1981 GS850G Power Curve

[SteveR]

My bike's carbs are finally close... I mean the pilot fuel/air screws are set for midrange and the plugs look good. My question is regarding the change in power from 5500-6000 rpm. It really turns on and begins to pull in this range. Is this normal or am I transitioning to the main needle and getting more fuel? I normally operate the engine in the 3000-5000 range so I rarely use the upper rpm range. How can I tell if the mixture is correct? Thanks

[Dogma]

Mine does that, but closer to 5000, I think. It seems to do it again at about 7000, but that goes by so fast I'm not really sure.

I don't actually know that it's normal though...

[tkent02]

Mine does that, but closer to 5000, I think. It seems to do it again at about 7000, but that goes by so fast I'm not really sure.

I don't actually know that it's normal though...

Do a series of plug chops at different throttle openings.
Then you will know, and not risk burning anything up.

I would have thought an 850 would have smoothly increasing power, not a sudden increase.
Haven't ridden a good one in years.
Could be it's going from an area of the correct mix into an area with the too lean mix which gives better power and burns valves and things. Or it could be moving from an area of too lean to run correctly into an area of perfect.
Do the plug readings.

[Steve]

Simply AMAZING, isn't it? :shock:

Yeah, that's about the way my son's 650 is, too, but his has pods and a pipe on it, too. I don't know if it's been re-jetted, but we have had no problems with it.

Easiest way to tell if the mixture is correct is to do "plug chops". This involves running the bike at a specific throttle opening (not engine speed) for a while, simultaneously hitting the kill switch and pulling the clutch, and coasting to a stop, hopefully in a safe area, so you can pull a spark plug to read the color.

It helps greatly to mark your throttle grip with a bit of tape, then also mark the housing with idle, 1/4, 1/2 and full-throttle marks. Idle is easy, you can do that in the driveway. Full throttle can be done in a higher gear, going up a hill to control speed. Half throttle is also good going up a hill. Choose a gear that gets you up the hill without getting run over by traffic. 1/4 throttle should be easy to do, the trick is holding it there for close to a minute, if you can. The other settings can go less, as they are burning more fuel, so they show results quicker.

.

[doctorgonzo]

That's pretty common for these engines I think. I mean that's what separates an inline 4 from a V-twin, and it's exaggerated in these GS engines. You don't start to really get into the power band until about 6000 RPM, but then you better hang on.

[SteveR]

Thanks for the replys.
I will do the plug chops at 4500rpm and 7000 rpm and see how the mix compares.

Does anyone have a plot of RPM vs Torque for the 850 ?

[Dogma]

Fascinating. Any chance a leaky intake system could be to blame? My intake sealing project should be done in a week or two. There were definitely some leaks in there. It sounds like I should brace myself for a general tuning project soon.

SteveR, is your intake system well sealed? If so, then I have no hope that mine will magically be in tune when I put it back together. Ah, I should just learn to do this. I only dread it because I don't know how. It actually sounds like a good excuse for an extended afternoon ride. "By the time I got it right, I was two hours away".

[tkent02]

Any chance a leaky intake system could be to blame?

Absolutely it could.

[SteveR]

No leaks. New intake boots and o-rings. , new air box connectors also. Carbs have been rebuilt and fuel o-rings installed, sync is good, valves are close.
Stock air filter and -8 NKG plugs.
I should pay myself for all the hours I have spent on carburetting... what a learning experience.

[Steve]

SOMEBODY has not been reading very well.

This involves running the bike at a specific throttle opening (not engine speed) for a while, ..

I will do the plug chops at 4500rpm and 7000 rpm and see how the mix compares.

You do not do the chops based on engine RPM, you do them based on throttle opening. This is because you can get to 7000 rpm in neutral with minimal throttle opening, probably on the pilot circuit. This will not be the same circuit that will be in use at 7000 rpm while on the road, so why check it? You want real-world conditions, and the different circuits in the carb are based on throttle openings and airflow, so that is how you check.

.

[tkent02]

You do not do the chops based on engine RPM, you do them based on throttle opening.

I wrote this on another thread a while ago, it is still good.


Three separate tests for three separate carburetor circuits. Before you start, put a little anti-seize on each plug, don't install the plugs very tight. No need to wear out the plug threads in the head, you will be pulling them out hot. Bring a pencil and paper, keep track of each plug at each test. The low throttle chop, just putt around a few minutes at about 20 0r 30mph or so, keeping the throttle open but just barely, in the correct gear for this speed. It takes a while to color the plug because there's not much going on inside the combustion chamber. This tells you about the mixture from the pilot circuit, the low power stuff. Pay attention to how it runs and feels, you can tell rich from lean. If it runs well and the plug isn't black it's probably doing OK. This one is probably the most important to get right, it sucks at have your engine screw up momentarily at the apex of a slippery corner.

Mid range, go out on a highway and run it at about 1/4 to 1/3 throttle for a while, up a long grade is wonderful but not required as it doesn't take that long to get a good plug read, it happens faster. This tells you about your mixture from the position of the needle. If it spits and pops that would be rich, if it surges, wanders or loses power for a while and comes back on that would indicate lean. If it runs smooth and nice it is close, looking at the plugs will tell you exactly. You need to see a little color here, a mixture resulting in lean white insulators can run fine but eventually may burn things up.

Full power is done in one hard blast, full throttle through the gears like a drag race, or a blast up a hill. Does not take very long at all to get the plugs colored, you don't need to do insane speeds. You do need to find a place safe to go faster, where you can see any cops, and yet still be able to safely pull over to remove the plugs. The mixture at full throttle is controlled by the size of the main jet. Definitely need to see a color other than white, too lean here will burn things up in a hurry.

For each of these reads, you need to actually chop the engine. A few seconds under closed throttle as you slow down will taint your readings. Simultaneously hit the kill switch, pull in the clutch and close the throttle. This leaves the plug looking exactly as it was during the run at the power setting you are testing.

To get the mixture right at idle, you adjust for highest rpm, you are looking for the peak combustion temperature here, it won't hurt anything to be lean at idle because there's not enough heat in the combustion to damage anything.

If the main circuit or even the needle circuit is too lean you can burn the valves and even pistons up from the excess heat.

A mixture a little too lean is very powerful indeed until something expensive burns up.

[bwringer]

A nice bump in the power curve at 6K rpm is perfectly normal on an 850. Many owners never spin them that high, sadly.

As long as it runs fine below that, it's a feature, not a bug.

[doctorgonzo]

A nice bump in the power curve at 6K rpm is perfectly normal on an 850. Many owners never spin them that high, sadly.

As long as it runs fine below that, it's a feature, not a bug.

I have the GS1000G and yes, it doesn't even really start to pull until about 6000. It runs fine below, but you know when you start to hit the sweet spot! I live for chances (few though they may be outside a drag strip) to run it up to about 8000 before shift and take it through the gearbox.

[SteveR]

Got it!
I will mark the throttle positions. I read it ... It just had not soaked in.

And thanks again for everyones input.

[Nessism]

On my ’82 there is a lean spot in the carburetion at partial throttle – about 1/3 throttle opening. When the bike is cold the lean jetting is more noticeable but as the bike warms up it runs pretty darn well. I’m going to try shimming the needles a smidge to see how this affects things, but the bike doesn’t need drastic action or anything and I hate to give up mpg with gas prices the way they are.

One interesting thing about the 850 is that Suzuki changed the cam timing and added an exhaust cross-over chamber in order to improve midrange power. Not sure what year these changes were introduced, 1980 or 1981, at the same time they switched to CV carbs I think. If someone had an early bike it would be interesting to try the later cams so see if they really make a difference.