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  1. #1
    CB750 Member Ricebiker's Avatar
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    Question Too lean or Too Rich, Who knows???

    It's most likely too lean. Removed the airbox and put pods on. Oval chrome cheapos. Ok, Vance and Hines 4 into 1 header pipe with Carpy stainless exhaust tip and baffle (Carpy's, So. Cal exhaust fabricator and bike restorer. World famous locally) May as well be wide open.
    With the above situation you would think that this bike is going to be lean. Probably is, but I'm getting this flamed backfire when I run it. Barely runs at that. I'm trying to find out if I can tell if the bike is lean from these conditions, because the more I read online, the more confused I get. Some say a flame always means too rich, but I'm also getting an overflow symptom out of #4 carb. Pissing out of the tube. I lowered the float level a bit and the flaming symtom and runability got worse. All this is pointing to a too lean condition. In my opinion. I think what's happening is, the high float level was actually helping with the runability. Even though the carbs were overflowing. A higher float level enrichens the mix, yes?! So, when I lowered the float level the leaner condition got worse and is causing more flaming action and a worse running condition. I moved up to 110 from 100 on the main jets, and 70 from 68 on the pilots. Anyone had success with bigger jumps, like let's say 100 to 150 on the mains? Any other success stories with similar situations?
    80 CB750F
    VB42's
    Main pilots, #70
    Main jets, #110

  2. #2
    CB750 Addict joe p's Avatar
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    There's only one person I know that had the patience and time to made that work and he is no longer with us RIP. It take a lot of work to get vacuum carb to work with pods. Years ago they had a kit that had different size drill bits for changing the size of the air hole in the slides.

  3. #3
    CB750 Member HEAD's Avatar
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    Quote Originally Posted by Ricebiker View Post
    It's most likely too lean. Removed the airbox and put pods on. Oval chrome cheapos. Ok, Vance and Hines 4 into 1 header pipe with Carpy stainless exhaust tip and baffle (Carpy's, So. Cal exhaust fabricator and bike restorer. World famous locally) May as well be wide open.
    With the above situation you would think that this bike is going to be lean. Probably is, but I'm getting this flamed backfire when I run it. Barely runs at that. I'm trying to find out if I can tell if the bike is lean from these conditions, because the more I read online, the more confused I get. Some say a flame always means too rich, but I'm also getting an overflow symptom out of #4 carb. Pissing out of the tube. I lowered the float level a bit and the flaming symtom and runability got worse. All this is pointing to a too lean condition. In my opinion. I think what's happening is, the high float level was actually helping with the runability. Even though the carbs were overflowing. A higher float level enrichens the mix, yes?! So, when I lowered the float level the leaner condition got worse and is causing more flaming action and a worse running condition. I moved up to 110 from 100 on the main jets, and 70 from 68 on the pilots. Anyone had success with bigger jumps, like let's say 100 to 150 on the mains? Any other success stories with similar situations?
    80 CB750F
    VB42's
    Main pilots, #70
    Main jets, #110
    The site cb1100f.net covers alot of this article of the DOHC CARB TUNING, using the search function.

  4. #4
    CB750 Guru amc49's Avatar
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    You DON'T JET to cure a messed up running engine. Lost, oh so lost. Make sure ALL engine parameters are fixed at OEM values before you EVER touch a jet, that is the mark of a...............well, I'm new here and I should shut up. But I damn well know what I'm talking about.

    ANY flaming you got comes from engine screwed up not rich or lean. Your 70-110 will work fine and if still doing it not the jetting.

  5. #5
    CB750 Member Ricebiker's Avatar
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    Now I'm running 140 mains and 80's and it's running pretty good now. Engine is fine, what do think of that! It was fine before I put pods and a 4 into 1 on. So clearly, it was too lean, causing excessive heat, causing the exhaust gas to reignite.

  6. #6
    CB750 Guru amc49's Avatar
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    140s are WAY too rich, I've never seen one ever run longterm like that and been there myself.

    You are clearly in no man's land. 80-115 works with pods all day long. CV carbs by virtue of the way they work do NOT need super big changes like direct lift carbs do. They self adjust much of it. I'd bet in a second you can't pull out to the full 9500-10000 rpm in top gear these will when the carbs are truly right. Another CV rule they never tell you about, the slides don't open all the way up when rich. Or, rider thinking he is on badass bike when it is a dog. CVs always open slide more slightly lean. I ran 125s for a while until I figured that out then dropping lower made the motor light up over 8000 rpm. The engine may well run fine like you got but wait a month until the tune settles in, then they don't. It takes a while for the motor to carbon coat rich to where it then begins to spit the jet setup you got out. I've been there plenty of times.

    Your whole idea of chasing a flame or backfire with jetting is flawed. It means slow combustion if out the pipe and fouled plugs if out pipe or carb. Lots of other things can cause it like timing, compression, cams off, etc. I can make an engine do it in seconds with perfect jetting.
    Last edited by amc49; 10-08-2015 at 05:39 PM.

  7. #7
    CB750 Member Ricebiker's Avatar
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    Thank you amc49, probably the most helpful advice I've gotten. So far. Yes, the low speed seems to be good (idles a little high though which makes me believe it's lean or vacuum leak) new intake boots installed. I just got rid of the chrome cheap pods and put k&n's on. Made the 1/2 throttle position worse. I think these pods let less air in than the previous ones. So yeah, too rich on the mains. Plugs look tan and dry though. So . . .
    I bought a manometer and a colortune.
    I'll get it right yet!

  8. #8
    CB750 Member Ricebiker's Avatar
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    Quote Originally Posted by amc49 View Post
    140s are WAY too rich, I've never seen one ever run longterm like that and been there myself.

    You are clearly in no man's land. 80-115 works with pods all day long. CV carbs by virtue of the way they work do NOT need super big changes like direct lift carbs do. They self adjust much of it. I'd bet in a second you can't pull out to the full 9500-10000 rpm in top gear these will when the carbs are truly right. Another CV rule they never tell you about, the slides don't open all the way up when rich. Or, rider thinking he is on badass bike when it is a dog. CVs always open slide more slightly lean. I ran 125s for a while until I figured that out then dropping lower made the motor light up over 8000 rpm. The engine may well run fine like you got but wait a month until the tune settles in, then they don't. It takes a while for the motor to carbon coat rich to where it then begins to spit the jet setup you got out. I've been there plenty of times.

    Your whole idea of chasing a flame or backfire with jetting is flawed. It means slow combustion if out the pipe and fouled plugs if out pipe or carb. Lots of other things can cause it like timing, compression, cams off, etc. I can make an engine do it in seconds with perfect jetting.
    The main reason I thought jets was because bike was running fine before pods and exhaust change. Naturally runs leaner when these changes take place. More air in, more air out. I did drop down to 120's and seems a little better. Problems at 3/4 throttle now, a little bog, chugging when I give it throttle all at once. How does the timing get "off" when it's electronic ignition, pick up coils, etc? I think also, too much mix? 2 1/2 turns out along with 80's probably too much. Anyway, no more flames. Runs pretty good overall.

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