tricky amals

I do not think the problem could be the choke lever, as I never ever use it, Michael. I just tickle the carbs thouroughly and it starts right away. When the engine is hot, I go through the same procedure and keep the throttle right open. Same effect: it starts spot on, giving me goosepimples because of its beautiful sound (NOT noise, definately).
One of my mates who rides a '69 750 S as well (mint condition) says my bike bike looks and sounds flawless.
It could be that, because of the vibrations of the engine, some electrical thingy has gone loose. So I will check all wires, cables and nuts and bolts.

The Boyer seems to be causing problems, according to some of the threats here in this fabulous forum.

Which batteries do you use to keep the power up?

Thank you for your replies!

There is a known problem with the Boyers fitted to a Commando, the iso's allow a lot of movement which causes the wires where they fit to the trigger plate to internally break down strand by strand. Fix is to replace the wires using solder and to tie them more securely to the trigger plate, a bit of foam underneath the points cover also helps.

This would give an intermittent fault that could affect a single or both sides.

Another is worn slides, this can cause too much variation in the amount of air being taken in at low throttle openings and cause hard starting when hot unless you start it with large thottle openings. When hot it should start with a very small throttle opening and quickly settle down to a good tickover.


PS if you have never used the choke lever still check it, it can close on its own accord and is easily missed.

I use a Gel battery but BMW riders have stated that Gels have low voltages, I have the blue capacitior in there too so maybe that helps.

Sudden onset also sounds more electrical, but could be something as easy as a main jet unscrewed in a carb, it will unscrew gradually but will not give a major effect till it drops into the bottom of the float bowel

Matt
Don't spend too much time chasing the Boyer, it either works or doesn't and it fires both cylinders at the same time.
With the limited information you've given, it sounds like a fuel delivery problem. Do two things check the float bowls for crud in them or that the float may be sticking also since you said the bike was recently restored drop the shut off valves on the tank (especially if you have a glass fibre tank) to see that the fine mesh screens are cleared. When the bike sits for a while fuel will fill the float bowls then you tickle it to get started so it's got enough fuel to run for a short time but insufficient fuel flow to continue running.
Anyway hope that helps, if it does save me a Spaaten if I ever get to Hamburg.
Scooter

thecallowyouth said:

my '73 850 runs pretty decent but will occasionally die when i pull up to a light. the idle just conks out unless i blip the throttle. strange thing is that when i give her the old kick in the morning, great running. but a little mild use and idling becomes dicey. im running original amal's with the pilot screws set at 1 1/4 turns out. idle is normally around 1000rpm. any advice? things to check?

Click to expand...

Hope that you have sorted out the idling /stalling problem by now? myself with similar problems was able to sort it after warming up the engine then a short blip on the throttle any black fuel emisions from each muffler can be overcome by leaning out the pilot jet anywhere from 1 1/4 - 2 1/4 turns out until no smoke is visable as you rev it briefly. Even remove one plug lead at a time to adjust the live cylinder, once idle jet mixture is good (no black smoke) re -adjust the idle speed lower. Then it should not flutter out out intersections again! Remember the pilot jet is used for the first 1/8 th throttle use, which can be around town in traffic stuff!!

norvil,

been awhile since i replied but the short answer is no, I havent fixed my problem. in fact, my Norton is running worse than before. fouling plugs, backfiring. its weird actually. when messing with the bike i found i was running with the choke partially on, so I turned the choke off and then the idle just went completely downhill and the bike refuses to rev past 4 grand. not surprisingly, when i put on a tiny bit of choke on, the idle is much better, but the running is not. anyway, don't know what to do--but i refuse to give up on my amals. i'll figure it out eventually. I've been looking through these posts and might have worn slides and needles. both should be easy to check.

start all over from the beginning and check everything again. I mean, if the throttles open at the same time, if the carbs are clean, jets clean, needles the same sort and length....everything. The Amals are a trick to get right, but they will most likely be able to be saved. Check even if the o rings are good on the manifolds, everything will be screwed up if they leak. Read the carb info on the home page of the forum....or some other Amal info that will tell you the initial setup, someone sent me a PDF that was good on Amals a while ago, but I can't find the thread now. You are going to find the problem is a small one that you have overlooked, and not, I bet, worn needles and jets...they would have to be worn to nothing before they would cause such drastic problems....backfiring you say too? Backfiring is often a timing, valve problem, not a carb thing....unless there is SO much gas being pumped into the cylinder that a lot of it ends up as raw gas, in the exhaust and sits there waiting to ignite by itself or hot carbon. Go through everything again, from the beginning, and don't forget that it is easy to forget to pay attention to something you see every day... as some devorced fellows can attest.
Let us know how it goes......

will do. I snagged that PDF on this site when i first saw it, so I'm not completely clueless about the amals. yup, backfiring, but I havent touched the points. it'll probably take a couple of days, but i've got a starting point. thanks for the advice....

Others have suggested you start at the basics with the carbys. I would start there also.

Firstly, I would make certain it is not a simple problem like a loose fuel bowl screw causing an air leak around the fuel bowl gasket. Are the carbys also "airtight" where they actually mount to the manifold & head itself ?

If that is ok then I would drop both fuel bowls & check that the jet holder is screwed in place with the main jet screwed in firmly also. These are easy to break if overtightened so be careful when u "nip" it up checking it.

If you haven't replaced the lil "friction o-rings" in the throttle stop & idle mixture screws do that also.

Next make certain that both slides are opening at the same time by adjusting the cable adjusters on top of your carbys. You have to remove the stock air filter to do this job.

The 1/14 turns out for the idel mixture you talk about is just a starting point, your bike may need to be 1/2 a turn either side of that general setting. You need to drop one spark lead & "listen" to your engine to get it right.

Get the both cylinders firing right & then readjust your idle.

If none of this "fixes" your problem, then look to the electrics. Your carbys will be right for a while at least. I have mentioned before that ignition switches can be erratic, so try bypassing the switch & see what happens.

If the switch is good, then you may have a loose wiring connection "somewhere in the ignition circuit.

Have you tried new plugs also ?

That is the best I can help you with without being there.

It can be frustrating finding these type of faults, but once you find it, it is easy the next time

Good Luck & please let us know what it is when you nail it.

HI, I purchased a 73 Commando 850 back in late March. The person I bought it from had to change the plugs because it would not start the day I was to pick it up, at least he was honest. It would not idle and the second time I tried to use it the plugs were fouled again. It had a brand new battery and Boyer ignition. I purchased the 34 MM Mikuna Carb and it solved the problems immediately. I also want to thank the persons who responded about my post on machine dating, I had a family emergency and have been out of the loop for several weeks, thanks to everyone for a great site.

I've been reading about amal vs. mikuni and I really think its just a matter of personal preference. im planning on keeping the amals since there are still parts available for them and people that do resleeving. I just need to take the time to get 'em right.

Update: It cant be my carbs. I've taken plenty of carburators apart before and the ones on my bike look just fine. The needles are not notched or worn, the jets are clear. I replaced the o-rings on the air/fuel mixture screws but the problem persisted. Bike wont rev past 3 grand and the left cylinder is backfiring and fouling the plug--right seems fine, nice plug and no backfiring. So far I've gone throught the carbs, plugs, checked for loose electics, replaced the air filter. Tomorrow I'll go through the timing.....maybe the auto advance??

sounds like you are moving forward. If it's just one side...plugs and coils have been switched, or?
You might think it sounds stupid...but I have a Vespa too that has a simular problem....runs great until I stop for a light, the idle then gets irradic...and the motor sometimes then stops dead, with no warning at all. I have been told that this is a problem with old Vespas, that the coil wrappings lose the insulation after years, and the coil grounds out at idle speeds....but only when hot. My Vespa will drive to the moon if you don't have to stop and idle it.....sounds a bit like what you said your machine was doing, back in the beginning of this thread....

Oh I wish I could go back when times were simple and my bike was just stalling out at lights. Now I cant even get the thing TO a light! Its just unrideable since it doesnt want to rev. Now this will sound stupid but I have the same problem with a rescued CB450. but with the honda i KNOW its the carbs. the norton carbs are just fine. I agree with you that it is something elecrical. tomorrow i'll check the timing, coils, kick the tires, do a rain dance...etc...maybe that'l work...

I acquired a very nice 74 850 last year that had all these symptons. The PO had done a lot of nice work, new HO stator, wiring harness, Boyer, carbs completely gone through with all new parts including slides, needles jets etc. etc.
His comment was the only thing he regretted was he couldn't get the &*^%$ Amals to work and should have put a MIkuni.
It was the only Norton I have ever had to need full choke to start.
Turns out he had the wrong needles. Most or all 850's have a cutaway spray tube which require a 4 ring needle. The 750 has a straight spray tube which requires the 2 ring needle. These are not the rings the clip fits in because those are consistent with 3 on all needles. These ID rings are above the clip rings.
I have seen cutaway tubes on 750's as things have been replaced through the years. I don't worry about that, but you have to match the needle to the spray tube.

Also, the new HO stator is not putting out enough. He has rewired the bike with new harness and Boyer Powerbox and has chosen not to put in the 2MC capacitor therefore it is constantly draining the battery. I suspected a poor rotor which may be but turns out the rotor is not centerd properly with the stator. I have ordered shims and I'll give an update. That probably should be another thread.

Hope this Helps

callow....
Let me know what is up with the electrics....and do check those needles as suggested...the float needles are ok? I am replacing mine tommorrow when I get to finish...hopefully..... the ring job. As you say...might be the timing too, the advances might not be doing thier job...theres a lot of members that can give you tips on those...I do the Boyer thing myself. Just don't give up....the Amals are great when sorted out.

Mike...
What shims? The rotor can't be so far out on the shaft, or so far in on it, not to have the magnatisim function...or? I do know that if the rotor makes even a bit of contact, when it turns, with the alternator, it will ground out the power it is busy producing and the batery won't get loaded so well....but that is solved by tweeking the mounting bolts, holding the thing in place...must be a certain clearance to work correctly....ask the others how much, I have just eyeballed mine.

All in all....two cases of just taking time and checking all things out....both problems will be found...have no fear...unlike us humans....these ladies are just mechanical...and can always somehow be fixed....

Mike
To help set the clearance between the rotor and alternator i use a piece of brass shim stock 3 " wide x 8 1/4 " long and 0.0010 " thick

Slide the alternator over the rotor and then insert the shim stock in between the alternator and rotor. This will give you an even 0.0010 " all around .Gradually tighten the nuts securing the alternator until you reach correct torque. When this is complete you should be able to slide the shim stock out with any real problem and your gap will be equal all around . If the shim stock is too tight you may just have to tap the mounting stud to shift it over .

It certainly saves time and you don't have to keep chasing the dimension as you do with feeler gauges.

This is a very handy little tool to have as you will undoubtably be in there again.

Ron,

I assume you mean .010" thick shim stock.

Jason

callow....
Just found the good carb info that someone sent me...setup etc. If you want it...it's 6 jpgs...take awhile to download...let me know through a PM...

Ya !!! sorry about the extra 0 . Must have been stuttering while i was typing. It should be 0.010 " thick