1979 GS750 E rejetting issues

[thumper1]

Hello to all. I'm a newbie to this forum and only found one thread that touched on my problem. I accidently posted this thread in the wrong place earlier, so I'm sorry about that. Hope someone can help. I have a 1979 GS750E with about 15k. When I bought it, it had stock airbox with 4-1 headers and a blown-out D&D muffler. It ran fair, but loud and weak mid-range. I replaced just the muffler with an Emgo short tapered muffler which looks and sound great. I can't get it to run right. Low end is great! It starts to weaken around 4000 rpm and will hesitate and sputter above that. It has the VM 26 carbs. I tinkered with the top air and lower fuel screws but never got it to run better. I now have the lower fuel screws set at 1 turn out, and the top air screws set at 1 1/4 out. I moved the needle both up and down and only made things worse. The needle clip is now at the next to bottom slot which is one richer than stock I believe. This is about the best I can get it to run.
I pulled the carbs and cleaned them thoroughly. They were not very dirty. I noticed that the main jets are #120. Pilots are the stock, #15. I did some research and it looks like the stock mains were #102.5. I think that these mains may be too big, but need some direction before I start buying jets. Again, it has the stock airbox. Any tips would be greatly appreciated. Thanks

 

[49er]

Welcome to the GSR.

Your main jets are way too big. Even with a 4-1 pipe, you need to be in the 102.5 to 107.5 region, assuming you are still tuning with the stock or a slightly modified airbox. With your fuel screws at 1 turn out, the airs should be around 1+3/4 to 2 turns out.

The trashed muffler would have caused poor performance too. These engine's need some back pressure.

Have you checked the condition of your air filter and all the seals that need to be air tight? There are many places in the intake area that can bleed air directly into the engine, causing leanness and poor throttle response.

But, check the colour of your spark plugs. With 120 mains, I'm picking they will be rather black with carbon.

There are many other regular maintenance issues that need to be addressed too, but others will detail them.