SOHC.co.uk Forums > CB750
Bogging down.
philward:
I have messed with jetting etc for some time on the CR750 Rep (road use) and now got it pretty much spot on across the range. I can quote what my settings are but there are so many variables on each build. I had no choice when building a CR Rep as you can't have a CR Rep with std airbox and exhausts but on the K6 Cafe racer project I'm building I've re-fitted the airbox and std exhausts (which was a pain as the previous owner cut the airbox and exhaust brackets off and I had to weld them back on!) to make the carb set up easy - ie, standard..
One of the main issues MarkCR750 pointed out was the step from the pod to the carb body that the pods create - ruining the smooth airflow. Mark adopted the standard airbox rubbers to the pods to maintain the airflow I think (there's a post on here somewhere). Marks build uses PD carbs though I think.
My CR runs standard carb with foam pods (no step to carb bodies) and ended up with 135 mains, 45 primary jets (I had to drill 40's out to 45 as no longer available), air screws 1 3/8's out and needle on middle groove. I'm running 4 into 4 race exhausts with K0 baffles in and standard compression 836 kit. This may not help as you will be running a diiferent combination but will give you an idea - took about 10 changes of combination to arrive at the right setting that gives a reasonable tickover, smooth step free mid range and good top end.
Hope that helps
brodsord:
--- Quote from: K2-K6 on May 12, 2020, 08:27:09 PM ---As the others have already posted, you can really chase your tail on this sort of stuff.
Basic problem is not that the engine will burn more fuel, but that the original airbox / intake etc cause more vacuum that pulls the correct amount of fuel from the float bowls. Going to pods gives you less vacuum in the carb venturi for the same engine speed, and consequently pulls less fuel up from the float bowls, hence the weak mixture.
As a test / see, you could lift the needle in main jets by dropping the circlip two notches, setting the float height a couple of millimetres higher, turning the idle jets in by 1/4 from standard setting and using resisted spark plugs with resisted HT caps as well.
It would push it toward where you need to be and at least see if it improves without significant cost.
Edit;- also make sure the plugs are at their minimum specified gap too.
--- End quote ---
Thanks so much for this info K2.
I’m running just the standard plugs for a k5 engine from DSS
Sent from my iPhone using Tapatalk
brodsord:
--- Quote from: philward on May 12, 2020, 09:17:12 PM ---I have messed with jetting etc for some time on the CR750 Rep (road use) and now got it pretty much spot on across the range. I can quote what my settings are but there are so many variables on each build. I had no choice when building a CR Rep as you can't have a CR Rep with std airbox and exhausts but on the K6 Cafe racer project I'm building I've re-fitted the airbox and std exhausts (which was a pain as the previous owner cut the airbox and exhaust brackets off and I had to weld them back on!) to make the carb set up easy - ie, standard..
One of the main issues MarkCR750 pointed out was the step from the pod to the carb body that the pods create - ruining the smooth airflow. Mark adopted the standard airbox rubbers to the pods to maintain the airflow I think (there's a post on here somewhere). Marks build uses PD carbs though I think.
My CR runs standard carb with foam pods (no step to carb bodies) and ended up with 135 mains, 45 primary jets (I had to drill 40's out to 45 as no longer available), air screws 1 3/8's out and needle on middle groove. I'm running 4 into 4 race exhausts with K0 baffles in and standard compression 836 kit. This may not help as you will be running a diiferent combination but will give you an idea - took about 10 changes of combination to arrive at the right setting that gives a reasonable tickover, smooth step free mid range and good top end.
Hope that helps
--- End quote ---
Thanks so much Phil, very helpful
Sent from my iPhone using Tapatalk
brodsord:
--- Quote from: Trigger on May 12, 2020, 08:54:15 PM ---
--- Quote from: K2-K6 on May 12, 2020, 08:27:09 PM ---As the others have already posted, you can really chase your tail on this sort of stuff.
Basic problem is not that the engine will burn more fuel, but that the original airbox / intake etc cause more vacuum that pulls the correct amount of fuel from the float bowls. Going to pods gives you less vacuum in the carb venturi for the same engine speed, and consequently pulls less fuel up from the float bowls, hence the weak mixture.
As a test / see, you could lift the needle in main jets by dropping the circlip two notches, setting the float height a couple of millimetres higher, turning the idle jets in by 1/4 from standard setting and using resisted spark plugs with resisted HT caps as well.
It would push it toward where you need to be and at least see if it improves without significant cost.
Edit;- also make sure the plugs are at their minimum specified gap too.
--- End quote ---
If you use resisted plugs and resister caps that with give a weak spark ;)
--- End quote ---
Thanks for this Phil, yes I think I’m going to do exactly what you have said here to see if it improves any, and move on from here if needed.
Sent from my iPhone using Tapatalk
brodsord:
--- Quote from: Nurse Julie on May 12, 2020, 06:23:06 PM ---
--- Quote from: brodsord on May 12, 2020, 06:16:25 PM ---Choke lever*
F@cking iPhone
Sent from my iPhone using Tapatalk
--- End quote ---
.
--- End quote ---
Hate predictive text Julie
Sent from my iPhone using Tapatalk
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version