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Carb sync and tune leads to high RPM

  • Thread starter Thread starter poppadre
  • Start date Start date
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poppadre

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Hey all.

Today I decided to sync up and tune my carbs.

After a full rebuild in the winter, upgrade to electronic coils and ignition the next step was to upgrade.

The bike is a 1978 GS750. With VM26S stock carbs and jets and pod filters. Stock 2-1 exhaust.

Before I synced the carbs it was burning fuel like crazy and getting about 26 MPG. I had to have the idle adjustment screw in quite a bit for the bike to idle around 1500. So much so that I had to take out the spring out in order for me to get the idle in all the way. When riding over 1/8 throttle the bike was pretty smooth and responsive. but when it idled it would jump up and down around 500rpm.

A few questions:

Does the bike have to be on the center stand to sync them? After syncing I took the carbs out to adjust the screw settings and noticed that the #1 throttle body is raised a few mm higher then #2 and #3. #4 is slightly higher than #2 and #3. Is this common?

As I started to sync the carbs the idle sky rocketed. I couldn't get it less than 4000 even with idle all the way out. What causes this?

I turned the air pilot screw to 1/4 turn out from bottom and they matched up with the factory notch on the carb body. I turned out the fuel screws 2.5 turns each. The bike seems to pull quite a bit smoother now on 1/8 and up throttle and I got the idle down but the bike is idling at 1500 with the idle adjuster just barely in. The bike seems to want to stall and is very up and down at idle. I fear if I adjust the fuel screws to optimum settings they idle will just get higher and I wont be able to get it back down to 1500.

Any comments or feedback helps. Thanks
 
The bike does need to be on the center stand or at least centered if yours is removed like mine is. The fuel level in the float bowls needs to be the same.

I'm sure more help will be along shortly, or worst case in the morning.
 
Could your throttle or choke cable have snagged on something since you said you removed the carbs? Are you sure you don't have any vacuum leaks from the intake boots?
 
Your adjustment screw settings are all screwy, too.

That might be in an effort to compensate for improper jetting.

With pod filters, you typically need to increase your main jets 4-6 sizes, you say yours are stock.

You likely also need to raise the needles, but pilot jets don't need to be changed.

For your setup, the fuel screws (the ones on the bottom of the carbs) should probably be set about 7/8 to 1 turn out from lightly-seated.

The air screws (the ones on the sides of the carb intakes) usually start about double the fuel screw settings, then fine-tune from there.

Why is the idle so high? Carb sync usually helps get all the carbs pulling the smae amount, which will smoothe out the idle, but look for tight cables or hanging linkage.

Why did you remove the carbs to adjust any screws? What screws were you trying to adjust? By "throttle body", are you talking about the slide? After synching the carbs, they should have all been EXACTLY the same height.

.
 
Thanks for the response guys.

It sounds like I will need to have another go tonight but put a puck under the kick stand to get the bike standing upright. This is probably why my slide (I called it throttle body) of carb #1 is sitting so high up compared to the rest of them. The Slide adjuster screw of #3 is pretty much bottomed out but was having a hard time getting it like the other 3.

Side note: Does anyone know where I can get another adjuster screw. Bikebandit says that part number is discontinued.

I replaced the O-rings in the intake boots, and the boots have been inspected with no cracks and are still pliable.

Choke on these is directly on the carb, and the throttle cables are new and unobstructed. I even had them disconnected for some of the time when I was constantly taking the carbs off the bike.

I was taking the carbs off the bike to get at the fuel mixture screws easier.

Tonight after work I will try your screw settings Steve and I will get the bike as upright as I can. I will give you an update then.

I do plan on upping the jet size over the winter when I have more time and money
 
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