I learned a lot in my adventure trying to track down myriad problems with my bike, but along the way I learned a lot about vacuum leaks. I thought I'd post. Others please correct me if I am wrong, and I will update this top post.
First a little bit of understanding of the BST33 carbs on the Mk1 GS500s (the two-jet carbs). It may apply to the later carbs too.
Air comes in (in theory) through a few controlled places depending on throttle position. At idle or very small throttle openings, air is drawn in through a little hole on the lower edge of the throat of the carb, and fuel comes in through the pilot jet. Once you get to higher throttle openings, the slide is drawn up and air comes in through the main throat of the carb, however much gets past the slide. Fuel is metered through the main jet depending on the position of the needle, which corresponds to the position of the slide. The slide moves up and down depending on vacuum. More vacuum == more slide opening, which means more fuel due to a higher needle position. The rubber diaphragm on the top of the carb beneath the plastic cover is what pulls the slide up. More vacuum under the top cover of the carb will cause the slide to be pulled up.
When you open the throttle, it creates vacuum on the cylinder side of the throttle plate, which results in pulling the slide up and opening the throat of the carb for air to come in, and subsequently fuel from the main jet. If you close the throttle, the vacuum condition between the throttle and cylinder inlet goes away and the slide returns down, the needle effectively closing the pilot jet and closing off air through the throat of the carb, and air comes in through the little hole across the pilot jet.
That's how it is supposed to work, if I understand everything correctly, with no vacuum leaks.
There are numerous places for vacuum leaks on the BST33 carbs. It's important to isolate vacuum leaks in terms of which side of the throttle plate, and whether they impact air coming into the carbs. But the point is, in any case, a vacuum leak means that two things are happening. 1) air is getting in through some place where it shouldn't, and if this air gets added in to the carb's main airflow circuit, then this results in a lean condition. 2) vacuum is reduced where the carb needs it to be, and this results in improper operation of the slide and diaphragm, so the throat of the carb may remain closed or open when it should be doing the opposite, and subsequently the main jet needle is in the wrong place at the wrong time.
At IdleAt idle, engine vacuum is very high, but the throttle opening is low so the slide won't be pulled up. A vacuum leak anywhere, including at the intake-boot-to-cylinder interface, will result in air getting in that is not drawn across the pilot jet. This means the engine will run lean and cannot be adjusted to not run lean. However, if you open the throttle even a tiny bit, then the amount of air coming past the throttle plate is far more than what leaks in through your vacuum leak or the air that comes through the pilot air port, so suddenly you may go from running very, very lean to very, very rich when you go from close throttle to small throttle opening. By the way, the idle adjuster knob will open the throttle enough to effectively negate the additional air coming in through your vacuum leak, which will result in an idle condition where either the bike stalls because of way too lean idle with closed throttle plate, or just a tiny adjustment for faster idle and the bike will idle at 2-3K rpm because of a way too rich idle mixture and too much throttle opening. This issue will have you chasing your tail.
At Throttle Tip-inWhen you transition from throttle closed (idle) to throttle open, there should be a big vacuum difference between the cylinder side and the intake side of the throttle plate because the slide is closing the throat of the carb. This vacuum should cause the slide to be drawn upward, allowing air in the throat of the carb and also pulling the needle out of the main jet which allows more fuel to come in through the main jet. If you have a vacuum leak, then you will get insufficient vacuum to pull the slide up, which may result in a lot of hesitation off idle or stumbling at like 2-3K rpm with part throttle openings. Since there is a lot of vacuum on the cylinder side of the slide, with the slide closed, then a lot of extra air will come in through the vacuum leak (primarily if it's the intake boot o-rings leaking), making the engine run really lean during this condition. Eventually the slide will slowly open and it will return to ordinary running, but this feels like a very slow throttle response. It's easy to misinterpret this symptom to be "lean main jet" and attempt to fix it by increasing the main jet size or shimming the needle. But this won't fix the problem, and in fact may make it worse because now you have a confusing transition from very-lean to very-rich as the slide opens in a delayed fashion.
WOT & High RPMOnce you get the slide open enough to get the bike to rev high with big throttle openings, there is so much air coming through the throat of the carburetor that the amount sneaking in through vacuum leaks is negligible and won't appreciably affect the mixture. This is where vacuum leaks can lead you to really screw up the mixture and jetting, because you may wind up over-jetting thinking the bike is running lean at smaller throttle openings or midrange RPMs, and end up with it running rich and bogging at high RPMs or big throttle openings.
So what to do about all of this?
FIX VACUUM LEAKS FIRST. You need to fix any and all vacuum leaks
before you attempt to adjust the mixture or jetting, because you can wind up chasing your tail and making erroneous assumptions if you have a vacuum leak. Since a vacuum leak in many cases allows extra air in at a point where it does not draw fuel from one of the jets, it will produce a lean mixture and symptoms of a lean mixture, but trying to address this lean condition by adjusting mixture will only make matters worse and harder to diagnose. In short, the mixture, idle and jetting cannot be properly set on a CV carburetor if there are
any vacuum leaks.
The main places where your vacuum leaks may happen will be:
- intake boot o-rings, cracked or poorly sealing intake boots, dirty etc. interface between cylinder and intake boot
- "little" o-rings on the vacuum ports on the top of the carbs
- warped/bowed/cracked plastic top of the carb, or otherwise poor edge seal to the diaphragm
- hole in the diaphragm
- caps on the vacuum ports leaking or the petcock vacuum hose leaking
- an air leak on the air filter side of the throttle is NOT a vacuum leak! so don't go nuts trying to seal the airbox.
I would not recommend spraying carburetor cleaner on the carburetor to diagnose vacuum leaks, since carb cleaner will both degrade or ruin rubber parts it comes into contact with and it will also ruin paint. I don't really recommend spraying WD-40 or anything else on there either. In fact, I suggest fixing vacuum leaks as a preventative measure, which is to find and inspect manually every possible vacuum leak source and remedy it first. Using the spray-something-on method can be misleading and leave a residue of gunk that you don't want anyway. I advocate manual inspection and simply replacing any suspect o-rings, vacuum hoses or caps, etc.
Hoping this helps everyone understand what it took me so long to figure out, so you don't waste two months chasing your tail like I did.
Have fun!
You have a very informative post, until you get to the end..."I suggest fixing vacuum leaks as a preventative measure, which is to find and inspect manually every possible vacuum leak source and remedy it first" <<< what does this mean, can you see them with your eye? I give your post an A+, but in the real world...sometimes you gotta spray a little chemical to find that split in the plastic fitting under that spaghetti mess of vacuum hoses that your eye can't see :thumb:
https://www.youtube.com/watch?v=oKVHqCJN2LU (https://www.youtube.com/watch?v=oKVHqCJN2LU)
Quote from: Suzuki Stevo on September 30, 2016, 05:44:17 PM
You have a very informative post, until you get to the end..."I suggest fixing vacuum leaks as a preventative measure, which is to find and inspect manually every possible vacuum leak source and remedy it first" <<< what does this mean, can you see them with your eye? I give your post an A+, but in the real world...sometimes you gotta spray a little chemical to find that split in the plastic fitting under that spaghetti mess of vacuum hoses that your eye can't see :thumb:
https://www.youtube.com/watch?v=oKVHqCJN2LU (https://www.youtube.com/watch?v=oKVHqCJN2LU)
The problem with that method is that it can give you a false-negative... that is, you can have vacuum leaks, especially beneath the plastic top of the carburetor, and spraying your WD-40 or carb cleaner or whatever on everything will not reveal it. So the you might go out thinking you've diagnosed the vacuum leaks and ruled them out, then try to fix the conditions with mixture changes and wind up making everything worse and harder to diagnose.
There's no spaghetti mess of vacuum hoses on a GS500. On the early bikes, there is only
one vacuum hose. That's not even the most likely cause of a vacuum leak, but anyway, I stand by my advice, but I'll add yours too: just replace the stuff likely to degrade and leak
first, because it's cheap and good insurance,
then if you still have an issue you can chase it by spraying WD-40 to find something you might have missed or a leak in an unlikely place.
In any case, GS500 carburetors are not going to suddenly go from running fine to under-jetted or running way lean due to mixture/jet/needle changes. Those are hard parts that either degrade slowly over a long time or just don't change unless you open the carbs. So if you find you have a lean condition problem that you know you did not cause yourself, like you didn't just put on a lunchbox air filter or you didn't just change the jets, etc., then most likely it's a vacuum leak and I say do preventative maintenance, replace $10 worth of rubber parts that are probably dried and cracked and old, then if the problem persists, at least you will know those things are good to go. But I have read (not participated in) hundreds of posts on this forum alone where someone shows up and says "my bike is doing xyz" like hanging idle or hard starting cold or stumbling off idle etc. and the collective advice is "it's running lean" then they run off and try to fix it with shims and jets when most likely it's a vacuum leak making it lean.
If your rubber parts are over 5 years old, I would just replace them. It's not hard and it's a lot easier than continually diagnosing mixture issues.
My spaghetti mess reference is derived from my truck automotive repair experience, when spraying chemicals on many occasions was the only thing that exposed the leak, diagnosis is the hardest part of repairing any kind of an engine issue, customers like results, mechanics need to move on to the next job, that was really my point.
My post was specifically fo gs500 vacuum issues. An 80s car is a whole different animal. I think it's just much easier to begin by replacing things likely to leak and then work from there.