Carb synchronizing question on a 1979 GS750

[oregonDave]

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.

[Hotblack]

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.

[oregonDave]

Thanks for the information. That's just what I needed to know.

[earlfor]

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

[stiksave]

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

Thanks,
Stiksave

[RJ]

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?

[Hotblack]

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

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)

[earlfor]

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

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

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)

[Jethro]

I keep mine all synched the same value. It seems to really smooth out the bike.

Caution! Carb synching can become addictive!

[AOD]

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!

[earlfor]

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

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!

[AOD]

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.

[earlfor]

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

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.

[alan]

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.

[AOD]

You dont need a flat spot. Drill a pilot hole with a 1/6" bit

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