Carb Tuning

Spub

Spub

Hero member
Location
Pasadena, California, USA (Left Coast)
My SFC (PHB carbs, K2 needle 3rd notch down from top, 175 mains, 52 pilots, stock two into two exhaust and X over) runs fine once warmed up with 52 pilots. Has nice crisp idle sound, and appears to be running perfectly. Plugs a nice tan color. Trouble is, getting it to being warmed up. It is a hard starter and a rough runner, not idling well until warm. Going to 55s takes care of this, making it much easier to start, and a much smoother runner until warm, however once warm it sound much less crisp, and smells a bit rich. Tried 54s and 53s and playing with the mixture screw, but they do not run like the 52s when bike is warm. I was thinking of going to a richer choke jet with the 52s to act as a bridge to get me to warmed up. Has anyone tried this? 
 
Spub,

Choke jet will only act when choke is engaged... big fat jet there anyway!

What plugs are you running?  Cold plugs definitely take longer to warm up.  Fit a couple of 7s or 8s (Champion 3 or 4) to see if they improve starting matters.  Don't go riding on them though, could end up ugly! :o

Which cam is fitted?  C6 doesn't really like idling, mates' SFC idled quite nicely........ at around 1500rpm! :D

piet
 
Spub, Have you got the Dellorto tuning manual? If not drop me a PM and I'll send you a copy, it's got some very useful set-up tips .
 
Piet, Bob:  C5 cams, running Champion N2s. Not looking to use the choke to tune, just looking for a "bridge" to get me to warm, since the 52s run best past this point. If this means leaving choke on for 5 or 6 blocks, this would be preferable to running hotter plugs, since bike runs perfectly without them once warmed up. Bob: do have the tuning manual, and will go through it once again, but thanks for offer.
 
Not being at home I had a look on Andy Wagners catalogue which specifies 70 for the choke jet.
I know that my SFCs carbs are by the book so that must be it. K1 needle, middle notch. My bike runs on the 2/1 so 180 mains. SFCs don't like the 2/2 as much but I seem to remember 170 mains with that exhaust. As for pilot, either 50 or 55, can't remember. They are very sensitive to pilot jets and the slightest bit of crud in there will prevent the bike starting and running at low revs. It's nice to know that it can tick over, but I hate to imagine those lumpy cams with just a hint of oil.

Ciao
Paul
 
Spub,

are you pulling the choke completely? Maybe that is to much... I found out that when I ran the 1000 SFC's still on choke, mostly it is enough to pull the cable just 2/3rd. I even started them without choke engaged and slowly pulled the choke until it fired up (that is how I found out 2/3rd was enough for my bikes...). Set the choke sliders to the same hight first and make sure they are fully open when you fully pull the choke lever. You can see if that's the case from behind the carbs, when looking into the intake and slowly pulling the cable...

If that does not improve it, think of different solutions such as: always close pet-cocks roughly 1/2 mile before you get home. this way the float chambers are mostly empty and when you start the bike again, fresh fuel gets into them. Has done it for me the past 15 years since I got that hint and never had starting issues ever since, even not after longer time (such as in spring after 3- 4 months of not using the bike).

They all run a bit rough when cold, but normally I can put the choke off after roughly 1 - 2 miles.

HTH
 
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