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Author Topic: 74 CB750 - Can't figure out high idle  (Read 9080 times)

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Offline flybox1

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Re: 74 CB750 - Can't figure out high idle
« Reply #25 on: May 24, 2017, 10:54:37 AM »
^^Yes, the screws are there so you can individually tune them for ideal air/fuel mixture.
But best to wait till you get those new boots installed, or you'll have to plug chop again...
« Last Edit: May 24, 2017, 10:57:10 AM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
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Offline zerodaydave

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Re: 74 CB750 - Can't figure out high idle
« Reply #26 on: May 26, 2017, 07:51:44 AM »
Just pulled apart the carbs to check float bowls again (26mm) and jet sizes #102 Main #40 Slow. I think stock for these carbs 064A is #105. I normally ride at 5,000ft altitude. The bike was running rich (fuel in the intake boots as well as black plugs) but I'm thinking it could have been the timing + air screws all the way open + vacuum leak. All of these have been fixed. The timing is now absolutely dead on. The bike is idling smooth at 1k. Before I sync the carbs one last time my question is about jetting. Should I bring the main back to stock even though the bike was running rich? Should I leave the idle alone. Bike is running pods and 4 to 1 as well. Thanks guys.
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Offline HondaMan

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Re: 74 CB750 - Can't figure out high idle
« Reply #27 on: May 27, 2017, 08:07:15 PM »
Quote from: zerodaydave on May 26, 2017, 07:51:44 AM
Just pulled apart the carbs to check float bowls again (26mm) and jet sizes #102 Main #40 Slow. I think stock for these carbs 064A is #105. I normally ride at 5,000ft altitude. The bike was running rich (fuel in the intake boots as well as black plugs) but I'm thinking it could have been the timing + air screws all the way open + vacuum leak. All of these have been fixed. The timing is now absolutely dead on. The bike is idling smooth at 1k. Before I sync the carbs one last time my question is about jetting. Should I bring the main back to stock even though the bike was running rich? Should I leave the idle alone. Bike is running pods and 4 to 1 as well. Thanks guys.

I recommend the #105 mainjet for up to 10,000 feet, but turn the air screw slightly inward as you go up, if for extended time (like a camping week trip?). For pods: tape off all but the last 1" of opening at the back to try to improve the airflow across the mainjets' emulsifiers and help pump the fuel up the throats more evenly between 1000-3000 RPM. Otherwise you'll get rich spots and flat spots, all caused by those pods.
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Offline zerodaydave

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Re: 74 CB750 - Can't figure out high idle
« Reply #28 on: May 28, 2017, 08:35:14 AM »
Hey Hondaman -

I'll put the 105s in but the issue we are having with it is it's still running rich. It cold starts on the 1st try without the choke. Timing is dialed. Valves are dialed. Vacuum leak is fixed. Idles smoothly at 1k.

Mixture screws are 1 1/8 turn out. Plugs are black, dry, and sooty.
« Last Edit: May 28, 2017, 08:45:41 AM by zerodaydave »
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Offline zerodaydave

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Re: 74 CB750 - Can't figure out high idle
« Reply #29 on: May 30, 2017, 07:34:36 AM »
After putting it all back together and taking it on a quick canyon run it looks like it's running a bit lean. I had the mixture screws running at 1 1/2 turns out I've just turned the mixture screws to 1 turn and I'll just keeping fiddling with it until i get it dialed in perfect. It does feel like it loss some power on 0-1/4 throttle but over 1/4 throttle it pulls HARD. Jets are 105 and 40 which are stock. I do have pods and a 4 in 1 but I think the main is right where it needs to be and I'm hesitant to go any bigger on the slow jet. I'm not going to mess with the jetting anymore.

I think I need to do a proper plug chop but right now the plugs look pretty decent. Thanks everyone for your help.
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Offline CowboyX

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Re: 74 CB750 - Can't figure out high idle
« Reply #30 on: July 22, 2020, 04:17:05 PM »
Reading from above these are supposed to be completely seated, right?

https://photos.app.goo.gl/efWdUtWGFdy69hXD9

Is the adjustment only at the top.  One post said there were supposed to be four threads.
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1976 CB750 SuperSport
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