Pinging, popping GT550B

[yeadon_m]

Folks,

I have a problem with my GT550B and I'm concerned about it. Would love to have collected wisdom on it please.

Right, full rebuild including crank, rods, replating the nikasil cylinders (Euro ported bike) and Suzuki pistons/rings. Engine was done at great cost by SEP Engineering UK, who did a great job years ago on my 380B (which has been sweet since 2007 and still is).

I've done about 400miles on the new motor. As far as I'm aware, ignition timing is spot on stock (Suzuki points, new HT leads, loom and battery in perfect condition). Carbs are very clean and running good needle jets (I was able to gather three perfect looking ones from two sets of carbs) - these are 174 P-4 and needles are 5DN1-3. Stock jets were 30 pilot and 105/102.5/105 mains (Reiner confirmed these from the stamped numbers I sent him from my VM28 carbs).

Initially on the road I found that stock jets were far too lean - I experimented and am currently on 37.5 pilots and 110/107.5/110 mains. I have the pilot air screws at 1 3/4 and it is slightly rich off-idle only, in the 1st 1/8th throttle. Probably better on 35 pilots. The motor was never as quiet as I'd hoped, with more piston / cylinder noise than the GT380 by far. Not like a Kawasaki, but not like a sewing machine at idle, as is my 380. Riding it, is does sound and feel quiet except as below.

Riding it, if I open the throttle 1/4 and up, its creamy, smooth, powerful and a delight. It doesn't chug much if at all when I close the throttle at speed. I can ride 60miles and find temps just above the exhaust clamps ~125C using an IR thermo directed onto black dots I've painted. Compressions cold are all 125psi. Plugs are '8s. Use of two stroke oil is as expected, with light coking in the baffles, not dripping with oil. Plugs look on the light side of tan, but not white-ish as they were with stock jets. Idles steadily at 1100rpm hot, starts easily hot and cold, runs cold on three pots on choke.

The problem: if at 2000-3000rpm especially noticeable in 2nd (as wind noise is low) if I crack the throttle about 1/8th, there is a dreadful racket in the headers. Sort of rattling, maybe popping, pinging. Definitely nasty sounding and not had this on any other triple. Reproducible, occurs each time I do this and am going slow enough to listen. Not sure if only when hot, but definitely occurring when hot.

Having done a bit of reading about detonation, pinging, preignition and the like I am both confused and worried. I do not want to have my motor melt down. I am aware even when I cannot hear the noise (riding faster) that a just cracked throttle results in an 'unpleasant feel', so I guess this noise - and whatever causes it - is occurring all the time at this throttle.

I should mention that very oddly, when I put the head on, using aftermarket gaskets, the pistons seemed to hit the head at TDC. I did not run it like this but noticed it with plugs out when setting timing - rotating the points cam, I could feel a very gentle thunk as it went over TDC. If I backed off the head nut torque, it rotated smoothly. I replaced the head gaskets with Suzuki, and the issue went away and I thought no more about it. The aftermarket were 0.9mm and the Suzuki, 1.4mm thick.

But I wonder if I should be this close to no clearance anyway, and might this have something to do with the pinging, rattling sound?

Reading suggests this noise has to be sorted or it will cause damage and might already have done so. Suggestions welcome! ignition timing, head gaskets (or base gaskets, also aftermarket), did someone skim the head, carbs, anything I can try? what is happening and how can I can diagnose and fix?

Thanks!
Mike

[tz375]

Mike, that's odd that it occurs only at low revs and small throttle openings. Detonation is not so common at light throttle but rather at full load unless it goes very lean just at that point and you say it's rich there. You could try screwing the air screws in 1/2 a turn just to see if anything changes. If that helps, try a set of slides with less cutaway (are they even available?).

Unless that makes a significant difference, I'd pop the head off and look for signs of detonation and mechanical damage. Suzukis usually have a bout 0.5mm of negative deck height - in other words the piston is 0.5mm below the top of the barrel at TDC. If your pistons are sticking out the top, I'd be tempted to fit new/thicker base gaskets. Pistons don't need to be below the deck, but they should not be sticking out the top.

[yeadon_m]

Richard,
Thank you for your thoughts. I already tried having the pilot air screws in further, say 1 1/4, and it was still 'noisy / popping / banging' in the headers, just off idle, and horribly farty rich too

I wonder if you're on with it with the decking. I did notice that the pistons seemed to protrude more than I recalled or expected, when I put it together.

Yesterday I had the chance to looks at a nearby pals 550 engine on the bench and felt that his pistons were as you say ~0.5mm below the cylinders at TDC. His is also a euro / nikasil motor.

I will next take the head off and see whats what. If I confirm that the pistons are not 0.5mm below the cylinder tops at TDC then clearly I need to raise up those cylinders by using Suzuki gaskets. Its only recently that I'd realised how important gasket thickness can be in certain applications, base and head being among the most important I guess.

Could this if confirmed be a cause / contributor to the noise I describe? I've never had it before so am uncertain wth it is and means (except: not good).

Thanks!
Mike

[Blokhead]

It almost sounds like you are describing severe 4-stroking. Or actually 6, or 8-stroking.

On my "73 GT750 with a new top end, I had what sounded like banging in the head pipes at very light throttle (very loud). It would happen when I tried to maintain a steady speed anywhere below about 50 mph (or about 1/16 throttle). The "explosions" were so severe it blew some of the Permatex out of my FPP collars, resulting in oil spatter all over. Bike would also buck on deceleration with closed throttle. It did idle fine, and accelerated fine on light throttle. I think the fact that I'm running thin chambers exaggerated the noise.

In my case I know exactly why this was happening; I installed UFO's in my original VM carbs. This is common (known) problem resulting in a very rich condition below 1/8 throttle. The official fix is to cut your pilot number in half, and then some. I went down to #10 pilots, but even that wasn't enough. Finally had to machine part of the UFO away to get them to work right.

I've heard many a 2-stroke that popped, and pinged over the years, but nothing like what my GT was doing. By finally getting the low speed mixture right, its made a huge difference. Seems to 4-stroke like it supposed to now.

yeadon_m, You say it was lean on the pilots; what made you think that?
If I were you I would try smaller pilots, and/or adjusting the air screws to lean out the mix. You might have to go smaller than stock to get it right.


Oh, and if it really was detonation, that occurs inside the cylinder, and makes the fins on the head really ring. I used to have some free air snowmobile motors that sounded like shaking a coffee can full of rocks when I ran them too lean. This only occurred at 3/4 to full throttle though.

This is from Wikipedia on 4-stroking;

Four-stroking is an undesirable operating condition of two-stroke engines, where they instead begin to fire every four strokes, rather than every two strokes. This firing is uneven, noisy and may even damage the engine if allowed to continue unabated. Four-stroking was often a cause of poor idling in two stroke engines.

And this;

Hazards of four-stroking[edit]
When a four-stroking engine eventually fires, the excess mixture from the previous stroke causes an excessive cylinder pressure. This can be nearly double the normal pressure, leading to excess noise and potentially failure of overloaded bearings in the connecting rod.[2]

[tz375]

4 stroking from being too rich is possible and Vintageman had a similar problem and concluded that his was too lean.

Going back to mechanical issues for a moment, I usually have the block machined to zero deck height i.e. the edges of the pistons are just level with the top surface (deck) of the block and I use the had gasket to set squish or machine the head to get the results. In an extreme case I might have to have the pistons higher than the deck if the only head gaskets available are super thick - but that's rare.

I would pop the head off and look for damage and measure deck height and I'd measure piston to head clearance before the head came off - just to be sure of what is going on.

[Craig380]

I'd measure piston to head clearance before the head came off - just to be sure of what is going on.

This is a great idea - using the old solder trick for measuring the clearance BEFORE removing the cylinder head. Probably worth measuring 4x per cylinder (front of piston, rear, left & right) to make sure the clearance is relatively even all the way around the piston crown.

Sorry to hear about the continued noise, Mike - as you know, mine is still making its own banging noise in the headers, and it's very frustrating. Mine's been doing it for so long now that I'm starting to ignore it, because in all other aspects the engine runs great with no noise.

Just a thought. Are you running the factory settings for timing? It might be worth retarding the timing slightly, just by 3 or 4 degrees, to see if that helps. My own bike does NOT like the factory settings, and has always pinged at low and mid rpm when set according to the manual. It likes to be about 0.3mm retarded.

[yeadon_m]

Craig,

Thanks for the input - the degree of timing retardation to try is really useful.

I will check timing plus squish as described.

Cheers,
Mike

[Vintageman]

This is odd. But a little more info please.

Do you run expansion chambers?

Does this occur only at light load (rocking between pulling and coasting)?

Only occurs if engine loaded ... pulling like a tractor at these lower rpms?

If you hold throttle fixed and climb up rpms I assume it goes away?

My issue small throttle (~1/8 ) was only higher RPM. Here the aftermarket chambers when boosting (~ 5K RPM) I guess is raising compression and I was too lean. I say lean for what I am probably really now doing is over richening things to help cool it down.... the GT550 as we know has domed/quiescent type head and prone to pinging. So far on my 73 GT550 I have solved this by a richer pilot jet and larger needle jet. I think a richer slide cut would have been spot on, but N/A.

I do not ping anymore but I am too rich at idle and very small throttle (1/16) like you and now 32.5 pilot. Slow city riding not as enjoyable as it could be is my complaint.

There is a big difference between the early GT550 and later GT550 pilot jet and one of the pilot body hole sizes. I have a spare set of early carb bodies. I am going to try and reduce my pilot jet size and open up the pilot hole closest to the engine more like the later GT550 (a size half way between the two model years as a starting point).

Back to yours. It maybe something mechanical versus pinging...
I await your reply the question above for some more possible clues

[yeadon_m]

Folks,

Right, more 'data' and probably now pointing to a mechanical origin. I'm quite cross about this if its what I think it may be.

Checked plugs - all three a nice light coffee.

Checked compressions - cold, WOT, 3sec on starter - all read 125psi on a screw in gauge which has a rubber pipe about 1 foot long with spark plug hole adaptor at one end, gauge the other. This is at or slightly higher than my other GTs are pretty much - generally I see ~120.

In turning the motor slowly with the kick starter, there it is again. darned 'light thunk / clunk' as it goes over TDC. Worse, I don't hear exactly the same sounds 3 x per rotation.

Used thin solder to assess squish. Well, there isn't any. About 0.1mm thick is how the solder comes out, every cylinder. The pistons are 'just clipping' at the top, which is definitely not right.

Removed the head. All noises on rotation go away.

Checked pistons at TDC. No marks on pistons or in head chambers. Pistons? Oh yes, they stick out, by an estimated 1.1 - 1.2mm at the edges, proud of the cylinder edge.

This is 100% NOT what I saw on another (untouched) euro GT550 motor at the weekend.

How can this happen? I do have a base gasket on each cylinder, the one the engine rebuilders put there. So even if Suzuki gaskets are a little thicker, this cannot explain why my pistons are 1.1mm+ proud of the cylinder at TDC when they should be ~0.5mm+ below it.

I can only conclude that my motor has been rebuilt using longer con rods. Anyone have an alternative opinion? everything about the cylinder tops and also head surfaces say they have not been skimmed.

Additional checks: at TDC the pistons do not twist to finger pressure, so thats good. These move from side to side only very slightly, about the amount I'd expect. But Left and Right pistons can be pushed fore and aft far further and when I do so I can hear a gentle 'thunk'. The very noise I was hearing before removing the head. I had noticed when running that I could hear a disappointingly large amount of piston/cylinder noise. Not terrible, but not whisper smooth and quiet like the GT380 right next to it.

I cannot think of any other course of action than to contact the engine builder (who received and built the intact motor). I don't really want to add extra head gaskets and continue to use it, I'm no longer sure its 'right'.

Cheers,
Mike

[Vintageman]

Rod too long as you said. Wrong piston (e.g. PE175?)

Pull a jug off and see what is written on the side of the rod. Also measure piston dimensions. I have some GT550 piston loose and can help measure.

[Blokhead]

This is odd. But a little more info please.

Do you run expansion chambers?
Yes, Factory Pipe Products

Does this occur only at light load (rocking between pulling and coasting)?
Yes

Only occurs if engine loaded ... pulling like a tractor at these lower rpms?
No

If you hold throttle fixed and climb up rpms I assume it goes away?
No, progressively got worse as rpm's rose, and the load decreased.



I do not ping anymore but I am too rich at idle and very small throttle (1/16) like you and now 32.5 pilot. Slow city riding not as enjoyable as it could be is my complaint.
Was very annoying to try to keep a steady speed, especially in traffic. People would stare because the popping, and banging was so loud.


Back to yours. It maybe something mechanical versus pinging...
Absolutely not mechanical. This has been fixed by leaning out the low speed carburation.

[yeadon_m]

I cannot post a pic of the con rod which I took before putting the motor back in the frame (the jugs were slipped in place over the base gaskets now on the bike so I lifted them to lighten the motor and took pics.

It says R0111 or RO111 and has a little symbol that looks a bit like a capital R made up of three smaller symbols. This is 'Royal' brand but not as in this link: http://www.con-rod.com.tw/atv-connectin ... -8001.html" onclick="window.open(this.href);return false;

So in the afternoon I thought nothing ventured. Put two stock head gaskets on and put the head back on, torqued it up. Squish is now ~1.1mm. No thunking / clicking as I turned it over without plugs in, by hand. Compressions cold dropped a surprisingly small amount, to 120psi. This is lower than previously by a needle width on the gauge.

I ran it and it sounds as before (which is as expected). I have not yet ridden it.

On the movement of pistons, I realise I have no reference to how much they 'should' move in fore/aft vs side to side directions. I can still hear more piston / cylinder noise than I expected. Anyone with a GT550 - does yours tick as it idles, or is it dead swishy quiet like my 380 is?

Cheers,
Mike

[jeff kushner]

I've worked on several GT550's and have never heard what you are describing on a stock engine. While not nearly as smooth as a 380, they are sweetheart engines. Have you cc'ed the heads to find out if they have been machined down? I have one on my tool-bench that I can measure for comparison if you'd like.

The secondary compression you noted at 125 is within reason and occasionally on some rod/crank/piston assemblies, as the piston transitions at TDC is will make a noise as the bearings take slack in reverse direction but nothing like you described. First, is this across all three cylinders or just one? Since you've got skills, pull the head and closely examine, with a magnifying glass if necessary, the heads and pistons to see what's going on. You should note pits or burn marks as well as the charge path on the piston and it should be fairly even across all cylinders. It's amazing how much you can tell by looking. You can first pull the plugs and again with the aid of a glass, check to see "dots" on the porcelain portion of the plugs. If you have the proper gauge, the end gap of the rings should be .15-.35mm, any tighter then about .11-.12mm and it will cause all kinds of issues!

Also, a really simple method of seeing rich/lean is to engage the choke when the noted condition is occurring. Obviously, if it runs better, its lean and if it gets worse, its already rich!

Just don't ruin your brand new engine by running too long without finding the cause. Bad camfers, hard edges on the piston, even mismatches between small end bearings/wrist pins can all cause weird noise but as the general rule of thumb, the Gt550 doesn't rattle nearly as much as any H! I've ever owned!!

I pulled apart one of my H1's a couple of weeks ago as I finally got around to getting it done and the previous owner had told me that he put new pistons in rebored cylinders. He said it had less then 100 miles on them. Thank God I pulled them before running it. The left cylinder piston was hanging, creating a long score mark......someone failed to properly prepare the cylinder and piston! You just never know so good luck!

jeff

[Craig380]

Sorry to hear about this, Mike - as Vintageman suggests, perhaps pistons with a different pin-to-crown height have been used in the rebuild. Hope you can get it sorted.

[Alan H]

I wonder if someone skimmed the cylinder head Mike? Sounds like it's skimmed too much if so.
Check with a pipette, how much liquid you can get in the upturned heads.
I'll have a look at a spare head I have when I can get it out of the garage if you like.