68 GT carb sizes

angonemoto

angonemoto

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Location
Chicago, USA
Hi all,
The more I tinker with this bike the more I discover. :D I found a 750 GT and S manual and it notes that the GT should have a left and right VHB 30 carbs. I just found out I have 2 VHB29D (right) carbs. I'm not too worried about them both being right sided, but should I try to get 30s to get the most out of this bike's tuning? I really don't have any idea of the difference or rarity of the 30s for that matter.

normal_2VHB29AD.jpg

 
I think the 68 originally had 29mm carbs.

Not sure if they were both RH & LH that year.

I have a head from that year with only 28mm inlet ports !
 
I thought all the pre '73 GT had 29s. I don't know if they continued with the advent of the big valve head.
I'll have a look at the odd 29 here to see if it is a D or an S. Those aren't the original petcocks.
 
jotacorsa said:
I thought all the pre '73 GT had 29s. I don't know if they continued with the advent of the big valve head.
I'll have a look at the odd 29 here to see if it is a D or an S. Those aren't the original petcocks.
Click to expand...

Scott,

I'm pretty certain that 30mm was introduced during 1969, if not from the start.

Paul
 
Hey Scott, are the originals the square cube kind that are on eBay like these?

http://ebay.to/2r69BuM

I may replace them depending on how successful I am with my leaky tank fix...see my other tech thread. :(
 
my understanding is that the 29's were for the 650 and that the 30's were for the 750, made all the more confusing because I have been told in the past that quite a few 650's were converted to 750 at the factory and before despatch to service the high demand for the bigger bike, maybe these ones are 750's with 650 carbs?????

I have also been told by similar sources that the 29's were very quickly "no longer available" which forced the change to 30's.

CLEM
 
Scott, Piet,

I have  GT n? 2312 with 29mm carbs.
It also has the first frame, Grimeca brakes and square mudguards.

I have the remains of GT n? 22.. which has 30mm carbs and also later type frame, round mudguards and first series Laverda brakes.

Once again, no precise corelation between numbers and specs on those early twins. That's for people who write books.

Paul
 
I believe that from the moment the 750S entered production, with 30mm carbs, the GT was fitted with same carbs. That was sometime in May 1969.

Marnix
 
MarnixSFC said:
I believe that from the moment the 750S entered production, with 30mm carbs, the GT was fitted with same carbs. That was sometime in May 1969.

Marnix
Click to expand...

Would seem coherent though as usual there would have been some overlap concerning use of parts.
My S had 27mm intakes, as pointed out by Phil Todd. I've since opened them out to 30.

Paul
 
I did some arm chair research over the last few days and per the photographic and reported archive from magazine articles online and owner bikes pics you can see through search it looks like the 29s are correct for the 1968 GTs and American Eagles. I also noticed that it's not unusual to have 2 right handed (D) carbs on the bike, per the plumbing of the carb fuel lines, from what I have seen. That said, I am going to leave them alone. I have a bigger issue with the tank leak to attend to anyway.
 
Moto Guzzi used the same VHB 29 and VHB 30 carbs on many of their bikes (also on the Benelli 650 Tornado).

The 850 Guzzi Eldorado came with VHB 29's and the 750 V7 Sport had the VHB 30's, the VHB 30 was also on the 850T, T3 and Spada so parts are readily available and cheap.

MG Cycle Moto Guzzi sells all the parts online for the VHB 29 / 30.

I don't think you would notice any difference between the two on your bike.

The VHB slides wear on the flat face that is up against the cylinder intake side of the carb body. Remove the slides and tape some 1500 grit to a sheet of glass and then wet sand the flat face of the slide until all scrapes and marks are removed.



Accelerator pump springs are often rusted through replace them, they are cheap and clean out the accelerator pump chamber and make sure the tiny white plastic ball is moving freely.





The carb top caps tend to warp over time, you may need to use two of the paper gaskets to get an air tight seal.



The new white plastic floats for these carbs are crap and tend to leak, I would keep the original green plastic floats if they are not leaking. Check the float height.



Choke plunges often leak at the underside of the plunger is a rubber base that seals against the carb body, these dry out and the choke circuit can leak, I would replace the choke plungers and the tiny O ring that the plunger cap has in a flange recess where it drops into the carb body.



The tiny O ring on the choke jet is also prone to rot



VHB carbs do not use O rings on the air and idle screws even thought he kits supply them



These carbs can work great but they need to be really clean, a couple hours in an ultrasonic cleaner with purpose made carb cleaner for an ultrasonic bath is a must.

Sharpertek makes a great ultrasonic carb cleaning solution.

 
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