GS750E Mikuni VM26 vs. VM29 Smoothbores

[Reddog78]

Got my hands on a set of VM29 Mikuni Smoothbores which were on a 77 GS750. The bike was stored unused for almost 10 years. Seller knows nothing about the bike's prior running condition other than it had velocity stacks and 4 into 1 exhaust.

Looking for opinions on whether or not it's beneficial to install them on my 78 GS750E. Stock engine, but considering future 850 conversion with OEM compression ratio.

My 750 is running original VM26's. With help of folks on this forum, I've got jetting close to perfect. Pulls well from low rpm and easily runs past redline. Individual K&N's with Yoshimura exhaust.

So I'm in the if ain't broke don't fix it school of thought with the current carbs. But it seems Smoothbores are coveted rare carbs promising 10% bolt on horsepower.

How difficult are these carbs to rebuild? What parts need to be replaced? I disassembled and cleaned my current carbs (Robert Barr's orings) with no issues.

Is the advertised 10% HP gain real on an unmodified engine? How streetable are the Smoothbores? Don't want a high rpm only ride.

Assuming these carbs were purchased new for the 77 750, were they pre-jetted by Mikuni for that specific application?

No vacuum port for petcock. Any suggestions for manual petcock?

I guess the short answer is bolt 'em on and see if you like them. I appreciate any observations.

 

[Agemax]

if they are indeed smoothbores, they will probably be ok on a stock 750 but will be much better with the 850 top end. they are easy to rebuild, not much different to stock vm carbs. just be careful not to strip the threads in the jet blocks.

 

[Nessism]

I doubt you would get 10% HP on a 750. Maybe on a 1000cc engine but even that's seems like a stretch unless it's been internally modified.

Going to larger carbs can have the effect of more sluggish low end but from what I've read, the VM29's are pretty streetable.

VM29's are easy to rebuild. I've redone several sets with good success. Z1 Enterprises sell all the O-rings and gaskets. Only other thing you may need is float seat/needle assemblies.

 

[chuck hahn]

29s take the same oring kit for the 26 that Cycleorings sells. Youll need to buy jet block gaskets though from another source.

 

[Chuck78]

Darn... I immediately recognized that you must be the one that beat me out to that bike in New York with all the cool vintage go-fast parts!

I wanted that Yoshimura exhaust so badly... Been looking for one of those for years and years.
I was his backup offer, you beat me to it by just a couple of hours... I was ready to drive 11 hours each way to pick that up to restore it as it was...
I told him just the other day to pass my number and email long to you in case you wanted to sell the rest of the parts off of the bike.

chuck_lambert78 is my yahoo.com e mail if you are looking to get rid of any of the rest of it.

 

[Chuck78]

The 29 smoothbores will give you a slight benefit with a stock cylinder head and 850 pistons, but you really need to have a head ported to open it up to a full 29mm to realize the full top end potential of the 29mm venturis. Otherwise you are running a $700 set of carburetors and only getting the smoothbore benefit, not the 29mm benefit. And the smoothbores are not quite as responsive down low as what you can tune the stock vm26 carbs too. But you can get the 29 dialed in pretty well though, but the crispness on the bottom end will not be quite what you would have with the stock carbs when properly tuned.
To fully take advantage of the 29mm venturi, you would really want some high compression 69 mm pistons and aftermarket camshafts on a head that was mildly ported and opened up to a full 29mm inlet, then the engine would definitely benefit substantially from a bigger carb. The stock engine or even a stock 850 does not really need any more than 26 mm venturi's.

I'm going to go ahead and assume that this bike you bought at the best deal of the year also has Yoshimura high-compression pistons which are either 844cc or I have read some reference to 860cc, 69.5 mm, but I have never had the existence of the 860 Yoshimura kit proven though. And it probably also has the same Yoshimura camshafts that I have, a hot street cam which requires aftermarket valve springs and aftermarket pistons with the deeper valve pockets cut into them. And also higher compression.

That bike had some sort of air shifter or aftermarket ignition box or something on it, that in combination with velocity stacks and 29mm smoothbores would make it highly likely that the internal engine parts are also definitely not stock at all, and highly likely to be exactly what I guessed they are.

Those are some sort of aftermarket mag wheels also, Lester's or Henry Abe or something similar. I wouldn't be so much interested in those, but pretty much all of the other non-stock parts on that bike and the gas tank would be of great interest to me. The max would definitely be a desirable item and you might be able to fetch $400 for them as a set. You would need to identify exactly who they were made by, however. The Honda CB single overhead cam guys love those vintage aftermarket mags especially.

 

[Chuck78]

Ditch the velocity stacks if you run the smoothbores, unless you are racing. You need more filtration than that, and if you put screens over them, they may not even flow as well as a set of K&N pods.
One might wonder why so many of these cylinders that I have torn down off of bikes have deep scoring marks in them that run vertically up and down the cylinders... Air filters are important! So is oiling up the cylinder bores for a long-term storage so that they don't get rusty from condensation being stored in a garage during cold and humid times in between seasons.