'79 GS1000S - 1085 Wiseco Kit - overheats at low speeds

[Guest]

The bike
1979 GS1000S, with Standard pipes (yes proper originals), and standard airbox and filter with lid on, Dynateck Electronic ignition (I've put a strobe light on the timing and it appears fine going fully advance at around 3500rpm).

The change
I've recently added a Wiseco 1085 piston kit. The bike runs like a train - lots of power, no flat spots, no hesitations up to redline, idles well.

The problem
If I ride for about 20-30 minutes at 60-80km/h 35-50mph on a COOL day with no stop-start traffic the oil temp gradually keeps rising (this model is the one with the oil temp guage) until it reaches almost 170C (around 330 deg F) before I stop and let it cool down. If I take it onto the freeway or non-urban roads and ride at around 100Km/h (60mph) or above the temp drops to around 100 deg C (212 deg F) or less.

The cause?
I've read on this forum that big bore kits make these bikes run lean. It seems to me that this is what must be happening - the plugs look very clean, but not burnt when I do a chop test.

Three Questions for the experts:

1. Why is it that the bigger capacity causes it to run lean with standard jetting?
2. I've booked the bike into a motorcycle workshop that has experience with these older bikes. What could I reasonably expect them to investigate/do to rectify the overheating problem?
3. It seems that many forum members who add a big bore kit also add an oil cooler. Should I be considering this as an option? I'd perfer not to as I like my bike looking stock (see below) and with the adaptor plate I would not be able to use the oil temp sensor.

Thanks for your time,

Ian

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[posplayr]

Big bores don't make bikes run lean; not rejetting for a BB causes the bike to run lean. You have to step up the jetting.

[IanR]

Big bores don't make bikes run lean; not rejetting for a BB causes the bike to run lean. You have to step up the jetting.

Thanks for replying. My question was really seeking an explanation for the need to "step up the jetting": what happens to change the air/fuel ratio on the inducation stroke after the installation of the big-bore kit?

[posplayr]

I can't tell you the precise technical reason but on my 1166 with cv carbs I needed about 4 steps on the mains.
bigger cams need bigger mains but a flowed head may need less. So a simple answer to your question might be hard to obtain. Most of my experience is using an afr o2
data logger.

i would not be riding the bike without increasing the jetting.

one other thing on temp, this will probsbly draw some fire from some 8v guys but the bb 16v guys know you can drop temperature by using a series r/r.

[Rob S.]

Adding an 1150 oil cooler (and filter cover) to my stock 1075, dropped temps so much I usually ride with the cooler covered up with cardboard. It still runs much cooler than it did before.

If your jetting is correct, sounds like you need an oil cooler. Doesn't a big-bore make more heat?

[Nessism]

An interesting result. My bone stock 1000S oil temp will get up over 300F when on the road at 75+ mph on a hot day, but it cools off when you slow down. Pretty much opposite of what's going on with this bike.

BTW, a big bore kit won't make the engine run hot all by itself. It's just bigger pistons. The larger engine has the potential to make more power, but when cruising the amount of power output won't be different than a stock engine, so it shouldn't run hotter unless the jetting is jacked up. You might want to try bumping the mains one step and lifting the needles. At cruise the engine won't be running off the main jets anyway but it won't hurt to hedge your bet.

[JTGS850GL]

I can't tell you the precise technical reason

I believe Bernoulli's Principle comes into play. Ironically you'll find that in most cases you actually need to reduce the size of the main when going bigger bore. With everything remaining equal, larger displacement means more air volume means larger signal to the carb = more fuel drawn from a given sized jet. Pilot circuit usually need to go up though. At least that's what I've found on carburetored automotive engines. Maybe one of the tuning gurus will chime in.

[rapidray]

Aftermarket pistons usually have more COMPRESSION than stock pistons, causing more heat. Go 2-3 sizes bigger on the main jet, re-adjust the mixture/air screws, to the "fastest idle" method & you should see a good improvement. I am a fan of oil coolers on bigger air cooled engines but to each their own.
Ray.

[60ratrod]

I believe Bernoulli's Principle comes into play. Ironically you'll find that in most cases you actually need to reduce the size of the main when going bigger bore. With everything remaining equal, larger displacement means more air volume means larger signal to the carb = more fuel drawn from a given sized jet. Pilot circuit usually need to go up though. At least that's what I've found on carburetored automotive engines. Maybe one of the tuning gurus will chime in.

wow, I haven't seen Bernoulli's Principle since I got my Enlisted Aviation Warfare Specialist. but it does make sense in this case

[posplayr]

wow, I haven't seen Bernoulli's Principle since I got my Enlisted Aviation Warfare Specialist. but it does make sense in this case

Well I'm sure Bernoulli will pop up his head somewhere trying to riddle the jetting question, but JTGS850GL has really offered no explanation that follows the facts. His automotive experience is exact opposite to these motorcycles, so I think we will need to wait a little longer for a "simple" explanation for the jetting behavior.

[John Park]

Well I'm sure Bernoulli will pop up his head somewhere trying to riddle the jetting question, but JTGS850GL has really offered no explanation that follows the facts. His automotive experience is exact opposite to these motorcycles, so I think we will need to wait a little longer for a "simple" explanation for the jetting behavior.

Automotive emulsion tube carbs are operating on the same physics as everything else but utilizing quite different principles.

My guess is that the reason for bigger jets as the wide open flow rate increases is that the increased pressure drop that results from more air going through isn't capable of delivering equivalently more fuel velocity due to the fact that the fuel is liquid and massively more viscous than air. IOW, the two curves don't overlap. If the air velocity/fuel flow relationship was a linear one, oh how simple carbs could be. In fact, they can be; my lawnmower is a one jet wonder, but it only runs one speed.

The oil overheat question is pretty strange; my only guess is that maybe the engine speed is too low [downshift?] to circulate the oil fast enough. There's also the possibility that the air passageways between the cylinders are plugged with dead bugs and leaves. In theory, the engine should run its coolest at about 50 mph.

[posplayr]

Automotive emulsion tube carbs are operating on the same physics as everything else but utilizing quite different principles.

My guess is that the reason for bigger jets as the wide open flow rate increases is that the increased pressure drop that results from more air going through isn't capable of delivering equivalently more fuel velocity due to the fact that the fuel is liquid and massively more viscous than air. IOW, the two curves don't overlap. If the air velocity/fuel flow relationship was a linear one, oh how simple carbs could be. In fact, they can be; my lawnmower is a one jet wonder, but it only runs one speed.

.

OK that is a good explanation (even using Bernoulli) but how does it predict a better flowing head needs smaller main jets? Do we need a composite theory?

[posplayr]

An interesting result. My bone stock 1000S oil temp will get up over 300F when on the road at 75+ mph on a hot day, but it cools off when you slow down. Pretty much opposite of what's going on with this bike.

Ed since his bike is building heat at moderate riding speeds and RPM, it would be a pleasant surprise if his operating temperatures did in fact drop with the SERIES R/R as the 1100E's do (I know you don't believe it).

[Fjbj40]

Larger displacement, as in boring the stock barrels, will equate to the engine running hotter. It really does not "make more heat" it just has reduced material to absorb the heat and dissipate it, therefore it will run hotter.

Like I teach my students "less meat equals more heat". One of the main reasons we do not bore out aircraft cylinders, which are 2 valve and air cooled. Quite similar to the GS1000 2 valve engine.

Do your engine/stock over bored cylinder barrels a favor, run an oil cooler. My 1085 has an oil cooler, the highest temp I have seen is 111 Celsius after the oil cooler, before the oil cooler was installed I could see as high as 150 Celsius, way to hot. But 111 is perfect, water boils at 100 C !

[posplayr]

Larger displacement, as in boring the stock barrels, will equate to the engine running hotter. It really does not "make more heat" it just has reduced material to absorb the heat and dissipate it, therefore it will run hotter.

I have argued that the convective heat dissipation capacity of an particular engine is fixed with smaller displacement engines being generally of the same surface area and therefore same heat dissipative capacity. The smaller engine runs cooler because it has a high dissipative capacity in relation to it's generated hp. When you bore (increasing cc s) the engine, you are certainly not improving it's ability to dissipate heat, as there is nothing changing on the outside. Since it has a fixed thermal resistance between the the air and the heat generation source, more heat requires a higher internal operating temperature.

The alternate argument is even the High hp engine only needs to put out the same hp as the smaller engine to go the same speed so all else being the same why does it still run hotter? I suspect it is the high ratio of losses to crank or RW hp, but it may be where this simple explanation (from an external view) breaks down.