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Bad running, white plug....

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by mattb, Sep 5, 2009.

  1. Went on a ride today but turned back just south of Wallan. As with last week, when I was riding along at 100kph with the throttle mostly open going uphill, the engine felt bogged, the power would diminish suddenly if mildly, and then return, in waves. While this was happening today, I decided to do a plug chop. So the throttle was open at least ¾ or above. The plug was white! Hence I called an end to the ride. I chopped it on the way back at half throttle and it was better, more of greyed tan, and at idle it is black. The engine has not felt right actually, in the last two weeks I've had it in - not quite confident or robust, which I had put down to the sprockets, but had wondered about the fuel flow or pinging; I found last week on the road from Yea to Seymour, with all those hills, that downshifting at 100kph, even though it means 5000rpm, helps a lot at those moments of power waver.

    The bike is a ’78 500 with a grey import 400 engine just fitted. I simply plugged in all the old 500 gear – electrics and carb (VM34 – believe the 400 runs a 32), and old pipe. Pipe is aftermarket and there’s a K&N. Awaiting new sprockets, ratio is currently high at 2.7333 as opposed to standard 400 ratio of 2.9473. Didn’t have this problem with the carb on the 500 engine, though it resembles the bad running it once gave with a sprocket ratio of 2.4117. The engine has a high pitched whine which is slightly discernable; don’t know if it’s just mechanical noise, but QuarterWit compared it to the sound of a fuel pump. I don’t know what pinging sounds like. It also has a high tinnie tapping, a coin against a glass, which is loud when the engine it hot but not so discernable when cold.

    So I’m assuming either that:

    a)this carb and filter set-up on the smaller engine has shifted the balance to a lean mixture, hence the plug, and the bad running is simply fuel starvation.

    b) Pre-ignition? Either the engine is just out, or maybe the ’78 500 CDI doesn’t quite work with the 400 engine (the SR guys aren't sure about fitting the 500 CDI, though an SR mechanic reckoned it's fine with an older 400 engine). I don’t know what that - pinging etc - sounds like.

    c) It’s got too much load because of the sporket ratio (but would that cause a lean burn, and is the ratio actually so high anyway?)

    d) The CDI's wrong - but how do I analyse this?

    Any suggestions? My responses at the moment are simply: I’ve already ordered a sprocket to change the ratio to 2.875 (if the shop can get round to getting it in – bloody mumbling dirt-biker-teenager counter boy, I just knew the order would go wrong!). I guess I could hunt around for an engine timer and give that a check. I could borrow QuarterWit's 400 CDI. And then I guess it’s a matter of opening up the carb (for the first time) and looking to adjust / replace jets for more fuel-flow.


  2. Sounds, at first blush, like your main jet is too small and either your idle jet is too large or your idle mixture screw is adjusted too rich (or both). On the bright side, your half throttle plug chop suggests that the needle isn't too far off the mark.

    Feeling bogged can be a symptom of lean running and running lean enough to cause the Bone White Plug of Doom could certainly cause pinking, particularly when hot, so my suspicions would fall upon carburettion.

    Even if your CDI is not quite right for your engine, I wouldn't expect the effects to be quite so radical. Excessive advance might affect the plug colour and pinking, but I wouldn't necessarily expect bogging or flat spots. However, if a 400 unit is available for trial, it's worth a go. I don't know what the bore/stroke dimensions of the two SRs are, but if the 500 has a larger bore, it might have a tad more advance than the 400 to give the combustion flame time to travel all the way to the more distant perimeter of the combustion chamber.

    A properly adjusted carb and ignition system shouldn't give the symptoms you describe as a result of any load you can apply to the engine, so I wouldn't regard the gearing as a primary issue. However, excessively high gearing will exacerbate any underlying problem as you will be running at larger throttle openings, which is where most of the nastier symptoms will show up.

    Usual rule with troubleshooting is to change only one thing at a time and note any changes. eg, get the bike thoroughly warmed up and go for a run to establish a baseline, check plug colour etc, then swap out the CDI and do exactly the same run. Note what changes you're making and you can always return to the original baseline and start again f necessary.

    Couple of points on mixture fiddling. Do a bit of Googling and you'll turn up half a dozen decent articles on tuning Japanese carbies which will give more detail than I can. Modern unleaded petrol and modern plugs do not allow fine mixture readings to be made from plug chops. Gross errors will show up though, as you've found. A crude test for a lean mixture is to provoke the bike to bog or pink and then to try the same thing with the choke on. An improvement indicates leanness. Might be awkward to do without the engine dying if you're already sooty-rich at idle though.

    Good luck with it.
  3. As always Pat, thanks!

    The choke - I should have thought of that! (There I was on teh side of the road in the misty rain, trying to tape tissues over my K&N so as to enrich things on the ride home!

    The bore is the same, different stroke - different crank (QuarterWit's having a 500 crank fitted to his 400 as we speak, and I've got one sitting there for the same purpose).

    QuarterWit also has a spare stock carb off his 400 (he went and bought some fancy big thing), tuned for a K&N and open pipe - might also try to grab it off him and chuck it on to see what happens - might tell me straight away if it's the carb. Theoretically, if it is the carb, then why would a change of nothing else but the loss of 100cc mean a lean engine? I would have though it'd be rich if anything.

    Remember that scene in Easy Rider where Jack Nicholson goes, "Oh! I've got a helmet!" Well, "Oh, I've got a carb description! I've got a real doozy!" http://www.siue.edu/~rsutton/SR/mikuni.htm
  4. If you've access to a CDI and a carb for testing purposes, it sounds like you're good to go.

    Given that the bores are the same, I wouldn't expect the CDI characteristics to be so different as to be troublesome, although they might not be optimal for the smaller engine.

    Thing with carbies is that a number of different factors interact and it can be almost impossible to predict, intuitively, what changes will have what overall effect. For example, if the smaller engine is demanding less volume flow of air, the air velocity through the carb will be correspondingly lower. If I remember the teachings of Signor Bernoulli correctly, this will result in the air pressure above the main jet being higher than previously and so will pull less fuel through, resulting in a lean mixture, where intuitively you might think that a jet sized for a larger engine would go rich.

    You also have the disadvantage that you've now got a non-standard set-up and so manufacturer's data is no longer valid so you're going to have to work it out from first principles.

    As I noted previously, the needle appears to be somewhere in the right sort of range. Richness at idle should be fixable by fiddling with the idle mixture screw, although whether it needs to go in or out depends on whether it's an air bleed or a fuel metering valve. The latter is more likely, but I've seen the former (though i can't remember where).

    Which really just leaves the main jet. Mikuni and Keihin jets aren't grossly expensive, but it can take a few goes before it's right which pushes the cost up. It's unlikely you'll find anyone who'll exchange jets because of the risk that the customer has taken a small drill to them.

    Which is another option. I've tried enlarging jets by scraping them out with bits of wire and sewing needles (when I was mucking about with the primitive Jikov carbs on my CZs, for which alternative sizes were not available) but didn't have much success. Jets (or maybe Czechoslovakian brass) are a lot more durable than I'd previously been led to believe :( . Micro drills are a better bet. I'm not sure of a good source though. Try a good model shop. Obviously this route is quite risky. Baby steps is the way, and, of course, if you go too far or snap a drill in the jet you're knackered. As is the jet, of course.

    But first try QW's carb, before you do anything irrevocable.

    Oh yes, and make absolutely, doubly sure you've got the right grade of plug in there for the 400 engine.

    Good luck.
  5. Thanks Pat. I bought a main jet last year in a confused state and it was $7, so shouldn't be too bad - will go two sizes up and then judge it from there (of course, if all is good on QuarterWit's carb, I can use that as a guide).

    And I'll raise the jet needle one (unless, again, QuarterWit's carb suggests otherwise). Or is a grey tan ok (I guess you can't tell without seeing - I'll load up the pic I took of it).

    I have no recorded specs for the 400, can't find any on the web - this was basically produced, from 1978, for the home market alone. I believe Deus ex Machina use a copy of the manual in Japanese! Didn't even check out the plug it had before tossing it, though QuarterWit has been using a 7, I'll ask him.



    Here we go. I remember looking at teh pic when it was taken and thinking it seemed darker on the camera screen than in the flesh, and now I'm not sure either way:

    The main jet chop:


    The half-throttle chop


    I guess the first isn't that bad (though it's not that good either).
  6. Lean but not super-piston melting lean. The question I'd be asking is; What in it's immediate history caused this condition. If you've been riding this bike for a while, and it's developed this condition over 50 ks, then something has failed, or a recent change has not worked.
  7. Those plugs don't look that bad. Not much paler than the Ural with factory jetting.

    As I noted earlier, modern fuels don't let you read plugs that accurately. You're really going to have to do this on how the bike feels. I'd agree that going up two sizes would be about right as a start.

    I'd also agree that lifting the needle a notch is worth a try and will do no harm. Again, final adjustment will be a feel thing.

    For the idle mixture, I'd recommend using a Colourtune (a rather nifty glass spark plug that allows fine tuning of idle mixture based on flame colour) if you can borrow one off anyone. Makes twiddling mixture screws a piece of the proverbial. Sadly it's of only limited use away from idle and none at all for the main jet, but still......

    Just remember, one thing at a time.