More Amal Troubles

Not open for further replies.
Jun 3, 2005
Hi all,

I've posted regarding the same subject on this forum many times before. Last year I seemed to temporarily cure the problems with bad running/ backfiring etc. by replacing several components and tuning the Amal 932's carbs as best I could.

However, this year the bike is running like a bag of hammers. The bike does not idle and it appears to be hunting but yet at 60 - 70 MPH it runs fine. I have been told air leaks could be the problem, I have replaced all seals for exhausts and stripped the carbs and manifolds replacing the O rings, everything has been cleaned and the joints sealed with gasket sealent.

The problems are identical to the issues I had several years ago, I am now in the same position of considering ditching the Amals for a 36mm Mikuni Conversion. I do not know the mileage the Amals have done but the bike has 40k miles on the clock.

Apart from the normal Amal problems, the left hand carb has a sticking slide when the trottle is opened right up. A slight tap on the side with a screwdriver body and the slide springs back. Timing was checked with a strobe light and was fine.

I have also recently carried out a compression test, left hand which was 120 PSI and the right hand was 125 PSI. The carbs have also been re-tuned using a colour tune. The bike is also fitted with a Boyer ignition.

After all the problems I have been driven to consider the bike's sale, after talks with friends and family i've decided to try and get to the bottom of the problem. I would like to receive some advice before i place an order for a Mikuni 36mm conversion. I don't want to spend £200 and realise the carbs are not the cause of the problems.

Thanks again,


Sounds like your Amals are shot but do a leak down test to see if there's a problem with the valves/guides. At 125 psi your on the low end indicating the motor has seen better days. Some of the 850's had soft cams too which could be causing some problems (you didn't say year or model).

Here's a place in Canada that can rebuild/resleeve your Amals for a reasonable cost or if your intent on Mikuni here's a place that sells the kits at a reasonable cost should be less than 150 GBP with shipping.

Neil, PM me and I will give you contact details for a guy who will bore your Amal and resleeve the slide at £25 a carb. It will be better than new.
Have you carefully cleaned the little "L" shpaed idle circuit? Most times when I've had idle problems with the concentrics, that turns out to be the problem.

Remember, there's a 90 degree angle that has to be clear or there will be no fuel supplied to the idle and you end up turning up the slide idle position and the bike will try to idle on the main jet. Plus, the idle jet is active througout the jetting range, although it's a minor player at higher throttle positions.

Blow air into where the idle air screw goes and see if it comes out the little hole above the float bowl and the small air hole in the venturi. You can see the circuit on the underside of the body. I use spray gumout first and lots of air, but often end up physically cleaning it with a very small needle I made up many years ago.

This problem is obviously most common after the bike has sat for a long period of time, but I've seen it occur out of the blue on bikes that are used regularly.

Good luck and don't give up on the Amals! If nothing else, resleeving them will do wonders.

Hmm, if the new Dyna coil doesn't fix my right cylinder idle, I'll try a sleeve job as well. If the other contacts don't work, I have info on a guy in Oregon who does this, PM me for info.
He did the amal276 for my '51 Goldenflash and it idles quite nicely now.

Hi Neil,

You may have more than one problem with the Amals. Resleeving the slides will do wonders, but you need to check the jet needles for wear also. Take one of the needles out and run your thumbnail over it. If it feels notchy it's worn out and you need to replace both jet needles and needle jets. Worn needles will cause uneven running below open throttle. Blow out the idle passages as suggested. Another possibility is your spray tube may have come loose and dropped toward the fuel bowl. This is the brass tube in the middle of the floor of the mixing chamber, needle goes through the middle of it.

Take a gradual approach doing things that don't cost money first and see if you get improvement.
Scooter62 said:

At 125 psi your on the low end indicating the motor has seen better days. Some of the 850's had soft cams too which could be causing some problems (you didn't say year or model).

That's slightly worrying. The bike is a Commando 850 MK2A Interstate 1975, I am the third owner from new. What should the bike be putting out PSI wise using the compression tester that is screwed into each spark plug socket while running the bike on one cylinder? I have found a local Triumph supplier who is willing to take a look at the bike, should give me a better idea.

Thanks for the response,

The actual compression reading is less important than the difference between the sides, 5psi is not much. A valve regrind would probably restore the compression, the later Norton's as yours is are very long lasting due to the excellent air and oil filtering, a rebore every 60 to 80k miles with 1 or 2 ring changes in between. The soft cam did not affect the MK2a's only the MK3's.
I phoned the T140 shop today. I described the problems the bike is having and was told to check the timing with a strobe light, as my bike has a Boyer electronic ignition he said to set it at 38° BTDC at 5000 RPM. I was also told to then try and adjust the carbs again, if this fails he said my best option was a Mikuni single carb conversion. He's recently fitted them to 2 Norton Commando's, the owners are very happy with the result.

I have decided to go for the single carb conversion. What size do you recommend for the 850 MK2A 1975? 34mm or 36mm? The black box that is fitted behind the Amals will have to be removed for the K&N air filter, does anyone how big a job this is to do?

Thanks again,

Depends on how you use your bike. Bigger is not always better. You have to rev the bike high to get any benifit from the bigger carb also the air cleaner will be a tighter fit. If you like low down pull and good MPG go with the smaller carb. I like going fast and have modifyed the 36 mm to perfom with twin Amals. norbsa
Hi, Stick to twin carbs if you can as the single carb dose not look the best! I have had similar idling and rough running problem a few months back, finally after blaming everything else, even coils / leads/ plugs i replaced the stail junk 91 fuel and raised the fuel octane from 91 to 98 / 100. Leaned the carbs very fine adjustment when warm after a run, and the 1973 Norton 850 has run like a dream ever since. checked for exhaust leaks also to prevent overrun backfire when running rich. Yes my left carb also sticks when fully open and requires the odd tap to free it, It has never been a problem and just the other day 90 mph on the speedo kept me smiling for a few days. ps have not touched the carbs or tune for the last 2K it is sweet. HOPE THAT THIS KEEPS YOU ON TRACK TO FINALLY TUNE YOUR ORIGINAL TWIN CARB NORTON. :wink: :lol:

AMAL carburetors are wonderfully elegant in their simplicity. They’re made up of only a few parts and are relatively easy to tune. Also, you can use your stock air box with AMALs. The downside is their poor wear characteristics. However, “sleeving” will extend the useable life of an AMAL carburetor.

So, don’t be too hasty to get rid of your AMALs.

Neil, Properly set up, a single Mikuni is easy to live with. If you do change, you will have to remount your left side cover as it attaches to the air box. I have only run 34's, so I can not comment on 36's. I will say both bikes I received with Mikuni's were not set up right, so it is not automatic, they need to be tuned.

One issue that keeps rising with my Amals are the idle circuits clogging after sitting awhile. Check these circuits.

The compression seems a little soft for a stock 850. They both are within 5 psi, that's good. Are your plugs oil fouled? Does it smoke? Take another reading, kick it through several times.
Have you checked your valve adjustments lately?

Timing, 38 is to much for a Commando, should be 31 @ 5000.

Good luck!!
Another thought. How's your battery? Do you ride with the headlight on? Have you installed a Halogen headlamp?
The Commando's charging system draws on battery at idle and low revs. The Boyer makes it even worse as it draws more than points.
As the revs increase, the alternator takes over and all is fine.
Thanks for your input MichaelB. I'm pleased to inform you all that I ordered and fitted a 36mm Mikuni single carb this week and the bike has never gone better. The booklet I received with the carb doesn't appear to have any information on tuning it, I noticed that the bike is running slightly rich. Does anyone know which adjustment screw alters the mixture?

Again, further to the comment about soft compression, the bike does not smoke and the plugs are not fouled - It uses very little oil. What should the compression be for a stock 850 as I am thinking about removing the head to check the valves when the bike comes off the road, would this not be necessary?

Thanks again,

Hi Neil, glad to here all is well. Mikuni's are quite sophisticated compared to Amal Concentrics. When you say rich, where?? Down low, mid range or upper. There is a mixture adjustment screw for low end. There are numurous jets within.
Your carb is probably close. Before you jump inside the carb, I highly recommend the Vitory Libraries Mikuni tuning manuel. This manuel is excellant. It helped me sort out two badly tuned Mikuni's.
The carb appears to be running rich on idle and low speed operation which I think is the pilot setting. Can this be adjusted by opening the air screw slightly? The carb purchased from Allen's Performance UK will hopefully be jetted for a Commando 850 but I suppose giving the amount of wear on each engine could differ the end result.

Thanks again,

Not open for further replies.