Topic: Jetting thread #3,648....

[Scott S]

 Working on a set of 087A carbs on a hopped up CB550 engine.

 These carbs were on an engine that always ran rich and started fouling #3 plug. I rebuilt that engine out to 563cc using Yoshimura high compression pistons. It has a CB650 cam, mildly ported and polished head, match ported manifolds, and a Kerker 4-1 (the good, old school kind!).
 I'm using the stock plenum with a K&N filter on the back in place of the stock filter box.

 I just got it home and have yet to ride it myself (crappy, cold weather down here). The shop reports that it needs some carb tuning. He thinks the emulsion tubes are "hour glass shaped" and says it's both rich and lean in spots.

 I have ENORMOUS success on my other 550K with replacing the emulsion tubes and needles. This completely cleared up a rich condition on an otherwise stock bike and made it perform better all the way around. Taking that into account, and thinking back to how this engine used to run, I tend to agree that the emulsion tubes are a big part of the problem.
 I now have four N.O.S. emulsion jet sets; tubes, needles and clips.

 I've jetted for a nearly identical engine before, but without the high compression and using the later PD series carbs. I was surprised to find I ended up very close to stock settings with a mild bump on the main jet.
 My unknown area here is the increase compression. This engine is running about 10.5:1 CR.

 What I have now:
 -needles in stock clip setting (to be replaced with NOS)
 -110 mains
 -38 pilots (but he has the air screws out 2.5 turns....too much, IMO)

 I have some 95, 98, 100, 105 and 115 mains.
 I have some 40 pilots.

 When I pull the carbs to replace the emulsion tubes, would you change up  the jetting? I'm thinking about actually going DOWN on the mains. I kinda wish I had a 107.5.
 Are there other pilot jet options beside the 38 and 40?

[flybox1]

Checked your bowl fuel levels(clear tube)? 

Do your e-tubes, first.  No other jet changes, and retest
Do you seen any oblong wear spots on your needle jets that might be allowing unwanted fuel to pass?

With 2.5 turns out on the air screw, you're close to or past the effective range to add more air
If the e-tubes dont tune it out, IIWM, id go down to #36 pilots and in 1-1.5 air screw turns.

[Scott S]

 Yeah, I was surprised at what a HUGE difference the emulsion tubes and needles made on the other bike.
 And looking back, this bike was exhibiting the same symptoms. That's why I pulled the engine and rebuilt it. The tubes and needles are definitely getting replaced.

[jonda500]

The pilot jets are available in #35's too.

Fitting correct 2nd hand 550F emulsion tubes fixed mine after literally months of trying to get it to work right. It went from only running well if I always kept accelerating (and then backing off - tedious!) to running great in all situations except for just above idle at very low revs with very little throttle after idling for a little bit... I have #35 pilots - I wish even smaller ones were available!

I also am curious about the effect having high or low compression has on the engines jetting requirements as mine is rather tired with low compression. (Definitely far from crisp performance but still pulls 150kph ok!) Will I need to re-jet when I restore the compression?

Best of luck Scott, I wish I had something more helpful to add,
John

[Scott S]

 Bumping this one up. Does compression change jetting results, all else being equal?