Author Topic: '73 CB750 Won't Idle. (Read 10580 times)

awwyiss · « **on:** August 31, 2012, 12:08:08 PM »

Last week I picked up a 73 750 from a buddy and I can't get this thing to idle for the life of me.

I pulled the carb bank out, took them completely apart and soaked in carb dip over night, jets aren't clogged. Actually it was pretty immaculate inside already.

This is how everything looked when I pulled it from the bike originally.

So like I said cleaned out the carbs, got everything put back in, replaced the spark plugs, changed the oil, and oil filter.

When I try to start the bike I'll give it about 1/4 throttle with the choke open and it fires up right away. As long as I keep on the throttle it'll keep running (although it sounds pretty rough). I've tried slowly closing the choke but usually as soon as I start doing that it'll die. If I let off the gas at all it'll die. Really if I do anything but hold on the throttle with the choke open it'll die.

No real leaks that I can see. Every now and then one of the float bowl over flow tubes will spit out a small amount of gas after a few failed idle attempts that's it though.

Any suggestions? $:-\$

stockscreamer · « **Reply #1 on:** August 31, 2012, 12:16:11 PM »

check the float heights they should be 26mm from the carb bowl lip to the top of the floats. If the floats are out of adjustment bike will run like shi-poopi. what are your idle air screws set at? they should be 1 turn out counter-clockwise from lightly seated.

awwyiss · « **Reply #2 on:** August 31, 2012, 12:22:07 PM »

I made a little float height tool, they're all set to 26mm as far as I could tell. For the air idle screws I screwed them in all the way then backed them out a full turn.

I followed the guide at salocal.com while working on the carbs.

Tugboat · « **Reply #3 on:** August 31, 2012, 12:54:53 PM »

You running an air box or open carbs? Could be lean.

awwyiss · « **Reply #4 on:** August 31, 2012, 01:09:26 PM »

Running the stock air box. Some more details after screwing around a little more. Almost no smoke coming out of the pipes, just a tiny amount of whiteish wisps every now and then while trying to get it to idle.

I backed the idle air screws out half a turn and it's aaaaalmost idleing. I still have to give it a little gas but with the throttle slightly open it'll over around 1300. As soon as I let off all the way though it dies.

I hooked up my carb sync tool and unless I'm totally reading it wrong it looks like when I reassembled them they were pretty close to sync'd. I'll try to get video of that later.

Also I only get that little squirt of fuel out of the over flow tube like once every 3 or 4 attempts. Is this me flooding it or is that a sign of an improper float height?

Xnavylfr · « **Reply #5 on:** August 31, 2012, 01:40:49 PM »

Did you bench sync them and do you have the Idle adjust screw adjusted in some?

Xnavylfr(CHUCK)

lucky · « **Reply #6 on:** August 31, 2012, 01:43:47 PM »

Quote from: awwyiss on August 31, 2012, 12:08:08 PM

Last week I picked up a 73 750 from a buddy and I can't get this thing to idle for the life of me.

I pulled the carb bank out, took them completely apart and soaked in carb dip over night, jets aren't clogged. Actually it was pretty immaculate inside already.

This is how everything looked when I pulled it from the bike originally.

So like I said cleaned out the carbs, got everything put back in, replaced the spark plugs, changed the oil, and oil filter.

When I try to start the bike I'll give it about 1/4 throttle with the choke open and it fires up right away. As long as I keep on the throttle it'll keep running (although it sounds pretty rough). I've tried slowly closing the choke but usually as soon as I start doing that it'll die. If I let off the gas at all it'll die. Really if I do anything but hold on the throttle with the choke open it'll die.

No real leaks that I can see. Every now and then one of the float bowl over flow tubes will spit out a small amount of gas after a few failed idle attempts that's it though.

Any suggestions? $:-\$

When you sync the carbs try to get 8 Hg vacuum.

awwyiss · « **Reply #7 on:** August 31, 2012, 01:47:45 PM »

Quote from: Xnavylfr on August 31, 2012, 01:40:49 PM

Did you bench sync them and do you have the Idle adjust screw adjusted in some?

Xnavylfr(CHUCK)

Crap didn't think about that. Well that's something else to look at now.

stockscreamer · « **Reply #8 on:** August 31, 2012, 02:03:27 PM »

+1 on the idle screw. Usually fuel from the over flow tubes indicates a float out of adjustment, if its only somtimes it could the float valve sticks open occasionally. Try lightly tapping a flat part of the bowl with a rubber handled screwdriver to unstick the suspected valve. As for the float height ive always used a joe blow caliper style micrometer with great success, make sure the floats are level all the way across. Just for #$%*s and giggles check the points gap and timing to rule out all variables..

Josh

awwyiss · « **Reply #9 on:** August 31, 2012, 02:31:49 PM »

I went in and kind of bench sync'd the carbs following that guide I posted earlier (adjusting the luck nut down two threads, it was sitting flush at the top) and put the idle air screws back to just one turn out and fired it up. It's idleing now. This is my first 750 so I'm not really sure what it's supposed to sound like.

I'm reading two different things on the forums regarding carb sync. 1. that the dials don't matter as long as all of the needles are in the same range, and 2. that the dials do matter.

I had to tighten up the valves on the sync gauge a decent amount to keep the needles from bouncing all over the place and this is what I ended up with...

youtube.com/watch?v=z7zTI4HRiLc&feature=youtu.be

awwyiss · « **Reply #10 on:** August 31, 2012, 03:24:34 PM »

One more question...the shop manual says to adjust the throttle stop screw. Do these exist on the 73 model?

lucky · « **Reply #11 on:** August 31, 2012, 03:58:21 PM »

Quote from: awwyiss on August 31, 2012, 03:24:34 PM

One more question...the shop manual says to adjust the throttle stop screw. Do these exist on the 73 model?

All carbs have throttle stop screws. Question is are you talking about the throttle stop screw for all the way open, or when it is all the way closed?

awwyiss · « **Reply #12 on:** August 31, 2012, 06:23:39 PM »

Either mine is missing or I'm totally looking in the wrong place.

My carbs don't seem to have anything that looks like what the shop manual says it is. What would the symptom of a missing throttle stop screw be? haha

70CB750 · « **Reply #13 on:** September 01, 2012, 03:52:03 AM »

Did you clean the hole that goes from air intake - it's very tiny, goes to the jet. That made the difference for me in idling.

harisuluv · « **Reply #14 on:** September 01, 2012, 10:45:54 AM »

Well from the picture you posted blown up you have the screw. If you are trying to find where it is I'll assume that you haven't just tried to turn the idle up a bit. This is good for you as you might just be able to adjust the screw and then be idling normally.

Slug750 · « **Reply #15 on:** September 01, 2012, 04:30:28 PM »

My K2 engine was rebuilt and tuned by a professional 750/4 guy with a good reputation. I heard it idel and rev when he bench tested it when picking her up. Now I've got exactly the same problem - fitted engine, started it and idled real rough. Now won't idle at all and sounds like it's running on two pots. That made me think coil issue which would explain two cylinders having same problem. Then I read this post. Also, left my fueal tap on for two hours by mistake with bike sitting. Came back and found fuel had leaked onto floor, assuming through overflow lines. Would you bother fiddling with carbs (given a pro had done them already) or look at coils? I'm a trier but no pro! Slug

ericmu17 · « **Reply #16 on:** September 01, 2012, 05:43:44 PM »

Sounds like youre actually turning the choke on when it dies. Pushing the lever down is turning the choke off...and up is choke on.

Stev-o · « **Reply #17 on:** September 01, 2012, 05:46:16 PM »

Quote from: ericmu17 on September 01, 2012, 05:43:44 PM

Sounds like youre actually turning the choke on when it dies. Pushing the lever down is turning the choke off...and up is choke on.

And there may be some miscommunication here...when the choke is open, it is off.

ericmu17 · « **Reply #18 on:** September 01, 2012, 05:46:50 PM »

Opening the choke is turning the choke off.... and closing it is turning it on.

awwyiss · « **Reply #19 on:** September 01, 2012, 10:15:08 PM »

Quote from: harisuluv on September 01, 2012, 10:45:54 AM

Well from the picture you posted blown up you have the screw. If you are trying to find where it is I'll assume that you haven't just tried to turn the idle up a bit. This is good for you as you might just be able to adjust the screw and then be idling normally.

Which one is the idle screw?

Slug750 · « **Reply #20 on:** September 02, 2012, 02:05:22 AM »

My normal K2 starting procedure from cold has always been - pull choke lever fully up (as in 'on'), about a 10% twist of throttle, fires up, then drop throttle back after about 20 seconds to half choke, continue to warm up at about 2000rpm for up to a minute before slowly pushing choke lever full down til 'off', by which time the bike would normally be ready to idle with the occasional throttle blip. Isn't this the norm? Slug

harisuluv · « **Reply #21 on:** September 02, 2012, 07:57:59 AM »

Sometimes it is on the other side depending on model but from the pic you posted that is the side it will be on.

Stev-o · « **Reply #22 on:** September 02, 2012, 10:28:14 AM »

My K4 requires full choke (up) when old starting. And no throttle until it fires.

lucky · « **Reply #23 on:** September 02, 2012, 11:27:10 AM »

Quote from: awwyiss on August 31, 2012, 12:08:08 PM

Last week I picked up a 73 750 from a buddy and I can't get this thing to idle for the life of me.

I pulled the carb bank out, took them completely apart and soaked in carb dip over night, jets aren't clogged. Actually it was pretty immaculate inside already.

This is how everything looked when I pulled it from the bike originally.

So like I said cleaned out the carbs, got everything put back in, replaced the spark plugs, changed the oil, and oil filter.

When I try to start the bike I'll give it about 1/4 throttle with the choke open and it fires up right away. As long as I keep on the throttle it'll keep running (although it sounds pretty rough). I've tried slowly closing the choke but usually as soon as I start doing that it'll die. If I let off the gas at all it'll die. Really if I do anything but hold on the throttle with the choke open it'll die.

No real leaks that I can see. Every now and then one of the float bowl over flow tubes will spit out a small amount of gas after a few failed idle attempts that's it though.

Any suggestions? $:-\$

If you left them in carb cleaner overnight some of the internal plugs can come unglued. Might want to check that.
There are some small round balls at the ends of the internal passage way ends that can come unglued. I will try to post a photo.
Another member on this forum already went through this problem.
It was solved by re gluing that part the red arrow is pointing to.
That is in addition to external parts.

Also if you did not put in new float needles then all bets are off.
The float needles are the most important part. You said one of the carbs keeps spitting fuel out of the over flow tube.
Make sure the brass overflow tube in the float bowl does not have a crack in it.
Very common in states with cold weather.

Click once on the photo fro a larger image.

awwyiss · « **Reply #24 on:** September 02, 2012, 07:28:50 PM »

Quote from: harisuluv on September 02, 2012, 07:57:59 AM

Sometimes it is on the other side depending on model but from the pic you posted that is the side it will be on.

Thanks for that. After messing with it for a few minutes the bike now starts on one press with no throttle and comes straight to a smooth idle at 950.

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Author Topic: '73 CB750 Won't Idle. (Read 10580 times)