Topic: slow runing problems

[Stormhunter]

Hi i have just had the carbs set by a bike shop as number 3 was jammed.driving the CB400/4 at 40+ it goes like a rocket no problems at all.When driving through town trying to keep to 30 or below the engine judders. i have set timing using a strobe gun and have set points.Bike has only done 13000 since 1976 no rust in tank have taken petcock apart and cleaned filter, drained carbs and found no bits in fuel.Can anyone suggest any thing it might be? its just like fuel starvation but plenty comes out of pipe.only problem with bike is that it keeps blowing headlamp main beam.
Cheers Chris

[TwoTired]

I believe any time there is some kind of performance problem with these bikes, a complete and through tuneup should be performed.  You've done most of it, already. But, cam chain tension and tappet clearances aren't listed in your report.  Also, I'd like some info on the spark plugs to; age, condition, heat range, plug number, proper gap.  The color of the deposits on the center electrode insulator might also give a clue.  Also, any differences in deposit color across the cylinders, might be interesting, as well.

A compression check would establish that the cylinder mechanicals are sound.  And, that a burned or leaky valve is not contributing to low speed run issues.

Is the exhaust and induction system stock?  Changes here can effect the separate carburator adjustments for different throttle settings.

Also note that the stock paper air filter can only be blown clean just so many times and then it must be replaced in order to get the proper air flow and restriction from it.  The air filter is supposed to be replaced every 6K miles, according to the Honda manual.  More frequently, if operated in dusty areas.

If you have addressed all the above, and there is still a problem, that leaves just carb adjustment.  And, you want all the above done beforehand anyway so the carbs are tuned to a properly adjusted engine and not compensating for something mis-adjusted.

Do you know if the carbs have been syncronized?

If you have eliminated everything else but the carbs, the bike shop ought to stand by their work and finish the job properly by completeing the tuning adjustment. 

Or, you can start with checking pilot screw settings.
At this point, you'll need to determine if the mixture is too rich or too lean.

Drive the bike at the throttle setting that annoys you (all warmed up), and apply partial choke.  If the symptom improves you are too lean.  If it gets worse with choke you are too rich.  And now, you know where the adjustments should be made.

Report back your findings and ask more questions if needed.
Cheers,

[Stormhunter]

Hi the spark plugs are brand new and were replaced before the carbs were syncd and set up. They are NGK D-8ESL and are gapped at o.8mm,cylinders 1&2 are slightly brown and 3&4 are clean.
Compression check was done on the 4 cylinders and they are fine. The exhaust is a motad 4 into 1 and came with the bike. The air filter is new and clean.
The only problem is setting up the points as cannot get the back plate round far enough to line up with strob marks,People who tried to get the bike going before i purchased it had used an impact screw driver on the plate and extended the bolt holes.
I have set the points up by feeler gauge and the set the dwell angle. It runs fine when accelerating but when you try to cruise at a steady speed it is like riding a kangeroo. The carbs were striped and rebuilt by richard stevens in devizes who specialises in the older bike.
I will check the tappets and cam chain tensioner over the weekend and see if that helps.
Thanks Chris

[TwoTired]

I'm curious as to the reason why 3&4 are running leaner than 1&2?
Are the air bleed screws set equally on each carb?  What is their setting?  Have you tried turning them in 1/8 or 1/4 turn to check for improvement?

Can you equate the run problem to specific throttle positions?

Was the problem you had with the points resolved to achieve the proper timing?  Did you verify that your mechanical advance was operating smoothly?

Assuming no problems are found with the preceeding checks, then the carb mixtures will need to be addressed.  Since you are happy with the performance above 40, we might assume the main jet is of sufficient and proper size.  But, there is still the slow jet and slide needle to address.  Since your carbs have mechanically operated slides, it is important to note at what throttle positions the undesireable symptoms occur.  You may consider temporarily marking your throttle so that you can easily observe and equate just what aspect of the carb is not performing.  Then we can focus on that area to make changes.

Cheers,