Can someone help me with my latest problem and explain something to me. I've just sorted a mis fire by replacing a plug that seemed a bit wet and black so now engines running sweet. Hurray!
However when it gets up to temperature and I rev it, when it tries to settle at idle it stalls.
Now prior to revving its idling fine at about 1100-1200 rpm. I've a feeling its down to the dynojet and stuff needing set up properly but I'm just trying to get my head around summat.
Whats the effect of moving the circlip on the needle and what's the effect on altering jets? I think its something to effect rich/lean mixture?

The clip settings are specific to your engine. My R1 has the clip set on the second groove from the blunt end.

You could try altering your mixture screws for now to see if it has an effect? (sorry if that should be Affect - I wasn't allowed in english/math/science lessons at school!)

ATB
Steve

Yes my clip is on 2nd groove from top too. I'm aware that the mixture needs leaning out a bit but would this possibly be the cause of the engine stalling at temperature?

Here is my set up

ZX9R C1 1998 Engine
TTS Foam Filter
MK 4-2-1 Exhaust, CAT and R1 Can

Carb set up:

Dynojet Stage 1 kit

138 Main Jets fitted
Main Air Jet Blocked
Dynojet Needles installed on fourth groove from the top
Fuel Mixture Screw 2.5 turns out
I also drilled an extra hole out on the slides as per the Dynojet Instructions.

Although I don't have a BEC, I do have bike carbs on a x-flow. Mine was running far too rich, and I didn't have a clue on how to sort it.

I was pointed towards a book from the library: "Motorcycle Fuel Systems Handbook" by John Robinson - publ. Haynes, ISBN 1 85960 514 1. It's written for students so gets a bit techy in places, but the practical bits are very useful, especially how Mikuni and Keihin CV carbs work, and how they're tuned.

I borrowed a copy from the library...

[Edited on 18/7/08 by David Jenkins]

Cheers all. I have the same set up as you Ian. I'll try dropping onto the 4th needle groove too and drilling the slide, didn't notice I had to do this.
I will have a look for that carb book too sounds really good. Cheers again.

[Edited on 18/7/08 by mistergrumpy]

Hi, I wrote this all once and deleted it by accident.
Its not too bad to work with these carbs when you understand them a little.
As you will have noticed the needle is tapered and when installed and at rest it sits through and almost totally blocks the main jet.
The needle is attached to the throttle slide and as you depress the pedal it lifts up through the main jet. Due to the needles tapered shape it begins to allow fuel to be sucked up from the float below the main jet, by the airstream rushing through the venturis. The quantity being sucked past increases at the desired rate depending on the rate of the taper.
There are as many tapers as swear words in the world and some clever tuners design their own to suit different engines and pipe set ups'/circuits.
By the time three quarter throttle is reached the needle is entirely clear of the main jet and fuel is sucked through impeded only by the main jet size itself.

So the carbs work in stages dictated by the throttle position as follows.

The first quarter is governed by the pilot screw(i think your problem relates to this being a little too far out-try 2-2 1/4) this screw partially obscure a gallery in the carb allowing a set quantity of fuel to be pulled past at low engine speeds and while the main jet is closed by the needle.

As the throttle is depressed beyond a quarter and up to approx three quarters the needle taper governs the fuel flow in an increasing rate as it lifts up through the main jet as previously explained.

Then as 3/4 throttle and beyond are achieved simply the increasing air flow through the venturis governs the mixture as it drags petrol up out of the float bowl through the fully open main jet. It does this because a partial vacuum is created above the jet by the speed of the airflow and this is enough to draw fuel from the float at a rat set by the size of the said hole. This size is what the jet number refers too.
With this info and some imagination its easy to trial and error your way through and you will soon be very good at taking carbs out.
The needle goes leaner as the clip progresses up the grooves because the taper is being lowered in the hole and obscuring it more.As a clue to start you off a needle tapers suitability is usually checked by observing the bikes/cars behaviour from corner apex to exit.


A lean needle will usually result in a lively car that wants to run quickly up the revs and spin up but actually lacks real grunt/drive, a rich car will bog a little anf feel reluctant to get on with driving out.

Be careful and systematic while working with your carbs and it will be fine, only go one clip pos at a time though as you cant hurt anything that way and simply decide better/worse. Same with jet sizes only one or two either way will not cause any catastrophe but improvements can be easilly noted and an optimum can be found with patience/trial and error.

A good dyno operator will sort things out in a jiffy for you with a mixture trace, a couple of runs being sufficient for an experienced guy to observe/adjust/check result.

Well done to anyone still reading and sorry to the many that already know this, hope it helps someone cos you struggle to find a car dyno with an operator that understands these carbs properly.
Given some experience and and the info above its easilly possible to get within 10% of optimum in the garage without turning a wheel then check on the road.
Sorry for open university epic, blush
Si

[Edited on 19/7/08 by simoto]

[Edited on 19/7/08 by simoto]

[Edited on 19/7/08 by simoto]

I would heed a little caution before drilling out the holes in the throttle slides.
I know the dynojet instructions tell you to do so, but remember, these are presuming that the vehicle is a motorcycle and not a BEC!
I remember talking to Andy Bates (AB Performance) about setting up my Dynojet kit when it arrived and he said under no circumstances should I enlarge the holes in the throttle slides with the drill included with the kit! (I seem to recall him saying that because of the extra weight of the car compared to the original bike, it affects the throttle response and opening said holes can ruin the running or something to that effect.)
Admittedly, the above is with reference to fitting a dynojet kit to 'blade carbs, but I'd have thought the same principles would apply to other engines/carbs?
Might be worth giving him a call to confirm?

Si that's fantastic, just what I was after, thanks a lot.
Andrew, unfortunately I've been and drilled the slides earlier and guess what, it's all gone to sh1t Struggling to start, struggling to idle and hunting a little. Admittedly I did alter the mixture screw as per the instructions but playing with it in 1/4 turns has just about got it starting and it's only on 1 turn now. I think I might try to get some undrilled slides and try again.
Thanks again Si
Ooh edited to add. When I rolled the sodding thing back into the garage I could hear the carbs gurgling. I removed the air filter and there was fuel intermittently dribbling from the jet and the throttle valves were opening by themselves What's going on? I checked for any ghosts pressing the accelerator but none there

[Edited on 19/7/08 by mistergrumpy]

Could be seals blown by the pump pressure, hard without seeing it mate. Does the symptom appear as the pump is activated and dissapear when power is cut?
The drilled slide thing is a non starter as its designed to improve throttle response in lightweight bikes. As the other poster remarked this is innapropriate in a bec as more time is taken to gain momentum on depressing the throttle.
Sure it would be worth at least trying standard slides. If your suitably equiped its possible you could epoxy and redrill maybe cos the slides are dear as you would expect. Prob got this to come with mine too, not even been in the carbs yet as so much other stuff i cant do distracting me.
Good luck again.
Si

Wahey! Getting somewhere, well at least back to where I was before this morning. I've stripped the carbs again and noticed that the 138 jets from the Dynojet kit are actually 140's! So i've put in some 136's that I have and that's helped loads. Just a bit of a bogging down after a few seconds if I keep the revs at 2000 eventually leading to a stall. I'll work on that though.

Ok that could be a pilot circuit issue try half a turn either way and note changes to under 2000 rpm steady throttle. Also worth checking plugs after stall.
New plugs are a revelation sometimes too.
If your unsure of yours then treat it to a new set now your getting near and unlikely to do any harm.
If the pilot screw has little affect set it at one and three quarters and alter the needle a clip either way and observe.

cheers for this si. By pilot screw do you mean the pilot jet alongside the main jet when the float bowl is removed?

Hi mate,

How are you getting on with setting your car up,alot of your symptoms I have been having too !!!

Sorry mate been away for a couple of days, by pilot i actually mean the pilot screw. The one some refer to as mixture screw i think.
Sorry again to have taken so long to spot this.

TBH mate. I've not had time to tinker yesterday as I was putting a new roof on the conservatory. I'm too knackered tonight so it'll probably end up being Thursday before I can play again. Ideally I'd like to get it running summat close then somehow get it to a rolling road, not sure how yet though. I'll deffo kep you updated though. I take it you're having that bogging down/stalling thing too then?

Si, no worries mate. Yes I've been playing with the mixture screws. They're very fine adjustments needed there eh? I can tell when it gets too rich now like you suggest. I'm just waiting on delivery of some carb balancers then I'm going to have a play with them too and a little more tweaking.
Thanks for getting back though.

Yeah,I think I have the pilot setup quite close as I get to around 3500/4000 rpm and it starts to misfire so I assume this could be down to clip position,I noticed when going down hill tonight the problem didn't seem as bad though,I'm currently running 170 main jets and have my e clip on the 3rd notch from the top,If I've read the info on this thread correctly I think It's a problem with being to rich,That was certainly the problem with the pilot screw as I had it set at 2.5 turns and ended up backing it off too 1.5 turns out,That made a big difference to how the car pulled.
Good luck all the same mate.

Process of elimination once you understand the stages of operation. Really glad someone has found it useful
I'll check this more frequently now, good luck. si

Hi,
Yeah cheers Si,Excellent info here.

You got any opinions in 170 mains,Maybe a bad starting point (too large)

Sorry to hi-jack the thread !!!

No worries, do seem a tad large but my experience is related to race bikes not becs sadly.
Are we talking about the same (roughly) mid to late noughties blade carbs?
Do you have a ramair box fitted or bare trumpets?
Sometimes they run much larger jets.
Cpould be your running issue is down to that, try holding a steady throttle at 3/4 thou and at 6/7 thou rpm.
If the engine needs coaxing to stay at selected rppm's like your chasing the dial up its prob rich in that sector.
If on the other hand it seems as if you are holding the revs down, like it wants to run up the dial without more throttle then it will be lean.
The rpm bands suggested are in order for you to decide weather it is needle to main transition or purely an overich main jet and may need tweaking for best result to be obtained.
Sometimes tuners prop up a lean needle with a rich main(by accident usually) can in some circumstances give seemingly better running condition when a lean needle was actually the prob.
The resultant overlarge main jet symptoms are not as obvious as an innapro' needle on the road.
Hope this helps, good luck.
Si

Its on an older zx10 motor I've been refering too !

The setup just now is bare trumpets,I've had a read through a few other posts regarding bike carbs and have not come across anyone running such a large main jet,I'm now begging to think my self that this could be a little large.
I will go for a test run later doing what you suggest,I will also raise the needle 1 position without touching main jet to see what difference that brings.

Cheers Mike

You can easily do the procedure on the driveway and make instant adjustment. then drive for overall effect. Actually easier static i think its about feeling weather the revs"want to" rise or fall asyou attempt to hold them level with a steady throttle

[Edited on 22/7/08 by simoto]

[Edited on 22/7/08 by simoto]

Ok I tried raising the needle 1 notch and the same problems are still there so I assume now my main jet is too big !!

I also noticed when changing gear the revs don't drop back to idle right away they seem to rise on there own accord.I.E If i dip the clutch while in 3rd gear at 1500 revs they will brielfy rise to 2000/2500 revs ...

Any ideas ??

Cheers

Well I've been messing with the damned thing again. I've tried needle settings at 2,3, and 4 with mixture screw from 1.5 turned in quarters up to 3 turns and it still won't play.
It's being a bitch to start. Having to give it full choke and just keep it turning over for about 15-20 seconds before it'll go then it idles fine. The problem seems to be that when I rev it slowly, at the very point that fuel comes out of the main jet, the engine stalls. There also seems to be alot of fuel coming out of the jet when I'm trying to start it and it'll happily splash me in the eye when it stalls. Any more ideas. I'm beginning to loath this sodding engine. There's very little fun left in this build

This is just a stab in the dark,I don't think you should be getting fuel from the main jet when you are trying to start it,Are you sure the needle in the float chamber is shutting when the fuel level is reached,also have you checked the float levels ??

Are you using the orginal bike fuel pump (maybe to much fuel pressure)

Might also be worth checking the diaphragm in the top of the carbs (make sure there not split etc)

I'm not sure if I should be getting fuel through the main jets also so I'll check them tomorrow. Yes I'm using standard fuel pump.
I read somewhere:
idle = mixture screw
mid range = needle
top end = jet size
So I was thinking needle height but I suppose if the jet is wrong then it can impose on the needles job. I'm all confused now

your mixture screw sets up low down rpm as per Si's post,the needle is mid range,I'm refering to the needle that is attached to the floats,when the floats reach the desired height the needle on the tang at the end should shut the fuel supply off.I don't think the size of the main jets will make any odds at this stage mate.Are you getting fuel coming out of the 4 mains at tick over or just 1 ??

Erm, I'm getting no fuel coming out of the jet at tick over.
It only comes out at start up, when I press the throttle, at which point it stalls and fuel tends to come out once the engine has stalled, quite a lot actually, just dribbles out.

Hang on let me have a read through, be with you in a minute.

Sounds like float level or seal problem at the moment and without seeing im a bit stuck myself.
I will u2u my tel no to you in case thats helpful.
Def shouldnt have weeping fuel from main jet and that is very indicative of float malfunction if its welling up as such.
Most of these carbs were designed to be gravity fed and even a low pressure pump can be a problem with internal seals being affected typically.
Sorry so long again im just back to work after long absence and very busy.
See u2u. regards
si

Si. No apology needed mate I'm just really grateful that you're taking time to help.
Locosaki. Help appreciated also. I'll crack on with stripping them again and update later.

I'm stuck between a seal issue and a possible mistake on reasembly(to include float level). Gurgling eh?
Was the ignition on/pump powered up at the time?

If not could be seals relinquishing the pump pressure, or slow recovery of slide diaghtams related to the drilling(obscure that)

I have seen these engines fill to the brim with gallons of fuel due to seal and float issues so please be careful and check your fuel isnt disapearing in large quantities and ensure a fire extinguisher is at least around and/or well ventilated area. Honestly, if your in a closed room breathing petrol fumes for more than about five minutes you will do well to identify your own family never mind a carb problem. Safety first. Feel free to ring me.

Pretty convinced that the slide drilling has been made a horlix of(sorry/blush) upside being its relativelly easy though poss pricey to sort.

A spare set of carbs would be real handy right now especially if near to you. As yet mine are an unknown quantity too so far from ideal for the purpose.

Anyone?

Good luck. Si

Well. Thanks to the advice given I set about stripping the carbs down again and giving them a real good clean. Took ages at it as I was feeling crap after having a wisdom tooth out earlier (nearly passed out at 3rd carb ) Anyhow one of the carbs had a slight jelly mix in it but what I did find that I didn't do last time is that around the breather hole that gets blocked off, there are 3 or 4 other holes and these were filthy. I just squirted them with cleaner till it came back out clean.
Re assembled it and started it up and......




fired first time Me and the neighbour were grinning like Cheshire cats. There's a very slight hesitancy around 2000rpm and the carbs still sound a bit lumpy but I'm going to balance them tonight or tomorrow. Over the moon though.
Thanks for all the advice given and U2U's it's massively appreciated

Good to hear your getting on better now mate,Hopefully now it'll just be a case of trying different needle position and mains untill you get the desired effect !!

I have been at mine all afternoon and now have it running near perfect,My problem (I'm assuming) Is my main jet has been way to big,I started with a 170 mian and I'm now down too a 140 main and it's going without any spluttering

So 2 successes in a day eh Great

Yay superb lads, there a doddle when you understand em. Just a trial kto keep taking them apart and requiring patience to be systematic.
Bloody well done again and if i was any help to either of you then im totally made up
Regards si.

Si mate,

Hats off to you,you have took the time to answer us with our problems and have helped me no end understand these carbs.

Thanks again,much appreciated

Mike

Again and as above mate. Superb. Thanks for taking the time.