Runs well, plugs foul....

Lineslinger

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According to Roy Bacon's Norton Twin Restoration: 850 mk2a option for 74 would be 220 mains. The carbs should be stamped R 932/33 and L 932/34 My 74 850 Interstate had 260 mains but when I changed the cam(Megacycle) required 270 mains. My guess and Onder's is that the floats are influencing the needles during riding.
Mine correspond including size of new jets.

You mentioned “floats influencing the needles during riding”.....as in opening too much or simply allowing an irregular/unpredictable flow?
 

Lineslinger

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I suppose next logical move will be to look into the float level adjustment “tool” in the link you offered.
I was fairly confident I had those floats tweaked properly but will make sure.
Thanks for your time.
 

acadian

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Is it rich at all throttle positions? You mentioned the
A bit lengthy but please bear with me, just making info as complete and accurate as I can.

1974 850 Cmndo. The engine starts and runs well through full RPM range...until the plugs start to foul...within 30 miles.

I am running twin Amal 932's that I have overhauled. New slides, 3 position needles, main jets (standard 220's), floats and float needles. Synchronization between the two is good.

Tri spark ignition with ign. pack, fresh plugs and leads with proper resistance...set dead nuts on at 28º BTDC.

While overhauling the head this fall, I went back into the carbs as the combustion chambers and plugs displayed rich mixture in color/deposits.
Upon reassembly I set the needles at their upper most setting, looking for a leaner offering.

The carb work was meticulous. Idle jets clear with smooth operation throughout. Reassembled everything and fired on 3rd kick. No air leaks around the fitting faces.

As I rode and started and tweaking the air adjustment screws to find an acceptable mixture I noted how far in the air adjustment screws needed to be turned from recommended 1.5 turns out, 1.10+ back in apprx., thus enriching the mix.
I attributed this, partially to the K&N air filter allowing more air intake.
Popping gone, decelerates smoothly, good throttle response.

Dirty/a bit sooty plugs found when starting became difficult.

My next step is to adjust the needles one setting, to the #2 position, richer, and reset the air adjustment screws to 1.5 out from seated and see where that takes things.
I have other ideas but would like to hear from those the have been there - done that.

I have a clue as to how carbs work, Holley's, Webers, SU's.
I really like the simplicity of the Amals and know there is a sweet spot in there somewhere. I would like to find said sweet spot rather than getting pissed off and end up sitting on the curb beating my dick with a brick because the beast won't run or start.

Thanks for reading.
If you're certain you've eliminated ignition as the culprit (not doubling up on resistor wires/caps etc.), this guide is your friend:

https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf

Make sure the problem is at all throttle openings, if so, the float level is the only thing that will effect mixture at all openings, if you can get your hands on a colourtune you may be able to quickly ascertain if this is the case.
 
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Try some stay-up floats, Mark. They are easily height-adjustable. Sounds like yours may be a little high. Does it idle without the choke before it's warmed up? If so, drop the float height a bit. It should almost die with no choke before it's warmed up. And I'll front you a pair of Champion N4G plugs. Much less prone to fouling than N7Ys or the equivalent NGK plug.
 
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Doubt it is electrical, the bike would 'rev out' early if the coils and electrics were not up to it. If it still blackens plugs at a stint at WOT then I doubt it is the fuel level. Not been mentioned yet, chokes? as this is affecting both sides, and I doubt both carbs are likely to carry the same fault, or build error if there is any on either carb.
 

tomspro

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One factor I have not seen mentioned is your altitude.
I have very similar setup on my 750 and use 200 main jets. Even 190s ran well but would not deliver at WOT.
I am at 5k so O2 is thin and fuel must be thinned to match.
 
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Upon reassembly I set the needles at their upper most setting, looking for a leaner offering.
Always ambiguous (for me at least). So when you state "needles at their upper most setting", I interpret that as the needles are higher thus richer transition and mid range, where most of you street bike business takes place.

If the needle clip is in the upper most slot, that is the leanest position. If your needle is in the lowest slot it is richer.

Did you change the needle position (to slot #2) as you had stated earlier and was there any noticeable change?
 
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Do you keep your battery on a tender, do you ride with lights on? Sounds you have the carbs covered, so maybe electrical, does tri spark need a good battery? I had a similar problem on one cylinder, turned out to be a bad coil after I had been down the fuel route.
 
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I asked about the exhaust because the book likely is predicated on black caps and cross over. 2a may be different spec. Check.
If they are Andover silencers they will be fairly unrestricted so Id guess 260 on the mains. Mind that is just a guess.
But the level of the float will affect how much you need to screw in or out the air mixture screw.
You need to run up a tube affair as suggested to actually know where the fuel level is not just where the float is. Simple enough to
do.
One of the problems of modern nonleaded petrol is you no longer can do a plug chop and get an accurate idea of carb tuning.
Remember that it isnt just the mains that affect things it is the whole shooting match combined.

You could test the coil with a multimeter just to KNOW what the ohms are on
primary coil and spark output.
 
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Agree that for normal street riding you arent into the mains at all.
 

Lineslinger

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Is it rich at all throttle positions? You mentioned the


If you're certain you've eliminated ignition as the culprit (not doubling up on resistor wires/caps etc.), this guide is your friend:

https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf

Make sure the problem is at all throttle openings, if so, the float level is the only thing that will effect mixture at all openings, if you can get your hands on a colourtune you may be able to quickly ascertain if this is the case.
I have a tough time interpreting mixture balance at specific RPM,s....

I’d be willing to buck up for a Colortune if it is a reliable reference. Plugs don’t “read” with Ethanol. Have you used it with success?
The instructional video looks encouraging.
I have been using the Princeton Amal PDF as my guide/reference.

Thank you for the help.
 

Lineslinger

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Doubt it is electrical, the bike would 'rev out' early if the coils and electrics were not up to it. If it still blackens plugs at a stint at WOT then I doubt it is the fuel level. Not been mentioned yet, chokes? as this is affecting both sides, and I doubt both carbs are likely to carry the same fault, or build error if there is any on either carb.
Chokes have been removed, little to zero cold weather riding and I just never used them.

Thanks for your thoughts.
 

Lineslinger

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One factor I have not seen mentioned is your altitude.
I have very similar setup on my 750 and use 200 main jets. Even 190s ran well but would not deliver at WOT.
I am at 5k so O2 is thin and fuel must be thinned to match.
I am in Missouri, 500 ft. Above sea level. I checked around, seems to be a non issue but thanks.
 

Lineslinger

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Always ambiguous (for me at least). So when you state "needles at their upper most setting", I interpret that as the needles are higher thus richer transition and mid range, where most of you street bike business takes place.

If the needle clip is in the upper most slot, that is the leanest position. If your needle is in the lowest slot it is richer.

Did you change the needle position (to slot #2) as you had stated earlier and was there any noticeable change?
The needle jets and needles are new with the circlip in the top, upper most notch of the needle seeking a leaner condition.

I have yet to move it until I can digest the various and helpful information you guys are offering.

I am still shaking this rebuild down so my riding so far has been very conservative until my level of confidence sets in.
 

Lineslinger

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Do you keep your battery on a tender, do you ride with lights on? Sounds you have the carbs covered, so maybe electrical, does tri spark need a good battery? I had a similar problem on one cylinder, turned out to be a bad coil after I had been down the fuel route.
New battery always on tender. Appreciate the sharing.
 

Lineslinger

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I asked about the exhaust because the book likely is predicated on black caps and cross over. 2a may be different spec. Check.
If they are Andover silencers they will be fairly unrestricted so Id guess 260 on the mains. Mind that is just a guess.
But the level of the float will affect how much you need to screw in or out the air mixture screw.
You need to run up a tube affair as suggested to actually know where the fuel level is not just where the float is. Simple enough to
do.
One of the problems of modern nonleaded petrol is you no longer can do a plug chop and get an accurate idea of carb tuning.
Remember that it isnt just the mains that affect things it is the whole shooting match combined.
You could test the coil with a multimeter just to KNOW what the ohms are on
primary coil and spark output.
I have yet to put the meter on the output side of the single coil conversion pack (from CnW) but will check.
I did get whacked by it the other day while pulling a plug wire at idle. Snapped me hard enough for some creative language, but no down side to double checking.
Interest is appreciated.
I plan on mocking up a float level check this aftn.
I keep leaning that direction as my culprit. Thanks.
 

Lineslinger

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Try some stay-up floats, Mark. They are easily height-adjustable. Sounds like yours may be a little high. Does it idle without the choke before it's warmed up? If so, drop the float height a bit. It should almost die with no choke before it's warmed up. And I'll front you a pair of Champion N4G plugs. Much less prone to fouling than N7Ys or the equivalent NGK plug.
Hey Dan,
My floats and needles are new, but not “stay ups”, do you find these more reliable?
Inlet needles are aluminum black tipped. New as well, came with the rebuild kit.
The chokes are gone and it does not idle well until some heat comes into play whereupon it smooths out nicely....but, it does tend to labor or stumble somewhat below 1,000 rpm, in other words it’s not smooth below 1K.
Might this tendency indicate too much fuel-not enough fair?

I am running BPR -7ES which I have been cleaning when fouling starts. I have a fresh set of NGK B7ES which I was going to install once I confirm my float bowls are accurate.

I need to get that horn to you anyway, trade you for the N4G’s?

Float bowl level is continually noted since I started this thread and I must admit I simply did not give it the attention it obviously requires, maybe the rich condition is that simple?
 

acadian

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I use a colourtune when I need to get a quick idea of whether the mixture is off at a given throttle position, about all it's good for but it does the trick.

Bear in mind that recommendations "now" being published by Burlen are a bit different than those from the past, or provided by Princeton/Bushman etc... The recommended "Fuel level" (not float height) tends to raise the float some distance above the bowl edge, though the carb body is recessed enough on the underside to allow this, just make sure there's no fouling if you get it to the suggested level. I've got my "float height" set pretty much level with top of bowl if not not slightly above, runs well, but I know it's technically still below Burlen's fuel level suggestion... these carbs are pretty rudimentary when it comes to "fine tuning", more wooden stick than Stradivarius

You don't need to mock up any fancy rigs, just get the fuel in the bowl till the valve shuts and measure down from top edge with a business card cut into a "T" shape.

Any chance you are running a kill switch?
 
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Lineslinger

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