New Amal 932 carb adjusting

Eric Bauer said:

Hi Chris - Not sure how new you are with these things so my apologies if my response sounds silly but if things are a little bit off - the bike will do awful things. I've been shocked by what being off on carbs (what you would think is not much) just will make the bike feel like its about to explode. If i were you I might try the Number 19 pilot jet and see if that helps. That seems to cause a lot of 850 bikes to run lean. The pilot jet and the slide height are the only things that will effect at ilde so the rest is basically irrelevant. I doubt a brand new carb is dirty. I used a 3/16" drill bit to set my slides and they are running about at that height now ( i marked it on the carb body). How quickly are you taking down the idle? If it's at 1300 and running fine after you start it - I might take it out for 5 miles like that then come back and bring it down slowly in exactly the same increments on each side. Have you already done an exercise like that? Also what do you mean by drowning in air when you rev it? If you rev it and it goes up in rpm and sticks there and takes a long time to come down it’s too much air. If you rev it and it drops back down below where you started and either slowly comes back up or dies it’s rich.

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Thank you for the detailed reply Eric

I am mechanically inclined but a newbie with carb tuning / syncing. I've not ridden as I make adjustments, I'll try that next.

You state you use a 3/16" inch drill bit to set your slide height, with 3.5 cutaways. Is this the cutaway height that sets the manifold-side slide height at the correct height... about 1/16"? I've been told 1/16" is the right slide height (manifold side) to create a consistent vacuum air speed at idle... yes?

I've set it with 3/16's bits (smooth end), with no luck starting it.

The easiest way to start my bike is to set the cutaway side height with a 1/4" dia. drill bit (smooth end) and setting my fuel screws at 1 & 1/4 turns out, then lowering the choke slides to full engaged. 3 to 5 kicks and it starts and idles (around 500 to 800 rpm). After a couple minutes I begin to raise the choke, very slowly - which is where the trouble begins - quite often it begins to stumble badly as I raise the choke (making me believe too much air is getting in the upsetting the idle vacuum?) Right now - my carbs falter without the choke engaged, or only very slightly raised.


By "drowning" I meant... while at a lower idle (approx. 1000), if I give the throttle a (fairly) quick twist, it dies immediately, as if adding air overwhelmed it... drowned it.

Typical slide height at the cutaway is about 0.200 inches, I think I used a 13/64" on mine. That'll get you in the 1000 ish RPM range. Air screws at -1/2 turns out to start.

Chris Larkin said:

By "drowning" I meant... while at a lower idle (approx. 1000), if I give the throttle a (fairly) quick twist, it dies immediately, as if adding air overwhelmed it... drowned it.

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It's been my experience that blipping the throttle will give you a stutter, although stalling is unusual. The cure is to slowly richen the mixture until it stops doing that. If the carbs have the correct idle jets it's usually only 1/8-1/4 turn in on the air screw to stop the stumble.

Chris Larkin said:

Thank you for the detailed reply Eric

I am mechanically inclined but a newbie with carb tuning / syncing. I've not ridden as I make adjustments, I'll try that next.

You state you use a 3/16" inch drill bit to set your slide height, with 3.5 cutaways. Is this the cutaway height that sets the manifold-side slide height at the correct height... about 1/16"? I've been told 1/16" is the right slide height (manifold side) to create a consistent vacuum air speed at idle... yes?

I've set it with 3/16's bits (smooth end), with no luck starting it.

The easiest way to start my bike is to set the cutaway side height with a 1/4" dia. drill bit (smooth end) and setting my fuel screws at 1 & 1/4 turns out, then lowering the choke slides to full engaged. 3 to 5 kicks and it starts and idles (around 500 to 800 rpm). After a couple minutes I begin to raise the choke, very slowly - which is where the trouble begins - quite often it begins to stumble badly as I raise the choke (making me believe too much air is getting in the upsetting the idle vacuum?) Right now - my carbs falter without the choke engaged, or only very slightly raised.


By "drowning" I meant... while at a lower idle (approx. 1000), if I give the throttle a (fairly) quick twist, it dies immediately, as if adding air overwhelmed it... drowned it.

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Sorry I misread that you were using the choke. I did not set my slides at the manifold, only the carb (fuel) side. I would not use the choke - that just adds a variable. If done right my British bikes will not idle cold. If they will that means your idle circuit is too rich. This is my opinion (I don't use chokes). I would mark where the carbs are synched on the idle adjustment screw, then crank it up a bit on each side the same amount so you can get it to idle. You can bring it down and you will know where that is but at least you can get it to run. I'd get it started and then take it for a ride to warm it up, then try and tweak it. I couldn't get mine to idle first off so I had to crank the idle screws up at first. Then once I got it running I set the air screws which will help the idle speed. Then I brought the idle screws back down once I got the air screws right. Lean and rich is tough. In my experience if you blip the throttle and it dies it's too rich. BUT if it's WAY too lean it will do that too. And that is one of the challenges. Another alternative is to do it the way you are doing it is before you take the choke off crank the idle screw in equal amounts up to where it is idling much higher - like say 1500 and see what happens as you take it off and then adjust the air screw then. Same difference - but if you set your idle cold it will be too fast when you get it warmed up - so you will then have to mess around with it again. If I can limit the tweaking I try to - it's very sensitive.

Also, I wouldn't get too frustrated - this stuff is hard. Maybe I suck at it but I spent a month tweaking my little T100R Daytona and I almost tied C4 to it and sent it back to the carburetor gods and it did the craziest stuff with the smallest of tweaks. There would not be 100,000 threads on the internet on this subject if it were that easy. And the Bushmans carb secrets are great but they wont help you when you're sweating kicking the crap out of it after it kicks back because its too lean because oh by the way the last time you tried to kick it the carb balance pipe came off and you were screwed but you didn't even know it Wonder where that story came from.

I think Htown nailed your problem on the last page. You have the wrong idle jet size in your carbs. It's happened numerous times before with new premier carbs. You have #17, and you need to replace them with #19 jets..

Without the correct idle jets, good adjustment technique isn't going to do anything to fix your problem


.

Out of curiosity, do #19 jets have an 0.016" (78 drill) orifice like the fixed jet concentrics?

maylar said:

Out of curiosity, do #19 jets have an 0.016" (78 drill) orifice like the fixed jet concentrics?

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I believe it is .019, so no.

The old 25 pilot bush has a 17 thou bore, a 16 thou drill breaks up any deposits but does not oversize the hole. So when they went to the premier pilot jet they stayed with the 17 thou hole and called it a 17 jet. But the new route for the fuel is leaner so they ended up going for a 19 jet which is 19 thou, best cleaned with an 18 thou sized drill.

If you use appropriate Mikuni needles. you probably have more ways of getting your jetting right :


Amal carbs are crude.

As you winde the throttle on, the needle gets drawn out of the needle jet and richens the mixture. It compensates for the loss of vacuum. With Jaoanese motorcycles there is a huge variety of MiIkuni needles used. And for some bikes, Mikuni has even supplied specials. With petrol, getting best power is about very fine adjustment. If you use methanol fuel the adjustments can be half as critical. To my mind, using Amal needles with petrol as fuel, does not cut it. The shape of the needle affects how fast you can winde the throttle on without the motor gasping. The port size has an effect on the shape of needle which is required. In my case, I just use the leanest (slowest taper ) needle and winde the throttle on in a controlled fashion.

It's simple physics, if you whack the throttle open on a crude device like an Amal carb you are going to have a slight bog or hesitation especially in a higher gear. The sudden increase in vacuum draws a large volume of air into the carb and the fuel takes a few seconds to catch up resulting in a momentary lean condition . The taper on the slide is an attempt to make it less of an on off switch. It's why newer carbs have an accelerator pump that when the throttle is suddenly opened sprays a shot of fuel into carb throat to counter the lean condition. You can jet an Amal richer to try to make up for this affect somewhat but then you are over rich for more normal riding or you can roll the throttle on instead of whacking it open.

They need to invent something less crude than the carburetor. But what could there possibly be? Amals are great. Of course they are crude. So are Mikunis. I think if you're going for perfection you're kind of missing the point and joy of old mechanical things. Just my two cents.

To avoid the lean condition if you pop the throttle open on a carbureted engine you either need a CV carb or an accelerator pump. AS noted, if you adjust so as to NOT have a lean condition when you go to WOT from low speed, your mixture is much too rich for normal operation.

“but they wont help you when you're sweating kicking the crap out of it after it kicks back because its too lean”


Dad always taught me: If it doesn’t start with a couple kicks/pulls/seconds on the starter, STOP. Figure out what it wants.

I’ve seen people spend HOURS setting idle mixture & speed on British twins.
Mindblowing.
10 mins. max.

concours said:

I’ve seen people spend HOURS setting idle mixture & speed on British twins.
Mindblowing.
10 mins. max.

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I've spent hours - I'm also new at it. There's no reason to make people feel bad about not understanding things - the point is to help bring people into the hobby and solve problems as a group. How long have you been working on them and riding?

Or you could go with the 850 short needle and slotted spray jet, this was needed to pass a noise test where at low revs the throttle was snapped open, with the previous 750 needle and straight spray jet the 850 just did not accelerate. The cure was to go for the short needle and slotted spray tube which gave acceleration with a snapped open throttle.

I have one on my B44 and it has improved low throttle response.

kommando said:

Or you could go with the 850 short needle and slotted spray jet, this was needed to pass a noise test where at low revs the throttle was snapped open, with the previous 750 needle and straight spray jet the 850 just did not accelerate. The cure was to go for the short needle and slotted spray tube which gave acceleration with a snapped open throttle.

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That was with muted silencers. With good breathing pipes the old carbs work fine.

Eric Bauer said:

I've spent hours - I'm also new at it. There's no reason to make people feel bad about not understanding things - the point is to help bring people into the hobby and solve problems as a group. How long have you been working on them and riding?

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I sure didn’t mean to hurt your feelings.
My (missed) point was that the procedure is not complex. If hours are being spent, there is something else at play besides idle mixture/speed.

concours said:

there is something else at play besides idle mixture/speed

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In my life long education since leaving high school, I learned what an analog computer was and how it differed from a digital computer. FACT: A carburetor is an analog computer. Just like a digital computer garbage in=garbage out.

This was reinforced the other day when ?Kara asked about dellortos. some one gave the link to a dellorto manual. I cruised through it to see the quality of the write up to compare with the amal booklet. I was very positively impressed at the slightly more description of the idle/low speed circuit.
The dellorto write up includes what I call "bleed" and "feed"(they call progression). The low speed function acts in two modes.
The low speed circuit has 1 fixed size fuel jet and 3 air holes.
The main idle mix orifice is on the engine side of the throttle slide. It passes fuel and air mix from the carb face open passage but idle mix air volume is controlled by the idle air needle screw.
In addition to the 3rd hole/3rd hole is in front of and is on the outside of the slide wall.

Next the 3rd air hole works first in bleed mode when the throttle valve is shut down to the idle position. Atmospheric air enters the hole in bleed mode and goes in the "mixing chamber" to complete the fixed fuel jet idle air mix ratio.
The air is from the adjusting screw AND blended with the 3rd air hole in the mixing chamber(little round aluminum slug in body.
The big AH HA
2nd next 3rd air hole act in progression or feed mode. Since the call for more speed, you raise the throttle there is now vacuum on the exterior of the 3rd air hole.
The idle/low speed fuel jet goes into full feed mode.
If the 17 is not enough in low speed mode it however will still work OK in idle mode by adjusting the needle.
If you run a richer 19 jet the performance is no longer limited/starved in feed low speed mode and can be adjusted for idle mode with a different air screw setting.

All of this analog functioning is to allow running up to the HIGH speed circuit which then takes over.
The dual mode port definition makes it easier for me to comprehend it's functioning. Seems simple enough to me why the 17 might be lean and the 19 is better .
The ignition advance setting actually is very influential to idle speed/vacuum so it can easily explain differences out side of carburetion parameters

Compared to my other Brit bikes the Commando's seem to "hunt" a bit at idle. I'm wondering if minor flexing of the timing chain is messing with the advance. The timing chains definitely have tighter and looser spots when adjusting tension. These bikes all have Pazon Surefire ei's. Maybe the timing chain flutter would be less noticable with points with centrifugal advance.