How to tweak and tune for good starting

[Terry Syd]

A weaker spring will 'ramp' in the fuel quicker. Likewise, if you re-drill the transition holes you can change the ramp. A larger hole closer to the throttle plate will ramp in the fuel quicker, putting the larger hole away from the throttle plate delays the ramp.

Those carburetor mods only affect throttle response, they don't affect the starting. I'd check the coil gap first to make sure I was getting the hottest spark for starting.

 

[CR888]

Do you only open up existing transition holes or in some cases drill new transition holes closer to butterfly. Terry

 

[Terry Syd]

I just reem out the existing holes. A micro-drill set is your friend.

 

[49er]

A partly sheared flywheel key was part of the problem. It ran but not well.

 

[davidwyby]

Quoting @Trains from another forum

There is a process that stihl outline that gives you correct position of the LA or butterfly idle position on the saws.
This can be adopted to most other saws as well.

Depending on the size of the saw depends on the rpm its set to, smaller saws have higher idle etc, but for a 70cc type saw, ie a 460 here is the procedure.
LA is butterfly position.
L is the low mixture screw.

Warm up saw,
Set the LA to 3300 then lean out the L screw for max rpm, clockwise, if the max rpm is over 3700, bring back the LA to 3300 and start again. and ensure the max rpm is not over 3700.
Then set the LA to 3300rpm, then bring down idle speed to 2500 by using the L and richening it up turning anti clockwise.

Make sure its got good crisp acceleration off idle, and that it is happy idling and not loading up over 20-30 seconds and your good to go.

This ensures that the butterfly is set correctly, and your not chasing your tail adding LA then L and then LA etc

there is also a good vid online that shows this process too.

 

[mettee]

So you are tuning the throttle plate opening to not allow over 3700 rpm with the low needle adjusted leaner for max rpm?

I assume that puts it in a sweet spot.

 

[davidwyby]

I haven’t tried it yet. I bet @Stump Shot has.

 

[Stump Shot]

I haven’t tried it yet. I bet @Stump Shot has.

You can physically look at the carburetor throttle plate and see if it looks closed all the way or if it has a good crack to it or not if it's giving you troubles 9 out of ten the throttle is closed and the L leaned out to compensate for idle speed. This will even affect hot starting and is one of the biggest bugaboos I find when tuning a saw. You have to wait until it's hot to get it set right as this will give you a too rich a setting, a hair rich is fine, normally a cold saw should balk at trying to run after first start for a few seconds until it warms. Other signs the settings are wrong are the saw falls flat on its face and dies or wants to die after letting go of the throttle, only starts with choke on or high idle starting engaged or manually holding the throttle open.
A crispy diaphragm set in your carburetor will also make it difficult to pull and give fuel, as well as a dirty fuel filter and kinked or leaking fuel and impulse lines.
You should be able to pull your saw over three times and it be running, one to prime the carb, two to prime the engine and third to fire up. First two on choke the third off. You can shorten this by slowly rolling the engine over to get the carb primed first and save one hard pull on a high compression saw. The above is true for most saws unless you have a McCulloch 10-10, then everything can be accomplished in one pull, don't ask me why, but I've seen it over and over again, even bone dry saws, just filled with gas, which some saws have a hard time priming for the first time.

 

[mettee]

^STUDMUFFIN