Topic: idle jet adjustment

[mechanic3]

Hi guys. Finally got back at my 1975 cb750f carbs for the second time.(idle problems) Carbs were in real bad shape when I got it and I cleaned them once with no new parts just to see if it ran. Had been sitting for more than 17 years. It ran but poor idle. Did all the usual things ie. plugs, set points, set valves, adjust timing chain etc. Off idle is very good but idle jets screwed in or out makes no difference. Purchased carb. kits and installed needle and seats and bowl gaskets. Also adjusted floats. Double checked all jets and ports and they all seem clean. Still no adjustment on idle jets. Also did a comp. test. 160psi on all but one and it was 150psi. What am I missing?

[pampadori]

what size are the pilot jets?  I'd check for vacuum leaks near the flanges using some carb cleaner or starter fluid. Also check that on a cold idle, all 4 pipes are warming at the same rate.  Maybe you have 1 cylinder that isn't in sync and is staying closed all the way instead of barely cracked to maintain idle. 

You didn't mention it, did you sync them?  At least bench sync?

[mechanic3]

Thanks for your reply pampadori. I just tried brake cleaner around carb hangers (rubbers) and no change in idle. Can screw jets right in and idle changes a bit but kept running. I did not sync carbs but did not take apart assembly. Have no vacuum gauges.

[mechanic3]

forgot to mention jet size in #40 and eyeball gauged sliders as open the same amount. Not very accurate I know.

[pampadori]

you sure the bowls are filling up with fuel when you have the petcock on?  I've had a float needle stick on one cylinder and leave it bone dry while the others were receiving their fuel.  Its easy to check for, pop the bowl retainer off and see if its empty or not. 
Just a thought.

[flybox1]

Hi guys. Finally got back at my 1975 cb750f carbs for the second time.(idle problems) Carbs were in real bad shape when I got it and I cleaned them once with no new parts just to see if it ran. Had been sitting for more than 17 years. It ran but poor idle. Did all the usual things ie. plugs, set points, set valves, adjust timing chain etc. Off idle is very good but idle jets screwed in or out makes no difference. Adjusting these effects idle mixture....can you hear mixture change?  Purchased carb. kits and installed needle and seats go back to stock pls....aftermarket stuff s&cks and bowl gaskets. Also adjusted floats. Double checked all jets and ports and they all seem clean. Still no adjustment on idle jets. Also did a comp. test. 160psi on all but one and it was 150psi. What am I missing?

you arent missing anything....you just need to move to the next steps 
if idle sucks, and all other tuning items are as they should be (cam chain, valves, timing), you then need verify proper fuel level in the bowls (clear tube test).  If you adjusted your floats, you need to verify they are correct.
Once checked...proceed with a vacuum sync of the carbs.
Finish it with an idle plug chop with new plugs, to help you dial in your idle mixture on each carb (with your mixture screws)
Your plug color (light tan) shows you your idle mixture is correct for each carb.
It should purr at this point.

[mechanic3]

Thanks flybox1. I am no familiar with the clear tube test. Am new to cycle repair. My background is in automotive. Been a tech for 41 years. Seems to be rich, plugs black and does not flair up when propane is added it just dies. Will it run good enough with stock breather box off to do more  diag. on each carb? Have no tried this, ran out of time.

[mechanic3]

Don't know when I will get back at it due to work. I run my own automotive shop. I thank you guys for helping walk me through this problem.

[flybox1]

The clear tube test is pretty simple.  It allows you to see the fuel level inside the bowls.   Just adjust your float height to get the fuel level correct.
You can make an adapter to insert in the drain screw hole.  I just use a pvc fuel line connector, and twist it in.
Best to test all bowls while the carbs are off the bike so adjustments are easy.  Your goal is 3mm below the bowl gasket seam.
read here:

[mechanic3]

Nice pics. flybox1. Will take another crack at it when time allows. Thanks