Bike won't start after running

[JamieNeve]

Hi Guys,

Been a while since I've posted as the Bike has been running beautifully over the winter!

However, over the last two weeks, since the weather started to get warm, I've had a slight problem starting my bike.

My bike starts perfectly in the morning, and if it has been left for about 20-30 mins, however if I go to start it after it has been ridden and is warm (tends to stay in the lower 'warm' section of the temperature gauge as it should when riding, it struggles to start so I have to bump start it. This is pretty annoying, especially when I have to get petrol as I have to bump start it to get it going again. When it is running, there are no problems whatsoever.

I have read up that it is a common problem and can be due to electrics, fueling or compression. The plugs are only 3 months old, along with the coils and battery so couldn't imagine it is a problem with them. Head Gasket is also 3 months old, and haven't had any other symptoms of that going which leaves me with fueling.

What is it that I should look for/adjust or replace when checking out my carbs, anyone had previous experience with this issue?

Thanks as always!

[banner001]

try to lean out your 4 pilot air screws, now its warmer, air is less dense, doesnt require as much fuel as before.

[jimsym]

Yep agree with banner's post.

I set my pilot jets exactly on spec with the manual and put the carbs back together again.

Come winter the bike fires into life almost the instant I press the starter, however in warmer weather it takes a good second or so, indicating the pilot is a tiny bit too rich.

I reckon unscrewing it barely 1 quarter turn would have it spot on for all conditions.

That said, once it does start, the engine runs and idles like a peach.

[banner001]

on the keihin cvk32 carb you turn the pilot adjusters IN to lean, and OUT to richen, not the other way around.

the pilot air jet on the top of the carb works alongside the pilot jet in the fuel bowl, unless you change your pilot jet or change your pilot air jet this always provides the same fuel to air mix, what happens when you screw the pilot adjuster in is you allow less fuel and air to get into the cylinder - leaning out the bike, when you turn out the screw you allow a larger amount of fuel to be sucked into the engine creating a richer condition.

at least this is the way it is according to keihin and factory pro...

[jimsym]

on the keihin cvk32 carb you turn the pilot adjusters IN to lean, and OUT to richen, not the other way around...

Apologies if I sound like Im teaching you to suck eggs, but are we talking about the bog standard factory carbs for the ZXR400?

They are adjusted via a pilot air screw - in that screwing it all the way in will fully constrict all airflow producing a very rich fuel mixture

[rmkd177]

on the keihin cvk32 carb you turn the pilot adjusters IN to lean, and OUT to richen, not the other way around.

the pilot air jet on the top of the carb works alongside the pilot jet in the fuel bowl, unless you change your pilot jet or change your pilot air jet this always provides the same fuel to air mix, what happens when you screw the pilot adjuster in is you allow less fuel and air to get into the cylinder - leaning out the bike, when you turn out the screw you allow a larger amount of fuel to be sucked into the engine creating a richer condition.

at least this is the way it is according to keihin and factory pro...

This isnt the case im afraid....and is wrong. Turn the pilot screw OUT to lean the mixture and IN to richen it, the correct name for this low end mixture adjuster is 'pilot air screw' it adjusts the flow of air from the pilot air jet which is next to the main air corrector jet on the back of the carburettor. The 'pilot air screw' has no control on the ammount of fuel, this is controlled only by the jet in the float bowl and the mixture is adjusted by increasing or reducing air flow..Fairly obviously screw IN to reduce air flow hence RICHENS it, OUT to increase air flow WEAKENS mixture.

[banner001]

Figure 4-17 although for harleys, is the same mechanism as our CVK carbs if im not mistaken?
http://jwvess00.home.insightbb.com/jay/1200s/cv/

[ManxMan]

on the keihin cvk32 carb you turn the pilot adjusters IN to lean, and OUT to richen, not the other way around.

the pilot air jet on the top of the carb works alongside the pilot jet in the fuel bowl, unless you change your pilot jet or change your pilot air jet this always provides the same fuel to air mix, what happens when you screw the pilot adjuster in is you allow less fuel and air to get into the cylinder - leaning out the bike, when you turn out the screw you allow a larger amount of fuel to be sucked into the engine creating a richer condition.

at least this is the way it is according to keihin and factory pro...

This isnt the case im afraid....and is wrong. Turn the pilot screw OUT to lean the mixture and IN to richen it, the correct name for this low end mixture adjuster is 'pilot air screw' it adjusts the flow of air from the pilot air jet which is next to the main air corrector jet on the back of the carburettor. The 'pilot air screw' has no control on the ammount of fuel, this is controlled only by the jet in the float bowl and the mixture is adjusted by increasing or reducing air flow..Fairly obviously screw IN to reduce air flow hence RICHENS it, OUT to increase air flow WEAKENS mixture.

Gosh thank you.

ive had real problems getting my bike set up right and this explains a lot.

this forum is reelly wonderful so much good information once you read everthing properly

[banner001]

This isnt the case im afraid....and is wrong. Turn the pilot screw OUT to lean the mixture and IN to richen it, the correct name for this low end mixture adjuster is 'pilot air screw' it adjusts the flow of air from the pilot air jet which is next to the main air corrector jet on the back of the carburettor. The 'pilot air screw' has no control on the ammount of fuel, this is controlled only by the jet in the float bowl and the mixture is adjusted by increasing or reducing air flow..Fairly obviously screw IN to reduce air flow hence RICHENS it, OUT to increase air flow WEAKENS mixture.

im sorry, but THIS is wrong, the method you are refering to is where you have an adjustment screw near the top of the carb, where the main and pilot air jets are located...we dont have that, we have a fuel screw, not an air screw, an air screw is located here:-


and does this:


which is correct, screwing it in reduces the amount of air hence richens the carb...its not what is on the standard carb, our carb is a fuel mixture screw (as its near the base of the carb where it enters the engine - well away from the pilot air jet), what we are doing is adjusting how much fuel/air mix goes into the engine...this is why if you need to screw your pilot screw out by more than 2.5 turns you need a bigger pilot jet, to flow more fuel. if it was the other way around you would need a smaller pilot jet (as you are saying that screwing it out leans the system, a smaller pilot jet would also do this).

[rmkd177]

Sorry to disagree 'Banner' but you are totally wrong and 'jimsym' for one knows his onions. I do not refer to other carburettors nor 'Harley Davidsons' and i have knowledge of many and have spent lots of hours on dynos setting carburettors up. Try this little test...next time you have a set of zxr400 cv carbs apart...take an air line and blow it down the 'pilot air corrector' ( which is situated to the left of the main air corrector at the back of the carb) whilst blowing air down adjust the 'pilot air screw' and see what happens to the airflow at the engne side of the butterfly in the venturi. Another test to disprove your theory is to adjust the 'pilot air screw' radically say from half a turn out to 4 turns out and test by seeing how much choke...if any the bike needs. The latter test is the way we use as a rough idea of the mixture at the bottom end...none or little choke 'too rich' full choke for a long period too weak. Just for reference the bottom end or slow running circuit is the last thing we adjust on the dyno, get it running correctly at the top (main jets) and work backwards.

[Deswilkie]

All symptoms point to it running rich low down (pilot). Since it's running fine once going the needles and main jets must be in the right ball park at least. Based on previous experience I'd have to go with unscrewing the pilot screw weakens the mixture based on well...... that's how it works on the ZXR CV carbs as jimsym sums it up perfectly; close the screw and there's little to no air flow hence rich.

Can't comment on the relevance of Harley carbs being a ZXR forum.

[banner001]

next time you have a set of zxr400 cv carbs apart...take an air line and blow it down the 'pilot air corrector' ( which is situated to the left of the main air corrector at the back of the carb) whilst blowing air down adjust the 'pilot air screw' and see what happens to the airflow at the engne side of the butterfly in the venturi.

do you mean the pilot air jet, or the pilot jet? so what should happen? i take it the airflow should stop.

Another test to disprove your theory is to adjust the 'pilot air screw' radically say from half a turn out to 4 turns out and test by seeing how much choke...if any the bike needs.

so what are you saying, that turning the screw fully out means you will need less to no choke, or it will need choke continually?

please enlighten me with your answers so i can compare them against mine, ill whip the carbs off this weekend, but i assure you that im correct.

there was not a sudden departure from protocol when making the cvk32 carbs, it is a fuel screw and it does work as ive said.

i would also like you to provide documentation to say that the carb behaves as you have mentioned...as i have never found any, what i have found is:-
http://forum.highlifter.com/Basic-CV-Ca ... 72049.aspx
http://www.thumpertalk.com/topic/489690 ... djustment/
http://www.amervoyassoc.org/Documents/C ... %20jet.pdf
http://www.factorypro.com/tech/tech_tun ... gines.html
http://kawasakiworld.com/zx-12r/50110-p ... crews.html
http://locostbuilders.co.uk/viewthread.php?tid=100287
http://www.advrider.com/forums/showthread.php?t=659109
http://www.advrider.com/forums/showthread.php?t=570047

[Deswilkie]

Quoting other forums isn't evidence. Search the internet hard enough you'll find 'evidence' of aliens.

Sorry but you are incorrect. I think you're just not getting this. Screw in restricts air therefore rich. Screw out allows airflow so weaker.

[banner001]

but its no where NEAR the pilot air jet!

that wasnt a hard search, i was all within the first few pages of a google search for "CV carb pilot screw", i find no mention of it working as any of you suggest. i also think that your concept of the internal workings is lacking. what you are suggesting is that there is no straight connection between the pilot air jet and the pilot jet, that the pilot air jet passage goes from the top of the carb, curves around the pilot jet in the fuel bowl and goes down to the mixture screw close to the engine, then back up again to the pilot jet. THEN the fuel air mixture travels back down again through another passageway in the carb and emerges at the bottom...crap, thats never been a design of a CV carb, and if it is i would like proof, again, not your word for it, proof, something from a book or a factory manual that describes it the way you are mentioning.

what you ARE mentioning is a carb with an air jet in the top on the airbox side, this controls the air flow (its also why it doesnt have a rubber o-ring) that mixes with the fuel that gets pulled through into the bottom of the carb venturi. with the screw fully in you have no additional air and the vacuum draws a quantity of 100% fuel from the pilot jet, when you open the screw you introduce air, which has volume, this decreases the amount of fuel that can be pulled through and "leans" out the mix. what you are actually doing is altering the A/F ratio from rich to lean, if you want to add a greater volume of fuel or air you change the pilot air jet or pilot jet.

With a KEIHIN cv carb your pilot mixture is fixed, and you vary the quantity entering the venturi using the adjuster screw near the bottom of the carb close to the engine, screwing it out allows more of the mix to enter and screwing it in allows less...thats what I dont understand how YOU cant understand it.

ok try this, screw your pilot mixture screw until it seats and blow air through the pilot jet, i guarantee you the air will come out of 2 places, the coaster enrichers near the butterfly, and the pilot air jet, if you were then to blow air through the pilot air jet it will emerge from the enrichers and the pilot jet as it did before as these passages are all linked and seperate from the pilot screw - none of it will come out of the pilot hole in the venturi. this proves that the mixture screw blocks the pilot hole in the venturi. if you are correct, and the screw restricts airflow, blowing air through the pilot air jet with the mixture screw fully seated should result in the air going nowhere - this you can easily test.

[Deswilkie]

Have a read of the Kawasakiworld post the you put up (and obviously didn't read). A member there (hubz14) explains it for you.

Also, can I point you in the direction of an excellent book on English and grammar called 'Eats Shoots and leaves'. I need to prove nothing to you and I'm done with this subject.

Good luck to the op, should get it sorted easy enough.