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  1. #1
    CB750 Member wkong's Avatar
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    Plug fouling and idle issues

    Iíve got a 71 (engine is from a 75 according to the case number) thatís fouling plugs. It doesnít happen if I just let it idle in between checking adjustments or if I pop around town for a bit, canít nail down when exactly itís happening. It also wonít start or run at all with the choke on (yes itís operating correctly and Iím using it right). I have to stay on the throttle for a few minutes or bump up the idle screw to keep it above 1500 then drop it back down as it warms up. Hereís what weíve got:

    7a carbs
    110 mains (keihin)
    40 pilots (keihin)
    25mm float height (hondaman recommended dropping the float heights since I have Ďaftermarketí float valves)
    Air screw 7/8ths out
    Electronic ignition first statically then dynamically charged at 2500 rpms
    New coils, plug wires, and caps (switching wires between 1&4 and 2&3 has no effect)
    Carbs synced which wasnít easy when it has a tough time idling at 1100
    Fuel flows out of the petcock and all four drain screws
    11.5v at coils
    4th set of plugs all gapped to .028
    Needles at the middle clip (shouldnít affect this, itís fine off the line)

    This bike was a drag bike, came with 40 pilots and 130 mains with pods and straight pipes. Iíve got a stock air box with k&n filter and 4-2 pipes with baffles. Idle sounds good out of 3 and 4 but not as smooth out of 1 and 2. Some sort of popping but itís a low pop not a high one if that makes sense. Canít tell which because of the 2-1 on that side. Itíll idle at 13-1400 all day, starts to have a tough time staying alive and the popping gets worse below that.

    It had intake leaks after I soaked the carb boots in alcohol, got new boots and still had leaks so Iíve put on some heavy duty clamps that donít show any leaks if I spray a moderate amount around the spigots, but if I douse it it revs up just a hair for a few seconds which I assume is the carb spray getting sucked into the air box.

    Itís great on throttle, just a little bit of popping that only happened above 100 mph. It pops on decel.

    I bought it and have essentially touched everything but the engine, compression test was within 10% across all four but I canít remember the numbers anymore. Only time I get smoke out of the pipes is on start up if it sits for a few days but Iím in the PNW and itís cold and humid so Iím not worried about that.

    The only thing left I can come up with is the bungs in the carbs loosened up one of the last three times I cleaned the carbs, since this is the type with I think three of those.

    Iíve been reading these forums for months and I was hoping to just post before and after pictures once it was finished and say thanks but Iíve gotta call in the experts. Iíve got a month left before I plan to drop it off a few hours away for the friend Iím rebuilding it for.

    So! Any ideas on how to get the choke to work and get the idle even and lower? I assume thatíll fix the plug fouling.

    And if anyone has a source on clamps that are something like 2mm smaller than the stock intake clamps Iíll try those instead of these torque clamps Iíve got.

  2. #2
    CB750 Member wkong's Avatar
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    Oh I did try 38 pilots but no change. That was before the sync though so I can try again Iíd recommended. Itís supposed to have an 836 kit in it but I really have no idea. I just know it had some left in it for sure at 105 mph and that was a pretty windy day.

  3. #3
    CB750 Member wkong's Avatar
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    ...D8EA plugs in it now. Fouled both D8EA and DP7EA-9s so far.

  4. #4
    CB750 Member wkong's Avatar
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    Sorry sorry sorry I keep forgetting things. 1&2 fouled, 3&4 show rich but not as bad as 1&2.

  5. #5
    Moderator dirtdigger's Avatar
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    Going to be kind of hard figuring out if you dont know exactly what the motor is and where we are starting from. Ex drag bike and think its a 836 makes me wonder. 836 with a big cam will never idle good or low and is a pain to jet. 110 mains for a stock motor is too lean, for an 836 it would be very lean. Need to go through and verify some stuff. Recheck compression so know what we are working with for numbers. Would be nice to know what cam is in it but would have to do some dissecting to figure that out. Also it is going to be hard to know what to set valve clearances to if you dont know what cam is in it.

    11.5 volts at coils is suspect, should be battery voltage.

  6. #6
    CB750 Member wkong's Avatar
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    Iíll recheck the compression and the voltage at the coils tonight.

    Judging by the state of the bike and everything around this guyís garage, I doubt he went to the expense of an engine with a new cam. But who knows. He said he bought the engine from some guy in Pennsylvania that only works on 750s, he didnít know exactly what was done to it just that it cake from this guys shop with an 836 tag on it... his buddy he worked on the bike with supposedly picked it up. The original engine also came with the bike and the compression test on it was good so Iím 50/50 on it actually being an 836.

    For what itís worth, early on before I got the dyna s ignition and replaced the coils I started it with only 1&4 plug wires hooked up and it idled great at 1100. I could pull the plug wires from 2&3 and ground some plugs to the engine just to test that again if it would be worthwhile to know if itís a problem in #2, but both 1&2 were fouled.

  7. #7
    CB750 Guru amc49's Avatar
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    With all due respect there is no such thing as idling great with 2 cylinders not working. That sort of judgement warps everything else posted, like the compression is good but with no numbers given to be more worthless than having worth, as it can now get in the way of helping rather than being a solid point to build on. One major fouling reason is a lack of compression and here it is an unknown, as the judgement of others is so often faulty that it cannot be accepted as fact. Just because an engine will run has no bearing on good compression at all. Low compression as well forces a higher idle when the engine has lost enough compression that it will not tick over slowly, slow enough it then has the weak cylinders trying to die.

    No insult intended or directed at anyone at all but it is what it is, and affects getting the problem sorted out. Need to positively know about any cam bigger in spec than OEM like said too, those can make a low idle speed impossible and push it toward fouling plugs like nobody's business.
    Last edited by amc49; 04-03-2018 at 09:18 PM.

  8. #8
    CB750 Guru amc49's Avatar
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    The popping noise is the fouled plugs misfiring down the sides rather than going through the gap, due to being wet. The choke not being needed means engine is pig rich, they go that way when the idle speed is jacked up to get it faster, it exposes the off-idle fuel compensation and then the fouled plugs. The 40 pilots are adding to that, as is likely the under one turn airscrew. Several things there to shove the fuel to the rich side and no wonder it's fouling plugs. 38 pilots of course do nothing with the plugs already wet, a situation like this requires new plugs (the damage is already done) or driving it a week once corrected to possibly even back out to run right. The K&N if overoiled can mess up the plugs like that almost as soon as it is installed.

  9. #9
    CB750 Member wkong's Avatar
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    Iíll restate it - it idled on just 1&4 at 1100 without popping or missing. I know that doesnít mean everything is great in any of the four cylinders just giving some info in case it matters. If it doesnít that doesnít bother me, better for me to give too much info than not enough. It was an accident as I had pulled the plugs to check spark and forgot to put 2&3 back in before starting it. Iíve since replaced the 2&3 coil out of fear of damaging it since I got 2.6 across the posts for that coil after. I get 4.8 across the posts and 26k ohms across the plug wires on both coils.

    While I canít remember the numbers exactly I know that all were between 127 and 135 with a cheap harbor freight compression tester. I donít know which cylinders were what numbers but those were the two extremes and that was after not running for four months without the spoonful of oil trick. I held the throttle wide open for it and had the kill switch to off.

    Iím really not looking to pull the engine but if the consensus is I need to in order to visibly check the cam then so be it.

    As for the plugs Iíve been replacing them when I see they are fouled. Iím on my fourth set.

  10. #10
    CB750 Guru amc49's Avatar
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    The compression is not great but it will run like that. I know DOHC mainly and those OEM numbers at 170 psi and 9/1 compression as the stated number, I expect the SOHC to be close to that. Just something to relate to but when in garage work I learned to count 100 psi as a cylinder that adds no power-they are dead by then and foul plugs very quickly to not run at all.

    I guessed that two plugs were missing in the 2/3 thing, with them in and compression it would not idle nearly so well. A four with only one plug installed and the other 3 missing will idle as well, there is little load on it then. Add compression and it changes.

  11. #11
    CB750 Member wkong's Avatar
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    The plugs were in 2&3 but the wires not connected

  12. #12
    CB750 Guru amc49's Avatar
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    Check what you posted................

    'I had pulled the plugs to check spark and forgot to put 2&3 back in before starting it.'

    You put wires back ON , not IN. Plugs go IN. Not going to argue past that; I've worked on hundreds if not thousands of engines and pretty much know.

  13. #13
    Moderator dirtdigger's Avatar
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    Again not knowing how the engine is built, with a big overlap cam those numbers can be good as far as compression. My big bore drag motors even when new wont go past 135psi even with 12:1 compression....again with big cams bleeding off cranking compression. They are mediocre in a stock motor but the sohc will run fine with that number. I have made 750's with 110 run good smooth just lower on power. The only thing that ties certain cylinders together is the ignition 1 and 4 are paired and 2 and 3 are paired. Carb issues will generally only affect that particular cylinder. If you have a 1-2 or 3-4 issue start looking at a mechanical issue...head gasket leak.

  14. #14
    CB750 Member wkong's Avatar
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    No leaks anywhere on the bike.

    Coil voltage test:
    13.2 at the battery

    12.8 at black and white wire in headlight
    12 at black and white wire going to coils
    11.8 at coil post

    12 at black and white wire in headlight with lights on
    11.1 at black and white wire going to coils with lights on
    11.1 at coil post with lights on

    I pulled the plugs which were replaced immediately before the last time it idled (and was synced a second time). I never turned the throttle past 1/8 to get it warm, rpms never broke 2k.

    1 - black
    2 - brown
    3 - white
    4 - white with two small brown marks that look like scorch marks

    Never seen something like 4 before. All four bowls have gas, originally thought 2 and 3 were low but realized I spilled some when I was draining the bowls. Iíll get clear tube to check with that method.

  15. #15
    CB750 Member wkong's Avatar
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    Amc, I misspoke. Understood on the meaning of on vs in, thanks. Bottom line, all four plugs were in, 1&4 wires were connected while 2&3 were not. It idled without popping or missing on 1&4 with all four under compression, and the compression test showed all cylinders were good. If any of that matters.

    When you say with all due respect, at some point you should probably mean it.

  16. #16
    CB750 Guru amc49's Avatar
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    ' My big bore drag motors even when new wont go past 135psi even with 12:1 compression....'

    That can be true of course and why one goes about decking things to get that compression back up, as well as fitting better pistons that fill the chambers up better. I have no trouble getting well over 200 psi out of them when I want it. The car ones can go over 300. You can degree to get back a bunch by simply advancing the cam to trap more in motor. As well, a whopping lot of generic supposed 12/1 pistons out there couldn't make it at all, they have to hold the pop-up ODs in a certain amount and the valve notches to fit more variables in the mostly OEM engines. Why they won't hit numbers even with shorter overlaps. I personally learned to ignore claimed numbers of compression and only look at the piston design itself.

  17. #17
    CB750 Guru amc49's Avatar
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    I likely misspoke as well, and as you can see I rise to the occasion. My bad there.

  18. #18
    CB750 Guru amc49's Avatar
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    To begin with no plug on the planet should be color graded at idle and off-idle only. Why? Virtually every plug on the planet is heat range matched to only be the correct color UNDER LOAD and idle is no load at all. Any dead stock engine before modern electronic ignition that will burn the ends off the plugs will foul plugs after starting and running an engine just barely up to temperature as in the case of say starting an extra car sitting just to be used in emergency or other situation. Do that say 3-4 times and then the plugs foul even though in normal use and car used every day it would then burn the plugs off correctly, it cannot happen in a short time like that. Idle is normally too rich anyway at least before EFI and oxy sensors came about. It has to be to keep running as the exhaust dilution of the fuel/air is great at slow speeds, the overlap is a detriment there. You even get mild EGR effect to make even less of the fuel ignite and then more left to foul the plug which is not at max heat then anyway.

    A plug trying to fire like that may get black, another weaker by only a bit one will not fire and simply washes to stay white yet it will look like diametrically opposed in fuel mixture when it's actually not. Why all plugs on the planet get read after a hard WFO run and engine cut instantly to coast to a stop. You can check them with normal driving as well but the engine MUST NOT IDLE any length of time at all before engine is cut off to check them.

  19. #19
    CB750 Member wkong's Avatar
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    Okay, I intend to do a check at WOT in second and cutting it coming up the hill to my house after hauling ass for a while. Just wanted to figure the idle out before moving on to the main jets. From the reading Iíve done a plug chop at idle should still indicate if the pilot jets are doing their job, plus 1 looking fouled and being on the side the popping/sputtering is coming from seems to make sense..

    Do those readings on the coils look okay? Thatís a helluva drop from the headlight bucket to the white/black coil wire with only one connection in between, but the connectors look solid. 2-4 either look good or lean according to the plugs so I assume both coils are working fine and they all have new resistor caps including #1. And switching 1&4 plug wires had no effect.

  20. #20
    CB750 Guru amc49's Avatar
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    I myself never use resistor plugs or plug caps, the reduction in resistance I use to bump the gap open a bit more to do better good. The kill switches often have resistance issues.

    You simply cannot read plugs at idle and a misunderstanding there. You are NOT reading what the pilots do anyway, they are modded as to richness by the setting of the airscrew so any plug chop wouldn't show what is needed at pilot anyway. The idle gets one fuel air ratio and often the pilot then supplies richer to the off-idle transfer holes that show as soon as the slide is lifted up at all. Why too high an idle speed makes the idle go rich, you are then pulling off those holes and not just the idle curb hole which is in FRONT of the slide. Pure idle fuel comes from there ALONE and it is modified by the airscrew to be leaner than the pilot puts out by itself. Why you can run slightly different pilots and still get same result provided the airscrew gets reset each time. It chokes back on the pilot fuel as accurate for pure idle alone. Then your true problem is not idle itself but just OFF idle when the fuel goes rich to match the pilot output. Or, holding slides up to get too high an idle does the same and foul city. Hard to do on bike but I used to on cars and Holley carbs. It involves drilling holes in the butterflies successively larger until you have the correct idle speed and then the carb butterfly (think slide on a bike) is backed off again to be in the correct spot in reference to those off-idle fuel feeds (on a carb they are slots rather than holes). That way you have the faster idle speed but not all screwed up at your fuel transfer holes. I've done it on a couple bikes by carefully drilling holes in every slide down close to the bottom to allow more air around the closed slide but VERY easy to screw that up.

    I don't care what you have read, you cannot plug chop on idle using the correct plug for bike-it WILL be inaccurate as the plug will not be hot enough to burn accurately under those circumstances. And even if you tried they will show too dark as idle is ALWAYS richer to again make up for exhaust dilution at idle, you simply cannot get away from that effect. Only a computerized fuel injection setup using O2 sensor can do that.
    Last edited by amc49; 04-04-2018 at 05:47 AM.

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