Wildly fluctuating warm-up rpm

[mgp214]

Hello all!
I've been lurking around here for a few months now since I bought my 1979 Cb750k because so many of my google searches have led me here, and let me tell you, ya'll have been a godsend! The two problems I'm wrestling with right now are a very high idle when warm, and wild jumps in the rpm while warming up.

A little background on my bike: I bought the bike a few months ago in a barely-starting condition. It was three quarters of the way through being rebuilt and a lot it wasn't done up very well, but with the help of the FSM and reading a lot on this forum, I've gotten her up and driving.

I know pods are strongly recommended against for these CV carbs, but it's what the PO put on and I don't have the original airbox, as well as the PO telling me he "de-tabbed" the rear frame. I'm using the K&N ovals taped up about 80% shut. She runs well with taped up pods and 75/110 jets for the most part, good response and power through most of the throttle range.

The high idle when warm is like this: After the bike is warm (say, a 5-10 minute ride around town) the idle rpm is around 3k to 5k. The warmer the bike gets, the higher (up to 5k) the idle sits. Some quick searching told me that it's a vacuum leak, so I sprayed carb cleaner around the carb boots and confirmed. Anyways, I ordered new boots, put them on, and the idle is about the same. It seems to need to get hotter than before to reach 5k, but it still gets there. I'd guess it's important to note that it doesn't seem to be a "hanging" idle/slow return to regular idle, as the bike will gradually climb to 3-4k if just let sitting and not given any throttle.

The second problem that I can't seem to figure out how to word for results on google is the wild fluctuations in rpm while the bike is warming up. Where I am in the country, it's usually about 10-25 degrees F. I put full choke on, start the bike, and gradually back the choke down over 10-45 seconds as the idle climbs. The problem is that without touching the throttle, and even when I don't touch the choke, the rpm will suddenly jump up to 4k or 5k for a few seconds, and then fall back down to normal (1k-ish) and repeat several times, randomly. I figure this is probably related to the other issue, but damn if it isn't bizarre.

Any advice would be greatly appreciated! I'm excited to finally be posting as I've felt like a member for a month or two now with the amount I read on here.

[TOOLS1]

I know you do not want to hear this, but you most likely have a vacuum leak. Do you have the vacuum port for the SOS (vacuum operated petcock) plugged? Also even new boots can leak. Another thing is, have you synced the carbs? That can mess up an idle if they are way off.
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[mgp214]

If it's a vacuum leak, that's a-o-kay, as long as I can eventually fix it!

The vacuum port you mentioned--is that the one that is a black plastic T in between carbs 2 and 3? If so, no, I left it open. Should I close it?

I haven't done a vacuum leak test with the new boots, I was going to try that sooner or later, I guess I'll get to it sooner if you think that's a possibility.

I did sync the carbs, or tried to at least. It was my first time syncing carbs, so I probably didn't do a great job, but I bench-sync'd them and then used a cheap vacuum gauge set to sync them.

[shinyribs]

Hey bud, that black tee is your vent. Just route a hose off of it kinda up-and-over fashion. Not really needed, but will ensure any fuel can never slosh out of there.

The vacuum port is a brass spigot off the main body of the all-important carb #2. Located on the engine side of the carb.


Also make sure the screws on the intake spigots ( where you attach your synch hoses) have crush washers under all of them. I had a bad vacuum leak there once due to not having a washer. Darn PO's...

[mgp214]

So here's a picture of my carb #2 right where the port is on your picture:


I checked my FSM and it doesn't look like my carbs should have a port there. Are you sure my model has one? I was told she's a 1979 CB750k limited edition, but there's been so many modifications that I'm not sure how to tell.

My intake spigots all have washers, but I might go get new ones just to be sure.

[18Bravo]

I have exactly the same bike.  I'll have to post a photo tomorrow. It's running pods, but the carbs are original.

[mgp214]

I look forward to it! Did you have any similar issues to mine by any chance? I'm planning on really thoroughly checking for vacuum leaks tomorrow so hopefully that will yield progress.

shinyribs -- the washers on my intake spigot screws don't appear to be crush washers; I had a hard time tracking down a place I can get metric crush washers and it looks like my best bet is a Cycle Gear store near me. Planning on checking it out tomorrow. Is there a preference for aluminum or copper for these?

[mgp214]

Update: I went out for a ride to get the bike nice and hot (since that's when I experience the issue) and stopped and repeatedly used carb cleaner on both sides of the boots, the intake spigot screws, and the throttle shaft (I've read the felt seal in there can age poorly and leak) and didn't notice any jumps in the throttle. It was idling at 5k while I was doing this.

I figured maybe this stems from the pods, so I tried taping them up even more. There are a lot of knocking sounds when I let off the throttle and coast or brake, so I'm going to check the spark plugs when it cools down to see if they look lean.

Is there anywhere else I could have a vacuum leak? Anyone have any tips for what to check next? I figure my next move will be warming up the bike and syncing the carbs, although I'm not sure if that's possible with the bike running at 5k rpm.

[TOOLS1]

No it is not possible to do at 5K. Also with it running at 5K, you will not notice any change with spraying carb cleaner on a vacuum leak. And the pods will not cause any change to idle. They will just make the bike run like CRAP while accelerating, and at high RPM.
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[mgp214]

Okay, so considering that it seems I have a vacuum leak that only presents itself while the bike is warmed up, how should I look for it?

[shinyribs]

Well, that explains it...your vacuum port fell off!


Check for vacuum leaks with starting fluid. Carb or brake cleaner isn't nearly volatile enough.

Spray around the tops of your carbs,too. The big top caps that look like aerosol pant can caps, they can leak there,too.

[18Bravo]

I have not forgotten your photos. I took them earlier today but just got back home again, so I may have to post them tomorrow. In the interim, I have a question. I have the same bike as I said, and the previous owner has pods on it as well.  My local expert says syncing the carbs is good for idle, but as you open the throttle it eventually doesn't matter if they're synced or not as they'll all be opened up, so much the differences will be negligible, especially when opened 100%, at which time they're synced whether they were or not at lower RPM.  What say you guys?

[shinyribs]

On CV's, I think that might have some truth to it. Considering that style of  carburetor has slides with , literally, a mind of their own, I can't see synching having any affect once the CV slides start doing their own thing.

On mechanical carbs, like roundtops, it's a little different. I noticed my 76 wasn't idling quite as smooth as it once was and decided to check the synch. It was off. After leveling them back out I felt some of the power I hadn't realized the bike had gradually lost.

So I say yes and no.  But in this case, concerning CV's, yeah. Sounds sensible to me.

[18Bravo]

So here are the photos.  Carbs are still as they were when I bought the bike - disconnected from the boots. They're coming off anyway for a cleaning.





[URL=http://s118.photobucket.com/user/18bravo_photos/media/DSC05358_zpsfeb2412b.jpg.html][IMG]http://i118.photobucket.com/albums/o110/18bravo_photos/DSC05358_zpsfeb2412b.jpg[/IMG][/URL]

[mgp214]

What jet sizes are you using that you run fully open pods? Or are they a little blocked up inside?

[18Bravo]

I don't know anything about the carbs yet. I haven't even tried to fire the bike up since I bought it in November. The prevoious owner said it sounded strong on full choke, but would not run correctly otherwise. It'll be interesting to see what happens once I get a battery for it.

[mgp214]

I decided to check my valve clearance just in case, and turned up some pretty crappy numbers:

intake: .004, .004, .005, .005, .005, .003, .005, .005
Exhaust: .002, .002, .004, .007, .003, .003, .004, .002

I plan on picking up replacement shims to get all of them to .005 or .004

Is it possible this could be causing or contributing to my issue?

[TOOLS1]

Too tight of valves can cause rough running, but not a fluctuating idle.
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