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Post by muffler82 on Dec 11, 2008 8:54:19 GMT -5
Hello everybody! I took advantage of last Sunday afternoon's sun to go out for a ride.  Strange thing has happened after about 10 km of trip: I slowed down at a stop sign and the engine... got stopped.  Tried various times to kick to re-start but nothing happened. At the end, I tried to kick after pushing the "hot start" white knurl on the carb's body (never used before as I remember!...) and finally the engine started. I decided to go back home, and when I slowed down at crossroads, roundabouts etc. the engine's RPMs slowed so down that I had to keep on turning the handgrip to prevent the engine from turning off. (Once at home, I noticed that the white knurl had reset itself during back trip.) When at home, I decided to inquire into the problem's cause. I tried to adjust the pilot adjuster screw: until that moment it was set to standard 1 - 7/8 turns out, but then I found that the engine started and kept running at idle only if I get the screw fully screwed in!  (With white knurl in OFF position, obviously.) If I tried to unscrew the screw little by little, RPMs slowed down to finally stop near standard 1 - 7/8 turns out. I tried to unscrew the spark plug too to see its colour (also if I think that it's not so relevant because of the many regulations): extremely black and sooty. I say first of all that during last days' rides nothing like this happened. I'm currently running with stock muffler, stock paper air filter and NGK BP7ES. My carb is the stock VM34SS, but without accelerator pump (it's been dismantled by previous owner, every passage in the carb that's related to te pump have been sealed): all jets, needle, screw etc. that are installed were bought as a Rebuild-Kit from Kedo. What could have happen? Regards Simone |
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Post by muffler82 on Dec 11, 2008 20:08:31 GMT -5
Update: I remember that when I overhauled the carb I set the float height to 22.0 mm +/- 2.5 (source: service manual) but now I discover that's wrong as Haynes says it's the height for the TT500 carb's float...  So I've dismantled the carb, adjusted the float height (I've set at the tolerance limit to try: 23.5 +/- 1.0 mm, I've set at 24.5 mm) and replaced the jets, needle, screw etc. with the old ones I dismantled when overhauling - they show very little wear signs on, but they're the original Mikuni, not like the new I had installed. I checked the needle's height, too. Couldn't warm up the bike enough because I had to come back to work and I hadn't time, results are surely invalidate by this thing. Situation doesn't look a lot better. The only positive thing is that the engine started without using the "with knurl" (maybe exactly because the engine was cold?!?). The rest: turning the pilot jet screw changes nothing, and fuel consumption seems to remain quite high (I've seen with my eyes because the fuel tank was dismantled, and I've feed the carb with a funnel and a pipe. Anyway some fuel could have evaporate because of open funnel, vibrations and heat from engine.) I've tried with Colortune too, but with cold engine I know it's been a waste of time. Anyway this is what it says: rich mixture, flame is orangeish becoming lighter when revving. Colour seems "quite" regular at idle. Just above idle, flame is a good blueish. ...  Greetings Simone |
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Post by StewRoss on Dec 12, 2008 1:03:44 GMT -5
Hi, These carbs are not so good when they get old (34s)...the 32's in my experience were never very good after a while at all...might try the pump lever to see if it's still connected... Otherwise try the 'replacing with a VM round slide' route. Once set up they're trouble free and you get a mild performance gain as well! Perhaps place the 34 on a shelf on display where they eventually all belong...heh, heh... SR  |
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altf4
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Post by altf4 on Dec 12, 2008 14:35:30 GMT -5
.:. but you haven't replaced the membranes, have you??? they can cause all sorts of grief...for the not so faint hearted there's also a possibility for carb modification that gets rid of the membranes altogether. maybe i should translate this one day... best, max ~ |
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Post by Just Plain Bill on Dec 12, 2008 17:11:48 GMT -5
.:. but you haven't replaced the membranes, have you??? they can cause all sorts of grief...for the not so faint hearted there's also a possibility for carb modification that gets rid of the membranes altogether. maybe i should translate this one day... best, max ~ Max, I think you are referring to the accelerator pump diaphragm as a "membrane" . Simone says that the accelerator pump has been disabled, so the membrane may have been removed. In any case, it's out of the loop. I'm not sure about this but I think the membrane on the carb body (enricher valve) is a decel diaphragm that opens an air circuit when coasting to prevent excess gas from being drawn into the engine due to the high vacuum caused by a closed throttle and high RPM. It would not be involved with a start/idle problem. The third membrane (on the float bowl) is a pump safety valve. I didn't know that before finding the Ron Sutton article on the VM34SS carburetor: www.siue.edu/~rsutton/SR/mikuni.htm (Sutton has compiled a lot of good stuff on SR500's.) As for your problem Simone, the best thing would be to start by checking the adjustment of the intake and exhaust valves. If they are out of adjustment, they can affect starting and engine idle speed so you have to have that variable under control before fooling with carburetor settings. Since you've been changing jets, it might be a good idea to go back inside the carb to verify that jets are to factory specifications too if you didn't check that during your recent work. Next, re-set the pilot (actually it's an air adjust) screw to 1 7/8th turns out. It has very little to do with idle speed. It's like the valves, you need to start with everything adjusted according to specifications and work from there. My service manual (Yamaha Lit 11616-SR-50, dated Aug 1980) shows float level for the SR500 as 23.5mm +/- 0.5mm, so at 24.5mm, you are a little on the high side. It shouldn't be a problem unless you are running at high speeds for extended periods of time. The earlier 22mm setting might have been involved in your problem since the fuel level is higher in the bowl and it might have been drawn into the engine through the main jet/needle jet circuit (your black sparkplug indicates a very rich fuel mixture). That's another thing. You want to be sure that your sparkplug is the correct heat range (N-7Y Champion or BP7ES NKG or equivalent) and that the gap is set between 0.7-0.8mm. You also need to either clean it properly (sand blast) or replace it with a new one as part of having all your variables controlled. The white "hot start" button merely cracks the throttle open a set amount to better control air/fuel mix at start. (Four stroke singles are notoriously difficult to start when hot.) However, the idle speed adjustment is made with the same screw, without engaging the button. If you've had the carburetor apart, be sure that the tensioning spring that fits over the screw is in place. Without it, engine vibration will cause the screw to back out, lowering idle speed. To increase idle speed turn the knurled metal wheel above the white button clockwise, turn it counter clockwise to lower idle speed. Once you have everything correctly set and adjusted, start the engine and let it warm up (one to three minutes). Use the knurled wheel to adjust idle speed to 1100 RPM. Check the throttle to see that there is free play in the cable (2-5mm is the recommended amount) and adjust if needed. Test ride. If that doesn't solve your problem, you will have to tear the carburetor down and soak it in a strong cleaner followed by blowing through the circuits and maybe even pushing wires through the circuits to ream and loosen varnish and other blockages. Bill |
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Post by StewRoss on Dec 12, 2008 18:54:58 GMT -5
Ooops, I didn't notice that 'disabling of the pump'...hmmm, sound like a great carby to get rid of to me.  The white push button is only there for hot starts...once you've started it, moving the throttle back to idle will re-set it to off again unless it is stuck up. SR |
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altf4
New Member
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Post by altf4 on Dec 13, 2008 1:59:39 GMT -5
.:.
re: diaphragms; bill your information is correct, although my bike showed the same symptoms as Simone's...and it drove me nearly crazy solving it...changing the membranes cured the problem though, don't ask me why ;)
then again, an old vacuum tube running from the fuel tap can cause weird idling, too, often resulting in a non-starter, also the tap itself can go wrong, there's another diaphragm in there. this short description reflects my approach to the problem - i'm curious as to see what was the cause of it all!
good luck, Simone!
max ~:)
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Post by muffler82 on Dec 16, 2008 8:26:41 GMT -5
Thanks everybody for answering! I've stripped the carb down again, re-checked float height, jets, needle, etc. Have blown compressed air into all passages, checked membrans, everything seems OK. I've also seen that the new jets I installed follow the factory specifications. I've checked intake and exhaust valves too: they are OK. Could not try the bike on the road because of the filthy weather we have here in these days when I'll try I'll disconnect and seal the fuel tap's vaccum pipe anyway to see if the problem is related to the tap's membran. Regarding to StewRoss's "VM round slide" idea: directing to a 34mm, as standard carb is, does it fit in intake and air filter manifolds as it is or modifications are required? (that's not a bad idea after all  Best regards Simone |
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Post by StewRoss on Dec 17, 2008 2:06:39 GMT -5
No the carby won't fit the outlet of the airbox but it will fit into the SR's rubber intake manifold... See pics here: sr500forum.proboards38.com/index.cgi?board=Fuel&action=display&thread=2462&page=2You can also use the 36mm Mikuni and possibly the 34mm as well... A relatively simple change all up. You can modify the standard throttle cables or buy one to suit form the US and maybe other places as well. You can also fit a MX style throttle and cable to suit an older MX bike...these came with a similar carb to these ones. Once set up and sorted, the new carb will be trouble free... SR |
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Post by muffler82 on Dec 20, 2008 18:52:18 GMT -5
No the carby won't fit the outlet of the airbox but it will fit into the SR's rubber intake manifold... See pics here: xt500forum.boards.net/index.cgi?board=Fuel&action=display&thread=2462&page=2You can also use the 36mm Mikuni and possibly the 34mm as well... A relatively simple change all up. You can modify the standard throttle cables or buy one to suit form the US and maybe other places as well. You can also fit a MX style throttle and cable to suit an older MX bike...these came with a similar carb to these ones. Once set up and sorted, the new carb will be trouble free... SR Perfect, so relatively minor changes are required. Thanks for answering. Here's an update for the Carb problem: Maybe I've found out. It seems that the aftermarket air filter I had installed:  (now replaced with this one:  like the one factory fitted) was too "closed", giving so a very rich mixture. Best regards Simone |
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Post by brownie on Sept 4, 2014 16:33:18 GMT -5
Hi everyone
Just after joining. I have just bought an 1983 xt 500. It was sitting in a shed for 5yrs. I got it going, full service. Plugs points etc. However it is not running right. Snatchy throttle and high fuel consumption. Striped and cleaned the carb but still the same. Fitted a carb rebuild kit. Will start without choke, but will only run on choke. When I turn off the choke the bike will cut out. When I rev it up it will backfire and splutter. When fitting the main needle. I did notice thar the emulsifier tube was slightly oval. Reamed it out to make it round. Has anyone ever come across this before. Any help would be appricated. Tanx
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Tried various times to kick to re-start but nothing happened. At the end, I tried to kick after pushing the "hot start" white knurl on the carb's body (never used before as I remember!...) and finally the engine started.
