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Carb synchronizing question on a 1979 GS750

  • Thread starter Thread starter oregonDave
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oregonDave

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I bought a Motion Pro vacuum carb synchonizer to synch the carbs on my 1979 GS750.

I hooked up the Motion Pro and noticed that the levels on the outside are way higher than the levels on the inner carbs.

I have a couple of questions. Should the levels all be the same? If I get them all the same, what setting should they be - high or low? I'm guessing that the higher the mercury rises, the stronger the vacuum.
 
They should indeed be all the same. I don't think the actual level is all that critical, as much travel as there is for the mercury I think it really only relates to a tiny range overall. Also the mercury level and hence the level of vacuum is related directly to idle speed, so it's just going to change any time you set idle anyway. From what I've gathered (and put into practice) you just set carb number one, or leave it, somewhere near the middle (to leave room for adjustability of the others) and then set the rest to it. If the mercury is really high or low on the guage I'm guessing your idle speed isn't where it should be, and the would be the first adjustment to make (so you don't ultimately end up with one or more of the slide adjusters close to the end of their travel). Warning, you hopefully know this, but do NOT juice the bike while those things are hooked up to the carbs - the vacuum can suck the mercury into the engine where it is expelled through the exhaust, where it can be conveniently inhaled by YOU.
 
Thanks for the information. That's just what I needed to know.
 
Average vacuum reading will be about 25cm hg @ 1200 rpm with the VM carbs on a 79 750. You can set all four carbs the same or as the books recommend set the 2 and 3 carbs 1 cm less vacuum than the 1,4 carbs.

Earl
 
Where do you buy the Motion Pro, and how much?

Thanks,
Stiksave
 
I have a factory shop manual for my '82 1100 and Suzuki's instructions say to set the levels higher on the outside than the inside carbs. I don't know if that pertains to all GS's but it seems like your bike is synched in a similar fashion. I don't know if the levels should be altered when running individual pods. Any advice out there in GS Land?
 
earlfor said:
Average vacuum reading will be about 25cm hg @ 1200 rpm with the VM carbs on a 79 750. You can set all four carbs the same or as the books recommend set the 2 and 3 carbs 1 cm less vacuum than the 1,4 carbs.
Earl
Click to expand...

I didn't see that in the Clymer manual, I really wish I had a factory manual. I'm going to try that next time, does it make a difference in your experience? Do you in fact do that? What is the theory behind it?

Sorry for the pelting of questions, but you're the man with the information. 8)
 
The info isnt in a Clymer. It comes from years of miles with a 79 750E. :-) The suggested difference in the books between the 2,3 carbs and the 1,4 carbs is because of a difference in heat retention between the inner and outer cylinders. In practice, I have not found the slight reduction in vacuum on the 2,3 carbs to be beneficial.

Earl



Hotblack said:
earlfor said:
Average vacuum reading will be about 25cm hg @ 1200 rpm with the VM carbs on a 79 750. You can set all four carbs the same or as the books recommend, set the 2 and 3 carbs 1 cm less vacuum than the 1,4 carbs.[/b]
Earl
Click to expand...

I didn't see that in the Clymer manual, I really wish I had a factory manual. I'm going to try that next time, does it make a difference in your experience? Do you in fact do that? What is the theory behind it?

Sorry for the pelting of questions, but you're the man with the information. 8)
Click to expand...
 
I keep mine all synched the same value. It seems to really smooth out the bike.

Caution! Carb synching can become addictive!
 
get your motionpro at CRC2, www.usedmotorcycleparts.com

I just bought mine there and used it for the first time to sync my carbs after a rebuild. It's the last time I can sync them too because the inner allen head bolt that plugs the sync hole broke off in the f'n hole when I was putting it back in. I didn't even torque it hard and it broke off at the tip. GRR!
 
The simplest thing to do would be to drill another hole beside the broken off allen, tap it for a new allen and youre back in business.

Earl

AOD said:
get your motionpro at CRC2, www.usedmotorcycleparts.com

I just bought mine there and used it for the first time to sync my carbs after a rebuild. It's the last time I can sync them too because the inner allen head bolt that plugs the sync hole broke off in the f'n hole when I was putting it back in. I didn't even torque it hard and it broke off at the tip. GRR!
Click to expand...
 
That may work Earl, but two things to consider:

1. There may not be enough 'flat' spot to drill and tap a new hole next to the existing. You'd have to drill out the broke bolt and retap the hole to say 6mm size fittings.

2. The GS 550 intake boots are not removeable with the engine in the bike. Maybe others have gotten them off, but when I tried, I could fit my impact driver in the open area, but there's no room to whack it with the hammer. Engine would have to come out. :(
 
You dont need a flat spot. Drill a pilot hole with a 1/16" bit

I have removed "resistant" intake boots a few times by removing the carbs and airbox and striking the impact driver with the side of a hammer. It normally does not take a severe blow, just a sharp rap. Before trying with the impact driver, take a small hammer and flatten the bolt heads slightly. Then take the bare bit, place it in the remaining screw head slots and strike it with a hammer to cut a clean, new phillips slot. (this is only good for one try usually, so when you use the impact driver, be certain you have it set to the correct rotation.) Be sure to hold the impact driver down TIGHTLY.

Trust me, :-) :-) :-) There is room. (practice your short stroke swing) LOL

Earl

AOD said:
That may work Earl, but two things to consider:

1. There may not be enough 'flat' spot to drill and tap a new hole next to the existing. You'd have to drill out the broke bolt and retap the hole to say 6mm size fittings.

2. The GS 550 intake boots are not removeable with the engine in the bike. Maybe others have gotten them off, but when I tried, I could fit my impact driver in the open area, but there's no room to whack it with the hammer. Engine would have to come out. :(
Click to expand...
 
I've taken mine off my 550 with an impact driver. It wasn't too bad, though I don't wanna do it too often.
 
You dont need a flat spot. Drill a pilot hole with a 1/6" bit
Click to expand...

Drilling into a radius (curved surface) results in an elipical (oval) shaped hole. You can then tap that hole and you'll still have an elipse. It may work if you use a sealant around the fitting when you screw it in place.

~Adam
 
Only during that portion the bit is above the tangent of the radius, but if that is a concern, you can flat spot the area with a dremel and 1/4" grinding wheel, or a couple strokes with a small file.

Earl

AOD said:
You dont need a flat spot. Drill a pilot hole with a 1/6" bit
Click to expand...

Drilling into a radius (curved surface) results in an elipical (oval) shaped hole. You can then tap that hole and you'll still have an elipse. It may work if you use a sealant around the fitting when you screw it in place.

~Adam
Click to expand...
 
One more question. If I have an automotive vacuum gauge And I read the first value, couldn't I do each one individually? Just being cheap.

Siksave
 
Technically, yes you could do it that way. BUT, there are 4 vacuum ports and when you check vacuum, none of them can be open, so you would have to cap the ports off and switch back and forth changing caps. also, when you change the vacuum level on any one carb, it will change the level in the other three. Thats simple enough, but the problem is it will not change the level by the same amount in each of the other three. so for example if your carbs 1 through 4 have vacuum levels of 20 cm hg, 10 cm hg, 30 cm hg and 20 cm hg and you start with carb #2 which had 10 cm hg of vacuum. If you raise the vacuum to 20 cm hg in carb #2, it could/would change the vacuum in carb #1 to 15 cm hg. You set carb #2 to 20 cm hg. Carb #3 is now 40 cm hg and carb #4 changed to 10 cm hg.
all this because you changed the value on the #2 carb. It can still be done, but you will probably be retirement age before you get them all set to equal values.

Earl


stiksave said:
One more question. If I have an automotive vacuum gauge And I read the first value, couldn't I do each one individually? Just being cheap.

Siksave
Click to expand...
 
I'm almost retierment age, so forget that idea. So I guess I'm old, and old school. If you look at some of the commercial setups, all they are is a board with 4 vaccum gauges and hose and adapters. The motion pro is a vaccum monimitor. I use monimitors at work and other pressure type gauges. I know that the cost of a simple 30# pressure gauge is minimal. So if vaccum gauges of the same quality could be dug up, couldn't I make a 4 guage pro setup. Playing with mercury and glass with my kid around sets off an alarm for me. Like a $50 one time use Motion Pro.

Just a thought from an old guy who's done everything himself restoring my Suzy. Why not make the tools to keep her purring?
 
yes, you can make your own using 4 vacuum guages. The biggest drawbacks to vacuum guages are they are more difficult to read accurately from a distance and you will need to calibrate them to each other each time you use the set. Considering the cost of buying four of them. I think the better choice is to spend the bucks and buy a Morgan carbtune II. It does not use mercury, but instead uses four stainless steel cylinders, requires no calibration and is easy to read. Accuracy is superb!
The downside is the four guage tool is about $80. Upside is John Morgan is a great person to deal with and he stands behind his product. Parts and service are available. I ordered mine direct from him in England.

You could also use water tubes instead of mercury. But, you will need a rather tall instrument for a column of water equivalent. :-)

Earl


stiksave said:
I'm almost retierment age, so forget that idea. So I guess I'm old, and old school. If you look at some of the commercial setups, all they are is a board with 4 vaccum gauges and hose and adapters. The motion pro is a vaccum monimitor. I use monimitors at work and other pressure type gauges. I know that the cost of a simple 30# pressure gauge is minimal. So if vaccum gauges of the same quality could be dug up, couldn't I make a 4 guage pro setup. Playing with mercury and glass with my kid around sets off an alarm for me. Like a $50 one time use Motion Pro.

Just a thought from an old guy who's done everything himself restoring my Suzy. Why not make the tools to keep her purring?
Click to expand...
 
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