Monobloc dead spot

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Yasser Fernandez

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Feb 2, 2012
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Hi everyone.

I own a Norton Dominator 99. Using a monobloc carb that starts and runs very smooth except in a point.

¨ The bike would still run well but when you slowly opened the throttle the engine would slow down before it speeded up. You had to open the throttle quickly to get by the dead spot. I was used to that it wasn't bad. ¨ I used the exact words I found on another thread because the are ideal as a fault description.

Ideas?
 
Fault on carbs are usually at amount of throttle openings to correctly diagnose fault.
Has your 99 got a magneto with a automatic advance and retard? The reason I ask is that at that age it may be sticking.
 
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Ok. I will give you further details.

I was using before a carb that doesn't have this problem. It was a modest carb in terms of performance, but it idles well and the transition from idle to 1/8 was smooth.

My Domi has a Boyer Brandsden installed. It sparks 8 degrees idling and 32 at high RPM, just as the manual suggest. I have checked this with a strobe lamp.

More than that...if via the air screw I made the idle mixture leaner, it idles erraticaly but erradicates in some way this dead spot. If I persue an ideal idle then the dead spot come back.

I repeat, it is just there if I raise the RPM very gently..

My though is that the slide is worn, because of the usage, and because the cable pulls up not by the center of the slide itself, there is a tendency to lean down. This produces a small gap where air comes trought. When you pull the throtle this gap desapears in some way so the mixture gets richer and then the RPM goes down.

If you pull the throttle faster, then the transition needs more gasoline so the effects are imperceptible.

Do you agree with me? Any thoughts to cure this?
 
As an experiment can you try another slide? IMO as you suggest off idle problems are often related to a worn slide. IMO one symptom is an idle that gradually increases in rpm without touching the throttle, but then decreases in rpm if you open the throttle very slightly.
 
Unfortunately I have no access to slides.

Triton Thrasher! You always saving my live!

I read a very good Amal document where they suggest smaller needle jet and bigger slide cutaway using straigh pipes.

After inspecting my needle jet, there was a huge gap between the needle and the jet, so I decided to change it with a needle jet from a 626 concentric I have. It was not as high but definitely the jet calibration was narrower.

Using the same slide I made the cutaway higher from 4.5mm to 6mm.

There result was that from idle to 1/4 the engine was much more smooth and tuned.

But unfortunately I am still having this small dead spot.
 
The good thing is that you are not clueless about how the carb components work, so it's just a matter of figuring out a solution which gives you a steady idle and removes the stumble... I don't know where the idle jet is located on a monobloc and if it can be changed out, but if there's a chance to experiment with different idle jet sizes perhaps you can find a solution there.

I'm very good with amal concentrics on my commando. I use a specific technique to micro adjust my idle air screw position to get the smoothest acceleration. I do think that the screw position for the smoothest acceleration is not the same screw position for the smoothest idle, but the bike still does idle without stalling. IMO, a bike that stumbles when you go to accelerate from a traffic light is dangerous, so correcting any stumble is a priority, because I don't want to be run over by the car behind me who doesn't see that my stumbling bike has stalled dead in front of him...
 
Yasser Fernandez said:
slowly opened the throttle the engine would slow down before it speeded up
Click to expand...
idle/slow speed jet is a touch small. All carbs have idle port/passage(engine side of slide) plus the transition port(outside the slide) . If it idles good but leans out slightly on slow throttle opening then a slightly bigger low speed jet is called for. Just my 2 cents and similar to classic 17 or 19 on premiers
 
This information you are providing is extremely valuable for me. I understand that with carbs a middlepoint must be found, but sometimes my mind is not prepared for this....in Cuba traffic jam is not an issue so I prefer a perfect idle and maybe becomming a trickier driver.

Yes Thrasher, I am the guy in Cuba. New Idle Jet??? hmmmm BIG WORDS!!!! I just can work on it to make it look like new...LOL

dynodave. My idle jet is a 25. It would be difficult to make it a "30" by drilling it. I will keep an eye on a proper and handy tool to achive this goal.

THANK EVERYONE.

I feel like surounded by Gurus!

All the best
 
Many years ago I would routinely soft lead/tin solder the jets and drill them with my 60-80 drill set to find approximate jetting. If wrong I would resolder the hole and try again. When I found the right size I would buy the right jet.
Of course you can not do this with old concentrics, only the new "premiers"
In a way you are lucky because your idle/low speed jet is removeable.
 
Another clue. If I completely unscrewed the air screw the engine won't suddenly stops. It idles very low. I guess that the slide free play is allowing air to pass through. So it is definitely the slide.

Now. At this point and even with all this clues, I am not completely sure the RPM slow down it is due to weak mixture, should I?

P.D at higher RPM is pretty easy to realize if mixture is weak or reach, but not at this point.
 
Sorry. I removed the throttle stop screw and the engine doesn't totally stops. After some strokes it will surely do, but not instantly as it should.
 
I understand it is a not a big deal. But certainly a clue to confirm the real cause of the dead spot is the worn slide. The bigger idle jet could be a cure but not the cause itself.

The manifold isn't leaking air.
 
Certainly the first thing to try is enrichment of the idling mixture, with the screw.

You may have a slide with too large a cutaway. I have, at times, experimentally glued a sliver of cork to the edge of the slide, to reduce the cutaway. 1/32” makes a difference. I used cyanoacrylate glue.

A small slice of cork is not too likely to damage your engine if it comes loose.

I had a throttle response problem caused by the throttle cable being too loose in the lid of the carburettor. That was a Concentric.
 
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