Topic: Idle question ??

[MikeJW]

I doubt this will interest anyone other than me, but I would like to know. When I bought my '72 750 a couple years ago, it was in running condition. I have still done a long list of maintenance fixes and parts replacements including carburetor cleaning & adjusting, timing, plug/plug wire replacement, adjust valves as well as many non-engine repairs: seat, brakes, wheel bearings, lights, etc., etc.
So, at this point in its life, the bike starts easily, runs well but I still have a question. Every time I start it I have to initially fiddle with the idle. That is, it idles too low. Then, after I have been riding for a while, it wants to idle too fast. I have to speed the idle up at first when it is maybe not quite warmed up and then dial it down when it is well warmed. Back in the old days when I had 4-wheel vehicles with carburetors, I wasn't constantly resetting the idle speed. How come my bike seems to demand this ?? I know it is not the end of the world and I could get used to doing it to pamper an old classic, but I just wonder why it seems to be a requirement on my bike.
One of you old time gurus please clue me in. Thanks
Mike in Memphis

[jimbojangles]

I'm sure someone will post a more intelligent answer, but in short. All of our bikes are that way. They will all idle faster once they are fully warmed up. (Insert intelligent post here about idle circuits and A/F ratio)  However, you shouldn't have to continuously set the idle screw. Set the idle where you want it when it's warm and then leave it alone.  There should be screw/knob/whatever on your handlebars next to your throttle. (It's not a cruise control!) Use this to lock your throttle in place while your bike is warming up in the driveway. Then loosen it before you take off.

However, I have a follow up question. What RPM do people have their idles set at on their 750s? Mine will idle super slow, but it sounds stupid.

[ekpent]

They are just cold blooded beasts with a pretty primitive choke circuit,no fast idle etc. built in.I usually just hold the throttle open a little to keep it going and then ride till warmed up. Mine usually like to idle around 1000 - 1100rpm or so when warmed and good. .

[dhall57]

MikeJW, Im no expert and don't claim to be, but I have a 76CB750 and these CB's are very cold natured. Cold starting my bike is always full choke on, bike almost aways fires right up lst try, but than playing with throttle I slowly take choke off and at the same time turn idle screw to speed up idle so it will idle by itself without throttle. Has it slowly warms up I will back off idle screw. At my lst stop light I have to adjust my idle more because engine is getting to operating temp. But once it gets  there and I make the last adjustment it will idle all day long around 1300 to 1500 rpms with no further adjustment.
But next cold start I have to go through the same thing again. Remember this is 1970 technology. It doesn't sound like you have carb problems, its more just the nature of the beast.

[MikeJW]

OK, I took that screw, under the throttle, off of the bike very soon after I got it (don't remember why). Maybe I should look for it and put it back if I can find it.
I can ride the bike within a couple minutes after I start it, but if I don't keep it revved up with the throttle it will die every time I stop. If I adjust the idle up to 1000, it's fine.
When the bike gets itself fully warmed up, it wants to idle at 2000 or slightly higher. If I adjust the idle back down to 1000, it's fine.
That's what I mean by fiddling with the idle.

[dhall57]

OK, I took that screw, under the throttle, off of the bike very soon after I got it (don't remember why). Maybe I should look for it and put it back if I can find it.
I can ride the bike within a couple minutes after I start it, but if I don't keep it revved up with the throttle it will die every time I stop. If I adjust the idle up to 1000, it's fine.
When the bike gets itself fully warmed up, it wants to idle at 2000 or slightly higher. If I adjust the idle back down to 1000, it's fine.
That's what I mean by fiddling with the idle.

Welcome to the world of riding a older classic motorcycle.
Don't you just love it

[TwoTired]

Why?
Cold engines don't atomize fuel as well as hot engines, and that makes them less efficient.
Carburetors don't automatically adapt to temperatures and are normally optimized for normal temp operation.

So, to get them to run when cold, you need fuel enrichment (choke), and a slightly bigger throttle opening.  Pretty much normal for all carburetors, without computer control.  It so happens that the computer for the SOHC4 is located several inches above the seat.

I recall watching my mom start up an old 40's era Chevy on a cold Illinois morning.  It had two knobs on the dash, one labeled C and one labeled T.  One operated the choke and one operated a throttle stop.  To start, you pulled them both, and then regulated them so the engine would keep running cold.  When the engine was at operating temp, they were both pushed in all the way.

In the 50s carbs got "modernized".  They got a bi metallic coil spring bathed in the carb manifold or exhaust manifold heat.  The spring was stronger cold than hot, so the spring actuated the choke butterfly and a fast idle cam that blocked the idle stop to a higher setting.

In the 80's, many vehicles got an added computer to further remove any driver toil keeping the engine running when cold.

In effect, the cars got smarter and the drivers, dumber.  World rejoices, more vehicles sell.

The motorcycle industry was just behind the auto industry in development for a long while.

Cheers,

[Gaither]

Two-Tired

Right again!

Since installing a new AGM battery with 360 CCA, the ol' 550 starts VERY quickly (what an improvement!). Then, I have to keep it running with the throttle and the choke. Within approx 2 minutes I can open the choke but still give it a bit of throttle to keep it running (not much). Within about 3 minutes, it'll idle at 1000. I never bother the idle screw.

FWIW, I cut my teeth on a '41 and '47 Chevie. Both were easy to start cold (using the choke and go pedal). Then, the trick was to keep 'em running til the heat riser warmed the carb enough to ldle WITH partial choke. 'Never used the throttle - far too easy! 'Had to do it myself. (My, those were good ol' days!)

[HondaMan]

I doubt this will interest anyone other than me, but I would like to know. When I bought my '72 750 a couple years ago, it was in running condition. I have still done a long list of maintenance fixes and parts replacements including carburetor cleaning & adjusting, timing, plug/plug wire replacement, adjust valves as well as many non-engine repairs: seat, brakes, wheel bearings, lights, etc., etc.
So, at this point in its life, the bike starts easily, runs well but I still have a question. Every time I start it I have to initially fiddle with the idle. That is, it idles too low. Then, after I have been riding for a while, it wants to idle too fast. I have to speed the idle up at first when it is maybe not quite warmed up and then dial it down when it is well warmed. Back in the old days when I had 4-wheel vehicles with carburetors, I wasn't constantly resetting the idle speed. How come my bike seems to demand this ?? I know it is not the end of the world and I could get used to doing it to pamper an old classic, but I just wonder why it seems to be a requirement on my bike.
One of you old time gurus please clue me in. Thanks
Mike in Memphis

Two things:
1. You mention "changed spark plug wires" [shudder]. How did you do this, if you have stock Honda coils? You didn't mention changin plug caps at the same time, which should have been the minimum. If you have coils with removable wires, which coils are they?
2. This issue with a K2 is most likely due to one of these 3 things:
   a. Minor vacuum leaks in the air hoses of the carbs, most likey at the head. If the clamps are fully closed already, get wider hose clamps or new hoses (Z1 Enterprises sells them in sets).
   b. Old condensors (presuming you are using points instead of a Dyna ignition). If it is a Dyna, it will be cold-blooded, period.
   c. Carb idle air screws should be set at 15/16 to 1 turn out at your altitude. If too rich (like 1-1/8 turns) it will idle at one speed cold and another hot.
 

[MikeJW]

Hondaman, you caught me. I didn't change the plug wires. I changed the caps that go on the plugs. Sorry. Two of my original ones were cracked and I soon discovered that one cylinder wasn't firing. I believe the carburetor connections are good because I bought brand new rubber connector hoses and clamps and I have them tightened down very firmly. I am using the original points & condenser. I had heard/read that condensers last a long time and the points look good on close inspection. Have thought about buying your pointless ignition module but I have had to fix so many other little items on this bike and I'm kind of poor so I have skipped trying to fix things that don't seem to be broke.
Perhaps I don't have the carbs exactly fine tuned just right. I need to check that. Thanks

[Stev-o]

I have read about guys changing plug wires but personally, I would not try it.

[Johnie]

Fine tuning the carbs via vacuum sync is the final step in the tuning process. There is not a bike I have owned that does not run better after the final sync. My 750's are the same as the rest of you, but I do not have to use the throttle screw on the bars. I just full choke it at start and keep it running for a couple minutes and slowly back off the choke as she warms up. Once warmed my idle stays anywhere from 1,000 - 1,300.

[Stev-o]

Johnie - your bikes do not run better after the final sync??

[MikeJW]

Johnie, I absolutely agree with you that I should vacuum synch my carbs but I just don't have a hundred some odd dollars laying around to buy the gauges. I wish there was someone here in Memphis I could borrow them from. I would do it in a heartbeat.
Stev-O, I didn't understand your response?? Was that a statement or a question?
Mike

[HondaMan]

Hondaman, you caught me. I didn't change the plug wires. I changed the caps that go on the plugs. Sorry. Two of my original ones were cracked and I soon discovered that one cylinder wasn't firing. I believe the carburetor connections are good because I bought brand new rubber connector hoses and clamps and I have them tightened down very firmly. I am using the original points & condenser. I had heard/read that condensers last a long time and the points look good on close inspection. Have thought about buying your pointless ignition module but I have had to fix so many other little items on this bike and I'm kind of poor so I have skipped trying to fix things that don't seem to be broke.
Perhaps I don't have the carbs exactly fine tuned just right. I need to check that. Thanks

The condensors are typicaly in good condition for about 3 years, after which they start to deteriorate. First they lose a little of their capacitance from the drying-out process, which lets the arcing increase slowly over time on the points: this raises the arc voltage which in turn attacks the insulation in the condensors, breaking it down until it starts to "leak" electrically. This then makes them act "cold blooded", amplifying the cold-blooded nature of the tungsten conductivity on the points themselves, and the bike needs more and more time to "warm up" before it will idle, changing to a more efficient burn as it does. That's the life cycle of the Kettering Ignition. 

My Ignition unit is actually not pointless, rather it uses the points so setup on the bike remains 100% stock per a Honda manual (for less confusion, among other things). It also leaves the points in command so that if the box ever dies you can just unplug the 4 wires to it, plug the point back in (less than 5 minutes' effort on the side of the road), and ride on. (No other 'electronic ignition' can make that handy claim!) While it operates, it reduces the voltage to the points to about 6 volts and their current to about 0.1 amp and stops all the arcing that eats them, so they don't wear out on the contacts and the arc-weld-break open cycle that wears the pivot is stopped. Thus the points lasts for many, many years. I am going on season #7 with my first test unit on my K2, no timing shift, gap change, nor points wear yet. This lowered voltage also ensures the condensors at least twice their normal life, probably more (but only time will tell that part).

I'm actually not attempting to 'sell' one here (Lord knows I'm having a hard time building enough of them!), but I have recently seen some confusion arising about how they work. So, I'm attempting to jump in here and there to re-explain what it does so an urban legend doesn't arise. Terry in AUS is my next visit: he was one of the first testers of it years ago, and one experiment was to run without condensors (because he had a bad one on his K0 at the time), which he still does. Obviously, this would make switching back to points on the side of the road a little tougher. I don't recommend this approach, although it will work in a pinch

[Johnie]

Johnie - your bikes do not run better after the final sync??

The bestest...

[chickenman_26]

If it is a Dyna, it will be cold-blooded, period.

Most of that is due to weak advance springs. The timing is set at full advance rpm. But the springs don't pull the weights all the way back at idle, so the bike is over-advanced at idle and runs as if it were too lean. Correcting the spring issue eliminates that...provided the bike isn't actually running lean for the reasons you stated.

Stu