850T: Installed a big bore kit. Carb suggestions?

[Semitone]

I'm in the process of restoring my 850T. Many changes, one being a 1000cc big bore kit. Didn't need to re-balance crank as balance factor for existing pistons was close to 55% and the added weight of new pistons brought it down to just over 52%, which seems to be the consensus for best BF  The existing Dellorto VHBs are tired and I want to step up the breathing to match the increased displacement. Whichever carb I go with I will have heads port and polished to match carbs, and will be willing to go with bigger valves if necessary. Does anyone have any suggestion as to proper carb size and/or recommendations on various manufacturers? Grateful for any thoughts. -Paul

[Dukedesmo]

LeMans, 1 & 2 use Dellorto PHF36. Mine has Gilardoni 'big-bore' kit so 950cc (1,000) and runs well with the PHF, I don't think it needs anything bigger than 36mm


If you want something more up to date, you could fit Keihin FCR? I fitted 41mm FCR41 to my (944cc) Ducati 900 Monster and it pulls like a train though I would think 41mm would be too big for the Guzzi? but they do make smaller models.


The PHF also come in a Left and Right sided version (AFAIK FCRs don't?) so setting up is easier on the Guzzi engine layout. 

[Frenchfrog]

I big bored my T 3 a long while ago and chose PHF 30's .You won't see a lot of performance gains from the bore change or the carb swap. Lots of people put 36 mm dells on small valve engines these days and that usually entails a manifold change depending on the carb. It would also work best by having light porting done so there is no step in the manifold but some dispute that . I'd almost certainly think that the performance would be better but also your fuel consumption will increase, The optimum carb size for the small valve engines was 34 mm carbs according to Amedeo Castellani of raceco UK.Given the hundreds of Guzzi engines he's worked on and tuned and he race sucess he enjoyed I'd tend to go with that advice. Thing is that 34 mm PHF 's are rare , raceco bored out 30's because of that.

[Murray]

I'f you are not too worried about the purity I'll secound the FCR suggestion they are a much much better carb that the delorto ever hoped to be.

[huub]

vm 34 mikuni's are dirt cheap , and work great.
I run those on my V65 lario

[Scout63]

My CX runs very nicely with 36 PHMs, matched intake, competitziones and a cam.  I also really like the 32 VHBs on the stock G5 1000 motor. No fiddling required.  If I were keeping the stock pipes and look of a T I’d probably go with the smaller carbs and leave the heads stock. Wonderful dilemma.

[moto-uno]

  Do Not Go With The 40mm dellortos ! Peter

[geoff in almonte]

I ran 38mm Mikuni flat slides on my big bore (950cc) , cammed T5.

G

[Semitone]

@scout63, wonderful dilemma indeed! Great info from all to put into the dilemma hopper! Needless to say, I’ve now more homework to do. Forgot to mention, the bike came to me with BUB exhaust. Don’t know when they were installed or what model they are, so I don’t know if they provide greater breath than stock. Crossover pipe still in place. I’ve also installed a slightly hotter cam. Will do more research on the BUBs first. -p

[Gliderjohn]

You have done similar engine mods as was done on my T-3. I used the 36mm and they appear to have worked out great.
GliderJohn

[SoCV]

You have done similar engine mods as was done on my T-3. I used the 36mm and they appear to have worked out great.
GliderJohn

 I can attest to that , John's T3 is pretty healthy .

 Dusty

[Turin]

Unless you are putting in a monster camshaft, and doing major head work 36mm carbs will be fine.
I looked into converting small valve heads to lemans spec for my 850 Lemans engine.  The difference between small valve and medium valve heads is not dramatic to the naked eye. Slightly bigger valves will require you to re-shape the combustion chamber.  With the reshaped combustion chamber you will need a piston with a bigger dome.
See where this is going? Keep in mind, A LeMans 1000 in good tune makes about 65 rwhp

Here is my 850 T recipe, and it's a blast to ride. Norris S / megacycle X9 camshaft. mildly ported heads with new stainless valves. set squish to .040 with copper base and head gaskets. delrin manifolds with 36mm Delortos. mildly lightened flywheel.

Mikuni VM's are about $300.00 for a set. you have to figure out the manifolds yourself.  Keihins are up to $1500.00 for a set.

Here are pics of a small valve VS 850Lemans heads. first pic will be small valve 2nd lemans











6 face dice

[Turin]

850 T-



free picture upload

[Semitone]

Unless you are putting in a monster camshaft, and doing major head work 36mm carbs will be fine.
I looked into converting small valve heads to lemans spec for my 850 Lemans engine.  The difference between small valve and medium valve heads is not dramatic to the naked eye. Slightly bigger valves will require you to re-shape the combustion chamber.  With the reshaped combustion chamber you will need a piston with a bigger dome.
See where this is going? Keep in mind, A LeMans 1000 in good tune makes about 65 rwhp

Here is my 850 T recipe, and it's a blast to ride. Norris S / megacycle X9 camshaft. mildly ported heads with new stainless valves. set squish to .040 with copper base and head gaskets. delrin manifolds with 36mm Delortos. mildly lightened flywheel.

Mikuni VM's are about $300.00 for a set. you have to figure out the manifolds yourself.  Keihins are up to $1500.00 for a set.

Here are pics of a small valve VS 850Lemans heads. first pic will be small valve 2nd lemans











6 face dice

Hey Turin, I also used the Norris S x9 cam. I realize now that larger valves triggers more mods and $$ than I can stomach. I like your 850T recipe. This bike also came to me with a lightened flywheel. Btw, who made the delrin manifolds? Before my latest posting, I was pretty much resolved to go with Dellortos PHFs. However, I'm not necessarily trying to stay original with this bike, and I'm still intrigued with other carbs folks have suggested. Thanks for your input.

[Turin]

Here you go.  https://www.mgcycle.com/index.php?main_page=product_info&cPath=279&products_id=179

  These are a direct fit for 36phf's from a sp3 https://www.mgcycle.com/index.php?main_page=product_info&cPath=62&products_id=1173

It's a nice set up and she hustles down the road.

I also have a 1082 CC Lemans 1000 SE that used to be a race bike. It has a wild motor and it sports a set of Keihin FCRs. Even on that they are a lot of carb. My friends dad built that motor back in the day and raced it with 40phf's.
I'm building another Lemans 1000 (1036cc) based engine with a 92mm Bore , 11.1 Wiseco pistons and major head work. I'm going to go with Mikuni's on this one.








It might be a monster, or it might end up being a slug.

[Klinkhammer]

I would consider also lightening the flywheel.
My 850T based cafe racer has a big bore kit, stock heads and cam, free flowing exhaust system and PHF 36 with matching intake manifolds from HMB. In combination with a lightened flywheel (also from HMB) it is a very nice running bike! Easy, straight forward setup!
Just tell Michael what you are running and he will set the carbs up with needles and jets. For my engine he was spot on!
https://hmb-guzzi.de/Inlet-manifold-set-CNC-for-PHF-36-carbs-tapered

[moto-uno]

  I too have a BUB exhaust on my Le Mans 2 , it was a few millimeters larger in diameter than the stock header pipes .
It's got either the BUB quiet(er) mufflers on it if spending time around town or the Conti replica megaphones which I
love , BUT , holy crap they can raise the dead on full throttle  . Peter

[Turin]

I went from double walled factory exhaust with bub mufflers to a mistral stainless exhaust with the same bub mufflers. The sound changed a bit. If there were any performance changes, they are minimal.

[Semitone]

Here you go.  https://www.mgcycle.com/index.php?main_page=product_info&cPath=279&products_id=179

  These are a direct fit for 36phf's from a sp3 https://www.mgcycle.com/index.php?main_page=product_info&cPath=62&products_id=1173

It's a nice set up and she hustles down the road.

I also have a 1082 CC Lemans 1000 SE that used to be a race bike. It has a wild motor and it sports a set of Keihin FCRs. Even on that they are a lot of carb. My friends dad built that motor back in the day and raced it with 40phf's.
I'm building another Lemans 1000 (1036cc) based engine with a 92mm Bore , 11.1 Wiseco pistons and major head work. I'm going to go with Mikuni's on this one.








It might be a monster, or it might end up being a slug.

Thanks for the links Turin. This does seem like the path of least resistance for me. Not crazy about the black theme for carbs and manifolds. Still figuring this out

[Semitone]

Hello again all. I had originally posted this query a few months ago. I decided to finish the build with the original VHB 30s and see how she ran. The bike is a 1975 850t. The motor received a Norris S x9 cam a big bore kit. It already had a lightened flywheel, Ram clutch and Bub exhaust when I bought the bike. Heads are still stock. No mods there. First start was a complete failure with terrible back firing through carb as I cranked it over. Having your ignition wiring reversed will do that. After correcting this she fired right up and was eager to go! So I’ve been breaking the engine in. The bike runs very nicely from idle to about a 1/4 turn on the throttle. However if I lean heavy on the throttle and load up the engine (I’ve only done this a few times since I’m still breaking it in), the engine labors and starts to misfire. My assumption is that I’m running lean since I haven’t made a jetting mods to account for the added displacement. Does anybody have a suggestion for this? I’d like the bike to run better until I get new carbs.

My initial posting of this topic generated a lot of great info. After a lot of consideration, I’m leaning toward going with Mikuni TM-34s or DO PHF 36s. Keihins are a bit out of my budget. I will get a port and polish but I‘m staying with stock valves so I kinda feel that I shouldn’t go larger than a 34mm carb? The only reason I’m considering PHF 36 mm is because I can’t find 34 mm.

Regarding the TM-34, in my research, I thought I read that some extra manifold mod had to be done because of float chamber was wider so carb interfered with engine housing? I cannot find this post now, so I’m not sure if this is indeed true with the TM-34 or if it was for a different Mikuni model.

Looking forward to any additional thoughts. Tnx -paul

[Furbo]

My initial posting of this topic generated a lot of great info. After a lot of consideration, I’m leaning toward going with Mikuni TM-34s or DO PHF 36s. Keihins are a bit out of my budget. I will get a port and polish but I‘m staying with stock valves so I kinda feel that I shouldn’t go larger than a 34mm carb? The only reason I’m considering PHF 36 mm is because I can’t find 34 mm.

Regarding the TM-34, in my research, I thought I read that some extra manifold mod had to be done because of float chamber was wider so carb interfered with engine housing? I cannot find this post now, so I’m not sure if this is indeed true with the TM-34 or if it was for a different Mikuni model.

Looking forward to any additional thoughts. Tnx -paul

Well, I'm not familiar with your cam. I have used a B-10 Cam in a small valve engine with PHF 36's and it runs great. Would at least grab a pair of PHF 30's as they're a tighter device than the VHB's. Unfortunately, the Bub Hyper system is now made of unobtainium as I consider it one of the best midrange exhausts ever made for a Guzzi. Good Luck

[Turin]

Don't like Black manifolds? Here's another way to skin the cat. They go from a 36mm intake and reduce down to 30mm at the head. You'll probably need some step studs.

https://www.officine08.com/p68647_intake-manifold-moto-guzzi-serie-grossa-carburetor-dell-orto-vhb-30-kit-conical-straight.html

I bought a set of the 40mm-36mm for my 850 Lemans engine. they are nice looking.

[Frenchfrog]

Plenty of manifolds her too...but hardly a cheap choice. https://hmb-guzzi.de/

[moto-uno]

  When in doubt , refrain from small incremental changes in jetting . Go at least 2 to 3 sizes up ( or down) and
see if it improves , or gets worse , that should lean you in the right direction , excuse the pun . Also , a lot of
cams don't always respond well to full throttle at lower (under 4 grand ) revs , particularly in higher 2 gears !
Have fun , tuning is a true test of patience :) . Peter

[Alfetta]

something to consider perhaps...

[Turin]

1. That would be expensive.
2. Where would your shins go?

[Semitone]

Well, I'm not familiar with your cam. I have used a B-10 Cam in a small valve engine with PHF 36's and it runs great. Would at least grab a pair of PHF 30's as they're a tighter device than the VHB's. Unfortunately, the Bub Hyper system is now made of unobtainium as I consider it one of the best midrange exhausts ever made for a Guzzi. Good Luck

The cam is by megacycle. I believe it is fairly similar to the B-10. Good suggestion about the PHF 30s, but I think I’ll up the jetting in the VHBs until I get the replacement carbs I want. I very happy with my BUBs and hope to get some more years out of them. Lovely sound!

[frozengoose]

What type of and where did you get a 1000cc kit for a T that didn't require rebalancing? I was told they all did.
Thanks, cause I have a T and was considering this.

[ccoli]

I may have a set of dellorto 34s. Pm me if interested.

[Semitone]

Don't like Black manifolds? Here's another way to skin the cat. They go from a 36mm intake and reduce down to 30mm at the head. You'll probably need some step studs.

https://www.officine08.com/p68647_intake-manifold-moto-guzzi-serie-grossa-carburetor-dell-orto-vhb-30-kit-conical-straight.html

I bought a set of the 40mm-36mm for my 850 Lemans engine. they are nice looking.

Hello again Turin. It’s more about the black Dellortos I don’t like, but thanks for link to the manifolds. I’m certainly going to need something like this when going to a bigger carb. Wonder if I’d be able to mate the Mikunis to this without too much fuss. P