Topic: Carb Air Screw Settings at Elevation

[cshanek]

Hello SOHCers,

I recently rebuilt my carbs due to a fuel leak that occurred when I woke my 750k6 up from her winter slumber.  I went through the syncing/tuning procedure outlined in the shop manual, and was able to bench sync (and fully sync) my carbs pretty well, though for some reason I could only get my vacuum gauges up to about 16-17 cmHG (really consistent across all 4 carbs). I fiddled slightly with the Air Screws and throttle stop screw but that was about as high as I could get. I live at around 6700 feet (bike is properly jetted for this altitude), so I wrote that off to elevation. I finally rode her into work last week and she was SEVERELY starved for power. I felt like I needed to rev her to about 7000 RPMs at a stop light just to get her to move forward and not stall. The shop manual mentioned that 1 full turn back from being seated is a good starting point.

Does anyone have any experience/recommendations with Keihin air screw settings at a similar elevation? Should I continued to back them out uniformly and test? The bike seems fine and peppy in the shop, but the second I get on it and take it around the neighborhood it's really obvious she isn't at 100% power.

-Shane

[PeWe]

Stock airbox with stock type filter and stock HM341 4-4 exhaust pipes?

If changed to something better flowing affect jetting. Need more fuel.

[RAFster122s]

HondaMan lives in Denver, he can advise you on what he sees working at your kind of altitude...
He wrote some info in the FAQ about tuning for altitude. Thoughts of HondaMan thread

[newday777]

Shane
What process exactly did you use to clean your carbs?
Did you get rebuild kits? Did you put the brass from the kits in your carbs or clean the original brass parts?

[HondaMan]

I felt like I needed to rev her to about 7000 RPMs at a stop light just to get her to move forward and not stall. The shop manual mentioned that 1 full turn back from being seated is a good starting point.

Does anyone have any experience/recommendations with Keihin air screw settings at a similar elevation? Should I continued to back them out uniformly and test? The bike seems fine and peppy in the shop, but the second I get on it and take it around the neighborhood it's really obvious she isn't at 100% power.

Like Newday says: did you replace any of the jets? If you used jets from the kits we get today, it is running very lean. The typical kit-jet is 7% lean for its number as compared to Keihin numbers. If your OEM mainjet was #105 (that is normal 750K6 size) and you installed a kit #105 then it is running with less than #100 mainjet size, so it will feel quite weak. Likewise with the idle jets: the kit jets act like size #37 despite the "40" stamped on them.

Here's the part that affects it all: if you used new float valves from anyone (Keihin or kits) they have much stronger springs in them. This causes lower float bowl levels by about 2mm overall when running. The OEM springs were much softer. So, set the floats to 24-25mm (stagger them, with the 'deeper' one toward the bike's kickstand side - left side) in the K6 carbs, measuring WITHOUT the floatbowl gasket in place, to the seal's surface.

At 6700 feet, use 7/8 turn for the idle mix screw and #105 (Keihin) for the mainjet.

[cshanek]

PeWe - K&N air pods, and MAC sidewinder exhaust (https://www.denniskirk.com/mac/performance-sidewinder-exhaust-system-901-1401.p4411031.prd/4411031.sku). I know the answers to those questions will help provide context to my air screw question, but she was happy and peppy with that setup last summer.

Newday777, I completely disassembled the carbs, put them in an ultrasonic, used carb cleaning wire tools, and sprayed a bunch of carb cleaner in each port multiple times (and back into an eye once).

HondaMan, I bought the following kit (https://www.ebay.com/itm/304794487466) mostly for the floats/gaskets. I recall being VERY surprised at how much I had to adjust the floats (using a float caliper) and thinking that they seem to sit so much higher (while adjusting), than my old floats. That kit comes with a #40 for pilot and #110 for main jet, but I *think* I reused my old, properly jetted pilot and main jet. As stupid as it sounds I need to head out to the garage to look at my carb kit to make sure I didn't swap them out. That would def explain some things and could be a candidate for head-slap of the year. 

[Jerry Rxman Griffin aka MuthaF'er]

I thought I'd seen that bike running around Briargate before  You may be close. Lexington & Dynamic. I'm a little above 6800'.

I don't have a stock engine or stock carbs on my 2 1975 CB750F's so I can't provide any useful info.

Jerry
719-963-3080