Ran well, now doesn't

I hate to be one of those guys that want their running issues to be diagnosed at a distance, but I'm getting to the end of my rope (and expertise)!

'75 MkIII that ran well. I've owned it for a little over 2 years, and haven't done anything much to it regarding engine. PO installed Boyer (mkiii) and dual coil and 34mm single Mikuni (pancake air filter). Most of the problems I've had with it had to do with loose wire connections; once a loose connection to the Boyer stator plate, and once the spark plug nut (on top) came off and I limped home, backfiring all the way.

Now, it starts and idles like a champ, but has a huge problem taking throttle. I've check compression, replaced tank contents with new gas, looked at plugs (which look ok to me). I've chased down the wiring to the ignition and can't find any issues. I haven't removed the Boyer stator plate; it doesn't have the ziptie around the wires, they are encased in what looks like clear epoxy, which I have assumed is an upgrade to avoid vibrations issues?

I've removed the float bowl; no crud, very clean. I've removed the main jet, nothing clogging it.

Today, I took it for a test ride and found no improvement. When I got home, I noticed that my beautiful chrome headpipes are now blue at the ports and gold underneath the engine. Lean running?

Typically, I don't know which way to turn, ignition or carburation!? It doesn't want to pull over about 2500 rpm unless I declutch it, then it will begrudgingly "clear its throat", but when load is applied it bucks and pops.

It had been sitting for about a month before the problem started. I went for a ride, had ridden about 4 miles from home with it running fine, then it started acting up and hasn't run right since.

Any help or suggestions would be greatly appreciated.

Thanks,

Michael

Sure there'll be lots of suggestions but I'd ask if fuel delivery is okay? Are the taps clear and flowing well?

I have had a couple of Mk 3 Boyers develop "dead spots" in the advance curve in the past, usually at 2500 and 3500 RPM, very similar symptoms as what you describe. Could you get a hold of another box and try it?

BTW - I can highly recommend the Mk 4 Boyer box, especially if you ever decide to use your electric starter in the future. If you do go to the Mk4, you will need to retime the bike, or else you will be running with about 12 degrees less advance.

EstuaryBoy said:

Sure there'll be lots of suggestions but I'd ask if fuel delivery is okay? Are the taps clear and flowing well?

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Yes, they flow fine. I used them to drain the gas tank. Although I guess that doesn't eliminate the fuel line...

Derek Wilson said:

I have had a couple of Mk 3 Boyers develop "dead spots" in the advance curve in the past, usually at 2500 and 3500 RPM, very similar symptoms as what you describe. Could you get a hold of another box and try it?

BTW - I can highly recommend the Mk 4 Boyer box, especially if you ever decide to use your electric starter in the future. If you do go to the Mk4, you will need to retime the bike, or else you will be running with about 12 degrees less advance.

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Thanks. I have been suspicious of the Boyer box. No, I don't have one to swap out. If it comes to that, I'll look into a Mk 4.

The mark IV is a real improvement , if in fact that is your bike’s running issue .... even if the bad running is not your Boyer , the Mark IV would still be an upgrade

Sitting for a month is usually a plugged pilot jet.
However, It does start and idle but you don't say if that's with the enrichener on.
Off idle sluggishness can be attributed to the enrichener in the on position.

I would check enrichener and pilot jet, then move forward.

MichaelB said:

Sitting for a month is usually a plugged pilot jet.
However, It does start and idle but you don't say if that's with the enrichener on.
Off idle sluggishness can be attributed to the enrichener in the on position.

I would check enrichener and pilot jet, then move forward.

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Thanks for the reply. It idles really well almost immediately with no enrichener. I use the enrichener slightly to start (it's warm here), and then shut it off. I've checked the enrichener cable to verify it has slack when off (reverse of Amal), and that everything is tight and properly closed. Since the bike started running poorly while riding, I considered bad gas or plugged main jet, both of which I've eliminated.

How old is the fuel in the bike? E10 fuel breaks down much faster than conventional gasoline. You could have an issue with the throttle needle coming out of it's retaining clip. If it falls down, that will create a lean condition. I would suggest dismantling the carburetors for a close inspection. While you have them apart, put them in an ultrasonic cleaning machine. Perhaps you have a friend who owns one. Lots of videos of cleaning small engine carbs on Youtube. Take a test ride. Pay close attention to the throttle position where the issue crops up. Is it off idle, 1/8 to 1/4 throttle, 1/4 to 3/4 throttle or over 3/4 throttle.

Charlie K

Possibly ignition advance issue? Do you have a strobe light to check it?

Bill C. said:

Possibly ignition advance issue? Do you have a strobe light to check it?

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Probably the next thing I'll check (also, probably should have before now). I've been assuming the Boyer is doing its job advance-wise, but I'm getting suspicious.

Do you still have the original points ignition components? If so, you could temporarily re-install them. That would allow you to verify/eliminate the ignition system [other than the ignition coils]. Retarded timing will cause the pipes to overheat and change colors. Happens because the fuel/air mixture is still burning as it leaves the exhaust port.

Charlie K

michael95453 said:

Probably the next thing I'll check (also, probably should have before now). I've been assuming the Boyer is doing its job advance-wise, but I'm getting suspicious.

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Just a guess. Other's here with more expertise. I know the Mikuni is pretty dependable if getting a good supply of clean fuel as it sounds like your's is.

Minor air leak (somewhere between the carburetors and head) in one or both intake tracts can present like bad ignition at take off. Bike can idle fine and run OK above mid range and up, but be slightly starved for fuel and run weak off idle for a second, or during low speed on off throttle events. Usually the last thing I check on my setup, but often should be the first thing to check.

Broken leads to Boyer pickup. Very misleading because the engine vibration is always varying, making that break intermittently worse/better. Easy to test.

michael95453 said:

Thanks for the reply. It idles really well almost immediately with no enrichener. I use the enrichener slightly to start (it's warm here), and then shut it off. I've checked the enrichener cable to verify it has slack when off (reverse of Amal), and that everything is tight and properly closed. Since the bike started running poorly while riding, I considered bad gas or plugged main jet, both of which I've eliminated.

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Very well, I would then verify the needle is moving with the slide, jets, spray tube are all tight, then move on to electrical.

As concours said, even if this isn't your issue, it's very worthwhile checking the wires on the Boyer stator as they have a long history of failure and it will catch you out one day. If your one doesn't have the piece of rubber under the supporting cable tie it is very likely to be the problem. But I believe they still fail even with it. If you snip the tie and bend the cables you will easily be able to feel if the strands are broken (normally under the top edge of the tie). This is how I fixed mine, but there are many other solutions on here.

I had that issue with a Boyer issue on mine (fitted before my ownership and probably quite old).

I was CONVINCED I had a carb problem until I sound the above by accident!

Question for the OP, when you say it has a problem taking throttle, what does that mean, from closed to 1/4 throttle? Won’t take WOT? Or...?

And are the symptoms any less / worse when the engine is warmed up?

I had one of the very first Boyers , fitted on xmasday !! in 72 excellent service for 20years and then was replaced by a mk3 . Bad mistake changed to Pazon which are similar to Boyer but of superior quality. Would suggest you have a broken pick up wire making intermittent contact

The wires can break just outside the points compartment too. There should be some slack or even a loop before the wires go into the points compartment. It is easy to check the wires with an ohmmeter while wiggling them. Use alligator clips to connect the meter securely then wiggle. It seems like timing has changed (retarded). Possibly the pickup PC board has rotated.