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That was a good post, I tell ya what.^
View attachment 33481
Me bad for clogging the thread, but technically speaking (tinfoil hat time) , that should read methane although I believe there is some hydrogen in there as well. In keeping with the technical theme.... There were some fellows that introduced a relatively small amount of methane into the carb of a running P&W radial. They were surprised how quickly the engine died. They were researching the disappearance of aircraft in the Bermuda Triangle and were speculating a cloud of methane belched out of the ocean floor. No idea why I have that rattling around in my head.
 

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Discussion Starter #882 (Edited)
ok so here is more proof I am an idiot.

Pull the #1 float bowl and pull the pilot and main jet. Main jet is clean as a whistle, but the pilot has some sediment in it. Not enough to block it entirely but enough to block flow. So I blow it out with carb cleaner and while I am at it, hit the passages as well. Fire it up and Cyl #1 is banging and heating up, GREAT! But now Cylinder #2 is out. I pull the plug and it is the most fouled plug to date. At this point I realize I am just fouling plugs and everything else is working as it should. So I pull cyl #1's main jet and it's a 120. Now for those of you playing at home, this is the part where you say "But stock is supposed to be 105". Yeah.

Now before you do what jag did and send me a text saying "you didn't look at jets this whole time!"....actually I did. I even noted it in this thread several dozen pages back. The issue is the carbs are off a 1972 and are 7A models which came stock with 40 pilot jets and 120 main jets. But the stock carbs for the 1975 cb750F are 086A and used a 105 pilot jet. The cb750F also have different cam profiles and compression from the 1972 models. So when I looked up the jets by carb model I saw 120 and thought no problem. The other fun thing I have been doing wrong is using the clymer recommendation on the air bleed screws which is 1 and 1/8 turns out, where as both the 1972 and 1975 use 1 turn out only and the 77-78 bikes use the later 1.5 turns out. So I pulled the original carbs for this bike off the shelf and they are 105 jets. I have them soaking and will install them tomorrow, clean all the plugs, and then fire this thing up and see if I foul anything more.

I knew this was a problem, I even recognized it here, and then I forgot all about it because it was like 6-10 months ago and in the back of my head I kept telling myself the jets are stock so size shouldn't be an issue. And it turns out it's an issue.

I also found a set of 110 jets which were used on 657A model carbs and also by most people with the 1974-76 bikes when they switched to a 4-1 and santee airbox, so if the 105's turn out to be too lean, then I have the 110s to throw in.

now I am off to finish an online defensive driving course because I got popped for speeding by a state trooper (or so he says)....FML.
 

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Discussion Starter #884

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Not the same (obviously), but a good carb sync will help the bike to idle pretty nice even if the mixture screws aren't spot-on perfect.

You can get the screws pretty close by adjusting until the idle settles down straight away after a rev (no hanging).
 

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Why would you run stock jets for any CB750 in this set up?
 

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Discussion Starter #888
Why would you run stock jets for any CB750 in this set up?
I don't understand the question. Obviously I would want to run what ever jets yield me the best fueling, however, I would want to start with stock as a baseline and move from there. There are several variations of carbs for cb750s and all have different setups so it pays to have a starting position to work from.

At the time I think my thinking was the 120 jet was appropriate based on this information from Hondaman on the SOHC4 forum:
http://forums.sohc4.net/index.php?topic=108988.0
If the carbs do NOT have screw-on float bowls, they are for the 1970-1976 "K" model bikes. These come in 4 basic models, identified by the little number stamped on the front top of the carb body:

7A - found on 1970 K1 and 1975 F0 bikes. The early ones have "A" calibration, with brass floats, 24mm float depth, 115 mainjet with needle clip in 4th notch from the top, or else 120 mainjet with clip in middle notch. The airscrews on teh early ones have a tiny hole in their tip, while the ones from the "F" bikes do not. Most (but not all) of the F0 bikes had plastic floats and 26mm float depth.
657A - found on later K1 and early K2 bikes. These have 110 mainjets with the clip in the 4th position from top, and tiny holes in the air screw tip. The float level is 25mm, measured from a small pedestal on the INSIDE of the gasket. (Same as 7A, BTW). These carbs will idle well at 850-950 RPM.
657B - found on K2 and later bikes, through K5. These usually have 105 mainjets, needle clip in 4th notch from top, floats at 26mm, air screw tips are solid. These carbs complement the Hitachi spark advancers with reduced advance angle, and idle around 1100 RPM, all have plastic floats. The float depth is measured from the notch in the outside edge of the float gasket area, is set to 26mm.
087A - found on some K5/K6 and F1 bikes: calibration is similar to 657B carbs. The idle air screw shape is different (to allow for a plastic cap to prevent you from adjusting it, probably now long gone from heating), choke lever on the opposite side from previous carbs, single hose feeds fuel to whole rack from the 1-outlet petcock on tank.

The PD carbs came in PD41 and PD42. The main difference seems to be the non-removable idle jet in the PD42, probably to make the EPA happy. Most of these have 105 mainjets and after 1977, non-adjustable needles.
So I thought 120 jets would be appropriate. However, thinking about it more - there really isn't much that is different from the bike that I have than a stock bike. The Kerker probably only flows marginally better than the stock 4-1 designed by honda, it is just a shit ton lighter.

you and I both know that jetting is trial and error. 120 isn't working for me and it shows. so why not try 105. If that comes out too lean, then I can go to 110. These are all jets I own and are stock for various years of the bike. Once I get close....I can then go out and spend for the perfect jets, but until then why not fool with what I have. If I drop $30 on a box o' jets for these carbs I am only going to get 105, 110, 115, and 120 anyway, and I will have paid $30 for 12 jets I won't use and 4 that I will. Why spend when I have it already. The only jets I don't have are 115 and if 120s are as rich as they are showing, I doubt 115 is going to be on the money, esp since all cb750s from 1973-1976 (except the 75 F0) used 105.
 

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Discussion Starter #889
Now go and apologize to your ignition system for ever doubting it.
actually I should go apologize to it for monkeying with some of the components and making things worse. However, the martek wasn't without it's own faults, it was a hair off and needed to be properly timed and something needed to be done about it walking forward. If I didn't doubt it I never would have found it.
 

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Discussion Starter #890
Just to give you all a heads up.....This thread comes to an end next weekend. Stay tuned for the thrilling conclusion!!!!!
 

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Taking old yeller out back?
 
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Discussion Starter #893
Based on pamps result selling his CB........ I hope there's still enough meat on the bones for you.
I will discuss it with the buyer how much information I can reveal. Till then I am going to sort this last issue out.
 

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I'll bet the 105s will get it running much cleaner but might exhibit a lean stumble at some point if you just wack it open or maybe some steady throttle surging. I vote 110s being spot on. All in with $0.02.
 

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Discussion Starter #895
I'll bet the 105s will get it running much cleaner but might exhibit a lean stumble at some point if you just wack it open or maybe some steady throttle surging. I vote 110s being spot on. All in with $0.02.
Well...we will find out won't we?

I can tell you from years of owning these bikes, almost all of them have a "lean stumble" when you whack the throttle open fast, and it's usually worse when the bike isn't fully warmed up (it takes about 30 minutes for a cb750 to fully warm up). This is why they switched to a VM style carb with an accelerator pump in 1977. I am pretty sure my 75 K bike is running 105s and if this runs half as good as that bike it should be no problem.
 

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Discussion Starter #897
do they use the main jets for a lot of the throttle opening?
Almost anything above idle. It's not a cv carb so the moment that needle unseats it's on the main jet to some degree. The only variance you have is the needle height.
 

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Discussion Starter #899
What's the hold up?
Sorry, the wife's birthday was yesterday and I had to tend to my husband-ly duties. She actually yelled at me for cleaning jets in the kitchen while I was waiting for her to finish getting ready for dinner.

I am looking to get them installed tonight, plus I have a major dr's appointment on friday because frankly my heath is going down the tubes fast, so I need to get it done and sorted before they bombard me with gamma radiation and turn me into the hulk (or give me massive tumors - whichever is based in reality).
 
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