GZ250 losing power on the motorway after downshifting - scary stuff

[maraudingkiwi]

Relatively new biker here - I've been riding a Suzuki GZ250 for just over a year now and generally it has been really good. However, I've had a pretty nerve-wracking issue with it occur a few times and was hoping someone here might be able to shed some light on it.

The circumstances leading up to it have been almost exactly the same each time - it's a cold morning on the southern motorway heading into the city (Auckland that is, for non-JAFAs) and I'm cruising along at 80 - 100, when the traffic becomes congested (usually this is around Khyber pass) so I need to drop my speed to around 50. I start applying the brakes, and when my speed drops to about 70 I squeeze the clutch in to drop into 4th. What happens next is the engine seems to completely lose power. It's still idling but on the verge of stalling, well below normal idle revs. Trying to give it any throttle at this point just makes it threaten to stall completely (i.e. it starts missing). Nothing I do can get the revs back up. I've tried letting the clutch out (i.e. engaging 4th) as well and this *does* stall it.

What this means is I end up rolling unpowered in congested Auckland traffic, and have to try and navigate across to the shoulder to pull over... not a pretty picture. It's usually hard to get started again once I've pulled over.

This has happened maybe 3 times since I've had the bike (around March last year). I've talked to a number of people about it, one place reckoned it could be the transmission but it would cost about the price of the bike to strip it down to find out. Another suzuki mechanic I talked to seemed to think it was from pulling the clutch in at too high of a speed (I'm not ruling out that it could be something to do with the way I'm riding it). Another mechanic reckoned it sounded like a fuel problem. What I did from then was switch from 91 to 97 at the pump and keep the fuel level from going below a quarter full, and I never had it happen again... until last week.

So far I've identified a couple of common elements - it's always on a bloody cold morning (like we've been having a lot of recently) and always happens after I've been cruising in top gear for a good while then need to quickly slow down. Any ideas? As much as I love my bike, having it cut out on the motorway is a bit of deal-breaker :/

[haydes55]

So the bike pretty much dies when the clutch is pulled in?

If you were in 5th, pulled the clutch in and stayed In 5th would it happen?

Try rev matching on the downshift. Give the throttle a blip before engaging 4th.

Sounds like a weird problem. Good luck. Sounds like you'll need it

[Rhys]

I don't know much about GZ's But the duc does the much the same, It has to very cold, below zero, i wouldn't have thought Auckland would get cold enough ? with the duc it is apparently the carbs icing up

[ducatilover]

Clean the slide on your carb and lube it

[iranana]

Hmm well I'm no four stroke expert or anything but this is my take on the issue. Of course, my analysis could be entirely wrong, but this is how I would approach things. Personally I don't think it'd be a transmission issue - once your clutch is in, your engine is disengaged from the crankshaft, so I don't see why it would be related to the transmission in any way (other than the fact that having the transmission engaged 'carries' the crankshaft and piston). Secondly, I don't really see how pulling the clutch in at a high 'speed' would be the cause of the issue either, seeing as your clutch is concerned with engine RPM not its rolling speed. Pulling the clutch in at high RPM might be a different issue though.

When you pull the clutch in this is roughly what happens (to the best of my knowledge): firstly, the crank no longer has the motion of the gears to carry it and give it momentum, and nor are you supplying the cylinder with extra fuel so things start to slow down to an idle. As this happens, your ignition timing changes (slows down) to compensate for the reduced engine speed. So, you've kind of got three main factors at play here: fuel delivery, ignition timing and the spark plug itself. Fuel is fed from the carburettor to the engine through valves that are controlled by cams and tappets or whatever (valves are unfamiliar ground for me, I dunno how these work). Ignition timing (presumably electric ignition on your bike) then determines at which point in the piston's journey (usually a few degrees before top dead centre) the spark should fire and ignite the mixture. If your spark plug isn't hot enough, or worn out, the fuel won't combust properly. If it's too hot, it can melt the piston crown/overheat the engine.

If the timing of the spark is retarded (too late), you'll experience a loss in power because the piston is already on its way down and the wave front from the explosion dissipates by the time is reaches the piston. If your timing is too advanced you get 'engine knock' because the piston experiences a whole lot of opposing forces which throw it off center. If your timing is quite retarded, you'll experience quite a dramatic loss in power and your engine will do funny things like backfire and bog down. After the exhaust valve opens and lets everything out, your intake valve opens and a fresh charge of fuel is sucked into the cylinder and the whole process happens all over again. I don't know much about fuel, but what I do know is that the higher the octane rating, the more compression the fuel can withstand before ignition, so ignition timing and octane rating correlate to some extent. Out of curiosity, why did you decide to change from 91 to 97 and how much of a difference did it make? I've read that going up in octane rating can help reduce engine knock/pre-ignition/pinging or whatever you want to call it, so going down in octane rating would (maybe, and in theory) do the opposite (but I don't actually know).

So the first thing I'd do would be to take out your old spark plug(s) and check them out, then I'd put it new ones and see if it makes any difference. Also perhaps consider your carburettor(s). When air changes temperature it changes the fuel/air mixture slightly and this makes a surprising difference (especially on two strokes, but I suppose it applies to four strokes too) as your fuel mixture will either become leaner (low fuel/air ratio) or richer (high fuel/air ratio)... So to recap, you're cruising along, you pull in the clutch, the revs drop like a stone but giving it throttle just makes it crap out entirely. This sounds to me that perhaps the fuel mixture inside the chamber isn't being burnt properly and your spark plug is having a hard time keeping up (perhaps your timing is retarded?). Giving it more throttle will just dump more fuel into the chamber, flooding the engine and making your spark plugs too wet to even spark, hence why it would be hard to start after crapping out.

So, what this long rant of a post was all about was to point out that an issue like this could actually be combination of small(ish) problems. I've never owned a four stroke and I know very little about the action of valves and cams and tappers or whatever, but I find that the two strokes I've had have been very touchy with timing, carburetion, octane ratings and spark plug temperatures. Correct ignition timing is absolutely integral to proper running, and things like a weak spark and small electrical issues will fluff things up big time... Like I said, I could be totally wrong in my analysis, but take the time to learn the processes by which internal combustion works and you'll feel much more comfortable about your bike when it does do weird things, and it'll make it easier for you to rule out possible areas of concern.

[ducatilover]

It really does seem like a sticking slide issue...

But, your spark plug is on the LHS and should have a D8EA in it.

[iranana]

I was wondering about that but I would have thought for the RPM to drop/the engine to misfire, it would have to either not be getting enough fuel or not igniting it properly. Assuming we're talking about a carb with a needle type slide, wouldn't the slide sticking just result in more fuel being delivered? And if the slide were to stick, wouldn't the throttle not return immediately? Or am I missing something here?

[ducatilover]

I was wondering about that but I would have thought for the RPM to drop/the engine to misfire, it would have to either not be getting enough fuel or not igniting it properly. Assuming we're talking about a carb with a needle type slide, wouldn't the slide sticking just result in more fuel being delivered? And if the slide were to stick, wouldn't the throttle not return immediately? Or am I missing something here?

Mikuni BS34SS CV carb. Slide and butterfly act independently. Slide staying up will cause it to be rich, but throttle butterfly being closed will not be allowing enough air through the venturi to make it massively rich and die.
Had it happen on one of my GNs (same motor/carb etc.)

Or something.
But, could also simply be carb icing. So, make it happen again (not on the motorway) and pull the spark plug out.

[iranana]

Mikuni BS34SS CV carb. Slide and butterfly act independently. Slide staying up will cause it to be rich, but throttle butterfly being closed will not be allowing enough air through the venturi to make it massively rich and die.

Ahhh I see, makes perfect sense now! Haven't seen that kind of carb before. Sounds more reasonable than my hypothesis

[jasonu]

Maybe it is the kerfuffler valve. Is it an import and does it have the left or right powerband installed.

[MarkH]

one place reckoned it could be the transmission but it would cost about the price of the bike to strip it down to find out.

I reckon that you should never go to that place for mechanical work ever!
I just can't see how what you describe would be a transmission issue.

To me it seems that if the engine can't run well when warm while the air is cold it is almost certainly going to be an issue with the carburettor, as in a air/fuel mixture problem.
Cool air is more dense so I would think that on those cool mornings it is running too lean maybe?
Or the cold air coming through the carby is causing metal to contract and making something stick.

Auckland doesn't typically get cold enough for frost so I would not think it is an issue with ice forming.

What I would suggest would be to find a good mechanic and get him to check out the carburettor as the prime suspect for the cause of this issue.
This could be a good time to arrange a visit from George from Motorcycle Doctors - I've seen plenty of recommendations on these forums for his services.

[Zrex]

Sounds like its running out of gas. maybe the petcock or a blocked vacuum line. Open the gas cap next time and see if the tank has pressurized.

[fridayflash]

Sounds like its running out of gas. maybe the petcock or a blocked vacuum line. Open the gas cap next time and see if the tank has pressurized.

thats not uncommon, i recently found my cap breather holes to be gunked up and were causing me major starvation issues.......

[Akzle]

errr. Dont use the clutch?

[nzspokes]

Auckland doesn't typically get cold enough for frost so I would not think it is an issue with ice forming.

I used to get that on cold mornings in Manurewa on my Bandit, not so in west auckland.