79 cb750 cv carb tuning help.

C

ct_racer

New Member
Last fall when I put the bike in storage it was acting as if it had a vacuum leak. Once warmed up, it idles around 3000 rpm, unless as I'm slowing I let the clutch out and pull the engine speed down. Once I get it close to normal idle it will stay there. The idle isn't exactly rough, but it isn't exactly smooth either.

Pulled them apart over the winter, cleaned them up, did a bench sync, and replaced the rubber boots to the intakes. Still have the same problem. I read somewhere on the interwebs that the top caps and vacuum slides can get out of round and stick creating a symptoms similar to what I'm experiencing. I pulled the caps and slides and they seem to be able to move freely and don't stick at all.

I also pulled the plugs and I'm lean. To me this seems like a vacuum leak still, but I don't know where one would be coming from on such a simple engine (as I mentioned, I replaced the boots).

I think I still have some work to do with the air Idle mixture screws to do, and I haven't yet done a sync... but I also don't think my hanging idle issue is related to those settings (just an only slightly informed hunch). Correct me if I'm wrong, but doesn't the idle air mixture screw only have to do with 25% throttle or less?

I do have good compression (170 +/-5 on all cylinders) and she really does run like a champ while cruising. Also, the bike is completely original (including the air box). I am hoping to work on this in the coming weekend, and I'm trying to build a game plan for what to do. I'm hoping there's some people out there that have more experience with these cv carbs than me that might be able to point me in the right difection.

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The manual doesn't really specify, but from doing some research on Google and the carb theory thread on this forum I've deduced that backing the air screw out should fatten up the mixture across the throttle range. Can anyone confirm this?

Also I am really out of ideas on my hanging idle. A stuck slide would have made a lot of sense, but I don't think that's it after checking the slides in the top caps... any thoughts?

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Correct me if I'm wrong, but doesn't the idle air mixture screw only have to do with 25% throttle or less? correct

Are you running pods or air-box?

Does the idle drop at all once the choke is disengaged?

These bike run best lean.
 
It does drop after the choke is pushed back in, and it runs correctly...ish until it warms up. Which now thinking about it might be an indication that it is not a vacuum leak, but not ruling that out yet. Once it warms up is when the idle hangs.

Yes, stock airbox. The plugs are too white for my liking. There's not even a hint of tan, she needs to be fattened up a bit. I'm going to lose the mixture screws a half turn each and see where that gets me in terms of the mixture. I don't think that will have much effect on the idle issue though... then again I could be wrong.

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Different carbs have different adjustments for the idle mix. Some need to be turned in for a richer mixture, some need to be turned out for a richer mixture. Set the idle mixture screws to what the manual states and then ride the bike at normal operating temp. Then ride for a few minutes at 1/8 to 1/4 throttle only and do a plug chop. Do not release the throttle, but just hit the kill switch, pull in the clutch and coast to a stop. Pull ALL the plugs and check the colour. Make adjustments accordingly.

These bikes can be lean in one part of the fuel circuit and rich in other parts. Really need to do the plug chop in all three main fuel circuit areas. Idle, mid range and WOT.

If you think you got an air leak, run the motor and spray some carb cleaner around the rubber boots. If the engine speeds up, you got a leak.

Old diaphrams in CV carbs can cause all manner of problems. They do perish over time, but still look OK. If in doubt, chuck them out. ;)
 
ct_racer said:
It does drop after the choke is pushed back in, and it runs correctly...ish until it warms up. Which now thinking about it might be an indication that it is not a vacuum leak, but not ruling that out yet. Once it warms up is when the idle hangs.

Yes, stock airbox. The plugs are too white for my liking. There's not even a hint of tan, she needs to be fattened up a bit. I'm going to lose the mixture screws a half turn each and see where that gets me in terms of the mixture. I don't think that will have much effect on the idle issue though... then again I could be wrong.

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Click to expand...
white is right it is not a 2 stroke
 
Thanks for the reference. Mine are far whiter than that currently. That being said, I've only checked the plugs after idle, so it may only be lean down there, and be fine in the rest of the range. I am hoping to start the plug chop process this weekend (been a busy short week)

I am also hoping to use carb cleaner or propane to check fo a vacuum leak. The boots are new so I am hesitant to think that there would be a leak there; though I'm not saying there isn't one yet. Can any of you think of another place to loom for a vacuum leak while I'm in the checking around?

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Bone stock. I've seen the filter in the box, thouh I've never pulled it. Based on the originality of everything else I've been through on the bike I'm assuming it's original. Because of that I'll assume it's dirty, which might account for some of my lean condition. I'll check it out either tonight or on my lunch. He bike does only have 7500 miles, but still, 35 years can lead to a lot of dust.

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Checked the filter on my lunch break. It's original, but it seems to be fairly clean yet. Will take a compressor to it to see what might come out, but my guess is not much.

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Blocked filter makes it run rich not lean.
Those plugs are running real rich at some point, the black inside body proves it
With stock airbox and exhaust they should be a light gray colour, pretty much same as background on this site ;D
Oh, one other thing, the three jet Kei-Hin is virtually impossible to tune without stock airbox, anyone who says otherwise is either lying or has no clue how bike should run
also had a thought about black inside body, it's probably from running on choke. Try setting float height 1mm higher than stock
 
You're right about a blocked filter fattening it up... good point. That said, filter seems fine, or at least close enough, and it is the stock box and exhaust. After going through the manual though I am not as close to having the pilot screws set as I thought. I am the initial setting out, but I have to keep backing each one out until I get a 50 rpm increase and then keep backing it out until it drops 50 rpm again.

Since I don't have a digital tach, and the stock one is unstable and inaccuarte, I think I will plug chop the idle circuit and then move on to plug chop the rest of the range. Since everything is bone stock I'm going to assume everything off the idle circuit is at least close enough for now; I just need to get it idling and synced properly and then see where I'm at.

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You back it out for 50 rpm drop then screw it in for 50 rpm drop. setting is midway between the two points
Most common problems on stock motor
1, corroded primary main jet emulsion tubes
2, faulty ignition units
3. 'long' cam chain adjustment
4, worn alternator brushes
5, advancer mechanism rusted/stuck
Not necessarily in that order, it's just what I thought of first to last ;D
 
1. Primary jets aren't new... but they're pressed in if I remember correctly. Is there anything I can do about that?

2. Spark appears to be fine. I haven't had any ignition issues.

3. I'll have to check the chains next time I'm in the head.

4. Stator was replaced before I picked up the bike.

5. Where is the advancer? Say it were stuck, could it be causing my high idle issue?


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The 'long chain' adjuster is underneath the carbs in middle of engine and often gets 'neglected' I always checked it with the cam cover off as it sticks. With cover off you can feel it moving
Primary/slow/pilot jets, you have to grab with a locking pliers and gently work them out. (it's a PITA but possible)
The advancer is under left engine cover bolted to starter reduction gear.
Even though you have electronic ignition, the advance is mechanical
 
ct_racer said:
You're right about a blocked filter fattening it up... good point. That said, filter seems fine, or at least close enough, and it is the stock box and exhaust. After going through the manual though I am not as close to having the pilot screws set as I thought. I am the initial setting out, but I have to keep backing each one out until I get a 50 rpm increase and then keep backing it out until it drops 50 rpm again.

Since I don't have a digital tach, and the stock one is unstable and inaccuarte, I think I will plug chop the idle circuit and then move on to plug chop the rest of the range. Since everything is bone stock I'm going to assume everything off the idle circuit is at least close enough for now; I just need to get it idling and synced properly and then see where I'm at.

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Click to expand...

If your going to do the plug chops for the entire fuel range you should start from the top. i.e. you do the plug chop for WOT first. Get the bike running properly on the main jet first.

Then do the chop on the needle position. This is for throttle position from 1/4 to 3/4.
When you get that right, then go to the chop for the idle position or closed to 1/4 throttle.

Here is a handy chart that shows what happens and where in the fuel circuit.
 

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Hot500 said:
If your going to do the plug chops for the entire fuel range you should start from the top. i.e. you do the plug chop for WOT first. Get the bike running properly on the main jet first.
Click to expand...

That makes sense as long as it is idling properly, which it isn't. I feel like I need to get that somewhat close first... that said, good news everyone, I fattened her up a full turn on the idle mix screws and my hanging idle seems to have mostly gone away. I guess she really was way too lean; I pulled the plugs after taking a quick ride after the adjustment and they now have that ever so slight hint of tan I was looking for.

I still can't get it to idle at 1000rpm once good and hot, but it will idle at about 1200. That is probably just a sync issue at this point, but that is pointless to do until I get everything else setup.

Tomorrow i will do some checks and maintenance as suggested above, and begin the plug chop process. It's getting too late to be WOT by me, and after the initial success I had today I hear some beers calling my name.

I should also mention I took it for a couple hour ride after i adjusted the carbs and it feels better off the line, though it does fall on its face if i blip it off idle. But I'm sure I'll get that figured out as I get into it.

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That 'handy chart' is worthless with Honda 3 jet carbs
The transition between primary and secondary main jets is impossible to fix without blocking various passageways and drilling new ones plus adding some brass tubes inside float bowl
Also, although 'main jet first' is the way to tune race bikes, street bikes are not full throttle everywhere so tune the circuits you are going to use - pilot and primary main to about 4,000 rpm, secondary main and 'middle of needle up/needle jet above 5,000rpm. The spot between 4~5 will never be right unless you fit different carbs
 
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