Suzuki 2 Strokes

Over the past few rides (couple of hundred miles) my 380's developed an unusual, highly specific noise. It's a popping / banging noise, almost like a muffled backfire that you might get from a hole in a header pipe or manifold. It seems to come from the exhaust side of the engine, but is much more noticeable when riding the bike with helmet on, than it is when standing next to it.

It ONLY occurs between 3,000 and 3,500rpm, either at VERY light throttle in gears 1 - 4 (i.e. when bimbling around town at 25 - 30mph or so, with no load on the motor) or on the over-run (shut throttle) when slowing down. These are the rpm / load conditions at which the engine would 4-stroke and pop a bit any way, and this noise coincides with the "ring-ding-ding-ding" that you normally get when shutting off the throttle, and it happens 3 or 4 times per second.

When revs drop below 3,000 the noise stops; if I snap open the throttle to accelerate, the noise stops. If I shut off the throttle at say, 5,000rpm or higher, the bike decelerates normally until 3,500rpm, when the popping / banging starts. Accelerate again, the noise stops.

The motor doesn't rattle or knock at idle, it doesn't rattle or knock when changing gear. Starts easily and runs great otherwise, with no loss of performance. Plug colours are good, no metallic traces visible on the electrodes, piston crowns look good, timing is set nicely. The bike has J&R chambers but is otherwise stock. It's run great with these pipes for 5 years.

I thought it was a pinhole or crack in an exhaust pipe but I can't see anything, and there's no obvious leak or blowing from where the header pipes meet the cylinder.

Any ideas, anyone? Thanks!

I have a similar noise on my newly rebuilt gt550. I don't know what it is either but I'm guessing its mixture related. It makes no sense to be mechanical. I'm watching this thread with interest!
Mike

I'd just put it down to a normal two stroke overrun popping and not worry about it at all.
Remember these bikes are 40+year old technology - engine and carburation are a tad less than exact FFS.
Stop wittling and enjoy the bike.
If you want to worry about popping on the overrun, ride a BSA Bantam or Francis Barnett (Villiers) Plover. They really did bang on rundown and shake the transmission in the process.
The Suzies were perfect in comparison, but ALL two strokes pop when slowing, it's the nature of the beast.
I know those two were singles, but I could ask you to compare an Arrow or 2/3/4T Villiers (Twins) or Wartburg Knight (991 triple). I've ridden, driven or owned them all in the past.
Mind you the Yam 1.7 litre reed induction 130Hp outboard is very nice. Been running one today and didn't notice much popping on shutdown but maybe it was quite a time since I'd had my 2 stroke fix that I just didn't give a toss.

Alan, if the OPs bike had always made those sounds, then fair play to you, but it didn't so it can't just be the design..,.summat has changed and he's unsure if it matters it not. I've less of an excuse. Mine is too early to call it a problem, I'm just super alert for issues as my cheque book is still bleeding:-)
Cheers, Mike

Craig380,

I sounds like the issue occurs at small throttle opening. I am not sure if pilot circuit (real small opening) or little more opening and on the Jet/Needle.

Popping out the exhaust when closing throttle from a higher rev then slowing down engine can be a too lean condition. Some 4 stroke like my 900F have a bypass enriching circuit to eliminate high rev closed throttle banging out exhaust.

We don't have that on 380. So something too lean. Using the small throttle fixed, real light, load can you accelerate to a high rpm or will it not (too lean?). You may have picked up a spec of dirt and that pilot jet is plugged. Even running filter I find I have to clean carbs every now and again and usually pilot.

The other issue is if you are a bit more than pilot circuit and running in needle jet then I would check the inside of the needle jet exit for pitting. It may not be atomizing. If it looks rough the walls of jet resist flow of fuel and/or droplets versus mist.

so I agree mixture. Too lean or not atomizing low speed circuits. Why you hear it so is it is kind of a violent pop and if you may have a small leak or loose connection that opens when it bangs. like header spring to exhaust. usually you hear it out back of pipe too. Maybe Also look for oil signs about head gaskets, may leak out there somewhere.

Also isn't a lot cooler now for you too? I noted the last few rides I took (too cold here now) my bikes we all too lean. Accept my rd400 which was too rich during summer. It was about 30F out and 3rd gear wheelies no problem too lean and mean and just decided to park it for I blew it up

Same story T500... freaking pinged like crazy about 1/5 throttle light load where summer @ +80F was just perfect everywhere.

Still thinking may be a clogged pilot or rough inside needle jet

my 2 cents I hope it gives you some things to try if issue is annoying.

Thanks for the input, everyone, I will play with the pilot screws first and see if I can reduce it.

@AlanH, you're right, there's a guy locally that has a lovely standard rigid-frame Bantam and the RACKET it makes is unbelievable. Accelerating it sounds like a V1 flying bomb, and decelerating it sounds like artillery fire

@Vintageman, the popping / banging lessens a little when the bike is properly warmed up (that is, after a fast 5 or 6 miles), which leads me to think there could be a gap / leak somewhere in the exhaust that almost closes when the engine is under load

My first bike was a 150 D3 Bantam - a 1957 model that my Dad bought for me in 1967 as a 16th Birthday present! Cost £6, which was a lot when you think that my first wage was only £5.18 shillings (£5.90p) for a weeks work.
With a Todd high compression head, it reached a heady 55 mph, which was pretty quick for a 16 year old recently used to pedalling for transport. Only had it for a few weeks and got knocked off by one of my old teachers! Bought a Suzuki M12 after that, much better and faster and let the way for many more bikes since.

Craig380,

Actually to me if it stops popping when warm another sign too lean. My son has a 2006 Polaris Fusion 600 snowmobile that will do the same thing until warm (just finished getting this ready for the seasons). On that sled if you try to solve it when cold or half warm you are just too rich when operating temp. Takes a while for the coolant to get to temp, etc. New sleds with fuel injection can manage this better with computer and sensors. Again not sure what temp it is for you now.

Your theory is mechanical gaps close up when warm... maybe. But I would think when cold you would here it all the time and rate would follow engine speed and not throttle dependent. Interesting you said you hear it more with helmet. Isn't funny how some helmets work as a sound amplifier or resonant chamber. With my helmet if I run without air box or even the snorkel it kills my ears. No helmet no pain

Here is my test, when warm, if you just crack the throttle open, on a flat light load, bike should rev way high and get there smoothly. But why would anyone hold small throttle and see how high it revs? Just me maybe but sure shows that circuit IMO is spot on.

If bike straightens out when warmed up no worries.

@Vintageman, the popping does only happen at less than 1/4 throttle so it's mostly the pilot circuit there. Ambient temperature is around 5 Celsius, and I first noticed it a few weeks back when ambient temps dropped below 10 Celsius.

I have not tried the test to see how high it will rev on a small throttle opening, I'll give that a try. Thanks again.

Well, after trying all sorts to resolve the noise, including:

- a complete strip and clean of the carbs
- new points, plugs and caps
- new exhaust gaskets and generous amounts of RTV sealer
- checking all head and base nuts
- checking clutch hub and spring nuts, and primary gear nut

and having no success, I'd pretty much given up trying to get to the bottom of it.

Then I took the bike out for a long run yesterday. After 15 miles I suddenly realised: no banging noise on deceleration. It never surfaced again during the ride. Nor did I hear it again on a 60 mile ride today.

I can only assume it was some crap in a carb airway that finally dislodged itself after nearly 18 months, or similar. Don't you just love old bikes?

Wierd little noises suck. Drives you crazy! I am glad it's gone. You can now relax and enjoy the beauty of classic two stroke triple! Can't buy a bike like that new even if you wanted to.

Sounds like, a bad tank of gas. Or perhaps, a rust particle temporarily clogged a jet and worked loose.

pearljam724 wrote:Sounds like, a bad tank of gas. Or perhaps, a rust particle temporarily clogged a jet and worked loose.

Problem was, it had been going on for 18 months and some 2,000 miles -- it varied in severity but it never disappeared.

500 miles ago I had the carbs completely apart, all jets were clear and clean (UK fuel isn't as bad as everyone says it is) and every drilling / airway hosed through with carb cleaner ... and it made no difference at all.

And then I take her out for a ride, and I suddenly notice she ain't banging anymore