Forum

[TimG]

Many of you will recall recent posts where I was trying to track down a problem with poor idle and carb blowback, but had traced the start of the problem to the time I replaced the points on my 500 Classic. I re-did these points at the weekend and, well, I never knew that a few thousandths of an inch could make so much difference. I thought I had gapped them correctly the first time, but something was obviously wrong, so I took another look at the problem. My issue was trying to find out the highest point on the rather peculiarly shaped points cam. Once I had taken the plug out and (with the ignition on) sussed out where the points opened and closed, the solution became obvious. The thing is that the highest point on the cam is not actually half way between the opening and closing points (as I had imagined). It is possible to use the cam centre bolt as a reference point, and doing this you can actually find the point quite accurately. Once I had positioned the points heel correctly, I found that the gap was too narrow...adjusting it up to 15 thou (the proverbial "light sliding fit" on the feeler gauge), and all is well, in fact amazingly better! Torque and acceleration much improved, motor eager to pull at all revs, idle speed is back to what it should be, and, although I am still apprehensive, the "carb" problem seems to have gone. Just goes to show that you can get used to anything over a few hundred miles. As a bonus, the motor sounds much better, with a "rounder" exhaust note (don't know how else to describe it) - not louder, just fruitier! Many thanks to all who put me on the right path. I will monitor things over the next few miles, and see what happens as things settle in...
Cheers
TimG

[Dennis C]

I really don't like to say I told you so, BUT, if you have changed/fiddled with something and a change occurs to the way the engine runs it seems pretty conclusive that what you have changed/fiddled with could be the first place to look for the cause of the trouble, but pleased you finally got there.

[TimG]

Hi Dennis
Of course you're right, but I was so sure I had the gap right the first time, that, you know how it is, I went off down another track, following the symptoms rather than the cause...not logical behaviour, but there you are...
Cheers
TimG

[trevorch]

Hi TimG,

Your findings are quite typical - incorrect gap leads to a change in ignition timing. Points wear, both on the heel following the cam and the points actually change the gap a little by the process of arcing, building a pip on one face and a pit on the other. This makes feeling the just sliding point with the old feeler gauge somewhat touchy. Points are cheap so rather that using a points file, I always fitted new points, at the risk of occasional problems due to incorrect assembly. Practice makes perfect. Was tempted to fit electronics to eliminate points, but fear of failure caused me to retain the old tried and tested points. However, now having an EFI, no choice but to accept what I have been given. So far so good. No kickbacks on starting by foot or electric starter, hence hopefully no sudden Sprag failure. Tickover is dead even, no traffic light syndrome, though you can still stall it. Other advance for the EFI is fuel efficiency - old cast iron bullet around 5 litres/km, new Bullet 2.6 and with about 20% more power. Moral is, don't neglect your points or your plug. cheers TrevorCH

[Alan R]

Hello TIM G---------just to add a bit of "tech" to the subject matter }------ setting the points gap correctly actually sets the DWELL ANGLE correctly and from this comes magnetic flux density within the HT coil designed to be at it's peak(Saturation point)---- when the points open( ignition )and also for the correct-rated capacitor to work in harmony to produce that nice fat blue zappy spark at the plug. All this helps to produce a more complete combustion and an increase in the MeP ( Mean Effective Pressure)applied to the piston crown. RESULT !!----- SORTED !!!! ................... PS, 0.015" isn't very big, is it ??

[John L]

TimG - Proves what many of us have always known - 95% of "carb" problems are electrical............. trevorch - a fuel consumption of "5L/km" wouldn't be difficult to improve on, would it ?--------My cast iron Bullet regularly returns about 3.6L/100km.

[ed.lazda]

Being keen to replace the points didn't work for me. I had some minor timing issues, the points looked a bit pitted, so I thought I'd replace them. I had two spare sets (neither, IIRC, from our hosts). One set wouldn't close. The other set was misaligned so that the two contacts overlapped by only about half the area. Neither worked.

So I put a grinding wheel in the electric drill, ground the faces of the old points flat, and put them back with the correct gap. They have now performed without a problem for 5000 miles.