Can I use 38mm carbs on a 787?

[pcmandevon]

I have a set of 38mm carbs for my 787, I remember seeing that these should run the engine just fine with the jets changed or the screw settings changed... I don't remember what they should be and I can't find the thread. So has anyone used 38mm carbs on a 787, and if so what jets do you have and what do you have the screws set at? I would really like to avoid buying the 40mm carbs. Thank you for the help. Devon. :bigear:

P.S. The fuel pump on the MAG carb is messed up, I have access to a dual Mikuni fuel pump and a block-off plate. Is it possible to over supply fuel to the carbs?

 

[Dr Honda]

38's will run your 800 just fine... but you will be on your own with the jetting.

Personally, I would make the internal pump work... but you can use a external pump if you want. Feed each carb separately. (not one into the next)

 

[pcmandevon]

Do you happen to know where I might find this info? should I try using the same size jets as the the manual states for the 40mm?

The fuel pump is a dual output so one would have to go to both carbs.

Thank you, Devon.

 

[Dr Honda]

I don't know of anyone trying to put on smaller carbs... so I have no idea of where to start. BUT... I do know that they will physically fit.


Personally, I would go with the stock 38 jets, and open the high needle an extra 1/2 turn. Take it for an easy ride, and get the low needle set, and then open up the throttle. If it sputters, and won't clear out... turn in the high needle. If it accelerates, and then feels like its slowing down slightly, or feels flat up top... open the needle.

From there... adjust the jets if needed.

Remember... the carbs don't know what they are bolted to, and on the last engine... it had the correct mixture for the volume of air it was supplying. With a larger displacment... you will be drawing a heavier vacuum, and in turn, more fuel will enter the carbs. Because the carbs are subjected to a "Pulse" as the rotary valve opens and closes... you may find you are OK. Most of the time when I put a "Big Bore" kit on a dirt bike at my shop... I find that the main jet needs leaned out to run. (if nothing other than the bore changes)


I do need to add one thing... what did the carbs come off of? If it was a 720 with a 159 rotary valve... bolt them on, and tweak the needles. If it was off something else... let me know.

 

[pcmandevon]

yes, it was a 717, but I don't know anything about the engine the carbs came from. If I had the money I would just buy the larger carbs and be done with it, but at this point I have had to throw a lot at this and I haven't even used it yet. To be honest I don't know if the rotary valve in the 787 I have is even right. Thanks again. Devon.

 

[Dr Honda]

To be honest I don't know if the rotary valve in the 787 I have is even right. Thanks again. Devon.

That's important.

Pull the manifold, and make sure it's the right valve, and that it's timed correctly.

 

[pcmandevon]

I'll take a look at the valve as soon as I can jump back into the boat. I'll let you know what I find. Thank you again. Devon.