jumping ...missing and backfiring

[billy192]

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.



 

[Bitsa (Ralph Wright - RIP)]

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

[mike the bike]

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.

[JamesH]

Hi Billy, also worth checking the impedance across each pair of coils, and whether you're running resistor or non resistor plugs.

[billy192]

the two coils are the same and the plugs are the same. 
thanks for replying

Billy

[Lobo]

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

[billy192]

ok so had a busy sunday  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

[ka-ja]

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

[billy192]

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

[JamesH]

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?

[Johnwebley]

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,

[mike the bike]

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.

[Bryanj]

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

[billy192]

New ngk caps but thanks I will do new checks this Friday


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[Sparrow1]

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.