Correct Jetting for XS650 bumped up to 750

[Nelson]

I am cleaning the B38 carbs on my 77 XS650D getting ready to reinstall them after rebuilding the engine. I have installed a Hayden Tuning 750cc big fin cylinder block and have turned the crank 83 degrees. What do I need to do to the jets on the carbs for the larger displacement? I am not looking for max power but rather more torque as I haul a sidecar with this rig. Since it is actually my wife's bike, reliability is important which is why getting max power is not a high priority. It has a 2 into one exaust out the left side which are somewhat larger diameter than the original pipes. Any thoughts on this would be welcome. Thanks

 

[WideAWAKE]

The 750 kit doesn’t seem to need that much of a bump from stock jetting. I’d start with a few sizes bigger and see where your at (a little bump for the kit and a little bump for the open exhaust)

There is a thread where people have posted their jetting and bike specs for the big bore kits.

How big is the diameter of your header pipes?

You may want to look into some torque inserts for them if larger than 1.5”

 

[Kevin Werner]

I have an unknown 750 BBC in my 77 and I suspect stock cam.. With 77 carb and the big 77 needle jet, my mains are at 130 from 122.5, my pilot is at 30, from 25 and my needle is dropped 1 notch. Midrange is really good, I have been developing this as times allowed. Now, the bike run really well but, I get a slight snap/pop around 5k rpm WOT and think maybe the main is 1 step too lean. Note, if you have chrome pipe it might be good to go rich and move toward perfect to prevent blueing. (I should edit an hour later. I have single stage Unis and flat track pipes which I have managed to muffle to "only little bit" too loud.) I should have learned from @grizld1 to post a complete and well thought out response the first time. RiP Dick

 

[Nelson]

The 750 kit doesn’t seem to need that much of a bump from stock jetting. I’d start with a few sizes bigger and see where your at (a little bump for the kit and a little bump for the open exhaust)

There is a thread where people have posted their jetting and bike specs for the big bore kits.

How big is the diameter of your header pipes?

You may want to look into some torque inserts for them if larger than 1.5”

Thanks. I was wondering if that would make a difference. I'll check this out but now I have another problem. The slides in my carbs are frozen tight. I have one spare BS38 that was disassembled and cleaned but not reassembled about 20 years ago. It looks to be in good shape. I might have one more BS38 somewhere but have not found it yet. I may have to find another BS38 on ebay but haven't entirely given up on finding my other spare carb or freeing up the slides in the ones that came off the bike I am rebuilding.

 

[Wingedwheel]

Soak it in hot water or boil it. That should soften it up.

 

[Nelson]

I have an unknown 750 BBC in my 77 and I suspect stock cam.. With 77 carb and the big 77 needle jet, my mains are at 130 from 122.5, my pilot is at 30, from 25 and my needle is dropped 1 notch. Midrange is really good, I have been developing this as times allowed. Now, the bike run really well but, I get a slight snap/pop around 5k rpm WOT and think maybe the main is 1 step too lean. Note, if you have chrome pipe it might be good to go rich and move toward perfect to prevent blueing. (I should edit an hour later. I have single stage Unis and flat track pipes which I have managed to muffle to "only little bit" too loud.) I should have learned from @grizld1 to post a complete and well thought out response the first time. RiP Dick

Thanks for the info. My new problem, discovered after attempting to take the carbs apart for cleaning. is that both slides are frozen tight in the bores. This sidecar rig has been sitting in the shop for about 20 years. it was only a couple of years ago that I started to rebuild the bike at my wife's insistence. Now I need to find a way to free up the slides or if that is not possible I will need to replace the carbs. I have at least one in good condition but not assembled and should have another buried somewhere in my shop. I suspect that gasoline mixed with alcohol is responsible for corroding the bores in the carbs. I am trying to soak them in penetrating oil but so far no luck breaking them loose.

 

[Kevin Werner]

Warm them then introduce PB blaster or WD40 or other penetrating oil. the cooling should draw solvent into the void. Remove thr diaphrams first if you can.

 

[Nelson]

Soak it in hot water or boil it. That should soften it up.

All is saved. I took your idea one step further...I prepped the carbs by inserting tight fitting wood pegs into the 2 holes in the bottom of the slide then put hot water in a metal coffee can on a single burner hot plate I use in my shop for chores such as this. I then made a wire bale to allow me to dip the carb into the can of water and retrieve it after it came to a rolling boil. I lifted the carb out of the boiling water, turned it upside down to empty the water in the slide and then poured ice water into the slide (the wood pegs were to keep the ice water from running out. As soon as I poured in the ice water I used a wood "pry bar" to try and lift the slides. On both carbs it worked like a dream and lifted the slides right out. I have some pictures of the process I will try to post here. Thanks for the help guys!