How to tune an inline twin?
06-17-2013, 11:20 AM
#1
How to tune an inline twin?
I have a 3W-140 inline twin. Discontinued engine but it has never seen fuel. It also has two walbro carbs. Question: how the heck do I tune the fuel/air mix? Manual says to start initially with one turn open on high and low needle settings, but how can I know if each carb is properly mixed/adjusted after that? Any advise and tips are appreciated.
06-17-2013, 12:27 PM
#2
My Feedback: (1)
RE: How to tune an inline twin?
Hi Mark, With my old Norton twin...seperate AMAL carbs I started at factorty recommended settings. Yor twin will tune the same way.Then by ear fiddle with only one cylinder's mixture both hi and lo as you would with a single. You'll hear what you're doing or use a tach to get the engine to run as good as you can make it. Then tune the other carb the same way....like an optometrist...better A or better B.
When it sounds about right run it for a while at normal throttle positions and then do a plug chop. By that I mean to turn off the ignition...not slowing to idle and choking or closing the carb to stop the engine from running. With enough running time to give the plugs some color take a plug reading on each. That's where you see if any other adjustments are needed.
Hope this helps. Mitch
When it sounds about right run it for a while at normal throttle positions and then do a plug chop. By that I mean to turn off the ignition...not slowing to idle and choking or closing the carb to stop the engine from running. With enough running time to give the plugs some color take a plug reading on each. That's where you see if any other adjustments are needed.
Hope this helps. Mitch
06-18-2013, 04:25 AM
#5
My Feedback: (2)
RE: How to tune an inline twin?
I think something that needs to be taken into account is "syncing" the carb's throttle openings? You could get a false lean reading on a cylinder that's carb is open further than the other?
On a snowmobile engine (just a bigger version of what you're talking about), we start by setting the high and low speed setings exactly the same, then the carb openings are set so they start opening at exactly the same time and reach WOT at the same time. Then tune from there....
After that, very often a cylinder reading rich isn't pulling as hard as it should be, or a lean one is pulling too hard? Carb mixture settings are always kept identical. If you richen one side, you match that adustment exactly on the other. FWIW -Al
On a snowmobile engine (just a bigger version of what you're talking about), we start by setting the high and low speed setings exactly the same, then the carb openings are set so they start opening at exactly the same time and reach WOT at the same time. Then tune from there....
After that, very often a cylinder reading rich isn't pulling as hard as it should be, or a lean one is pulling too hard? Carb mixture settings are always kept identical. If you richen one side, you match that adustment exactly on the other. FWIW -Al
