p-8) and needle.
"I get 50 mpg. and I ride 45 miles a day round trip to work.
I also backed the pumper adjustment way off for better economy.
It won't really buck until I lug it under 2750 rpm. My old bike with stock carb would buck at anything below 3 krpm.
11 discs in the Trapp."TM-40 or HS-40Engine side:43mm, Aircleaner side:55mm Steve's suggestion for a HS-40 (same as TM40) pumper flatslide:
I don't know if this will be any help but the HS40 that I just put on had a #150 main (actual size=1.50mm) and 17.5 pilot jet but was slightly too rich at full throttle so I just went down to a #140.What about all of these snowmobile VM36 and VM38 (mostly VM38s) carbs sold on Ebay? What is the electrical cord for? Heat? I would guess you might have to change every jet in them. Cliff |
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Post by mochimochi on Sept 16, 2008 1:32:43 GMT -5
Thank you Solo - much appreciated. I'll comb through the information.
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Post by andy on Sept 16, 2008 8:11:38 GMT -5
Hi Mooch,reckon i've just about cracked it,gone back to a 20 pilot and weakened the needle to 2nd from top,its so much better right across the rev range,picks up really well from 3000 and is quite happy on lower throttle with no lumpiness.I've always erred on the richer side when making changes i,e bigger pilot raising the needle,reversed this thinking and its been a big improvement and i'm sure the fuel consumption will be better.What i've found with the pilot is when i,ve gone bigger i find i'm screwing the adjustment maybe 3-4 turns out to get the best tickover,the 20 feels dead right.Sounds like your getting there now but remember theres infinite carbs/settings depending on your engines internals,this is why i recommend taking the bike out and making small changes in quick succesion so you can easily compare the differences,also you won't piss your neighbours off! good feeling when you hit the sweetspot not to mention the satisfaction of sorting it out yourself.Keep tinkering.
Regards,Andy
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Post by mochimochi on Sept 16, 2008 10:58:24 GMT -5
Thanks Andy. That's interesting data and experience. The thing that has been a challenge for me is remembering how different carbs are than say fuel injection systems. The carb having different circuits, idle, part throttle, full throttle - each needing tuning and adjustment. Change a carb size, use a different cam, or open the exhaust up a little and everything changes.
All the settings I'm playing with right now are very different than my old SR500 which had a stock exhaust, cam and a 36mm mikuni. The simplicity and singular nature of the SR's engine seems to exaggerate settings changes that would in other engines result in subtle variations.
I'm happy the bike does not die when the throttle is closed. Important progress has been made there. I think the main jet has helped a bit in the upper end. I know its running rich though. And since I have not been cranking it out at full throttle I think I can safely say the next area of tinkering is the needle. I'm going to adjust it one step leaner. If that does not work there is no option but to replace it and go down a size.
While there is always a temptation to "try a bunch of stuff" all at the same time, I'm staying restrained and focused. One thing at a time.
As to my neighbors. I think I was being quite restrained there. I mean its not like I was reving high or running the engine hard. It was not late at night. Perhaps they are just philistines unaware of the beauty of the SR;) But I'll hold off on further harsh critiques of my neighbors. The bike does not sound particularly loud to me. But my neighborhood is very quiet and does echo a bit. Perhaps too much so for the distinctive thumping bass note of the SR. I've located a couple of nearby areas where I can test without disturbing anyone. Having a test area seems very important in this particular case since running over the same stretch of road conditions lets me evaluate subtle changes in a controlled and repeatable fashion.
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cliff
Junior Member
Posts: 42
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Post by cliff on Sept 18, 2008 8:45:32 GMT -5
Having recently revived one of more SR's and struggling with carb settings from a new carb I am going to offer only a few suggestions that may make your life easier.
1). Make sure your valves are adjusted correctly. Incorrect adjustment of the intake can cause irratic behavior.
2). Make sure your timing is correct and the timing is advancing correctly. My CDI was advancing far more than specifications but it would still cruise at 70 easy but the behavior was irratic causing me to tinker with the carb more.
3). If the carb has been sitting any length of time with fuel in it you probably have changed or sealed off orifices that require proper running. You will have to replace or clean the carb. The only clean that has worked for me is a 3 day dunk in chem-tools carb cleaner and that is after taking off all non-metallic parts.
Just offering my experiences.
Cliff
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Post by mochimochi on Sept 22, 2008 1:56:37 GMT -5
Thanks for the tips cliff. The carb is new and the previous owner drained the bowl after each ride. The valves need a little adjustment - I need to close them up a little.
But good news! A steady process of small and progressive adjustments is making headway and turning things around. I started with the pilot air adjust and idle tweak. That worked nicely.
Today I made an adjustment to the needle. It was set in the middle of 5 possible settings - so half way up/down. I moved it down one setting to make the carb run leaner at part throttle. then took the bike for a test run. Immediate positive results. The rough running at part throttle improved. It did not entirely disappear, but operation was considerably improved.
Low speed operation was much easier and safer. The throttle response had been very touchy and the rough running at part throttle made it feel very awkward in low speed turns and parking lot negotiation.
Dropping the needle one stop caused an immediate improvement. After running for a while I noticed that the problem had not entirely been remedied, but the rpm at which the roughness would kick in was increased to 3000 - and when it came on the symptom was less severe. Below 3000 things were fine. The exhaust noise on deceleration at part throttle had also quieted down.
I ran out of time and daylight today, but I'm going to drop the needle another stop as soon as possible. I'll check the engine behavior and see how that works. If there is more improvement but dropping 1stop does not make this problem go away I'll go to a smaller needle and seat.
This is a real relief. The bike is starting to feel right.
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Post by jdbrees on Sept 22, 2008 12:53:00 GMT -5
Put a piece of tape on your throttle grip. Put another piece of tape on the stationary throttle housing. Make marks on both pieces of tape that are aligned when the throttle is closed.
Next, hold the throttle wide open and make a mark on the stationary tape that aligns with the mark on the throttle grip.
Between those two marks on the stationary tape, make marks to indicate eighth, quarter, three eighths, half, etc.
Now instead of looking at your tachometer during troublesome engine performance, look at the throttle opening instead. This will help you find which carb circuit is being used when the problem occurs.
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Post by mochimochi on Sept 23, 2008 1:50:39 GMT -5
Great suggestion jdrees. I've been keeping an eye on the throttle twist, but I have not making absolute measurements and comparing them as I make the adjustments. Given that this is largely throttle position based not measuring and comparing is an oversight. I'll remedy that  Thanks for the reminder. |
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Post by mochimochi on Sept 30, 2008 1:09:49 GMT -5
Progress update and more questions:
So since the last installment I've focused on the needle. The needle has 6F8 scribed on it. There are no other numbers on the needle itself... but there is what looks like Square Box "logo" with a smaller box in the lower left corner. More details... just in case... I noticed that a lot of needles are called out with an extra number lit /2 or /3... but not this one.
The recap is that the needle was originally on the middle setting. I dropped it one setting to make it leaner. That improved the roughness that I was noticing at about 2500rpm. The roughness shifted to near 3000rpm and diminished in intensity. Checking the spark plugs showed that it was no longer black, but it was a nice light to medium brown.
Over the weekend I did more testing. I dropped the needle one more notch to the lowest (leanest) setting. The result was that deceleration cleaned up on slight down hills. Part throttle roughness at 3000k rpm was almost completely gone. The throttle setting for this was less than 1/4. Closer to 1/8 throttle. So I'm pretty sure I'm still in the idle circuit or at least in that cross over area between the idle circuit and the needle circuit.
BUT! there was a small problem. Opening up the throttle to a 1/2 setting brought out a leanness problem in the needle circuit. Under some load and throttle settings there was detonation (pinging). Not a lot of pinging - a couple of detonations then it would go away as rpm increased.
I pulled the spark plug and did a reading. It was a bit on the ashy white side. So we were a little too lean. I'm going to test it a little more at that previous slightly richer needle setting to make sure my results are consistent with previous runs.
But now I'm starting to wonder about the cut-away. Someone correct me if I'm wrong here but it seems like the cut-away is the means of transitioning from the Idle Circuit to the Needle Circuit. I'm guessing that my cut away is a little on the rich side.
It would seem like my needle circuit on the 2nd from top setting is pretty good - not too rich or lean. My idle circuit seems ok too (no more stalling - no roughness there - making it leaner only makes the idle drop and sputter - dialing out the screw make the engine idle well). It's in the transition area that the problems arise. A leaner needle seems to help but that can't be sustained above 1/4 throttle because of the detonation. A leaner cut-away could lean out that 1/8-1/4 throttle range. Or so it would seem in my understanding of carb operation.
Does anyone have any experience with this? Is my understanding of the cut-away correct? And does this sound like the right direction to pursue as a next step? I'll need to get in and find out what the cutaway is on my throttle.
Also from what I have read leanness increases with a larger cut-away and richness increases with a smaller cutaway. So a 3.0mm cut-away is leaner than a 1.0mm cut-away.
Thank you in advance.
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Thanks for the reminder.