Topic: Can i run a '77/'78 carb setup on a 1969 motor (CB750)?

[Kudzu]

I have a set of PD42B carbs and a 1969 motor.  Is it ok to run these carbs on that motor?  Sorry, i'm never tried.  I don't have the carbs from the 69 motor.

[Jerry Rxman Griffin aka MuthaF'er]

Which 77/78? You will need the later intake boots unless you attempt to ream out the 69 boots (presuming that is what you have) for the larger throat 77/78 carbs.

[Kudzu]

The keihin PD42B’s

also i have the boots for the newer carbs.  I have the whole bike.

[Don R]

Yes. But if you have a real 69 you may want to find some 69 carbs just to preserve the value of it. Out of curiosity, what are the engine numbers and do you have the corresponding 69 frame? A 69 throttle will only have a pull cable, not pull and push.

[Tews19]

I'd stay away from slapping on  77/78 carbs. I may have a set of later round tops for sale if you are interested. You need 5 post before you can send or receive PMs.

[70CB750]

69 are hard to find, but at least round tops would work better.

[maxheadflow]

I'd stay away from slapping on  77/78 carbs. I may have a set of later round tops for sale if you are interested. You need 5 post before you can send or receive PMs.

I'm curious as to why you don't care for the 77/78 carbs.  Is it simply a visual thing?

[Kudzu]

It's not a sand cast engine. 

It's a custom build and the engine happens to be a '69.  I also have another cb750 that I use to scavenge parts off of.  That's where these carbs came from.  The carbs currently on the custom bike are the cycle x dual carb setup and it's a nightmare to get tuned in a way that allows all 4 cylinders to get the same amount of fuel and/or air.

I'm not trying to maintain the time period.  I'm just trying to get the bike to idle and run smooth. 

I would love a set of round tops.  Are they rebuilt or would I need to rebuild them?

[Tews19]

It's not a sand cast engine. 

It's a custom build and the engine happens to be a '69.  I also have another cb750 that I use to scavenge parts off of.  That's where these carbs came from.  The carbs currently on the custom bike are the cycle x dual carb setup and it's a nightmare to get tuned in a way that allows all 4 cylinders to get the same amount of fuel and/or air.

I'm not trying to maintain the time period.  I'm just trying to get the bike to idle and run smooth. 

I would love a set of round tops.  Are they rebuilt or would I need to rebuild them?

They are actually on my 69 750 at the moment. I can clean them.

[Don R]

 I get that, but real 69 engines don't have to be sandcast to be desirable.  Good luck with the carbs, I'd jet them up a size or two, the 77/78's were pretty lean.

[ekpent]

  Round top carbs are super easy to tear down and work on, lots of parts and jets etc. are easy to get for tuning and I wish all motorcycles carbs had those lovely snap on bowl set-ups ! No choke cable to deal with also.

[maxheadflow]

  Round top carbs are super easy to tear down and work on, lots of parts and jets etc. are easy to get for tuning and I wish all motorcycles carbs had those lovely snap on bowl set-ups ! No choke cable to deal with also.

Thanks for the thoughts on this. I assume they were for my benefit.   Those are all good reasons.   The fixed pilot jets are definitely a PITA.

I kind of come from a performance background. Once the carb is tuned, is there any difference in performance?

[ekpent]

  Round top carbs are super easy to tear down and work on, lots of parts and jets etc. are easy to get for tuning and I wish all motorcycles carbs had those lovely snap on bowl set-ups ! No choke cable to deal with also.

Thanks for the thoughts on this. I assume they were for my benefit.   Those are all good reasons.   The fixed pilot jets are definitely a PITA.

I kind of come from a performance background. Once the carb is tuned, is there any difference in performance?

Not specifically at you but as my wife tells me "anybody that will listen with my big mouth" 

[Don R]

 If you have them, put 'em on and let er rip.