Not weak spark, turned the lights off and it's a fat blue spark. I think this image belongs in this thread
uh oh... What I initially thought was bad fuel is indeed not. There is a severe flat spot at idle-2500rpm. It revs quick, drops back down to idle quick. Thought I had it tuned properly. It's alright on flat ground, but hill starts are impossible. It will start going, then get stuck at 2000 (Opening throttle changes nothing) until I pull the clutch to rev it, then slowly slip it till it's beyond the flat spot. It *seems* to behave well at other rpm (haven't gone above ~12k) Is this a pilot screw setting issue? The carb boots are new and I don't think there are leaks. New pilot screws, seals, washers and springs. The screws are at 2+1/8th turns each (stock = 1+3/4), I used Litetek's carb tuning by ear guide (plus throttle response). The bike needs choke to start, and dies quickly with choke on once it's warmed up. Is this set up properly? These are VP20A carbs. Any thoughts appreciated.
It might be that your 'ear' is not as good as you might hope. Set the mixture screws at 1-1/4 and see what happens. Then set it at 1-1/2, keep going until you find the sweet spot. All this has to be done with a warm engine of course. "The bike needs choke to start, and dies quickly with choke on once it's warmed up." That is how it should be.
Also make sure that petrol passage from pilot-screw to carb-venturi and its bleed hole is clear. Scrub passage with small brushes using fuel-system cleaner with high dose of PEA. Gently poke through bleed hole with soft copper wire from venturi end.
Hi, Got the bike running nicely now. Here is an audio clip: On decel, there isn't popping/backfiring, but a bit of a gurgle. I like the sound of it, but does this suggest something improperly tuned? Around 65 seconds is a good example. Thanks for all the help guys!
It doesn't sound too bad. A re you still running your "loud" exhaust? If so. It might be contributing to a bit of lean out when you back the throttle off. Try adjusting the idle screws initially to make it a bit richer when the throttle is closed. Unfortunately, as soon as you change anything from standard you will need to tweak the tuning a bit. Good luck, but making good progress.
Idle? Beautiful Throttle response? Quick Wide open? Awesome 1/5 to 1/4 throttle? Nah Seriously... I can't figure this out. I have tried 1.5, 1.75 and 2 turns out on the pilot screws. All this changes, or what I can perceive as change, is the idle speed/throttle response and if it hangs/dies after revving. 10 steps forward 9 steps back. At 1/5 to 1/4 throttle the bike behaves exactly how it does when it's running out of fuel. I'm not sure if it's all 4 cylinders or just two that do it. At the moment I have to slip the clutch enough to keep the revs up and use lots of throttle to get past the dead-spot. The video I posted was 1.75 +1/8 turns out. Can I fix this myself? Should I take it to a carb shop? Budget's tight... which circuit is responsible for delivering fuel at this throttle opening? Jet info: 105 102 102 105 mains, 35 slow? jets. Currently at 1.75 turns on the pilot screws. I believe this is stock. 150psi compression on all cylinders. Also, anyone know why spark plug insulators crack overnight? Always #1 or #2 plugs. Happened 3 or 4 times now. Replace it with a working plug and it runs fine. I'm not over-tightening them. Just snug. Washed the bike today.
Now I know it might seem like a stupid question but have you ever ridden another 250 4? They have no power at all down low.
Yes, I have owned two bandit 250s and have ridden a hornet 250, as well as having this bike as my daily for just over a year (till I wrecked the engine) I'm not kidding when I say it's as if it's running out of fuel. Like when you need to swap over to reserve. There's more power at 1/8th open than there is at 1/4. I can feel the low spot if I slowly go from closed throttle to WOT
Is the fuel tank breather working ? .... open the cap a little and see if it does the same .... it will create a vacuum if its blocked Does it have an inline filter ? ... is it new ? ... installed in the correct direction?
Already tried the tank cap thing. No fuel filter... It's not an issue of fuel flow into the carbs. I've filled them without the tank on (using a funnel) and it does the same going up the driveway.
Yep. Bench sync'd then used cheap vacuum gauge setup (each gauge was calibrated using #1 cylinder, then attached to their respective cylinders) to fine tune. Got them apart now... giving them a soak in threebond... again. Double checked, all the jets are stock size. Nothing was blocked. Slides move as they should. Needles in good condition. Threebond came out the pilot circuit air intake when sprayed down the slow jet hole, when the air inlet is covered and sprayed down the slow jet hole it comes out the pilot screw hole, and when sprayed down the pilot screw hole it comes out the venturi side. Will post update when they're back on bike.
What is the condition of the intake rubbers? They do not have to leak much to really stuff things up on these little motors.