Rough at startup, then not

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May 12, 2008
My '75 MK3 has developed problem that is driving me nuts. Starting is kind of tough, but if I give it some throttle while hitting the starter button, it eventually fires up… with a bang. The idle is a bit erratic, but after a few minutes calms down. However once I get on the road, the first several minutes are a mess. Loud backfires, lots of hesitation. Then after about 2min, mysteriously, all becomes normal for as long as I am out and about. If I let the bike cool down all the way, the problem comes back. I’ve cleaned the pilot within an inch of its life, the timing seems fine (Pazon electrics), good strong battery, plugs look normal… I am at a loss. She never leaves me stranded, but I’d really like to figure this thing out. I let her warm up for a good 5min and don’t keep the choke on for more than 2-3. Am I rushing the process too much? Any suggestions? Thanks in advance.
Helps to diagnose to if there is a little more info.



and anything else that is not stock (for some people it might be easier to list what is stock) :mrgreen:
Sorry! The bike has Pazon electronic ignition, Podtronics regulator, plugs are BP7ES, dual coils, original pos grounding, and a single 34 Mikuni. I also installed a Yamaha breather valve (got rid of the Motormite... it died after about 100 miles). The alternator is one of the high output types and appears to be working fine.
Are you using the ticklers or just the chokes?
Is there any smoke from either exhaust during warm up, do both exhausts warm up at the same rate?
Hi c,

We have nearly identical set-ups on our mk3's. Can you answer the following?

1, Is your Mikuni choke stock (up-down on-off lever on the carb) or have you installed a continuous lever contol?

2. Are you fully choked during the rough-running initial period?

3. Are you choked at the magic 2-minute point when the engine smooths out?
No tickler on the Mikuni, but there's usually a little smoke along with the initial backfire. But only for a second... The carb has the original lever choke. After 2-3 minutes of warming up I take it off the choke, unless it starts to die, but usually after 3min it is idling fine at about 800rpm. However, when I start down the road at minimal throttle, it hesitates and backfires. If I roll the throttle back it surges and hesitates, with lots of backfires. The weird thing is that I can 'ride through it'. Within about 2-3miles, it settles down and runs like a dream, tons of response. This is a fairly recent development too. Earlier in the year it ran great from the get-go. The ignition system and setup seems fine. If something's clogging up the carb, I shouldn't be able to just ride through it, right?
Hi c,

I live on an island with a circumferential road 13 miles long with no stop signs or lights; perfect for the "sea trial" phase I'm in with my recently rebuilt Mk3. I replaced the on-off lever choke with Stan's (Rocky Point Cycle) Mikuni kit and a Doherty air lever on the handlebars, so I have a finer adjustment of the choke. Here's my routine and experiences regarding choking:

The bike starts quickly on full choke and immediately runs up to 2500 rpm or so. I adjust the choke lever to bring the rpms to below 2000 rpm and immediately begin riding. Halfway through the ride, say 10 minutes in, I would remove the choke, and within 30 seconds or so the engine coughed and wheezed, acting as though it was fuel starved. If I failed to apply more choke, the engine would actually stop. The engine did not fully warm up until the end of the first loop, 13 miles and 2o minutes from start-up. At that point I could remove the choke completely;the bike would idle and run smoothly without choking on the second loop of the island.

Stan confirms that the Mikunis are properly set up for a fully warmed-up engine, and that could easily take 15 minutes or more. Before that, some choking is necessary to avoid the problems that I experienced. Jetting the carb to relieve the need to choke at, say, 10 minutes from start-up, condemns the engine to always run too rich when fully warmed up.Your experiences sound similar. You might consider the cable kit to better regulate the choke; it's cheap and easy to install.
Ok, so I may be rushing things. Tomorrow I will run on the choke longer and see if that's the problem... This may be the least expensive issue yet!

And I have to ask... if my math is right you live on a island about 4miles across! Whoa... 13mile ring-road, no lights or stop signs, Commando in the garage... you must get dizzy whipping around there. :mrgreen:

Hi Paul,

Well, the island is the tip of a submerged mountain chain, so there is at least some elevation to the ride. Mind you, this is only for my sea trials. I figure 2-3 more excursions to reach full confidence in the Norton. I'm connected to the outside world by floating bridges and I-90.
I have seen this many times. Change to some more foul resistant spark plugs. Autolite AP-64's work for me. the cooler 63's work too, gap them about .035. Yes all platinum plugs are resister type but what they take away in power they seem to give back in toughness. There are other choices Bosch Wrp7's come to mind a little smother softer feel. Some find the 64's a bit harsh make the bike feel more vibration in the seat.
Some may say this masks the real problem but if you lean burn you need more fire to light that kind of mixture. It's worth a try if it clears the problem you will at least be on the right pathway to tuning the carburetor to a better mix. Hey for about four bucks you can get to the bottom of it.
Rick and Norbsa,
Thanks for the tips! I choked her for a few miles this morning (a capital offense in some states). She was good and warm (man... this is sounding like the script for a snuff film) and... well... same problem. Children were crying, mothers were shouting... when she backfires its like a cannon. After work I will get some of the plugs Norbsa suggested and give that a try. She was running a bit lean last year, but I thought I'd dialed in the mixture... maybe not. Thanks again and I'll let you know the results.
If it were me, I'd be at the substitution phase.

Have you got a spare battery ? Can you borrow one ? Do you still have the old ignition system or the Amals ? Can you hot-wire to the ignition and replace the pick-up wires ? Even a Pazon can't work with poor connections.

At least these will let you rule components out.

In my experience, big backfires are more usually electrical. That it's worst after turning the starter and improves a bit would make me suspect the battery.
Here's the latest: when I checked the plugs after a 60mi ride at speed they looked fine. Today I spent about 20min putting around town never getting out of first or second, and they are covered with soot. So I replaced the old BP7ES plugs with the Autolite AP-64 and will take her out for a ride in a bit. This is looking more like a mixture issue, correct?
Have you dismantled the Mikuni and checked if its OK? The first start cold running with excessive period of choke on is symptomatic of too small a Pilot Jet. It should be a 35 or 40. Rarely do they work well under a 30. The bike should start with choke full at the 1st or2nd kick with good battery and plugs. It should run at a fast idle, around 2000RPM. After 30 seconds or a minute, depending on how cold the day is, the choke can be turned off. Check that the float needle and seat are working OK and the main jet is secure. Also check the flange mounting is secure, with no possible air leak. Apart from that the Mikuni is a very dependable carb.

So I took her out for a 20mi ride at speed. Absolutely did not want to start from cold, had to bump start it. Ran rough again, but then seemed to settle down... until I took a corner at no throttle, at which point she died. Had enough speed to pop the clutch and get underway again, but I had to keep the revs way up just to keep going. Under throttle it ran fine, just would not start and couldn't seem to handle low revs. Took out the new Autolite plugs and they looked fine. The battery 'seems' fine (reading 12.8v), but I should probably test it under load. I had the carb off awhile back and cleaned out the pilot jet, but didn't really do much else. Tomorrow I'll tear into it again. It seems to be getting worse, so that might be a clue... Thanks for the suggestions, I need all the help I can get.
Where is the air screw set? Have you moved this at all? Try setting it at 1 1/2 turns out. Do you know what pilot jet is in it? I just went thru a hell of a time getting my Mikuni dialed in. But once you do they are great carbs, Mine would start great and run fine up to about 2,600 rpm and then just spit and sputter. But once it was warmed up it would run better, Or when I just opened the throttle to almost full it would just take off like a bat out of hell. It was set good at idle and just above but was way rich in the middle. What I did was lowered the needle, This is done by raising the clip on the needle and this will allow it to set down further in the jet. Man did that make a difference, That got me close and then after a different needle jet and a few more plug chops I have a real hot rod. But I think I must have pulled out a lot of hair. The biggest problem for me was I have a two into one free flowing exhaust and a Megacycle cam in a Combat motor which changed all the settings that should have worked. :roll:
Ran rough again, but then seemed to settle down... until I took a corner at no throttle, at which point she died. Had enough speed to pop the clutch and get underway again, but I had to keep the revs way up just to keep going. Under throttle it ran fine, just would not start and couldn't seem to handle low revs.

I had a similar problem last year. I think it was a combination of a too-small pilot, and maybe some rusty crud in the tank. The small holes in the pilot would clog up and I would get a dead engine or poor idle at low revs. Trying to throttle through it wouldn't help until I got the main jet going, so it ran fine down the highway and like crap in town. I found myself cleaning the pilot all the time. I put in a 35 pilot and have yet to coat the tank, but that's on the winter to-do list. In the meantime, I removed the petcocks and sloshed some gas around and emptied it repeatedly until my fuel looked crystal clear. I haven't had this trouble since.
I am still struggling with fuel-related problems on my Matchless G12 and some of the symptoms described here sound familiar. My bike runs OK at times but occasionally runs out of fuel at highway speeds. But generally only after 20 miles or so of running perfectly fine. I initially suspected the float needle getting hung up or float level being too low. I checked float level and it is fine. I also replaced the float needle housing and float needle. Now I am starting to suspect petcocks. I installed a set of cheap aftermarket petcocks which don't feel right to me and don't always completely cut off the flow of fuel. I am thinking that they may disintegrating inside and be obstructing fuel flow and this may be happening intermittenly. Lots has been written about crappy petcocks on several different forums. I think you need to pull your tank and clean it out really well as well as check out your petcocks. Do your petcocks have mesh filters on them? Do you have an inline fuel filter? One major weakness of Mikunis is that they do not have a built in mesh filter like Amals do so you definitely need an inline filter. I use a Pingel two-into-one cleanable bronze filter on my Commando. From the description of your startup routine, the bike is running way too lean. This lean condition could be caused by carb settings but fuel flow into the carb could also be suspect. Unless you know for sure that you have good fuel flow from the tank, your could fiddle with carb settings until the cows come home and never solve the problem.
Big back-fires to me usually mean temporary loss of spark. When the spark resumes unburned fuel in the pipes is ignited with a bang. I reckon you should be looking at the ignition, coils and plug leads, you may well have a weak spark.

Best of luck.

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