Carb issue

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Stephen Hill

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I recently installed anodized slides, needles, and new needle jets in the Amals on my 1972 Norton Commando. The slides, needles, and jets were the same spec as the parts I removed.
The bike ran fine before I installed the new parts.
Now, when I am roll the throttle on hard, the engine loses power, starts to miss, and when I back off the throttle, it surges ahead. I think the missing sound is called 8 stroking. Which apparently can be caused by an excessively rich mixture.
Should lean the mixture by raising the needle? Or putting in a smaller main jet?

Stephen Hill
 
In my humble opinion the float needle jet is the most important thingy to replacy. The new type rock , super light.
 
yes, you are now too rich with less air being let in the mixture with the new parts, I would drop the needles one notch and test ride
 
Stay up floats, aluminum float valve, check float height and drop the needle. Main jet is only really applicable when at the wide open end. Pretty much what everyone is saying above. Personally I think Norton’s run rich and always have plugs on the sooty side.
 
Actually having re read your post, you might be to lean. Old parts = rich, new parts (tighter tolerances) = lean. Raise the needle! Talk about sitting on the fence! At any rate first thing to do is play with needle heights both up and down and see how you go.
 
My 70 commando with a 21 front sprocket would get up to about 83mph and hit a wall. More throttle didn't make it go faster, but IF I gave it more and then backed off, it would speed up slightly before it backed off...

My theory was the same as yours. I was either too rich or too lean in that throttle range. Since, in my case, the range was wide open throttle I suspected the main jet size was incorrect. I had 240 mains in the bike at the time. (it's a stock 750, but has a little higher compression from flattening the head and barrels) It's spec'd to be 220 main jets as the stock size. I had numerous jet sizes in my parts bin, but I figured I'd try the specified 220's first. I have a jet "key" tool and the type of float bowls where the main jet can be removed through the drain hole. I tested the 220 main jets first and there was no need to test anything else. I zoomed right past the 83mph mark and have hit 100mph since then with the 220's in place.

that's my single data point on amal carbs.
 
I agree with the “old parts = rich” comment as jets do wear, which makes them bigger, which makes them richer.

However... the OP said “when throttling hard... when I back off the throttle, it surges ahead”

Now, it really depends what the OP means by “throttling hard” but if he means being at, or close to WOT, then that is a text book symptom for being too lean on the main jet.

I would urge the OP to tell us exactly what jets and needles and slides he has in the carbs, then folk with similar machines can comment if they think anything is amiss.
 
Just to throw this out there, wouldn’t worn slides cause a lean condition except in the WOT postion? I agree worn jets and needles will result in a richer condition. It almost seems that as everything wears out as a unit they offset each other, but I bet the needles/jets wear quicker than the slides. Just sitting there ratting around at idle has to beat the needles and needle jets up pretty bad.
 
If by hard throttling you mean wide open the main jet sounds too lean based on the symptoms. The main jet dominates above 3/4 throttle. This is from John Healy.
"To check to see if the Main Jet is too small, experienced tuners will do what is called a “Roll Off” test. “This is based upon the fact that as the throttle is closed the air/fuel mixture momentarily becomes richer. The Roll Off test takes advantage of the momentary richness created as the slide is closed. If the bike gains power the Air/Fuel mixture is too lean and you need a bigger Main Jet."
If it happens below 3/4 throttle that is where the needle and needle jet dominate. Chances are your old needle jet was worn and the bike was probably rich, although the needle position might have been set to compensate for it. A new needle jet would lean the mixture. In that case you want to raise the needle.
To get an idea where your problems are put a piece of tape on the handle bars and mark the throttle position, 1/4,1/2,3/4 and WOT.
Here's the article Johns quote is from.
https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf
 
So there are three parts which jet to the throttle opening - the idle circuit and slide cut-away, the needle jet and needle, and the main jet.
If you have replaced the main jets with the same size jets you should not have a problem when you use full throttle. The way to get the mid-range jetting correct is to lower the needles until the motor coughs when you ride the bike changing up and down through the gears. After you get the motor to cough, raise the needles one notch. This is AFTER you have checked your float levels and made sure you have got adequate fuel flow into the float bowls. If you have a blockage in the idle circuit, you should know - you won't get a clean pick-up when you start to open the throttle and you would have trouble getting the motor to idle properly.
 
A blocked tank breather can cause a partial vacuum to form in the fuel tank and cause the carbs to starve of fuel.
 
The staggering on heavy throttle, with its distinctive miss, followed by a surge when the throttle is rolled off, sounds a lot like what is called 8 stoking. Which apparently takes place when the mixture is too rich. I think I will lower the needle and see what happens. Worse case scenario I need to remove the top of the carbs and raise it.

Here is another quote from John Healy: "8 stroking is when a 4 stroke engine is so rich it needs to go through another 4 cycles to get enough oxygen to produce a combustible fuel to air mixture. Instead of the engine producing a smooth brrrruuuummmmm... as it accelerates, it goes bah, bah, bah, bah. Often the 8 stroking will be accompanied by black smoke coming out the tail pipe."
A buddy rode behind me and told me he could smell gas in my exhaust. I'm thinking too rich.

Stephen Hill
 
What jets, needles and slides do you have?

What bike: 750 / 850? Combat, none Combat? Standard / modified?

It’s highly unlikely that your bike is unique. If you tell us the a above, you’ll get almost instant repsonses from those with good running / correct settings and you’ll save yourself a ton of guesswork and experimentation...

Although these carbs are simple, it’s still incredibly easy to make a real balls up of it !
 
If you want to definitely find out what 8 stroking at full throttle is then remove the main jets completely and go for a ride, at WOT it will 8 stroke and you then know if before was 8 stroking or lean.
 
It is very possible that when installing the slide/needle combination that the needle and its clip travel up the spring resulting in a very rich mixture on one side. Check here first. Is one plug blacker than the other?
 
It has to be something such as the needle hung up on the spring. If you have replaced everything with the same spec. components, it should work perfectly. You cannot get 8-stroking caused by too big main jets unless you have got something radically wrong. In normality the richness would only occur at 3/4s throttle. On full throttle the mains would meter correctly. It takes really oversize main jets to get 8-stroking on full throttle, I don't think it can happen. The only other thing might be one of the float bowls flooding.
 
Remember, that, here in the US, the 10% corn gas has LESS thermal energy per volume than proper fuel 40 years ago. So, going bigger on mains (versus stock) is a good idea to get the power output/stoichiometric ratio we want in the old engine.
 
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