Flat spot between 4800 and 7000?

[Guest]

1979 GS750 W/stock VM carbs
I just replaced the O-Rings on all Intake Pipes. All were brittle with 3 & 4 having an 8th in. gap in them. #4 was sucking air thats the reason for the change. Before the change, the engine ran real good upto redline. All plugs were a nice light brown. After the change all plugs are still burning correctly with the exception of #4 which is sooty. Starts right up when cold and idles around 1200. Poping has gone away at idle.
Now it bogs down and hesitates intermittenly from around 4800 up to 7000 or so as if not getting enough gas. When it's warmed up, it idles at around 2200 and after a minute or so it will idle back down and then up again and back to around 1200. Has the engine been synced to fit the bad O-Rings? It really sounds like a lean condition but the plugs look good with the exception of #4's sooty condition. I'm confused:?.
I know I need to aquire a sync tool. Where to get one?
Can the pilot fuel screw be adjusted outside of the carb or do I need to remove the bowl?
Any ideas on what my problem is??
Appreciate any help you can give me.

[mark]

You can adjust the pilot screws while the carbs are on the bike. I use a mirror and a screw driver that I rigged up that fits. It takes a little getting used to, but it can be done.

When your checking your plugs its probably best to do a plug chop. Run the bike at the RPM where you're having the problems and hit the kill switch, then check the plugs. Don't drive it around at lower rpm's and test the plugs, that won't help you too much.

You're probably right that the bike was synched to leaky intake manifolds. If its as bad as you say that could be the whole problem right there. It is odd that the bike ran so well with the leaks. Hopefully it wasn't re-jetted as well?

Most people here use the MOTION PRO carb synchronizer. I have some vacuum guages that I use and feel comfortable with.

[HammerHead]

Thanks Mark, I'll get back to you and let you know what I ended up with.
Appreciate,
*Steve*

[KEITH KRAUSE]

You can't synch the carbs with any intake leaks and you can't compensate for an intake leak by trying to achieve more vacuum. The leak won't allow it, unless very small.
If your bike is idling at around 1,200 cold and 2,000 warmed up, you probably still have an intake leak. Once in awhile a slide can start sticking after it heats up but I think you still have an intake leak. That has to be fixed before you do anything else. On your 750, you may have installed the manifolds in the wrong order or they may be cracking. You may also have installed the manifold o-rings wrong or the carbs aren't "popped" into the manifolds correctly. Use only the correct slim style clamps for the manifolds too. No standard "hose clamps".
Be sure the carbs are cleaned and all old o-rings inside are replaced.
Set your float levels to .94"
Bench synch the slides. Vacuum synch later.
Then initially adjust your pilot fuel screws (underneath) to about 1 1/4 turns out from lightly seated. Set your side air screws to 1 1/2 turns out.
After warmed up, set the side air screws using the highest rpm method.
Is this bike completely stock? Air box lid still on? What jetting set up have you got? You may need to re-jet?
Is the bike in otherwise good tune? Have you checked the ignition timing? If points, are they clean/not pitted and the dwell set? Good spark? Valve clearances been checked, etc?
It's best to tell us what specific throttle position a problem shows up at instead of rpm. Carburetion is more about throttle position than rpm. Plug reads at minimal, 1/3, and full throttle positions would help too.

[HammerHead]

You can adjust the pilot screws while the carbs are on the bike. I use a mirror and a screw driver that I rigged up that fits. It takes a little getting used to, but it can be done.

When your checking your plugs its probably best to do a plug chop. Run the bike at the RPM where you're having the problems and hit the kill switch, then check the plugs. Don't drive it around at lower rpm's and test the plugs, that won't help you too much.

You're probably right that the bike was synched to leaky intake manifolds. If its as bad as you say that could be the whole problem right there. It is odd that the bike ran so well with the leaks. Hopefully it wasn't re-jetted as well?

Most people here use the MOTION PRO carb synchronizer. I have some vacuum guages that I use and feel comfortable with.

Mark
Keith

I pulled the carbs off tore them completely down and cleaned them out.

#4 pilot fuel screw was set at 2 1/2 turns the rest were between 1 and 1 1/4.
I also got grit out of the #3 and #4 fuel screw port.
I bench synced them to GS'er specifications, set #4 at the approximate adjustments as the others and it runs Great!!!
I still need to get a sync tool, and as Keith suggested, do a plug test at different trottle levels to determine burn color.

Thank you guys for your help, appreciate.
*Steve*

[mark]

Its great to have success once in a while.

When you do your carb synch, make sure you have a couple of fans available to keep your engine cool. Also, remember to plug your vacuum line to the #3 carb and rig up some way to get gas to your carbs. I just bought a long rubber hose so I can set my gas tank on a work bench and run a gas line to the carbs, other use a little plastic gas container that they rig up.

Glad its lookin' up.