CB500 Four - engine cuts off in idle

[Siloxious]

Hello everyone!

I had an awesome weekend, I was finally able to get my CB500 Four running! After a year and a half of work on the bike, it came to life. While wrestling with the ignition, I noticed that the previous owner had installed the cam of the ignition advance 180 degrees wrong. After fixing this, the engine came to life with only one kick with the kickstarter. Now, another chapter of the restoration begins!

The engine starts brilliantly, no problems with starting and runs on all four cylinders (the exhausts become hot). However, it cuts off when idling. When I use the throttle, the engine reacts. But beneath 2000 rpm, it begins to sputter and beneath 1000 rpm, it cuts off. It just doesn’t idle at all. I noticed that cylinders 2 and 3 are hotter than 1 and 4. Would anyone know what the problem could be?

The next thing on my list is doing a compression test of the cylinders as I’ve not gotten around to it yet. And if I can manage to get the engine to idle, I’d like to do a carburetor synchronisation.

Overall, it was a good weekend!
Cheers!

[Oddjob]

Sounds like some of the idle jets or slow jets as they are sometimes called are blocked. Maybe some of the passageways that supply them with air as well. I presume you've tried using the thumb screw on the carbs to keep them going?

[Siloxious]

Just so we're talking about the same screw: if you say thumb screw, do you mean screw number 42? I've put these in and gave them 1 and 1/8 turns from when they're seated all the way (as the shop manual mentions an air screw opening of 1 and +- 1/8).

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[deltarider]

First things first. Compression test and synchronisation can wait.
For how long has bike been inactive and during that period, were the carbbowls empty or did they contain fuel? In case of the latter, inspection and cleaning of the slow jets is almost inevitable. On a CB500 you can leave the carbs where they are and just remove the floatbowls. Slow jets can then be removed by sliding a  small screwdriver sideways into the slit to loosen them a bit. From there your fingers can unscrew them. Carb #1 can be a bit difficult because of the nearby clutch cable.
But I'd first begin with the ignition.
When you have the engine running, have a close look at the sparkplugcaps and see if you can detect any intermittent (!) arcing from the cap or its wire to the head. This is best seen in the dark.
Does the advancer move freely? Do you have a stoboscopic timinglight to verify a good functioning of the advancer and to check the timing?
The thumb screw is #58. You can reach it from the right side under the tank an adjust the general idle rpm with it.
#42 Are the airscrews. Don't touch them! At least not now!
#73 are the slow jets aka idle jets. They may need cleaning. May. First check and doublecheck the ignition.

[Siloxious]

Thanks for the answer!

1) The bike (apparently) stood still for 2.5 years.

2) The carb bowls were dirty and there was a lot of gunk inside, so I would think they contained fuel.

3) I have already rebuilt the carbs by the instructions of Classic Octane a year ago. I've used a kit, so the jets are new. This is the video I followed: https://www.youtube.com/watch?v=JFTOfYl5BSc&ab_channel=ClassicOctane.

4) Half a year ago, I was ready to start the bike. I used a secondary fuel tank and no exhaust. I had a little accident with some leaked fuel :(. Nothing broke but I definitely learned that I should work safer. I then decided to first fully restore the fuel tank and fit the exhausts, so there would be a small chance of fuel leaking.

5) Now, with a clean fuel tank and exhausts, I started the bike again and it worked. Because I tried starting it half a year ago, some fuel ran through the carbs. So, it could be possible the jets are clogged. Would you suggest that I clean the bowls again?

6) The ignition advancer moves freely. When I noticed the cam was turned 180 degrees wrong, I decided to clean the advancer while I had to take it apart. I've lubricated it sufficiently.

7) Tomorrow, I can work on the bike. I'll check for that ark that you're talking about. I sadly don't have a stoboscopic timinglight, so I'll have to do with a static timing. I'll check the static timing tomorrow (I use a multimeter to test it, since I don't have a static timing light).

8) I've not touched the thumb screw in any way since I've bought the bike. So I guess I'll try it tomorrow.

[Bryanj]

Get the original brass ultrasonic cleaned and refit as the kit parts are notorious for not being to Honda spec especially the needles and needle jets.
If it has points use a points file to clear oxidation and a SMALL amount of lube on the pivots

[deltarider]

[...]This is the video I followed: https://www.youtube.com/watch?v=JFTOfYl5BSc&ab_channel=ClassicOctane.

The best on the subject is Motormac. On Youtube: Motormac Honda CB500 four Vergaser. You don't have to see all of them. Language is German. But he's the best.
[...]

Would you suggest that I clean the bowls again?

Personally I'd refit the original parts. Mine ran 140.000 km and still don't wear.

I've not touched the thumb screw in any way since I've bought the bike. So I guess I'll try it tomorrow.

That should be the first step. Who kniws? Aim at an idle of 1150-1200 rpm.

[Oddjob]

The Honda recommended idle is 950-1050rpm for the 500, check the workshop manual if you are in doubt. If the engine is in fettle, as in good condition, it should quite happily tickover at that speed. Increasing the idle speed is like turning the radio up in a car to disguise engine noise. You really want to hear the noises, not mask them, if you can hear it you can work out wants wearing or what's going on inside the engine. An engine is top notch condition will tickover at less than 600rpm with little noise.

The thumb screw or number 58 lifts ALL the throttle slides on the carbs, if it's too low the engine will NOT tickover, almost an instant stall.

As Bryan says, put the original jets/needles etc back in, aftermarket jets are quite frankly awful, poorly made, lack of precision etc. Especially the float seat parts, they leak for fun as a rule.

[deltarider]

The Honda recommended idle is 950-1050rpm for the 500, check the workshop manual if you are in doubt. If the engine is in fettle, as in good condition, it should quite happily tickover at that speed. Increasing the idle speed is like turning the radio up in a car to disguise engine noise. You really want to hear the noises, not mask them, if you can hear it you can work out wants wearing or what's going on inside the engine. An engine is top notch condition will tickover at less than 600rpm with little noise.

Oddjob, with all due respect, what you suggest, is for much, much later.
Let's stay on the safe side and first have it idle at around 1100 rpm.
Oh and I'd welcome a video where a CB500 will tickover at less than 600 rpm. When I had everything top notch, I managed to get at an acceptable idle of 800 rpm. A Honda mec persuaded me to have it idle at around 1100 rpm. Allthough he didn't explain in depth, he said camshafts wear most at idle.

[Oddjob]

Mine used to tickover quite easily at 600rpm DR, it would go as low as 400-500 before the engine started to knock a little, at no point did the oil light flicker.

Once it's rebuilt I'll gladly take another vid of the engine idling at very low speeds, it's just the sign of a really good engine IMO.

I'd agree, set a faster idle until it's actually running right, then lower it until it runs smoothly without too much noise.

[deltarider]

Looking fprward to it.

[philward]

I can get my 500 tickover down to 500 rpm - assuming the rev counter is accurate!

[Oddjob]

Sounds like your engine is pretty well setup Phil. You need good compression, accurate timing and really well set up carbs to achieve it in my experience. Mine had just been completely refreshed inside, new rings but not a rebore, new bearings and a bank of carbs that were as clean as a whistle. Timing was set using a dwell meter and a good strobe. It was exceptionally quiet. Started it up once at Matlock Bath and I could hear people in the crowd around the bike asking if it was running and they were only 6-7 ft away.

[Johnny4428]

I’m just happy if it starts and gets me out and back without too much fuss!😂 I must say I’ve never heard these old bikes idling at 500 but it sounds impressive to get them tweaked to perform to that level.

[McCabe-Thiele (Ted)]

I like an idle speed of around 1100 when warm on an old Honda.

At under 700 rpm they never sound happy imho.