Author Topic: Honda CB500 Can’t Go Over 55mph  (Read 3486 times)

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Offline Nomad

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Honda CB500 Can’t Go Over 55mph
« on: March 22, 2019, 05:47:52 PM »
I recently purchased a CB500 Four that I thought was running like a champ until I took it out on the highway. When I reached 55mph I could hear a slight stuttering noise and the bike could not go any faster. Any suggestions?


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Offline Charles T

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #1 on: March 22, 2019, 07:45:36 PM »
Have you tried the usual? Fuel blockage, carb tuning, timing, spark quality, plugs, compression test? Mine was gutless, but ran well. Had low compression due mostly to valves. But I haven't quite got it on the road yet to try it out for sure.
Regards Charles T

Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #2 on: March 24, 2019, 12:29:38 PM »
I’ve found it is not so much the top end speed as it is the RPMs. No matter the speed I am driving, it starts to stutter at about 4200 RPMs and the tachometer cannot go any higher. The plugs appear fine. I would have to take it somewhere to see if the carbs are correctly synced. I added some Seafoam to the gas in the event there is some sort of blockage in the fuel delivery.


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Offline TwoTired

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #3 on: March 24, 2019, 03:45:20 PM »
The plugs appear fine.
Pleas describe this better.

Carbs don't know about RPM, only air flow and applied vacuum.

Does it run better with the fuel cap lifted?  Have you checked fuel flow from the fuel tap?

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
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Offline BenelliSEI

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #4 on: March 24, 2019, 04:27:12 PM »
TwoTired has a great point. Last time I chased a similar problem for days. The fuel cap vent was plugged!

Offline campbmic

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #5 on: March 24, 2019, 04:28:00 PM »
Get on the highway and get to 55mph. Do you still have more room to turn the throttle?
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Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #6 on: March 24, 2019, 04:43:45 PM »
Get on the highway and get to 55mph. Do you still have more room to turn the throttle?
No, the throttle is opened up all the way, no additional room to turn it any further.


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Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #7 on: March 25, 2019, 10:26:52 AM »
Regarding more description of the spark plugs, the plugs are D7EA plugs and they did have some black carbon build up on them but it brushed off easily.  From what I've read about plugs with black carbon on them it could be a sign of the plug not running hot enough and in some cases it is recommended to switch to a plug that burns hotter.  Not sure if my situation necessitates that or not.  I have not tried running the bike on the highway with the fuel cap open, but will try it and see if that makes a difference.  I've replaced the fuel hose from the tank to the petcock, but will disconnect the hose from the petcock to the carburetor to make sure it is not obstructed.

I checked the adjustment of the throttle.  When I turn the throttle all the way open and look at the throttle underneath the tank, it is open as far is it can go.

Offline TwoTired

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #8 on: March 25, 2019, 12:05:20 PM »
It is important to know where the carbon is collecting.  The base ring is normally black as that is where is contact the cooling head.

Black on the center electrode insulator is quite another matter.  And where that black is is important.  Black deep in the well on the insulator is also more or less ok, as that is also closest to the cooling head material.  Black near the tip electrode is very bad.  As this soot is conductive, it can shunt away spark and make it impossible for the cylinder to fire properly.

Soot is unburned hydrocarbons (fuel)  This is a result of more fuel to air mixture than can be burned in a combustion cycle.  If it is occurring at the spark plug tip, the mixture is simply too rich.  A hotter plug may aid.  But, it will not cure a too rich mixture delivery.

The plugs usually clear with highway use.  Power and heat will clean plug normally.   Excessive choke and low temp operation is a recipe for soot buildup on spark plugs.

Cheers,
Lloyd... (SOHC4 #11 Original Mail List)
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Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #9 on: March 25, 2019, 12:42:52 PM »
The carbon appeared to be collecting mainly on the treads of the spark plug.  The insulator did not have build up of any kind on it. 

Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #10 on: March 25, 2019, 01:13:55 PM »
I'm new to this forum and am seeing people reference velocity stacks.  I don't know if it is helpful to mention or not, but my bike does have velocity stacks rather than the stock air filter.

Offline Gene

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #11 on: March 25, 2019, 01:35:15 PM »
I don't have the skills to describe what you need to do, but you'll need to tune the carbs to that airflow which is different than the stock. There are a bazillion suggestions on the forum here but one thing you will see over and over is - put the stock air back on and see how it runs. THEN work with the modifications.

Anything else different from stock?

Also - are you certain you're firing on all four cylinders?
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Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #12 on: March 25, 2019, 01:56:12 PM »
Correction....my CB500 has Air Pods, not velocity stacks.  I do not have the stock air to put back on to test how it runs with it.  The bike runs perfect under 4,200 rpms but I will check to make sure each cylinder is firing and begin researching how to tune the carbs with air pods.   

I've seen references being made as to changing "jets" and "pilots" but I am new to all of this, so will have to research the basics of these terms and what that means in adjusting the carburetors.   
« Last Edit: March 25, 2019, 02:32:06 PM by Nomad »

Offline TwoTired

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #13 on: March 25, 2019, 03:58:08 PM »
Airflow is secondary to carb throat vacuum.  The carb venturis don't exert much in the way of throat vacuum until there is a pretty good velocity going through them.

The dominant vacuum in the carb throats originates in the piston cylinder when the piston falls and the intake valve is open.  Nature abhors a vacuum and that negative pressure tries to get equalized with air from the atmosphere via the intake ducting.  Stock the carbs are about 1/2 way between air source and piston cylinder.  For simplicity sake lets say the pressure or vacuum is also halfway between piston cylinder and outside pressure.

The pressure is important to carbs because the throat pressure to outside air pressure differential is what pulls/pushes fuel through the fuel jets.  Pressure is applied to the fuel in the carb bowl.  And negative pressure or vacuum is what pulls it from there though the various fuel jet orifices in the carb.  Fuel volume is then determined by the pressure applied and the orifice size.  It's still a pressure equalization effect even when applied to liquids.

Velocity stacks and pods both shorten the air duct between piston fall source and air inlet.  This places the carbs closer to the equalization source (outside atmospheric) and effectively reduce the throat vacuum in the carbs.  So, the carbs now draw less fuel through original sized jet orifices.  Since there was no displacement or valve timing change, the carbs no deliver less fuel than with the original duct and air filter box for the same volume of air.   The engine runs lean.  Too lean and the engine can't make power under load conditions. 

Venturi effect adds to the basic pressure drop as air velocity increases.  However, if the basic pressure drop originates too low, it cannot compensate for the loss on it's own.

To restore the correct air fuel balance, there is no choice but to increase the fuel orifice delivery size when shortening/altering the intake duct tract.  This is far easier than moving the carb closer to the falling piston vacuum source.

Very unlikely your stock engine will make more power with pods or stacks.  At least not until you approach and/or exceed redline where the stock filter box begins to add oxygen restrictive factors.  1-3% is important on the track.  It will be louder without the stock air box, and this tends to enhance the butt dyno effects rather than actual measured improvement.

Now you know...

Cheers,

Lloyd... (SOHC4 #11 Original Mail List)
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Offline Stev-o

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #14 on: March 25, 2019, 04:13:47 PM »
Is it the "Sammy Hagar Edition"?!
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Offline TwoTired

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #15 on: March 25, 2019, 04:44:29 PM »
Is it the "Sammy Hagar Edition"?!
Er..  Sammy couldn't drive under 55, as I recall.  Perhaps you meant the 80's federal edition?
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Offline dave500

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #16 on: March 26, 2019, 01:23:00 AM »
look at and be sure your ignition is spot on first!start with the ignition and get it out the way first or youll be chasing your tail,especially with pods.

Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #17 on: March 26, 2019, 04:55:20 AM »
With all the variables out there, maybe I should just replace the air pods with a stock air box and filter?


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Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #18 on: March 26, 2019, 08:50:06 AM »
look at and be sure your ignition is spot on first!start with the ignition and get it out the way first or youll be chasing your tail,especially with pods.
Any specifics on what to check regarding the ignition?


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Offline lafindufondu

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #19 on: March 26, 2019, 10:15:40 AM »
Check your timing, make sure your firing at F for 1/4 and 2/3.  I've never run pods before but I think you may have to go up a size or two on the main jets.

Offline Gene

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #20 on: March 26, 2019, 04:11:58 PM »
Timing instructions are in the manual. If you don't have one, def get one. it's a lifesaver. But yeah - if your spark is firing when there's no fuel to combust it's just a light show.

Also - easy check to see if all 4 are firing, start the bike, let it run for a minute and GENTLY touch the header with your hand. Is it hot enough to burn the snot outta you? Firing. is it kinda warm to the touch but you could still grab it and hold on with impunity? Not firing. you can also get a laser thermometer to read it, but your hand will do just fine.

lastly, don't panic, you're gonna be fine. these bikes are damned near impossible to kill.
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Offline scunny

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #21 on: March 26, 2019, 10:41:33 PM »
As Gene said, you can verify if the cylinders are firing with the finger test. To avoid burning your fingers, just spit on the finger tip and gently touch the header for a short time. If it's firing, sizzle, if not, no sizzle.
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Offline Nomad

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Honda CB500 Can’t Go Over 55mph
« Reply #22 on: March 27, 2019, 06:20:25 AM »
I confirmed all cylinders are firing. I’m thinking the bike is running too lean by taking in too much air from the air pods. Rather than trying to guess what size of jet to move up to for the carburetors, I think I will locate and purchase a stock airbox/filter to replace AirPods. Hopefully that is the source of my problem. The bike runs great under 4200 rpms.


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« Last Edit: March 27, 2019, 06:22:19 AM by Nomad »

Offline TwoTired

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #23 on: March 27, 2019, 10:46:16 AM »
I’m thinking the bike is running too lean by taking in too much air from the air pods.

I give up.
Lloyd... (SOHC4 #11 Original Mail List)
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Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #24 on: March 27, 2019, 10:48:44 AM »
I’m thinking the bike is running too lean by taking in too much air from the air pods.

I give up.
Did I somehow offend you?


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Offline robvangulik

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #25 on: March 27, 2019, 02:16:27 PM »
Reread reply #13 and then either find a stock airbox or get larger main jets....

Offline Nomad

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #26 on: March 27, 2019, 02:58:03 PM »
If I can find an airbox in reasonably good condition, I'm thinking that is the route I will go because I don't know exactly what size jet to move up to.  I've read going up one or two sizes on the jets when running air pods, but would like to avoid guessing.  Thanks for your input on this.

Offline Nomad

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Honda CB500 Can’t Go Over 55mph
« Reply #27 on: April 16, 2019, 09:24:59 PM »
Issue resolved. Main jets were changed to 110 and it runs great. Interestingly enough, it was found my carbs actually had 125 main jets rather than the stock 100 jets. I’ve taken the bike up to 85 mph so far and it had no problems. It wanted to go faster but I backed off as I was in a 45 mph zone.


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« Last Edit: April 16, 2019, 09:26:35 PM by Nomad »

Offline Gene

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Re: Honda CB500 Can’t Go Over 55mph
« Reply #28 on: April 17, 2019, 10:31:33 AM »
good news.
*1973 CB750K3 (Bow)