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View Full Version : How to: Quick carb lean or rich condition vacuum diagnostic



Jetblack
02-10-2013, 1:15 PM
This random carb tip comes from some guy named Mac; I found it while randomly browsing around, thought I'd share it.

"Shoot a bit of WD-40 at and about the carb with the engine running..

Forst shot right into the carb... just a spritz... listen to that.... what happens? Idle go up or down.... I am not talkin about a long spritz either..... just a quick blast...

If Idle goes up the engine is lean and so would lack some power..... If the idle dips your about right... IOf the engine stalls you may be running too rich, and so again lack power."

surfbum
02-10-2013, 1:19 PM
good stuff. seems like a good common sense approach

BrokenSprocketGarage
02-10-2013, 2:50 PM
Why wd-40? Why not use gasoline?

Jetblack
02-10-2013, 5:00 PM
It's supposed to help check for vacuum leaks too to see if you hear hissing, less chance of igniting everything up that way. The propellant burns up too fast to get out of control... At least that's my best educated guess from having lost eyebrows and got lit up a few times with starter fluid etc.

I'd tell a story of how I was walking around once; and set some curtains on fire with my ass, that I didn't know was ablaze for over a minute...but that's more for the JP.

2ndAmendmentEnthusiast
02-10-2013, 5:42 PM
That's a good one, my favorite "trick" has always been this:
After the engine is at operating temp, go to full choke. If it improves, you're lean. If no change, you're rich. It works well when you're riding too--if you run fine at idle but shit out at a certain throttle setting you can do this to figure out which way to go.

BrokenSprocketGarage
02-10-2013, 6:13 PM
I think that what you are describing is a technique to check for intake manifold leaks. Not vacuum leaks, Not jetting.
I have done this test with brakleen, silicone spray, carb clean, etc.
You need to fully heat up the engine and then spray directly at the intake manifold seals. If the engine speeds up or stalls, then the seals are suckin in air. since this occurs after the carb, it creates a lean condition.
This test in no way is an indication of proper jetting.

Jetblack
02-10-2013, 6:27 PM
I think that what you are describing is a technique to check for intake manifold leaks. Not vacuum leaks, Not jetting.
I have done this test with brakleen, silicone spray, carb clean, etc.
You need to fully heat up the engine and then spray directly at the intake manifold seals. If the engine speeds up or stalls, then the seals are suckin in air. since this occurs after the carb, it creates a lean condition.
This test in no way is an indication of proper jetting.

I don't do that, I find the best main that pulls hardest at wot then work backwards Idle last. I light up a smoke to find intake leaks and watch were it gets sucked in, used to see my grandpa do it.

Sounded interesting but hard to make out the garble of what he was saying some what. But yeah lean or rich air condition in the carb...not jets.

Jetblack
02-10-2013, 6:29 PM
That's a good one, my favorite "trick" has always been this:
After the engine is at operating temp, go to full choke. If it improves, you're lean. If no change, you're rich. It works well when you're riding too--if you run fine at idle but shit out at a certain throttle setting you can do this to figure out which way to go.

Thanks that's a nice one. I dial em in before I put them on then go from there...this one sounded interesting so I plopped it.

heretic530
02-10-2013, 6:50 PM
every older mechanic has these tricks. if you really want to get good at them, spend a littie time at the local drag strip.
Every strip has that one crusty old guy who can tune carb and timing by ear , and really make an engine sing.

Jetblack
02-10-2013, 6:56 PM
every older mechanic has these tricks. if you really want to get good at them, spend a littie time at the local drag strip.
Every strip has that one crusty old guy who can tune carb and timing by ear , and really make an engine sing.

I love those little nuggets too, smarter not harder.

2ndAmendmentEnthusiast
02-10-2013, 6:59 PM
every older mechanic has these tricks. if you really want to get good at them, spend a littie time at the local drag strip.
Every strip has that one crusty old guy who can tune carb and timing by ear , and really make an engine sing.

I don't disagree, but we both know that there are plenty of guys on this board who aren't "older mechanics". Hopefully shit like this will help those guys out.

BrokenSprocketGarage
02-10-2013, 7:05 PM
It's a lost art to hand tune a carb. reading spark plugs. changing heat ranges. listening to engine noises. engine temps. swapping jets and making WOT runs. FUN stuff

Jetblack
02-10-2013, 7:20 PM
I like my caramel light. I also love working on carbs; both lean and rich rob power...guessing a huge main then trying to balance around that is a bad idea.