bike hesitates on first opening of the throttle

G

Giles C

New member
Location
United Kingdom
Hi all.
Thanks for your help regarding the main jetting on my 500. It turns out it needed a 108 not 115. It now races up the revs nicely.
The issue I seem to have is that it hesitates when I open the throttle. It's when it's opened from closed.
I don't know if it's related but the idle is high too.  The idle screws are nearly out and seem to make little difference.
It's a standard alpino s. Standard air box jetting (apart from mains) and std
exhaust
system.
Thanks a lot.
Gc
 
Try increasing the pilot jet sizes.
What size does it have at the moment?
 
Sorry I can't remember.. whatever is stock..
I've just been reading about riching up the air mixture too. It's on 3 turns out at the moment.  is it out for richer?
 
Yes.
If for example it is running 55 pilot jets try 60s.
Usually this will reduce the required tuns out of the mixture screws to normal range, usually 1.5 to 2 turns out.
To me 3 turns out means the pilots are struggling to flow sufficient fuel.
 
When you had the bowls off did you check the idle jets to see if they were clear?
If the mixtures are out 3 turns then suggests weak idle mixture so could be jets or vacuum leak
What idle speed have you got? Also are the plugs ok as a weak plug can give you issues also!
 
I would check the float hight first. If the floats close to early, there is less gas in the bowl which leads to leaner running in idle.

make sure they are correct, then start from there...
 
Thanks for the replies. The floats are spot on. The idle is about 1.5k but slow to fall. Both plugs are good.
The manual says 3 turns out but if you think that's a bit much ill back them in a bit. The manuals main jet recommendation was too rich so I guess fuel has changed a fair bit.
 
So you have it pretty rich on the mix screw,3 out, but it has a hanging idle,that usually indicates a lean condition.Kinda odd.What helps a lot is making quite big changes that have an obvious effect,if that effect works back it back a bit.Pulling the choke while holding a set throttle while riding helps indicate stuff as well.
 
You should check all your carb settings, chokes, synch etc. before you start changing things though. I would do as Grant said and go up 5 numbers on the pilot jets. Backing in your mixture screws will just lean it off even more, which is the symptom you are fighting. 3 turns out is pretty well the limit for the fuel mixture screws on a Dellorto. Ideally they are within half a turn or so of one and a half turns out. The actual pick-up from idle bit is the slide cutaway. Unless someone has changed from the standard slide the only way to affect it is with idle mixture.
 
just to avoid confusions: are the mixture screws on the intake side (compared to the slider) or on the engine side?

If on the intake side, out is rich, in is lean
If on the engine side, Out is lean, in is reach if I remember right...

make sure you do not have an air-leak on the connections between the carb and the engine. You can use a spray that would burn like brake cleaner or so and spray it to the connections... If engine reacts to it, you have a leak there...
 
I think he before said was all standard? Which if the case should therefore mean that he will need to go down in the idle jet! But he has the screws out and still feels it's lean so it's idling way too high at 1500 therefore it either has a vacuum leak or is too rich on the idle mixtures so he is compensating by have to have the slides open too far hence it's starting to run on progression ports. Hence would also then explain why the hesitation.

I would check for vacuum leaks at the manifolds, also that the little o rings in the mixture screws are there first and secondly not old hard and not sealing. Then check the idle jet size and if standard as per the book then drop down by around 5 as a base, drop the idle back till the slides bottom then say bring them up around a turn or so and see how it runs. You should be looking for a smooth idle around 800 or so for one of these.


 
I trust you have made sure the pilot jets and airways are not clogged up with clag?
That might explain why it is showing signs of running lean, in turn needing the mixture screws wound out further than ideal.
What pilot jet sizes are in it at the moment?
 
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