Topic: CB750 rejetting pilot jets? (linked roundtops)

[Garage_guy_chris]

Hey guys. So on tuesday I got the new engine back up and running for the first time. It started right up and ran on the first crank. Initially it didn't run the best and hunted around and wouldn't settle into an idle. I went to time the ignition and noticed the timing was jumping around badly. You could see the advancer trying to advance at 1000 rpm. I remedied this by trueing the advancer shaft as it was out by 0.012" i got it to 0.003" and i also installed a tighter advancer. After this i was able to time the ignition and get good stable idle base timing out of it. 

The carbs are the linked roundtops but are odd "064A" versions. They started out in very good condition  and got fully cleaned ,rebuilt, floats adjusted, and bench sync'd. After getting the ignition setup i vacuum sync'd the carbs (bench sync was pretty close) and started playing with the air screws. I tried them anywhere from full closed to 2 turns open and didn't see any real difference in the running condition still hunting around and running a little funky.

Currently the pilots are #40 genuine Keihin originals the needles are in the 4th position from the top and currently #128 mains which will be re-assessed after i ride it for the first time tomorrow. But Im just wondering if I should be considering a change to #38 or #42 pilots? It doesn't seem like people generally need to change the pilots?   The engine details are as follows

-Ebay cruzin image 836cc 65mm kit
-Lightly ported and de-shrouded head
-F2 cam
-Hindle 4-2-1 exhaust
-Original unmodified airbox (will be opening up the openings)

[seanbarney41]

I would totally be looking for carb boot leaks because your engine combo does not sound like anything that should need a change of idle jets.

[Garage_guy_chris]

I bought new Cruzin Image boots. the boots look good and fit fine. but the clamps are soft metal and kinda underwhelming I have already been looking around for something better.  I did poke around the boots with a un-lit propane torch but didnt notice any major difference. Sticking it in the air box opening definitely affected the running.

while idling i do see the carb bank pulsing in and out and can feel the boots shrinking with the suction of the intake pulses ?!?!!? 

Im not too worried about it yet id like to get it out on the road tomorrow (monday)  and ride it a little let the engine break in a little and play with all the jetting as i ride.

[Henning]

while idling i do see the carb bank pulsing in and out and can feel the boots shrinking with the suction of the intake pulses ?!?!!?

WTF? That doesn't sound right or good, it may be throwing something out of kilter, so I would seriously think about getting the Honda boots. Never heard about anyone needing to move away from the stock #40 idle jets, so I would stick with those.

I will take a stab at it being too rich at idle, so I'd set the needle to the middle position and put in some 115 or 120 main jets. I know these are more for mid and top end mixture, but I'm pretty sure they have a certain effect on the idle mix as well. And maybe a better starting point for later tuning? Idle mixture screw 1 to 1 1/2 out, shouldn't need to be more than that. I'm at 1 1/2 but not the same setup.

[evanphi]

I would totally be looking for carb boot leaks because your engine combo does not sound like anything that should need a change of idle jets.

Agreed!

Pulsing is normal. Mine do that.

You want new clamps! Get these: http://www.amazon.com/gp/product/B0036R4UDC?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00

[70CB750]

I would totally be looking for carb boot leaks because your engine combo does not sound like anything that should need a change of idle jets.

You want new clamps! Get these: http://www.amazon.com/gp/product/B0036R4UDC?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00

Those are great, used them myself.