Carburettor Testing (Get it Going)

[Rustynutz]

Small Engine Diaphragm Carburettor Tests
QUICK TEST
Prove metering system works
Put 10psi on fuel inlet and see it hold there, (will creep down slowly) put tube on diaphragm vent hole and blow, watch inlet pressure drop
If it holds and drops that’s working

Test Main jet and Check Valve
Screw low Needle in until snug fit (Do Not Over tighten you will damage seat) then remove High Needle, get red nylon tube from WD40 spray can or similar and place in jet hole (takes some fiddling to get a good seal) when you think you are in the jet blow then suck you should feel the valve opening and closing as you do this (easy to blow harder to suck) if your not getting a response try sucking and blowing a little faster, if still not noticeable attach your WD40 and squirt some in (look in chamber and you should see it come out of the main jet, then try the suck blow test again (remember to clean the tube or you will get a mouthful)

Test Low Jets
Remove L needle put tube in and spray with WD40, look in chamber you should see it coming out of both holes.

NB
Using WD40 like this is OK for short periods (Don’t leave it in there for to long) I usually flush through with petrol after the test, A Quick blow on the supply tube will prove fuel can get there.
Put both needles back in till snug then back off 1 and ¼ turns (most working Carb’s will start at this setting)

General Notes
Main jet is in the centre of the chamber. Low Jets are each side of the throttle butterfly (Engine end). The H high needle (shorter one) is always closest to the air filter end and the L low needle (Longer one)is always closest to the engine. I made my pressure tester with a bicycle pump that had a gauge or you could use a car tyre pump and make an adaptor with an old valve stem and some fuel line (just remember not to go over 10psi or you might damage the diaphragm)

If it passes all these tests it should start and run now???

Set up the low jet first to get the engine ticking over nicely.
Increase the tick over screw so the engine runs without stalling, now by moving the L Jet in till it starts to die then back ¼ turn should be ok, now turn the T O screw till the engine runs a nice speed below the clutch cut in speed. Now open throttle and adjust H needle till you get a nice response as the engine speeds up and down (H needle should be set with a hot engine) now this will probly be to lean a mix, with full throttle screw in till engine starts to falter (4 stroke) then turn screw back ¼ turn. (this should be somewhere handy)
Now I usually run a couple of tanks through just to get everything to stabilise then reset everything again. Just remember to rich it might flood or foul the plug but to lean will kill your engine (so get it right!)

Quick Tip
Most carb faults are caused by storing machines with fuel in, I found if your lucky and its not damaged just clogged up, you can warm it up with an air dryer then flush through with WD40 followed by petrol, found this to work well on a lot of carbs without dismantling anything.

Hope this helps!
Russ…

 

[Brian VT]

Nice job !
I use carb. cleaner instead of WD40, although WD does taste better.

 

[Rustynutz]

Get it Going

Thanks Brian
The only reason I use WD is the red tube is just right to seat in the jet screw thread, I guess any will do the job, just don't leave it in there for to long.

Squirt Blow and Flush

oww and don't forget to wipe after!!

 

[USMC0802]

Squirt Blow and Flush

oww and don't forget to wipe after!!

That's what she said.

 

[mheim1]

Very nice thread.
Thanks for sharing.

 

[Rustynutz]

Get it Going

Thanks Guys
I've seen some great carb explanations here, but most seem to require lots of expensive vacuum gauges and equipment, I wanted it to be simple things most people have got laying around. and lets face it most of us just want it running not set up carburetta repair facility.
The other thing I forgot to mention is to only fix the part that's broken, say if its a brittle diaphragm just fit that, I know you get all kinda bits in the repair kit but that don't mean you must fit them, and in fact what usually happens is you make it worse and inject more faults, making repair much harder.
fit what's needed and keep the other bits for spares. I'm sure you will find them useful in the future.

Remember the old adage: if it ant broke don't fix it

 

[GlenM]

My cleaning process has evolved over the yrs to pretty much how you described.
Use WD-40 now, quit with carb spray, don't put high pressure air to blow, use from a distance, also, oftentimes only replace metering diaphragm.
Carb spray and high pressure blowout seemed to be destroying the idle check valve in zamas....
thanks,

 

[heimannm]

Nicely done Rusty.

Are you really that new here (3 posts?), in that case welcome.

Too many guys believe there is a lot of voodo involved in carburetors, and maybe with some of the more moderns ones they may be correct. Most of the time though, back to basics is all that is needed to make that saw purr again.

Mark

 

[Rustynutz]

Get it Going

Yes been looking more than posting

Just thought I would share this with you as most of the faults on small engines seem to revolve around the carb.

I agree whole heartedly with Glen :agree2: "no compressors or air lines" lung power is all you need

Great site BTW

 

[willbarryrec]

Thanks Rusty!
Will def be bookmarking this.

 

[lesorubcheek]

Great post Rusty, and welcome . Some wonderful tips for sure. Just wanted to throw one thing out in regards to Brian's comment of using carb cleaner. I've heard (not expereinced), that certain carb cleaners can expand the rubber used on the tip of the metering needles. Don't know if this is true or not, but hearing this persuaded me to make sure I only use carb cleaner on a disassembled carb. I usually flush with WD afterwards. Again, no first hand problems with carb cleaners, and I'd suspect it depends on multiple factors, but wanted to bring this up.

Dan

 

[sefh3]

Great job. Easy to follow and to the point.

Can we get this as a sticky........ or do I need to add this to my signature too????

 

[Rustynutz]

Get it Going

With regard to cleaners
I was advised not to use carb cleaner on the small Walbro and Zamma's as it can do damage and as a result I always use WD40, but my thinking go's this way it really don't matter what you use as long as its in out and away, rubber and nirile will all expand if its submerged for any length of time, so my advice is to use what you need to do the job then flush it through with clean fuel
that way your safe.

Just something I've noticed if nitrile rubber is submeged for a few hours and expands taken out and washed returns back to its original size.

and as a footnote, WD's cheap and everyone's got some

Happy Buzzing :greenchainsaw:

 

[AUSSIE1]

Good job Nutz, but you screw out (anticlockwise) to four stroke.

Personally I set the low when warm also.

 

[Rustynutz]

Well spotted

The point is an engine running at it's fastest probably way to lean and will burn out (ouch) that's just my way of setting the H Jet. wind in till it starts to flag then back a 1/4 turn.
However I'm in the process of getting a Colortune plug, you know the one with the window in it so you can see inside and look at it running, there great.

What's your way of setting it???

 

[AUSSIE1]

Well spotted

The point is an engine running at it's fastest probably way to lean and will burn out (ouch) that's just my way of setting the H Jet. wind in till it starts to flag then back a 1/4 turn.
However I'm in the process of getting a Colortune plug, you know the one with the window in it so you can see inside and look at it running, there great.

What's your way of setting it???

That plug sounds interesting.

I turn in slowly the high speed till I loose most of the four stroking then back it out to a sound I'm happy with.
You can back out the high to a fat four stroking and do some cutting, leaning off the high till it stops four stroking in the cut.

 

[Rustynutz]

Yes I agree, the best tuning is tweaking it when it's worked
That's just a starting point for me, as I said in my article I usually do that setting then run a couple of tanks of fuel through then tweak some more.

Have a look here, http://www.carbtune.co.uk/colortune.html

with these you can see the burn colour go from light blue/yellow (lean mix) to bright orange/red (rich mix) the optimum is in the pink for two stroke and light blue for four stroke engines, (Sweet)

Anyway this thread is more about getting them to run in the first place than tuning them up when they are running.

R.....

 

[1steve]

very nicely explained.......great job, rep sent

 

[roncoinc]

Did you see the price of them plugs ??

On the carbs,USC and a fresh kit ,ALL of the kit,if one part worn the rest cant be far behind.lever adjustment,never a problem..
10 psi is a ballpark number,some go off at 8,some at 12,varies.
as has been said,no majic to carbs but proper procedure will go a long way to insure a good job.
carb cleaner is made to clean carbs,oil is not..
if a carb has rubber parts they are included in the rebuild kit.
you want a good rebuild, NO shortcuts..do it right.

 

[Hedge Hacker]

Ive never seen those things before. Thats worth every penny. Although im sure once i get a little better at tuning the carb i wont have a problem. Ive always just cut untill i had a problem and took it into the dealer. But since i found this site i quit taking it to the dealer!