Topic: CB500 hestation, 3800 rpm, 1/8 throttle

[rtbmrgl]

Hello, My 1973 CB500 I just restored runs great except for a minor flat spot at about 3800 RPM with 1/8 throttle. I rebuilt carbs, all jets are stock, slow jet #40, main #100, needle in the middle with the stock air filter and housing. The only modification was done to the exhaust,(got it that way).

I was told from a long time Honda tech that making the exhaust opening larger will possible effect the performance, so I plan on fabricating some sort of washer to cover up the drilled holes and leave the center opening. I thought about making a temporary fix like a soup can with a hole drilled in the center and see if it made a difference. I want to eliminate this before I start messing with jetting.
Maybe a beer can would be better, well at least for me.
I posting a pic to see what the muffler looks like.

I would appreciate opinions and suggestions.

Mark

[TwoTired]

First check the pilot screw setting, number of turns out and equal among them. 

How does it recover from "flat spot", burbling, or sudden clean power?  If the latter, then try turning in the pilot screws 1/8 turn at a time a drive for effect.

I will ask if the emulsion tubes are known to be clean.  And have the orings where the intake meets the read been renewed.

It would also help if we knew the that the head pipes were all evenly hot.  And, what sort of spark plug deposits appear on the insulator porcelain.

Finally, have the carb been checked for vacuum synchronization, and when was the last 3000 mile tune up check list performed?

[rtbmrgl]

Two Tired, thanks for getting back,
I have been turning the pilot screws in increments, no change. The carbs have been sync. The intake o-rings and boots are new,  I have an inferred temp sensor and they all seem to be even, Ill check again. Spark plugs are all a medium gray, as soon as you accelerate passed the the flat spot, which is about 4K, the engine smooths out and no more flat spot. The plugs are new, timing ok, valves adjusted. To me it feels like one step larger pilot jet or raising the needle on step (lower the clip).
 I've owned bikes all my life but I'm not familiar with quirks on the CB500, if any.
One thing I didn't mention is that in the cool 55deg AM the engine has less of a flat spot, if any. When heading home from work, about 85-95deg the flat spot is more noticeable. Which tells me there is a lean condition.
If fixing the exhaust doesn't work I think I will raise the needle and see how that works. I don't like the way the muffler was hacked, but it does sound good. 

[rtbmrgl]

Update, I installed washers to make mufflers back to stock, Hesitation is now gone. I silicone them in for now to test, I will tack weld in later.