750 carbs on my 650 question

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stangkid1987
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750 carbs on my 650 question

Postby stangkid1987 » Tue Feb 23, 2016 12:46 am

hey guys so i rebuilt a set of round top cb750 carbs with 120 mains and 42 slowjets and installed them on my bike yesterday. The bike bike fired up almost immediately so i knew this was a good sign because my factory carbs were just a pain in the a$$. but now im not sure if i have my fuel lined running correctly because as soon as i hit 1/4 throttle when riding it sputters and cuts like it has no fuel. but when i sitting still and crack it it is super responsive and revs all the way up. I tried not to have any " uphill bends " in the lines. but the lines are running on the air filter side of the carbs. does this matter or should i try to run them like the oem 750 on the engine side. also i am running pods which i was also running on the 650 carbs and didnt have a problem, i just hate the factory pd carbs and thats why i wanted to try the 750s. Thanks for the help guys

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Volker_P
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Re: 750 carbs on my 650 question

Postby Volker_P » Tue Feb 23, 2016 4:16 am

stangkid1987 wrote:now im not sure if i have my fuel lined running correctly because as soon as i hit 1/4 throttle when riding it sputters and cuts like it has no fuel.

Try this again with the choke. Should be at least weaker if it was a fuel (or air) issue.

I would not think that the fuel lines path could cause something like that. Just make sure fuel lines are not blocked or kinked.
You may pull the fuel line from the carbs and put the free end into a jar. The open the petcock to see if and how much fuel comes out there. :wink:

stangkid1987 wrote: also i am running pods which i was also running on the 650 carbs and didnt have a problem,

Congratulations. PD carbs on a 650 with pods without problems, that's rare enough. :roll:
Possibly the 750 carbs need even larger jets with pods. What is the stock jetting for them?

According to viewtopic.php?f=4&t=10321&p=71605 the spacing is a bit off. So worth checking for possible air leaks.
A few more details to the 750 carbs on the 650 is written here: viewtopic.php?f=4&t=722

stangkid1987 wrote: i just hate the factory pd carbs

I just loved them. 25 years without any trouble. But I know US fuel is different.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum

stangkid1987
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Re: 750 carbs on my 650 question

Postby stangkid1987 » Tue Feb 23, 2016 3:12 pm

Im sure its not a air leak the carb boots are brand new just put them on with the new carbs. Im not positive on the oe jets for the 750 my carbs had 110 mains and 38 slow jets. Screws out at 1 1/2. Im also wondering if it has anything to do with the fact that i only had literally one gallon of fuel in the tank( maybe not enough gravity flow for the dual inlets. Im going to try again fri when im not working and as far as the choke it chokes oyt and dies when i engage it.

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Volker_P
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Re: 750 carbs on my 650 question

Postby Volker_P » Wed Feb 24, 2016 3:47 am

I think the 750 carbs have the choke without cable? So you must not fully engage the lever but just try out if it gets better when you close it a bit only. If it dies then, too, this could be a hint that you might be on the rich side.

If you still worry anyway about getting enough fuel into your carbs, plug a transparent hose on the release nipple of the carb bowls and guide it upwards along the floater bowl without kinking it.
The open the release screw and watch the level of the fuel in the hose. It should give a hint to your actual floater/fuel level.
You may start this also with dry carbs and open release screw by opening the petcock.
Then you get an idea how long it takes until your floater bowls are filled to a hopefully reasonable level. :wink:
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum

cb650
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Re: 750 carbs on my 650 question

Postby cb650 » Wed Feb 24, 2016 5:58 am

isnt anyone going to ask what years we are dealing with? 79/80 650 can use 78 and older 750 carbs. 81/82 650 can use 79 and newer 750 carbs.
Did you clean the tank out?!?!?!?!

stangkid1987
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Re: 750 carbs on my 650 question

Postby stangkid1987 » Wed Feb 24, 2016 12:26 pm

Its a 79 with 73 750 carbs

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stangkid1987
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Re: 750 carbs on my 650 question

Postby stangkid1987 » Wed Feb 24, 2016 5:40 pm

Volker_P wrote:I think the 750 carbs have the choke without cable? So you must not fully engage the lever but just try out if it gets better when you close it a bit only. If it dies then, too, this could be a hint that you might be on the rich side.
If you still worry anyway about getting enough fuel into your carbs, plug a transparent hose on the release nipple of the carb bowls and guide it upwards along the floater bowl without kinking it.
The open the release screw and watch the level of the fuel in the hose. It should give a hint to your actual floater/fuel level.
You may start this also with dry carbs and open release screw by opening the petcock.
Then you get an idea how long it takes until your floater bowls are filled to a hopefully reasonable level. :wink:

im going to give that a shot. I really think it's just not flowing fast enough. Maybe it's the tee between both carbs. But i am rebuilding my 650 carbs just incase i do get fed up

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Volker_P
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Re: 750 carbs on my 650 question

Postby Volker_P » Thu Feb 25, 2016 2:12 am

Restricted fuel supply typically results in a few seconds of full boost followed by sputtering and cutting out (as your floater bowls are usually filled at the beginning, otherwise it won't start and stay alive at all).
But anyway it is a good idea to check out this issue before you tear apart something else again.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum


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