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Discussion Starter #1
I've got a customer who's looking to have a full fairing "GP" racer built, he's more concerned with the look, but wants a faster engine as well, I'm itching to do an alloy fairing, so I'm biting. I've been told by someone that the cb550 is slow and heavy and probably won't be a good candidate, I was looking at the gs550 as a possibility. It looks to be about as heavy and slow as the cb550, but has very low compression stock and a dohc, so would it be a potential candidate for a hot cam, overbore and high compression pistons? I would probably model the body around something like the 50's augustas, so the dohc would look proper too
 

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Handles weird. Even a stock engined gs550 with appropriate jettings (rad: fix EPA bullshit) makes them damned fast.

How fast does he want it? I'm building some 675's right now that should be (quite a bit) more than 70hp at the crank with gs550 bottom ends.
 

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I'll tell you exactly what will do it:

Remove airbox lid (leave in filter)

Main jets to 112.5 to 120's, depending on location.

One size up on idle circuit jet. Maybe two, but doubtful.

That's it. That's all it takes.

The gains for doing this on the GS550 are absurd. No other bike I've fucked with has the same kinds of returns with a simple slight derestriction and jetting change.


(The above is for the 80 and later gs550's with CV carbs. I haven't done the earlier ones, nor ridden one, with regular slide piston carbs. The earlier ones may not respond as well. Head is a bit different, mainly on intake tract dimensions.)
 

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The early slide carb models needed a lot of work, but, you could get ports a really good shape and flow.
I spent about 12~14hrs on the first intake port then the others went quite quick (about 25~30 hrs in cyl head)
Set of cams, real race exhaust, carbs bored 1.25mm, big bore kit (forget how big, 718cc?)
It was a LOT faster than Honda VF 750 (geared for 175mph would almost pull red-line)
Speedo never read over 93mph ;)
 

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quote:Originally posted by crazypj

The early slide carb models needed a lot of work, but, you could get ports a really good shape and flow.
I spent about 12~14hrs on the first intake port then the others went quite quick (about 25~30 hrs in cyl head)
Set of cams, real race exhaust, carbs bored 1.25mm, big bore kit (forget how big, 718cc?)
It was a LOT faster than Honda VF 750 (geared for 175mph would almost pull red-line)
Speedo never read over 93mph ;)
Well there ya go.
 

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Seriously don't need to go big bore for someone who isnt going ot race it. If you just do the stuff I said, assuming it's 80 or later. And you can try with the earlier ones, too.
I reckon mechanical slide piston carbs would take to velocity stacks a fair bit easier than the CV's.
 

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He's building it for someone else who wants a GS550, Joe.

If all I wanted was a "fast bike" and a "dual sport", I wouldn't have a garage full of 30 to 45 year old bikes. I'd have a gixxer 750 and a BMW f800gs.
 

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Was there any stipulation on how old the core vehicle had to be? How many cylinders? Did he narrow it down for you, or did you guys already hash that out? The GS 550 is an acceptable choice, but the KZ550 or Seca 550 are also twin-shock frames and IMHO would be better candidates.
 

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Both of the bikes Emit mentioned are lighter and better handlers. They don't have near the headroom the gs550 does engine-wise though for performance.
The gs550 is an absolutely physically huge bike for a 550.
Hmm, GS550 powered KZ550...?
 

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My opinion was mainly based on aftermarket and oem performance support for the platforms. You can hot-rod a KZ engine with GPZ parts, for example, and the Seca can have the same done with FJ/FZ/Radian 600 stuff. Those are relatively simple factory parts swaps.

Now, on the other hand, a GS is a good platform if the guy wishes to start with a wire-wheel bike and keep it kind of period-correct as far as that aesthetic. I just don't think the early GS engine has good representation in the performance engine parts spectrum. Mark, if you say they can be hotted up reasonably with simple mods, maybe that would be enough for the guy. Or, perhaps the hybrid Mark mentioned? Too much work?

Does this customer have a basis or point of reference on what would be considered "faster" than a stock GS550?
 

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The work is relatively simple. Katana/GS650 top end. Die grind the top case to allow the larger 650 sleeves. Cut your own base gasket from 1/32 gasket material.
Instant 70+ crank HP.

Of course, every go fast part for the Katana 650 2v (cams, mainly) are a drop in, if you want more power.
 

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Discussion Starter #18
quote:Originally posted by joe1028

i read it as he is looking for a full faired gp racer and does not even have a bike...well i would think most of us here have old bikes, that is a given but if you have never tried a modern bike you do not know what your missing....and you only need one of them in the fleet to keep ya happy...
joe
naw, he's got 3 other bikes, two for his sons and one for him, I did the alloy tanks, seats and exhausts for them, he's looking to have me do a whole bike and liked the idea of a fairing too

as far as fast goes, I think he'd be pretty happy with a bike a little faster than a stock cb750, but lighter, he likes the stock engined 550 he has now, so my guess is he doesn't get all twitchy for wheelies
 

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a well sorted stock k0 through k2 cb750 will still be faster, and they weigh about the same, though I imagine you'll be lightening it some.
 
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