Normally float level only effects high RPM.

 

Interesting. I have the float level at about 7mm (1mm above recommendation). 108 main jet. Adjustable needles that taper to <1mm at the tip (stock is closer to 2mm). These needles were originally in the bike with 112 main jets. The needles are set to the second highest position with one washer (similar to how they were set on the original carbs, can't remember if the washer was in there or not). Pilot is stock 35, Pilot screws are two turns out. Haven't played with them any further as the bike idles pretty good. No hesitation, no fluttering at mid RPM ranges and very hard acceleration over 5k. So all in all, it feels pretty good. I have done the stick coil mod which has definitely eliminated some idle surging. Not exactly sure what it has done in cruising and high rpm situations (hard to judge as so many things were changing on this bike getting it running good). I also have the MOSFET regulator modification that was recommended and it shows a steady 13.5-14v even at idle. Before I raised the float level I had the needles at the MAX out position which helped the bike cruise better but still had some hesitation. Once I raised the float level, the bike was obviously too rich as it would flutter at > 1/2 throttle below 4.5k. But lowering the needle again fixed that and now it is smooth across the RPM range (well up to 6-7k anyway, haven't done much above that). So all evidence leads me to believe that the hesitation was fuel starvation.

 

But you could be compensating for small or misadjusted pilot. The overlap is difficult to tune. Increased fuel at needle lift by moving the needle position could compensate for not enough fuel coming from the pilot. Idling isn’t the only thing the pilot circuit does. It provides fuel up to about 1/8 throttle when the needle begins to lift when vacuum increases and moves the diaphragms. Depending on how fast the throttle opens the overlap can vary due to response.

if the float bowl is only half full it won’t create a problem until you reach the RPM that the incoming fuel can’t keep up which would be much higher than what you have been discussing. It won’t effect running until that point. The fuel/air ratio is controlled by the orifice sizes of the jets and in the case of the pilot in conjunction with the pilot screw position. Have you ever run a carb dry when you put a motorcycle into storage. They run fine until the carb runs out of fuel. The float adjustment should never increase fuel delivery to the engine under normal conditions...it just allows fuel to be there to be fed thru the jets to the engine. Fuel starvation occurs at high rpm when the float(s) are out of adjustment. The RPM you have been discussing would never see fuel starvation due to float.

 

I get your point. This article doesn't quite agree with that however as it does discuss adjusting the float level to get good low RPM performance without stumbling. My experience seems to support this.
https://cbrforum.com/forum/stickies-...-guide-101969/
I think the needle height adjustments were compensating for low float level, as it was low, I measured it at 10mm and over. I have played a lot with mixture screw adjustments which should effectively change the pilot jet's function and this had zero effect on the the stumbling problem. Actually, leaner seemed to be best; but it was inconclusive. I do still get a small miss sputter just accelerating out of 1st gear, perhaps pilot jet or mixture screw would help with that. The carb is back to being very close to where the original carb was with respect to needle height and float level. But the original carbs were impossible to get right. They acted like there was a vacuum leak even though I did everything to make sure that was not the case. All the jets, floats, needles from those carbs in this other set was what got me on he right track.

 

By the way, here is the contradictory statement to my situation. "Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm". This is the behavior I had, but making the adjustment I made is opposite of what it recommends. I can't say it showed signs of richness, I can only say it started missing as it warmed up and was still good at 4k and higher.

 

Not sure that sticky has anything to do with this. It’s about valve adjustment.

As I have followed this thread the one thing you didn’t do was change the pilots. I also didn’t see where you adjusted pilot screws per manual when the engine is running and hot. It’s hit or miss when they are adjusted static. It’s also the best way to determine if pilots are sized correctly.

 

Sorry. Here is the carb adjustment page. CV Carb Tuning Procedures
You are correct, I did not change the pilot jets. I did the mixture screw adjustment when the engine was struggling. I have not done it again since it has been running good. But if my float level, etc is really wrong now, not sure how this will help. Pilot jet adjustment is the last step in the tuning guide after you have the float level, needle etc. correct. I may experiment more later, but since it is running good with what is pretty much the original carb settings, I will probably leave it alone. I was just looking to understand the bike running good for about five minutes and then struggling and that being fixed by float bowl level increase. Seems backwards if that really would make it richer. Seems odd if the float level really shouldn't fix low RPM carb performance, such as lower speed cruising (30-40mph) at ~3000 rpm. Because this change was definitely helpful; even if it has overcome a problem with a pilot jet being too small.

 

Its sometimes difficult to determine what actually improved a situation, particularly when more than one change is made at the same time. Float level IMO is not likely to effect low rpm and transition running unless it is extremely out of adjustment. The fuel picks up from the bottom of the bowl. Engine vacuum brings the fuel from the bowl thru the jets and into the engine. The volume of fuel is controlled by the size of the jet orifices. There is no “pressure” pushing the fuel from the bowl. The float level won’t provide more or less fuel to the carb, it only insures that there is fuel available. Low float setting will effect high RPM
when the bowl runs dry (fuel starvation).

I know you feel that the float adjustment had a positive effect on the stumble, but IMO that is not likely. One of the other changes likely helped to improve it. It’s also possible that the pilot jets still had some gum in them and running it cleared them out. On these carbs the pilot circuit cleanliness and adjustment is critical to get smooth idle and throttle up to transition. More so than most people think until they work with these carbs for a while.

 

Ok. I made one adjustment at a time and moving the floats was where the biggest improvement in this particular problem was seen. Then adjusting the original floats to this same level was done and the problem was still fixed. This broke full throttle at 2500-4000 rpm which was then fixed by raising the needle.