Topic: 750K3 idle all over the place, what would you do?

[fmctm1sw]

I've had this bike in my garage for about 5 years now.  It's to the point where I can start it and ride it but I have this nagging idle issue.  The slightest adjustment to the idle speed has drastic results.  When revved it wants to hang around 2000 RPMs or so too. 

I just had the carb rack off again and made sure everything was clear.  Currently, all the pipes get hot and I have the carbs balanced with a Morgan carbtune.  I removed the throttle cables and raised the slides manually and it does the same thing. 

At this point I guess it's finding it's own air source.  Would you guys replace the carb insulators?  I sprayed them with WD40 and it didn't make any difference but the middle ones are pretty hard to reach.  Also, I can turn one of them a little with my hand even when tightened.

The bike has a stock airbox and had idle screws that were drilled but I soldered them shut.  Idle jets are stock to the best of my knowledge.  The exhaust is aftermarket, if I could find a decent stock set for a good price, I'd have them on it...

Ideas/opinions?

[DH]

Do the carbs stay in sync or change after revving? If they change, check for bent needle plates.

[flybox1]

How do you control/adjust idle mixture if the mixture screws are soldered shut?

[Sdsbassist]

How do you control/adjust idle mixture if the mixture screws are soldered shut?

Say what?

[jonda500]

How do you control/adjust idle mixture if the mixture screws are soldered shut?

solid mixture screws are adjusted in exactly the same way as drilled ones - you prob start with them turned out a little more, maybe 2 turns?

[evanphi]

How do you control/adjust idle mixture if the mixture screws are soldered shut?

solid mixture screws are adjusted in exactly the same way as drilled ones - you prob start with them turned out a little more, maybe 2 turns?

No they are still 1 turn out. Early carbs had solid screws.

[fmctm1sw]

How do you control/adjust idle mixture if the mixture screws are soldered shut?

solid mixture screws are adjusted in exactly the same way as drilled ones - you prob start with them turned out a little more, maybe 2 turns?

No they are still 1 turn out. Early carbs had solid screws.

Yeah, what he is saying.  I knew there was a reason I soldered them.  I caught this bike on it's way to being "customized" and returned it as much as I could to stock setup.  I think I read something at some point (HM's book maybe?) indicating the screws for those year's carbs were solid.  Right now, I have them adjusted for best idle, I think they are around 1-1/2 somewhere...

Do the carbs stay in sync or change after revving? If they change, check for bent needle plates.

I revved it a few times and it stayed in sync but that's something I wouldn't have thought of...  I'll try to look at it this weekend..

[John Eberly]

The problem might not be carb at all. Does the idle hang high after revving it?

Classic problem is the advance springs weaken and don't pull the advance weights back in. This causes a fast idle condition until you slow the engine down - like by letting the clutch out a little with the brakes on and the bike in gear. If this brings the idle down you should look at the advance mechanism.

I have not found a source for the springs by themselves but I think you can still find replacement advance mechs.

[fmctm1sw]

The problem might not be carb at all. Does the idle hang high after revving it?

Classic problem is the advance springs weaken and don't pull the advance weights back in. This causes a fast idle condition until you slow the engine down - like by letting the clutch out a little with the brakes on and the bike in gear. If this brings the idle down you should look at the advance mechanism.

I have not found a source for the springs by themselves but I think you can still find replacement advance mechs.

I read about those springs in another thread here last night so I'll check that too.  Now that I think about, that's in HM's book too. 

[70CB750]

I did cut my advance springs short 1/2 of a loop, they were really tired and it certainly made it come back to idle much faster. 

I remember feeling a big difference.

[fmctm1sw]

I think you guys were onto something here.  I cut down those springs and the situation is much better now.  Aside from a leaking tach gear seal (which I really hope I can get to without taking the motor out), I think it's a matter of getting this thing out on the road and seeing what happens.  I really wish I could find a stock or stock-like exhaust though...

[chewbacca5000]

I think you guys were onto something here.  I cut down those springs and the situation is much better now.  Aside from a leaking tach gear seal (which I really hope I can get to without taking the motor out), I think it's a matter of getting this thing out on the road and seeing what happens.  I really wish I could find a stock or stock-like exhaust though...

Interesting.  Is it cured? 

[fmctm1sw]

I think you guys were onto something here.  I cut down those springs and the situation is much better now.  Aside from a leaking tach gear seal (which I really hope I can get to without taking the motor out), I think it's a matter of getting this thing out on the road and seeing what happens.  I really wish I could find a stock or stock-like exhaust though...

Interesting.  Is it cured?

I haven't gone far enough to consider it cured but it is noticeably different when it comes to returning to idle.  That leak prevented me from leaving it run very long.  Another thing I noticed is that the timing is steady, it used to bounce around a bit at idle.  Part of me worries I overdid it though on the springs.  I don't seem to have a smooth transition to full advance.  I DO have an old 550 advance though with uncut springs...   

My 550 has a little miss at idle every once in a while.  It almost makes me wonder if I shouldn't look at those springs too....

[chewbacca5000]

So I am working on my 75 restro-mod today and could not get the timing dialed in.  1,4 close, 2,3 way off.  Remember this thread and switch out the advance for a different one and now 1,4 spot on!

Does not make any sense as the springs "seemed" to be ok.  Idle is decent as well. 

I found this using my $8 flee market dwell meter.  1,4 was pretty steady, but 2,3 jumped all over the place.

Now going to recheck both with the dwell meter and fidle a bit and really get it dialed in.

From Mreicks post and what Pewee says having both 1,4 and 2,3 match at 23 degrees BTDC is huge.

[greenjeans]

It is amazing what brand new HONDA idle jets (not K&L etc), new airbox boots and air filter can do to any idle issues.

I've started buying new jets now as I've used up my supply of good used ones.  Alot can happen to jets in 40 years...