Sputter at 2500 rpm

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Stillreel

Stillreel

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71 Commando 750. Boyer electrics, connections look solid, good spark. Fresh plus. New AMAL 106 and AMAL Needles. I'm getting a bad sputter and occasional backfire as I throttle up at 2500 rpm. Once hot, everything settles down but initial driving is really rough. Plugs are getting fouled quite quickly too.

What might be the cause of this. Time for those Premiere Amals?

BC
 
could be a number of things

but I would first verify the carb air screws are set at about one and one half turns out from fully seated

one or both may have vibrated more closed, enriching the mixture too much and fouling the plugs

if the air screws are set right then take off the tops of the carbs and pull out the slide to see if the needles
are still in their clips (middle?) as they too can get jostled out of place

based on what you said this does not seem to be electrical so do the easy carb checking out stuff first

check tightness of all carb mounts, manifolds, while you are at it
 
I've played with the air carb screws almost daily trying to get that right. No setting prevents plug fouling, which to me means rich mixture (however may be a leaky valve guide as well).

But the sputter is a new development. Even with clean plugs, a consistent misfire at 2500. Auto advance issue? Seems odd. Boyer was fine and connections appear solid. I'll look for air leaks again tomorrow.

Thanks
BC
 
If ya ain't replaced the boyah trigger wires or checked after dark then all the above and following wisdoms may waste as much time as it did my learning curve balls. Of course can have more than one thing at once doing the same thing. Air leaks should always be kept in mind carb to exhaust as well as keeping Earth Positively connected. Does add some spice on how far away you dare fly on such funny fragile things. Try key on , plugs on head to earth, out of gear then flick flop jerk and pound loom top to bottom, listening watching for a snap to tease ya in right direction.
 
Stillreel said:
What might be the cause of this. Time for those Premiere Amals?

BC
Click to expand...

My slides are really worn, you can hear them clicking at idle. Bike runs and idles fine. Clean the idle circuits mechanically.

Running OK when the bike warms up makes me think mixture. What throttle opening would you say it is worst. Did you check float levels ?

Then again, most carby problems are electrical.
 
I've checked and re-check all of the Boyer connections and the grounds; they all appear solid. As Hobit suggests, I'll try the same (plugs out, grounded to head) in the dark looking for a rogue spark but why would this be an electrical issue if the sputter only occurs at 2500 rpm (approx 1/4 to 1/3 throttle)? The start and idle seems okay.

How does one test the Boyer with a volt meter?

BC
 
here's why one should still check the boyer wires even if the connections appear fine

because I once had your exact same problem, and I found the culprit only when I removed the wires from inside the points cover and pulled them out of the backside of the timing case

and found it, a partially fractured wire that apparently reached its harmonic stutter rattling against the inside of the timing cover, I have since learned to pack silicone around the opening

in another case one of the boyer wires had rattled for years against the frame tube under the zip tie
and had worn its insulation off

however I don't think you are out of the woods yet as far as the carbs are concerned, your motor will start even if one of the needles is dislodged from its clip, I would pull the slides out and verify the needles in their proper clip position just to rule that possibility out

you may even have some crud partially blocking your pilot jets or air screws, both easy enough to take off and either run a thin wire through or blow out with compressed air, this can cause your plug fouling
 
I put thick axle grease on worn to fluted slides/bores to find it solved fast idle creep up and some stumbles in mild operation rpm range. Seemed to last most a Roadster tank before washed out w/o hint of smoke or chamber yuk, as got to do a ring job shortly after d/t carb booties leaks.

https://www.google.com/#q=amal+anodized+slide
 
What type head is it?
RH4 - RH10?
I ask cuz the RH4 is know to crack along the intake runner.
Spray carb or brake cleaner around the head to rule out leaks.
But if a consistant sputter at 2500 then I would suspect electrical.
I'm no expert,,,,,,just so you know.
 
Stillreel said:
the sputter only occurs at 2500 rpm (approx 1/4 to 1/3 throttle)

BC
Click to expand...

1/4 to 1/3 throttle is mainly needle height with some slide cutaway and some idle screw.
 
Stillreel said:
71 Commando 750. Boyer electrics, connections look solid, good spark. Fresh plus. New AMAL 106 and AMAL Needles. I'm getting a bad sputter and occasional backfire as I throttle up at 2500 rpm. Once hot, everything settles down but initial driving is really rough. Plugs are getting fouled quite quickly too.

What might be the cause of this. Time for those Premiere Amals?

BC
Click to expand...

Premiere's .... just do it. If your carb/s' are suss another few hundrred $$ dont mean SFA... Somebody on this site will do a good deal for you... Trust me... :lol: :lol:
 
You say it runs OK when its hot, some bikes will splutter when you try to give them to much throttle when cold, but if this is just a new thing then I say it could be a fractured wire in your pick up by looking at the wires you won't see the fractures, I have run my Norton with worn slides the only effect it had was on idil but ok when running, clean your carbies out and look for leaks, could also be a earth problem with the Boya, just do one thing at a time and you will get if.

Ashley
 
So I tried the plug out, grounded to the head in the dark last night and it all looks good; no un-expected spark and the spark at both plugs looks good. As much as I would like to believe its an electrical issue, it does not seem to be the case. The carbs are out now (float bowls were a wee but high) and one slide was quite worn BUT not on the misfiring side). I'll clean and re-install them tomorrow. If that doesn't work, I'll either re-sleeve them or get new Premiers (likely buy new). Solid 12.6V on the battery, BTW.

Still puzzling over this one. I hope the head doesn't have to come off. Lucky the flippin' '68 Triumph started on the first kick.

BC
 
Stillreel said:
So I tried the plug out, grounded to the head in the dark last night and it all looks good; no un-expected spark and the spark at both plugs looks good. As much as I would like to believe its an electrical issue, it does not seem to be the case. The carbs are out now (float bowls were a wee but high) and one slide was quite worn BUT not on the misfiring side). I'll clean and re-install them tomorrow. If that doesn't work, I'll either re-sleeve them or get new Premiers (likely buy new). Solid 12.6V on the battery, BTW.

Still puzzling over this one. I hope the head doesn't have to come off. Lucky the flippin' '68 Triumph started on the first kick.

BC
Click to expand...


If you haven't wrung out the Boyer pickup wires with an ohm meter, do so before checking anything else. They fracture inside, leaving the insulation intact and produce exactly the problem you are describing. A visual inspection will not discover the problem. There's been a ton of posts made about this problem and the fix for it. If you are having misfires below 3,000 rpm and have a Boyer you can pretty well bet one of the pickup wires are fractured.
 
Stillreel said:
...but why would this be an electrical issue if the sputter only occurs at 2500 rpm (approx 1/4 to 1/3 throttle)? The start and idle seems okay.
BC
Click to expand...
That's right about where the isos begin to work, allowing the engine to perform its crazy dance apart from the frame. The engine-to-frame wiring flexes much more then. That, and the carbs suffer more, for the same reason. I wish I could see what the petrol in the float bowls looks like right about then.
Hobot makes a good point about using grease on the slides (sparingly) to check for loose slides. It works! That's part reason I went to a single Mikey after showing that my Amals were the cause of the elusive consistent idle. If this proves out to be carbs, the Premiers sound like a great alternative, save for the expense.
Nathan
 
exactly ^

grounding the plugs against the head to check for spark is just that - a non running, vibrating motor

when you have the carbs cleaned and needles securely in their clips and still have the problem

then I would next replace the boyer wires, the ones going through the back side of the timing chest

take fracturing seriously, many of us have found this to be the solution
 
The wires you want to replace are those short leads from the Boyer pickup plate. They are soldered to the pickup coils. The solder wicks up the wire for an inch or so. This is where you'll find the fracture. There are some detailed posts on how to effect a proper repair. Soldering new wires onto the board is only a temporary fix. Some have had luck with strain relieving the wires. The ring lug (crimp, do not solder) and screw method completely eliminates the problem. I would have thought Boyer would have fixed this problem long ago. As far as I know, they are still putting out the same failure prone pickup plate. Kind of reminiscent of the British motorcycle industry prior to it's demise.

I recall when Los Angeles area Triumph dealers gave the buyer a drip pan with his new Triumph. For roughly the same money you could get a new Bonneville with a drip pan or a Honda CB 750.
 
33 years with my 850 motor in a Featherbed frame and a Boyar ignition and I have never had any problems with fractured pick up wires and poeple say Featherbed frame Nortons vibrate more than Commandos, but then I did have some slack on the pickup wires as well a bit more insulation where it goes throught the hole, but I have seen it happen on my mate's Commando, lucky it was the first place that I looked at and found one pickup wire with a rub mark where it went throught the pickup hole and it was misfiring on that cylinder when I moved the wire.

Ashley
 
ashman said:
33 years with my 850 motor in a Featherbed frame and a Boyar ignition and I have never had any problems with fractured pick up wires and poeple say Featherbed frame Nortons vibrate more than Commandos, but then I did have some slack on the pickup wires as well a bit more insulation where it goes throught the hole, but I have seen it happen on my mate's Commando, lucky it was the first place that I looked at and found one pickup wire with a rub mark where it went throught the pickup hole and it was misfiring on that cylinder when I moved the wire.

Ashley
Click to expand...


Not doubting your experience, but many Commando owners running Boyers have reported the pickup wires fracturing, creating a misfire below 3,000 rpm. If I were to guess I would guess the isolastics are the difference. Obviously, the frequency of the vibration has a lot to do with the misfire, which will usually quit after ~3,000 rpm. I doubt Decent Cycles would offer a Boyer fix-it kit if this were not a frequent issue.
 
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