Topic: Idle accelerates - CB500

[Matte Black]

Hi all, my first post here!

My '73 CB500K cold starts and idles fine. When it heats up, and I accelerate the RPMs even a little, it starts to accelerate dramatically to the point where I have to kill it before it exceeds red line!

With the choke almost closed, if I drop the idle screw while the RPMs increase I can get a stable RPM idle although it's not at a reasonable idle speed. Like 3 times a normal idle (I don't have any gauges, I'm going by ear).

As you can probably imagine, I overheat VERY fast to the point where I'll stall after about 1/2 hour of riding in the city, and lose ability to change gears, mostly to neutral.

I've had the bike in front of multiple mechanics and a ton has been rebuilt and configured by them and myself, and the problem still occurs consistently.

My last attempt to fix, I've installed a immaculate and brand new bench synced carburetor, new manifold boots, and new points (which I didn't install).

I just spray tested for vacuum leaks and it's very solid!

The only thing I can imagine that is causing this would be the timing advance. It's the only thing that I hav't checked and think that I should check for sticking er something.

Here's where I'm a little clueless. I don't know much at all about points, aside on how they work, and I wasn't the person that installed my new ones. I confess I am a bit intimidated in removing the points plate to check the timing advance. When I attempt to remove it, it won't go past the center rotor without me having to remove the points (I'm sure you know). I've searched like crazy how to check the timing advance with no avail. I know these parts are turned and set precisely and not comfortable going further without instructions. Plus once I expose the advance, how do I check that it's OK so I can check that off as a possibility?

Other than this task, anyone have any other ideas on what could be causing this? Summer is ticking by and I'm desperate at this point.

Thanks!






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[flybox1]

Welcome to the forum.
Always check the FAQ section of the board, first, as someone has had your problem before.
Here are a few links to get you started.

http://forums.sohc4.net/index.php?topic=5752.0

http://www.motorcycleproject.com/motorcycle/text/sohcign.html

http://www.hondachopper.com/garage/carb_info/timing/timing1.html

[Matte Black]

Thanks for the reply flybox1 but  my points IMO are fine, and was having trouble getting to my timing advance to make sure it was ok. I ended up checking it out and it's action seem perfectly fine and I think I can rule it out.

I've been searching and reading forums for months and can't find any solution.

Here's a post that's almost identical to my problem but it doesn't state a solution as well:
http://www.motorcycleforums.net/forum/honda/47132-engine-races-once-bike-warm.html

I've put thousands into fixing this and getting nowhere and need your help.

Recap:

-In a cold start, I can adjust the idle screw to have a very reasonable idle speed. It starts fine with choke on (closed). After about a minute I have to open the choke a bit to keep it running. In this state, and in neutral still, I can rev the throttle and when I release it, the RPMs drop down to a reasonable idle speed successfully. Seems perfect.

- As the engine warms, still in neutral, the idle RPMs begin to rise, then, they race very high and keep increasing. At this point I either hit the kill switch or turn the idle screw down multiple turns to get back to a sustainable but high idle speed. If I were to slow the idle to a reasonable sounding idle, it will eventually stall in a matter of seconds if I don't pull on the throttle a bit. At this point I can either keep the idle low and consistently pull the throttle, or set the idle high. These are the only methods to keep it running at this point. If I turn on the choke (closed) the engine dies immediately.

- If I now put it in gear and begin to ride, the problem gets worse over time. The hotter the engine gets the lower I have to set the idle as the RPMs keep increasing. I continue to lower the idle the hotter it gets, and in about a half hour of riding, the idle is very low and the engine is VERY hot. So hot I can't get into neutral.

Everything I've tried, in order:

1) Clean / sync carburetors
2) New points
3) New fuel lines
4) New manifold boots
5) All new refurbished bench synced carburetors
6) Checked timing advance action
7) Checked for vacuum leaks using starting fluid

I've read many things that could cause this, from vacuum leaks, sticking timing advance, clogged idle jet, needing to adjust pilot screws, etc. All seem to point to a it running rich, but no solutions.

Final note: I've replaced the plugs multiple times, and in some cases they were wet.

Please give me some homework guys, I'll try anything.

Thanks!

[TwoTired]

Bench sync is often not accurate enough for a stable idle.  Vacuum sync allows all cylinders to produce equal power pulses at idle so they don't fight for dominance of the crank speed.

The idle knob setting is made AFTER the engine has reached operating temp.  Choke off, lever down.
You will have to hold the throttle open when cold and during warm up.  Nature of the beast.  Cold engines are less efficient at atomization, and there is no automatic mixture adjustment connected with temperature.

Must know fuel level in carbs, usually determined by float height.  Can be checked with the clear tube method.

The carb slides must have the lower cutaway facing the air inlet.

Must know slide needle setting and brand installed.  They have number on them.  Report.

The emulsion tubes must be clean and the air jet passageways and the pilot circuits must be "proved" to flow.  No guessing or assumptions are allowed.

Must know pilot screw setting.

Must know what main jet size is installed and if it has been drilled or not.

Must know induction type stock filter arrangement or cheap, cursed PODs.

Must know exhaust back press of exhaust.  Stock type or unknown aftermarket substitution.

Must also read the spark plugs for mixture/combustion conditions.  Carbon fouled plug will cause run issues all by themselves.  It's usually cause by carb issues, but if the plugs aren't clean or now self cleaning, een proper carb mixtures won't run right due to random spark shunting. 

If you need to rejet the carbs you will need a test track or a dyno, or subject yourself to endless trial and error part substitutions until you find another project more interesting.

[goldarrow]

ignition advancer sticky? throttle cable sticky?  i'm sure something very simple

[Matte Black]

TwoTired:
I'm aware that there is a warm up period before she idles right, but whats killn me is that about a year ago it had no issues with idling even during warm up. This is something new. Thanks for the checklist of things to know, its helpful. It's my goal to know all the states of my carbs better. Here's the thing, (and I'm not ruling the carbs out) I went through everything you mentioned multiple times myself and with dif mechanics with no outcome. And out of frustration I ended up installing all new (immaculate) carbs and the SAME issue occurs on them! This is why I'm now thinking it could be something other than the carbs/jets if the new ones didn't change the idle issue at all... In fact she runs very much the same before and after installation. I'm possibly overlooking a very small vacuum leak that gets worse the hotter the engine gets... Smoke test?

goldarrow:
- I checked the ignition advance and it snaps fine: https://www.youtube.com/watch?v=twPn7JLmWQM
- The throttle cable isn't sticking and snaps back fine as well. Here is a video of the RPMs racing after I pull on the throttle a bit. You can see the throttle snaps back just fine on the carbs but the RPMs accelerate: https://www.youtube.com/watch?v=Y-awZjbTxSI

[goldarrow]

I see pods in your video.  Are your carbs jetted right?

[Matte Black]

Yeah, I believe they are. When I got the new carbs they were setup for pods + 4 into 1 exhaust.

I want it to be the jetting but I think it's something else. The bike ran great for years with the jetting/pods and I replaced all the carbs and the issue is exactly the same.

I'm getting desperate now and need to start eliminating. The only parts that hav't been replaced/checked are the coils/plug wires, and exhaust.

Anyone have a better way of checking for vacuum leaks? Like smoke? I want to check when the bike is hot and perhaps the issue is multiple little vacuum leaks that get worse when it heats up.

[iron_worker]

What jetting do you have in there now? If it's over heating that fast then it's probably lean.

Have you checked your timing with a light? Does it advance to full advance by 3500RPM and return to the "F" mark at idle?

Also did you check for vacuum leaks at the head? There are o-rings between the head and carb spigots on the 500/550 that are prone to cracking with age and heat cycles.

IW

[Matte Black]

100 main jets. If its a jetting issue then both my new and old carbs would have had the exact same jetting issue. I find it hard to imagine.

I admit I hav't checked the timing with a light as I just had all new points added and it didn't change anything. That is unless the shop didn't check with a light and just replaced the points... I guess its worth a try to do so. I don't have a tach or a strobe though.

I havn't checked for  a leak in the head as I just had the head and valves cleaned and it didn't change the issue. But again its worth a test. Can I attempt it with starting fluid?

[goldarrow]

What size is slow jets?

[nhg2112]

Mine acted similar to this when it was running super lean. Turns out the issue was float height AND I had a bad oil pump. Engine was lean and not getting oil, the more I ran it it would get progressively worse, cooking oil on the camshaft and all sorts of fun stuff.

First advice I've got is if it's getting really hot, dont ride it. These things get hot but not crazy hot - if you think its too hot then it probably is. Riding/running it hot can ruin lots of stuff - learned this the hard way.

Second, sounds like you've already done some of it but you need to just make a checklist of systems & subsystems on the bike and go through each one methodically, making sure they're functioning properly.

These bikes aren't super complicated - you've just got to be patient and troubleshoot bit by bit. You'll rarely have an 'aha' moment and know exactly where to look (unless you're one of the old guys...).

Even on super clean carbs your float height could be jacked up, needle positions off, etc...Just have to personally verify everything is how it is supposed to be and you'll find the issue eventually.

[Matte Black]

Goldarrow: Sorry, Duuu, well slow was 40, but the new carbs I recently put on are from a CB550 which uses 38. But again the idle issue occurs with both idle jet sizes.

nhg2112: I havn't looked at the oil pump/filter recently and its due. The bike does heat up very fast, perhaps caused by the high RPMs, or the other way around. I dunno...

Whats the best way to check for vacuum leaks in the head?

[goldarrow]

i don't believe 38 or 40 is enough for PODS.  but i'm saying this out of my rear end    may be someone who knows better about pods can help out

[TwoTired]

i don't believe 38 or 40 is enough for PODS.  but i'm saying this out of my rear end    may be someone who knows better about pods can help out

40 is enough for stable Cb500 (or550) idle, maybe not for throttle advance operation off idle with pods.  But, surely NOT with the stock pilot screw setting.  ...which has never been reported in this thread.  Honestly, I don't know what the OP thinks to gain in this thread without reporting accurate details about his carbs and bike.

Sometimes one does find a silver bullet.   Perhaps lighting will strike and provide illumination, like a light bulb overhead?  Eureka!!   Hard to overcome a belief system unfounded in science.

Let's change 20 things and see what that does!

[Matte Black]

Not sure what "inaccurate details about my carbs and bike" I've given... What do ya need to know man? "My goal for this thread":
- To be able to idle without having to lightly pull/hold the throttle.
- Not have to constantly adjust my idle screw. My fingers are now jacked up.
- Do the ton. JK

My bike runs really good other than this.

Ya'll think if there was an issue with my oil pump or filter could effect my idle?

Update:
- Last night I heard my cam chain flopp'n around, so I re-adjusted my CCT. No more rattle.

Weekend:
- Will look at my oil filter & pump.
- Check for vacuum leak in my head.
- Strobe-check points

Thanks all!

[TwoTired]

Not sure what "inaccurate details about my carbs and bike" I've given... What do ya need to know man?

read reply #3