Poor sparks on front cylinders

[MadDogRoger]

"I thought the poor spark issue still existed. Or has that been ruled out?"
The plugs on the front cylinders spark like hell now, after having cleaned, wiped and dried them with compressed air. Look I'm past 50 so not so young any more, I have reading glasses. So standing bent over, holding the spark plug in the cap against a ground point (try not to get jolted) while trying to kick the starter firmly enough to create a spark and keeping the spark plug in focus of my reading glasses is getting quite a challenge. My son is already making fun of me so spare me the jokes. I actually didn't see any sparks until the bike kicked into life and then I saw a lot of sparks. The same applies for both cylinders (yeah had to actually start the bike before I could see the sparks). So the good news is that it isn't an electrical problem.

Today the weather is not so good so I can't take the bike out and run some test but as soon as it's dry I'll have a go with the suggestions you all gave me.
That info about jets is very interesting. Apparently they have a reasonable spread in their process.

[silverstrom]

That is good news. Electrical problems often present as fuel problems and can be very difficult to sort out. Having passed that point you can now focus on what remains, which should be an easier task than electrical, but likely a more complicated solution.

If it was mine I'd start with a compression test on the lower cylinders. Rule that out first because it's such an easy test and tells you so much right away.

I suppose I'd next remove the lower pipes and make sure they aren't the problem due to being plugged or full of fuel and oil. Then move on to the lower carburetors and make sure all ports and jets are clean and the main nozzles are not plugged, that it does have the correct main nozzles and needles, and the needles are clipped in the correct position. If all that checks out then look at doing a leak down test. I prefer to do the carbs before the leak down test because going through the carbs if you have never been inside them, or haven't been for years, is always a good idea.

Ensure you have good fuel flow and the petcock is working properly. Poor fuel line routing can adversely affect the lower cylinders. It has been discussed here before.

[MadDogRoger]

Hi Silverstrom,

I actually already did a compression test a couple of days ago. All cylinders measured 6 bar, all within a couple of tenths of each other.
Today I took of the air filters. Well not the actual air filter under the tank but that thingy bit on the carbs. With them off you have a good view at the carbs. Took out what I think is the pilot air jet?? Why am I not sure? Well on page 5.1 of the workshop manual there is a graph with numbers but I'm sure not all the numbers match up. Since number 8 in the graph is the float, but according to the numbers, number 10 should be float. So in that graph I took out the number 9 screw. Very small screw. On all 4 carbs it was clean with no obstructions. Cleaned it anyhow and put it back.
Start the bike, pretty much the same result. This time the right exhaust was a bit warmer (so cylinder working a bit more), but remember I cleaned the spark plug.
I couldn't seen a noticeable difference between the working of the carbs, but then I doubt that a rich mixture is visible. At least none of the carbs was overflowing or anything.
Then I tried some brake cleaner. Just a little bit in the air intake didn't do anything at idle. I assume it didn't get sucked in.
Spraying a bit more would stall the engine. I would have to kick it 10-15 times before it would start. You could say it was flooded.
Spraying with a little revs (2000-3000) would cause a drop in revs, but it would keep running.
Am I right to assume that this means that the carbs are rich at idle, since adding fuel makes it bog down?

Petcock is working fine, after revision because it was shot when I got the bike.

The next hing I'll do is pull the lower pipes and see if they're full of fuel/oil.
Greetz,
Roger

[phildu31]

You removed the pilot air jet.
Fuel jets are inside the bowl.

[MK]

You removed the pilot air jet.

And you didn't mention what's written on them. 1.1 or 1.4?
Once again: 1.1 would be the rich ones and as you're running bloody rich enlarging them to 1.3mm wouldn't be the worst idea.
If you're filthy rich, Mikuni also sells them new.

[silverstrom]

Hi Silverstrom,

I actually already did a compression test a couple of days ago. All cylinders measured 6 bar, all within a couple of tenths of each other.

6 Bar is only 87 PSI. If that is your actual reading you have bigger problem than you think.

Typically we like to see around 120 PSI in each cylinder. 110 PSI in all cylinders would be good. If you see less than 100 PSI in all 4 cylinders you have a problem.

[MK]

Only if you assume that the gauge is showing the right value and the procedure was carried out the right way.

So another homework: unscrew ALL spark plugs.
Screw in the gauge to one cylinder.
Hold the throttle wide open.
Kick around 20 times as fast as you can.
Repeat 3 times for the other cylinders.

PV in the lower /upper position makes around 0.5 bar difference.
Not going to WOT makes a up to a few bar difference.
Also kicking slower will give a lower reading.

[silverstrom]

Proper compression testing instructions are easy to find. Do your own research and find something that works for you and make sure you understand the basics.

[MadDogRoger]

Hi all,

As some of you suggested I pulled the lower pipes today. Left hand pipe was dry. Right hand pipe tough: I poured about 100cc from that pipe, fuel mixed with oil. That's not good.
About the compression test. I know I didn't do it correctly (no WOT, kicking 5-6 times, other plugs in), it was more of an initial test for me to see if all the cylinders are more or less equal or if the right front has a problem. Them all being the same was a little reassuring. Once in a while I need some dopamine to keep going since it hasn't been going great finding the rootcause.
But thanks for the clear instructions, I will do a proper compression test shortly.

Up next are the carbs. Some of you suggested blocked idle jets. I will have a look at that. But can someone explain how that caused rich running. I mean if it's blocked fuel doesn't come out so wouldn't it be lean and run poorly at idle???

And another question: any hints and tricks for removing or actually putting back the carbs. Last time it took me 3 hours to get the fuel pipes connected since they can only be connected after you put the carbs in place but putting the carbs in place obstructs access to the fuel lines and connectors. I must say that I didn't pull back the tank. Is the clou to pull up the tank and remove air filter which allows you access to the fuel lines? I'm really not looking forward to another 3 hours of messing with pliers, torches, scuffing your hand etc.

I do hope it's a simple thing like a blocked jet.

Greetz,
Roger

[phildu31]

Yes, blocked idle ( fuel ) jet will run lean, that's what I first thought.
But if you run rich, that could also be a worn idle jet ( cleaning with ... a bad tool for example ).
Anyway, all is possible.
I think your fuel hoses are too short. Carbs are easily disconnected without removing fuel hoses ( just remove the elephant ears and unscrew the long screws that tighten the carbs ).
There should be enough free play to work on the carbs without removing hoses nor cables.

[MK]

There is an easy solution, if you're fed up with RD500 carb maintenance.
Sell it, buy a 350 and invest the rest for your pension.

Seriously : I had a RD500 for several years and it always annoyed me how stupidly everything was designed regarding maintenance. On most roads the 350 was more fun, so I still have it, but sold the 500 long ago.

Fun fact : a few years ago a former local Yamaha dealer contacted me. He was retired and a former customer asked him for a maintenance. But he hadn't done one for decades and didn't want to screw it up, so he asked if I could do it.
I knew I'd hate it and doing that job proved me right.

[MadDogRoger]

Hi all,

Did a proper compression test today. All plugs out, WOT, PV closed, kicking like hell 20-odd times. And here are the results of the jury:
Left front cylinder: 6.8 bar or 99 psi
Right front cylinder: 6.4 bar or 94 psi
Left rear cylinder: 7.0 bar or 101 psi
Right rear cylinder: 7.0 bar or 102 psi.

Pilot air jets are 1.1mm, that is 1.0 drill fits and 1.2 doesn't (don't have a 1.1 drill). There's nothing written on the jets themselves.
I had a look at the old internals of the carbs that were replaced when I revised them. They are as follows:
idle jet: 22.5
Main jet: 165
Needle jet: 475 with 0-0 after that
Float jet: 2.8
Needle: 5014 with circlip in third groove from the top (so in the middle).
I assume the new parts were the same size. I didn't take any notes but I can't remember them being different. So that should be in there now.

I also checked the float level. First putting the bike on a paddock stand so it stands upright (not on the side stand).
Drain hose up and opening the drain screws. I can't hold a meter next to it but it is clear to see that the upper cylinder are lower than the lower cylinders. These lower cylinders are too high.
I hope this works with the images:





BTW an RD350 was my first bike, so I know what's that like. A lot easier to work on, ample space

To be continued.

Greetz,
Roger

[phildu31]

Yes, good point : no one float level is good.
Another check you must do is : put fuel on PRI and check that your levels don't move overnight, that will tell you how your float valves are. There is a oring seal inside which is the cause of overflow when worn.

[MK]

Your compression values are on the low side.
This changes the fuel demand of the engine and using the nominal values may not match your engine.

Nevertheless: If the air jets were 1.1 mm, you have a cheap and easily reversible way to test what your engine likes. In my case I ended up with 1.3mm (which is a tad richer than the latest models have 1.4mm)
Get a 1.3mm drill and change the bore.
If it fails: The jets are available at https://www.mikuni-topham.de/

As already mentioned all your fuel levels are off spec. The uppers are too low (= cylinder will run leaner), the lower ones are too high (= cylinder will run richer).

If it's still rich after trying/fixing the above things, you may change the needle position from 3 to 2 (higher clip position = needle gets a bit deeper into the needle jet)

[MadDogRoger]

Hi all,

Finally got my nerves up to taking off the carbs. The good news is it isn't the pilot jet, they were fine.
Although I changed the ones that were in there now for the old ones that came out when I revised the carbs.
Because the finish on the old ones is much better:

New one on the left and old one on the right. The old ones has neat chamfering on the holes which the new one doesn't have.
The old ones looked good, no scratches or anything. The old ones have size indications, the new ones don't although they look the same size.

Now then what is the problem you ask. Well maybe it's this:

Take a look at the O-ring on the float valve seat. It looks decapitated. I can imagine that this is not sealing properly and causing running rich.
So this has to be fixed first.
Does anybody have the size of these O-rings so I can see if I can get them locally?

Oh BTW, this isn't the only carb on which the O-ring looks like this, there are 2 more.
So first fix this, reset float height and see if this cures the problem.

Greetz,
Roger