GS550 doesn't like prolonged high speeds - fuel starvation or something else?

[roeme]

As the subject says. My GS550 is capable of doing the ton (at least per the speedo) on certain roads (cough, cough), i.e. with some downhill stretches, but not for very long – after a while, I get a power/thrust loss. The first few times I encountered that, I rolled onto the shoulder, waited perhaps a minute or so...bike then started up again easily and I continued on my way with no further incidents.

Last time, I just eased off the throttle until the bike pulled again properly.

I can't really correlate the issue to anything else (road inclination, hot/cold weather, ...) other than speed and subsequently engine rpm.

Now I'm not sure, am I overheating the engine perhaps? Or is a simple cause of "the flimsy petcock and fuel line rally can't feed four carbs that fast - or rather have not been designed to do so" ?

I don't have experience with overheating engines tbh. and wouldn't know what the symptoms are, other than seizing up...

 

[Rich82GS750TZ]

This may not be likely, but I suppose your vented gas cap could be partially blocked, so that at the extreme demand doesn’t allow enough air in to keep up with the fuel draw. And I suppose you could test this by attempting a “ton” run with the fuel cap removed, but I wouldn’t advise it.

 

[Rijko]

Does your other GS550 run well ?

I am not very familiar with the differences between 78/79 .. but if it is an option to swap the gas tank
that would be an easy way to check/rule out if the issue is in the gas cap or fuel tap, or something else.
Same for the carbs.

 

[zed1015]

If the bike pulls better when backing off the throttle at high rpm it can be an indication that the main jets are too big or you have a clogged air filter but you should first check that the fuel supply isn't being restricted and that the fuel tank vent and any fuel filters are not partially blocked.

 

[Rijko]

Oh just remembered, my buddy recently had an issue with losing power full throttle.
Would run perfect on auxiliary fuel bottle, but full throttle on several test drives
would give issues after a few miles.

He checked everything he could think of, he is meticulous and it could not be the carbs he
just cleaned to perfection, right ? ​

Turned out he had water in the tank, which would go down into the freshly
cleaned carbs ... just a few drops but enough to mess up full throttle riding.

 

[Cipher]

As the subject says. My GS550 is capable of doing the ton (at least per the speedo) on certain roads (cough, cough), i.e. with some downhill stretches, but not for very long – after a while, I get a power/thrust loss. The first few times I encountered that, I rolled onto the shoulder, waited perhaps a minute or so...bike then started up again easily and I continued on my way with no further incidents.

Last time, I just eased off the throttle until the bike pulled again properly.

I can't really correlate the issue to anything else (road inclination, hot/cold weather, ...) other than speed and subsequently engine rpm.

Now I'm not sure, am I overheating the engine perhaps? Or is a simple cause of "the flimsy petcock and fuel line rally can't feed four carbs that fast - or rather have not been designed to do so" ?

I don't have experience with overheating engines tbh. and wouldn't know what the symptoms are, other than seizing up...

Float height.? Your not using pod filters are yiu?

 

[rphillips]

Possibly vacuum pet cock, as eng. makes less vacuum at wide open throttle (I think). First I'd do is give it a try while on PRI. Running on PRI by-passes the vacuum function of the pet cock. Also check for kinks or tight bends in fuel line that may restrict fuel flow...Also what Rich said, be aware the tank vacuum will show up waaay faster with completely full tank than with lo gas in tank. Your 550 "will do the ton"... A ton is 2000lb., I'd think a ton would be 2000 mph. who decided the ton is 100 mph.? Hey, keep us updated.

 

[roeme]

Hey all, thanks for the replies, I'll try answer your follow-up questions summarily:

* While the bike currently is a bit hard to start, the issue existed for way longer, basically since I acquired and re-built it - and it started well then
* When running, the bike runs well (but see next point)
* Due to time constraints, a shop that I trust has cleaned the carbs approx. a year ago and reverted everything back to stock - it started up very well back then, just high speed acceleration is dissapointing, but that's because the PO installed bigger jets, and the shop reverted to stock. In any case, the issue showed up before than, and kept appearing after that.
* Stock airbox, no pod funkiness

* My other GS550 is in pieces, waiting for another bike to be finished, so unfortunately I can't really compare. Lacking that, if it were a known issue, then surely the GS hive mind would know about it, hence the post.

The tenor of your answers seem to be vent and/or fuel line restriction. So I will investigate this as well.

As for "the ton" - yeah I wondered the same. Just read it in multiple places...seems to be of british and/or NA origin. And since it's not like imperial units make any sense anyway, I just go along with it ^_^

Since for various reasons (not the bike, that's fine) it's always a bit iffy doing these speeds, I don't encounter the issue often and that's why I only post now.

I'll keep you guys posted!

 

[Rich82GS750TZ]

Do the Ton.
Here's a fun article on the history of the usage of the phase, if no real explanation of its origin or any relation to an actual unit of measure.
https://caferacertv.com/history/the-history-of-do-the-ton/

 

[Rob S.]

Your 550 should do 112 mph. (Aka "ton and a dozen"). :nevreness:

 

[Suzukian]

If your valves get tight, this will happen as the bike warms up and the valves don't close completely. They kind of float when the engine gets hot. Stopping lets the engine cool down, and you get the acceleration again. Make sure you valves are correctly adjusted, this will give you a level playing field.