carb, old bike= dog chaing its tail

[redcoat]

Hi,

1978 CB 400 twin. More miles than the Starship Enterprise.

Gets terrible gas milage, about 60 miles to 2 Imperial gallons (24 mpg-US)), or 110 km per 10 litres. Blows black, not blue smoke, on acceleration.

Rather than fettle the carbs myself, I took them to the dealer, and asked for them to be set to factory spec- proper sized jets in the proper holes- as when I put it together I had a box of mismatched bits and $10. I hoped by having them set up as per spec, I would remove one variable from the equation.

They assured me that they had been cleaned through out, all the little orifices blown out with comp air, new jets.

There is no way to adjust fuel height- the floats and arms are blown plastic.

Fuel height might be slightly affected by worn fuel cut-off needles- the rubber cone gets worn and the jet gets crammed higher up the hole, and the float arm gets to float higher. Hence, the cut-off needles were replaced with new OEM items. Pattern parts are sometimes less than exact.

Bike idles fine, runs around town fine, burbles a bit- suspecting rich mixture. Removed air filter. Ran a tiny bit better.

Doesn't need choke to start.

Back fires a bit on deaccelaration on the freeway.

All of you must by now be screaming

"Main jets are too big!"

But I paid the dealer to do it right...

1 Is there something else on an old, worn bike that would create a similar problem? intake valves open too much/too long? (Hardly the case I'd guess, old valves pocket and don't give enough mixture)

2 Fix the symptoms rather than the cure, and fit smaller secondary and and main jets?

If I could fit an Amal monobloc carb, I would in an instant.

Justin

[honda750]

dude
1. go on ebay
2. buy an old carb
3. its not worth buying all the damn parts.
4. i have carbs for an old 1974 cb550f
PM me if u want to buy them.
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Chilean Forum

[redcoat]

Ummm...thanks for your offer, but the linkage and throttle mechanism is completelt differet.

I intend riding this bike for the next thirty years. I have spare engines, wheels, frames etc.

My bikes have survuved 30 hard years thus far. New parts should get me close to another 30.

I have replaced the main .110 jet with a .105 and have noticed a very small improvement.

A new air filter reduced efficiency by 10%- but at least no dirt in the bores, right?

I have ordered needles and seats from David Silver Spares in the UK. It is the last ossible thing it could be that is causing such fuel guzzling over the entire rev band.

Other ideas?

[honda750]

sounds like you've ruled everything out. post again when u put in the new "poo poo" if it still isnt running right.
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Opiate Rehab Forum

[dragsta]

that bike should get over 70mpg.

ignition problems can mimic carb problems.

check the plugs, timing (if adjustable) spark and do a compression test then post the results.

[redcoat]

I know, I know. It hurts every time I pump gas. I've read 57-59mpg, but that would be US gallons, not imperial. I'd be happy with 45-50!

It's a CDI unit- no points, and a coil, all solid state. No room or place for any adjustment. The ignition systems on these things are faultless- they always work, and on the odd time they don't- it's a plug, and very rarely, a bad CDI- and they work, then don't work. Simple enough. None of that intermittent stuff from points and condesors (but way more expensive)

I'm really coming to the end of my tether on this one

I've got clouds of black smoke, wet black plugs...it HAS to be mixture, right? What could possibly be another reason?

Could you be right? can the timing of the spark change with a CDI? Pulse generator nudged?-

[MrShake]

Sounds like the carbs arn't syncd/tuned well. I would not suspect the jets until you have a REASON to suspect the jets.

1. Put the air filter back in
2. Start the bike w/o choke and run it at 3500ish rpm for 45 or so seconds
2. Pull the plugs and describe them

Are they black and sooty?
Are they covered in white ash?
Are they a nice chocolate brown?
Are they damp?

The plugs will tell you if you are running rich or lean, and on which cylinder. Adjust your pilot mixture screw to fix the problem. In is leaner, out is richer.
(I'm assuming a cv carb here).

Go from there. As well, check for vaccum leaks and run a compression test.

[redcoat]

You know, it might be helpful to read the posts I have made on the subject, then you'd know I've given you your answers about 4 times thusfar in this thread.

The carbs were synched last June after the rebuilt carbs went back on.

[flynrider]

I've got clouds of black smoke, wet black plugs...it HAS to be mixture, right? What could possibly be another reason?

Yes. Black smoke and wet plugs are a clear indication of a rich mixture. The tough part (as you know) is figuring out why it is so rich. If you're sure that the jet orifices are correct for your bike, then the only thing left is the float system and the intake system. You mentioned that the floats are non-adjustable, but have you checked to see that they fuel level in the float bowls is correct. There should be a simple procedure for doing this in the manual, using a clear tube attached to the bowl drain.

That said, I do not envy you. I've run across similar carb problems over the years and was only able to solve them by redoing all of the carb work that had supposedly been done correctly the first time (i.e. tearing down the carbs and rebuilding by the book from scratch). Good luck.

[redcoat]

If I attach a clear tube to the float chamber drain, can i get an accurate result while the carbs are still on the bike? I was thinking of the clear tube from each carb, putting the bike on the centre stand, and taping them to the fuel tank in a big u.

My only fear is that the bike sits slightly nose down on the centre stand, and hence, the fuel may show this incline? Am I right?

Or should I do a bench test?

And if they are wrong, then what? The floats are not adjustable...

Damn I wish i had an old Amal monobloc carb on it...