3-4k stutter

Forums Technical Two Stroke: Help needed 3-4k stutter

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    • #13767
      Matthew Nichols


      I’ve been trying to track down a problem (or just an annoyance) i’ve had for the last few thousand miles on my 251.

      A stuttering/pinking (??) feeling between 3-4k at steadily increasing throttle. This only really is noticable in the lower two gears at a lower speed.

      I’ve ruled out (as best I can) electrical issues and I am intent it is carburettor related. I’ve had similar problems in the past that was the result of the pilot screw screwing inwards inadvertantly. I know this rpm range is the intermediary periodbetween pilot and main jet.
      I’ve cleaned out the carb a few times (nothing to see) with carb cleaner and 150psi of compressed air and there’s no real difference.

      It’s possible that there is carb wear, but i’ve felt a much worse carb slide movement on a perfectly running bike. Carb is an original BVF 30N3 with 46k miles on it, and i’ve put the last 17k on it so I know when something feels off.
      The problem has seemingly come suddenly so other than straight up swapping the carb with that from my other working 250, it’s difficult to determine whether the carb is ‘worn’ or i’m going down the wrong rabbit hole.

      Just wondering if other people have 3-4k suttering in the past and what they’ve had to do to smooth it out? It’s a little annoying!!

      Carb is fully standard jet size and needle size/postition. I’ve leaned and richened out both pilot screw and main jet needle position but it doesn’t necessarily change anything.

    • #13771
      ‘AAA Dummy

      As stated earlier I had a similar prob with my cb77 twin. All fine till it got into the higher rev range, particularly on the motorway. Like you, I was convinced it was the mixture. Played with the jetting for about a year even though I had bought new coils just in case. Wanted to keep it standard like but when I did remove the coils I found the problem side had leaked its oil into an almost solid goo. Sorted !
      So try another coil, not expensive.

    • #13772

      I’d be tempted to clean the exhaust. Probably won’t sort it but narrows the field down and it’s never a bad idea to have a clear exhaust.

      I’m with Rob on never underestimating the capacity of bikes to mislead. If you’ve got them to hand try a “new” carb, coil, exhaust etc.

      It could be an issue with the ignition module, even just a bad earth.

    • #13773
      Matthew Nichols

      Hi both,

      Thanks for the replies.

      These sorts of problems can make you go down the wrong rabbit hole.. I do have a spare coil, which might be easier for me to just swap over as my first thing to do.

      If it still feels odd, and no change then I’ll nick the 30n2.5 from my etz250 and see what that does.

      As for the exhaust, i’ve already cleaned it out thoroughly about 3k miles ago- and a little bit of crap did come out. I know that the exhaust block can cause the problems I have, because my Suzuki TS185 also has the same symptoms- although the difference there is that the symptoms are worse when cold, and then start to gradually go after the engine has been running for an hour or two and everything is nice and hot and you’ve blasted it a little bit…

      Coincidentally, I’ve already swapped exhaust with the 251 trying to find another problem with a different bike (actually a restrictive exhaust), and the 251 felt no difference with a known recent and nicely flowing exhaust..

      Will keep this updated.

      • #13775
        Nigel Barraclough

        Have you checked the HT cap ?

        I have had issues with failure of the suppresion resistor built in to the cap & this gives “odd” running. These are typically 5 K Ohm & can be measured by unscrewing the cap from the HT lead and checking the resistance bewteen the lead side & plug side of the cap – the ones I have had problems with have gone open circuit. HT caps are cheap enough to fit a new one if yours is old.

        Another source of “odd” running can be accidentally fitting a resistor spark plug (if an NGK, a plug designation with an “R” in it) with a resistor plug cap fitted. Easily done if you fit an Iridium or Platinum plug, which only seem to come in resistor flavour – a non-resistor HT cap is required with these.

        As the old saying has it – 99% of carburation problems are ignition related & vice versa.

        Nigel B.

    • #13777
      Matthew Nichols

      Hi guys,

      Have changed coil for a known working one- no difference.

      The bike in general does feel much peppier but only since cleaning out the carb thoroughly. It does feel a lot better, but don’t get me wrong, the bike only does the ‘stutter’ when the throttle is at a steady position and the rpm is going up on the engine in a gradual and civilised way (until anywhere between 3-4k) when it will require either a gear change, or more throttle to circumvent this. Most prevelent when cold, but it’s a lot better now that I cleared out the carb. If I am actually trying to get somewhere fast, then you don’t really even notice it- and of course- i’m tainted and biased now though – my senses are in tune now to find any fault in that range.

      I’ve been looking at the slide, and although there is very little play slide in bore, there is significant scoring, piston side. It’s had these since I know, but of course with an extra 17k on it in my ownership wear will always occur.

      I’ll be looking to find a low mileage original slide first, because I hate mucking around with bikes that are working perfectly, – but if I cannot then I will definitely be taking that other carb.

      New spark plug cap and HT lead were installed only a few months ago- I can check the resistence in the cap- but normally when caps have resistors with high resistence it mainly causes a lumpy idle- but it’s a perfectly valid thought. I’ll check it.

      I use Denso W22FSU sparkies, and i’ve already tried a handful, including of different makes; champion, ngk, beru- but no effect.

    • #13779

      Possibly a quick thing you can try is to remove the air filter and the housing if poss. this will weaken the mixture. Or block part of the inlet off which should enrichen the mixture. I don’t expect it to sort your problem but I would expect to see/feel a change if it is the carb.

      I don’t know 251s but I presume the carb fitted is essentially the same as fitted throughout the two stroke era. If it has it I would inspect the cold start rubber as if that perishes then it enrichens the mixture. A bit like running with the choke on.

    • #13834
      Matthew Nichols

      Been there, done that Brian.

      As a commutor bike, this sort of problem bugs the hell out of me so I did indeed put the 30n2.5 from my etz250 onto the bike in question and well.. it was perfect. Reved cleanly and smoothly and it was great.

      The ultimate test would be to put the 30n3 onto the etz250 and see if that also gets 3-4k stutter. But it’s fairly conclusive.

      I would like to try a replacement original 30n3 carburettor slide before , if anyone has one spare they would like to sell before I blame the carburettor inner bore perhaps…

      I know a lot of people blame wear on carburettors, but who else has a genuinely well working original carburettor with 50,0000 miles on it? It seems odd, because there is scoring on the slide sure, but it’s not too bad- and it doesn’t exert much play.

    • #13836
      Derek Reynolds

      I know a lot of people blame wear on carburettors, but who else has a genuinely well working original carburettor with 50,0000 miles on it?

      I take it you meant 50,000 and not 500,000 !

      Please Sir – me. Yes, there’s always one.

      Honda 400/4, 55,000miles no issues.
      Three Honda CX 500’s, all did over 100,000 miles each, no issues.
      BMW R80/7, 211,000 miles, periodically replaced the rubber diaphragms on the Bing’s (3 times in all).
      Moto Guzzi V50III, 308,000 miles, replaced the needles once. Dellorto.
      (Plenty of other issues, but was my favourite bike to ride).

      However, the Amal carbs on the BSA’s used to wear their slides and well under 50,000 miles. Don’t the C15 had done 40,000 when part ex’d, and the A10 – whilst a nice engine – was often with the head off. Single carb on the A10.

      Do singles have something to do with it? Shouldn’t do, as all the high mileage bikes above had one carb per pot.

    • #13838
      Matthew Nichols

      Well yes, ONLY 50,000 miles… My question was mainly pertaining to MZ’s on original Carbs.

      I myself am coming up to 90k miles on my own CX500, Derek. Despite the other faults, fuelling has never really been a problem there… The slides are still pretty much perfect- infact I’ve never done any repair work on those carburettors..

      Of course- it could simply be the metalurgy at that period of time in that particular location WRT the british made carbs and the BVFs. Singles could still have something to do with it though(?) as although a multicylinder has a carb per cylinder, they do not typically exert the vibration that a single does- esspecially with the MZ’s and the excessive movement allowed by using soft engine connections (would it help, or exacerbate). The only thing I could relatively do without much more invasive and perhaps machining work, would be simply to find a low mileage slide as the cheapest option first.

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