Topic: Ignition Timing vs. Carb Tuning

[pbr23n]

I have been struggling to get my (non-sohc) xl350 tuned right.  The plug reading looks spot on but there's a huge flat spot at the lower RPM range.  The bike idles great, runs spot on at 1/4th throttle but, has a flat spot where the bike boggs down from 1/4 to 1/2 throttle, then runs perfect.  I have adjusted pilot jets, mains, needles, clip position, and float height and can't seem to get rid of the flat spot.

Could this be a timing issue and not carb related? 

[flybox1]

They can easily behave like one another.
Ive always made sure timing and full advance was done first. 
This can be done no matter how bad the jetting is.  The bike just as to be running.
Once this is complete, then dig into the carbs.

[Deltarider]

Some set the idle by leaning out the mixture to achieve the highest RPM. Although this can be good practice for many motors, other (older) bikes that lack an acceleration jet in their carbs, need a pretty rich idle to have a good driveability in the range where you have your issue. Not saying this is true for your bike (I don't know what type of carb it has) but I know quite a few bikes that need a rich idle to pick up well. These bikes were marketed before the US became hard on emissions at idle.

[HondaMan]

I have been struggling to get my (non-sohc) xl350 tuned right.  The plug reading looks spot on but there's a huge flat spot at the lower RPM range.  The bike idles great, runs spot on at 1/4th throttle but, has a flat spot where the bike boggs down from 1/4 to 1/2 throttle, then runs perfect.  I have adjusted pilot jets, mains, needles, clip position, and float height and can't seem to get rid of the flat spot.

Could this be a timing issue and not carb related?

Do you happen to know...what the piston-bore clearance is? Reason I ask: 4 years ago one of those bikes came to me because after the rebuild he had done with "parts from a California performance shop", it acted just like yours, and was also hard (almost impossible) to start. I discovered almost 0.005" piston clearance! It came as a (oversize piston + cylinder) kit from a 'specialty shop' in California who sends out pre-bored cylinders with pistons. I contacted them and got another set, after specifying 0.0012" piston clearance: he sent me another set with nearly 0.0060" instead. I called him up and REAMED him on the phone, explaining my background with these bikes in some detail: he had heard of me. He sheepishly admitted he wasn't watching the guy who bores his stuff (in his own shop!) that closely, and searched the USA and Canada until he found me a new cylinder and match-bored it to my required spec. After reassembly, the bike started on the first kick and pulls the front wheel up in 3rd gear: no other changes were done.