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Friday, June 25, 2010

CARBS DOUBLE-CHECKED

Don't know about you, but I always doubt myself before I doubt others! Feeling completely stumped by this problem, I decided to take the refurbished carbs to a professional mechanic to have them checked over. If I'd screwed something up and he found the problem, then apart from feeling a bit embarrassed, I'd at least have sorted the bike out.

Today I got my carbs back from the mechanic and basically, they're spot on!! So God knows what to do to resolve this issue. Think I might just let it all take a bit of a back seat while the sun shines and come back to it when there's bugger-all better to do.

Thanks to everyone for their tips, advice and general comments.

21 comments:

CB Paul said...

Hello just sorted out the problem with my bike it will not rev over 5000 rpm and plugs are black. Checked timinig and it was out changed CDI no different check rotor and when in gear the rotor moved but not the back wheel so the woodruff key has sheared. Just got to get tool to remove rotor and replace key hope this helps you.

MadDogMcQ said...

Hi Paul,
Are you saying that with the bike in gear, you could turn the rotor around and around without the rear wheel turning?

Effectively, the rotor was spinning on the crankshaft??

Klaas De Craemer said...

Hello Tom,

What about bringing the bike to a garage to have it tuned? I'm really convinced your carbs need syncing (it can't be done properly with the carbs out of the bike). You're not in a hurry so they take their time. :)

Cheers,
Klaas

MadDogMcQ said...

Hi Klaas,
I have the 4-piece Morgan CarbTune but can't use them to balance the carbs because the damned thing won't even run steady.

Like you mentioned, I'm in no hurry at this point, so I'm going to leave things alone until I'm in the mood to sort it out.

It seems that the British Summer is over now, so it won't be long before I'm stuck in the garage again LOL.

Anonymous said...

I managed to get mine running beatifully by putting in jets from a 400 into my 250 carbs.
My jets looked OK but they were obviously not fully clear even after a damn good clean and a blow thru with an air hose.
Are you sure your jets are spot on Tom ?
I was determined my Slug was going to run come hell or high water and Im happily commuting a few days a week when the sun shines.
I know the garage has checked your carbs Tom but for my peace of mind please check your jets are as clear as they should be.
Yes I know Im a PITA :¬P
GL Matey

CB Paul said...

I was wrong about the rotor key being sheared it was ok but do have some good news might be worth a try. My bike didn't rev over 5500rpm and plugs were sooty so running very rich air screws undone 1.5 turns removed air filter and bike ran spot on brown plugs no sputters so all electics and timming ok. washed out air filter of oil tryed again but run rich again undone air screws 4.5 turns and it ran 90% small splutter.

CB Paul said...

so not fixed yet but alot betterso carb trouble it is cant get air flow though carbs when on main jet

Klaas De Craemer said...

Tom,
This might be a very late addition, but did you ever do a compression test on the engine?

Klaas

MadDogMcQ said...

No I haven't done a compression-test yet because my compression gauge doesn't have the right fitment for these size spark plugs.

Mind you, the engine pulled strongly prior to the stripdown, so I don't think there's too much cause for concern.

I'm going to get back into it pretty soon. I fitted a brand new brake master cylinder to it yesterday cos the old one was leaking.

Klaas said...

Well, mine also pulled quite well but does not want to keep idling for longer than 15s (and it runs very hot). When checking compression, I only had 9 bar of the 13 specified in the manual. After removing the head, I even noticed a small piece was broken off of one of the intake valves.

Just saying... assumption is the mother of all f**k ups :)

Anonymous said...

Tom,
your symptoms are very similar to those caused by a leakage of air on the engine side of the carbs. The manifold rubbers do suffer quite badly with age and could be at least part of your problem. Good luck.
John

jesselaing said...

Hi there, have you had any luck with your bike yet? I have stumbled upon your blog trying to sort out this very issue! I have a 1982 cb250n Superhawk (dream)I also have tried 99% of what you have but to no avail at all.....

MadDogMcQ said...

Sad to say that no, I haven't sorted it yet because I haven't even been near the bike. I keep thinking of taking another look when I get home from work each night, but it's so bloody cold in that garage!

"George" posted what seems to be some very valuable technical information (below) and I'm going to check it out as soon as I warm up LOL.

GEORGE's POST:
Clean all conectors between Earth, Alternator, CDI and Coil. Undo the bullet connector on the short black/white wire from the CDI to override immobiliser (run and key switch) faults. If you havent got a spark that you can see in sunlight, youve got an aftermarket coil or the ignition winding on your alternator needs a recon. The spark coil must be genuine Honda. Cheap aftermarket coils are wired different and give a weak spark and bad idling. The original Nippondenso CDIs never burn out. Ultrasonic cleaning fixes a load of carb problems. David Silver sells photocopies of the Honda shop manual for £16. Job done mate.

Alternator resistance checks from Honda shop manual 1980.
CB250N and CB250T
Green-White 200-500ohms(Low speed coils),
Blue-White 4-7ohms(High speed coil),
Green-Light blue 100-200ohms (Pickup),
Green-Brown Not applicable,
Green-Pink 10-30ohms (Advance coil).
CB400N
Green-White 387-473ohms,
Blue-White 77-95ohms,
Green-Light blue 95-116ohms,
Green-Brown 76-92ohms,
Green-Pink 126-154ohms.
CB400T
Green-White 400-500 (300-400)ohms,
Blue-White 75-130ohms,
Green-Light blue 75-130ohms,
Green-Brown 73-130ohms,
Green-Pink 120-180ohms.

jesselaing said...

Thank you very much for your reply! I have just resistance tested as per info, and low and behold, "Green to Pink" resistance, none..... Im picking the advance coil has shat itself and may explain why it wont rev out beyond that 5-5.5k rpm range. has been a head scratch as this problem only occurred after changing a snapped throttle cable. I have had the carbs apart no less than 10 times trying to see whats going on... I will remove cover and flywheel shortly and do a little more digging. Thanks again for the info, I will let you know how I get on and will be keeping an eye on your little beasty also!
Regards
Jesse

jesselaing said...

I removed the CDI unit, peeled back loom, inspected all wires, all ok.
Removed all connectors from within plugs, sanded clean and reassembled.
I checked the advance trigger for CDI again, was supposed to be between 10-30 ohms, but this time was reading 15.8 meg-Ohms??
Couldnt remove flywheel as my strap spanner is at work, so dealt to the electrics/coils behind it with contact cleaner.
Started it up and straight away I could here it was sounding "happier".
Re-checked the "green to pink" advance wires resistance and it is sitting on 31 ohms which is very close to where it should be.
So it was an advancing issue after all, nothing to do with the carbs as I have had so many people telling me. The cause and true solution to this bloody problem still somewhat eludes me. I cannot provide a concrete answer apart from dirty terminals,connections.
Took it for a ride and its running the best it ever has. Thanks for your help, it has probably saved me an arm and a leg paying an auto sparky and/or mechanic to sort it out. Perseverance pays off!!

Anonymous said...

Had this very problem with my 400.Turned out to be a sticking valve caused by the stem oil seal deteriorating. Initially cured by taking off the rocker cover and giving the little sod a clout with my trusty hammer.New seals and it runs sweet again.

Fishbait said...

Tom, Did you check the breather from the petrol tank?

MadDogMcQ said...

Yes I did John. In fact, I ran the bike without the tank (using a makeshift reservoir) and it made no difference.

Fishbait said...

Tom, Are you sure the cables to the spark plugs did not go conductive? Have you looked at the bike in the dark to see if there is arcing?

MadDogMcQ said...

Yes John, I checked the plug leads and have even replaced them with the new coil. I've got 3 bikes in the garage right now, so as soon as my son sells his Kwak, I'll have more room and try to get to the bottom of this.

MadDogMcQ said...

Jesselaing - glad to hear you got yours sorted out 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.