Is it Boyer or is it Amals

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May 22, 2004
Hi folks..

Mine is a 73 Commando 850 with 632 MK I amals.

Here is the challenge: Idles fine.. starts fine.. goes like a stung bull.

Back off to mid range ~ 1/4 to 2/3 throttle ~ 2500 to 3000 rpm.. and it pulls and surges like the final gearing is too tall.. :shock: The problem has degenerated in the last 200 miles.. :(

The carbs have stainless steel inserts.. I have had quite bit of dramas with sticking slides and tightness.. I POLISHED this out of the slides.. now a nice fit.

Its works fine.. The LH carb gives the sytems of having a too low a cutaway.. rich plug.. VERY rare back spit on the carb.??

The symptons to me read as running 12V coils on a Boyer ignition..

But I believe the coils are originals and 6 volt. I have overhauled the carbs and used the proverbial fine tooth comb..

I have ordered a full set of needles jets.. etc and plan to replace all and sundry..

I live on top of the Great Dividing Range in Northern Australia at approx 2000 ft above sea level.. :twisted: 8)

The reason I no longer have a Boyer ignition is because I experienced strange carburetor-like problems at 2,500 or so RPMs. The Boyer was replaced with points and condensor and the problems disappeared. But I don't think your Boyer is at fault here.

Instead it sounds like carburetion. At 2,500 RPMs you are probably running on the carb needles. Have you checked your spark plugs after riding at this speed for a mile or so? You may have to adjust (raise or lower) your needles based on what your plugs look like.

If you replace the needle, be sure to also replace the needle jet.

My 850 runs well with: 3.5 slides, 106 needle jets, needles adjusted to the full rich position, 270 main jets and pilot air screws 1.5 turns out. The bike has a K & N air filter, stock Peashooter silencers and no crossover pipe. These settings should work well at 2,000 feet on a stock 850.

Good luck,


I wouldn't rule out your first theory, it could be the Boyer.

I have had 3 boxes on 3 different bikes do the same thing. The common occurances were older boxes on bikes that had sat unused for a few years. The oil in the capacitors dries out if not regularily exercised and the caps go south, leaving a dead zone in the advance curve. The bike would act like the carb's were acting up and have a hardtime pulling through the 2500-3000 rpm range, especially in the higher gears.

If adjusting the carbs doesn't help, replace the box and the problem should disappear.

My $0.02,

Thnks Guys..

Still waiting for the carb bits..

As for appearances.. the Left plug is running rich and the right is text book colouring..

Both needles are set on the lowest setting at this point..

The bike has spent quite abit of time 'loafing' in various sheds over the past ten years and only been back on the road on and off since 2000.

It sounds more and more like a carburetor problem to me. However, as Derek pointed out, those diobolical electronic ignitions can offer up some strange carburetor-like problems.

At this point you could rule out the ignition system by swapping the various ignition leads and blackbox wires around - left becomes right and right becomes left. If after the swap you run the engine and the right plug shows a rich condition, then you can assume the ignition system is at fault. If the plugs look the same as before, then you have a carburetor problem.


PS: Are you near Exmouth?
Again thanks ~ Jason

I hav ebeen doing a lot of homework.. and I am STILL waiitng for my jets needles replacement etc . I found an entry in teh NOC service notes that typicfy the problem ~ suggesting that hte Needle jets are only good for 10,000 miles or less..(?)

So IF that is the case my machine has 11,000 miles and the I beleive the jets are original ~ so hopefully ~ it will prove to be an easy fix.

Exmouth ~ No ~ west of Cairns on top of 2000 ft of mountain range..
Amals or Boyer


Just to second Jason's oppinion, my '72 combat roadster had a mid range surging problem that new needles and needle jets cured. I now have about 5,000 miles on these new parts and already suspect the needles need replacing again. Previously the jets were so worn that at 9,000 feet elevation the engine bearly would run being so rich. Another upside was my fuel economy rose from 35mpg to current 55-60mpg. I'm no exagerating the mileage. Yesterday again I went on a 200 mile NCNOC ride and got 61mpg.

Yes ~ after this time ~ the new bits have shown up.. but his week is out .

BUT on consideration of the facts.. I figure..

The fact that I moved to this place of residence only a relative short time ago.. and previous to this I lived on the "western slopes and Central Australian region (Ayers Rock) , which I think may even be BELOW sea level.

I am beginning to believe that a LOT / most of my problems are a combination of altidtude extreme, tropical environment.. and the fact that I have some 11,000 mles on what I believe are original needle jets.

So when I find the time ~ hopefully this weekend.. but 'we' are dashing to Townsville some three one way and back again .. on Saturday.. so hopefully Sunday I will replace the old bits and get 'out there' once more..

I will give detailed report ~
Thanks SSS
SSS ~ NOT Guest ~


Thurs ~ New carb bits and Tridium NGKs all set to go and a time frame bit after working huge day ..

Have look at this ~ ... ic_id=1070

Say NO more ~ they are the same size Needle Jet 106's! (Aprox mileage 11,000 )

Got it al back to gether and one fuel bowl pisses out fuel pull it off. Renew inner fuel pipe to carbs link...

Crank 'er' up.. first (1/2) kick ~ as I would expect.

Instantly notice a different tone to exhausts.

Perfectly even muffler note/s unlike stated previously!

Idle settings FAR more responsive on the adjustments!

At this point I have settled with the original 260 mains and will road test in the next few days.

Weather is classic Australian North QLD tropical paradise .. totally 10/10..

(Probabaly find the warm weather will agree with the beast .. as opposed to cold snap of the last few weeks.)
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