Hanging throttle/High idle

[asjwalsh]

Hello all,

I've just finished rebuilding/restoring a 79 CBX. I had the carbs totally rebuilt by a very renowned CBX guy. Bike started up fine, but then suffers from a problem that seems to pop up here all the time. The only thing is people rarely post their fix. It revs freely, but instead of snapping back to 900RPM it sits at 2000RPM for 10-15 seconds before dropping. But other times (especially if I blip the throttle and revs don't exceed 3000RPM) it drops immediately to 900.

So I contacted the carb rebuilder and he gave me a big list of things to check.
1. Choke cable installed correctly
2. Sync
3. Air leak
4. Ignition timing
5. Mechanical issue (throttle cables, idle screw binding, linkages)
6. Tank rust/petcock/hose cleanliness
7. Less/more air test
8. Check cylinder compression
9. Fast idle linkage

So I'm waiting for the Carbtune sync tool to arrive. Choke cable is installed correctly. There are no air leaks. Static ignition timing is spot on. I don't have the tools to do a dynamic test but my gut says it is fine. The throttle cables all move freely and the carb linkages do not move when the revs back down to 900. Tank has been re-lined and the petcock is brand new. I've checked fuel flow from the tank and it is normal.

I haven't done the less/more air test yet and I don't have compression numbers yet but again, my gut says that mechanically the engine is fine. The fast-idle linkage is a last resort since it involves removing the carbs.

Does anyone who has experienced this remember how you fixed it? The engine runs great apart from this annoying problem.

Cheers,

Andrew

[steve murdoch icoa #5322]

Quite a few threads on the subject and almost all of them say a carb synch is key.
viewtopic.php?t=1134

[asjwalsh]

Steve,
Thanks for the response. Syncing on the bike is my next step, although I’m having trouble sourcing the adjustment tools. I ordered the long one from partzilla only for them to tell me it’s out of stock.
I’ve already read that thread you mentioned. Unfortunately the OP never returned to comment on how they fixed it.

[steve murdoch icoa #5322]

Not seeing follow-ups or conclusions can be frustrating. A lot of us are guilty of it.
Once you have been around these bikes for a bit and doing some searches you will be able to tell which members are the "smart guys" and follow their advice regardless of the original posters actions.

[asjwalsh]

True..Happens on most websites.

What are guys using for the shorter carb adjustment wrench? The part number brings up no available tools...

[daves79x]

Your problem is almost certainly synchronization. It would be much easier for you, for just a one-time sync, to find someone that can do it for you. Failing that, you likely can get by with just the long tool. The short one only comes into play for #5 and 6. That one is still available from Honda, I think. Gauge syncing can be a daunting task if you are not familiar with the process specific to CBX carbs.

Bench syncing usually has the carbs behaving pretty well, but occasionally this will happen. Just be absolutely positive your choke cable is not installed too tightly. Even a fraction too tight will cause this.

Dave

[asjwalsh]

Thanks Dave,

Great chatting today. I'm going to give it a go and if I make a hash of it I'll take it to Preston :)

[AshishNJ]

As a side question, where do you folks recommend those synch brass tubes can be had for CBX that screw on to carb body. Also what do you do with the carb that feeds vacuum to fuel check valve ?

[steve murdoch icoa #5322]

Many places for the brass tubes.
Z1Enterprises, DimeCity, Motion Pro probably Randakk

[asjwalsh]

Quick update: I had the carbs balanced by a well-known to this community expert. It improved the issue but didn't eliminate it. Now it only hangs at 2k for 5-10 seconds before dropping to 900rpm.

[steve murdoch icoa #5322]

You did say no air leaks.
Are you running stock airbox and filter?

[daves79x]

I've seen this a couple of times. A throttle plate not perfectly centered in it's bore will not allow immediate closing of it individually, and when connected to the rack of carbs, the others also. They will close, but not immediately. I thought of this after we talked. Sorry this has been such an ordeal. Call if you like.

Dave

[tevan]

Dave,
Do you think he could shove a Borescope thru the air cleaner and see this?

[daves79x]

No. Unfortunately, you can’t really tell if this is happening unless the carbs are off. Then if you see a delayed or sticking return to the idle setting, you’re stuck guessing which one is the culprit, unless you separate them all.

That said, was there any issue syncing the carbs? I assume they all came in at the correct idle speed?

Also, don’t know if we talked about it, but are you absolutely sure you advancer is assembled correctly and functioning correctly as verified with a timing light?

Dave

[hondaman160mph]

I have an 82 CBX that also had a problem with hanging high idle. I have a 6 into 1 Kerker header with a straight through canister muffler. Carbs were rebuilt and synced
with the idle screws set at 2 1/4 turns out. I reset the idle screws at 1 1/2 turns out and no more hanging high idle.