Pinging, popping GT550B

[tz375]

Pistons sticking out of the top suggests one or more of the following: Long rods, tall pistons and/or barrels machined top or bottom.

zero squish is clearly not right. Did you look carefully at the head gasket/s for signs of pistons clipping the gaskets?

1.1mm deck is WRONG and needs to be addressed. I would pull one barrel off and check the height of the piston and the barrels. I can borrow a set to measure as a reference if Vintageman can't get to his. If it's the rods, that raises all sorts of other questions of course, but start with facts.

While that one cylinder is off, check the piston to bore clearance and see if the machine shop screwed something up.

Good luck

[yeadon_m]

Thanks everyone. On close inspection there were no marks on either pistons or head. Nevertheless, before taking the head off, I found just 0.1mm clearance between piston and head at the cylinder wall, all around, on all cylinders (cold - I've no idea what happens when its hot).

Its clear that as the pistons protrude, yet there is no evidence that the cylinders or head have been skimmed, and that the rods are 'Royal RO111' (or Royal R0111) tells me the rods are slightly longer than the stock rods. Theoretically, it could be the pistons, but the bill of work states these are Suzuki NOS and I'd be surprised if they're not, since I understand they are unique to the nikasil plated cylinders (the plating was renewed as part of the job).

0.1mm being too small, and 1.1mm being too large, what should the gap be? it is dangerous to ride it with 1.1mm clearance?

On the 'noises' noticed previously and which prompted me to go looking for causes:
-generally, its just ain't as quiet at idle as I hoped and expected. Its not H1 like at all (I heard one last weekend) but its ticky ticky as it idles, though this cannot be heard when riding and its creamy smooth with the exception of the below. This noise is surely excessive piston / cylinder clearance. It made the same noises on first start up and have not worsened. If 'they're all like that, sir' then no issue. I sense that the left cylinder is slightly the noisier but if so its not a big difference and I also assume that small differences in carb balance at idle might emphasise one pot over another, so I'm not making firm statements on whether one pot is noisier. Should I be able to push the piston fore and aft more than I can push it side to side, in the bore, with my fingers? I ask because I definitely can. There are no marks in the new nikasil plating that should not be there. I can still see the cross hatching in most places.
-just off-idle, in low gears (so I'm going slowly enough to hear it) I could hear the rattle/popping coffee beans in tin sound in the headers - I think. If I open the throttle >1/8th, it goes away and sounds nicer and nicer as it accepts more throttle. It does not do it if I shut off. So this aspect sounds like carbs or timing (maybe compression?), rather than mechanical.

Cheers for any advice.

Mike

[yeadon_m]

Alan,
I don't think the head is skimmed and in any case that would not explain the pistons sticking out of the cylinder!
I looked at a stock 550 head at the weekend and the machined surfaces were the same amount above the casting as they are on my head.
I'm more or less convinced the builder has just bunged in rods which are 'close' but not identical in dimensions to stock.
Not sure where this will all go. Once I have enough collective wisdom from the experts I will at minimum be calling the builder.
Cheers,
Mike

[Craig380]

0.1mm being too small, and 1.1mm being too large, what should the gap be? it is dangerous to ride it with 1.1mm clearance?

For setting up tuned engines with squish-style heads (Yam RDs, etc), a 1mm clearance is usually recommended as that gives plenty of room for expansion etc. So I believe there's no danger in running the 550 with 1.1mm clearance.

However it would be interesting to know what a bog-standard 550's clearance is. I'd guess it's probably around 2mm, but then again, Suzis have always been set up very mildly (to account for production-line tolerances, crap low-octane fuel in certain markets, etc).

In terms of being able to rock the piston in the bore, I don't know what the specified piston-cylinder clearance is but it's got to be somewhere in the 1.5 thou' to 2 thou' area. I would've thought you should be able to rock the piston a tiny bit, but not much at all.

[yeadon_m]

Craig,

Useful info. I am unclear whether I should be concerned or not by what I've found. That the rods are aftermarket and slightly longer is pretty clear to me, but as the shop I've used does many GT motors, they 'should' know what they are doing (I used them with my GT380 and that is perfect).

I just found this info in my files (the rods listed here are not the ones I have in my motor though).

Cheers,
Mike

GT550 crank rods replacements KTM
[GT750] Re: GT550 Crank Rods
Hide Details
FROM:
• kjsisterson
TO:
• GT750@yahoogroups.com
Message flagged
Tuesday, 29 May 2012, 17:07
It all depends on the current decking. One of the first performance things you should do to a two stroke is deck the top of the cylinders so the piston actually comes to the top. Usually it's about .5 to 1mm off. IF the rod is 1mm longer it might not need anything at all.
A spacer plate under the cylinder or a thicker copper head gasket would do the trick.
Kevin S.
GT550 Crank Rods
Hide Details
FROM:
• chris@antics.co.nz
TO:
• GT750@yahoogroups.com
Message flagged
Monday, 28 May 2012, 21:23
I'm aware that rods for GT550 are long gone, unobtainium, rocking horse doos.

I was surprised to learn in a recent ebay add that a gent had rebuilt his crank using KTM200 rods.
I enquired with him and then got in contact with the shop who did it who in turn have given me this link.

http://www.mitaka.co.uk/OTHER%20RODS/CT.KTM200.htm" onclick="window.open(this.href);return false;
The picture shows a rod, Royal brand, marked R031 or RO31.

Since i have a backup NOS full crank shaft i dont need the above but i'm throwing it out there as a lead for others that rocking horse doos do exist.
The shop says the KTM200 is 1mm longer than a GT550, but doesn’t elaborate what extra work they did and how they coped with the 1mm (thicker head gasket etc or what), or whether porting was a big deal

Anyone throw more light on the subject ? Particular to this fix.

[Craig380]

As TZ Richard pointed out, it's a big oversight that the pistons protrude above the barrel at TDC. Suzi strokers don't do that.

It woudn't be too much of a problem if you'd been told about it, and instructed to use extra base gaskets etc. Best to have a word with the builder and see what they say about what components were used - longer rods, exactly which pistons etc.

Running double head gaskets isn't ideal in the long term because of the potential for compression leaks etc. I believe it's better to jack up the barrels using a thin spacer plate or an extra base gasket. Assuming the clearances are all correct, longer rods won't affect running as the actual stroke stays the same, and the port timing will be close to stock, or maybe a little better as the ports may end up half a millimetre higher.

The nikasil barrels sound like they are still in good condition, and if the pistons are good too, hopefully raising the barrels will sort it all out.

[Vintageman]

+1 on raising barrel. Exhuast may have some play and still mount OK

Did you have the cylinders bored OS or were they honed and pistons refreshed?

Also. curious if the GT550 with the extra transfer ports above intake port had a different piston than the ones without extra ports? You know like the GT250A change bulletin... it got a new piston for they had to move the ring locator holes so the ring gap ends did not snag into the new extra ports.

[Alan H]

If the rods are longer, then whatever the 'extra' length is, will be covering the ports at BDC, so you lose at both ends of the stroke. It will need a thicker base gasket then.

[yeadon_m]

These are 'nikasil plated' bores so they can only be stock size. Suzuki only supplied one size piston and not only these, but the rings, are special and not the same as those used in the iron liner, US-type model.

They were re-plated during the rebuild and the pistons / rings are new. In fact, everything is new.

Sounds like I'm close to having all I need to call the builder, get them to tell me what exactly is in my motor, and how to resolve the fact that my pistons are peeping out of the top of the cylinder

Checking other bikes base gaskets, these are often ~0.5mm thick. So if I add another one, and then go back to a single head gasket, I think I will be back to a squish of ~0.5mm which I am informed is about right. I have no idea. I'd not heard of squish with any comprehension until the last few days. Whether or not my fag packet maths is right, there seems a path to fix this part of the problem.

Then, and maybe separately, I can address why my motor is pretty swishy just as it warms up (for a few mins after the chokes have been shut off) but once fully hot, it tick ticks irritatingly.

If they can fit over long rods and not bother to have told me, they could as easily have plated wrongly so that the piston to cylinder clearance is larger than it should be.

Cheers,
Mike

[Craig380]

I can't remember if the 550 heads have a squish band as such, or if they are simply a shallow dome, but for me personally, 0.5mm is a bit tight, 0.8m - 1mm seems to be the accepted figure but there are many others here that know more about it than me.

[jeff kushner]

. I would pull one barrel off and check the height of the piston and the barrels. I can borrow a set to measure as a reference if Vintageman can't get to his. If it's the rods, that raises all sorts of other questions of course, but start with facts.

While that one cylinder is off, check the piston to bore clearance and see if the machine shop screwed something up.

Good luck

I've got a set on the shelf and can measure them too........................

jeff

[yeadon_m]

Does anyone has an Suzuki source for the cylinder / piston clearances on a nikasil plated GT550?

I have these for iron liner motors but would prefer the real McCoy for the euro setup.

Cheers,
Mike

[old racer]

As said above,If the rods are 1mm longer then simply raise the barrel 1mm.

[tz375]

+1

[pearljam724]

Wouldn't there be a good chance also. That his over all timing is just slightly too far advanced ? Causing pinging ? I realize he just had motor work done. But, wouldn't you want to diagnose that before tearing into the motor ? Aside from exhaust tone. My 550 is extremely quiet in reference to motor noise.