cylinder misfire: coils?

hi, I am starting to understand: it's not at all about riding, it's about fixing...

in any case: turn the bike on, all good for the first 30 seconds. then start having misfire from the left cylinder.
changed the spark plugs, changes the caps and leads.
have Boyer (old stuff), have original coils. the whole electric wiring is crap (poorly re-done recently).

It does not seem to be carbs. it turns on easily, works great for 30 secs (till it's warm...?) and then misfire the left cylinder. then turns on again very easily after 5 mins. etc.
Tomorrow I'll switch the coils and verify if the problem is there.

Any other hints?
If it's the coil, do I to buy the original ones? (I like the stock look) or is it mandatory to move on to something more modern (are there real benefits?)? in case what is the latest choice available?

(i've recently changes my plug caps and leads, has this anything to do with it? bought basic stuff from andover-norton)
all suggestions welcome.
thanks

If you already suspect the wiring system, check that the Boyer is receiving a full 12 volts at the box. If not check the battery voltage, then the kill button. These buttons often corrode, especially if the master cylinder begins to seep. The keyswitch is another possibility. Also make sure you have good ground connections.

If you are confident in the wiring, then the next likely would be the pick up wires. Boyers are often prone to breaking the wires from vibration at the trigger plate. If you are handy, you can replace the wires on the plate. I like to replace the soldered wires with brass screws soldered to the plate. Greg (Decent Cycles) has a kit you might want to take a look at. Otherwise, you could replace the Boyer with one of the newer ignitions (Tri-Spark, Arc, etc.).

I always found fixing was secondary, but necessary. But then I am using points. When you go to the EI, things change a bit and you have to start looking around, but coils are always suspect. Replacement is a good option. Good luck with your trouble shooting, you just have to be methodical.

Dave
69S

Is is an 850? If it is, I would check if the left cylinder pushrods are lifting enough to open the valves sufficiently. The original cam was very soft and would wear down. Years (decades) ago I spent at least a year screwing around with carburetion and ignition only to find that a worn cam was the problem.

If it turns out to be a coil, Old Britts have modern versions of the oem coils that are excellent but they are a bit pricey.

Ron L's advice about the Boyer pickup wires is a prime suspect. These wires break inside the insulation, so a visual inspection will not uncover the fault. Need to check them out electrically. Preferably with a meter. 90% of carburetor problems are electrical. If you are running a Boyer, 90% of the electrical problems are the Boyer.

montelatici said:

Is is an 850? If it is, I would check if the left cylinder pushrods are lifting enough to open the valves sufficiently. The original cam was very soft and would wear down. Years (decades) ago I spent at least a year screwing around with carburetion and ignition only to find that a worn cam was the problem.

Click to expand...

it is a 1972 combat.

Something else I would check, is that the stator connections are are clean & tight.
Also check the stator continuity, as a broken or disconnected wire in or from the stator will cause the the bike to misfire.

Had a terrible misfire on a 69/70 750s 30yrs ago that turned out to be a broken wire in the stator.
Also saw someone pull the head & have the valves done as the misfire was like a burnt EX valve.
I was there when it was started after the valve job & it still had the misfire.
I looked down & spotted a stator wire not connected. When we hooked it up the bike ran perfectly.

sweenz

Update, spent a few hours on it:

Visual checks (yeah, I know I should measure as well... not the right tools for the time being).
-Stator connections are good.
-Kill switch is all right.
-KeySwitch is all right.
-Wires are ... sort of good.
-Swapped the coils. it is not a coil probs.
Still get the same:
-left cilinder, after 1 mile (please note, AFTER 1 MILE) starts misfire till the bike goes off. If I leave it idle for 2 mins, all is good again for another mile.

Sparks between caps and plugs are "stronger" on the right cilinder. But I still get sparks on the left one (weak sparks: if you put half an inch distance between cap and plag you get no sparks, whereas you still get sparks if you do the same with the right cilinder).

I am still liking the fixing, although i'd like to add some riding too...

I know you said

It does not seem to be carbs.

Click to expand...

Just an easy couple of checks; Are the manifolds heat insulating joints fitted, and are you getting good fuel flow?

Cash

yup it is there.
The reason why I think it is no carb issue is that I do not have a problem turning the bike back on each time it stops after having a misfire on the left cylinder. Only thing I have to do is to leave it there untouched for 5 mins. then it goes on at the first kick and run smoothly for half a mile...
if it was carb I would have a hard time turning the engine back on (I think).

Perhaps there's a fuel flow issue to the left carb, a blocked line or stuck float?

It appears that you have identified the problem yourself, noticeably weaker spark on the left side.

So, what is the possible solution?

Reading what you have already checked, I would now move to the boyar unit in the timing cover.

You could well have a fractured wire that is sending intermittent or weak spark to the left plug.

You have pulled the plugs, cleaned and checked the gaps?

Maybe now is the time, you will have to do it sometime anyway, to rewire the boyer.

Nothing wrong with replacing the two wires that go from the sending unit all the way up the frame tube and to the

solid state boyer unit. Check the condition of the solder that connects to the wires inside the cover. This can fracture.

90% of carb problems result in electrical solutions!

In the case of a weak or nonexistent spark on one side, the Boyer can't be the issue. The unit fires on both sides simultaneously, and as far as the Boyer is concerned, one side is the same as the other. If you are still thinking ignition, start by swapping the plugs, then the coils and see if this swaps the problem from the left to the right cylinder. Look the wiring in and out of the left coil, looking for chafe or bad connections. Make sure the high tension lead from the coil to the spark plug has not chafed away its insulation and is grounding on the engine. Not much clearance in there and lot of opportunity for shorting, which will come and go at different revs, making you think it is carbs. If you decide it is a coil, there is probably no overwhelming reason to go to a new style of coil. If you are replacing the coil, make sure you end up with two 6 volt coils.

Stephen Hill
Victoria BC

cyclegeezer said:

Perhaps there's a fuel flow issue to the left carb, a blocked line or stuck float?

Click to expand...

hey... this makes sense!
-> everything goes all right till the petrol finishes in the left carb. then in 5 mins with the engine off, the petrol slowly leaks back in the carb and I can re-start. for another half mile... and so on.

I've just checked, the left petrol pipe gives a very poor flow - drops, like 1 every 2 seconds. the right one more (but this should not influence the left carb, as petrol from the right tap can still reach the left carb...).
I emptied the tank, all good.
I took apart the petrol taps, the black rubber inside is half broken and partially blocks the flow. tried to fix, not 100% succeeded -need new taps.
In any case partially fixed and went out for a ride: some problem. actually worse. BUT: the battery has gone down to zero (zero...) so that can actually play a role now too.
(is this the famous domino effect?)

(PS. the battery went to zero as I messed around with it. not because I have some issues with the alternator)
BTW I have a related question: how many wires are connected to your rectifier? I got 2 from the alternator (white and yellow + green and yellow), + Brown and Blue from the battery. that makes 3. although on the rectifier there are 4 slots. three in a row on top of it, and 1 in the middle below. (not sure it's understandable).
Does anything go there?

A red wire would plug onto the top by the bolt head.

Phil

Why not spend some time on the wiring and fix it up. Bad connections or even wires will heat up over time (maybe a mile of riding) and could cause problems. Matching resistance high tension wires with plugs and coils are important. But I doubt that would be the issue as the one side runs "good". Just pick the spark that is weak and work your way back to the battery. But maybe this information is useless if your fuel taps are plugged. I had plugged fuel filters and the bike would not be able to fill the carbs fast enough and would start to choke up. Just run with both taps open :lol:

Orsonoce said:

hey... this makes sense!
-> everything goes all right till the petrol finishes in the left carb. then in 5 mins with the engine off, the petrol slowly leaks back in the carb and I can re-start. for another half mile... and so on.

I've just checked, the left petrol pipe gives a very poor flow - drops, like 1 every 2 seconds. the right one more (but this should not influence the left carb, as petrol from the right tap can still reach the left carb...).
I emptied the tank, all good.
I took apart the petrol taps, the black rubber inside is half broken and partially blocks the flow. tried to fix, not 100% succeeded -need new taps.

Click to expand...

I'd suspect something in the carb itself, if it was a general fuel flow problem you'd have both carbs running out at about the same time. Does the left carb flood easily with the tickler? Maybe the float on the left side is sticking closed or there is a restriction keeping the bowl from filling faster than you empty it.

How old is the battery? No matter what your primary problem is you must have a good strong battery.

Idea.....run the bike OUTSIDE, and when it stops immediately open the drain on the left carb. See how much gas comes out. Also, have you checked the bowl filters?

this is the plan:
-make sure the battery is ok.
-make sure the carbs get enough fuel.
-follow the wires from the left spark and see if there's anything wrong there.

thanks all!