5000rpm rough spot

[The1970's]

I got my 1978 gs750 a few months ago and it has always seemed to run good. The plugs however have always looked black like it's running rich and I've always gotten bad gas mileage. I took the 120 plastic jets out and put in 117.5 to see how it would run. That created a rough spot at about 5000 rpm. Runs good above and below. To try to fix it I lowered the clip on the needles. They are now on the lowest setting. Any ideas what to check next? I did check the timing. Cylinders 2-3 are a little bit off but I can't adjust it due to my points being worn. I have already ordered new ones just waiting on them to get here.

[chuck hahn]

OK...so you have screws on the sides of the carb bodies that stick out at around a 45 degree angle and screws on the bottom of the barbs at the back of the bowls. Set the bottom ones at 3/4 to 7/8 out from lightly seated and walk away from them..period.

You fine tune the carbs with the side MIXTURE screws. Some call them air screws because they regulate the air let in to the fuel to air mixture. Anyways, turning the side screws OUT will lean a cylinder and IN will richen a cylinder.

Whats the airbox and exhaust setup??? stock?? pods?? 4 into 1 pipe?? A few more details will help in suggestions on jets you should have.

[The1970's]

I forgot to mention it has an aftermarket 4 into 1 but I am unsure of the make. The airbox is stock with a uni replacement filter. I wouldn't think the mixture screws would effect anything other than the idle and low RPMs correct? I haven't touched the mixture screws since I got it. I've been waiting till I got a sync tool to sync the carbs first. Also the pilot jet is a 15 and I think I remember a number 5 on the needle but could be wrong that's just from memory.

[chuck hahn]

The bottom pilot screws affect idle to about 2,000 RPMs. I have had several 77 and 78s and find the bottom setting i provided to be as about spot on as the factory set them. Once those are set you NEVER fiddle with them again. All your tuning is done via the mixture screws and jetting after that. Your problem is well above the range of the pilot circuit and is happen ing on the main circuit..or in other words the main jet.

100 is the stock size and you said youre at 117.5. That may be a tad too hot. BUT BUT you dont have them vacuum synced yet either, so do that first off. secondly, set then side mixture screws at 1 1/2 out from seated and start from there adjusting fuel to air ratios. I have a tool called a Colortune thats the cats azzz on VM carbs. What it does is allows you to see the flame color in nthe cylinder with the bike running. I would be willing to loan it out if you would like to use it. PM me your address and i will PM you my phone number.

[The1970's]

100 is the stock size and you said youre at 117.5. That may be a tad too hot. BUT BUT you dont have them vacuum synced yet either

But what I don't really understand is how it ran great with the 120's and I only had the problem when I went back to the 117.5's

[chuck hahn]

Also, youll need to do the valves adjustment to be sure your valves are within specs and not being held open a tad from a tight shim we have a shim club and I have a bunch myself so chance are you wont need to buy any shims. either swap them with Ray at the shim club..thread is in the services section..or I can send you some if I have what you need. Store this place and select the early 8 valve service manual from the menu.

http://members.dslextreme.com/users/bikecliff/

[chuck hahn]

Maybe the 117.5s are too small. I sent you my number..give me a call.


EDIT...I posted and hadnt seen you once had 120 in there,,,my bad