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Mixture screw woes (Hornet 250)

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by jack_1313, Oct 27, 2011.

  1. As my Hornet 250 was occasionally dying while idling, I decided to clean my carburettor (after replacing the air filter, having a mechanic check the valve clearances + a few other things). The bad news is that the jets were all completely clear.

    The worse news is that, despite my determination not to, I stuffed up the mixture screw settings.

    This resulted from confusion about how the screws are set. The information I read initially said that they should be screwed in until they “bottom out” in order to find the current setting. With this in mind, on the first three screws I continued screwing in past the point where they first gave some resistance, expecting to reach a clear bottom.

    After about 1/2 or 2/3 of a turn past these first points of resistance (not really forcing the screws, went quite easily), I got worried and stopped, but the new points that the screws had reached became the points where they first gave resistance. At this time I removed the screws; they all looked fine. I checked a few more websites and found that mixture screws are screwed in only until they are “lightly seated,” not until they “bottom out” - whatever that means.

    When I put the bike back together I set all screws to the setting of the 4th screw, which was 2 ¼ turns out from “lightly seated” (ie first point it gives resistance). However, the bike now doesn’t want to idling for more than thirty seconds or so and otherwise sounds like its struggling to idle.
    1. Can anyone tell me what the correct mixture screw settings are on a stock Hornet 250 (or even check for me if you have one and know what you’re doing)? The internet seems to be completely void of this information and the only workshop manual is Japanese.

    2. What happened when I screwed in some of the screws a half turn or so further than the points where they first started giving some resistance, and the new points the screws reached became the place where they first gave resistance? My guess is that the spring underneath compressed a little and didn’t depress instantly when I relieved the pressure? In that case, removing the screws and springs completely should have returned them to their original condition, so that they will all “lightly seat” at the same point again?

    3. Is there any reason to think that the screws might have been intentionally set differently on each of the four carburettors?

    Thanks for the help!
  2. 1. 2 to 2.5 turns out from lightly seated is fairly common, without any further info, I'd say you're in the ball park.

    2. Yes, removing the screws & replacing should sort that out.

    3. Generally, I wouldn't expect it on a water cooled i4, but it is possible.

    Other things to check would be float height (you'll need to know what they should be though) & balancing the carbs.

    Hope that helps.
  3. MVs right. You'll never be able to get the mixture setting right if the float heights aren't set up.

    If you screw past the point of light resistance then you are risking damaging seats and o-rings. It shouldn't go more than about 1/2 turn anyway. You can be firm, but don't grind it home.

    Once you've got it running roughly right, do the balancing and then come back and get the idle mixture and speed set up right.

    You may need to bump your idle up to keep everything running to start with.
  4. Ah yes, the simple thing I forgot about :facepalm:
  5. What you may have done is enlarge the 'ole wot the tapered bit on the mixture screw works in to vary the fuel flow. I'm not familiar with your specific carbs, but on many that I've seen, the pointy bit on the screw merely seats against the soft alloy body of the carb. If there was no positive stop on the screw it would be very easy indeed to force it into the hole without really being aware that you're doing it.

    If that is the case, it would mean that 2.5 turns out from lightly seated will be giving you a richer mixture than it would previously. In order to reestablish the correct setting it would be necessary to check the idle mixture on each pot with something like a Colourtune.
  6. Hi guys, thanks for the help.

    Regarding float hight - on the Hornet 250 the float height is non-adjustable, meaning that if anything is wrong with the float heights it's because the floats themselves are damaged and need to be replaced. I didn't measure the height (with no service manual, I don't know what the height should be), but the condition of the floats looked good - no signs of damage.

    Regarding the carb balance - it's next on the list, I just need get the adaptor needed to attach the hoses to the vacuum ports (which seem to be on the cylinder head and not the carburettor itself?).

    Regarding having possibly enlarged the hole, I wonder if counting the turns that it takes to screw the mixture screw in (from completely out) might help determine if this has happened, if the threads are the same across all four carburettors.

  7. hhhmmm, it's just a metal tab you bend a bit more or less rather than any screw adjust.
  8. Hi ibast,

    When I opened up the carb I wasn't able to find a metal tab where I expected one to be. Instead, the float valve just seated right into a slot on the rubber float. So I had a look at the Hornet 600 manual, which has a carb that visually looks very similar to the 250, which suggests inspecting the level but goes on to say:
    "The float cannot be adjusted. Replace the float assembly if the float level is out of specification."

    Riding the bike about 200ks over today and tomorrow - trying not to let it idle long, just turning the engine off at the lights. Bike otherwise rides as per usual.
  9. What's your idle RPM set at?
  10. dont consult the 600 manual consult the manual of the CBR250R as it has the same engine as the hornet 250. I would say thats your best bet for infomation but its hard, our bikes are rare and not much info is out there on them unless your japanese
  11. If there is a particular part of a Japanese manual you want looked at let me know. Would make for a good learning experience for me, although I have no intention of tinkering with the innards of my Hornets carbies! (Will leave that to the pros.)

    I was a Japanese - English translator in a past life.
  12. Hey guys, thanks for the help. Rode it about 250 ks over the last two days. Quick post now because I need to go to work.

    Idle was originally at about 1300 cold, maybe 1500 if they engine was quite hot. The correct setting is 1300 +- 100 rpm. I bumped up the idle to 1900 (hot) to keep it running.

    Bike rode well, but seems to be getting a bit hotter than usual (fan doesn't seem to cool the bike faster than the idle heats it at the lights). There is a ticking sound at idle, which I suppose is a spark plug firing... not sure what that means.

    I've been using both the CBR250RR and the CB600 manual.

    Glekichi, that would be great! Really I just want to know what the stock standard setting for the screws are. Then I can set them to that and see what the effect is. I'm not convinced they were at the standard setting to begin with, because the choke has never done anything other than kill the engine with a slight application. The probelm is that I can't seem to find a Japanese manual.

    I can't balance the carbs right away because no shops seem to sell the vacume port adaptors (tried Peter Stevens and another place yesterday), meaning that I have to wait to get some delivered. Nonetheless, the mixtures do definitley seem to be wrong, since the bike was at least idling ok to begin with.

    Can't see how the mixtures could be adjusted by ear, since the screws are completely inaccessible while the carb is on the bike. Maybe a long 90 degree screwdriver would do it.

    I'm hoping the metal washer and rubber oring would have protected the screw housing from damage from slight tightening...

    Thanks, Jack
  13. Ticking could be valve adjustment. Or do you happen to have a big clock on the bike?

    Running hot could be a symptom of poor mixture - usually running lean

    You can usually pick the difference between rich and lean by examining the state of your spark plugs.
  14. Ahh now we're getting close. It this is a sharp tick, rather than the dull tick of tappets, then it could well be you have a cracked lead and this would explain your problems.

    Run the bike in the dark and have a good look around. Use a spray bottle to spray some water near the top of the engine.
  15. Just a follow up on this now-old thread as it’s been an ongoing story. Thanks for everyone's input!

    The original problem: Occasionally stalling at idle. Opened up and cleaned the carbs but got a bit confused with the exact mixture screw settings. Set them all to 2.5 turns out. Bike continued to give a weak idle and occasionally died but otherwise rode well.

    I was busty, it was proving to be too much of a pain in the arse to track down the vacuum port adaptors for the carb sync, and as the bike needed a new chain and sprockets anyway – so I took it to my mechanic and asked him to balance the carb, inspect the screws for damage and tune the settings.

    When I eventually got the bike back it was running fine with a nice smooth idle. Mechanic advised me that one of the slow jets was blocked (even though I had earlier cleaned them all thoroughly). He said still had all screws set to 2.5 turns.

    However, within about two days the bike was faltering. It would wind down and die while idling at every set of lights (would last about 10 seconds to two minutes). It was also giving me no power low in the rev range (even for an I4).

    So, *knowing* that the carb was clean, the air filter is near-new, the valves were just recently checked, and that it was idling well two days earlier on the current pilot setting, I started thinking about an ignition problem. Swapped the plugs (old ones looked good), no change. Bought a multimeter and inspected the ignition coils and leads, all good. Started reading up about timing problems.

    I decided to open up the carb again and double check that everything was in good working order. Was surprised to find that one of the slow jets was clogged dead! Somehow the bike had clogged a jet in about two days. Cleaned all jets, put it back together and installed an inline filter on the fuel line.

    Bike idled and rode nicely, problem solved! Until later that day when I started feeling like the bike was lacking power at the top end. The next day I determined that the bike was reaching peak power at 7k revs and then giving me nothing more between 7 and 16k. So I start wondering if the bike has somehow clogged a main jet in a few hours.

    So off come the carbs again last night. When I take the float bowl off carb no #2 the main jet falls right out. I must have been too conservative when I reinstalled it (the jet heads are starting to show signs that they’ve been tackled with a screwdriver quite a few times) and it rattled loose and ended up sitting in the float bowl. Verified that everything was still clean and put the bike back together. Checked to make sure that the petrol filter on the inside of the tank was undamaged.

    A test ride at 3:00am this morning had the bike running nicely. Hopefully that’s the end of the story, especially since I'll be touring on the Hornet in about a week. The good news is that I now know my way around carbs pretty well.
  16. :worthlesspics:


    Wait, are you a dude or a chick?


    Good news, might be worth investing in an inline fuel filter or being a bit more picky about where you fill up with petrol.
  17. Already done!