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Constantly fouling plugs

Discussion in 'Technical and Troubleshooting Torque' started by mattb, May 20, 2008.

  1. Hey All.

    My bike had been running fine for a year; indeed it used to stall a bit at the lights last winter, but since summer even that problem had ceased, until a couple of weeks ago. Now, in the last week, the plug will foul up with soot almost immediately. It will constantly surge and is sluggish, the idle jumps unstably between 1000 and 2000rpm, it wants to stall constantly at the lights, and will begin to stall and misfire at stand-still even when I rev it, at which point it dies. It is also backfiring a little when I roll off the throttle; and it can be a complete pain to (kick-)start.

    The weather has gotten colder - winter has arrived - and it does seem to have co-incided a little with that, though we're talking moderate Melbourne cold. But other than this there has been no changes done to relevant parts of the bike, aside from a new air filter a week ago. She is burning a bit of oil from time to time in smoke out the back on start up - could this via bad rings be fouling the plug? I figure oil-fouled is quite different to dry carbon fouled. My assumption is that the bike is running rich (always has a little). I did the fuel/air screw up all the way in (it is on the engine side of the carb). Still fouled up. I'm guessing the general cause to suspect is rich fuel mixture? And that means perhaps not enough air somewhere. But could worn rings be another possibility? I run it on BP Ultimate, always have, and I take it off choke almost immediately and do not over-use the choke in starting.

    Damn, she had just got to teh point of being a fine starter and non-staller! :cry:

  2. Doug from the Canberra SR500 chapter has the same problem with his 500, and was speaking to Stew about it when I was down there...

    ... Buggered if i can remember what Stew said though. Sorry mate :oops:
  3. My mechanic said stop using PULP as he has seen heaps of VTR's (my ride) sufferring fouled plugs and the common denominator is Premium Unleaded. I have only used regular unleaded fuel since- no problems.
  4. It could be any number of issues.

    It could be the petrol, though the fact you run for a while and it was fine suggests it's not entirely that. I'd still be coming back to unleaded or 95 to eliminate that from the equation as it is a likely contributor at least.

    My best guess would be a slightly dirty float needle seat or a worn float need. Put a new float needle in, give the bottom end a good clean. Set your float level (critical).

    I'd be putting a new plug in anyway.

    After the carby is right and you have a new plug in and the carby is tuned as good as you can get it, do some plug chops. It may be you are running to cold a plug and the cold weather has made it worse.

    Also check for strength of spark. It may be a lead, a plug cap or even a coil.

    It could even be an ignition module.
  5. I'd look at the carb as first suspect, based on the rich symptoms and the unstable idle.

    +1 to everything ibast said.

    Also, be sure that the idle mixture screw isn't an air bleed screw. If it is, screwing it in will richen the idle mixture, which sounds to be the opposite of what you need.

    A little smoke on start up (probably) doesn't indicate enough oil to foul a plug. Given a healthy ignition system, modern plugs are pretty resistant to oil. If it is oil fouling, the plug will look and smell oily, not sooty. Trust me, I used to run a Land-Rover which used a litre or more every 100 km. I've seen an oil fouled plug or two :grin: .
  6. Thanks guys.

    I probably shouldn't be bothering with the higher octane fuel on this old girl anyway. I doubt it is the problem as many SR500 riders use it, but I'll treat that as something to change and observe. Certainly I've fouled the plug on the SR185 using the higher octane stuff.

    I've gone through four spark plugs in the last four days! I'll buy a bunch more (and now I've eliminated the battery I can store them and tools on board the bike). The standard is BP7ES, but I've been using BP6ES for a long while - my stalling issue disappeared in concurrence with my use of the hotter plug. I think it's time to open up the carb and give it a good clean / service following the advice here. I've never done so before, and was too nervous to do it on my only transport when it is running well - I guess this gives me the excuse for the next learning curve! I'll be changing the carb to a new Mikuni when I can anyway....
    (That's on the list of the meagre-budgeted one-at-a-time extensive rebuild:
    new carb
    top-end rebuild - soon
    Hyperpak CDI
    New ingnition coil
    coil rewinds at flywheel area (don't know much about this)
    new wiring throughout
    Swing-arm bearing rebuild - soon
    elimination of parts - ignition switch, stop button, battery...
  7. Carb rebuild never hurts anything, it could be a float height or sticking intel needle. I'd throw new plug leads at it, and also go over the low voltage wiring on the coils, just to eliminate them. Give teh low tension wires a good dollop of grease after you reassemple the connections to keep them moisture free, silicone grease if you can get it.
    It could also be carb icing in winter. The GTR will ice up at idle when it's 0 degrees or below for about the first 10 mins of running. Once teh engine gets some heat soaking through it, it is gone for the rest of teh day.
    Carb icing will drop one cylinder out, then another etc till the bike stalls, but the engine will still happliy run over 2000 rpm or so.

    Regards, Andrew.

  8. :roll:

    I suppose he was using Optimax as well.
    PULP could be a factor but a small one.

    If your plug fouls up so quickly check your compression and your oil consumption.
    Is the plug oily?
    If so then it "could" be a ring
    If not what Ibast said is the most plausible issue.
  9. Not a bad idea. Now I come to think of it I've seen a couple of LT ignition faults very succesfully masquerade as carb problems.

    Which is certainly an inconvenient problem on a single :grin: .

    Incidentally, matt, I've still got that near-new coil sitting on the kitchen table. If you're still interested, PM me your address and I'll stick it in a Jiffy bag for you.
  10. Hell, I forgot all about that! PM about to be sent. I'll put that thing in asap, with new leads.

    I'll be opening up the carb tomorrow. I've done a bit of reading on it and - moreso after the research - I fear I'll be a bit like a Neanderthal with a wrist watch!

    Incidentally, if it's a ring that's worn out, can that simply be replaced or is a rebore necessarily, or at least usually, required?
  11. In most cases, a worn ring can simply be replaced. A rebore is only indicated if:-

    (a) there is detectable damage/scoring to the bore. Anything bad enough to warrant a rebore will be easily visible to the naked eye or detectable with a fingertip.

    (b) there is excessive bore wear. Having seen inside quite a few elderly Japanese engines, the bores seem pretty resistant to wear. Without mucking about with an inside micrometer, which you won't have and which require practice to obtain meaningful results, there are a couple of rules of thumb for judging this.

    First is to have a look down the bore with decent lighting and see if you can still see the original honing marks (a diagonal criss-cross pattern in most cases). If you can still see 'em, congratulations, you've got a usable bore :) , if not, don't panic.

    Next check the wear ridge at the top of the bore where the piston ring doesn't reach. A fingertip is better than an eye for this. If there's no detectable ridge, there's no wear and all is good :) . Again, if you've got a bit of a ridge, you're still not doomed.

    Moving on to a more "precision" technique, invest in a set of rings. I'll assume for the time being that your bore is standard. If it's not, the oversize should be stamped on the piston somewhere. Check before spending money :) . Have a look in your manual to see what the ring gap should be and make sure you've got a set of feeler gauges that can measure it. Very carefully (they're astonishingly brittle :shock: ) put one of your new rings in the bore at the top, just below the ridge. Use the piston to ensure it's square in the bore. Measure the ring gap. Do the same at the bottom of the bore. If the ring gap at both ends of the bore is within spec, for most practical purposes you're good to go. It's a good idea to repeat with all the rings as a check. Once upon a time you often had to adjust ring gaps with a file or a stone, but these days they seem to be pretty much spot on from the factory.

    There are other aspects of bore wear, like ovality, which this method won't pick up, but, by and large, it will tell you if your bore's acceptable.
  12. It's a bit daunting when you read about them only, but once you've got one in front of you and start pulling them to bits, then they become pretty obvious.

    Ignore the choke circuit and top end (diaphragm etc) for the time being, to make life simple.

    It's really a case of; fuel goes in there, is regulated on the way in by that mechanism, at low speed it goes through that port, at higher speed it goes through a different port and on the way out it is regulated by that needle.

    So just think about what the bits are doing as you pull it apart and you'll soon see why certain bits need to work properly.
  13. The only advice I can offer about carbs is that they really aren't complex, buy some paint thinners to clean the parts in (no rubber or plastic parts though) and blow out all teh passages with compressed air before reassembly. Carbs, especialy bike carbs, are fairly simple things, adjusting the throttle cables etc on reassembly is usually trickier! ANd I know I go on about it, but put a filter on teh fuel line if you don't have one, especially after teh rebuild.
    If you do end up pulling teh head and cylinder off the bike, if it's a single, I'd probably hunt round for an oversize piston and rings. It'll cost you bugger all being a single.

    Regards, Andrew.
  14. That being a big motivating factor when I decided on what to buy last year: one piston, two valves all up, one carb, etc etc. Mind you I think it makes the trouble-shooting bit harder, compared say to a twin which might still be running and running properly on the other piston.

    I was fortunate enough to come across this "tour" of my specific carb.

    I think I find a likely-looking lad! I opened up the air filter box, and the air filter had come loose, leaving upwards of one centimetre's gap to let dirty air in! The inside of the box is always a little dusty anyway, despite my efforts, so I suspect that dirt has got into the carbi and blocked...the air jet (does that sound right?) or something like. My air filter is held in place by a bent piece of metal and garden hose that is pushed between the air filter and a piece of molded plastic; time to improve the system. I only changed the filter a couple of weeks ago, so it has happened since then and co-incides with the onset of the problem.

    I didn't have time today to open and inspect the carbi, but I blew some Honda carb cleaner stuff (aerosole) into the float bowl vent and the air inlet, got it going - still wasn't easy doing so! - and then started it again having put carb cleaning stuff into the petrol. I rode for five kms in circles around my house until I experimentally allowed it to stall, and it wouldn't start till half an hour later. The bike is no longer surging, and the idle is no longer erratic, however it's lost a little zest in the acceleration, and tends to stall as I drop the revs coming to a stop (it drops too far in revs, whereas it is stable at 1500rpm when idling); and it's harder to start when hot (first kick again when cold). Plug is still getting carbon coated, and it still blows black smoke when I rev harder. I guess either the aspect of the carb that looks after low revs is still nedding attention, or the carbon fouling is getting the upper hand at low revs (or both). I can't go any leaner on the fuel/air mix screw - there is a limiter for emmision purposes, which I will try to file off as others have done on SRs. The whole thing still needs an inspection and cleaning asap as per posts above. I might wait a week and take it to a car mechanic I know who can combine an inspection and service with a lesson in how to do all that. I'm still thinking seriously about a Mikuni VM34 replacement over the hassle of this 30 year old over-complex thing - others experience an immediate improvement in starting and non-stalling; maybe a pod filter will also deal with the dodgy air filter system. Any where I should go in Melbourne to buy and get advice it for the new Mikuni? A new carb and coil...mmmm :) (Gotta be cheaper than the chiro bills!)

    Thanks guys, these has been a really helpful replies for both the specific problem and future work on the bike!
  15. If you want to send teh carb away for a rebuild, I recommend Carburettor Services in Burwood, Sydney. Have been in business for ever, and always do good work. I would get teh carb rebuilt, it'll be lots cheaper than a new carburettor.
    As for your air filter, just buy one of teh single pod/cone air filters for it, snd get rid of the factory airbox. The old airboxes crack and fall apart, and finding original filters for old bikes can be a pain in teh ass. Teh pod/cone filters are washable.

    Regards, Andrew.