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Tuesday, June 08, 2010


The new eBay carbs arrived today so I installed them, replaced the oil and filter and fired her up. It's worse than before!! She would barely tickover! Attempts to rev the engine just resulted in a stall. With very careful and steady application of the throttle, I can get it to 7000rpm but it's missing like hell.

The left exhaust downpipe was stone cold and the right was hot, which obviously indicates that the left cylinder isn't firing, and yet when I disconnect the left hand plug-lead, the bike stalls immediately!

There's a spark going to the left plug, and if you examine it, it's wet as if it's not firing, and the downpipe is stone cold, so why the hell does it stop running when I remove the plug-lead???

This thing is doing my head in! LOL


IanPike said...


I remember many years ago when my Dad used to tinker with cars he used a thing called ColourTune. It was a glass topped spark plug that you looked into when the engine was running that allowed you to adjust the mixture. Not sure if you can still get them but it may help you see if it is a fueling problem.

By the way my problem the other day was that when I put the two carbs back together one of the butterflies was slighly open whenthe other was fully closed.

Keep at it Tom.

MadDogMcQ said...

Hi Ian,
I've got a Colourtune, but for these to be of any real use, you have to have a running engine.

I'm getting close to calling in a Honda Mechanic. I value my leisure time too much to spend it pi$$ing around with this annoying problem. I ain't too proud to know when I'm beat, LOL.

Paul said...


I think that both spark plugs fire every 360 degrees of crank rotation, so they fire when they are needed and when they are not. If this is true you could swap the spark plug leads over and it should still run. If the same exhaust downpipes are still hot / cold you have ruled out the electrics. If it changes over to the other way around I would think that it is electrical.

By the way, I liked your idea the other day about using the clear plastic tube in the carb drain hole to see the float level.


Klaas De Craemer said...

This is so exactly the same as the problems I'm having. But mine did run nice one time (only once...) after standing in the sun for half a day. It was warm and revved well. Have you tried the same?

MadDogMcQ said...

Hi Paul, I already swapped the leads over but it made no difference. It's a bugger!

Klaas, regrettably I live in Bolton. There is no sunshine in this town. LOL

Michael said...

Hello, I have been following your blog for sometime as it is of great interest to me as a fellow nightmare owner! Just read your latest post and this is exactly the same problem as i had with by 1982 CB250NB. I fixed this by washing the CDI and ignition coil in POT cleaner ( thanks to Maplins) and also giving every electrical plug a good wash too. It might be a dodgy earth. Good luck!! I have a working spare CDI if you're interested. Michael

Anonymous said...

Hi I'm a newcomer to this type of think, but typed 'CB250N ignition problems' into google and got blog.

Mine is a 1981 bike with similar problems. It starts OK and runs but won't rev over about 5000 rpm when riding. Seems to pull quite well at these limited revs, so my view is it can't be a serious mechanical fault.

I have rebuilt the carbs to no avail, and feel strongly that it is an ignition problem. I have swapped the CDI unit with one I bought on Ebay, but that's made no differnece either. I am now thinking that the coils/leads may be breaking down under pressure, or possibly a fractured wire or conector in the CDI system.

I saw the comment about both cylinders firing/sparking together, which is correct. In essence, one cylinder fires at the correct time and the other on an 'empty' cylinder at the top of the exhaust stroke. In theory you should be able to swap plug leads and the bike will run as normal - assuming no problems exist, so if there is an ignition problem in one cylinder it will swap to the other cylinder.

Not sure if this useful to anyone, or just me having a frustrated ramble!!??!!, but good luck anyway.


Klaas De Craemer said...

Hello Tom,

I got my CB back from the garage, and it runs very nice now! The main issue was the need for synchronisation of the carbs (and a reset of the valve clearance).
Good luck!


andrew said...

Hi tom,now got my superdream up and running.18 months later .last few months been having same sort of problems replaced carbs I striped and cleaned every jet then used a air line to blow out every airway . yours seams to be a problem with the carbs not the ignition . can now get back to the stag 3lt v8 can sort problems on this quicker .the superdream has been slowing me down hope you can sort this soon andy

Anonymous said...

reason its getting wet is the plug may work fine until under pressure then gives a weak spark. try swapping them over see if the problem changes sides

Paul Lungy! said...

HiTom,just starting to do up a 250n for a mate.It's a 78 model and had'nt been ran up for years.New plugs fitted with some fuel plus a new battery, amazingly it fired up with a little choke but not running very well,the n/s down pipe was hot but the o/s was cold. Disconnect cold side plug it would stall and would only run with the two plugs connected.Anyway carbs off a little stip down, not total strip and a good going with some carb cleaner. Carbs on 7 ran up,now the o/s pipe was hot.Next, tank off & took off carb tops i polished the pots @ pistons,main jet needles were cleaned with fine wet or dry.It fired up straight away without any choke,both downpipes hot,hot,running nearly at 100%,try it mate!

Bike Details

Honda CB250N Superdream
Reg No : ORH746W (ORH 746 W)
Reg Date : 1980
Orig Colour : Blue
New Colour : Black
Current Owner : Tom McQuiggan
Current Location : Horwich, Bolton, UK.