1962 Norton 650SS. Chopped 376 Monoblocs. Rough idle when hot. Change pilot jet?

Hi:

I've been working over the last few years to get my 1962 Norton 650SS back on the road. Have encountered quite a few issues along the way, but am finally getting close - in no small part thanks to some excellent advice on this forum! Engine has been rebuilt with new pistons, crank, cam and rockers etc etc and overall now runs very nicely. Carbs are two new Amal 376 (1 chopped) monoblocs from Burlen LTD, (376/288 and 376/289) that I believe are correct for my bike. Jet sizes etc are the same as recommended in my Norton owners manual.

The issue is that the engine does not like to idle when hot. Absolutely fine when cold, but engine tends to want to splutter and die when approaching traffic lights or similar. Issue is often accompanied by a small amount of backfiring from at least one of the carbs. Starts up again fine afterwards, (if the throttle is opened quite wide when kicking it over), is just obviously a bit inconvenient. Spark plugs after a run at normal rpm's are light brown and look fine. However, if left to idle the plugs get quite sooty. Pilot air screws are at about 2 1/4 turns out.

Seems to my tiny mind that I am running too rich at idle. Consequently I am thinking of trying smaller pilot jets. Currently using 25cc, am thinking of trying some 20cc ones instead.

Essentially I am trying to find out if this is an issue that others have faced. I'm just a bit perplexed as everything should be in excellent condition and setup to factory specs. Am I missing something? I'm very familiar with SU carbs, but still learning the ways of Amal.

Thank you!

Kind Regards,

James

I think idling shall be adjusted on a warm engine. That usually means that it barely runs when cold so you have to use the choke when cold.

Hi Mike40m:

Thank you for your input. I do not need the choke for initial cold startup. It idles smoothly almost immediately while cold with the air slides (choke) raised. More evidence I suppose that I am running rich at idle. The question is how best to address the problem.

Kind Regards,

James

Do you have an air filter? What kind of ignition do you have? What plugs are you running? Is the problem with both cylinders? In my experience the biggest issues with chopped monos is getting to, and seeing the number of turns, on the airscrew on the left carb. It's so difficult that it that it just asks to be mis-ajusted.

Britstuff said:

Hi Mike40m:

I do not need the choke for initial cold startup. It idles smoothly almost immediately while cold with the air slides (choke) raised. More evidence I suppose that I am running rich at idle. The question is how best to address the problem.

Kind Regards,

James

Click to expand...

Another question, are you running the stock slide cutaway?

No air filter. Have tried trumpets, cheese graters and nothing at all. Same hot idle issues in all cases. Magneto and points. NGK B7ES spark plugs. I assume it is the correct cutaway as it is is brand new from Burlen.

I think I have the pilot screws optimally adjusted for the current configuration. I say that because it took some time / a number of pilot screw adjustments to get both plugs to show a nice tan color after a run. Even before I got the pilot screws adjusted I had issue with hot idle. I assumed that once I got the pilot screws adjusted all would be well, but apparently not so.

Not absolutely sure if it is both carbs or not that have the problem. However both sides have nice tan pugs after a run and sooty plugs if left to idle.

Don't know anything about Monoblocs, but basic carb tuning says you are on the right track.
The pilot circuit is too rich.
I would....
1st. Verify proper choke operation.
2nd. Verify have correct slide. Too shallow cutaway affords rich running. IE 3.0 vs 3.5, 3.5 being the leaner one. Don't assume anything is correct as delivered.
3rd. After I knew I had the correct slide, reduce the pilot jet till air screw works approx. 1.5 turns out.

Slides all work smoothly. Just checked the cutaway sizes, they are apparently correct: 3 1/2. Thank you for your thoughts. I have read that that 1.5 turns is the ideal. I am assuming that as I am getting the best mixture at 2 1/4 turns it is another piece of evidence that my pilot circuit may be too rich?

Just checked and see that I went to 20 pilot jets. I remember doing this because I had to screw my air screws out many turns to get any kind of idle. I also think I dropped my needles one groove. (I'm not good at keeping tuning notes). My thinking was that the standard settings are for a bike without an air filter. The filter was an extra cost option in the UK. I think with a filter the bike runs rich. I think it's good practice to start with the standard settings but, between different compression ratios, non OEM cams and modern gas formulations, I don't think the original specs can be regarded as gospel. I've had my 650SS for many years and have always had difficulty getting it to idle reliably, although with the 20 jets it's ok. I find that even using ATF in the primary the clutch will drag if you set the idle much above 1k rpm. Mike40M's point about setting the idle when the engine is hot is a good one. FWIW I tickle until I see gas and never use choke on my featherbed twins. No choke or tickle when warm but about 1/4 throttle seems to work for hot starting. Just my experience FWIW.

Hi Bodger:

Thank you very much for checking, very helpful!

I suppose I probably should not make assumptions that factory specs are sacred and should think more of them as starting points - particularly when it comes to old British stuff. The numerous times I have had to play with SU carb needles over the years, (on four wheeled British iron) should probably have told me something!

My bike seems to be very sensitive to tickling. Anything over a few seconds when cold and the engine floods. I'm guessing the relatively sharp upward angle manifold makes things a bit more interesting.

I have not run into any clutch drag issues yet. Still, I'm sure I have all that to look forward to. Currently running 20/50w in the primary chaincase.

So far I have done around 400 miles on the rebuilt engine. Looking forward to when I have her run in so I can give her some beans!

I think I am going to get some 20cc pilot jets and try those. I think I am also going to double check the fuel level / left hand carb float level.

Cheers,

James

Britstuff said:

Hi Bodger:

Thank you very much for checking, very helpful!

I suppose I probably should not make assumptions that factory specs are sacred and should think more of them as starting points - particularly when it comes to old British stuff. The numerous times I have had to play with SU carb needles over the years, (on four wheeled British iron) should probably have told me something!

My bike seems to be very sensitive to tickling. Anything over a few seconds when cold and the engine floods. I'm guessing the relatively sharp upward angle manifold makes things a bit more interesting.

I have not run into any clutch drag issues yet. Still, I'm sure I have all that to look forward to. Currently running 20/50w in the primary chaincase.

So far I have done around 400 miles on the rebuilt engine. Looking forward to when I have her run in so I can give her some beans!

I think I am going to get some 20cc pilot jets and try those. I think I am also going to double check the fuel level / left hand carb float level.

Cheers,

James

Click to expand...

Good luck with the bike. No disrespect to the Commando aficionados. I have had the good fortune to own two 650SS as well as a Commando and an Atlas. When a 650SS is on song it is truly a special bike - - and worth all the aggravation it takes to get it there.

Is your mag ok?

If bike starts ok hot and cold and ignition does not fail when mag is hot, the mag is not likely to be the source of the problem.

Number 20 pilot jets are the recommended size for an Atlas according to my Norton factory manual. I believe the 650 SS is the same.

I am an advocate of reverting to factory specs when tuning, then going from there if necessary.

Slick

texasSlick, if your manual is P106/P, it says that your float needle seat is raised 1/32 by a washer. The higher fuel level might explain the 20 pilot jet. For 650SS the manual says 25.
I've found that some new Burlen carbs need fuel level adjusted. Might be worth checking. As the manual contains a number of pages on tuning carbs, the factory specs are not written in stone but a starting point. As you describe how your bike runs, smaller pilot jets might solve your problem. My six British singles all have bad idling when cold. But on singles idle speed is easily adjusted with the throttle adjusting screw.

Hi All:

Thank you all for your comments!

My manual, (P106/P) definitely states 25cc pilot jet.

Mike40M: I note that my float needle seat does indeed have a fiber washer, and that removing it would logically lower the fuel level. Until I saw your comments I must admit to dismissing it's significance, as I assumed the reduction in height would be too small to affect any real change in fuel level. After reading what you had to say I came across the following blurb from the Burlen website:

"Every new Monobloc Carburettor is built to provide a fuel level at a nominal setting utilising a fibre washer under the Float Needle Seating.For the majority of applications this nominal setting is perfectly acceptable. However for use with modern fuels tuned or modifed engines alternative setting of the fuel level may be required to obtain the engine?s optimum performance.This kit comprises of 4 aluminium shim washers of 0.15mm 0.25mm and two 0.5mm thicknesses. Replacing the fibre washer with a series of different combinations of these washers allows the user to vary the fuel level and thereby select their preferred setting."

So apparently a small change can make a difference.....

I now note that in the Norton P106/P manual there is a section on tuning that might be helpful. Unfortuntely I only have a free downloaded copy which is missing the applicable pages. It would be rather embarrassing if this proves to be a case of I should have RTFM! I'm going to buy a complete copy and will play with fuel heights and pilot jet over the next few days.

I will update this forum if I find out anything interesting.

Kind Regards,

James

The missing pages in the manual, pages 48-50 contains basically the same as Monobloc hints and tips on http://amalcarb.co.uk/downloads/
There was a thread on monobloc float level at https://www.nortonownersclub.org/node/12672

The fuel level in the float chamber changes by a multiple of the shim thickness change under the fuel needle seat because of the mechanical leverage of the float from its pivot point, versus the needle from the pivot point.

Slick

Mike40M said:

The missing pages in the manual, pages 48-50 contains basically the same as Monobloc hints and tips on http://amalcarb.co.uk/downloads/
There was a thread on monobloc float level at https://www.nortonownersclub.org/node/12672

Click to expand...

The comments by George Farenden in the thread referenced by Mike40M are very helpful. As he points out (and I never thought of this) you can use the fitting for the connector hose between the carb with the float and the chopped carb body to check the float level. I supposed you could attach a longer piece of clear hose to the barb on the carb with the float and voila.

Thank you again everyone for your help / comments!

After some experimentation the hot idle on my bike is much improved. Not yet perfect, but with a fast idle is way more manageable around town / waiting at traffic lights etc. Interestingly it now appears to be easier to start the bike - hot and cold - often with just a single kick! Plugs are light brown after a run. Pilot jet air screws are at 1 1/2 turns. All in all I'm rather pleased.....

I changed the fuel height by removing the fiber washer and replacing it with an approx .018mm steel rocker shim. I'm using one of the new fangled black "stay up floats". As advised above I used a piece of clear tube over the pilot jets to verify fuel height, which for me is on the dot under the word "Amal" on the float chamber side cover.

Second I changed the 25cc pilot jets for a pair on 20cc's

Cheers,

James