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GS Cylinder Head Differences?


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So I've broken down and ended up on the forums...

I've been a browser for quite some time and I know there are lots of talented people here. So maybe someone can help me out with a question.

I have a '79 GS1000 that I'm in the middle of doing a complete build on. Engine is apart and frame is stripped.

So the other day I decided I needed a bike to ride until the other one is built so I started looking. I unintentionally ended up buying another GS. It was just in the right place at the right time. The new bike is an '81 GS850G. It's been in storage for like 40 years and is in pretty great condition aside from the disintegrated intake boots. So I removed the carbs with the intention of replacing the boots and cleaning the carbs.

Here lies my conundrum. I happened to take a look at the intake runners on the 850. They're big. At least, bigger than the 1000. The 1000 head has 29mm intake runners which, as far as I've been able to discern, was standard on the 79. The runners on the 850 are 34mm. What?!

I've already ported the head on the 1000 and it's ready to accept the throttle bodies I've picked up for it. 

My question is: did I miss something?

I'm pretty sure the 850 is 100% original and I'm also pretty sure I'm the first to have the 1000 motor apart as well. But if I'm building the 1000 motor and the 850 head has 5mm bigger runners, does that head flow that much better? I'm not opposed to swapping the heads. The 850 is a fully dressed cruiser so I don't care if it's fast. The 1000 needs to be built to have some more fun.

Do later heads all have bigger runners?

Do they have bigger valves as well?

Can they even be swapped? Dimensionally they look identical. But would the 1000 cams be able to be put in the 850 head? 

Some expert here who's had a few of these motors apart will probably be able to help me.

Thanks in advance guys!

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From the 1980 model year, the GS1000 and GS850 changed from slide carbs to CV carbs. CV carbs flow less for a given choke size due to the permanent obstruction of the butterfly, so they're usually bigger for any application.

The GS1000 went from 26mm or 28mm VM slide Mikunis (depending on model and market) to 34mm BS CV Mikunis. The size and spacing of the inlet ports was increased to facilitate this.

There is no significant change in performance, although the bigger ports can help on a tuned bike, but you need carbs, cams etc to make use of them.

The GS850 1980 model also changed, going from 26mm to 32mm IIRC.

The 850 and 1000 heads are not compatible  as the bores are different sizes.

There's no performance advantage to the bigger ports on their own.

That's the gist of it.

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Welcome to OSS

The 1000 and 850 (as above) won't swap over, you can't swap the carbs ever as different port spacing. The 850 and 1000 cams will swap but you will need to change the cam chain sprockets. 

The best GS head to tune is the gs1100g head, as it's a CV head and has the D ports that are the best for a good flow. 

Like all engine tuning it's not just one thing but a package of exhaust, carbs, filters, cams, bore etc etc etc. 

Your best bet with your new 850 project is to buy some new carb rubbers (runners as you call them) and a new carb seal kit, get the carbs Ultrasonic bathed cleaned fit the new carb seal kit and ride it, save your time for the gs1000 

(y)

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