Warming up a GT 550

[pfg49]

I've looked at this section of the forum for a while, but I bought a GT 550 so I thought I'd better join.

There seems to be a people with good knowledge of two strokes, but when it comes to say what combination of things work to make a bike faster/slower/unrideable, people seem to be a bit coy. I have a fair knowledge of four stroke tuning, having been involved professionally for about 30 years, but what I'm reading on this forum, I'm struggling with, it's just so completely different! I bought the Bell "bible" of two stroke tuning, but its hard work reading it, because a lot of it just doesn't seem relevant to what I want to do.

Like lots of people I would like to have 60(ish) GT 550 rwhp, and I'm sure there are a few people who regularly contribute here, who know exactly how to do that. What I'd like is a simpletons guide to achieving this. "Put a 10 mm shim under the barrels/jugs, skim 10mm off the top of them to restore compression, do this to the exhaust port, do that to the transfers, etc, etc, and you should get what your looking for".

I think that if someone can do this, and I follow the instructions, I will then start to understand what I'll be doing as well.

Any volunteers please?

[Alan H]

Put a 750 H2 engine in it, that should give what you want.
The GT550 is rated at 40/50 hp at the crank - depending on year, model and general health of the engine.
If you chuck a couple of grand at it you might get 60hp at the crank - if you're lucky.
It's a GT - grand tourer. It's not a racing bike. You've got the wrong bike my friend - and I've had 550s since they came out in '72, and after taking advice from Eddie Crooks and Frank Whiteway then, got another 5 mph out of it after a lot af work.
Ride it as it's meant and you'll enjoy a different kind of biking.
Oh, my current 550K still pulled about a ton on the IOM this year, coming off the mountain towards Creg-ny-Barr but that was about it, with only a bit left.
Fast enough on 70s frame and suspension as well, I might add!
It's 40 + years old ffs.

[Craig380]

Hi and welcome,

the easiest way to give the bike a little more oomph is a set of expansion chambers -- Gibson and Higgspeed do new ones, or you can trawl eBay for a newish used set or a set of period J&Rs, Microns or Allspeeds.

Chambers do perk up the triples nicely as the factory exhaust is restrictive, as heavy as an anchor, and after around 40 years, likely to be full of baked-on carbon / loose baffle plates.

It's easiest to run chambers if you retain the standard airbox and filters. This way, you may not need to rejet the carbs, or at worst just go slightly larger on the main jets.

The engine is in a soft state of tune and you can skim the heads, raise the exhaust ports a couple of mm etc to liberate an extra couple of horses, but it's time consuming (or expensive if you get someone else to do it for you). Also if your bike has the later, SCEM plated cylinders (if the mounting stud at the rear of the cylinders is higher than the front, you've got these) then you need to be careful with any porting work. The liners have a special plating (like Nicasil) which can easily chip / flake off.

Let us know how you go on with it.

[pfg49]

It's 40 + years old ffs

Alan,
I'm 60+ ffs, and I can still do it, so if the bike is only 40+ffs, then it can, and will do it! I've just got my 50+ BSA Lightning to behave like a lunatic, and that's unreliable old British Iron, old Jap iron should do much better!

[Alan H]

It's 40 + years old ffs

Alan,
I'm 60+ ffs, and I can still do it, so if the bike is only 40+ffs, then it can, and will do it! I've just got my 50+ BSA Lightning to behave like a lunatic, and that's unreliable old British Iron, old Jap iron should do much better!

Oi, just because you're an old git, don't think you're the only one!
I'm retiring next month (early, I hasten to add), so it depends how many +'s you are past 60.

What I meant to say, was that the bike is knocking on, not us.
If you want new performance out of a 40 year old bike, you'll knack it up quicksticks, even if it's a Jap 2 stroke.
Yes you can get a bit more, but not a lot more and still be rideable on the road.
Remember that the 550 was never raced (except AFAIK, by Crooks Suzuki as a production racer in 1972) but it was in the 750 class, and while a bit of tweaking let it keep up, it would never be a winner when losing out on 200ccs.
As Mike says a set of spannies gives a bit, but mainly in weight saving. You can put a 19" rear rim on and chuck the centre stand away so it handles a bit better ground clearance wise, skim the head and tweak the ports and some other bits, but it'll still only do 8000rpm (ish) and the horsepower gains are really minimal for the expenditure. Even standard, the 550 is a better scratcher than the GT750, so you can still upset the big boys on their Hippos as I proved back then with my new GT550J, and more recently with the Kettle boys on rallies. You'll be surprised how it rides with just a good smartening up - or have a look at THIS THREAD for some ideas.
Where are you in the UK? I'm in South Yorkshire and there's lots of 550s up and down.

Looks like I'll have to get some more 'Old Fart Biker' shirts made. The older we get, the better we were.............

[GT750Battleship]

Alan,my "Hippo"will show a clean set of heels too my mates 74L GT550 any day of the week ! on a freeway at least,everywhere else they are pretty equal
Cheers,
Roger

[Alan H]

Exactly Roger. The 750 is a better tourer with the extra torque, but the back roads are very different!

[Vintageman]

Check the cyclinder in this older eBay post

http://www.ebay.com/itm/Suzuki-GT550-Cy ... 7675.l2557" onclick="window.open(this.href);return false;


When I asked Mr. Miler he claimed porting was stock. It does not look stock to me. The transfer port inlets at the bottom of the cylinder were widened to match a wide set of cases (not all case have wide tranfer ducts). All ports look like the have been cleaned up and widened some in all directions.

The post said HAS FRESH BORE FROM DAVE AT A&M MACHINE . I think they are in the same CT town as paul miller. I just may call them yet

If you cleaned up ports, widen all a bit in each direction and add chammbers, you will get a gt550 with decent power without changing it great midrange torque. That is what is amazing about this bike wide strong midrange. Compression helps that even more, but start to ping soon If you turned it into top end only machine it would suck for streat riding.

[Alan H]

Check the cyclinder in this older eBay post

Water cooled GT550 barrels eh? I think they should start to check what they advertise.

[Vintageman]

Water cooled GT550 barrels eh? I think they should start to check what they advertise.

Alan H, scroll all the way to the bottom of the post and you we see the three later model cast liner air cooled GT550 cylinders. Look carefully at the underside transfer passages and also you can see the part of the EX and transfers. These to me don't look stock and were dressed up a bit and done nicely.

Someday I'll contact “HAS FRESH BORE FROM DAVE AT A&M MACHINE” and ask him what he did

[Alan H]

Ahh. Didn't see those, only the 750 stuff. Small screen on this tab thing!

[Vintageman]

Someone made the transfer inlets wide as if were the later European platted version with the extra transfer ports which it's not. Researching it seems the later cases in use where transfer feeds are wide to match the Extra port version but had the single version cylinders mounted.

http://www.ebay.com/itm/1975-SUZUKI-GT5 ... 2W&vxp=mtr" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

http://www.ebay.com/itm/1975-suzuki-gt5 ... Km&vxp=mtr" onclick="window.open(this.href);return false;

I am not sure this transfer widening helps standard cast cylinders due to extra volume. But as far as flow resistance it looks like the person even reached into the transfer port existing into cylinder and elongated them a bit. Also the exhaust port looks opened up some and made oval. Same with intake.

Stock 550 I see the intake passage restrictive too small area and that should be opened and maybe thin divider and re-center or lower brass oil feed pipe. Shorten the SRIS tube in one of the transfers per cyl so it doesn’t F up flow

Raising the Ex port 1 - 2mm ( I went 1.5mm) and widening 2-4mm should help power without changing the bike awesome street manners. To me the transfer stock look like an enduro This is most likely what is holding this engine back. so that should be elongated 1-2 mm if not cleaned up at least. That is what impresses me about those cylinders. So maybe raining the cylinder a 1-2mm gives a little more Ex and Scavenging time. . I would open intake and retain intake port timing as stock or a few degrees more. Ensure the compression is restored. This engine midrange responds well to extra compression, but you can’t go much unfortunately due to head design, so losing any by raising ports will affect midrange as I have discovered.

Fun stuff

[pfg49]

Thank you all for your comments and help.

It does occur to me that I may be wasting my time. My 550 is a European A Model with the Nikasil coated bores, so..... if I start mucking about with the ports I'll probably damage the Nikasil, or will I?

Anybody done this?

[tz375]

Nicail barrels already have more port area in the transfers and also transfer heat more effectively.

If you port it, be careful with that plating, but if it needs to be replated anyway, then you might as well port it and then get it plated.

[pfg49]

Should I be thinking about modifying the transfers, and if so how? Should I think about cast iron liners, and who does it in the UK? Who does Nikasil Plating that's known to be good?