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Science of vacuum leaks

  • Thread starter Thread starter mhardig
  • Start date Start date
M

mhardig

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Can someone explain to me the science behind the O-ring or boot leak?

My 81 GS450T idles high, around 3000 rpm. The idle screw is almost all the way out. Conventional wisdom says the carb boots or O-rings are bad. I've ordered the rings, and will get the boots next, but can someone explain to me how a leak after the carb can cause a high idle? use little words, I'm dumb.

Just trying to unnerstand.
 
I'm not sure I understand why a vacuum leak can cause a high idle condition either. However, it can cause all sorts of tuning problems across the board. Not just idle. These carbs rely on vacuum as the gauge to tell how fuel used to maintain the correct air/fuel ratio. If there is a leak after the carb, then it's un-metered air getting into the system and will probably result in a lean condition. However, I'm not sure if a lean condition at idle could cause a high idle or not. Guess I'm not helping much...
 
hmmm...

hmmm...

Well, have to think about what causes an engine to accelerate - that's right, adding more air (opening the butterflys). I don't know if it's just being more explosive at lean condition or if the extra air after the carb causes another effect that makes it accelerate.
 
In a nutshell, gasoline+oxygen=fuel. The purpose of the carb is to mix the gas and O2 into fuel in the proper ratio. If you add air after the mixture is made, the mixture won't be correct, it will be lean.
 
In a typical piston-powered airplane there is a seperate mixture control (to compensate for lower air density at higher altitudes). Once you work with one of those (I'm a licensed pilot) you have a "eureka!" moment when you finally understand the air/fuel ratio (mixture) thing.
It's a little counter-intuitive, but here's how it works: When the air-to-fuel ratio is gradually increased you initially get more power (combustion is more forceful) - this is great for power but can seriously heat up the engine if the ratio is too high (leading to high combustion temps) - it can also lead to back-firing at wider throttle openings where the air-to-fuel ratio will typically get greater in typical carbureted engines (even though there is some attempt to keep it relatively constant) - and this effect will be exaggerated greatly with an air leak. As you increase the ratio even more, the combustion power (and heat) will eventually peak. Then as you increase the ratio even more the combustion power will actually start to decrease till eventually the ratio is high enough to result in back-firing/popping. As you increase the ratio further the mixture gets to a point where it will not combust at all and the engine will stall (this is actually how you typically shut down a piston-powered airplane, like the Cessna 152, after landing/taxi/parking).
The goal is to compromise such that the ratio is high enough to give you reasonable power at all throttle settings without overheating the engine.
 
I'll certainly take your word for it

I'll certainly take your word for it

but it'll take me a while to get my head around it.

Should have nhew O'rings later in the week, and if they don't fix the situation, new boots.
 
By the way, do NOT assume that the only leak area you need to worry about is the join (via the boots) between the carbs and the cylinder head. Your bike has CV carbs - extremely sensitive to air leaks at any point in the system.

Here's some advice I gave another member:

You will find a billion posts on this forum about how CV carbs wil NOT tolerate any air leaks on either side and how important the airbox is for CV carbs. CV carbs rely solely on pressure differential to lift the slides. I learned this the hard way.
Also, another thing many people (including myself) don't realize at first is how large an effect an exhaust leak can have on the pressure situation.
It took me weeks of fiddling to finally realize how critical air-leaks and exhaust leaks are. Once I got those resolved my bike ran great and I realized that all those extra carb cleanings were not the solution to the problem.
Another very important fact that people overlook is that coil performance degenrates at higher RPM. So a coil that works just fine at low RPM may not work well at high RPM. I found this out the hard way too. I put a pair of "known-good" coils on my CB750 and its high-end performance improved DRASTICALLY.
You're almost there - seal the leaks WELL (including aibox lid) and get a decent pair of coils (I'm assuming yours are the originals).
 
Last edited:
Thanks

Thanks

So far, coils are good.

I do have a tiny exhaust leak in the righ side, I intend to yank that and spot weld it this weekend. I just broke all the fasteners loose and replaced them with new grade 8 chromey stuff, using plenty of anti-seize.

I have doublechecked the airbox seals, they do seem to be fine. The boots cracked away from the original plates, and I glued them back until I could get new ones. I put new grommets on the vacuum ports. WD-40 doesn't change anything.
 
Carb Air Leaks

Carb Air Leaks

When I worked as technician we use some form of flamable spray to determine the area of the leak. Use (very very carefully) WD40 or brake clean to spray around the tubes and see if the idle increases usually when the vapor hits the leak it will make the mixture richer. Sometimes its a matter of tightening a clamp, sometimes it calls for replacing the intake tube. Be sure you have no source of spark or flame when doing this, you don't want to create an explosion or fire. Always keep the appropriate fire fighting device close by. I have never had a problem but I like to be prepared.
 
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