carb issues, excessive backfiring thru mufflers, and more....

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joe czech

joe czech

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having a bit of carb issues (?) on the 74 Mk2. performance wise, when you're on it, pulls strong from idle to WOT - all gears. however, it kind of feels like i'm getting a bit of, or a slight misfire under steady state, speed / RPM and fixed throttle position. when backing off the throttle to full close position, i'm getting loud, excessive backfiring through the mufflers. it idles fine, starts easy, and does not seem to have any other issues. i've played around with the pilot circuit and nothing seems to help. one note - the LH plug appears to be clean, almost too clean, the RH plug has excessive black soot. fresh rebuild on the carbs. specs - 260 mains and .106 needle, clips middle position, 3.5 slides (both carbs). a bit of a noob here, and i'd like a little input before i tear into things. for some reason, i'm thinking i may have a weak coil that maybe breaking down or has intermittent problems, however i could be out in left field with that thinking. any ideas....

edit - ignition specs - new tri-spark and tri-spark MOSFET rectifier/regulator. new champion resistor plugs, new, straight copper core, HT leads.
 
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I've just fitted new premiers and had issues with fuel level out of spec one side and a plugged pilot jet. Giving lots of "popping" left side on throttle roll off situations similar to your description.
Had to poke a very fine wire into the tiniest pilot jet hole in throat (guess thats the forward most hole?) Before it would actually spray a jet of carb cleaner vertically instead of dribbling.
This and the float bowl level seems to have improved the popping issue substantially.
Found a few more potentially contributing items today, loose joint silencer to header onright side. Right side pipe cooler after stopping than left side, so maybe the clamp torquing will solve or maybe a mixture difference still to dial in.
 
Just because your carbs are set the same don't mean they both will be the same when tuning, to get the bike to run the same on both cylinders both carbie settings can be different and even the needles can be set in different positions to each other, but of course new carbs have to be set up and tuned to each cylinder, you say the bike is running good and starting easy so it could be a few things that is causing popping from the exhaust, a slight air leak in the joints, a slight leak in your carb manifolds, a slight blockage in one of your carbs, you just got to do the simply things first and work your way till you find the problem, its also a way to learn all about your own bike and remember one carb adjustment will be different to the other carb.
As you say the left is clean and the right side plug is sooty, so right side might be your problem side.

Ashley
 
Some good suggestions tendered, do check them out. Also as mentioned above the idle air trim screws don't need to be at the same setting.

Could also be slides out of sync, possibly different cutaways. I is not uncommon for one of the needle clips, possibly both, to have "climbed" in a return spring?

You mentioned that one plug looks OK and that the other presents sooty, how long did you let the engine idle before checking?

Keep in mind that the idle system doesn't sign-off once you get metering from the needle and on up; it is all-in all the time and can effect all stages of carburetion, most notable at steady state throttle settings; this diminishes when the needle jet and main jet come to bat, but whether or not you noticeable it, it is still there.

The Bushman's guide has a lot of information. I'd venture your issues are related to sync and air trim screw settings. All best are off if the measurable compression diviates by more than 10%.

Best.
 
In my experience, backfiring on rolling back throttle is an exhaust leak. Usually engine to pipe joint.
Easily felt blowing holding your hand close to different parts of the exhaust.
As in modern instruction leaflets: Touching the exhaust can cause burns.
Another cause can be too retarded ignition. Rolling back both throttle and ignition twist grips at the same time did produce magnificent backfiring on the 1930 Harley I had 55 years ago.
 
As always.... easy things first... is that black plug fouled? Have you tried new plugs (I don’t mean ‘how old are your plugs’ I mean ‘have you tried different, fresh, new plugs)?

It would help you get good feedback if you could post pictures, how black is ’black’... how clean is ‘too clean’. Which one is wrong? Which one is causing, or is the effect of, your symptoms?

Mikes point about leaking exhausts is bang on. So the first things I’d do (one at a time) are:
1. Fit new plugs (always my first step at such times)
2. Check all exhaust joints
3. Check I haven’t got a tight tappet thats holding a valve open
4. Double / triple check the ign timing

The ‘too clean’ plug could be a sign of weakness. Your missing at a steady state can also be a sign of weakness, as can be back firing. It would be great to mark the throttle to see at which throttle openings it happens at, otherwise you‘re guessing. In order of easyness I’d check:
1. Inlet manifold leaks (at gasket or cracks) by spraying WD40 around the areas whilst running to see if it effects the running
2. Spend time checking and double checking the pilot air screw adjustment and the effects of adjusting it.
2. Check a needle hasn’t become dislodged or isn’t seating correctly
3. At the same time as 2, check and double check the slides are lifting evenly
4. Beg or borrow some number 3 slides and try them (when most 650 / 750 Triumphs run best on number 3 slides, even though they need smaller mains, I struggle with the fact that 3.5 slides are correct for a ‘free breathing‘ 850 Norton IMHO, although the difference between them is small)
5. But here’s the most likely IMHO... you say that playing with the pilot circuit doesn’t make much difference? It should make a big difference. If it’s not, it’s very likely a sign the pilot jet is blocked, either fully or partially. This is a very well known issue with original Amal’s. Spraying carb cleaner through is not enough, that does NOT show if it’s partially blocked or not. Lots of posts on here about how to clean the pilot. Unless you did this properly when you rebuilt the carbs, I suggest you gotta do it now.

The black plug could be a sign of richness. I’d check:
1. Again, is a needle dislodged
2. Pilot. If one side is blocked it’s quite possible you’ve inadvertently over enrichend the other ‘innocent’ side, ending up with one side too weak and t’other too rich
3. It could also be a sign of burning oil. But let’s park this bleaker possibility for now.
 
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FE is correct but as previous post noted "bets are off if the measurable compression deviates by more than 10%."
Do a differential compression test. You will see and HEAR any problems with leaky 1. intake 2. exhaust
and the ever popular "sound of air from the crankcase"...ie the rings.
Go forth and collect data.
 
you guy's are super - big thanks to all, and a double to "the fast one". much to digest. time to tear into things - really need to start with the basics. been wanting to double check valve settings for some time now, so it should be a good place to start. fairly sure timing's spot on, but it won't hurt to throw a strobe at it. almost positive i don't have an exhaust leak - things appear to be tight, but will double check. as much as i hate to, i'm going to pull the carbs, and give them a very careful (and anal) inspection, especially the pilot circuit and float levels. BTW i did run a compression check a while back - 145 right, 150 left.

ONE THING TO ASK - don't know if i'm overthinking this, and it may have nothing to do with the problem at hand, but when i installed the carbs - as called out in the parts manual, i used a thin phenolic (?) heat insulator to the head/carb spacer mount. i never thought these made very good sealing surfaces, so i added a correct, 32mm, thin paper gasket to both sides of each insulator. any issues here??? i never have seen any detail instructions on carb mounting in any manuals. basic common sense tells me this shouldn't have anything to do with anything, but then again, i ain't the smartest guy on the planet either - :D
 
Before you pull the carbs, check for intake leak by spraying WD40 or similar around joints at idle.
Rise in RPM, intake leak.
 
Where's the Amal carburetor troubleshooting sticky? Sure would save some bandwidth and typing repetition. :)

Joe,
Taking the insulators off shortens the intake tract and passes more heat to the carburetor bodies. The shorter intake tract might make a minor difference you could only see on a dyno, but reducing the heat passed from the head to the Amals is not a good thing.

When I used Amals, I sealed the ID on the insulators with epoxy. Back in the olden days the insulators did have a tendency to degrade and weep a little. At least that is what I told myself when I smeared the epoxy on. Probably totally unnecessary, but I can't resist reminiscing, like the has been back marker racers do.
 
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Before you start tearing stuff down try leaning out the rich running cylinder and enrich the lean side. A lean condition can create a backfire as well.
An infrared thermometer reading on the header pipes coming out of the head can get you in the ball park as far as matching up mixture for each side, at idle anyway. Try to get temp readings as close as you can.
Worn slides can drive you nuts as well, even if they appear to be OK.
 
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Schwany said:
Where's the Amal carburetor troubleshooting sticky? Sure would save some bandwidth and typing repetition. :)
Click to expand...
http://www.jba.bc.ca/Bushmans Carb Tuning.html

Schwany said:
Taking the insulators off shortens the intake tract and passes more heat to the carburetor bodies. The shorter intake tract might make a minor difference you could only see on a dyno, but reducing the heat passed from the head to the Amals is not a good thing.
Click to expand...
Aren't there (or weren't there) different thickness insulators available? What I see available now are called "thin", and I recall some thicker ones in the past.

Schwany said:
When I used Amals, I sealed the ID on the insulators with epoxy.
Click to expand...
I use Yamabond on carb and manifold joints.
 
Joe, please don’t tear your carbs down again yet. If you do, I clearly wasted my time responding! My list is meant to be progressive, meaning you start from the top and work down it, and only go as far down it as needed.
Please do the first 4 items first, they’ll take almost no time. You might solve the issue at that. If not, at least you rule them out of your further enquiries. Either way, you win!
 
Fast Eddie said:
Joe, please don’t tear your carbs down again yet. If you do, I clearly wasted my time responding! My list is meant to be progressive, meaning you start from the top and work down it, and only go as far down it as needed.
Please do the first 4 items first, they’ll take almost no time. You might solve the issue at that. If not, at least you rule them out of your further enquiries. Either way, you win!
Click to expand...
will do....
 
maylar said:
Aren't there (or weren't there) different thickness insulators available? What I see available now are called "thin", and I recall some thicker ones in the past.
Click to expand...
from my parts manual, only one, but i vaguely recall, old britts listed two thicknesses. i used the p/n called out in the manual + standard paper both sides. really don't think i have a leaking issue associated with the gasket/insulator mounting, but then again....
 
I have two thicknesses of insulators. Got them from Eurojambalya. Ive used thicker one on the head/manifold joint in the past to great success preventing carb bowls from getting too hot to touch within minutes after shutdown. Had a carb slide jam open once from this heat soaking before I used insulators. I will not run bike without insulators now.
The thicker ones at head/mani mean less threads from the allen head bolts into head. So either slightly longer bolts or move to thinner insulators there.
I use Hylomar blue on insulators and no leaks, easier clean up when reinstalling than Yamabond sealants.
 
Fast Eddie said:
As always.... easy things first... is that black plug fouled? Have you tried new plugs (I don’t mean ‘how old are your plugs’ I mean ‘have you tried different, fresh, new plugs)?

It would help you get good feedback if you could post pictures, how black is ’black’... how clean is ‘too clean’. Which one is wrong? Which one is causing, or is the effect of, your symptoms?

Mikes point about leaking exhausts is bang on. So the first things I’d do (one at a time) are:
1. Fit new plugs (always my first step at such times)
2. Check all exhaust joints
3. Check I haven’t got a tight tappet thats holding a valve open
4. Double / triple check the ign timing

The ‘too clean’ plug could be a sign of weakness. Your missing at a steady state can also be a sign of weakness, as can be back firing. It would be great to mark the throttle to see at which throttle openings it happens at, otherwise you‘re guessing. In order of easyness I’d check:
1. Inlet manifold leaks (at gasket or cracks) by spraying WD40 around the areas whilst running to see if it effects the running
2. Spend time checking and double checking the pilot air screw adjustment and the effects of adjusting it.
2. Check a needle hasn’t become dislodged or isn’t seating correctly
3. At the same time as 2, check and double check the slides are lifting evenly
4. Beg or borrow some number 3 slides and try them (when most 650 / 750 Triumphs run best on number 3 slides, even though they need smaller mains, I struggle with the fact that 3.5 slides are correct for a ‘free breathing‘ 850 Norton IMHO, although the difference between them is small)
5. But here’s the most likely IMHO... you say that playing with the pilot circuit doesn’t make much difference? It should make a big difference. If it’s not, it’s very likely a sign the pilot jet is blocked, either fully or partially. This is a very well known issue with original Amal’s. Spraying carb cleaner through is not enough, that does NOT show if it’s partially blocked or not. Lots of posts on here about how to clean the pilot. Unless you did this properly when you rebuilt the carbs, I suggest you gotta do it now.

The black plug could be a sign of richness. I’d check:
1. Again, is a needle dislodged
2. Pilot. If one side is blocked it’s quite possible you’ve inadvertently over enrichend the other ‘innocent’ side, ending up with one side too weak and t’other too rich
3. It could also be a sign of burning oil. But let’s park this bleaker possibility for now.
Click to expand...

joe czech said:
from my parts manual, only one, but i vaguely recall, old britts listed two thicknesses. i used the p/n called out in the manual + standard paper both sides. really don't think i have a leaking issue associated with the gasket/insulator mounting, but then again....
Click to expand...
Norton basically had 3 types of Heat insulators The thick ones are usually 28 and 30 MM ID, the thin ones were 32MM ID, But my parts supplier has more different types. I never use gaskets on them , as I surface the head and manifold and put on a very thin smear of RTV. If the surfaces are flat, there should be no problems.
 
Tornado said:
I have two thicknesses of insulators. Got them from Eurojambalya. Ive used thicker one on the head/manifold joint in the past to great success preventing carb bowls from getting too hot to touch within minutes after shutdown. Had a carb slide jam open once from this heat soaking before I used insulators. I will not run bike without insulators now.
The thicker ones at head/mani mean less threads from the allen head bolts into head. So either slightly longer bolts or move to thinner insulators there.
I use Hylomar blue on insulators and no leaks, easier clean up when reinstalling than Yamabond sealants.
Click to expand...
Am I right in thinking the thicker ones were perhaps from the pre Commando 750's??
Sure my P11 used them, what 'manifolds' there were sat on three studs and one allen screw, unless my memory has screwed up (not totally out of the question!)
 
EstuaryBoy said:
Am I right in thinking the thicker ones were perhaps from the pre Commando 750's??
Sure my P11 used them, what 'manifolds' there were sat on three studs and one allen screw, unless my memory has screwed up (not totally out of the question!)
Click to expand...
My P11 used 4mm thick insulators and 1/2 inch (12.7mm) thick intake manifolds, plus gaskets when I used Amal carbonators. I just went out and measured. They were originally 30mm I.D., but I took a Dremel to them and matched the 31mm ports on the head I have.

Is that far enough off topic to get a new set of frilly knickers? lol
 
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