Black smoke and hanging idle after a full rebuild

[thomaspigeon]

Thanks for your reply guys.

I doubt any problem is coming from the engine as I rebuilt it and done all the test to make sure everything was working ok.

Now changing for #35 as a pilot could be a good idea. But I’m running pods which si seems very small, though those jets are #38 and might have worn out. That can be considered.

Also lambda sensors are definitely something I might start to consider too

Thanks
Tom

[fogrider]

My bike lent itself to fitting a Lambda sender as it is the 4into1 style exhaust with an aftermarket silencer - that fixes to the end of the 4into1 with an adaptor sleeve. I simply brazed  the sender boss into that sleeve.
I already had the Lambda system too, which helps, so I did'nt have to pay for a couple of dyno runs.
It's not a cheap or easy route to take, but if all else fails, it's a valuable extra tool to use, especially if you have other vehicles and can make more use of it in the future. I've used it on 4 different vehicles now, with good effect.
I'm not sure I'd have bought it just to sort the CB550.
Personal choice .

[thomaspigeon]

I've ordered a repair kit for the CB350F carburettors. I rode it today and their is still a bit of smoke when hot so I guess I have to reduce some jet size somewhere!

It will arrive next week. Hopefully that will do the job. I'll keep you updated

[Trigger]

Black smoke is nearly always the piston to bore clearance, worn oil rings, valve guilds or seals 

[fogrider]

So when a float punctures, or an air filter is clogged, the choke sticks on, dirt on a float needle , a main jet falls out etc etc, that gives blue smoke ?
Or maybe I'm colour blind !

[Trigger]

Why would a main jet fall out, it is held in by the spring and compressed by the float bowl 

Once upon a time i built a 550 motor,  the bores were with in spec so, i just put new rings in with a light X hatch. The carbs were fully rebuilt but, the bike smelt very rich and had a small amount of black smoke. After 3000 miles and forgot how many times i had the carbs off, i decided to strip the engine back down again and check the head and barrels. Everything was with in spec and as i had a set of 0.50 pistons and bores sitting on the shelf, i decided to fit them.

Now that bike has never smelt rich or given any black smoke since and Julie is still knocking up the mileage on it 

A bike is many components and not just a bank of carbs 

[thomaspigeon]

So this CB has been rebored at +0.5mm with new pistons fitted, new valve seals, valve have been  lapped too. I’ve done a compression check and it has 145psi (everything is new I assume the compression will raise after the run-in period).

Hard to believe I have a leak somewhere. I had the valve play inspected by a specialist, he said it was ok.  Unless I forgot some lapping paste on a valve (I also cleaned everything with cotton buds and petrol) I doubt it’s possible I have any leak.

[Trigger]

145PSI is too low if you have had the top end done.  Running pods will never help you as, these old girls don't like them 

[royhall]

Agree with Trig. Hands up anyone who has got these bikes perfect with pods fitted. They usually end up as some sort of compromise.

[hairygit]

145PSI is too low if you have had the top end done.  Running pods will never help you as, these old girls don't like them 

Running pods, especially some of the ones I've seen have appalling filtration, basically they can knacker a rebuilt motor in a 1,000 miles as abrasive dust, grit etc gets dragged straight into the motor. All very well assuming that because racers use them they must be a good bet, but most engines are rebuilt after every race!

Sent from my JNY-LX1 using Tapatalk

[thomaspigeon]

So I also forgot to mention. My compression tester is a rubbish one. It doesn’t have a valve on the spark plug side  but on the gauge side. Meaning my compression reading should be a way higher with a proper tool.

Now I’ve done other engine rebuilds with this tool. And I always had a low reading with it. I never bothered changing my tester as the engines were running strong all the time after my rebuild.

About the pods. As I said previously, the previous owner discarded the airbox, bracket included so I didn’t have the choice really. I’m running K&N pods with oil on them. I run K&N filter on my africa twin and it’s spotless. I agree that velocity stack will knacker any engines. But pods are not supposed to if they are from a good brand

[thomaspigeon]

Now I know pods are a compromise and I’m ready to be wait until I get the right balance. I’m sure I’ll get there. It’s just a question of time

[fogrider]

Thomas, something to check while the carbs are in bits: mine was way over rich the full range, one of the things that seemed to make a massive difference was the seatings for the emulsion tubes. I was a bit slow spotting this , but a minute examination of the emulsion tube alloy turret showed that there was ally gunge on the seating face, also minutely on the brass seating face of the tube. (needle jet end ). It took a multi-facet milling cutter (6mm) turned carefully with fingers only, to clean the ally seats  up. Also, the turret had minor surface faults inside where the main jet O ring sat that needed carefully polishing out.
In addition, I felt that the main jets were not fully in despite being pressed to where they felt "right". I put a main jet in without its O ring and clocked the depth it sat. I found it needed a steady push to make the jets sit fully home. I'm certain that all the above  cured some fuel leakage past the seating face in the depths of the alloy turret.
Can't prove it, but I think its an essential thing to make sure is spot on.