CB 500 Four - engine problems

[Siloxious]

Hi everyone! I'm back here again as I'm a bit lost.

At the suggestion of one of you, I have cleaned my carbs and assembled them according to Motormac. I've really (really) cleaned them well.
My original problem was that the engine cut of in idle and generally didn't idle well. Also, the engine made a ticking noise. When I touched the exhaust pipes, I felt that all cylinders were hot, but cylinder 4 was I bit colder than the other three. So I thought that this ticking noise could be due to the cylinder not firing properly due to a clogged idle circuit. You guys thought the same, and thus I cleaned the carbs.

I hoped my problems would be over after the reassembly of the carbs. I have put in new brass jets. The engine idles a bit better, but there are still three problems.

1) The exhaust from cylinder 4 is still a bit colder than the other three. This could be connected to problem 2. Because I have really cleaned the carbs well, I would be surprised this is still a carb problem. Although, I don't exclude the carbs completely.

2) The ticking noise is still present in the engine, it is especially noticable at low rpm. In a video from a couple of months ago, it can be observed. <https://youtube.com/shorts/NGVH7g6JEZg?si=RLU1ZoAWtI8Ji8E2>

3) When I open the choke after starting, the revs shoot up to the 6000 rpm range. I suppose this could be a carb problem.

Does anyone have an idea where these problems come from? I have already checked the ignition timing and valve clearances. Can anyone give me suggestions to what it is? (I'm really hoping something isn't loose in cylinder 4, could this be the case?)

Thanks in advance!

[McCabe-Thiele (Ted)]

I have listened to the sound on your video those noises are not good as you know - you mentioned timing the engine statically - did you do this for both the 1/4 then the 2/3 timing?

Have you bench syncronised the carbs or have you balanced them with vacuum gauges as well?
I know my 400 ran much smoother after I had the timing the same for 1/4 & 2/3 using a strobe to set them up individually.

I think the engines run much quiter when all four cylinders are balanced and firing correctly both in terms of spark delivery as well as fueling.

Have you done a compression check to eliminate any possibility that one cylinder is out of range compared to the other three?

[Bryanj]

Cant watch videos so here is my 2 penorth
Points gap at 14 thou and timing spot on the F, it takes a lot of time, patience and practice to get BOTH spot on.
Sounds like the carbs need syncing, there is arie up of mine on here somewhere
Possibly tight, or loose tappet clearance.

You say you have cleaned the carbs, have you split them and had them ultrasonicaly cleaned?

[Siloxious]

Thanks for your answers!

Yes, I have split the carbs and cleaned them ultrasonically.
The ignition timing has been set statically. I have not yet done a dynamic sync (of both the carbs and ignition) as these tools are quite expensive to only use one time. But maybe they are necessary…

To be concrete: a dynamic sync with vacuum gauges of the carbs is to ensure that the carbs are in sync. A compression test of the cylinders is to see if there are problems with the engine? Does a problem with a carb have influence on the compression test?

[McCabe-Thiele (Ted)]

Compression test is done with the throttles fully open and all spark  plugs removed so not affected even if all the carb jets were blocked.

When you did the static timing that is a good starting point did you check both sets of contact breakers not just 1/4 ?

[davidcumbria]

I wonder if the rattling noise is due to the well documented issue of wear in rocker shaft bores of the valve cover due to rotation  of the shafts. This will produce inconsistent valve clearances and can be seen looking at the end of the rocker shafts. This can easily be checked by removing the small end covers in the first instance. Later bikes had pegged shafts to stop the rotation and I have a spare such cover.

I solved my cool number 1 cylinder by reducing the pilot jet to 35.

 Another possibility for the noise is related to the cam chain tensioner. The blade on mine had eventually broken off and was found in the sump.

[K2-K6]

Listening to audio, through headphones, and it sounds like one or more cylinder isn't running at idle, it doesn't start as you open the throttle either.

What this usually causes is to adjust the rpm at idle too high in keeping it running, then if tge cold cylinder starts the rpm simple goes to tge higher position from having all running.

From characteristics shown, there's intermittent running of some cylinder which will need the cause identified to progress.

[K2-K6]

Diagnosis, the need to seperate electric from fuel is priority in helping to solve this.

Start from cold, run  20 sec then switch off. Check exhaust for cylinder temperature to find which are cold. Swap a "hot" plug with a cold one, then repeat to see if the problem transfer across between those two.

If it stays in the same place (the fault) then you can swap plug leads over. If fault is on #1 then swap the plug lead over with #4 to, again, see if the fault moves. Same if it's one of the two middle cylinder 2 & 3 that show fault.

If you've two cold and consistent with 1 coil of ignition, e.g. both 1 & 4 then you're ordinarily looking at that whole set of ignition. The points, condenser, leads and coil &  caps for that set.

If absolutely none of this has any effect, then it's likely in the fuel system.

[Charliecharlcomb]

I had a misfiring problem that wouldnt go away and after cleaning the carbs and replacing the brass, I realised that the needles need to be changed as well as the replacements are subtly different. I went back to my original brass and needles and after all the cleaning and balancing it runs beautifully again.

[Siloxious]

Hello everyone!

After finding some time this summer, I've done a compression test of the engine and found out that cylinder 4 does not have any compression at all... I've since rebuilt the top end of the engine and placed new gaskets on everything. Eventually, I found out that the exhaust valve of cylinder 4 was bent, so the exhaust valve was open all the time (hence the no compression). This explains the ticking noise as the valve would not move that well. I've fit a new (straight) valve, did valve grinding and all the works for a top end rebuild.

Some problems I still encounter:

Immediately after rebuilding (so before placing the gas tank on, but after rotating the engine a couple of times), I have done another compression test.
The values were really dissapointing. All of them were around 90 psi (but now cylinder 4 has compression hooray!). Is it normal to have such a low compression immediately after a rebuild?

After this, I've ran the engine. It started right up, but again, it would not idle as well. With the choke in the closed position (lever up), the idle rpm is 'okay' at around 1500 rpm. But when I open the choke (lever down), the rpm shoots up to 3000-4000-5000 rpm. Shouldn't the rpm drop as the fuel air mixture has more air in it now? I would think that the rpm would be lower when the chokes are open, but I could be wrong. What would be the facts here? I will also do another compression test now that the engine has run for a few minutes.

Additionally, there is an oil leak at the right side cover of the cylinder head. It is because of a stripped bolt and the side cover does not stay on very well. I will place a helicoil insert to fix the thread.

Would anyone know the answer to my questions?
1) Is it normal to have low compression immediately after rebuild?
2) When opening the choke, shouldn't the rpm drop? What could cause this?

Thanks in advance!

[McCabe-Thiele (Ted)]

The only question I feel qualified to answer is that  the compression  test should be good right away.

Have you tried squirting some oil down the plug hole then test immediately afterwards. If you then get 140 psi or more that points to pistons. Can't remember if you have gone deeper than the top end. Low compression tends to give poor starting from cold particularly overnight, then once it's fired it will re-start immediately for the next few hours.

[Bryanj]

Is this a 500K3 with the godawfull PD carbs?

[Seabeowner]

Don't think so Bryan as he mentions choke lever.
Did you do the compression test with the choke flap open and the throttle wide open (wot)?
Has this compression tester given good results on motorcycles in the past?

[Siloxious]

I believe they are the 069A carbs from Keihin.

The compression tester is good, it measured my last compression readings quite okay without problems.

The choke flap was in the open position and the throttle was fully open. I’ll do the compression test again, now that the engine has run for some minutes. I’ll post my findings here. 

[Murf]

Assuming all the valves and seats are reground and good then lack of compression would be due to worn piston rings or bores or both. Was there much bore wear, indicated by ridge at the top of the bore.
Did you change the piston rings, if you did, did you hone the bores to remove the glaze and give a cross hatch to allow rings to bed in.
After a ring change or rebore it can take a bit of running for the rings to bed in and give best compression readings.

Does the cam show signs of wear on any of the lobes, if so is the wear mostly on one side of the cam lobe.
Did you check the rocker shafts in the rocker cover for any up and down movement. These shafts do wear their mounting points oval, as already mentioned, it’s a problem and I doubt that it is possible to get the engine to run well once this has happened.  Cam wear on one side of the cam is a good indication that the rocket shaft mountings have worn on one side and the shafts and rocker are at an angel to the cam face.


Did you use good quality replacement carb bits, there is a lot of cheap useless crap on sale.  It’s often far better to clean up the original jets etc.  Same with spark plugs lots of fake ones around, use a trusted supplier for them, not online auction sites.
Good luck