• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Help: High idle can't even warm up

angrymonkee

CB750 New member
Messages
1
Reaction score
0
Points
1
Location
New Jersey
I need some help. I've been struggling with this problem for about a month. I have read various posts and articles continue to suffer from a high idle when my bike is cold. I bought this bike and it didn't run. It's a 1979 Honda CB750F (or so I was told). When I bought the bike there was a hole in the block so I decided to rebuild the engine since it was going to be mostly disassembled anyway. After doing that it didn't make much sense to not do a carb rebuild as well. I did a bench sync on the carbs and installed them. I tried firing up the bike and it ran like sh*t, but it ran. The mixture seems very lean because I noticed the idle climbed to 5k. I turned the pilot screws out to 2 full turns. I tried cleaning the pilot Jets three times and still I'm having this issue. My carbs are the 79 model so it has the pressed in pilot (whatever the f*ck you call them).

As you can tell, I'm a bit frustrated at this point as I have run out of ideas. Please have a look at the link to my video for a sample.

Any help is greatly appreciated!

Link to video showing my pain: https://youtu.be/nfqIXAMJF8Y
 
There is high idle linkage built into the choke. Not adjusted right can do that. High idle is NOT from lean and where did you get that? It's from excess air leaking that the throttle butterfly does not control, it can ONLY go high though as long as there is fuel to cover it, when you run out of fuel then engine dies. Or if not then every engine that ran out of fuel would suddenly idle high and you would never need a gas station ever. Think. There is an idle speed knob under the carbs in the middle, have you found it yet? And if you didn't break the carbs apart to rebuild you wasted your time, as the aircuts can mess your day up just like now. High idle on those commonly being vacuum leaks.

Bike is not an F, has what looks like K tank, single disc and spokes. F has none of that. Look at the riveted tag on the steering head for model.

A normal start on one of those is choke knob pulled fully out and start engine then you chase the high idle when it shows up by slowly closing the choke a little bit at a time until it is fully closed at the handlebar and open at the carbs. You set the idle speed knob under carbs for HOT ENGINE ONLY and then leave it alone, you never use it to adjust cold engine speed.
 
Last edited:
HEHE, welcome angrymonkee!! It's hard not to giggle. But that's sort of how the bike is supposed to operate. Open the choke, and it revs high. .....sorry....but AMC is correct....Let it warm up...even if it idles at 5k. What are you worried about? Nothing is knocking or banging. Besides, I regularly bring my bike to 9K RMPS from 2-4th gear. Warm that bad boy up, and set the idle by reaching under the carbs and slowly adjust counterclockwise to lower the RPM. Do small turns and let the idle settle. You might not have any problems. From there, if you have idle issues, then post another video. I wouldn't idle in the driveway for 15 minutes, but 3 minutes is OK from a cold start. Plenty of time to get he warm and set the idle.

Use this guide to rebuild your carbs if you still have problems. Also, after the bench sync, you can get yourself a carburetor synchronizer. But first things first. Your bike sounds pretty good. Don't panic.....Yet

http://www.cb750c.com/publicdocs/Sea...anual_revG.pdf
 
5K is too high to run at engine cold, the pistons have not swelled yet and you wear the skirts out prematurely like that. Better to use like a car 2500 rpm or so.

I keep telling people to NOT use the idle setting under the carbs for cold idle, you are simply screwing yourself around doing it, you can control cold running speed simply by using the choke knob combined with the throttle. A cruise control on throttle helps even more. The idle setting under carbs is for hot idle ONLY. Every time you mess it up you have to re-zero in on the very fine last setting to get idle speed just right, it's moronic to have to do that over and over. Also, leaving it alone forever makes it a tool that can be used to tell if the engine is off, say the idle goes whacky, if you just moved that you now haven't a clue if the engine is different or you simply messed it up yourself.

We can be smart about it then there is the other way.
 
Back
Top