Get those rev's under 3K

[FastCletus]

I cant seem to get the idle under 3K.  Am i turniung the wrong screw?  There's the adjuster knob under the carbs, and I have it backed out all the way. Already backed it out to much and had to take the whoel damn thing aprort to get it back in.  Then I thought choke was to tight,  adjusted that.  I have 2 vaccuum hoses and both are capped off.  Throttle cables arent to tight.  Did the air leak check with brake fluid, and nothing jumped up.  Any thougts

[shinyribs]

The two vacuum hoes you have capped,are you sure they are vacuum hoses. If they are bowl vents it may be causing the carbs to draw in excess fuel. Just a thought.I'm not up on CV carbs,but I dont think those bikes have any vacuum operated functions,so I was actually just curious what those vacuum hoses would be for.Petcock?

[icerigger]

Yeah, at least one of those hoses is the bowl vent, and blocking it off should actually make it run lean as you'd get a vapour lock kind of situation where the low pressure in the throat drawing fuel from the bowl is now fighting the low pressure created in the bowl.

The other one might be the SOS hose and must be blocked off if you've bypassed or replaced it. Which bike are you working on? The neither of your bikes should have the SOS unless you've got carbs from a later year.

A picture would help.

[FastCletus]

Here are both hoses I have capped off image.jpgimage.jpg The first one is easy to see./ The second one is a black tube and I have a white pen that is pointing to where its going into the carbs

[FastCletus]

image.jpg

[FastCletus]

its for a 1980 CB 750 F.  As far as I know carbs are stock and no mods have been done to them and i have stock pipes as well.  I don't know if this makes a difference but when running down the street it runs flawlessly

[Beekeeper]

The first picture looks like the vent hose that comes off a plastic  "T" and on my 79F, clips on top of the airbox.  It's left open to atmosphere.    I took a quick glance back at the rebuild instructions and looks like if that nipple is on the front of #2 carb, it can/must be blocked to eliminate the SOS fuel pectcock. My 79 hadn't been started since 1988, and after the rebuild, idles perfectly at 1100.

[FastCletus]

went and rechecked everything.  made sure boot seals are good which is the only thing I can think of.  found that the choke is sticking a little.  Pushed it back down and then fired it up.  Idled around 1500.  ROde around the block, whacked the throttle a few times, and now its back to 3K rpms.  Is there a heat sensor or something. It seems to idle fine when cold, but as soon as its warm it starts climbing the revs?  

[Beekeeper]

I believe there is a fast idle cam that the choke linkage operates.  I'm not sure how you would check it other than pull the carbs and make sure it is functional.  Maybe make sure the linkage is snug and not allowing the cam to "walk" or vibrate into a fast idle position and stick there.   Just a WAG since I'm fairly new to working on this beast of mine.

[Half-Caf]

From what I understand a little idle rise is normal with these carbs. However, going up to 3k is definitely high. I used to have issues with a racing idle until I synced my carbs. This helped out in a bunch of areas.
My idle now starts out right at 1k, but goes up to 1.5 when warm.
This is with stock jetting of 102 and 68 with 4-1 exhaust and stock airbox.

[icerigger]

Beekeeper's right,the hose off the plastic tee is the bowl vent and should be open. If you're worried about crud, cover it with a bit of pantyhose mesh.

The nipple plugged with the cap screw is for the SOS, leave it.

The fast idle tang on the choke linkage may also be a problem as Beekeeper has pointed out. Any slop at all will cause the idle to be low when the choke is engaged. Easy to compensate for by adjusting the idle, but when it warms up and the choke is disengaged, the idle is too high.

With the engine fully warm and idling at the 3K, adjust the idle down to 1K and shut the engine off, pull the fuel tank and adjust the fast idle tang so there is no play. Its a bitch to get at with the carbs on the bike, but I use a dental pick to check the play and a right angle screwdriver to adjust. There's a good picture in the carb rebuild manual somewhere on this site.

[FastCletus]

Thanks all. I slept on this ( not physically, just mentally) and I think something is hung up on the choke linkage.  when I first got bike I took apart carbs and just did some general cleaning, and I recall having an issue getting them put back together.  I think I am going to take carbs off and just lubricate the levers and such.  I am getting pretty good at taking these carbs off.  Heck I might as well rebuild them while I have them out.  If its not one thing it another!!!

[icerigger]

The MacGregor rebuild manual on the DOHC forum has a really good section on getting the linkage back together.


http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&t=13611

[icerigger]

Better yet, try this link.

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

[FastCletus]

OMG that is the coolest document I have ever seen!  Who in the world took the time to do that?  I need to study it like it says a few times.  The big words, or better yet words i cant define are trying for me, so not sure If I will tak e this whole project on my own without adult supervision.  Although the up close and personal pic of carb linkage is key.  Plan on checking that out tonight, as I think thats the issue.

Anyone touching carbureators should print out this document@!!@

[FastCletus]

Well here is where I stand on this issue.  Finances running lean so I had to slow down work on this bike.  I did just recently put new carb boots on.  The old ones were like rocks.  I had a glimmer of hope that the new carb boots would fix the issue but after just installing them the same issue is going on.  Not a bit of difference.  Rides great down the rode, bike pulls and goes very well. no coughing or lagging, just get up and go.  But the revs still sit at 3K after its warmed up. I think my next issue will be just to rebuild carbs.  Found a neighbor that has an old SOHC Cb, and he seems quite knowledgeable so I think I will have him supervise.  

Might Synch carbs first, see what happens, and if no change then rebuild.  

[seestheday]

Have you checked for a vacuum leak?  while it's running, spray some carb cleaner or something flammable (unlit propane torch, etc) around the carbs.  If the revs go up, you have a vacuum leak somewhere.  The CV carbs can leak even somewhere in the body of the carbs.  I only mention this because you only recently replaced the rock hard old carb boots.

It could be a problem that is only presenting itself once the bike has warmed up.

[FastCletus]

 I have done the carb leak check with the spray and didn't notice anything substantial.  What I might check now is to do it when engine is hot,  spray more on the actual carbs as opposed to the boots

[seestheday]

I am pretty sure I have 2 leaks.  I'm in the middle of a teardown right now and am fixing a bunch of stuff.  My carb boots crumbled when I took them off, so that was definitely one of them (previously identified with a spray test), and I believe I have one in the middle of my carbs either in #3, #4 or between them.  I'm planning on pulling the carbs apart soon to find the leak in the middle of the carbs.

That being said, my bike was idling at 1400 and wouldn't climb after it warmed up and during around the town riding.  If it I rode it on the highway for 30+ min then it would like to stay up at 2K for a while before slowly coming back down.  It would stall if I adjusted the idle down to factory spec (1K +/- 100).

[FastCletus]

Before I tear this bank apart.  Can anyone tell me if this linkage looks right?  I have only taken off the  choke spring.  But it looks like one of this thingamabobs is bent?  Cant think this is why I am having my issues, but thought i should ask first