Topic: Maduncle's brat style CB750F - now called "The carb eater"

[hapakev]

**yeah, I am slightly mad - hence my forum name** I was calling you Ma Duncle. lmao

[maduncle]

**yeah, I am slightly mad - hence my forum name** I was calling you Ma Duncle. lmao

HA - yeah I get that a lot.

The Lectron looks interesting...

... if I am going to do something stupid to the bike, might as well use high quality insane parts.

[Syscrush]

LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON!  LEC! TRON! 

 

I don't know how much you know about the technical history of that log manifold, but a Posa or Lectron is the only logical choice IMO.

[maduncle]

Logical?

Logical??

Now c'mon - if I was going to be logical about having a bike I would be riding a brand new one.

But - I have contacted Lectron and asked for their advice on which carb to get to suit the POSA log manifold and my stock street engine.

[Syscrush]

I am getting f'n excited about your induction setup.

[maduncle]

And I am getting f'n scared!

The more I read about the Lectron carbs the more I steer safely back towards putting a 38mm Mikuni on the POSA manifold. I keep reading things like

 "the Lectron set up is fine for racing or drag bikes where the throttle is either off or flat out open - but no good for street bikes when you want partial throttle and low down power".

Still - I will wait to hear back from Lectron themselves - if they have a good street application carb then I will consider it - otherwise I am back on a Mikuni.

[Retro Rocket]

The Mikuni will be far more adjustable. Some GSXR's had 40mm Mikuni's i think, check the bike dismantlers for a crashed late 80's GSXR....
Just a thought....

[maduncle]

The Mikuni will be far more adjustable. Some GSXR's had 40mm Mikuni's i think, check the bike dismantlers for a crashed late 80's GSXR....
Just a thought....

Thanks for that - I will see what's around.

A brand new 38 is not that expensive anyway, so if I need to I can buy new.

[Terry in Australia]

I like Lectrons Cliff, mainly because they have clear plastic float bowls!

[maduncle]

Oh yeah,

I can see how useful a clear float bowl will be...

... if it were mounted between my speedo and my tacho when I have the throttle open on that carb!

Seriously, I am leaning towards a Mikuni 38mm as I do want to ride this bike at a controlled speed on the street.

Question now is Tm or VM, and do I need an accelerator pump on the carb?

[Terry in Australia]

Oh yeah,

I can see how useful a clear float bowl will be...

... if it were mounted between my speedo and my tacho when I have the throttle open on that carb!

Seriously, I am leaning towards a Mikuni 38mm as I do want to ride this bike at a controlled speed on the street.

Question now is Tm or VM, and do I need an accelerator pump on the carb?

Yeah, I suppose that as much as it'd be good to be able to check your fuel level without removing your floatbowl, the majority of folks here will tell you to buy the Mikuni, so fair enough. I had a Lectron on my Yamaha XT500 trail bike back in the 1980's and it worked fine, (and I liked being able to check the fuel level without removing the float bowl) but I'll bow to the expertise of the previous posters. Get the carb with the accelerator pump. Cheers, Terry.

[maduncle]

Onya Terry - I will get on to eBay and get that Mikuni.

This evening I installed my repainted second hand Koni rear shocks.





They look way better than the old stock shocks - but how is the ride you ask?

Well, I thought I might as well try to start the bike so I jumped on, flicked on the fuel and gave the starter button a push WITHOUT OPENING THE CHOKE and hey presto the bike roared into life without even the sound of the starter!

Go figure - it has never ever started this good. I put it down to no choke on start, I mean what else can it be.

So I took the bike for a 1/2 hour ride and the rear shocks are great, certainly harder but the rear of the bike feels firmer.

[maduncle]

Okay:

Mikuni VM38 or Mikuni TM38 - neither carb has an accelerator pump as far as I can tell - do I need one?

Otherwise I will need to get a Mikuni TM40-6 as this has a pump.

Any suggestions folks?

[70CB750]

Is your seat on hinges?  If yes, could you provide details?

Thanks!

[Syscrush]

This evening I installed my repainted second hand Koni rear shocks.

Well, I thought I might as well try to start the bike so I jumped on, flicked on the fuel and gave the starter button a push WITHOUT OPENING THE CHOKE and hey presto the bike roared into life without even the sound of the starter!

Those are good shocks.

Seriously, looks great.

What did you hear from the Lectron folks?  Have you given up on the idea of a Posa/Lectron setup?

I would really want an accelerator pump for this kind of setup.

BTW, what is the stock carburetor size?

[maduncle]

Is your seat on hinges?  If yes, could you provide details?

Thanks!

My seat is held on by two long bolts with wing nuts. This is so because I made this seat from scratch - it is a 'one off'. I did all the seat pan fabrication work and then I got it upholstered locally.

If you look back through this thread there are photos of the seat under construction.

[maduncle]

This evening I installed my repainted second hand Koni rear shocks.

Well, I thought I might as well try to start the bike so I jumped on, flicked on the fuel and gave the starter button a push WITHOUT OPENING THE CHOKE and hey presto the bike roared into life without even the sound of the starter!

Those are good shocks.

Seriously, looks great.

What did you hear from the Lectron folks?  Have you given up on the idea of a Posa/Lectron setup?

I would really want an accelerator pump for this kind of setup.

BTW, what is the stock carburetor size?

I have moved away from the POSA/Lectron setup as I fear that this is too much of a 'flat out or nothing' kinda race setup and I want a more subtle road application.

If I am geting a Mikuni with a pump, then it looks like being the TM40-6, 40mm is the smallest Mikuni I can find with a pump and 40mm is as big as I can go on the POSA manifold.

As for the stock carb size - good question, I think the stock Keihins are 28mm.

[Syscrush]

As for the stock carb size - good question, I think the stock Keihins are 28mm.

I thought they were in that range. 4 28mm carbs will breathe about the same as 1 56mm carb.  Are you sure that the 40 isn't too small?

[grachman]

Where'd you get those saddle bags?

[maduncle]

Where'd you get those saddle bags?

I designed them and had them sewn up by a local handbag and luggage maker.

I made a cardboard mock up as a template for one side, and I was able to use it as a template for both sides by unfolding it and re folding it inside out (as the bag design is offset).

Although I had two bags made, I only have the left bag on as I still have the stock oil tank. If I change over to an oil tank somewhere else I may add the other bag.

[maduncle]

As for the stock carb size - good question, I think the stock Keihins are 28mm.

I thought they were in that range. 4 28mm carbs will breathe about the same as 1 56mm carb.  Are you sure that the 40 isn't too small?

Well - it is not going to get much bigger on the POSA manifold, the carb inlet ID is 40mm.

As it is , further investigation of the 40mm Mikuni shows it has a 44mm dia fuel outlet, so that may be too big!

All depends on the flange adaptor, I might have to drop to a 36mm Mikuni with accelerator pump (there is nothing betwwen 36 and 40).

[Retro Rocket]

(there is nothing betwwen 36 and 40).

There is if you have it bored out....

[maduncle]

(there is nothing betwwen 36 and 40).

There is if you have it bored out....

Sure thing - but I will save that for a rainy day.

Meanwhile, the saga of the incredible shrinking carb continues.

I have just measured the POSA manifold and the ID of the fuel inlet is 36mm, with the 8mm diameter slotted flange mounts ranging from 48mm to 60mm centres.

Which means I think I am down to a Mikuni TM33-8012 flatside carb with pump and a M-VM34-200K flange adaptor to suit.

This carb has a 40mm diameter fuel outlet, according to the specs. But the specs measure outside to inside, so I reckon it is closer to 38mm.

So - will a 33mm single Mikuni be enough?

[Retro Rocket]

There's a few of us running or going to run Mikuni RS34's X4 so i think one 33 will not be enough.....  What about an SU or something similar..?    Terry? anyone.?

[Terry in Australia]

Nah, it'll run fine Mick, the POSA injector was smaller than 33mm, from memory. Over the years I've owned plenty of big 6 cylinder cars in excess of 4000cc with one pisssy little carb with a throat diameter of less than 40mm and they all started, ran, towed trailers and did hundreds of thousands of miles with no major issues, and more surprisingly when I think of it now, all the spark plugs ran the same colour, so the fuel distribution must have been pretty similar in all cylinders.

As Cliff has stressed, he's not doing this as a performance mod, he just wants to try it out. Here's an interesting article about POSA injectors, the author even describes riding a CB750 with the POSA setup and he was that impressed that he thought it was gonna be something that we would see on more bikes in the future. 

http://www.slickschoppers.com/TechArticles/Posa4.htm

Oh, and I'm just as curious as Cliff so I've just bought this on EBay, so Cliff and I will be able to compare modern carbs with 40 year old POSA injectors soon. Cheers, Terry.

http://www.ebay.com.au/itm/220903091032?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649