Amal question

I have a motor using the Jim Schmidt cam JS1 and his lightweight pistons approx 9:1. I have 932 Amals for carburetion.
50 miles on the new motor
The carbs have the standard 750 jets, straight spray tube, two ring needle set in the middle position, etc. The motor starts and runs well but I have not tried the upper RPMs yet.
My question is the idle. It is a bit erratic, that and just off idle. Then it clears up and is very smooth after that. I tried the needle in the richest position and it was too rich. I am assuming the cam is contributing to the erratic idle.

Has any one had experience with this cam? I am considering a pair of 107 needle jets and/or the 850 combo of cut away spray tube and 4 ring needle.

seattle##gs said:

I have a motor using the Jim Schmidt cam JS1 and his lightweight pistons approx 9:1. I have 932 Amals for carburetion.
50 miles on the new motor
The carbs have the standard 750 jets, straight spray tube, two ring needle set in the middle position, etc. The motor starts and runs well but I have not tried the upper RPMs yet.
My question is the idle. It is a bit erratic, that and just off idle. Then it clears up and is very smooth after that. I tried the needle in the richest position and it was too rich. I am assuming the cam is contributing to the erratic idle.

Has any one had experience with this cam? I am considering a pair of 107 needle jets and/or the 850 combo of cut away spray tube and 4 ring needle.

Click to expand...

This cam comes with a lumpy idle at no extra charge. It is what it is and there is no secrets from Jim. Even with a single carb, your cammy idle will be at 1500. The JS1 may be a stage one cam but it is surely on the hot side of stage one.
I suggest getting it set as smooth as you can around 1500 rpm and thoroughly enjoy what this cam will give as soon as you come off the line all the way to redline. You will find 3800 to 4000 RPM particularly interesting as an obvious yet smooth performance transition is observed.
Honest, learn to appreciate the idle and know that anyone who knows what the sound represent will take note.
You now have a hot rod.

It is a bit hard to get the idle smooth with a lumpy cam, I have a stage one cam in my hot rod Norton, it idles but not smoothly, I like it like that as when stopped at lights or parked while idling it makes poeple look, but once you get over 4,000 rpms hang on as thats when the cam starts to kick in and the fun begins.

I have been running a stage one hot motor since 1980 (850 in a Featherbed frame), in my workshop manual with the high performace section in the back it warns you to watch out as the cam will run very freely and will keep going till something goes bang in a big way.

Ashley

At idle, the idle air screw seems to have nearly a full turn where the bike runs pretty similarly. It almost feels like it's not doing much as you turn it while the bike is idling, but if you raise the throttle body ever so slightly as you turn the air screw back and forth, you will notice that the air screw's position has a greater effect on how smoothly the engine transitions from the carb's idle position to slightly open throttle postion.

Find the lowest steady idle you can get with the idle stop screw, then go to the idle air screws and turn them as you gently lift the throttle a little bit and then drop it back to idle. Keep doing that on each carb until you have the smoothest continuous transition from idle to 1/4 throttle. At that point, you've probably done all you can do by turning screws. I don't think needle height or position is going to effect the idle and low rpm range

You could try retarding the ignition timing a degree and see if that helps you idle too.

seattle,
First I would talk to JS and ask him in regards to the cam.

Regarding the Amals:

1. Start with the float height and get them right. The sweet spot is around 1 1/2 turns from full in on the idle screw. If you can get one or borrow from a friend, use a Gunson’s Colortune and look for a Bunsen burner blue just off the fence side of rich. Basic Amal must do for starters. That’s all you can really do with the idle circuit other than make sure there is no blockages… which you must check FIRST.
2. Get both slides working together.... disappearing to the top of the opening with the air cleaner off use your fingers and feel. Wiggle that throttle and make adjustments at the top of the amal where the cable come in.
3. Double check that both are lifting exactly at the same time off idle by using 2 drill bits like 1/8” or something of equal diameter and watch when the throttle is turned that these are dropping exactly at the same time. See our tech help in this forum section. If you can beg borrow a carbtune Pro Vacuum Balancer to balance between the two cylinders this helps aid that both cylinders are pulling together. I use a harmonizer made by a dude at avrider and it works a treat. But that’s another story.
I would then play with the Main jet while driving and pulling up a slight hill and do plug chops and examine the plugs. This is for ¾ to full throttle only. Again use the tech section here that L.A.B. was so kind to post up top.
Play with the needles based on your altitude or how it is coming off of the idle circuit ¼ to ¾ throttle.
This is all based on the assumption that your Amal Mk1's are “NOT” worn out…..lf the slides are worn the needle jet is worn and you must fix this first before.... or you will be chasing tail like dog looking for local entertainment. :roll: https://video.search.yahoo.com/video/pl ... zilla&tt=b
Cheers,
Thomas
CNN

seattle##gs said:

Has any one had experience with this cam? I am considering a pair of 107 needle jets and/or the 850 combo of cut away spray tube and 4 ring needle.

Click to expand...

I am not sure if you are aware of this, gut these items have little or nothing to do with idle.

If you've got the float levels right, float needles seating properly and no flooding, also all passageways in the carbs clear - the only difficult part of tuning Amals or Mikunis is the needle and needle jet. The 3/4s throttle tuning can be achieved by recognising the correct needle setting is one notch richer than the cough. If the mains are too rich, the bike will perform OK but might be a bit sluggish at full throttle - do a plug chop and read the spark plugs. Setting the idle is the last thing to do. Getting the optimum taper needle is a bit more difficult. I always use identical jetting in both carbs. If you've done your best and the bike is still running lumpy at idle, take note of where the cam spot is occurring. If it is above 3,500 RPM in a commando, I'd be checking the cam timing with a degree disc. If you ride the bike in about second gear with a bit of throttle on to make it pull, watch the rev counter and find out where the power comes on when everything starts to work. Cams are all about harmonics. Jim Schmidt will be able to tell you how his cam should behave in your motor.

Thanks all....I am very familiar with Amals. These are brand new, same as the motor. I have a carb synch tool and I have these carbs are about as close together as they are going to get. The idle was still swinging back and forth and I could not get it to settle down like the rest of the range but it was close. I will try to adjust the air screw as you recommend, I had not thought about it at the transition point. At the end of 20 miles I adjusted it by color on the plugs.

My main question is about using the 750 or 850 needle and spray tube set up. Any thoughts? Also, the needle jet has quite a bit to do with starting and idle if using a Mikuni.

A area which might need improving on this motor is just off idle, perhaps to 1/8 throttle. It revs very freely but it sounds like it is breaking up a little in this area both accelerating and decelerating...but I am not used to a cam like this so I do not really know what to expect. I am also using racing fuel provided by its owner ...$16/gal and fabulous stuff. A lot of variables going on here. I built it over the winter and it has about 60 miles on it now.

As for asking Jim Schmidt...he suggested asking you guys.
I am going to check the head torque...I am using a flame ring head gasket.

Seattle

seattle##gs said:

I have a motor using the Jim Schmidt cam JS1 and his lightweight pistons approx 9:1. I have 932 Amals for carburetion.
50 miles on the new motor
The carbs have the standard 750 jets, straight spray tube, two ring needle set in the middle position, etc. The motor starts and runs well but I have not tried the upper RPMs yet.
My question is the idle. It is a bit erratic, that and just off idle. Then it clears up and is very smooth after that. I tried the needle in the richest position and it was too rich. I am assuming the cam is contributing to the erratic idle.

Has any one had experience with this cam? I am considering a pair of 107 needle jets and/or the 850 combo of cut away spray tube and 4 ring needle.

Click to expand...

Ok here I am assuming you have a 750 commando??????????????

seattle##gs said:

Thanks all....I am very familiar with Amals. These are brand new, same as the motor. I have a carb synch tool and I have these carbs are about as close together as they are going to get. The idle was still swinging back and forth and I could not get it to settle down like the rest of the range but it was close. I will try to adjust the air screw as you recommend, I had not thought about it at the transition point. At the end of 20 miles I adjusted it by color on the plugs.

My main question is about using the 750 or 850 needle and spray tube set up. Any thoughts? Also, the needle jet has quite a bit to do with starting and idle if using a Mikuni.

A area which might need improving on this motor is just off idle, perhaps to 1/8 throttle. It revs very freely but it sounds like it is breaking up a little in this area both accelerating and decelerating...but I am not used to a cam like this so I do not really know what to expect. I am also using racing fuel provided by its owner ...$16/gal and fabulous stuff. A lot of variables going on here. I built it over the winter and it has about 60 miles on it now.

As for asking Jim Schmidt...he suggested asking you guys.
I am going to check the head torque...I am using a flame ring head gasket.

Click to expand...

Now what is this???
Is your bike a 750??? Or an 850???

Mikuni????r


T
CNN

Well, If you know amals well then you would know that main jet size isn't going to effect the idle to 1/8th range at all (as Pete V said previously)... but if we're just talking about carb adjustments that most effect idle to 1/8th throttle, we're talking idle stop screw, Idle air screw, and float needle seat height

Adjust the idle air screw the way I said previously. You'll probably notice that the transition from idle to 1/8th will be smoothest on the richer side of the idle screw range of where the idle seems good (as I said previously, the air screw will turn a little bit and seem to have almost no effect on the idle until you get to a point where it will stumble) You are looking find the exact spot where the screw makes the transition to 1/8th throttle the smoothest... You do that by adjusting, then lift, move the screw a little more, lift again,.... Did it get smoother or rougher?? keep doing it until it feels smooth going back to the other side after you do them both to see if you can make it better with a tiny adjustment either way...

tuning a carb by the color of the plug has nothing to do with the idle circuit. That's more related to jet sizes and needle jet/needle heights which comes in more open throttle positions, so forget plug chop tests for idle adjustment.

I don't think you can adjust the idle mixture by plug colour. I only ever set the main jets that way after a high speed run and a plug chop (no idling) . Jim Schmidt should be able to tell you where the power band occurs in most motors when his cams are used. It doesn't usually depend on jetting, however the exhaust system can make a difference. The only time low speed engine performance usually becomes a problem is if the cam spot is too high in the rev range and is savage as it comes on song. Even then a lot depends on what type of gearbox you are using - a multi ratio close box tends to cope better with a motor that is a bitch.

Hi seatle.
My bike is a 750. I now have stepped spray nozzle carbys and just fitted the 4-ring needles to suit. The manifolds are 32->30mm tapered. The 4-ring needles are 2mm longer at the parallel section so the transition from needle jet to main jet is now later. The spray nozzle pulls fuel better at earlier throttle openings as the air is rushing around a greater and lower area rather than across the top of the flat nozzle area at a later throttle opening. However, during the parallel section of the needle the 107 needle jets now pass too much fuel. I dropped in 106's and the AFR improved. Tomorrow I will fit 105's to see if that finds the sweet spot during the needle parallel phase.
I have also improved the pilot circuit to allow idle without stop screws.
Ta.

I have a JS1 and 10.5:1 CR in my 850 and it will idle comfortably at 1000rpm. Maybe the FCR carbs help with that?!

I tend to set it higher than that though, partly for 'comfort' but mainly out of respect for the cam etc that relies on oil being flung around for lubrication.

JS describe the JS1 as having a 'nice idle'. It is the JS2 that is described as having a 'hot rod idle'.

So in summary, IMHO, it is not your cam that is the root of your idle issues.

You state that your engine has 50 miles on it? If so then there are a lot of mechanical relationships that will change until the engine is fully broken in; it is not unusual for the cylinders to show significant leak-down differences up to 500 miles (possibly less), assuming that the engine was built correctly. At 50 miles I would be doing my second oil change and re-torqueing all engine fasteners and re-setting (checking) the valve lash; I would not be considering playing in the upper end of the tach, although I would use full throttle very briefly between 2000 and 4000 RPM to speed ring seating. How you do this is strictly up to you, but I wouldn't be faulting any carburation until things settle down. I always carry a small screw driver and adjust the carbs almost every chance I get until the idle and off-rest performance is 1) acceptable and 2) doesn't get any better, usually about 700 to 1000 miles.

Best wishes.

Hot rods with over lap cam idle issues do not draw in low volume air very good so tend to like adv ilde spark and an extra air bleed through closed butter fly or slide to idle slower So finer tune can be gotten with float level too.

The JS1 is hotter that stock and a bit rumpity but its still streetable and well behaved enough for anyone who wants more zip and a power band that comes on stronger but a little higher in the RPM band. It comes on cam at about 4000 RPM but will still putt around town. The JS0 is the cam that is close to stock idle and performance.

I spent many years tuning Amals and even when rebuilt and re-sleeved I generally had a flat spot just off idle. They are very simple and cheap to make but they are not as precise or well made as some other brands of carbs. You can do the best you can with them but finding perfection may be very difficult.

jseng1 said:

The JS1 is hotter that stock and a bit rumpity but its still streetable and well behaved enough for anyone who wants more zip and a power band that comes on stronger but a little higher in the RPM band. It comes on cam at about 4000 RPM but will still putt around town. The JS0 is the cam that is close to stock idle and performance.

I spent many years tuning Amals and even when rebuilt and re-sleeved I generally had a flat spot just off idle. They are very simple and cheap to make but they are not as precise or well made as some other brands of carbs. You can do the best you can with them but finding perfection may be very difficult.

Click to expand...

Yep I aggree but after putting Jim's PWKs on my Norton it idles pretty good, the Amals did idle but not as smooth, the amals did have that flat spot just off idle, but the PWKs are smooth running off idle and all the way through, but I also run the Joe Hunt with these carbies, I didn't have the J/H with the amals at the time, I was running my old Boyar then.

Ashley

I guess I did not mention that this is a 750 motor.....all Steve Maney and Jim Schmidt stuff. It is not running badly at all and streetable, though I would not venture into downtown Seattle with it. It has only been on the road once, 20 miles on the freeway and 20 miles back at night when the traffic and temps dropped. A friend drove the van in case anything happened. It performed with flying colors and only developed two small oil leaks which I can easily solve. It starts in one kick, sometimes 1/2 kick. the plugs were almost a perfect match so I leaned the left side by 1/8 turn trying to match them. It has the stock exhaust. The sound is unlike any Norton I have owned, high pitched, raspy instead of the usual Norton rumble. It could be geared too high...at 65 on the freeway I was in third gear. I was also trying to keep the RPMs up in order to seat the rings. If I dropped a tooth on the countershaft the whole motor would feel more punchy...and really...where can you go much faster than 80 these days with all the traffic?

seattle##gs said:

the plugs were almost a perfect match so I leaned the left side by 1/8 turn trying to match them.

Click to expand...

An 1/8th turn on the idle air screw??... That's not how amals work...

they are brand new. Give them a few months and they will go downhill.