1984 Marathon 2PG Carb Adjustment

[CP241]

1 turn out, I was pretty close! My default for pretty much everything has always been 1.5 but the book has spoken

[BrucecurB]

The results are IN...

The 'Pilot Screw' was 2.5 turns ..out.. after my carb cleaning expedition. So yesterday, I meticulously adjusted it to 1.0 turns ..out.. The engine seems less 'peppy' and slightly rougher running, nothing drastic but just a touch off.

I'm a life long 50 year veteran of Automotive/Farm Equipment/Small Engine carburetor adventures and understand precisely what each passage and adjustment does. However on this Mikuni, does that 'Pilot Screw' meter 'Air' or 'Fuel'. In other words, is 1.5 turns out 'richer' or is it 'leaner' ?

Good times....!
Bruce

[Dabble Inn]

Do you still have the original Mikuni? If so very good, but you might want to look at the name again. A lot of the Chinese cheapies try to fake you out with names like Mokuni, Mukuni etc. Just curious......

[cart around]

Originally Posted by BrucecurB

The results are IN...

The 'Pilot Screw' was 2.5 turns ..out.. after my carb cleaning expedition. So yesterday, I meticulously adjusted it to 1.0 turns ..out.. The engine seems less 'peppy' and slightly rougher running, nothing drastic but just a touch off.

I'm a life long 50 year veteran of Automotive/Farm Equipment/Small Engine carburetor adventures and understand precisely what each passage and adjustment does. However on this Mikuni, does that 'Pilot Screw' meter 'Air' or 'Fuel'. In other words, is 1.5 turns out 'richer' or is it 'leaner' ?

Good times....!
Bruce

Look at figure J-7. I think we can safely call it the "throttle valve bypass pilot air jet". The air inlet is before the throttle plate right where the venturi starts. The air outlet is behind the throttle plate. It looks like its job is to allow a small, metered amount of air through when the throttle plate is completely closed.

I'm not sure why.

[BrucecurB]

Veerry Interesting...

Mayhaps, my 'impression' on drivability was just that... an impression! I was actually more worried about running the engine with too lean a mixture, and wreaking havoc!

I'll leave it be for a while and see... I use my EZ for a 'Farm Truck', so it has worked for a living the last 20 years I've owned it. I run Tractor Chevron pattern rear tires, stock size, and the sucker will pull a 5x10 trailer with a load of limbs or whatever. An amazing piece of machinery!

Thanks again...

Bruce

[Dabble Inn]

Here's my theory;

We know Pilot jets are for idle. I've heard them called 'Pilot jet', 'Idle jet', and 'Transitional jet'. They do two things, not only meter fuel or air (in this case mostly air) for idle (hence the bypass around the closed throttle plate) but they also create a ('head start' if you will) for a smooth transition to a higher rpm. Since these don't actually idle in normal operation than it would seem to me it's there for the second purpose of a smooth transition to a higher rpm, especially when the choke is closed and before the throttle plate is open. Wouldn't it draw fuel from the main jet with closed choke plate?... Thoughts? Huh? Huh?......

I will say I can actually make mine idle at a stop sign, I leave the gas pedal pressed ever so slightly keeping the micro switch engaged and also it opens the throttle plate just a hair. So yeah, it's not using the pilot jet for idle. And no it's not just turning on the starter, I know what that sounds like. It is actually firing, but it's a slow enough rpm to not engage the clutch. I do this a lot just because it makes for a quicker go when in traffic

[BrucecurB]

Dabble...

I've got hilly terrain with some slopes about 200 yards. I figured out years ago that Throttle Pedal actuated Ignition Switch was terribly annoying, causing 'bucking' on slopes by engaging and disengaging the Ignition. I also didn't want the engine to be pushed all the way down hills without firing the Mixture. My fix was a Bypass Toggle Switch beside the Key switch that feeds constant ignition when the Key switch is 'ON'. The I slowed down the 'Idle' to the point the clutch disengages. I can stop with the engine firing, then take off quickly as you said.

Bruce