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Help Across Running on Three Cylinders?

Discussion in 'Suzuki 250cc In-line 4's' started by Laceysnr, Nov 30, 2022.

  1. Laceysnr

    Laceysnr Active Member

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    Around a year ago I dropped my Across off with a shipping company, they shipped it to the UK, and then I got hold of it again back in August. It was running ok, got it road legal, and when I picked it up from the MOT (a safety test checking brakes, lights etc.) it was running like garbage. I don't think the guy did anything, but he did bounce it around a lot to check the suspension and I think it triggered some issues with the carbies.

    I took them off, cleaned the float bowls etc. and put them back on but it was still running quite badly. Had them off again, tightened the boots, re-installed and checked everything was tight, sure enough she was back to idling and running well. Had a long ride, had fun.

    Two weeks later I washed it, she started, sounded good. Rode it a week or so after that and it sounded pretty horrible, sure enough it was running on three cylinders. Now it still sounds a bit rough, but all four exhaust headers are hot (and feel the same) so not sure it's consistently running on 3.

    Question is, what's a good procedure for diagnosing what's going on with it? Want to tackle it in a logical order and work my way through. Guessing the best place to start would be with the plugs? It is due to have the value clearances checked etc. but I'm a bit wary of taking the head off - that said I do want to learn how to do it and have recently acquired a second bike once more so I can take it off the road for a bit without being stranded. UK roads aren't exactly pleasant in the winter anyway!
     
  2. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    I don't own or know the Across, but work with your basics.
    All four exhaust headers are getting hot, so there's spark on all 4 at least, but from your description, that's not consistent.

    If possible try and do one thing at a time, I know that's not always practical as removing tank/bodywork is a PITA and sometimes two things get changed whilst they're removed.

    Unscrew the plug caps from the leads and check for corrosion, if they are corroded, trim a few millimetres from the ends.
    Whilst you have the plug caps removed, measure their resistance.
    On the FZR250, the resistors internal to the plug caps themselves are accessed by unscrewing the brass insert which the plug screws into.
    You may be able to do the same on the Across plug caps.

    Also with care [I emphasise with care as on the FZR250 the clips which hold the plug leads in the coils are brittle] you may be able to do the same with the plug leads at the coils.

    Check/measure the resistance of the coils also.

    A basic one which catches people out occasionally, a carbon bridge on a plug will cause it to misfire. What sort of condition are the plugs in.
    Plugs are a service item and inexpensive.

    I will assume it's wasted spark [2 coils for 4 plugs] so assume the ignition control unit is firing the coil(s).
    Check/inspect/measure the things from the plugs backwards before even considering the ignition control unit.

    With the carbs, from your description cleaning them seemed to help, if albeit only briefly, a blocked jet will give issues.
    [I'm swerving towards carb boots BTW as to remove/clean and then re-install them, the boots are a common factor.]

    Compressed air, carb cleaner or some soft thin copper wire up/down/into fuel and air jets to clear them if there's any buildup.
    I did just an interesting article where a blocked main air jet was causing issues on a bike, causing one cylinder to run horror rich with un-aerated fuel on medium to large throttle openings.

    I just found this document on the Across carbs
    http://teamghettoracing.com/mechanical/carb-efi-work/suzuki-across-gsx250f-carb-overhaul/

    Valve clearances should be checked, BUT before you get to that, Carb -> manifold boots - air leaks: have some aerostart, brake cleaner or other volatile cleaner in an aerosol can to hand, start it up and spray it around the boots to see if the idle changes.

    You mention tightening the boots, was this both the boots -> cylinder head as well as the carbs -> boots themselves?
    One forum user had a 3 cylinder issue, he'd forgotten to properly tighten the boots to the cylinder head of one cylinder.

    That's the simple stuff as from your description, intermittently it's running on 3.

    Valve clearances should be checked, and adjusted if necessary, however the cylinder head doesn't need to be removed for that.

    How many Km's on the bike and how long have you owned it?
     
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  3. jmw76

    jmw76 Well-Known Member

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    Good advice from Ruckusman.
    Some other things to look at.
    Start by throwing your spark plugs in the bin and getting some new ones.
    Successfully cleaning and reusing plugs on these little motors is very hit and miss.
    The carb boots bolt to the head, like on the bigger GSXRs. There is an o ring seal at the head joint. Air can leak around here. You might need to remove carbs and boots and replace the o rings. Check as indicated in the earlier thread.
    Make sure your ignition is in good shape (plugs and leads primarily) before going in search of other issues.
    Check connections to coil and all the way back to the ignitor box. Water can get into the connectors and cause issues. Make sure they are all clean.
     
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  4. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    All good tips from the guys there.

    The Across came with the Mikuni BSW27 (2-barrell) carbs. In my opinion, these are often the issue with many engine issues with this bike. I have an Across and it will, from time to time, drop to 3 cylinders and then pickup the fourth for no reason at all. When it drops to 3 cylinders I tend to keep the engine revs high, so the idle jet is bypassed and that seems to help. The article ruckus referred to is gold. There's a tiny jet in the bottom of the fuel bowls that I find gums up at the drop of a hat; and it's a pain to get to and clear. Good luck.

    http://teamghettoracing.com/mechanical/carb-efi-work/suzuki-across-gsx250f-carb-overhaul/
     
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  5. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Just putting it out there, this is the most helpful place on the internet.
    That idle circuit tip is GOLD!
     
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  6. Laceysnr

    Laceysnr Active Member

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    I got the bike mid-2019 - it had been off the road for 5-6 years at that point so I'm quite familiar with the carbies, had to have them off the damned thing at least 5 times before I got it running right. Also had to get the tank out of it which was a huge pain in the arse.

    Yeah I know about that little bugger - was blocked on both when I got the bike and it wouldn't idle at all until I got them cleared. I could blow air through them last time I had the carbies off, but will double check.

    You're not wrong there! Thank you all of you, got a great list of things to start working through this weekend! Will report back with progress. I tightened the carby boots onto the head but didn't know about the o-rings behind them, will get them off completely and have a gander.

    Just for kicks, here's a pic of an Across way out of it's normal habitat! North coast of Devon, with Wales just about visible in the distance.

    PXL_20221023_114226539.jpg
     
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  7. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    Those carbs haunt my dreams. They were also used on the first model GSXR250 (that my wife had)...painful, very painful.

    I will see your "Wales in the background" and raise you a "Tamar River in the background"; I think your pic is far more picturesque, but Tasmania does have elements of the English countryside, including muddy dirt roads. I'm looking forward to your outcome report. Cheers

    Tamar River.jpg
     
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  8. Laceysnr

    Laceysnr Active Member

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    Not much by way of an update here yet - I figured before doing anything Is hould replace the battery since it was completely knackered and not charging. Did that, took it for a test yesterday and it felt better, but it's still on 3 in the middle of the rev range. The fourth kicks in again no problem at 15k suddenly giving it a boost!

    Will keep on investigating now I know the battery is good.
     
  9. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    Bummer. I hope you sort it out as I'm curious to see what's causing it. Seems really odd that the cylinder only picks up after a certain rev. I wonder if that cylinder is getting too much fuel and only starts to work when the mixture starts to lean out?
     
  10. jmw76

    jmw76 Well-Known Member

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    Carb balance?
     
  11. Laceysnr

    Laceysnr Active Member

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    They were balanced a couple of years ago when I first got it sorted again but thinking I'll need to check that. Going to futz with the mixture screws first - given it's seemingly more of a low-mid revs issue I'm thinking it's probably related to the choke circuit or something. Will pull the plugs before doing anything though to see if that gives any clues.
     
  12. Laceysnr

    Laceysnr Active Member

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    Honestly... these bikes. Got it out on the weekend to have another go - first thing I did was reconnect the starter as I'd accidentally left it disconnected before. It struggled to start, so I screwed down the mixture screw on the non-firing cylinder and reset it to 1 1/2 turns out. Still wouldn't start, wound it out another turn and she fired up. Sounded good, so took it out for a ride and had no problems.

    Don't believe the mixture screw on that cylinder really made a difference because that shouldn't affect the higher revs (I believe), but either way, it's riding fine again. Must be the way the wind is blowing or something.
     
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  13. Frankster

    Frankster Grey Pride...Adventure before Dementia Staff Member Premium Member Ride and Events Crew

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    Personally, I think it's more...these carbs, but I agree with your overall appraisal. It's always a fingers-crossed moment when I press the start button on mine.
     
  14. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    Thing to do would be to put a GSXR250 head and carbs on it, but then you have to change the air box to the GSXR one as well while worrying about physical clearance under the tank, er, storage boot.
     
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  15. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    The idle circuit doesn't become completely irrelevant at higher revs, it just contributes less fuel than the jet needle and main jet as revs increase, BUT, if it's already borderline running too rich or too lean, that little bit too much or too little might cause it to struggle in the transition phases between carburetor fuel circuits - idle <> jet needle <> main jet.

    For example, Keyster kits for the FZR250 had the inner/outer main jets in the wrong boxes, @Joker couldn't get his bike, with freshly rebuilt cabs, to rev cleanly past ~10K RPM IIRC, so that's all 4 carbs out by 2.5 on a main jet, the inner pair too rich by 2.5 and the outer pair too lean by 2.5.
    Small differences can have big effects
     
  16. Laceysnr

    Laceysnr Active Member

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    That's a fair point!

    Ok I don't think I appreciated quite how sensitive these things are. I reeckon a rebuild kit is on the list at some point along with new jets. Replaced one jet a couple of years ago, but one of the enamel tubes just spins in the carb body so can't get the jet out of that one.

    Buying a ultrasonic cleaner has been on my list for a while, might have to step up the priority on that, not least because I'm going to check out a '92 CBR1000F tonight - like my Across was, it's been in storage for ~5 years but was apparently running when put away. Hoping it'll be a case of doing fluids and consumables and then it'll be right to go.
     
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  17. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    If changing that idle mixture screw changed the behaviour, it's worth checking the basics on the carbs as they can lead to all sorts of weird symptoms that can lead astray.
    Very easy to fall for, I know this from experience...
     
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