GT550 Engine noise

[Bluehaze]

I have just had my Suzuki GT550 engine restored at huge expense, consisting of crank, con rods, pistons, rebore, seals....the lot.
The first few miles I was advised to premix the petrol with 2T oil until the CCI pump took hold which I did, now after 100 miles on the road all traces of oil in the fuel tank have gone and I am running just on the CCI pump (maybe a little richer than norm).
When the engine is cold the engine purrs smooth and lovely, after about 10 miles and the engine is up to temperature I get a clattering noise only on the overrun (carb shut off) which frightens the life out of me knowing my cost layout. When the bike is on power (open throttle) nothing no noises smooth and lovely. After the bike has cooled again no noises on overrun until it gets hot again.
I have a boyer bransden electronic ignition fitted and I am wondering if it is firing unburnt gasses or is pinking somehow. I would say it could be running richer than normal, as I am a bit cautious of seizing. I have changed the transmission gear oil to 20-50W to see if that was an issue but no change and re-packed the baffles with clean wadding.
I am a bit stumped! Have you any ideas you can suggest?

[dinogt]

try to put one more turn at yours pilots air screw...

[Vintageman]

Sounds like the same issue I have. See my “Needle Jet GT550” in other tech section. I think it Pinging Pre detonation due to too lean (timing maybe too advanced?)


Turning in pilot jet will help. I had the problem fully solved by turning screw all the way but 1/4 in and needle at last/5th clip. Gas mileage sucked real bad and put needle clip to 4th, some ping returned.


I have a 1973. I am wrong in my other post for at 74 (Not 75 as is said) used larger needle jet and clip position, but at 74 air box changed too

I am confident I increase richness at around closed to 1/8 throttle it will fix this. This is the pilot jet and needle area... one more so than the other but not sure where I am at the time. Its when I back off throttle everywhere else is perfect.

I have new 32.5 pilots (stock is 27.5). I plan to put them in. Also I am waiting for new P-0 needles versus O-8.


My goal is to make these changes and return clip to third position. I am hoping this rids the pings when I back off, but does not over rich midrange and thus poor gas mileage.

If this doesn't work fully, I'll back off timing advance a bit. I use the nominal value now.

I have Chambers, but don't think that is the main issue.


I'll keep an eye on this post too.


I am going to call Jet R Us now to see where my NEEDLE Jets are. I bet the pilot jet size increase alone may do it too, but P-0 jet should not hurt… maybe should have gone P-2 or 4. We’ll see

[Buffalo-guy]

The fact that you have chambers, explains a lot. On my hot rod 750, I had J&R pipes, later cylinder on a J bottom end (more aggressive porting, but still stock) and J carbs with 115 mains (102.5 stock) and I had to go all the way up to 47.5 pilots to get the bottom end to work well (35 stock). With this jetting I was able to use the stock needles and needle jets, 3rd clip groove, and 1 1/2 turns out on the air screw. Definitely no pinging, and OK fuel mileage considering it would loft the front wheel at 3500 rpm with no clutch work. Very un-buffalo like. You are heading in the right direction with your jetting, as chambers really change the whole dynamics of the bike, and richer jetting is required to make them work well. Keep us posted, and good luck. Cheers.
Fred