Topic: carbs hitting tank

[Davy D]

Hallo all,

After a lot of work, I'm almost finished with my cafe build. I rebuild the entire wiring, engine, carbs,...

Yesterday evening I wanted to install the fuel tank and then the sh*t hit the fan, big time. I can't use the cabs as the tank comes too low on the nr 1 and 4 carb.

Changing the tank is out of the question. So I tought of changing the carbs. I read som good reviews of the keihin CR29's and am willing to pay for it, but not after I'm sure they don't com any higher than te stock carbs.

Does anyone have a drawing or dimensions of these carbs? Other suggestions on solving this problem are also welcome.

[Terry in Australia]

No mate, the CR's won't make any difference (see pic, below) but you can move your carbs up or down a bit by loosening the carb rubber boots, and rotating them. Give it a try.

cb750k-3-625x625 by terry prendergast, on Flickr

[Davy D]

Thanks Terry,

I was hoping on another answer.

I think loosening the rubbers won't be enough, I only have 5mm between the tank and the cabs now.

[Terry in Australia]

Understood mate, but because the rubbers are "bent", rotating the loosened rubbers can move the carbs up or down a fair bit.  Also, you can move the engine around a fair bit by loosening all the engine mounts and jacking it up from underneath. Give it a try!

[innovativems]

Well you may be able to shim the back of the tank up but it may not look right then or may not give you enough room. 

The best option might be to have a skilled fabricator modify the alloy tank to clear.  Probably be a quick work for someone


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[minimo]

Is your engine mounted to the frame correctly?

In addition to padding up the rear fuel tank rubber holder, maybe consider repositioning the mounts on the tank that insert onto the frame at the front. This is a common mod for tanks that are from other bikes to make fit. Be sure to empty out all fuel before chopping and welding

[Davy D]

Thanks for the answers.

I tried to rotate the rubber boots, that gave me an extra 10-15mm. Still not enough to completely open the throttle.

Next I tried to pad up the rear of the tank with an extra 10mm, not enough too.

Last I fitted the front hooks of the tank 40mm higher and that, combined with the rear padding gave me enough clearance.


Thanks for the help guys, appreciated!

[WhyNot2]

Can you show pix of the whole bike?

From the little bit shown, looks purdy neat.

[754]

77 or 78 PD carbs will fit and end your problem.

[Davy D]

Thanks for the hint about the PD carbs. I found some PD41 carbs in Germany, do they fit, or preferably the 42's? As for now, I only find 42's in the USA, but import taxes will be around 30%.

Hereby some quick pics of the bike. This evening I will try to make some better pics.


Thanks for the support,
Davy.

[seanbarney41]

Hi DavyD, I just scored that same exhaust at a swap meet.  Is it from Carpy?  How do you like it?

[Davy D]

It is indeed the carpy exhaust. As I haven't been riding yet, I can't tell how it does, but it fits perfectly. Installed it in no time.

[754]

77 and 78 used PD 41 and PD 42, can't remember which goes to the year.
 The 77 ones have adjustable needles.

[Davy D]

Good evening,

Thanks for the golden hint about the PD carbs! They fit perfectly under my fuel tank.

I also found the difference about the year the carbs are made on the website of CMSNL in their parts lists :
 - PD41A are from '77 K
 - PD41B are from F2
 - PD42 are from the '78 K

I still have one problem: it looks the cable for opening the throttle from my F1 is too short. I think it has to go entirely around the underside of the large disk on the main axle of the carb rack. Is this correct, the cable seems to be 2-3 inches too short? I wanted to be sure before ordering new gas cables.
If it matters: I have the PD41B carbs.

Thanks a lot,
Davy.

[Terry in Australia]

I'm pretty sure that any CB750 "pull" cable will work mate, also, if you're going to run anything but the stock airbox, you'll need to go up a couple of main jet sizes, those PD carbs were designed to run lean to pass EPA Testing, so without the "bolt on choke" (OEM airbox) they will run super lean.

[seanbarney41]

Disagree with Terry about the throttle cables.  I have tried to interchange PD and earlier 750 cables and it did not work.  The free length is quite different.  Do agree with him about the leaness of the PD's though.

[Terry in Australia]

I'll defer to your experience Sean, I did have PD's on my K1 many years ago, but I also had left over cables from my F2 resto.