I have a 1979 Yamaha yz400 that is hard to start,
I bored it out to a 426 and put a 78' yz400 piston in it since I couldnt get a 79' piston because its an odd year and they dont sell those pistons anymore, I did all the research and the piston works perfect as long as the cylinder is bored correctly. The reeds are good and the carburetor has been tuned to work good, it has no air leaks aswell, once its warm it starts up 1st kick no problem, please help Im at a loss. Thank you in advance -Davin W.

Here's some simple things I came across from my time at the track way back way back when...  some sounded like they wouldn't matter, but I saw and/or experienced all of them work.  Some are basic, please don't be offended.

Push kick starter through slowly to bring piston up on compression stroke...  once you just start to feel compression, stop and reset kick starter to top.  Now give it a good strong kick through.  It takes a bit of "feel" to tell when piston is just coming up on compression...  if you go to far, just keep going slowly until it just starts to come back up on compression again before resetting kicker and trying to start. 

Spark plug gapped correctly?  (racing CDI's of that era did not have a super strong spark at cranking speeds...  new plug gap out of the box is usually quite a bit too wide.)  Gap of 0.024" seems to work well.

Is carb set to idle at correct speed (not too slow).  Sounds silly but sometimes the 250 and larger MX bikes are hard to start cold with idle set too low.

When starting, don't twist the throttle.  Starting circuit does not work right with throttle twisted open.

For the next one I'm assuming it's hard to start cold because of not enough fuel...  not flooded because petcock leaks or is left on or something.  I've had several bikes that respond well to turning on the petcock and then leaning the bike WAY over for a second...  almost on its side and standing it back up.  Then kick get on and start.  I had one 250 that was nearly impossible to start cold but started first or second kick cold after leaning over first. 

 

You need to work out if choke is doing its job or if its blocked in the bowl choke cavity &/or the tube that leads up to the choke piston.
Has a very small fixed jet in the bowl.
Also need to decide if it's flooded up & if so, why.
So if flooded then no choke & full throttle kicking.

If everything is normal then choke & NO throttle, don't flick it, don't move it.
But--"& if choke is working as it should"--after 3 or 4 kicks with throttle shut & nothing happening then close the choke & go to full throttle.
All easy to say without being there i know--me keyboard expert lol.

I had one New in "79 but of course it started easily--except it broke third gear into 3 pieces on the first lap down the paddock, just one faulty gear & had to be me that was the unlucky one that day.
Had good times on that bike, long stroke motor one year only but should have used that bore/stroke combo on all the 400's in my opinion.

A video of your starting procedure would be great, & to see how it sounds--ie--starts up flooded or lean.

Sorry, wakeup me. I'm sure MarkT has covered it all.

Old Husqvarna with Bing carb and the tickler ain't enough have to lay the bike on the side then they start pretty good. old racer trick

Yup.  Leaning over is old Husqvarna trick.  Learned it by watching a factory Husqvarna mechanic that came to the bigger races and sometimes hung out at Malcolm Smith's shop when I was a kid.    

The trick works on other bikes too. 

On any older bike with CDI type ignitions, that are hard starting, if the original CDI unit is still fitted, the spark isnt likely to be that great.

Over the years the charge capacitor degrades, which means lower output, and a spark which is not comparable to when the CDI was new.

This isn't something that is often addressed, as the systems still spark, and issues such as poor starting from cold, and reduced performance, are sometimes the only problems.

A new CDI will mean better starting, improved performance, and lower fuel consumption. CDI's from other bikes, will often work, and in some cases cheap Chinese parts can be used.

That's something i'd never considered & always thought weak spark was mostly caused by lowered resistance of the source coil culminating with no spark at all when super low or of course open circuit.
But you're talking weak spark with good source coil?.
All info is good info & thanks for posting.

Charge capacitor degrades over time, and means a weaker spark. Source coil issues, are mostly related to insulation breaking down, when the bike gets hot, which will mean no spark.

Low capacity bikes will generally start and run, even if the CDI is well past sell by date, but larger capacity ones can be very hard starters, until they are warm, when starting is often then OK.

Fitting a new CDI has pretty much the same effect as a new carb, and is something that will always make a noticeable difference, especially so on larger capacity bikes.

The truth is that anything can fail, but 1970's era capacitors are pretty darn reliable. The "capacitor crisis" started in the 1990's.

I have and had several CDI bikes and points ignition bikes with original condensers (condenser is just a capacitor) that spark and run fine. In fact, in my experience it's the modern condensers that tend to fail quickly on the points bikes. I never change an original condenser unless it's failed.

Side note, if a points condenser capacitance get out of spec, the result is point contact metal transfer.... which is something very rarely seen when using an original condenser.


The truth also is that all you need is enough spark energy to light the mixture. More spark energy does not get you a darn thing. No matter how "logical" someone makes it sound.

The biggest concern with age on these magnetos (points or CDI) is the degradation of the magnetic field.

It can be frustrating to have a bike that is hard to start. Throwing money at it with new ignition systems and carburetors when nothing is wrong with the originals is more likely to increase the frustration, unnecessarily lighten the wallet, and increase the frustration. It's also the kind of advice you'll get from an unscrupulous repair shop.