At a Loss with my Carbs

[Da-Bubble]

Ok tonight I finally synchronized my carbs...I think. I made the vacuum gage tool found in tech tips. Is there a set amount of vacuum they should pull. When I started #1 & #2 were at 6psi when synched and #3 & #4 were at 10psi. When I synched 2 to 3 they ended up about 7psi across all 4 carbs.

Ok here's what I know. When the bike was running, the number 3 cylinder had a nice tan spark plug. It's float was set to 24.35mm (I believe) so I set them all to match. I have stock 137.5 mains and 42.5 pilots all Mikunis. Idle screws are set to 1.5 turns from seat. I pulled the plugs after synching and riding for 5 minutes and the #4 plug was soot black. All others appear clean. I checked for spark by pulling the plugs and watching each one spark and they do. I have a low speed sputter/miss. It idles smooth but when revving in my garage you can tell it's missing. I hoped that synching would remedy this but it did not. I guess it's not completely low but probably low and mid. Even at 5,000 rpms in 3rd gear, it's popping and raspy.

What do I do now. Please help! I want to ride so bad but I can't figure out the carbs!

[81xsproject]

I recommend having a look at your idle mixture screws. The 70's are more prone to having the screws smashed from being seated too hard. They should all have a nice even taper. If they have been smashed it creates a lot of inconsistency from on carb to the next to where setting them all at 1.5 turns doesn't mean they are all getting the same mixture.

[fredintoon]

Hi Scott,
my sympathies, niggling little problems are the worst to diagnose. At least if (say) a rod is punched out through the crankcase you KNOW what's wrong.
How did you measure the 7 & 10 psi of negative pressure? My understanding of the homebuilt vacuum gauges is that they are water-filled U-tubes. To measure 10 psi on a water gauge you'd need to see an ~20 foot water column? Are you sure you are reading psi and not inches of water?
You have also set the float heights to be equal at 23.45mm (and how you measure the height to that degree of accuracy is another puzzle) But the float set height is one thing while the actual fuel cut-off height can be quite another. From what you describe, #4 plug being all sooty and the rest not, I'd suppose #4 float needle wasn't sealing and thus allowing #4 carb to run rich.

[mro]

Float height stock...25.7 mm +- 1mm
You have a 4/1 exhaust and stock air box with stock jetting???

After your synk, adjust the pilot screws.
Can be done by "ear" (1 1/2 turns out is the starting point)
Think there be a good discription in tech tips.

Possible higher fuel height in bowl is only partially makeing up for needing different jet (5000 RPM stumble)



mro

[AlanB]

Hi Da-bubble,

If you think that your settings are close, get it out of the garage and RIDE IT!..... If you want to turn a beautiful, smooth running engine into a stumbling, misfiring mess, the best method is to park it in the garage and let it idle for a while. A spot of messing about with the carbs and some revving against no load will complete the transformation.

Fortunately, a bit of a blast on the road will reverse the process, and give you some useful information on how it is ACTUALLY running, and what you need to do next.

most of us will have experience of how much better ANY engine behaves after a good long run, compared with driving round the city for a while; the effect of running while sitting still in the garage is much more obvious. It will need a good "clear out" to really show the effects of your changes.

It also helps motivation enormously if you can take it out!

AlanB