Topic: Rough idle, won't rev... Carb help?

[Borkunit]

I am experiencing a rough running condition on my 750K4 that caused me to park the bike a couple years ago.  I have been trying to resurrect the thing, solved my no spark situation, but am still experiencing the same issues with running idling.

The bike will start with choke, but runs somewhat rough.  At first it is reasonably smooth, but gets rougher as is gets warmer.  It will initally rev up, but as it warms up it wants to rev less and less.  Eventually it gets to the point where it will barely start and will only idle if I hold the throttle open fairly wide.

I had installed new plugs, and after having run a total of probably 45 seconds they are pretty sooty.  See the attached pic.  I am thinking this is definitely a carb issue, and it seems like its running rich based on the condition of the plug and the way it's running. I should note that I am not seeing any smoke out of the pipes.

When I began the process of trying to get this thing up and running again, I pulled the carbs and opened them up to see if they were dirty inside.  I had left the carbs wet when I parked it, so I figured they might be a bit cruddy.  The previous owner had recently rebuilt the carbs when I purchased it, so I figured they wouldn't be too bad...  They were utterly spotless inside.  I pulled all the jets and verified that I could see through the passages.  Never the less, I sprayed everything out with carb cleaner, re-assembled, checked the float height, bench synced(per instructions in the FAQ) and reinstalled.  I am currently running some cheap pods, but I am planning on going back to the stock airbox setup soon.  The bike ran fine when I purchased it and for several weeks thereafter, went away on vacation, came home to this running condition and parked it for two years.

So, do the facts of the case bear out my suspicion that I am running hugely rich?  Is the progressive poor running of the engine the plugs getting loaded up and fouling out?  Are there any suggestions or tricks beyond what I will find in the Clymer book?

[bistromath]

Honestly to me it sounds like fuel starvation. Check your fuel lines and petcock. Make sure there's enough gas in the tank. Make sure the gas tank cap vent is working (just open the cap to rule that out).

[Borkunit]

Hmm... That was my thought at first, which led me to look at the carbs to see if they were crusty. 

Fuel lines are clear.  Rebuilt petcock with new screen and seals (assembled correctly now).  Enough gas in the tank.  I'll check the cap vent next time I'm working on it.

Thanks!

[TwoTired]

How long do you run it with the choke on?  That will soot up the plugs.

If the soot is collecting after you remove choke, then it is definately too rich.

Do you have stock jetting?  Have the jets been drilled?

Where are your pilot screws set?

Are you pods clean.  Could they be over oiled?

Are your choke butterflies opening and closing fully?

Cheers,

Soot buildup on the spark plug insulator will shunt spark away from jumping to the ground strap.  As it builds up, it gets worse.

[Rob M]

Sorry to resurrect this post by someone else, but I am having this same proble with the cheap pods that I have on my 74 CB750.  I always assumed that pods meant increased air flow, hence the upjetting, but using the stock settings for the air/fuel mixture screw (1ish turn out) and even raising the needle jet to the middle notch, I am still fouling out plugs.  This is sooty dry fouling too, which is pure rich running, but I can't seem to wrap my head around how increased airflow results in running rich.  I just ordered a airbox off feebay, and some D6EA plugs just in case, so I am going to go stock to see if there is any difference.  Just to note there was a puff of black smoke just off of idle everytime which is why I raised the needle one notch)  All jets were stock, and undrilled.  All I am left with is that the pods, foam cylinder type, and cheap, are just crap.  I hope going back to stock is the miracle everyone says it is.

[TwoTired]

I always assumed that pods meant increased air flow, hence the upjetting,

That's a common misconception.
1- "pods" isn't a specification.  It's a style.  Not all pods behave the same on the inside, even if they look similar on the outside.
2 - They *may* help increase airflow at and above Red line RPM.  Intake speed is determined by displacement, cam, and valve size until the induction begins to produce limiting factors related to high air speed.  At anything below max limits, the actual airflow is exactly the same.
3 - The reason why jetting and adjustments need be changed is that the most pods reduce the carb throat vacuum, which is what pulls fuel from the jet.  Less suck, less fuel.  To get more fuel with less suck you need to have larger fuel delivery orifices.  You can see this for your self.  Use a small diameter straw in your milk shake, then a large diameter straw.  The larger straw provides more delivery or the same effort.

but using the stock settings for the air/fuel mixture screw (1ish turn out) and even raising the needle jet to the middle notch, I am still fouling out plugs. 

Ok, are you saying you ran these carbs with the stock air box and it didn't foul plugs?  Or, have these carbs always produced fouled plugs in your experience.

This is sooty dry fouling too, which is pure rich running, but I can't seem to wrap my head around how increased airflow results in running rich. 

It's not the air flow.  It's the throat vacuum that draws fuel.  If you actually increase air demand (volume and/or air flow rate), then the venturi could make more vacuum.  But, a filter alone can't make the engine ask for more.  You'll need a turbo or supercharger for that.

Just to note there was a puff of black smoke just off of idle everytime which is why I raised the needle one notch) 

Perhaps a terminology issue.  Raising the needles make for a richer mixture. Raising the needle clip leans the mixture.

All I am left with is that the pods, foam cylinder type, and cheap, are just crap. 

I can't argue that, especially for street use.

I hope going back to stock is the miracle everyone says it is.

Miracle?  You mean like all the stock units sold with the original bike?   
I'm not going to talk you out of the stock air induction system  It is superior to 90% of the "for looks" pods.

But, how do you know the carbs are clean (emulsion tubes).  What jet numbers do you have?  Are they Honda factory items or aftermarket needles?  What number appears on them?  What is the set up number stamped on your carbs?
How confident are you that the air jets and the internal passageways are clear of corrosion and and other crud?

Restore to stock and it will run well like stock.  But, it all has to be in stock configuration.
Um, what muffler do you have?

Cheers,

[Rob M]

Thanks for your attempt to decifer my earlier post, believe it is much better language than I was using when I lost spark again on a short ride this morning.  Unfortunately I have never ran this bike with the stock air box.  The box it came with was done in by what looked like a wiring fire.  I got the bike fixed to this point, but the lingering mixture problem has been driving me nuts.  I have a air box coming from feebay, so I hope it does the trick. 

Just to note there was a puff of black smoke just off of idle everytime which is why I raised the needle one notch)

Perhaps a terminology issue.  Raising the needles make for a richer mixture. Raising the needle clip leans the mixture.

I did raise the needle clip, which would lower the needle itself.  It is now in the middle clip position.  Still puffing black smoke off idle though.

Slow jet was 35 if I remember right, and the main was 110 needle clip was on the secong up from the bottom.  I checked the emulsion holes in the slow jet twice, but they were clean, sprayed out the throat inlet for the idle circuit well, but I am unsure if it is really clean as all get out in there, couldn't see, but i did what I could and figured that if it were only one or two that were clogged it would be the culprit, but not all four.

I have the 4 into 4 pipes with the 5 baffles, I am pretty sure, it's an early bike,  titled 72, but it may have a later 74ish engine and carbs. 

I can get you exact numbers and specs tomorrow if you are willing to help me sort this thing out.

[TwoTired]

You know the mains/needle jet also has an emulsion tube, right?  Those premix air into the fuel before getting sucked into the carb throats at the slide needle position.

Also see:
http://www.motorcycleproject.com/motorcycle/text/specs.html


Cheers,

[Rob M]

Okay, slow jet 40- cleaned the emulsion tubes though they were clean
main 110- did the same
needle jet- on the second up, so maybe 4th down
resynced the carbs together.
Exhaust stock 4-4
Checked the float level all at 26mm
Retimed to right at the F mark

Running with no filters at all and still fouling and faint black smoke from the exhaust.

Going to check the valve clearance just because it seems like the last thing that is even adjustable. 

Any other ideas, now that I have ruled out the obvious for you, hopefully.

[Spanner 1]

There is a similar thread here, this week, with the same symptoms as your bike....... loads of carb suggestions were tried to no improvement....... turned out to be bad plug caps causing a misfire... 1 bad cap can cause a miss on 2 cyls. on our 'wasted spark' ignitions.
( sometimes a missfire can be hard to hear/detect and not very apparent except that it blackens/fouls the plug IMO ).

[TwoTired]

Are your certain that the slide needles are stock.  More than a few posters had issues with aftermarket needles that had a different taper profile causing over rich conditions.

Cheers,

[Rob M]

Wow, Spanner, I think you may well have nailed it.  I don't know if or how bad it may have been misfiring, but I was doubtful that this could have been the case with both 1-4 and 2-3.  So I checked, one on the 1-4 read 5 the other 20 ohms on the 2-3 one read 5 the other 10 ohms. That's what I get for picking a random lot at the salvage yard and not checking.  I'll be getting a new set shortly.  Thanks.  Hope it does the trick.