Topic: Do I have a vacuum leak, or . . . ?

[Honda-Rog]

My CB500 is behaving strangely,and frankly I'm baffled, so maybe one of you wise guys can put me right.

It starts fine from cold with choke, idles OK, and runs well, with great acceleration, until:- once it's properly hot, anything above about 1/4 throttle gives rise to spitting and popping, with a lack of power - it feels like fuel starvation. Then  sometimes it seems to recover and accelerates well. At the same point the idle becomes very erratic and cuts out after a few seconds.

I may have disturbed something, but have checked and rechecked:-

Carbs have been out several times, all jets and passageways visually checked (although I've never disturbed the needles and the top half in 5 years). Main and idle jets were replaced in the past couple of years. The emulsion tubes were all clear before and after this issue started.

Floats have all been carefully set to 22mm above the rim, as per manual. Float valves all work, no sticking springs and were replaced 2 years ago.

Air screws are all set to 1.5 turns out, but I haven't gone back to adjust these individually, as it wouldn't account for the issues above idle anyway.

All the carb boots were replaced a year or so ago, as were the O rings on the engine mounts.

There's no problem with fuel flow from the tank, and new fuel pipes have been installed.

I've tried my best to vacuum sync the carbs, and I think they're all pretty close, although I wouldn't claim to be very competent at this.

Timing has been reset and checked and is spot on, with auto advance operating normally.

New coils recently ( but I'm sure this isn't electrical, it has to be fuel-related).

What else could I have missed, or done wrong - this bike was running much better until I fiddled with it!

I plan to take the carbs off yet again (in my sleep this time), check all the boots, check all internals for blockages again, refit and resync, but I fear this won't solve it.

HELP!!!

[flybox1]

Do you have your original keihin jets and can you put them back in and give it a go?

Are you sure you dont have a fuel delivery issue in the sense that the bowls are not filling quickly enough? 
(check and see if you get a good, constant stream out of each carb drain screw)

IIWM, id do ^ and then raise the float height to 21mm, and see if it improves.....

[Vinhead1957]

Petcock does need cleaning as part of maintenance

[Honda-Rog]

Petcock already scrupulously cleaned, but thanks for the suggestion.

[Duanob]

Have you pulled your 'sparking' plugs to see what they look like? Too rich, too lean, etc.

[dave500]

check the resistance through the spark plug caps,have you got spare condensors to try?

[tkarahalios]

I was having similar symptoms with my CB550. After I replaced my points and reset them she started running much better. If you have points, take another look at them.

[bjbuchanan]

Don't overlook a bad battery. A bad battery causes all types of misleading weird symptoms. Corroded connections from age plus a so so battery could mean bad charging once the bike gets hot.

Make sure all of your ignition components spec out properly. Plug caps read proper range

[DanEarl]

I agree to check the plugs. Maybe they are just fouled out.

[Honda-Rog]

OK, gotta new gel battery, and plugs do look OK, I've given them a good clean. I'm still focused on the fuel side, anyone got thoughts there?

I'll report back once the carbs have been out again and I've checked the boots.

[dave500]

how did the caps measure out?

[Honda-Rog]

Between 5.11K and 5.48K, they are NGK 5K caps, only about a year old. I guess it's a process of elimination . . .

I've ordered new DR7ES plugs, as the centre insulators look fairly black.

Just had a thought - I fitted Hondaman's electronic ignition, and it had been running just fine, but could this in any way be causing the problem? HM, if you happen to read this, welcome any advice, thanks.

[iron_worker]

Usually if it runs worse at is it warms up then it points to a rich condition since as the engine heats up the fuel atomizes better in the intake runners and burns more efficienctly ... therefore less is needed at operating temperature. Your black plugs would tend to support this theory.

Problems that occur when warm can sometimes be electrical as well such as a coil or condensor failing with heat. I highly doubt it's the HM unit and if you're suspect of it I believe it can easily be tested by bypassing the box and running on the original points again.

Do you have stock exhaust and and intake? Clogged air filter perhaps (or over oiled)? Is your choke fully opening?

IW

[dave500]

using resistor plugs youll double the resistance,use non resistor plugs.

[Duanob]

OK, gotta new gel battery, and plugs do look OK, I've given them a good clean. I'm still focused on the fuel side, anyone got thoughts there?

I'll report back once the carbs have been out again and I've checked the boots.

What I meant by check the plugs is what color are they when you pull them out? White, black, nice shade of dark bown? That will tell you if its a fuel mix problem.

Update: I just read this

as the centre insulators look fairly black.

So as iron worker is on the right track about running rich. A few reasons to run rich. Non OEM jets, either after market or drilled out, needle clip position too low, crud on the needles holding them up out of the emulsion tubes and not seating properly, floats not set properly, air passages in the carb bodies plugged up, etc, etc, etc.

[Honda-Rog]

Hey, thanks guys. The emulsion tubes themselves are clear, had them out and cleaned every tiny hole, but haven't checked the needles. I've ordered the overhaul kits from cruzinimage on Ebay, heard good reports of these, so maybe a wholesale change of all those tiny carb components will sort it. Of course I will recheck all through the carbs for any other problems.

Iron_worker, thanks for the tips - probably time for a new air filter, but I can test for richness by running without it?

I should have said, apart from the HM ignition and relays for the coils, everything of significance here, including exhausts and intake is stock on my bike.

Can't help but think I must have changed something, as it was running generally well, but I'm always trying to improve the idle, which has been a little erratic for years. Must be my age, or something . . .

Any thoughts on fuel pipes - I change mine as they seemed to get a bit soft at high temperatures, and the new ones are maybe a tiny bit smaller internal diameter - surely it couldn't be that critical?

[dave500]

if your not running the bike around 4000 plus all the time itll be running in the richer part of the fuel circuit,if your in stop start traffic mostly thatll be your problem?

[Deltarider]

if your not running the bike around 4000 plus all the time itll be running in the richer part of the fuel circuit,if your in stop start traffic mostly thatll be your problem?

Advised by a Honda mechanic who owned a CB500 himself, I've made it a habbit to have the motor run @ 7000 or plus for at least some minutes every ride.

[Vinhead1957]

With the Hondaman kit aren't the condensers disconnected

[iron_worker]

If you run it without an air filter and your symptoms improve then yes that confirms that it too rich.

Dave may be on to something. They tend to run rich at idle and lower throttle openings so if you're not running it hard then maybe you're just not getting into a range where the plugs clean off.

IW

[Honda-Rog]

I understand that, and this is why I think it's odd. When I left home with a cold engine, everything seemed normal - choke to start with, then off, runnning fine for maybe 5 miles, including a high rev run up the local bypass at up to 70mph; after that the idle went bad, plus it wouldn't rev. properly throughout, much spitting and lack of power. It got to the point of being a safety issue, when I couldn't pull away at a roundabout., and at every junction it would then stall.

Should get the carb kits very soon, have stripped them down ready so will try to find a local source of ultrasonic cleaning.

Saddest thing is, once this is resolved, my lovely other half wants me to sell it!

[Blackhole]

Is the fuel cap vent hole clear?  If you give it a minute or two shut down after it starts acting up does it get any better when you start it back up?

[dave500]

you said you checked timing how about the point gaps?

[Honda-Rog]

Update:-

Carb kits arrived from cruzinimage within a week, fantastic. So I fitted all the new jets, float valve, etc., having first jetted out the carb bodies with carb cleaner, including the emulsifiers, which weren't replaced.

Bench synced, bike fired up easily, vacuum synced more easily than before. It started, idled well, and I thought "at last, a solution".

So I went for a 20 mile test ride out in the country - up to 70mph, nice ride, and the engine felt so smooth and responsive; but when I got back the idle was UGGHHH - up and down, almost cutting out, then rising up - I'd set it at about 1200rpm on the tacho, but it was going from 1000 to about 1400. Excuse me while I go and weep . . . 

. . .  that's better. I let it idle for a few minutes, then checked the (new) plugs. Two were fairly sooty, so I wound out their airscrews half a turn. This has made no difference. And even at cold/warm the idle is now not that steady.

I sprayed WD40 around the carb boots, and pretty sure there's no vacuum leaks there. The boots are all sound.

There has to be an explanation. Could it just be the airscrew settings, and I have to keep going at those? I suspect it may still be idling rich.

I will recheck points gaps and then timing, but surely this would affect performance throughout the range?

I think I also need a new air filter - but again, this would affect the whole performance?

I hate it when any kind of machine or technical device isn't working properly - I really can't stand it . . . especially when it's my Honda! 

I'm close to finding a bike shop somewhere who can deal with these oldies, and paying them to fix it - that would suck . . .

[Duanob]

Did you check the needle settings when you had the carbs apart?

Did you take Iron Worker's advice and run it with the air cleaner just to elimnate that possibility?

Saddest thing is, once this is resolved, my lovely other half wants me to sell it!

Well then don't fix it completely, just enough to enjoy riding it but not enough to "get top dollar". Or just ask a rediculously high price and tell her nobody is interested. I see that all the time around here