CB 750 carbs checklist before install

So, last year I did a full strip down and clean of my CB750 carbs. I'm getting ready to start buttoning things up here. Does anyone have a checklist of sorts what I should do, or check, or test before I put them on?

I really dont want to be pulling them out over something silly that is often overlooked :)

They need to be bench synched - planning that for this weekend sometime.

I have bypassed the vacuum petcock thing so fuel line should go directly to #2. what about the accelerator pump device? .... is that supposed to be left as is, or are people disabling that as well for sake of simplicity?
 
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Jimbonaut

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Did you install new gaskets and o-rings? Always a good look. And stick some fuel line on the drain nipples - I didn't once and fuel pissed out all over the engine. Make sure you attach the throttle cables before installing the carbs (install from the left-hand side of the bike).

Speaking of the accelerator pump. Hook up some fuel to the carbs and activate the pump by pressing on the pump's stem. You should see gas spraying out of each accelerator nozzle in the throat of each carb toward the front of the carb (towards the engine).
 
Did you install new gaskets and o-rings? Always a good look. And stick some fuel line on the drain nipples - I didn't once and fuel pissed out all over the engine. Make sure you attach the throttle cables before installing the carbs (install from the left-hand side of the bike).

Speaking of the accelerator pump. Hook up some fuel to the carbs and activate the pump by pressing on the pump's stem. You should see gas spraying out of each accelerator nozzle in the throat of each carb toward the front of the carb (towards the engine).
Yeah new gaskets, new o rings. I think the accelerator pump wasnt in the best shape as far as I can remember. I'll test it out.

Also, the fuel comes in to brass fitting on #2. But what's the black rubber hose I now have between 2&3? It comes in to a T fitting and goes to carbs 2&3. I cant remeber if this was a part of my removed vacuum petcock assembly .... and not sure what to do with it now if it is :/

This black line, and also that brass fitting on #2? Maybe that was part of the vacuum petcock? Been a while and cant remember what I did.

20200715_223619.jpg
 

pidjones

Well-Known Member
I believe the brass fitting is for operating the vacuum petcock and should be plugged. The other is carb vent and there should be cross-connects to the other csrbs. Really, test the accelerator pump and make sure it sprays well into each throat. I use isopropyl alcohol to be a bit safer - any left over will mix and burn. This also gives a chance to test the float valves. Bench sink on anything risks damage to the butterflys. Better to adjust based on the tiny idle holes exposed. If a carb doesn't spray on the accelerator pump, a high E electric guitar string held by a pin vice and steady hand can be reached back through open butterfly to clean it out.

When remounting, lightly lube isolators and carb snouts with silicone grease and heat the rubber to warm bath water temp with a heat gun or hair drier. Sometime a ratchet strap helps get equal press on them, but I've found the lube and heat gun enough even with old hard isolators. And make sure the airbox is there before installing the carbs or they will have to come back out to install it.
 
I believe the brass fitting is for operating the vacuum petcock and should be plugged. The other is carb vent and there should be cross-connects to the other csrbs. Really, test the accelerator pump and make sure it sprays well into each throat. I use isopropyl alcohol to be a bit safer - any left over will mix and burn. This also gives a chance to test the float valves. Bench sink on anything risks damage to the butterflys. Better to adjust based on the tiny idle holes exposed. If a carb doesn't spray on the accelerator pump, a high E electric guitar string held by a pin vice and steady hand can be reached back through open butterfly to clean it out.

When remounting, lightly lube isolators and carb snouts with silicone grease and heat the rubber to warm bath water temp with a heat gun or hair drier. Sometime a ratchet strap helps get equal press on them, but I've found the lube and heat gun enough even with old hard isolators. And make sure the airbox is there before installing the carbs or they will have to come back out to install it.
Yes, there is connects to the other carbs. What do i do with it? Hell, i think i left that black hose as is last year as well because i also didn't know what to do with it lol.

I tested the accelerator pump nozzles last year when cleaning and carb cleaner sprays out of them nicely. The pump itself i think maybe had some damage or just looked worn on the diaphram i can't remember. Might take it apart and look again.
 

pidjones

Well-Known Member
As I remember, the vent tube just goes over the top of the airbox or maybe there was a teet on the airbox to plug it in.
 
What are these two holes that are close together in the carb, engine side.
When I bench sync am I supposed to have the throttle valve cover both or one or what?

Currently the valves cover both holes when closed

20200717_192638.jpg
 

pidjones

Well-Known Member
Found most of the info in here:

http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revG.pdf

hopefully that has everything I should need.

still wondering what I should do for the full sync. Dont want to spend $100 for a sync tool. Been looking at some of those vacuum gauge DIY setups. Might do one of those.
Yes, bench syncing you adjust idle stop and the sync screws so that part of one hole is exposed. This is where fuel is sucked in and atomised for idle. Make sure the hokes have open paths to the pilot fuel area! They are a lot like GL1000/1100 carbs. Look f9r a Motion Pro mercury tube setup for live syncing. I store mine hanging on the wall in the shop and use an old photo tripod to hang it on for use. Just be careful reving with them hooked up so you don't suck the mercury in when the throttle snaps shut. Really, a good bench sync (don't use wires or bits as you risk scaring the butterfly) using the holes and then adjust idle stop as needed once you get it warmed up (they can take ten minutes or a couple miles to warm up). This will get it VERY close balanced.

SeanG's instructions are excellent.
 
Yes, bench syncing you adjust idle stop and the sync screws so that part of one hole is exposed. This is where fuel is sucked in and atomised for idle. Make sure the hokes have open paths to the pilot fuel area! They are a lot like GL1000/1100 carbs. Look f9r a Motion Pro mercury tube setup for live syncing. I store mine hanging on the wall in the shop and use an old photo tripod to hang it on for use. Just be careful reving with them hooked up so you don't suck the mercury in when the throttle snaps shut. Really, a good bench sync (don't use wires or bits as you risk scaring the butterfly) using the holes and then adjust idle stop as needed once you get it warmed up (they can take ten minutes or a couple miles to warm up). This will get it VERY close balanced.

SeanG's instructions are excellent.
So I read Sean Gs manual again and I still cant tell how to bench sync! Which hole am I supposed to leave partially exposed? The hole closest to the engine (first) or the second hole in?

The black and white picture in his manual just isnt clear enough to answer it for me :(
 

pidjones

Well-Known Member
When syncing it doesn't really matter which hole as long as you use the same one on each carb. Then the throttle stop is adjusted for idle which will move all four together. I seem to remember using the first hole. Make sure your choke when fully actuated causes the butterflys to open some to give fast idle for starting and warmup. Then as you push the choke in the throttle shaft will settle to the throttle stop. If the idle is too high or liw, you then adjust the throttle stop for that but leave the balance screws alone unless you have the balance gauges on it. Maybe I lucked out, but after bench syncing mine they didn't need any balance adjustment, just idle.
 

teazer

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I would use the hole closer to the engine because that's easier to see when they are different. Open and close the throttle a couple of times as you are setting that, to be sure they snap back to the same spot.
 
Are the choke plates supposed to close further than this?

This is where they sit at the end of the normal range of motion via the lever. I can get an additional 1/8" closed if I push with my hand .... wonder if I need to do something to get them to close more than this or not
 

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Jimbonaut

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Pretty sure they should close more than that, but not sure. My carbs are installed now but without the airbox - when I'm in the garage tomorrow I'll pull the choke cable and see if the butterflies close all the way.
 
Pretty sure they should close more than that, but not sure. My carbs are installed now but without the airbox - when I'm in the garage tomorrow I'll pull the choke cable and see if the butterflies close all the way.
Sounds good. Thank you!!
It's almost like that thin, tightly wound spring isnt really doing its job 100%
 
Is this spring put on wrong? I thought I had the horizontal and vertical hooks of the spring in the correct spots, but when I close the choke lever fully, the "O", the "X" doesnt seem to follow all the way and creates a gap. Is there supposed to be something that pulls "X" further to close the choke plates more?
 

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Jimbonaut

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Can't confirm or deny the spring position - did you split the bank when you cleaned the carbs? Meaning, did you take the carb bank apart to clean them, and disassemble the choke linkage? I've only done it a few times myself and yeah, can be a bit tricky to put back together again. One thing I found when I rebuilt my last set was that the choke butterflies didn't close completely, as one of the butterflies was binding up against the throat of the carb. Removed the butterfly, cleaned off some surface pitting, replaced it and it seated fine. Might be worth checking.
 

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