Synching Carbs - Low Compression

[2BRacing]

I have been using a set of the dial type vacuum gauges since 1981. They were good quality and have a restrictor with a very small hole, which was pressed into the metal mounting inlet of the gauges. I mounted them on a piece of aluminium plate. I have found these to read within one small scale division of each other when connected to the same cylinder, so the calibration is good enough for me.

I recently obtained a set of the OEM Suzuki gauges, and found that I get the same results as with the dial type gauges. However, the dial type gauges are easier to use, as I can lay them flat on the saddle, whereas the OEM Suzuki gauges cannot lie flat, or the balls will just run along the tubes. The OEM gauges also have to be calibrated individually to ensure accurate results, and this process takes extra time.

[Steve]

Can you do us all a favor?

Please compare the readings of the Suzuki gauge to your dials. The manuals all show the vacuum levels to be "half a ball" different for the inner two cylinders, but none of us has the original Suzuki gauge.

What we would like to know is: what is the vacuum difference in "half a ball"? Your gauge is calibrated in kPa, but we can convert that to units that we know and use better than we can convert "half a ball".

Here is what we see in the manual, what is the difference between the inner and outer vacuum levels?



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[2BRacing]

I was synching my GS300 carbs today with vacuum gauges and I couldn't get them anywhere close to the same on each side.

What was a little bit more frustrating is that the right cylinder had some really low/sporadic readings. Needle was all over the place but way below normal readings.

Nothing I did to the synch screw seemed to help.

I recently checked the clearances, but maybe I should again?

I thought maybe the possibility of a burnt valve because when I bought the bike the PO had put a big cinch in the right header and I don't how much it restricted things, but honestly I don't know exactly what a burnt valve is or the symptoms of one are.

Any thoughts or suggestions?

Reading the replies to your original question again, nobody has really answered this. You seem to have a couple of basic concepts mixed up. The carb synch process is to ensure that the butterfly valves both (or more if 4 cyl) open exactly the same amount, allowing the same amount of air to enter into the cylinders.

Compression is a totally different concept, and you do not see this on the carb synch gauges. The gauges are actually showing a reading of "vacuum" or suction, which is the opposite of compression!

Compression is measured with a compression tester gauge screwed into a spark plug hole. It measures the pressure built up inside the cylinder by the piston. If a valve is burnt (like the one shown in the picture) it cannot seal properly against the valve seat, and some of the pressure will escape. This will result in a low compression reading.

The fact that your one carb gave a lower reading on your gauges (vacuum) simply means that carb's butterfly was more closed compared to the other carb. So as explained above, it has nothing to do with "compression" and the condition of the valves.

Make sure you are turning the correct synch screw on the linkage between the two carbs, because there will be a change in readings as the position of the carb butterflies is changed (maybe you were turning the idle speed adjustment screw - the big one - by mistake?)

[2BRacing]

Can you do us all a favor?

Please compare the readings of the Suzuki gauge to your dials. The manuals all show the vacuum levels to be "half a ball" different for the inner two cylinders, but none of us has the original Suzuki gauge.

What we would like to know is: what is the vacuum difference in "half a ball"? Your gauge is calibrated in kPa, but we can convert that to units that we know and use better than we can convert "half a ball".

Here is what we see in the manual, what is the difference between the inner and outer vacuum levels?



.

Hi Steve

Yes, I was actually going to do just that out of curiosity! During the earlier years I never had the correct manual for the CV carbs, so I always used to set all 4 carbs to the same readings on my gauges.

However, when I found GSR and BassCliff's wealth of information (including the correct manual!) I saw that 2 and 3 should be "half a ball" lower than 1 and 4 (the ball is about 6 mm diameter).

As I only obtained the Suzuki gauges a few months ago, I also never knew how much the offset had to be on my gauges. I will now be able to do the comparison, and will let you know.

[hjacobmiller]

Reading the replies to your original question again, nobody has really answered this. You seem to have a couple of basic concepts mixed up. The carb synch process is to ensure that the butterfly valves both (or more if 4 cyl) open exactly the same amount, allowing the same amount of air to enter into the cylinders.

Compression is a totally different concept, and you do not see this on the carb synch gauges. The gauges are actually showing a reading of "vacuum" or suction, which is the opposite of compression!

Compression is measured with a compression tester gauge screwed into a spark plug hole. It measures the pressure built up inside the cylinder by the piston. If a valve is burnt (like the one shown in the picture) it cannot seal properly against the valve seat, and some of the pressure will escape. This will result in a low compression reading.

The fact that your one carb gave a lower reading on your gauges (vacuum) simply means that carb's butterfly was more closed compared to the other carb. So as explained above, it has nothing to do with "compression" and the condition of the valves.

Make sure you are turning the correct synch screw on the linkage between the two carbs, because there will be a change in readings as the position of the carb butterflies is changed (maybe you were turning the idle speed adjustment screw - the big one - by mistake?)

Arg I didn't think even of that! Of course the gauges are for vacuum and not for compression.

Wow, I feel REALLY dumb. I wish there was an emoticon for me hitting myself in the forehead. As Homer would say, DOH!

No I definitely was turning the sync screw. Just a little two cylinder so only one sync screw to adjust.

OH man that makes me feel a lot better.

I almost guarantee it was a problem due to no restrictors in the hoses then or a leak in them.

Will double check once I get the exhaust back on. I'm waiting for new exhaust gaskets which will come in next week.

I feel REALLY REALLY dumb.

[hjacobmiller]

Just a thought - wouldn't it also have bad vacuum if it had bad compression though?

[2BRacing]

Just a thought - wouldn't it also have bad vacuum if it had bad compression though?

The intake stroke on a 4 stroke engine occurs when the piston moves down, and the inlet valve is open.

The compression stroke occurs when the piston moves up, with both inlet and exhaust valves closed. So "leaks" at valves or rings would result in low compression.

However, the rings would have to be really non-functional to result in a bad vacuum. Under such extreme conditions the compression would probably also be low. However, an engine at such an extreme stage would probably sound like a bucket of bolts rattling around!

Regarding your earlier post about feeling dumb, don't knock yourself! You are asking questions here and folk are giving replies in order to help you learn and increase your knowledge base. Only people who think they know everything and are afraid to ask, are the real dumb ones! I am probably a couple of times your age, but I have learned a heck of a lot about GS's on here, and I am still learning things every day!

[hjacobmiller]

Well I went back and threw a set of compression gauges on now that I pulled my head outta my @$$.

The right cylinder held steady at 160 and left was the same.

Don't think I have any reason to worry about a burnt valve on either side do I?

[2BRacing]

Good to have compression numbers as close as possible to each other across all cylinders, so all is in order there.

Now you need to sort out the vacuum gauges to get a proper carb synch. Check that the adaptors that screw into the inlet manifold tube seal properly. I use a tiny rubber o-ring on my adaptors to make sure they seal. Check that the hoses fit securely onto the adaptors. Check that the hoses fit securely onto the vacuum gauges. Make sure you have fitted the restrictors either in the vacuum hose or in the base of the gauges to damp the fluctuation of the gauge needles.

Warm up engine and try carb synch again. The actual numbers of the readings on the gauges does not really matter - as long as both cylinders are the same. The actual numbers will vary according to the revs anyway, so don't worry about the actual numbers. Just adjust the synch screw to get both readings the same.

[hjacobmiller]

I still can't get the carbs to sync. I must have a vacuum leak somewhere

I switched the hoses and still got low vacuum on the right side so I know it't not the gauges.

I also got a T and put both lines on one cylinder and the gauges pulled down the same.

I triple checked all of my connections between the gauges and the adaptors, had the tiny orings around the adaptors and also had the copper washer on the adaptor. I don't know where to go from here.

I'm wondering if I put the intake boots on upside down or something. I couldn't decide if there was an orientation of them or not, and I know the parts fiche lists different #'s for each side.

I adjusted the sync screw until I got it run as good as I could. I held my hand at the end of each muffler and seemed to be the same flow on each side. I know that really doesn't tell me anything but turning the screw definitely made it run like crap one way and better the other.

I checked all the boots to make sure the clamps were down tight, also cleaned and oiled the air cleaner, and tightened up the lid on the airbox.

The 300 doesn't have the same type of airbox as others so I don't know how to seal it up. I would also think that the vacuum would be low for both sides if it was leaking.

I did notice something though. There is an additional hose that goes back towards the airbox. From what I understand this is a hose that keeps pressure from building up in the carbs, correct? It isn't a vacuum line and I noticed the if I covered it it would stall the bike.