running rough on neutral throttle

Twosheds

Twosheds

Junior member
Location
Surrey. England
Hi
I guess this is a carb probem on my Jota. Its done it all the time I've had the bike so it's not a new problem but it's about time I sorted it. I'm a Dell Orto dunce so some pointers would be appreciated.

Symptoms are that it start fine. It accelerates strobly and smoothly all the way to the red line. It decelerates on the overun fine but on a neutral throttle, holding a constant speed, it runs real rough especially at between 2000 and 5000rpm it is rough as hell. It doesn't do it on the stand in neutral though. Any ideas?
Maybe this is just what Jotas do but its a pain in the backside and my 1200 doesn't do it.
 
Sounds like the carbs may not be balanced.
Running strongly to and from the redline masks any carb balance issue.
Does it have rubber link pipes between the carbs? Are they perished?
If it has them that makes carb balancing even easier as you attach the vacuum gauges to the brass pipes once you pull off the rubber tubes.
When was the last time they were balanced?
When was the last time the carbs were stripped and cleaned?
How long is it allowed to sit without starting?
Is there any crap in the fuel filters in the carbs or fuel taps?
When last were the spark plugs changed (and what grade and make are they)?
When were the spark plug caps last changed? When you do change them trim 1cm or so from the cables before screwing in the new caps.
Are the accelerator pumps squirting the correct amount each time you twist the throttle?
What jet and needle sizes are in the carbs?

Mid-range revs under load put a different strain on fuel flow and the ignition system than under no- to light-load.

Look at all these areas and let us know what you find. Try the search function in this forum to check out previous comments and threads on this topic.

I think the answer is in there. If not, or you are unable to check these things take it to someone who knows. Where in the world are you?
 
chrisk said:
Sounds almost normal to me, engine is coming onto song at 4500> with Jota cams. Just rev the thing harder.
Click to expand...

Is that right? Anyone?  Is it normal that it should not run smoothly unless accelerating. I've not had that problem before on other bikes with lumpy cams.

 
chrisk said:
Sounds almost normal to me, engine is coming onto song at 4500> with Jota cams. Just rev the thing harder.
Click to expand...

???

Carb tuning has very little to do with revs. Its mainly about throttle opening.

Since I?ve been on the board I?ve noticed a tendency for the bikes to be over jetted.

Surging at  constant throttle up to about 1/3 open is often a sign of too rich a mixture during progression.

How many turns out are the mixture screws for best idle?

I?d drop the idle jet down and if the main is way over OEM I?d drop that down too. See how that goes.

Rod

Rod
 
Could be an idle/transition circuit blockage. Throttle openings can be quite low when cruising at higher rpm because the engine wants to make power, but is being held back with a low throttle opening.

Try this:
Take air box and rubbers off the back of the carbs - warm bike - 3,000 rpm - splat the palm of your hand against the back of one carb at a time.

The sudden draw on the idle circuit can pull shit out.

Alternatively, your idle screws could be set way too lean, causing lean surging.

HTH, Paul
 
Tell us what main jet, pilot jet, needle type and needle position.
A (UK) Jota would most likely come with 55 pilots, up to 145 mains and K1 needles in the middle position on the slide.

With that set up the mixture screws should be around 1.5 to 2 turns out.

Of course, wear and tear, blockages and such will also affect things.
 
So:

1). Starts and ticks over fine - that's idling and choke circuit out of it.
2). Pulls strongly and smoothly all the way to the redline - that's throttle well open and so that's main jets out of it.
3). Doesn't like partial throttle - so maybe you should concentrate on the primary jets and primary circuit blockage?

Just a suggestion as I am not a carb guru. However I do have experience with primary circuit.

Of course carb synchronisation, balance pipes etc.
 
If the problem is between 2000 - 5000 rpm then if it is a carb problem then its more likely to be a needle problem.Are they all the same and set the same?The needles might just be worn?

Andy
 
OK.... lots to check and think about here but I'm going away for a couple of weeks and I'll get back on it when I get back. Thanks everyone for your interest and suggestions. Together we will get this thing running properly.
BTW... it's a 76/77 bike running on the standard Bosch ignition.
 
Ah, just remembered.

I did have exactly the same symptoms once! One of the header pipes on the exhaust had come loose. Tightened it up and it ran smooth as silk.
 
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