View Full Version : Minor stumbling at small throttle openings
Quick one
New engine's in and running well (or as well as can be...) but it has a bit of an issue on tiny, tiny throttle openings. If you just barely crack the throttle at low RPM (Under 4000 or so), it feels like one of the cylinders is misfiring, with the bike jerking. No problems at all at higher RPM or throttle openings, no problems at idle.
There's currently an exhaust leak on the rear header (which I have the gasket to fix, just need a new front one to match), the throttle cables aren't routed well (but work), and there are no vacuum leaks that I can find. Spark plugs are new, and HT caps have been reattached to freshly cut leads. Any ideas?
Alpinestarhero
25-01-10, 04:33 PM
is it carbed or injected? If carb'd, check the carb balancing and choke is operating...if injected, then throttle body balance and TPS?
yorkie_chris
25-01-10, 04:37 PM
Sounds a tad lean, what have you got the carb low speed screws set at?
Not a clue, YC, but I know that they're rounded off, because I had a looksee when they were off for cleaning a few months back, so doubt I can easily adjust them. For A*H, carbs are balanced and the choke's spot on - recently had the plungers out and greased 'em up good, plus packed the splitter with grease.
Looks like I may need me a big hammer.
Having a similar problem. I thought mine was down to contaminated fuel but it isn't as I have a new tank and fresh fuel. When filtering through traffic at 4k revs on 1/8 to 1/4 throttle its really err lumpy. Open the throttle and it smooths out instantly. Its getting worse too so I suspect I'll find out the cause soon! Sometimes it clears too and you can really feel the difference, its much smoother and picks up cleaner and quicker as well. BMC filter dynojet and mixture screws 2.75 turns out. Picture of the plugs attached. Looks like one cylinder is richer than the other? I did put a new choke cable on and cleaned the plungers but the choke operation doesn't seem smooth?
In my experience you really need to adjust the idle mixtures individually while the engine is running in order to get the "tip-in" just right. Setting to a specified number of turns is only approximate due to tolerances. The screws do need to be clean and free to turn, if they're not you'll have little success. It is a bit of an art, needs practice to get the feel.
Having said that, unless you have a right-angled drive adjuster tool, it's pretty tricky on an SV.
First obviously all the other adjustments need to be correct first, at the very least carb balance, no leaks, needles set in the right grooves, chokes closing off properly etc, and in a perfect world valve clearances too.
I have found a thin cotton/thermal type glove will let me get my hand in there just long enough to adjust the screws a bit before it gets too hot.
Practice while the engine's cold.
If all else fails, just richen them 1/4 turn and see what happens, if you stick to 1/4 turns and keep a note you can always go back where they were.
If you're stuck and can't make any progress, at least put in some iridium plugs, they are more tolerant of lean mixture excursions (rather the lean limit is extended a bit with them). They don't fix the problem but can make it easier to live with.
Unfortunately for me, my mixture screws aren't going anywhere :(
Definitely no vacuum leaks, carbs were balanced very recently, choke definitely not sticking, valve clearances all within tolerances. The needle...not sure about, but it was never a problem before very recently....
The rear cylinder has an iridium plug in it. Unfortunately the front one I left in the old engine before it got sent back to the breakers, so it's got a standard one in there, but it was brand new and gapped correctly, at least.
Might have a bash at getting the mixture screws to move at the weekend, but I'm pretty sure they're fully rounded off. At the very least, I need to get the throttle cables routed properly anyway.
If you want a spare set of carbs to try Neeja give me a shout I have a set in a box, not great but worth a go if you fancy it
Might be worthwhile. If you pass 'em over to me I can strip+clean them and then give 'em a go...worst case scenario then is you've got a clean set of carbs ;)
yer no probs, I brought them to take em to bits to see how they worked, on one of them the pilot screw will be out and erm not screwed back into the same position as I had a look to see what it was, basically I got em for a fiver cause I wanted to see what was what before taking mine to bits. Pm you your address m8 and I'll drop them off or pop round tomorrow after 7 to pick em up, I think you have my address. (or now if you're up lol)
Mine was weird this morning. Didn't really want to start and kept spluttering and cutting out. Eventually it did start but coughed and spluttered. As I worked the choke in it started to sit at a fast idle which it hasn't done for some time?
On the ride into work it seemed to run better after each mile. There is still a hestitancy there at part throttle but no where near as bad as it was. So I'm guessing I either had a stuck choke plunger or a partially blocked pilot jet?
I'll pull the carbs at the weekend and check both things. If I find anything i'll report back and let you know Neeja in case it is the same issue as yours.
Late reply; grandfather's funeral today, so wasn't online. I'll give you a shout sometime soon for the carbs, 5hort5, but today the bike seemed much better. Go figure.
Neeja I've stripped my carbs down this morning and found a partially blocked pilot jet. You couldn't see light through it but could blow through it just. Tried to capture it in picture but macro is crap on my point and shoot camera.
That's one of the things I'm suspecting, but I had new pilot jets put in about...probably 3 months ago, with a full carb clean then. I'm going to get around to tearing them down at some point soon, honest, but lots of crazy stuff going on at the moment means I've barely got time to take a leak. Let me know if this fixes things for you, as it'll save me running through a checklist of other stuff first.
Well my bike starts and runs a lot better overall. But I now have a stumble if you open the throttle after steady cruising at around 4k. The carbs have a dynojet kit fitted and the mixture screws were at 2.75 turns out. I thought that seems very rich and backed them in to 2.5 turns. Looks like i'll have to put them back to 2.75 again! http://img32.imageshack.us/img32/8749/dohq.gif
Update on this:
Today my fuel-light came on with only 100 miles since last fillup. That's about 25 miles sooner than normal. Am now suspecting that one of the choke plungers isn't quite as it should be. Will tear the carbs down at the weekend.
Where did you get to with this? Have you sorted it yet?
When I pulled the tank and airbox off I discovered that the rear plunger didn't seem to be sitting quite right. A quick fiddle with it showed that it was was being pulled downwards, probably enough to pour excess fuel into the rear cylinder pretty much constantly.
I took the carbs off, rerouted all the cables from scratch and refitted them. I'm pretty sure I've now got a fairly small airleak in the front carb, as it backfires through it occasionally when the engine's started from cold and not running on choke...but so far I've got 110 miles out of this tank of fuel with no fuel-warning light, and it's a lot more pleasant to ride on partial throttle openings.
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