What determines redline in these motors?

[Guest]

Just curious, is it valve float, piston speed, timing chain stress, oiling, harmonics? I know any of these can wreck an engine, but what is the weak link in a GS twin?

[tkent02]

There's no weak link. Wind it out. Redline is merely a suggestion.

[Steve]

No idea about the twins, but if you take a 650 up to redline and miss a shift, the valves and pistons get REAL "friendly".

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[tkent02]

Done it thousand of times, maybe millions. Hasn't hurt anything yet. The twins seem fine with it too, although I haven't done it as often.

[GregT]

Just curious, is it valve float, piston speed, timing chain stress, oiling, harmonics? I know any of these can wreck an engine, but what is the weak link in a GS twin?

Absolute bog standard, the valves will float and touch pistons before anything else breaks...

With good (aftermarket) springs, piston speed is the limiting factor IMO.

But I'm talking well above std redilne.

[Steve]

No idea about the twins, but if you take a 650 up to redline and miss a shift, the valves and pistons get REAL "friendly".

Done it thousand of times, maybe millions. Hasn't hurt anything yet.

Well, I wasn't there when it happened, and the owner did not readily admit that he missed a shift. He kept saying "I don't know what happened ...'' but I finally got him to admit it. Told him I didn't care that he missed a shift, I just needed to know how deep to go into the engine, based on what happened. He couldn't remember if it was the 1-2 shift or the 2-3 shift, but he missed the gear (I'm putting money on the 1-2 shift and hitting neutral), gave it some gas and winged it well past redline. No telling what the actual speed was, he said he was too busy watching the road to bother to look at the gauges. Final damage was only three intake valves were slightly bent.

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[robertbarr]

Excepting rider error, as in missing a shift or revving the fool things with no load on the engine, 'redline' takes place after these things have quit making torque anyhow, so you get more acceleration by just shifting before the tach-suggested redline.

[Ric]

Robert, you make a most excellent point!

I admit that I ride my wife's bike like it's a diesel, mainly because I'm leery of locking it down( could have to do with driving a dump truck for a living, too) on the way home. Also need to love on the carbs a little to richen the upper mid to full throttle after looking at a few plug chops, but that can wait till pods are ordered.

I tell ya for a 300, this is a torquey little thang at half throttle!

[tkent02]

No, wind it out. They like it.

I have been winding them well past the redline since these bikes were new, seen no damage whatsoever from it.
Probably the most abused was my first 550. Beat the heck out of it daily, all of my friends had 750s or bigger, I made this 550 keep up for years and years. When we tore it down to fix leaks at 120,000 miles it had no wear on anything, rings, pistons, everything else were no where near service limits. We replaced no parts except seals and gaskets and it ran for a few more decades at least.

I think maybe the 1000s and 1100s might be less tolerant of the high revs, but I haven't seen any damage on them either. ON those it's no so necessary to wind them up so tight, they actually have some useful low RPM power unlike the little bikes. You can actually pass a car or merge onto a highway safely without going so deep into the revs.

[tkent02]

Excepting rider error, as in missing a shift or revving the fool things with no load on the engine, 'redline' takes place after these things have quit making torque anyhow, so you get more acceleration by just shifting before the tach-suggested redline.

Except that when you shift into the next gear it isn't in the power band yet, you have to wait for the power to start all over again.
Riding a 300 you need to know how to make it go, especially if there are other vehicles around. If it keeps pulling harder and harder you aren't there yet.

[Grimly]

Yep; even though I've been riding bikes most of my adult life, my mechanical sympathy kept me from routinely going above 6k on most engines, reasoning that if it was limited to that by my right wrist, the engine would last longer. Then I realised that with the very short stroke small capacity Japanese engines, the piston speed, the mechanical loadings, the material stress was actually lower at high RPM than some of the old long-stroked crapheaps I'd been brought up with, and had revved to 6K quite often, with nothing breaking.
Game on, from that point
I still believe the overall longevity is influenced by treatment, but with modern oils there's a huge extra range of acceptable stresses the parts can take and shrug off. Also, bear in mind, the GS range was built to last by a company determined to make a name for reliability.

[Ric]

They have certainly established themselves as reliable, Grimly.

Tkent, when I do wind it out it pulls hardest to within the neighborhood of 6000 rpm or so, at a little less than WOT. At WOT it's a tad less brutal and violent, and it just kinda marks time past 6500ish. Plug chops confirm it's running lean at WOT. I know running lean kills engines - that's actually what makes N2O so destructive, not the N2O itself. So till I get her podded and jetted, I shouldn't get too excited with the throttle, right?

[John Park]

Before you go searching for the missing horsepower with pods and jets, I'd look at the possibility that the 300L motor may have cams that favour the lower end of the rev range. The 250 cams were pretty revvy, but there's no guarantee that they didn't sabotage the high end rush to make the bike have a more beginner friendly power band like a DR200. Suzuki had some really anemic cams on the shelf for the restricted power bikes for the Euro market.

If they are 'lawnmower' cams, you could easily swap in some 250E ones, I'd suspect. Also look into the compression ratio and valve sizes before you start swapping things around. There's also the possibility that they sabotaged the air box to neuter it. IOW you have to look at the whole 'food chain' before you start spending time and money.

[tkent02]

They have certainly established themselves as reliable, Grimly.

Tkent, when I do wind it out it pulls hardest to within the neighborhood of 6000 rpm or so, at a little less than WOT. At WOT it's a tad less brutal and violent, and it just kinda marks time past 6500ish.

Whether or not it's too lean to run, timing is messed up, exhaust is clogged or whatever else it may be, if it isn't screaming past 8,000 RPM or so there is something seriously wrong with it. I wouldn't run it at all until I found out what was the cause of all this. For one thing it wouldn't even get out of the way of some school bus full of nuns if it had to, for another the chance of doing damage is good. If you know it to be lean at WOT your first job is to find out why. Some previous owner changed jets, set the float level wrong, put on a very restrictive fuel filter, some fuel passage is blocked, there has got to be something causing it to be lean. From the factory the pilot circuits were very lean but the main jets were only a little bit lean, they would make power all the way up...

What has been altered?

[Ric]

Tkent, I put a pair of Dime City's 17" shorty mufflers on it. Other'n that, everything is stock, no fuel restrictions, I cleaned the carbs myself, stock jets. Only oddity worth noting is it never had any pilot jet plugs. I didn't even know it was supposed to have them till I read BikeCliff's carb tutorial link.

Conventional wisdom tells me the shorties are what leaned it out, and it's not by an awful lot - rideable, just not right. It certainly hasn't lost any power, in fact it pulls a lot better in the range I already mentioned compared to the stock exhaust. Could that have to do with the missing pilot plugs?

John Park, you just got me thinking of a frankenbuild of sorts - a 250 top end on the 300. The 250 had 10:1 CR compared to the 300's 8.9:1 CR. Hmmm....