Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: billy192 on March 21, 2015, 07:26:39 PM
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well,
had the spark plug repaired engine back in and started ok. but it runs v lumpy. the faster i went the better the engine pulled. at 70-80 ish no miss. below 4k misses splutters and now and then a small backfire, or pop out of the left side.
apart from the plug blowing out the bike was running great before i had the head off.
things i have checked.
timing both static and with strobe
valve clearance
carb bowl clean
checked the needle position on each carb also
timing again
new petrol (getting desperate)
it seems as if it is misfiring on the left most cylinder (sitting on the bike)
how / can i check the coil
could it be one of the condensers?
when the manual says number 1 is that to the left, is that as you sit on it or looking at it from the front? just a daft question i know but i am not sure.
:'( :'( :'( :'( :'(
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sitting on it no 1 to the left.Doubt condensor as 1 and 4 would have the same problem.Coil lead only and cap of course,Sure no air leaks at carbs?
Cheers
Bitsa
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Try swapping 1,4. And 2,3 Ht caps and also swap the coil primaries (blue and yellow) if the problem moves to 2 or 3, then you've got a coil or HT fault.
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Hi Billy, also worth checking the impedance across each pair of coils, and whether you're running resistor or non resistor plugs.
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the two coils are the same and the plugs are the same. :(
thanks for replying
Billy
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Hi Billy,
The backfiring, lumpy, popping, plug blow-out are all suggesting (to me) a timing issue, and due that 4000 RPM threshold, perhaps a problem with the advance / retard unit... which is possibly 'stuck' in the advanced range? Check it's working properly, is smooth turning, the springs in good fettle, and most importantly of all, it's set up to the correct timing marks. And obviously, if one set of points is mis-set, that too will show up as a pair of mis-behaving cylinders.
(Ballpark, the advance retard kicks in @ 1100 RPM, and is fully advanced by 2500RPM.)
Apologies if you're familiar with setting up, but just to re-affirm, the static & strobe timing marks (low idle) are the 'F' mark, whilst above 2500 RPM or so the strobe should be illuminating at the two marks 23.5-26.5 degrees ahead of those F marks.
Er.... enjoy your Sunday!
Simon
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ok so had a busy sunday 8) advance is working fine,.
decided to check my coils so watched a utube vid
and i have 4.9 on the primary (ohm contacts on the yellow and black/white)
the vid said after switching to 2K on the multimeter keep the contact on the yellow wire and the other in the plug cap and you should have a reading of around 10. i get nothing :-\
then on to the other coil and 4.8 primary and nil again from the secondary
i put the plug on the head and spun on the starter and there is a tiny spark you can barely see it (inside the garage).
has anyone tested there coils like this?
Billy
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Hi,
Try the 2nd test again in one HT lead with the meter set to 20K with both plug caps off the plugs on duel output coil, as 2K is too low for 2nd circuit, and "lesser" meters will not automatically shift upscale and will show no reading---Ken
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thanks Ken, i am sure i did try on 2K,20K,200K,as i remember the dot moving across each time i changed it, but i will re test next Fri (work away all week). fingers crossed
Billy
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Billy, you need to break it down a bit and isolate the plug caps from the coils. Remove the plug caps and meter across the pair of HT leads on each coil (so 1&4 and 2&3). Should get 15 kOhms from memory. Then meter across each plug cap and if they are resistor caps you should get 5 kOhms. So total of 25 kOhms impedance across each coil with the plug caps fitted. This assumes you're running non resistor plug caps.
But this is based on 750 experience....maybe someone on here can validate correct for the 500/550?
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I recently replaced the plug caps on my 500,as the resistance had increase to sky high rating,
age and use does cause the caps to burn out,
for less then £20 a set,worth getting new,
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Think of a coil like a transformer; it has 2 coils,a primary and a secondary..
The primary (yellow to black white and blue to black white for the other coil) is around 5 ohms.
The secondary (from one HT lead to the other) is around 15k.
There should be no resistance between yellow and a Ht lead, they're not connected at all.
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The vid is probably a single output coil where the other end of the HT is earth or frequently the points side of the LT side. The Honda twin output coils have LT and HT as two seperate insulated windings so second test needs to be between the two HT leads on the same coil.
Weak spark is common if cap is going high resistance ALSO id resistor plugs are used with reietor caps
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New ngk caps but thanks I will do new checks this Friday
Sent from my iPhone using Tapatalk
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Hi Billy,
I would look very closely at potential air leaks. These are very similar symptoms on my 750 and found the inlet tube/s weren't sealed properly. They were, in fact, the wrong ones!! Lower the demand on the engine the poorer it ran. Particularly around town. I sprayed a soapy liquid onto the tubes and area it which should be sealed and the engine picked up and ran sweetly for a second or two on no's 1/2. Those tubes were incorrect and one was split.
I hope this may be a more simple solution and fingers crossed for you.
Nige.
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Thanks Sparrow1
they are the original that have been on the bike for the past 2 years when it ran great. i will still check to see if i damaged any of them putting them back on.
Billy
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just tested the coils they look fine. each one registered 10.3 to 11.4 and the new NGK caps were all at 4.9.
next i am going to strip the carbs and check in case some dirt got in them while they were off the bike. i will also look at the rubber boots while i can.
Billy
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Hi,
Still reckon you have an air leak between carb and the cyl. head----Ken
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well just spent a fortune with DS for new rubbers and new clips :-[ :-[
lets see
Billy
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Billy
If it were me sit back and look at what you have done.B4 fixing the plug you were happy with her yes?So you have removed the airbox,carbs,disconnected all the wiring blocks took the engine out stripped down to the head off and fixed your valves and plug thread and cleared all swarth yes?Now reassemble VALVE TIMING SPOT ON?All connectors checked,carbs back on no problems went on fully home?I assume you did not leave shite fuel in the carbs?So still does not run right ask yourself why not.Coils etc don't just stop working overnight(though some would disagree) otherwise every winter I would need new ones for my two they stand for best part of 5 months.
I wish you the best
Cheers
Bitsa
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I was looking at this last week but couldn't reply as my temp wi-fi link wouldn't keep me on long enough to write it, I was over in your neck of europe bitsa but a fair way from you in annecy/moutier/frejus area and drove back yesterday.
My initial thoughts barring any simple air leaks/fuel jet blockages etc are those that Bitsa says above in bold, are you sure the cam timing is right?
If the cam was timed a little in advance of spec then it would step toward what you have experienced I'm not saying it's the only thing you should look at but you'd need to be confident it's right to avoid chasing your tail through lots of other stuff.
I know this ignores the exhaust cam timing (which would also be out) but if you run the inlet cam in advance of the correct setting it would have the effect of compromising low speed running to potentially give gains at higher rpm, similar to using a wild cam in a race engine.
The inlet cam should open at low rpm late enough to prevent any mixture being drawn in and being fired by the wasted spark on the exhaust stroke of the engine, this would happen (the small amount of mixture being ignited and come out of the inlet because the valve is open too far) if you have got the cam in advance of the correct timing.
I was trying to think of a way to check the timing without removing the engine and the logical way is to use a degree disc on the crank to measure against spec if you are familiar with the method.
If not you could try a rough and ready setup to see if you get an improvement by backing off the inlet tappets to twice our three times manual spec's (should retard the inlet valve timing) and retarding the whole ignition plate by about 5 degrees (this would move the wasted spark further away from the inlet charge entering the cylinder) and then run the engine to see if the spitting back has been reduced, this is not for a solution overall but to see if the motor will run smoother at low rpm and give you more reasonable cause to remove the motor if required.
See what others on here think, please shout if anyone disagrees with the above.
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thanks Guys,lots for me to look at. when i put the cam back in i had the timing mark on the crank and the notch on the cam lined up. is there another way. surely if the notch is pointing up and the two lines are horizontal also 1-4 is at T then the cam is timed? or am I wrong? i can just about see the notch in the cam with the engine in the frame.
Billy
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That's correct; the 2 marks on the cam gear level with top of head while at TDC
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Hi 'Billy'
This is john here from Caerphilly, is this the beast that I did the carbs for? If so then I can say that the carbs were definitely on the bike tidy, the carb balance correct and the ignition timing and points dwell spot on.
I do remember that the engine had a habit of racing up on it's own for no apparent reason and a it came back down when it felt like or after you blipped the throttle a few times, that might have been the fuel supply as it was a temporary arrangement at the time.
The only other thing I can think of that would cause that is a sticking advance retard mechanism. You say that you have checked this out, but did you remove it and clean up, lube and re-fit or did you just check with a strobe. If you checked with a strobe it still might be temporarily jamming up and that would cause misfire/popping but this would be on all fours pots. The other thing is the springs, are they the correct ones as I seem to remember they are different strengths.
The problem with these old Honda electrics is that they have a very weak spark to start with, if there is an electrical fault it won't take much to upset the spark output.
Just a thought.
john
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I had almost the same problem. Turn out there was air leak between carbs and admission. Also, i had problems with my points.
After new o-ring for the admissions, timing and tappets tune up all is good!
Also check for the following:
- good battery
- problems with the fuel line
- good sparks and spark plug
- good air filter
- carb sync
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Yes as you say Billy with the marks as you describe, after i've done that i'd install the cam chain tensioner and then turn the crank (plugs out) with a spanner at least two rotations forward and recheck that the horizontal lines are still in line with the cam carrier joint to make certain that it is correct when in the fully assembled running condition.
As the other posts have said, if the ignition stays at advance and doesn't go back to base/static setting at low rpm then you'll have a similar effect with a conflict of inlet valve opening and spitting the first part of the intake charge back out of the starting to open inlet tract.
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thanks Guys, yes john (Caerphilly) the bike ran great the intermittent revving settled down after 400 or so miles the out popped number one. well i have a nice 4 day stint to sort this out thanks to the forum i have a list of stuff to change / clean adjust. i will post my findings and hopfully sort this out.
Billy
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Ok so today i put on the new carb to head rubber and new carb to air box rubbers with new clips...and still the same :(
as a last resort new condensers and points and checked the auto advance unit put all back together timed with a bulb (static) and fired her up and perrrrrrrrrrrrrrr like a pussy. 8)
will take her for a blast in the morning gut great no miss no popping
Billy
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The thing to do now is refit the old condensers to determine whether the fault was caused by the points or the condensers.
I don't blame you if you don't feel like it though.
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Pleased for you Billy.
LEAVE IT! If it now runs sweetly...😉😄 Go out and have a bit of fun her!
Nige.
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+1 with Sparrow on this one :)
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It was just a thought. A way of improving knowledge and experience that makes one an expert.
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no i did not refit the coils. i did do nearly 200 lovely miles Sunday. it has a small missfire at around 3K like a hesitation but other than that it runs great. i will re check the timing when i have five Min's. I called in Abergavenny on Sunday and when i described the hesitation at 3k a chap there who said he had a Honda dealership back in the 70-80s said the autoadvance springs were a bit tired. and a fix they used to do back then was to chop off the eye of the spring and fashion a new eye thus shortening the spring by a tiny amount made up for the slack in the counter weight. has any one hared of this?
Billy
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No but if you could get hold of a strobe light you could check if the auto-advance is working properly, which might be better than trying something irreversible... :)
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Agree with Tom.Seem to remember there is a topic about this on the states site
Cheers
Bitsa
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I'm with Tom here.... there's no going back when it comes to destructive testing...
This thread discusses springs a bit.. if of any use.
http://forums.sohc4.net/index.php/topic,114960.0.html?PHPSESSID=s38h0m3sbn7e2uqcpa1cofnhp1
.... and if you find a nice / new replacement source let me know... my K2 runs fine, but the springs do have that 'Dolly Parton' look about them.
Simon
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.... and if you find a nice / new replacement source let me know... my K2 runs fine, but the springs do have that 'Dolly Parton' look about them.
;D ;D ;D
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Well I found the fix
The auto advance was semi seized (I may have sprayed dry chain lube in it) it was stiff as hell but did move so it was all or nothing so when I strobed it the marks were not at all static now as I rev they come it to view as the revs rise
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Awesome :D