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Bike tune up

Discussion in 'Bling and Appearance' started by blaringmike, Jan 6, 2008.

  1. Hi all,

    I'm going to be booking my bike in for a tune up tomorrow and was wondering what should I ask the mechanic to do to it.

    I was thinking about getting the carbs cleaned, balanced and tuned. I'm not really sure what this involves can someone explain what they do and how much appox this would cost.


  2. They take the carb out, take out the needles and clean them with carb cleaner + high pressure air sometimes. Basically getting any built up crud out, which can cause problems if its obstructing flow. When they put it back together they have to synchronize the openings of the butterfly valves (that let air/fuel in) so that they are all equal for each cyl.

    I'd get valve clearance check too. You probably dont know the last time it was done, so best to be on the safe side and get it done now so that you can relax about it not being correctly set up (can case extra engine wear/damage if valve clearances aren't right).

    What servicing are you doing at home? engine oil, plugs, air filter clean, etc? If not, get them to do that stuff too.

    If they're a trustworthy mechanic, just ask them to do all the general stuff, and to ring you if theres anything costly needing doing, before they do it.

    FYI, I got new fork out, springs, spacers, steering head bearings, carb clean, valve clearances and general other adjustments and checks, all for $580. And the suspension stuff was at least $150 of that cost.
  3. A good mechanic will know what to do. There should be a mileage-due service to be performed unless you have specific problems.

    How long have you had the bike?

    Are there any issues with it?

    No need to clean or service carbs if the bike is running nicely. They don't have oil to be changed or anything like that. You could just undo the drain screws on each carb to clean out any muck, for your own peace of mind.

    There is no tuning, really, to do on modern carbs. The idle mixture adjustment is about all, and this won't normally get "out". This adjustment really ONLY affects the mixture at idle and just a little above, contrary to what many people wrongly believe. This adjustment does not affect fuel economy or power in any measurable way.

    The only other carb adjustment is idle speed. It should be set to whatever the manufacturer recommends and left at that. Too slow and the uneven idling can wear out certain engine components before their time.

    Carbs do need to be synchonised from time to time using a vacuum device of some sort. If the engine is running smoothly (the average expert mechanic can tell by ear) you don't even need this.

    The bike, if it has done over 10,000 km should also have the valve adjustment checked, but this is very expensive on a 4 pot screamer. A 10,000 km service with these things included can cost $1,000 (that's what I was quoted). This valve clearance adjustment is far more important for the running and long-term life of the bike than anything else, apart from oil changes. It is seldom done correctly, or at least at the correct intervals.


    Trevor G
  4. Sorry Trevor that's not quite true. It effects carbies up to about 1/4 flow rate. On larger capacity bikes that's most of your riding. Most carbie performance problems experienced on initial acceleration can be attributed to idle mixture.

    On my bike at 100km/h the throttle is barely cracked. If the idle mixture isn't right it will bog down when I accelerate.

    Maybe you are getting it mixed up with the idle speed screw.
  5. No, I stand by what I said earlier. I am not, in this case, at all mixed up.

    I have been working with and studying carburation for more years than you might realise. Just read any carburettor manual - try searching the Dellorto Carburettor Manual - it's available online. See Page 4 especially.

    In particular note the bottom of the explanation about flow reversal as the throttle is opened off the idle stop - there simply is no flow of any value through the actual idle bypass circuit, which is the only one affected by the mixture screw.

    Any simple throttle slide carburettor makes an easy patient for examination. The pilot jet itself will affect mixture up to about 1/8 throttle opening. After that the slide cutaway is the predominating factor.

    The idle mixture screw controls the quality of the fuel which exits via the pilot bypass opening which is an extremely small hole on the engine side of the slide itself. Typically the size of the bypass is about 1/2 the smallest hole used by the idle/pilot jet itself. The larger hole of the pilot jet outlet is often at least twice the diameter of the smaller hole.

    The pilot jet fuel mixture exits by one or two holes which are just on the "inner" side of the front edge of the throttle slide. There is no external mixture control for these fuel mixture outlets. You change the size of the pilot jet (very few people ever do, or have to) to affect the mixture up to the 1/8 mark, once you are off the idle stop.

    The pilot jet and main jet circuits are completely separate. It is easy to test your theory. Block the main jet completely and see how well the bike runs on the pilot jet.

    I can tell you why it doesn't run very well at all - when the engine is revving at 3 or 4,000 rpm the amount of mixture that can be drawn through the pilot jet circuit is just too small. However, at that speed, even with the throttle at just 1/8 opening, the main jet circuit supplies most of the fuel mixture. It's the one with the flow capacity.

    This, too, is easy to test. Just block the pilot jet completely. The bike will still run, but not very well at really low throttle openings. However, by 1/8th throttle you should not be able to tell that there isn't a pilot jet circuit.

    All the best

    Trevor G
  6. Trev mate...

    Come tune my bike! :grin:
  7. Trevor,

    I'm not saying it is the main control for mixture up to 1/4 throttle, just that it influences it, though I admit after 1/8th it drops off pretty quickly.

    The idle mixture definitely affects throttle response at initial throttle crack, from any speed.

    Keep in mind I have Mikuni CVs in mind (as that is what most Jap carb bikes have), so slide cut aways are a different story.

    I've tuned quite a few bikes too (doing mine again at the moment) and believe me, poor idle mixture settings will create a pig of a bike to ride.

    Mine at the moment has a hesitation, particularly when cold, and a pop on closed throttle which tells me it's running a bit lean on the idle jet. I'll got down at lunch time and richen it up half a turn and see how it is on the way home.

    I have seen diagrams about that demonstrate the overlap between the 3 primary circuits (not choke). Basically idle does up to 1/8, then trails of to 1/4. Needle starts at 1/8, is dominant from 1/4 through to about 3/4 and main is completely in control from 7/8 up.

    The main does influence through a lot of the needle circuit as the needle fuel flow is though the main first, but because initial flow is not at all restrictive, it only has minor influence at half throttle.

    So I agree with the main jet blocking theory, as the needle wont be getting fuel. As I noted the the needle jet is important after 1/8 flow.

    If we are talking about experiments to prove a point, then lean your idle mixture out 1 turn (bump up the throttle stop to compensate) and then go and ride in Sydney traffic.

    I think we are basically agreeing. I was just taking acception to "This adjustment ONLY affects the mixture at idle" as it does effect initial throttle response too.
  8. It might affect the way it idles, but idle mixture itself does not really affect the way the bike responds or performs above idle. Please look at the Dellorto Guide to see why.

    I first proved this some years back when I had a flat spot off idle on a 4 cyl Kawasaki. It was near new, everything was standard, but it had this noticeable hesitation, literally just off idle.

    The mixture screws ran best at 3/4 to 1 turn out, instead of the usual 1.5. That's a good way to know whether the pilot jet (20 in this cae) is the right size. No amount of changing mixture from lean to rich and then testing brought any worthwhile change in the hesitation.

    I bought a set of Yamaha 17.5 pilot jets - hey, that's getting small! (Kawasaki had no alternate pilots at the time.) They were physically slightly different - the drillings in the atomiser tube (the thin tube above the base of the jet) were slightly smaller, meaning that, if anything, these leaner pilot jets would actually run richer.

    In fact, that is exactly what happened. I had to screw the mixtures in to about 1/2 a turn out, and the flat spot was more noticeable as well. Out with the jet drills and I carefully matched the size, quantity and placement of the atomiser holes to match the original pilot jets. Since they were all Mikuni jets, that meant that the actual jet hole in the base of each jet was correctly corresponding.

    Once I refitted the pilots the mixture screws were best at the 1.5 turn position and there was no more flat spot. The fact that the pilots were too rich (at the point that they influence/control the mixture just off idle) was the governing factor.


    Trevor G

    PS. If you reread what I first said I did not say anywhere that mixture settings do not affect throttle response at idle. However, the most important thing is the size of the jet itself. Once off idle the only way to change the mixture is to change the size of the pilot jet.

    PPS. CV carbs work in exactly the same way. The pilot and bypass outlets are in exactly the same places, just on the intake side of the butterfly and on the engine side, respectively. The cutaway on the throttle slide still has its effect at the 1/8 to 1/4 opening.

    The throttle slide and jet needle also are the only influence on mixture control between the 1/4 and 3/4 mark, as with a standard slide type carb. The vacuum operation simply prevents wide throttle/high slide openings at low speed, which otherwise can lead to poor carburation until the engine speed catches up with the throttle opening.

    The vacuum operation does not affect the mixture other than by controlling how high and quickly the slide can lift. Opening a slide type carb at a sensible rate gives equally as good results as any CV type.
  9. You appear to have a basic misunderstanding of the idle circuit. The Pilot jet governs the maximum flow and the pilot screw is an adjustment within that flow range.

    So if changing your pilot jet affected your flat spot, then changing the idle screw setting would too. It was probably just that the original jet was out of range for the screw.

    The rule of thumb is once you get more than 4 turns out on the pilot screw, then you need to go to a larger jet. If you go below 1 turn out then you need a smaller jet.

    The fact that you got a change in an off idle flat spot by changing a pilot jet proves that the idle circuit influences off idle throttle response.
  10. I should point out that I am talking about carbies where the pilot screw adjusts the fuel.

    On some carbies the pilot screw adjusts the pilot air rather then the fuel. In which case out is lean and in is rich.
  11. I don't. Just read the Dellorto manual. It's no different to any other manual for a slide-type carb. They are all the same. It's readily available. Look on page 5 to understand the idle circuit.

    The pilot jet mixture screw works/has an effect maybe up to 1/16 opening. If the pilot jet is the wrong size the mixture screw will still allow idle but won't be able to help at the critical 1/8 region. Just look at the diagrams.

    Note the flow reversal that occurs to send the pilot jet mixture through the progression hole rather than the bypass once the throttle is opened. Have a look in a carb, any carb, and see the difference in size between the idle outlet (in front of the slide) and the larger progression outlet(s) which are just behind the leading edge of the slide.

    Once the slide lifts the airflow is no longer concentrated just over those outlets - the more the lift the less effect these outlets have on mixture control, especially the front one, idle bypass mixture outlet.

    Remember, I didn't invent the idea, I'm just reporting on it.


    Trevor G

    PS. It should be easy to demonstrate on a dyno with a load on the engine to simulate an 80kmh cruise. Even at 1/8 throttle it's the pilot and cutaway which are working, not the bypass mixture.

    PPS. My double proof was a mistake I made on one carb changeover - I left the pilot jet out completely by mistake! It didn't want to idle at first and was very fluffy down low. I changed the slide cutaway from a 3 to a 4 and adjusted the idle speed screw and mixture control so that it idled, and it ran fine.

    Only when I replaced the original carb after it was repaired did I discover the mistake. I did not have to change the main jet because the pilot circuit was way too strong...
  12. Dellorto Manual page 5

    Dellorto Manual page 5


    Each stage overlaps the next to some extent:

    1/16 Idle mixture

    1/8 Pilot jet

    1/4 Cutaway

    1/4 - 3/4 Needle jet and jet needle

    3/4 - full Main jet


    Trevor G
  13. Trevor,

    You keep trying to give me a lesson on carbie theory. I do have a thorough understanding of carbies.

    With Mikunis the slide cut aways are not generally considered a tunable item. That is a keihnen and Dellorto thing. though I imagine race engineers used to muck around with it.

    I also think you are underselling the overlap.

    I can tell you now 1/8th of a turn on the pilot screw makes a difference on my bike in terms of off idle response.

    For those that are interested here is a view of the Mikuni CV set up


    With this setup the butterfly sit pretty much on top of the open jet and thus the jet sees a high velocity at idle compared to the screw jet. once the throttle is cracked the screw jet sees a proportionally higher velocity.

    This explains why the screw jet is more important on these carbs

  14. They still perform the same task as on any other carb - they modulate the mixture at that same 1/4 opening. And allowing for overlap, of course.

    I believe it. Not sure whether you thought I had said that wouldn't be the case. Idle fuel mixture screws (ones which are on the engine side of the carb, rather than on the intake side of the carb) do have a better control of idle mixture than the air control idle mixture screws.

    What the mixture screw won't do is affect either the measurable power (on a dyno) or the measurable fuel consumption in normal use.

    That's what I said in the beginning and what I will repeat ad infinitum. Ad nauseum, you might say ;-)

    Have you got that actually printed in the manual? Is it online somewhere? That explanation is the exact opposite of any explanation I have ever read.

    The Mikuni diagram is not perfectly clear, but it looks exactly like the Dellorto system - when the throttle is closed air passes into the progression circuit (the one above the pilot jet) and through the pilot bypass outlet (the one with the mixture screw).

    Because the progression hole is much larger, once the throttle is opened a little it takes over delivering the mixture from the fixed pilot jet. The amount of adjustable mixture able to flow through the much smaller idle bypass mixture outlet is much less.

    The venturi effect of a closed or partly open throttle slide or butterfly means that the hole closest to the venturi (the butterfly in this case) is the one which sees the greatest "suction". There is no way that the much smaller idle bypass circuit comes into operation the more the throttle is opened. There is no way you can draw more air/fuel mixture through a smaller hole.

    Have a look on any Mikuni CV carb - the hole above the mixture screw is considerably smaller than the hole just behind the butterfly. The same is shown, as far as I can tell, in the diagram you have posted. Even Mikuni are subject to the laws of physics.


    Trevor G
  15. What you wrote was:
    "This adjustment ONLY affects the mixture at idle, contrary to what many people wrongly believe. This adjustment does not affect fuel economy or power in any way. "

    You are now conceding it does effect off idle response, which to me indicates it does effect power in some very real way.

    That was all I was taking axception too. Fiddling with the mixture screw has a very real world affect on the way a bike performs and not "ONLY" at idle.

    whether it is perceivable on a dyno is irrelevant.

    As I mentioned before I think we a basically in agreement, I just think this original statement was misleading.
  16. There. :)

    I changed it.

    I think folks who are at all interested will get the idea.

    Those who aren't will still, like their parents and grandparents before them, think that the idle mixture is important for power delivery and fuel economy purposes.

    Worse still some will continue to pay good money for what just about anyone, with the right screwdriver, can usually do for themselves. And without getting their hands dirty, in many cases.

    Synchronisation: no. Idle mixture: yes


    Trevor G
  17. Yeah what are those screwdrivers called? When i had the carbs balanced and the mixture screws adjusted the other day, they used a really long screwdriver that when you twisted the handle of, it rotated the tip only, which meant they could poke it through the virtually non-existant gaps between the frame and the Carbs. (and the bike still runs like shit BTW)