Runs Poorly For 2 Miles

Cburg74 · Mar 30, 2014

Hello Everyone - I'm a newbie as of yesterday and I'm very happy to be a member

. I have a restored 74 Commando 850 with Boyer electronic ignition and single Mikuni carb . The bike kicks over fine (usually) and I let it warm up for a minimum of 10 minutes. After start up it idles fine too. Here's my problem.... the first two miles or so it runs terribly, every time. It missed, it pops, it backfires and then all of a sudden it come's to life and runs flawlessly. This happens whether it sits for a day or for 2 weeks. Since I am not the most mechanically inclined I have no clue what to do. Any advise would be appreciated. Thanks.

1up3down · Mar 30, 2014

how long do you leave the choke lever pushed down?

why do you feel you need to wait 10 minutes idling in order to leave your home?

I suspect your plugs are partially fouled, pull and clean then, take choke off 15 seconds after starting and keep idle going with throttle hand

SquareHead · Mar 30, 2014

Welcome and congratulations on your new purchase!

A couple of questions: How long had the bike sat before you bought it? Was it ridden regularly or did it sit for awhile?

Also, I would caution you to be careful about letting the bike warm up for 10 minutes. That's a pretty long time, in my opinion, to let an air cooled bike sit without air flow. In the right circumstances, it could easily overheat and cause some pretty severe problems. I usually let mine warm up long enough to put on my riding gear (gloves, helmet, etc.), maybe 2-3 minutes, at most...

Chris

pete.v · Mar 30, 2014

You say you have restored a Norton 74 850 and you are not mechanically inclined? I feel you are selling yourself a little short my friend.

Anyhow, it seems when anyone one says "poor running" and "Boyer" in the same sentence, it is suggested to look into the battery and/or charging system. Check volts DC to the battery at different RPMs like 2000, 2500, 3000 and 3500 and report back. Do a load test on the battery to see if is holds charge just as you would with a car. The other things are the timing setting and clearing the carb jetting of any obstructions.

I realize this is pretty general but you have to start somewhere and the devil will be in the details.

There is lots of info on tuning a Mikuni, you just have to Google it.

As others have said already, leaving the choke/enrichener on for more than a few second can foul the plugs. It is possible that you blackened the plugs a bit and it take some run time to bring a good spark back to them.
I use the throttle stop screw to raise the idle a little and after a stop sign or two or a mile or so, give it a flick back to normal.

rvich · Mar 30, 2014

+1 on the battery. I had a similar thing going on last year until I swapped out my battery. Mine was an AGM battery from a "reputable" manufacturer but it was having some kind of internal issue that was not apparent from checking the voltage. However, it felt unusually warm to the touch after a ride. A different battery cured the problem so I didn't spend much more time trying to figure that issue out. If your charging system is up to snuff, you could disconnect the battery (protect the leads from contacting) and go for a ride.

Russ

concours · Mar 30, 2014

pete.v said:
You say you have restored a Norton 74 850 and you are not mechanically inclined? I feel you are selling yourself a little short my friend.

Anyhow, it seems when anyone one says "poor running" and "Boyer" in the same sentence, it is suggested to look into the battery and/or charging system. Check volts DC to the battery at different RPMs like 2000, 2500, 3000 and 3500 and report back. Do a load test on the battery to see if is holds charge just as you would with a car. The other things are the timing setting and clearing the carb jetting of any obstructions.

I realize this is pretty general but you have to start somewhere and the devil will be in the details.

There is lots of info on tuning a Mikuni, you just have to Google it.

As others have said already, leaving the choke/enrichener on for more than a few second can foul the plugs. It is possible that you blackened the plugs a bit and it take some run time to bring a good spark back to them.
I use the throttle stop screw to raise the idle a little and after a stop sign or two or a mile or so, give it a flick back to normal.

Man says he "has a restored Norton", never said he did it himself.

concours · Mar 30, 2014

Cburg74 said:
Hello Everyone - I'm a newbie as of yesterday and I'm very happy to be a member . I have a restored 74 Commando 850 with Boyer electronic ignition and single Mikuni carb . The bike kicks over fine (usually) and I let it warm up for a minimum of 10 minutes. After start up it idles fine too. Here's my problem.... the first two miles or so it runs terribly, every time. It missed, it pops, it backfires and then all of a sudden it come's to life and runs flawlessly. This happens whether it sits for a day or for 2 weeks. Since I am not the most mechanically inclined I have no clue what to do. Any advise would be appreciated. Thanks.

Hi, welcome. You WILL be mechanically inclined... or be a long time pissed off. :mrgreen: It may be as simple as lean jetting that runs better fully warmed, or perhaps you've left the enrichener ("choke") on too long and sooted the plugs. More info please....

pete.v · Mar 30, 2014

concours said:
Man says he "has a restored Norton", never said he did it himself.

Oops, you are right, Concours.

Cburg74, if you are not machically inclined now........[video]https://www.youtube.com/watch?v=NPOb3DlB7WA[/video]

Biscuit · Mar 30, 2014

my idea is a combo of what others have said. Choke on too long can foul the plugs a bit and take some time to self clean (when it starts running good). Or the idle circuit could be a bit rich and 10 min of just idling can foul the plugs that way even with the choke off. Or it's really ok but a bit lean overall so some real heat has to get into the engine for it to be happy. But I agree with the too long a warm -up period.

ML · Mar 30, 2014

Cburg74 said:
single Mikuni carb . The bike kicks over fine (usually) and I let it warm up for a minimum of 10 minutes. After start up it idles fine too. Here's my problem.... the first two miles or so it runs terribly, every time. It missed, it pops, it backfires and then all of a sudden it come's to life and runs flawlessly. .

The Pilot Jet is too small, often size 25 a mistake when supplied as the new carb. Commando needs a 35 or 40 Pilot Jet, easy to install.

Cburg74 · Mar 30, 2014

That is all great information everyone. Correct, I did not restore it alone, my friend did most of it for me. It was in pretty rough shape when I bought it from a different friend. I likely have well over 10K into it ( I know, crazy!). Some CNW, some Rabers (Mike's awesome!), some Rocky Point. All I did was order the parts and write the checks.

I think I'll check all items mentioned and report back with my findings. PS... Any advise on plugs? I'm currently running NGK Iridium-ix BPR7EIX.

I think I need a new set of wires too. One of the contacts on the right side looks like a piece cracked off or something. Maybe that could be my problem. My dumb ass is probably fouling the plugs with too much choke and sooting the plugs AND I have a bad wire. Come to think of it, I had a snowmobile that use to do the same thing. In one weekend I went through 13 plugs.

Thanks again! Please let me know on the plug and wire recommendation. Cburg74 Out 8)

Hobot · Mar 30, 2014

My dumb ass is probably fouling the plugs with too much choke and sooting the plugs AND I have a bad wire. Come to think of it, I had a snowmobile that use to do the same thing. In one weekend I went through 13 plugs.

Now that sounds like a seasoned Norton-neer's thinking of all the things in a row that can all do the same unpleasant thing. Hope that the end of the list for a while.

Cburg74 · Jun 22, 2014

I thought I had this problem licked when I found a bad spark plug wire. The bike ran great for a while. Then it seemed the same problem started up again early last week. On Friday I went for a short ride and one big POP! later and the left exhaust lock ring, washer and tab washer blew right off the head. The only thing keeping the pipe attached was the muffler bracket. The treads on the exhaust port look a bit mangled and cross threaded. Yeah! Question... what happened, what do I do and how much might it cost?... to do both sides. I'm in Southeastern WI and not to far from T.C. if it matters. Thanks in advance for your help and advice. - Cburg74

trident sam · Jun 22, 2014

Cburg74 said:
Hello Everyone - I'm a newbie as of yesterday and I'm very happy to be a member . I have a restored 74 Commando 850 with Boyer electronic ignition and single Mikuni carb . The bike kicks over fine (usually) and I let it warm up for a minimum of 10 minutes. After start up it idles fine too. Here's my problem.... the first two miles or so it runs terribly, every time. It missed, it pops, it backfires and then all of a sudden it come's to life and runs flawlessly. This happens whether it sits for a day or for 2 weeks. Since I am not the most mechanically inclined I have no clue what to do. Any advise would be appreciated. Thanks.

If it won't run without choke for that long I would say your pilot circuit is partially blocked, take the carb off, remove float chamber and find the round casting in the carb body . Remove the pilot jet from here and clean it, just spraying carb cleaner will usually not work.
As mentioned, it isn't good practice to let an air cooled motor idle for long periods
sam

les ward · Jun 22, 2014

I had this problem when I got my Mikuni. Needed choke (enricher) on for a long, long time or it would pop and bang and misfire. When warm was perfect; just took a long time. Larger pilot jet sorted it for me. From memory 35 or 40, but I am sure others will correct me if my memory is shot.
Hope this helps.

dave M · Jun 22, 2014

If you read the most recent post from Cburg74 you will see that he solved the initial problem and is now looking for advice on where to go to repair his exhaust port threads.

Cburg74 · Jun 22, 2014

Thanks Les. Does anyone have any idea what size jet that might be? Les thinks 35 or 40?? Also with regards to the muffler situation... any ideas on who does resleeving or what is all involved. What kind of cost can I expect?

rvich · Jun 22, 2014

As for the exhaust thread, do a search for thread repair, or exhaust thread. There is lots of information. One of our members, comnoz, does a bronze insert that is really a great repair. If you go to advance search you could enter "exhaust thread" and then put his user name in the next box down to seek his posts. I don't know how much he is charging these days, but you might as well send him your head for repair and evaluation of the valves and guides while he has it. Maybe helicoil inserts for the studs too. You will end up spending a thousand dollars or so (ask Jim) but you will only be frustrated if you get the threads repaired and then have to pull the head again in a short time to take care of something else.

Russ

Cburg74 · Jun 22, 2014

Perfect. Thank you Russ. BTW, what might have caused that much pressure to blow the exhaust pipe off like that??

kerinorton · Jun 22, 2014

Cburg74 said:
That is all great information everyone. Correct, I did not restore it alone, my friend did most of it for me. It was in pretty rough shape when I bought it from a different friend. I likely have well over 10K into it ( I know, crazy!). Some CNW, some Rabers (Mike's awesome!), some Rocky Point. All I did was order the parts and write the checks.

I think I'll check all items mentioned and report back with my findings. PS... Any advise on plugs? I'm currently running NGK Iridium-ix BPR7EIX.

I think I need a new set of wires too. One of the contacts on the right side looks like a piece cracked off or something. Maybe that could be my problem. My dumb ass is probably fouling the plugs with too much choke and sooting the plugs AND I have a bad wire. Come to think of it, I had a snowmobile that use to do the same thing. In one weekend I went through 13 plugs.

Thanks again! Please let me know on the plug and wire recommendation. Cburg74 Out 8)

If you are going to ride hard, then those plugs are good. If not, go to bp6es or y plugs. I run these or n9yc plugs. I don't screw the ass off my bike though.

Runs Poorly For 2 Miles

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