GT550 J/K carb needle jet

[GTandcbr]

Thanks all. I have put my timing back to normal as I didn't like the way it affected the performance. The one size bigger pilots are still in and the bike hasn't pinged since. I am going to decoke when the new head gaskets get here. Then maybe I'm going to put the smaller pilots back in. That should determine what was at fault hopefully.

[Vintageman]

If you have heavy carbon on piston tops for example it may just be one of the main problems. But the one thing I can contribute to society is this... If your Needle Jet, inside for the short distance it meters fuel with the jet needle, is not smooth, that roughness adds extra resistance/turbulence to fuel flow and you run lean.

A bike built in 1971, that has sat in our Oxygen and Water based planet since, chance are they are not pristine. If eroded a-lot you run rich or rough due to poor fuel atomizing.. But again, a little rough you run lean. You need good eyes and bright light to see inside this short distance.

My 73 GT550 with chambers and stock air box runs all stock jets (O-6 vs 0-5 needle jet though) does not ping. It did, was a pain, and was due to carbon + plus worn needle jet.

Oh, just to cause stress, if the connecting Rod small end for example has rust stains that eroded the metal out from running since, that ping may be the wrist pin needle bearing flopping around. Sounds the same to me. Should be very repeatable at light load right between the point the engine is pulling and coasting as a clue.

Clean carbon and verify all jets and float height and Jet Needle is pristine too, Verify the carb body passages are clear. e.g. the two little pilot hole passage in the carb.

[tz375]

Let's back up a bit and clear up some terms. Pre-ignition is where the mixture starts to burn before teh spark arrives. It can be caused by a few different things but usually a hot spot.

Detonation is quite different. Detonation is where the mixture starts to burn normally but changes to a shock wave that travels at the speed of sound and is heard as knocking or pinging. Detonation is essentially caused by the the burning mixture being too hot for the fuel used. Octane is just a measure of knock resistance, so a higher octane fuel will tend to resist detonation better and may be sufficient to eliminate it.

If timing is too advanced, pressure will rise too early and can create excessive pressure for the fuel.

Too high compression ratio and lack of squish (turbulence) also tend to cause detonation in the right (or wrong) circumstances.

Likewise a mixture that is lean will tend to create excessive heat and a propensity to detonate. In some cases it is necessary to use larger jets just to bring the air :fuel ratio back to where it should be. If the mixture is already perfect, sometimes a little extra fuel will act to reduce combustion temperatures and can be a useful tool until the mixture is so rich that it loses power.

A clogged baffle can also cause excessive combustion chamber temperatures and result in detonation.

The usual solutions are:
- Higher octane fuel
- Retard ignition timing
- Richer mixture

Better solutions include:
Using a squish band optimized for that motor
Improve heat removal (clean radiator/fins etc)
Clean the exhaust
Change the exhaust tailpipe to reduce back pressure ( a misnomer, but easy to understand)
Check timing on a dyno to see what the motor needs and to determine when it pings to determine the appropriate solution.
Jet it on a dyno and fine tune on the street

And so on. Kevin Cameron wrote a great article (or two) on detonation - see if you can find a copy

[Vintageman]

Some additional good maintenance points.

But since a 72/73 GT550 CAN run good from the factory, if mechanically OK, and they are known to be pretty tough ... if not subjected to poor storage over the years... I am of strong opinion pinging is simply a maintenance issue and/or those soft carb parts have weathered, plugged up and eroded over time (been a long time)... and they need to be so precise to meter properly.

A fresh top end does wonders when it's due (look into exhaust port at piston skirt for signs of very dark carbon build up caused by ring blow by for example)

Make sure jets, carb passages and settings are perfect. Needle Jets inside don't last typically and so critical.

Sync carbs (this can really can affect things unless you run at or near WOT all the time)

Set timing stock

Decoke cumbustion chamber and exhaust

Make sure cooling fins are clear. (oil leaking out of exhaust to cylinder fitting is another spot fins get clogged up)

But I will say. I have heard of a lot of early GT550s ping. And, I have heard many get bad gas mileage too.

The GT550 requires regular maintenance and sensitive to things out of tolerance. Seams more so than some of my other bikes. Old school! But when it's correct it is wonderful 2 stroke triple. Many period reviews of the time appreciated how well Suz tuned that broad torque engine that stretches out your arms.

Pinging sucks.
Poor MPG sucks.
Constantly having to work the throttle to avoid bad spots in a jetting circuit while trying to cruise or accel sucks


Service/Owners manual is good place for maintenance information that we may forgotten. If Worn carb parts ... no maintenance can make your bike run as the creators intended.

https://www.youtube.com/watch?v=6V1zIYO1BYQ" onclick="window.open(this.href);return false;

Fun Stuff

[ConnerVT]

Yes, there is a difference between pre-ignition and detonation, I agree. But many times, one will become the start of the other. Pre-ignition will raise combustion temps, making the detonation shock waves that more likely to occur.

Though I see we are in agreement in both the "easy' solutions to the problem, and the more involved ones (mechanical changes to improve chamber scavenging.

The usual solutions are:
- Higher octane fuel
- Retard ignition timing
- Richer mixture

Better solutions include:
Using a squish band optimized for that motor
Improve heat removal (clean radiator/fins etc)
Clean the exhaust
Change the exhaust tailpipe to reduce back pressure ( a misnomer, but easy to understand)
Check timing on a dyno to see what the motor needs and to determine when it pings to determine the appropriate solution.
Jet it on a dyno and fine tune on the street