Part Seven: Carburation

[Nutball]

What's the best way to apply epoxy to a venturi? I don't know if anyone has some type of syringe/dropper device for nice even application, or do you guys just use a toothpick or some stick?

 

[ferris]

Does some of u guys bore out carbs?
For in example, u can’t fit a large carb without making a lot of adjustments.
Are you boreing out the carb for more airflow?
What do u use for it?
Drill bit?

 

[MustangMike]

I only played with an AM carb on and Asian 440 Big Bore. The venture's are smaller than OEM, so I made it a little larger with my grinder.

Was not perfectly round, but I did as well as I could, and it did run better when I was done with it.

Could not use something full round because the fuel jet is in the way, had to work around it as I did not want to touch that (the one I modded did not run worth a crap).

That is the extent of my carb playing!

 

[Mastermind]

Does some of u guys bore out carbs?
For in example, u can’t fit a large carb without making a lot of adjustments.
Are you boreing out the carb for more airflow?
What do u use for it?
Drill bit?

Boring bar. But if I do anything like this it's not usually the venturi I enlarge. Making the throttle plate and choke bore bigger seem to make far more difference.

 

[ferris]

Boring bar. But if I do anything like this it's not usually the venturi I enlarge. Making the throttle plate and choke bore bigger seem to make far more difference.

if you increase the area around the throttle plate and choke. how do you make sure that not too much air goes by when they are closed, at idle for example.

 

[Mastermind]

if you increase the area around the throttle plate and choke. how do you make sure that not too much air goes by when they are closed, at idle for example.

Usually I use the choke plate in the place of the throttle plate, then make a new plate for the choke.

 

[Nutball]

Weird carb. I'm not sure how an open jet will work with a primer bulb, but this carb uses a check valve or something to let the fuel into both needles. The primer is connected directly to under the diaphragm. I don't know how else they would normally work, but this hole I drilled may allow for faster priming, idk.

How do you guys power micro drills? The .029" bit was the smallest I could use by hand, and it was sort of slow, so I attached a .020 and .013 drill bit to a little motor each with superglue. For a 35cc engine, so I'm starting small.

 

[Terry Syd]

There are very inexpensive 'pin vices' available that can hold the drill/reamer securely while you work your way down through the jet. - Yes, the first run through a jet with a .3mm reamer is a tedious job, but if you want it done right, you have to pay the dues.

Check out Ebay for some micro-drills and pin vices.

 

[Mastermind]

Running a micro drill too fast can enlarge the hole beyond the drill size. Ask me how I know. LOL

 

[Nutball]

I kept the rpm at a minimum that still effectively got work done, around 500-3000rpm. I started with a large drill and went smaller if, but I didn't get the drill bits quite perfectly centered on the motor shaft, so I knew there would be a slight reaming.

 

[Mastermind]

I kept the rpm at a minimum that still effectively got work done, around 500-3000rpm. I started with a large drill and went smaller if, but I didn't get the drill bits quite perfectly centered on the motor shaft, so I knew there would be a slight reaming.

Drilling brass jets at anything above hand speed has been a disaster for me.

Your mileage may vary.

 

[Nutball]

What did you use to power your drill? I had problems where I'd get a good straight hole going deep enough that it would bind up, and I'd have to go back to a larger bit or do a bit of intentional reaming with the small bit.

 

[Mastermind]

What did you use to power your drill? I had problems where I'd get a good straight hole going deep enough that it would bind up, and I'd have to go back to a larger bit or do a bit of intentional reaming with the small bit.

Rechargeable drill with a hand drill vise chucked up in it. The brass balled up on the tip and made the hole too big.

To drill holes in aluminum, that setup works well though.

 

[Lightning Performance]

Every jet I've ever drilled was a disaster for large carbs even Holley 100, 85's and up, all of them.
It was far easier to keep a full card of new ones in steel, anodized steel and yellow brass. Expensive but very handy at the track. Then order more of your favorite flavors.

These tiny carbs suck. If I'm doing more that contouring the inlet area, shaft thinning or smoothing the bore with a little bit in the throat area it gets new parts or a bigger carb. That always brings it's own tuning issues but jets won't be one of them most times.

 

[Nutball]

These tiny carbs suck. If I'm doing more that contouring the inlet area, shaft thinning or smoothing the bore with a little bit in the throat area it gets new parts or a bigger carb. That always brings it's own tuning issues but jets won't be one of them most times.

Well, it is. I can't get enough fuel. I'm also going to decrease the venturi some with JB

 

[RI Chevy]

I prefer just to change the jets and jet up a couple. Easier for me.

 

[huskyboy]

I’ve never had to drill a tillotson yet on a worksaw. Certain walbros and zamas yes.

 

[Ketchup]

I’ve been arguing with a buddy about larger carbs at altitude for years. It came up again today and I thought I would bounce it off you guys.

He thinks larger carbs make flooding at altitude worse. I think you have to lean the larger carb more, but the bigger bore allows more of our thin air in and can produce a lot of power and help an underpowered saw at elevation. Is my logic flawed?

Maybe I have a poor understanding of carburation in general. Will a larger carb (or a bored carb) put more of anything into the case? Is it only helpful if the original carb was too restrictive?

Back to the altitude stuff, can a larger carb actually create more pressure in the crank case? Or is case pressure basically always ambient when the intake closes?

 

[huskihl]

I’ve been arguing with a buddy about larger carbs at altitude for years. It came up again today and I thought I would bounce it off you guys.

He thinks larger carbs make flooding at altitude worse. I think you have to lean the larger carb more, but the bigger bore allows more of our thin air in and can produce a lot of power and help an underpowered saw at elevation. Is my logic flawed?

Maybe I have a poor understanding of carburation in general. Will a larger carb (or a bored carb) put more of anything into the case? Is it only helpful if the original carb was too restrictive?

Back to the altitude stuff, can a larger carb actually create more pressure in the crank case? Or is case pressure basically always ambient when the intake closes?

Bigger carbs will usually make more RPM if the saw wants to run above what a smaller carb would let it. Point in case, if you took a standard 372 with the HD 12 carburetor and a 36 inch bar and dropped it into an oak log, my guess is that it will turn a similar RPM as if it had the RWJ on it from an X torq. But put it in an 8 x 8, and the larger carb will be faster

 

[huskihl]

When you put a bigger carb on a smaller saw, the slower velocity and volume of air coming through the Venturi doesn’t pull as much fuel as the larger saw it was intended for will, so the carburetor usually needs to be richened to compensate. But at altitude, there’s already less oxygen, so they normally run rich. I would think that a larger carburetor and altitude combined should work pretty good together