Engine revving up at Idle (New question about main needle)

[rebellion ind]

Here's the deal after my bike is warmed up really well say 15 or 20 minutes or so and I come to a stop the bike climbs from 1 grand in rpms to 2 then 3 at idle. I know the float on my right hand side carb may not be set at the correct level since I replaced it a month ago after the original developed a hole in the seam, so after 24 hrs or so I need to kick the bike 5-7 times to get it to start I am guessing the fuel has drained from the carb. Would the float also cause a revving issue at idle?

 

[Big R]

Re: Engine revving up at Idle

Sounds like an air leak to me. Check around the rubber carb mounts. Most likely the engine is warming up the rubber enough to make it TOO pliable.

 

[GRHebard]

Re: Engine revving up at Idle

i had a similar issue (still do a little bit)

i replaced my pilot jets with bigger ones, and the issue isnt as bad now.
MikesXS says that hovering idle is due to a lean fuel mixture, and your fuel mixture at idle is dependent on your pilot jets and the idle mixture screw.

airleaks will make your bike run lean as well

 

[Big R]

Re: Engine revving up at Idle

Speaking of which, what kind of bike is it?

 

[rebellion ind]

Re: Engine revving up at Idle

Thanks at least some things to check into. BTW the bike is a 1972 CB350. Hopefully I can get some time this weekend to dig in a little further. Maybe I should also mention that I just replaced my filters with UNI's not Pods really but not direct replacements either they are a long foam filter that replaces the airboxes and still utilize the side covers, however I had this revving problem before I changed to the Uni's so I don't think this is the issue besides creating maybe a slight lean condition.

 

[demian]

Re: Engine revving up at Idle

hey man. we have similar threads going. you may want to check your spark advancer behind the points plate. clean it up and make sure it's working properly.

let's make it a point to follow up after we've figured it out.

good luck.

 

[wtskm]

Re: Engine revving up at Idle

Hi guys,

I have the same thing happening with my 72 CB450. Once and a while at idle the rev's jump to 2 or 3 thou. As soon as I partially ingage the clutch the drop down, then are fine. 2-3 mins later the idle jumps again and then it's o.k for a few days.

I'll check for the vac leaks also and the spark advancer

 

[demian]

Re: Engine revving up at Idle

hey all.

so i fixed the spring in my advancer, reset my timing, and replaced the carb manifold gaskets. i started the bike. reset the idle and then it started raining. although i haven't taken it for a ride, things sound a lot better already. nice even idle at 1200 rpms. the throttle just seems to respond better as well. i will report back after i rip around a bit. i don't have any fenders so, as excited as i am to test things out, riding in the rain really sucks.

some things to make sure of with all of this stuff:
- make sure when you replace those gaskets that the surface of your carb boot and manifold are totally free of even tiny bits of the old gasket. i got the one side pretty damn clean and when i started the bike up the carb began chirping. i went back and really took the time to get it as best as i could, added a little non-hardening gasket sealant, and reinstalled. no more chirping. some people have different opinions on the gasket sealant... more experienced mechanics should chime in.

- this one sounds stupid but make sure everything strapped to your carb is tight. i have been rejetting my mikunis for a while - taking things off, changing stuff pretty frequently. because of the position of my battery box, the pods sit at a slight angle. after frequent shifting and changing, plus the bumpiness of riding around shitty philadelphia streets - i really needed to tighten up those pods and the clamps where the carb sits in the boot.

i'm feeling good about this, but i'll report back after a longer ride.

 

[rebellion ind]

Re: Engine revving up at Idle

Well I have got as far as taking the carbs of the bike and setting the float height as guessed the one side was off and turns out the float was twisted a bit so it measured 2.5 mm more on one side than the other. I also found some speckles of rust in the bottom of the float bowls so the screen in the petcock isn't doing a great job, I guess I'll throw on some in lline filters as much as I don't want to (I have heard that sometimes they cause fuel starvation issues). One question that I do have is about my main needle and the little plastic hat that goes on top. In the bottom of the slide/diaphram tube there are 3 holes one is for the needle the other two I am not so sure, but anyway after you drop the needle in you put on the plastic hat looking piece with 4 holes in it, should these holes line up with the holes in the bottom of the slide or not. In one carb they line up and in the other they don't thus covering the holes. Any help would be appreciated. BTW I looked at my rubber boots that go between the motor and carb and while I haven't spray and WD40 on them yet while running they look and feel to be in great shape.

 

[fairfield]

This thread needs to keep going, I've got similar issues with my 72 CB350. Idle stays stuck at 3k if I rev it close to or over it. Does this on neutral, and whenever I pull in the clutch while riding. The problem starts when the engine is hot, though it's not always there.
Could the float level be off? One float has been replaced, not sure which one. That makes it hard to say where to set the prescribed float level.
Could be my intake boots, though I can't see any damage on the inside, the exterior is dry and has a nice crack all around the boot. WD-40 didn't do shit.
Gotta check the spark advancer.
There seems to be an air space between the plastic needle holder and the vacuum slider, I don't think it matters if the holes line up or not. I might be wrong though.
Just passed safety today after being in storage for 20+ years. It's my first bike, love it.

 

[Kev Nemo]

Easy visual thing to check-there is a spring that the idle screw goes through and keeps idle constant. PO left that spring off my carbs and I went crazy trying to figure out why I couldn't keep the idle constant.