Points Set-up

[ecurbruce]

Hi Brian,
Yes you do see a brass nut in that pointer assembly, no bearing in it though. Doesn't need the support, the pointer and shaft is not that heavy... what It does have is a packing insert between the brass nut and pointer shaft, for oil control while engine is running.
I use a jamb nut on the end of the pointer shaft to secure the bolt in the end of the crankshaft, just to be sure it doesn't move. CCW rotation of the crank may keep the bolt in place, but this way there's no question that it doesn't move, maybe under deceleration of the engine?

Bruce

[gregwils]

Jordon,

I got my motor sparking, but would still not fire. It was backfiring through the exhaust. Thinking that the engine was running so lean, I finally installed the Mikuni that I had purchased last ('15) summer. Still backfiring out the exhaust, then I remembered this post of yours as a comment to my challenge. Son of a gun, I flipped the advance mechanism and the bike fired up on the second or third try.

"Long shot, but are you sure you are on the compression stroke when the points open?
It's possible for the advance/retard mechanism to be installed 180 degrees wrong, which because it turns at half engine speed would open the points at the wrong cycle. If that is the case, remove the points main baseplate, then the screw/washer that retains the adv/ret mech, then pull it out a few millimeter and turn it 180 deg before reassembling. But you said there is no spark at all, so this is unlikely?"

I still have an issue, but feel like I will be able to work through it. The bike fires and runs when it's cold, then after about 30 seconds of me goosing the throttle, it backfires, stalls and won't restart. I haven't had much time to troubleshoot, but thought I would throw it out there if there is a logical first step to determine the cause of the backfire. How do I know if it's too rich or lean if it doesn't idle for very long? I am not very familiar with the VM26 and not anxious to screw it up. Thanks.

[DucMonzter]

Hi Greg,

In regard to your question about running rich or lean, you might try this product:

http://www.gunson.co.uk/product/G4074
http://www.carbtune.com/colortune.html

Although I cannot give a first hand endorsement, a friend just recommended it to me for tuning my engine - so I'm just passing along the info. The concept seems ingenious - Colortune allows you to see the combustion flame through a transparent spark plug. You can then dial-in the correct fuel:air mixture by optimizing the color of the combustion flame. Perhaps this isn't news to everyone but I had never heard of this product until about a week ago. 'Hope this helps.

Jeff

[Jordan]

Son of a gun, I flipped the advance mechanism and the bike fired up on the second or third try.

Yippee!

The bike fires and runs when it's cold, then after about 30 seconds of me goosing the throttle, it backfires, stalls and won't restart.

Check the basics first - fuel starvation, that could be caused by blocked filters or a clogged breather hole in the tank cap.

[Nick]

Ask around and see what size jets others are using on any 250cc singles with the same carb. Those settings should be close to what you need on your 250. Hell, you could prolly just bolt a carb on there from any 250 4-stroke and be good to go. (I do much of my carb shopping at bike junkyards.)

Or, call Sudco (the Mikuni people) and ask for their recommendations.

Try to get needle and needle jet numbers, pilot and main jet size, and if possible, slide cutaway.

Story: racer buddy of mine was having trouble getting his Mikuni dialed in. He takes his bike to Sudco and this Japanese tech guy comes out, starts the bike, revs it to wide open a couple of times, gives the guy the jets to use and walks off. Bike ran perfectly.

Regarding the timing, a degree wheel and strobe (or timing marks and strobe) are the best way to time it. However, to simply get the bike running all you need is a piece of cellophane. Put it between the closed points and gently rotate the engine using the rear wheel (engine in gear) while tugging lightly on the cellophane (yes, this takes some coordination). For bikes with auto advance units, the points should release the cellophane just before TDC on compression. If, after getting the bike running, you get lots of kickback on starting (or hear detonation), the timing's probably too advanced.

[gregwils]

Thanks Jeff - I will investigate those tools.

Jordan - Thanks. The fuel taps/filters are new, and I installed a new in-line fuel filter, but would have never thought of a clogged breather hole. I assume I can stick a pin or thin gauge wire in the hole.

Nick - Thanks. I am not really connected to a local bike community, so the only people I can ask about jets are on this forum. My bike is a 250, not a 450. I talked to Chad at Sudco, told him the bike was a Ducati 250 single. He said he would jet it accordingly, however, I believe it was Jim F who said he changed his jets to improve how his bike ran. I know nothing about the Mikuni - not even how to adjust it - so moving slowly. I do have a degree wheel and timing light to set the static timing, and need to verify the timing after flipping the points.

It does seem like the backfire is a fuel issue, so will concentrate my efforts there. I will laugh if it is something as simple as a clogged breather vent. Thanks guys, appreciate the direction.

[Jordan]

See JimF's Mikuni jetting info here:

viewtopic.php?f=3&t=1239&p

[gregwils]

Thanks Jordan. I had not seen this thread, so this will be very valuable.

[Bevel bob]

Poor Running and backfires in the exhaust are usually caused by intermittent ignition. This can be from oiled ,dirty points ,gap closed up ,bad connections, dirty plug etc etc. it can also be the case that there is no ignition fault except dirty plug from carb or oiling up problems.Start with a new plug laid on the head, turn the motor till the points are closed, flick the points open with your fingernail ,you should see a BLUE spark at the plug and a spark for every flick as fast as you like. A red spark is no good. This test assumes you have battery ignition and a charged battery. If you do this test in the dark it will be easier.A dirty plug does not always mean you have oil or carb issues, it can come from bad ignition. Thats why its best to start with a good regular spark.If you have the orriginal no battery system then either change to a battery system or polish the bike and mount it in the lounge!.

[gregwils]

Thanks Bob, sounds easy enough. The bike is a Monza, so 'yes' to the battery question. My newer battery is fully (trickle) charged, my spark plug is brand new (clean) and the coil is new, but the lead from the coil is probably original. I have installed a plug tester to verify I am getting a spark when kicked, but never operated the points arm manually. I have a plug tester like the one below, not sure I can get to it tonight, but will attack it tomorrow. Actually, a great thought, thanks for sharing.

I will order a new lead, at a minimum it's fifty years old.