Topic: 1973 CB500 Four - Idling High

[alb]

Hi all,  new to motorcycles and new to the forum, but I have a nagging question to ask.

I got a 1973 CB500 Four from my wife for Fathers Day this year.  She earned some serious points!  It has about 8600 miles on it, and it's very clean and in good shape.

It ran fine right away, but one thing was bugging me.  When it started up, it idled okay, but once it was warmed up (about 5-10 minutes), the idle would race at 3-4K RPM.  It was also a low on power, especially when I was starting from a stop.  It felt like I had to hold the clutch in the friction zone for longer than I should have to.

Not knowing much about motorcycle maintenance myself, I wanted to get it in front of someone who knew what to look for.  I rode it for a couple of months, and finally got it in with a local vintage motorcycle mechanic in July.  6 weeks later (I guess when you're the only guy in town there's little incentive to work quickly), I got it back.  It does run smoother and has more power, but the idle thing is still happening, and I'm ready to see if I can tackle it myself rather than lend it to my slow mechanic for another couple of months.

He gave it a full inspection and lube, replaced the points, spruced up and tested the battery, replaced the fuel line and filter, and replaced the plugs, which he said were pretty fouled up.  He also adjusted the intake valves.

What could be causing the idle to rise once the bike is warmed up?  Is it something that I can solve, or should I just get used to adjusting the idle screw when I ride?  I'd appreciate any stories or input you all have.

[flybox1]

First of all, welcome to the forum, and 2nd....NICE BIKE AND EVEN BETTER WIFE!  
here's what i suggest.
Get a Clymer Manual for your bike.  
Download the shop manual too.
go thru the 3000mi maintenance steps to learn your bike.
Your Mech only did a partial service. 
Timing needs to be set.
Air filter needs to be new or cleaned(if it can be)
Check fro crud in your tank.  dump out the gas as you slosh it around.  see what comes out.  if its dirty, you need clean it out.  rusty, and check back here for derusting tips.  any debris in your tank and it will find a way to f*#k up stuff downstream.
Carbs need to be pulled, all jets, emulsifier tubes and passages cleaned spotless,  bench sync the slides.
Set air/fuel mix screws to spec.
reinstall the carbs and do a vacuum sync.

i think the vac sync will help your idle issue, but the other stuff i listed abouve needs to be adressed first.

The search function is your best friend here.  the FAQ section is awesome help for getting started. most of what you need is already posted. if you still need help sifting through the mire, let us know    someone always has a light on.

[alb]

Thanks Flybox.

Actually, I think he did replace the air filter, and the tank looks good.  I got the clymer manual and the shop manual, so I think I'll be taking apart the carbs as a winter project.  Just need to get my arms around what tools and equipment I need to start accumulating.  The vac sync looks like it takes a special setup.  Any tips on where to find tools/equipment?

[flybox1]

google:
Morgan Carbtune, ~$100, but probably the best rated tool for the job
Motion Pro Carb synchronizer
on ebay, search for carb synchronizer.
a 4 dial setup goes for about $50. and will do a pretty good job.

here's the SOHC4 parts -n- stuff database...
http://forums.sohc4.net/index.php?topic=743.0

[Deltarider]

google:
Morgan Carbtune, ~$100, but probably the best rated tool for the job
Motion Pro Carb synchronizer
on ebay, search for carb synchronizer.
a 4 dial setup goes for about $50. and will do a pretty good job

.
Can't believe my eyes. The man wrote

It has about 8600 miles on it, and it's very clean and in good shape.

8600 miles only and immediately someone is advising him to buy a synchronisation set...
With only 8600 miles on it, how much chance is there somebody has played with the synchronisation screws or has bent float valve tabs??? That engine just finished breaking in.
What's this preoccupation with syncing and tabbending?
I've did 45000 kms before I checked synchronisation for the first time. Wasn't even necessary, just wanted to play with my new tool.
Forget it, adress the other points and your engine will run fine.
The high idle is partly normal after the engine is warmed up. When returning to normal idle is not possible by turning the idle adjuster (under the tank, right hand side) it's an indication for running lean. Search this forum for detecting air leaks and cleaning pilot jets. Mechanics usually don't like to do this.

[flybox1]

you'll always have to adjust the choke and idle set screw after the bike warms up. 
the preoccupation, Delta, is that we have no idea what the PO, and or mechanics did to our bikes before we got them. 

as far as i'm concerned, there's nothing wrong with completely going thru your bike, as a NEW owner, and making sure all settings are as they should be. this will always give a newb a baseline of knowledge of their particular bike. his mech obviously didnt adress all aspects of properly tuning his bike to run the way he expects it to.
of course...it may not need to be done. He asked about carb synch tools.  i answered.
if someone here answers a question on JIS's, you gonna jump on and say its a specialty tool not needed, so just use a standard phillips?   

[Deltarider]

8600 miles, 8600 miles!

 

[flybox1]

 
could have been all in the first year, 2-up on dirt roads or sandy dunes, then sat for 30+ 

[Deltarider]

Alb, another thing,

If, if your mechanic has adjusted the airscrews, he could be the victim of false information. Recently I saw some pages out of a booklet with specs (American Honda Company 1977) that advises 2 turns out. That's even half a turn more than the 1¹/₂ out, which was already a mistake, IMO. It should be 1 ⁺/_- ¹/₈ out. Unless American Honda Company counts 180^o per turn. That would be the same as 1 turn out.
This 1¹/₂ or two turns out seems only to occur in USA. Maybe a mistake is copied, maybe Honda America had to comply with pollution regulations at idle. I don't know.
Mind you, the bike will definitely idle better with 2 turns out BUT will accelerate poorly. These models need a rich idle for good acceleration.

Are you satisfied with how your bike accelerates? If not, you can do the following (no need to have the engine run):
1 Make notes of what position the idle screws are in by gently turning them in. You may set them back to where they were.
2. If they're 1¹/₂ out or more, no wonder acceleration is poor.
3. Experiment by setting them to 1 turn out and compare acceleration with the situation before.
4. Whatever you do, make notes, so you can always return to previous settings.

[TwoTired]

What could be causing the idle to rise once the bike is warmed up? 

Uh...the carburetors.
In the days before computers ruled machines, carburetors were normal equipment.  Carburetors are dumb.  They don't have clue as to whether the engine is hot or cold.  Carburetors atomize fuel, dispersing it into the inlet air.  The physics of this means that heated engines atomize fuel far better than cold ones.  So, a cold engine in more inefficient than a hot one.  Choke helps with the atomization and enrichment thing, but because of the inefficiency the engine turns slower.  Some carburetor designs bump the idle up during choke operation.  The CB500 does not have this device.  Instead, Honda thought the computer above the seat was smart enough to hold the throttle open during warm up (or the friction adjust knob under the twist grip will hold the throttle in any position).
The main idle knob is set so that after warm up, the engine idles at about 1000 RPM.  That knob setting will not allow idle when cold.
The bike does not need a stationary warm up.  It will drive with some choke applied, and since your hand is usually on the throttle while riding, holding it open during the few blocks need to warm the engine really shouldn't be much of a problem.  The left hand can reach down periodically to bump the choke toward off, until it is off.  And by then the bike idles on it's own.

The engine is not a "torque" or low end grunt engine.  It needs RPM to make power and I don't think much is available 3500 RPM.
If you need to scoot, drop to lower gears (even 2 or 3 down).  Red line isn't until 9200 and anything below that is a perfectly acceptable operating mode.  These engines like RPM, so use it.

If you still think power is an issue, make certain all the head pipes heat up equally.  These have pretty poor power curves when only three cylinders are working.  But, your mech should have found that if he was worth spit.  But, if you still have a cold head pipe after paying him, a saliva deposit is probably in order.

Cheers,

[Deltarider]

Normally a 500 can have it's chokes opened up again, immediately after starting. A 500 will need a bit of throttle though. Better not to ride with chokes closed, not even partly. 500s differ from later 550 models in this respect.

That bike is beautiful BTW.

[Hondawggie]

I always check the simplest things first.

If your slow mechanic adusted the idle (look in your manual to locate it on the carbs) he *may* have adjusted the idle when the bike was cold.

So do this, this is 'checking the simplest thing first' before diving in to some of the good advice given above:

1) read your manual to locate the idle adustment -- on my '72 cb750, this is a round knob that faces down and is about 3" in from the left side of the tank, near the top level of the carbs (hanging downward).   I don't know where this knob is on your 500, but find in in your manual.

2) ride the bike long enough to achieve the high idle you speak of.


3) Now with the idle too high and the bike warmed up, adjust the idle downward with the adjuster you found in step #1 above to 1000rpm or so, maybe 1200 rpm.


4) Let the bike go cold.   (few hours of sitting)


5) Start the bike and run it until it's hot (10+ minutes or so).  If you still have a high idle, it could well be an air leak, and it's not the idle adjustment.


I just went through this on a '72 cb750 I bought.  It idled fine when it started but after 10 minutes it was 2500 to 3000rpm.   I adjusted the idle *when the bike was hot* and now the idle is fine.

Good luck!

[Deltarider]

I absolutely agree. First do what Hondawggie wrote. On the 500 you'll find the round knob under the tank RH side. Warmed up, set the idle on around 1200.
Could be, you don't have a problem at all.

[TwoTired]

Normally a 500 can have it's chokes opened up again, immediately after starting. A 500 will need a bit of throttle though.

No surprise that is normal for a bike with the inlet snorkle.  Not so for ones without.  If your bike has one of these below, you probably don't need much choke.

[Deltarider]

No surprise that is normal for a bike with the inlet snorkle.  Not so for ones without.  If your bike has one of these below, you probably don't need much choke.

TT, with all respect, but you must be joking if you're suggesting that that snorkle, that allows plenty air for topspeed, works like a choke when the carbslides are almost closed.
Besides the procedure I described, is recommended in the 500 owners manual and has been advised in this forum by others before.
But... if, if a 500 accidentally has the air screws out to many turns however, I can understand that you'll need the choke for a longer period.
I still don't understand the advised airscrew settings of 1¹/₂ or even 2 turns out found in Honda America sources. Those bikes must accelerate badly.

[Duanob]

Wow! As said before SWEET BIKE and even sweeter wife! I thought I was lucky when my wife said it was OK to but a bike.

"What could be causing the idle to rise once the bike is warmed up?"

As all things that burn gasoline, vacuum leaks are not your friend. Low miles is one thing but when a bike sits for decades even in a garage, rubber O-rings and gaskets tend to dry out, shrink, and crack. You have a few tiny ones inside the carb and you have some bigger ones between the intakes and the head. All of my intake o-rings dried out and had spaces for lot's of vacuum leaks which can result in high idling and poor acceleration. Mine would only ruin with the choke on. The intakes are easy to reach. You might try checking the easy inexpensive stuff first. Then move on to the carbs. Before you dig into the carbs buy an OEM gasket set with all the gaskets and o-rings you need. Make sure all the little pieces make it back into their respective carb. Good luck and for god's sake POST A PICTURE!

[TwoTired]

No surprise that is normal for a bike with the inlet snorkle.  Not so for ones without.  If your bike has one of these below, you probably don't need much choke.

TT, with all respect, but you must be joking if you're suggesting that that snorkle, that allows plenty air for topspeed, works like a choke when the carbslides are almost closed.

Not joking at all, actually.  Air volume is a different parameter from air pressure and it's the air pressure that draws fuel though the jets in the carbs.  That snorkel increases the pressure differential in the carb throats, which is why you need to have #78 mains instead of the #100s that the rest of the world uses without the snorkel.  Without actually seeing/measuring the 649 carbs that you have, I expect there are other differences between those carbs and the 627 set ups that are seen in countries different from yours, but we weren't exposed to those here in the states, and I am unable to find them to examine.

You must be ignoring physics/fluid dynamics to expect that your set up and operation is the same as the CB500s found in the rest of the world.  (or joking   )

That snorkle adds about 5-6 inches to the inlet runner over which the pressure must equalize between inlet and intake valve.  This effectively moves the pressure equalization curve throughout the duct toward that inlet restrictor.

In short, it makes everything in the inlet channel suck harder, just like a choke plate does, and every air and fuel port exit into the carb throat delivers more of what it has for a supply.

It would be easier to explain with pictures/drawings, etc.

Cheers,

[alb]

Wow.  Thanks for all the response and encouragement.  Sounds like I'll just keep fiddling with the idle adjustment knob until I get it down.  I still think I may do a little tinkering this fall to try to ensure that the carbs aren't gummed up.  Is there any additive I can run through the system to clean out the carbs, or is that a big no no?

[TwoTired]

Many on this forum swear by Seafoam.  I've used it, too.  But, it didn't live up to the hype, IMO.

It is by no means a substitute for overhaul with actual carb cleaner.

I guess the best thing I can say about it is that doesn't seem to hurt the carbs.

[njclimber]

Got a 76 550 w/ less than 5000 miles for sale in New Jersey!!

[Scott S]

Got a 76 550 w/ less than 5000 miles for sale in New Jersey!!

 What the everloving hell does that have to do with a six year old thread about a high idling 500?!?!?!


 There is a For Sale section here, ya know....