Timing advance issue?

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I have a stock 75 roadster with a 2 into 1 exhaust.

The issue:
At about 3000 rpm it backfires (flame out the pipe) and runs very rough.

My limited knowledge leads me to think the timing is not advancing as it should (should it?). I have never checked the timing. I guess I better buy a light. Is checking the timing a one person job? I don't see how I can hold 3000 rpm on the throttle and hold the light/read the timing mark at the same time.

Is there something I can check to see if it is an advance issue, since the problem occurs at exactly the rpm that the manual says to check the timing at?
 

ILLF8ED

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MkIII backfire

You might just play with the timing advancing or retarding slightly to see if it changes the backfire condition. If no change, start playing with the carb needle height. You didn't say if you have points or electronic ignition. If points you could have an auto advance unit not advancing.

Now, if you have a Boyer ignition, you can have a backfire at this rpm due to the reluctor low tension leads broken inside thier insulation...this happens and is hard to figure out.

These aren't the only causes, so if these don't work come on back to the forum.

David

David
 

L.A.B.

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Captain B said:
I have never checked the timing. I guess I better buy a light. Is checking the timing a one person job? I don't see how I can hold 3000 rpm on the throttle and hold the light/read the timing mark at the same time.

Basically, what you have to do is get the engine speed higher than the point where the ignition stops advancing, so you don't have to look try to look at the revcounter and timing marks at the same time, as you also need to check the ignition is advancing reasonably smoothly as well as reaching full advance.


If any type of electronic ignition is fitted in place of points, then the book RPM setting must be disregarded, as a Boyer for instance, needs to be set to 31 degrees BTDC at 5,000 RPM.

Commandos do tend to move backwards due to engine vibration when they are revved hard on their main stands whilst on a hard surface, so be aware of that. Placing a thick rubber mat under the stand does help. Otherwise chock the back wheel or get somebody to hold on to the bike as you do the strobe checking.
 
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get somebody to hold on to the bike as you do the strobe checking
.

With it off the main stand or both of you will be chasing it round the place :lol: Borrow or buy the best strobe you can, the time lapse on a poor quality strobe will effect the timing significantly the higher the revs go.
Cash
 

maylar

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Even with 2 people it's a chore. I set timing with the bike on its wheels and my brother on the seat. A third person (nephew) with sharp eyes to view the strobe while I tweak the timing plate saves me from having to circle the beast.

Backfiring out the exhaust could be timing or a lean mixture. Check timing first.
 
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Re: MkIII backfire

illf8ed said:
You might just play with the timing advancing or retarding slightly to see if it changes the backfire condition. If no change, start playing with the carb needle height. You didn't say if you have points or electronic ignition. If points you could have an auto advance unit not advancing.

Now, if you have a Boyer ignition, you can have a backfire at this rpm due to the reluctor low tension leads broken inside thier insulation...this happens and is hard to figure out.

These aren't the only causes, so if these don't work come on back to the forum.

David

David
It has points. It is stock, so I'm guessing it is not Boyer. Did they come with Boyer? Vin # 330454
 

L.A.B.

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Re: MkIII backfire

Captain B said:
It has points. It is stock, so I'm guessing it is not Boyer. Did they come with Boyer? Vin # 330454

No, they always had points originally, however fitting an electronic ignition was, and still is, a popular modification.
 
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I'd want to know a few other things. Did it ever run right? Did it start doing it after sitting all winter? Did the problem start on the road?
What carb does it have?

If the problem started after sitting it is often corrosion or bad gas hardening, or just bad gas. If it never ran right it may have not been re-jetted after the exhaust change.
If it started on the road, broken wires, bad connections, wear on carb, etc.
 
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My Dad put it up knowing it wouldn't see the road for a while. Drained the tank and carbs (twin Amal) and removed the battery. 20 years later I filled the tank, made sure the oil was the right level and hooked up a battery and she fired right up. Ran pretty good for about eight (10 minute or so) trial runs around the block (about two trial runs a week). Then one day a little roughness around 3000 RPM and a backfire or two. I decide not to mess with it till after I swapped on some Vincent style handlebars. After the bar swap it was idling real high and backfiring. I discovered the throttle cable was pulled tighter than it should be (because of the bars). I got that worked out, it idled at 900, but still backfired at 3000. Removed and cleaned the gas tank, fuel line, and filters. I put in fresh 93 octane and it still does it.
 

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Cookie is right-on with his questions; a good diagnosis will depend on obtaining those answers.


Just a note about the effects of poor winterization. Now is the time when owners are bring their machines to life after a hard winter. Todays gasolines, even with stabilizers, are so full of aromatic compounds that evaporate quiclky, like various ethers, that they are prone to clog small carb passages, even main jets over time. If you store you carbureted machine in a tempature controlled environment you can get away with emptying the tank and draining the carbs, if your machine is subject to temp swings you either fill the tank (and drain the carbs) or oil the tank (and drain the carbs). I have been of the fill the tank group before I had temp controlled storage, in that case I would drain the tank in the Spring and feed the crap to me least favorite 4 wheeler with a dose of Gum-out or Techron.

RS
 
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If it was my bike the first thing I'd do is a standard ignition tune up. This would be replace points, condensers, lube and inspect advance, plug wires, clean every connection in the system, measure voltage of resistor if fitted, and critical connection points. Then I'd fire it up and set timing and check for advance.
After I had the ignition as close to perfect as I could get it I'd seal the tank and check petcocks and screen ( this problem could be caused by crud blocking a petcock) and then go on to carbs.
I've actually been doing much the same on my own Norton since she has been stored for a while.
 
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Good advice from Cookie on this, also bear in mind that with points ignition the left and right cylinders can be out of sync with each other, as they are independently adjustable. When you check the timing you must strobe both the left and right plug. This is good practice in any case but is actually not necessary on most bikes with electronic ignition.
 
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