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What am I missing?

  • Thread starter Thread starter Firewall
  • Start date Start date
F

Firewall

Guest
I've got a 77 GS750 with a Mac exhaust and K&N drop in air filter. Here's what I've done:

Sealed the air box
New boots on both sides of the carbs
New O rings between the carb boots and engine
Carb dip 24hrs, with new o rings
Valve adjustment
carbs syncd with vacuum tool

Petcock is original, but seems to work as designed (only flows fuel with vacuum, no fuel in vacuum hose)

I'm working my way through tuning the fuel/air screws with the color tune. I've done cylinder #4 and now I'm on #3. The bike starts up and idles great, far better than it ever has and it is getting better as I make adjustments with the color tune. But what concerns me is the adjustments I'm having to make to get the blue flame I'm supposed to be getting. On #4 I had to back the fuel screw out almost 2 full turns to get the air screw to make any difference. Right now I've got it about 1.5 turns out on #3 and the air screw makes no difference (I can screw it all the way in and nothing happens).

I'm assuming this means there's an air leak some where but I feel like I've covered all the basics in that department. Everything looks tight and the way it should be

So what am I missing? Or is this OK?
 
How far out are the fuel screws (the ones on the bottom)?

With your setup, they should be about 1 turn out.

As a rule, the air screws will end up about double the fuel screws, so 2 turns is about right.

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I set them at 1 when I put the carbs back together but so far I've had to increase that by 1 turn on #4 and atleast 1/2 turn on #3. I havent touched #1 or #2 yet.

Edit: For the two I've worked with, when the fuel screw is at 1 turn out the air screw makes no changes no matter what I do
 
Firewall said:
For the two I've worked with, when the fuel screw is at 1 turn out the air screw makes no changes no matter what I do
Click to expand...
When you had the carbs apart, did you verify that the tips of the screws were not broken off in the holes?

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Unless I damaged them putting it back together they were sharp and pointy.

I was just skimming the carb rebuild guide and noticed it says set the float height to 24mm. I'm pretty sure this is what I did. However, the carb spec chart on Basscliff's website says 26mm. Which is correct, and could float height create the issues I'm having?
 
According to the chart that I have for reference, the correct measurement for your carbs is 23 mm.
That is listed for the '78 models, I don't have anything that mentions '77 specifically.

I just looked at the service manual on BassCliff's site, it says 26 mm.
shrug2.gif


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Arg, I think I know the problem but I can't find a picture of the stock airfilter to verify.

I opened my airbox and there is a gap between the filter and the top metal plate on the sides. No matter how tight the screws are this gap appears, infact tighting the screws seemed to make it worse.

If memory serves, isn't there supposed to be a bottom metal plate too? Meaning shouldnt the filter be sandwiched between two metal plates?

My stock filter is long gone so I'll have to buy that bottom plate if that's the case
 
Sigh, found the pic of the stock air cleaner i needed. Don't know that it's *the* problem but definitely seems like it would be *a* problem.

Thanks for the help anyways Steve
 
Stock float settings for the 77 carbs is 26mm. There not the same as the later models. I still think you could get away with a +/- of 1mm even though the book doesn't specify that like the later models.

I'll also add, before you get everything all tuned up, you'll likely find that you're going to need a larger main jet and to move the needle clip down a notch for your KN and pipe
 
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