Topic: Mikuni RS34's & CB 750

[tweakin]

That is frustrating Dave, Maybe I need to get some 31's, lol.  Once I get a few more miles in on my build I will try to get it on a dyno for some results.  Thanks for all the work you have put in on the 34's...  I am kind of glad I don't have the easy access to the dyno you have, this has been troublesome for you to say the least.  Not knowing what I have as far as numbers might be a good thing.


It will still get a dyno run sometime soon.

[Doctor_D]

The curve does look good but the loss of 8HP is unacceptable. What was the AF Dave?

It's the light blue line that starts slightly above and finishes slightly below the dashed red line -- which is 13:1.  My A/F is about 13.75:1 for the first half of the run and just under 13:1 for the last half.

[Doctor_D]

That is frustrating Dave, Maybe I need to get some 31's, lol.  Once I get a few more miles in on my build I will try to get it on a dyno for some results.  Thanks for all the work you have put in on the 34's...  I am kind of glad I don't have the easy access to the dyno you have, this has been troublesome for you to say the least.  Not knowing what I have as far as numbers might be a good thing.


It will still get a dyno run sometime soon.

Forget CR31's, CR33's are what you need. 

Anyway, from my runs we know you're in the ballpark for jetting.  That alone makes me feel a bit better.

[Jerry Rxman Griffin aka MuthaF'er]

Sudco can whip you up a set of CR33's from what they've told me! Don't remember the details but you may need a head with the larger intake spigots.

[tweakin]

That is frustrating Dave, Maybe I need to get some 31's, lol.  Once I get a few more miles in on my build I will try to get it on a dyno for some results.  Thanks for all the work you have put in on the 34's...  I am kind of glad I don't have the easy access to the dyno you have, this has been troublesome for you to say the least.  Not knowing what I have as far as numbers might be a good thing.


It will still get a dyno run sometime soon.

Forget CR31's, CR33's are what you need. 

Anyway, from my runs we know you're in the ballpark for jetting.  That alone makes me feel a bit better.

I definitely owe you a beer, or several.

[buca]

Here are some photos of the Mikunis that arrived yesterday.
I wasn't sure what I was going to use for the rubber manifolds on these as the spigots are 40mm.
I had a new set of Honda K rubbers which were tight to get on, but they have snapped on perfectly with a small amount of lube and some even downwards pressure.
Mick

Hi, have one question, if anybody have some experience ..  About instal Dell Orto carbs PHF 36 A ( SD)  on CB 450 . Engine is stock with slightly ported heads. Main jet is, on this carbs 138. Is that too much? Bike is modified for classic racing. Clubs level.  Thanx   Borivoje

[Doctor_D]

Knowing as little as I do about Dell Orto's, it's hard to make a good recommendation.  But, if I had to guess, I would say that's too much main jet.

A 130 main works well on a fairly high-performance 2-valve with roughly the same displacement per cylinder.  Given that you say it's a stock motor with light porting, you might want to start with a 120 main.

[turboguzzi]

Buca, I dont think jets are going to be your problem

36mm are going to be very much on the LARGE side of big..... think about it, people here fit 34's to highly modified 250cc cylinders (i.e. 1000 cc kit)

you want to fit 36mm to a 225cc cylinder in an almost stock motor, dont think they will be easy to setup, would go with 34's or even 32's.

BTW, 36 dellortos is what i have in my 850 le-mans, just to give you an idea....

TG

[buca]

Buca, I dont think jets are going to be your problem

36mm are going to be very much on the LARGE side of big..... think about it, people here fit 34's to highly modified 250cc cylinders (i.e. 1000 cc kit)

you want to fit 36mm to a 225cc cylinder in an almost stock motor, dont think they will be easy to setup, would go with 34's or even 32's.

BTW, 36 dellortos is what i have in my 850 le-mans, just to give you an idea....

TG
I thought might be to much..I have 38 on my BMW 998cc. But I was asked what I thing about this, and I ask you, just to be sure. Thank U

[Roland]

Does anyone know how to remove the broken tips from the carb??? I have the exact same issue...not good.

It wasn't until I got to the idle fuel orifice, a tiny hole on the bottom of the engine side of the carb body, that I noticed an issue.  The guitar string only went through one of the ports.

Seems someone, odds are it was me, over tightened the pilot fuel screws on 3 of 4 carbs causing the delicate tapered tips to jam in the idle port and shear off. In my defense, the springs and o-rings on the screws create some resistance, making it hard to know exactly when the screw has bottomed out.

Never having had all the screws off at the same time, I didn't notice that the tips of the screws looked different.  Pretty obvious once they're on the table together though.



It's not an unusual problem, so I'm not beating myself up too badly.  However, getting the broken off tips out of the carb bodies isn't the most fun I've ever had.

In any case, this explains some of the problems I was having, and new screws are on the way.

The other issue was one of the accelerator pump sticking.  This was solved by using a felt polishing bob on a Dremel, and some Mother's to clean out the pump piston bore.  Much nicer now and it returns fully every time.

As for jetting, the needles that come with the carbs might be fine for a stock GS750, but on a modified 836 they're too lean.  9DZH03's are much better, curing the huge lean-out that was occurring right after the end of the accelerator pump stroke finished.  127.5 mains were too lean with my open 4 into 4's and 50mm velocity stacks, but with balky pilots and a lean needle it's not an accurate measure.  As soon as the new screws are in hand, we'll get back on the dyno and try again.

[johno]

Super D is the man, send him a pm Roland, I reckon being mates with a watchmaker or dentist would be a help

[johno]

Speaking of trying things out, I bought a set of Mikuni TM 32 flatslides last week, they should arrive this wednesday hopefully.
Didnt go the million dollar R version, just the poor mans version , 4 units with KO 4 to 1 cable and some new rubbers.

I hope they work OK, cant see why not, not exactly smokin it up out the corners hey ! 

[Doctor_D]

Does anyone know how to remove the broken tips from the carb??? I have the exact same issue...not good.

It wasn't until I got to the idle fuel orifice, a tiny hole on the bottom of the engine side of the carb body, that I noticed an issue.  The guitar string only went through one of the ports.

Seems someone, odds are it was me, over tightened the pilot fuel screws on 3 of 4 carbs causing the delicate tapered tips to jam in the idle port and shear off. In my defense, the springs and o-rings on the screws create some resistance, making it hard to know exactly when the screw has bottomed out.

Never having had all the screws off at the same time, I didn't notice that the tips of the screws looked different.  Pretty obvious once they're on the table together though.



It's not an unusual problem, so I'm not beating myself up too badly.  However, getting the broken off tips out of the carb bodies isn't the most fun I've ever had.

In any case, this explains some of the problems I was having, and new screws are on the way.

The other issue was one of the accelerator pump sticking.  This was solved by using a felt polishing bob on a Dremel, and some Mother's to clean out the pump piston bore.  Much nicer now and it returns fully every time.

As for jetting, the needles that come with the carbs might be fine for a stock GS750, but on a modified 836 they're too lean.  9DZH03's are much better, curing the huge lean-out that was occurring right after the end of the accelerator pump stroke finished.  127.5 mains were too lean with my open 4 into 4's and 50mm velocity stacks, but with balky pilots and a lean needle it's not an accurate measure.  As soon as the new screws are in hand, we'll get back on the dyno and try again.

It's a PITA to fix these.  I used sewing needles that I had snapped in half (to give them a flat-ish working surface) clamped in needle-nose vice grips, which I then rapped with a small deadblow.  Be careful as you can actually peen the brass tip into place.

[Old Scrambler]

DrD................Did you ream the insides of the intake boots to size?  The I-D on the F2 spigots is 31mm and can safely be bored to 33mm.............the carb is 34mm..........the connecting boots are 28 - 29mm and may be acting as a restriction, especially at the stepped edges.

On my Triumph T20 Salt-Flats bike we married a 34mm carb to a 32mm I-D alloy intake tube on the head and used a K-boot to attach the carb. We measured a notable increase to flow and required a larger main-jet after we reamed the boot and smoothed the edges.  We still do not have maximum power but attribute that to an under-powered spark system. 

[johno]

For any carbs with larger spigots, like webers , mikuni 32 flat slides etc  I have purchased 3 sets of rubbers from Japanese ebay over the last couple of years from the the same guy. Everything that is bad about them for a stock carb is good for the modified carbs, ID is 32 mm so dont have to grind out, very soft so they flex over the larger spigots , and Cheap at $35 per set of 4 with clamps.
johno

[Retro Rocket]

For any carbs with larger spigots, like webers , mikuni 32 flat slides etc  I have purchased 3 sets of rubbers from Japanese ebay over the last couple of years from the the same guy. Everything that is bad about them for a stock carb is good for the modified carbs, ID is 32 mm so dont have to grind out, very soft so they flex over the larger spigots , and Cheap at $35 per set of 4 with clamps.
johno

Do you have a link Johno..?

[johno]

Try googling " Cruizin Image "   he has a ebay store happening, if not I'll have a look for the link tonight.
cheers johno

[Retro Rocket]

Try googling " Cruizin Image "   he has a ebay store happening, if not I'll have a look for the link tonight.
cheers johno

Already got him in my link list mate, i'll check them out....thanks...

[PeWe]

Try googling " Cruizin Image "   he has a ebay store happening, if not I'll have a look for the link tonight.
cheers johno

-These that have 32mm ID?
http://www.ebay.com/itm/Honda-CB750K-K1-K2-K3-K4-K5-Carb-Intake-Set-4-Intake-8-Clamps-CI-CB750KINBT-1-/131120059802?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item1e875d559a

[Old Scrambler]

This site is now competing with the CIA for the best and fastest international intelligence source.

[johno]

Try googling " Cruizin Image "   he has a ebay store happening, if not I'll have a look for the link tonight.
cheers johno

-These that have 32mm ID?
http://www.ebay.com/itm/Honda-CB750K-K1-K2-K3-K4-K5-Carb-Intake-Set-4-Intake-8-Clamps-CI-CB750KINBT-1-/131120059802?pt=Motorcycles_Parts_Accessories&vxp=mtr&hash=item1e875d559a

Hi PeWe, 
Yeh the ID of the rubber is 32mm as opposed to the stockers with 28 to 29mm,   saves a whole lot of emery drum action. 
They are soft compared to the stockers so dont know how long they will last,  but at that price I'm happy if they are disposable.
cheers johno

[Old Scrambler]

These units may also be of some help................http://www.ebay.com/itm/320866775401?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

[johno]

Good spot up Den, I might buy a set to have on standby.
These are my new Mikuni TM 32 flat slides, they arrived today.  Unfortunately my gamble isnt paying divedends yet as they dont fit, yet     I'll show you some pics why

[johno]

[johno]