High Idle / Self Revving Engine

[Lowell]

Hi there. I've been using this forum with help to rebuild my 81' CB750c.
I've rebuild the carbs and have managed to get the bike started with no issues.

I've searched the forum but can't find my exact problem listed anywhere:

The bike starts up and the engine comes up to 3000rpm immediately. From there, on its own, it self revs up to 4000 and then back down to 3000. Repeat Repeat Repeat over the course of 10 seconds between.

Checklist:
High Idle screw is bottomed out.
Throttle cables aren't pulling when throttle closed
??

So essentially I'm trying to get rid of the combination high idle and self rev.

Any advice?

[shinyribs]

Are the throttle plates completely closed with the idle screw backed all the way off? Maybe that needs to be adjusted at the synch adjustment screws. Otherwise, tha's classic vacuum leak symptoms. Get a can of starting fluid and spray it around your carb boots while idling. If you have a leak it will show you exactly where when it sucks in the fluid and the rpms will raise. Small doses is all you need. Just spurts. Don't spray a full stream. It will evaporate quickly too, so it won't hurt your paint, but I'd keep it as localized as possible just in case.

[Lowell]

Before I put the carbs back I made sure the throttle plates were together and synced after my carb rebuild. So they should be in order.

I forgot to mention a big aspect in the last post. There is essentially 0 throttle response. The bike starts up but when cranking the throttle, barely a blip on the tachometer. I can see a vacuum issue but would it be bad enough to have such a massive impact to ruin all throttle response?

I'll have to check about the vacuum symptoms tomorrow around the boots.
Thanks shinyribs

Update tomorrow

[shinyribs]

Hmmm..well if the engine is drawing air from somewhere other than through the carbs (vacuum leak) I suppose it would make sense that open the throttle might not have much affect on it since it is already getting a source of air elsewhere.

Having your blades all bench synched and even is good, but you still need to make sure that they are able to close fully. You can have them all perfectly even, but if the adjustments are run way too far in,they may not close. I think? Just a thought,but worth taking a peek at I reckon.

[Lowell]

So I bit the bullet and pulled out the carbs again.
Turns out when I put them in the linking point for the throttle valves got budged and they were slightly open.

Tweaked the idle screw and she idles like a pro now. Followed up with a vacuum test and that is all good as well.

Thanks.

[shinyribs]

 Good deal!


Betcha don't make that lil mistake ever again.