Poor sparks on front cylinders

[phildu31]

Yes, that's exactly what I meant when I suggested to put fuel on PRI and check that the bowl fuel level doesn't move.
You're lucky in the way that these o-ring are often worn although it can't be seen.
These seals are in needle valve sets.

[steveho]

Does anybody have the size of these O-rings so I can see if I can get them locally?

Ditto this, it would be handy to know the sizes......

[MK]

Any jet without marking will go directly into the scrap bin.
It's extremely likely, that THIS was one of the main problems (together with the float height).
Aftermarket jets are known to have different flow compared to OEM.
As you haven't explicitly written it : The new jets carry a Mikuni stamp AND a 22.5?
Anything that has a number but no Mikuni sign: dump it and get new ones from Topham.

A leaking o-ring at the float valve will bypass that valve. As the valve is adjusting the fuel level in the float bowl, bypassing it means, your fuel level is likely too high in operation.
And a leaking o-ring could overflow the fuel into the cylinder / exhaust pipes which you also observed.

The tip with setting the fuel petcock to PRI would emulate a running engine and would likely make the level even higher.

[MadDogRoger]

Hi MK,

To answer your question: no the new jets don't carry the Mikuni stamp nor the 22.5.
Some people also PM'd me so now I have a couple of options to get new O-rings (original, good ones).
So there is light at the end of the tunnel. Will try to get a measurement on the old O-rings tomorrow and see where to order.

Greetz,
Roger

[silverstrom]

Replacing just the o-ring does not offer any guarantee that you won't have more problems with the needle valves. At this point you should consider replacing the complete needle valve assembly (needle and seat). That will give you 100% confidence for years and eliminate any future problems. Replace all 4 and be done with it.

[craig_johns469]

Replacing just the o-ring does not offer any guarantee that you won't have more problems with the needle valves. At this point you should consider replacing the complete needle valve assembly (needle and seat). That will give you 100% confidence for years and eliminate any future problems. Replace all 4 and be done with it.

Agreed. I bought aftermarket rebuild kits of which I replaced some parts but had issues, mainly due to the float valve assemblies including the o-ring issues you're seeing. I was hesitant in buying genuine due to the cost but haven't looked back since. I've also replaced the fuel tap with a new one after finally getting the mod to seal, but I can now sleep knowing I won't have a puddle of fuel on the floor when I next go in the workshop.

[MK]

To answer your question: no the new jets don't carry the Mikuni stamp nor the 22.5.

So how do you determine that this is a 22.5 jet according to Mikuni specification?
I can't stress it often enough. Throw these away as far as you can and get new ones from Mikuni.
Topham ships fast and one of those idle jets costs around 7 Euro, an idle air jet maybe around 5 Euro.
So we're talking about some 50 Eur for 4 x 22.5 idle jets and 4 x 1.3mm idle air jets to fix most low range fueling issues once and for all.

If you still say it's not worth it, I whish you good luck in further testing.

[MadDogRoger]

Hi all,

It has been a couple of rain filled days since the last update. I ordered original Mikuni parts for the carbs: air jets in size 1.3 and complete float valves, consisting of seats, needles and O-rings.
Mounted everything today so the carbs are now back to original specs/parts, apart from the size air jets (now 1.3 iso 1.1).
Set all the float heights to 24mm, made a small jig for that from metal plate, because measuring is near to impossible.
Put everything back together again. Just as a precaution put a couple of bowls under the overflow tubes and then primed the carbs.
Goddamn, left bottom carb starts overflowing. After a little tap with the back of a hammer (that's the wooden handle), it stopped.
Start the bike. It starts right up and runs idle a bit higher then previous. Left bottom carbs starts overflowing again, so tapped it again. It stopped.
However there's no change, whatsoever. Lower two cylinders are not working as they should. The only fire occasionally. The pipes remain too cold, exhaust gases feel also cold.
Took out the spark plugs again to clean and dry (they were new plugs), but no change.
So now after spending €150 on parts and a weeks work, what do we learn:
- It's not an electrical problem: CDI was switched, ignition coil replaced, every other electrical component bypassed.
- The battered O-rings in the picture are NOT the problem. The new O-rings from Mikuni were the right size and smaller than the O-rings from the no name revision kit. Mounting these latter ones (since they are too big 1.5mm to 1.3), causes them to be "scraped" when mounted. They did however seal. Also the no name needles worked. I'm still in favor of mounting the original ones but this isn't the problem.
- Fitting original Mikuni parts didn't stop the left bottom carb from overflowing.

I didn't yet check the float height, because after the disappointing result I was a bit fed up. I mean I set them all 4 to 24mm and they're all the same but I didn't check with the overflow tubes to see what level is actually in the float.

There are not a lot of things left to check. I don't believe that the two lower cylinders are in need of overhauling because they have the same compression as the other two that are running fine.
Only thing I can think of is a leak test.

Greetz,
Roger

[MK]

How about focusing on the important things first before opening other battlefields?

You didn't mention, if you bought original Mikuni 22.5 idle jets. So I assume you're still using the parts of unknown size?

And if you're not using stock needles the same applies.
They have a diameter and taper that has to be very precise. The straight part at the top adds to low end partial throttle behavior.
Around 5/100mm difference in diameter equals one size of needle jet (like going from N-8 to O-0).

Last but not least overflowing is something that would enrich the affected cylinder if it happened during riding.
So this is something you also should take care of.
Have a look at the actuation lid on the float part. Often that location is worn and carries traces of the float valve pin.
If that's the case, grind it flat and polish.
Secondly there's a second lid preventing the float going completely down when open. Adjust that lid that it allows the float to open but not going down too far.
This prevents it getting stuck in the "low" position.

Some people recommend polishing the inner surface of the brass float valve with a q-tip and an electric drill to make the aluminum valve slide easier in the brass cylinder surface.

[phildu31]

You must first of all fix the overflow. Not the hardest.
For example, swap parts between two carbs and see what happens.
Also check the float itself. Put it in fuel, you could have a surprise ...

Checking the fuel ( rather than float ) height is irrelevant until you have fixed the overflow.

I mentionned idle fuel jets some time ago now in my first post...

[tacky1]

Leaking fuel and having to tap the carbs is a problem. You need to strip the carbs and fix the problem.

[silverstrom]

Are you running from an aux fuel bottle?

Is the bike's tank spotless inside and has no fine particles? Those fine particles will go through the tank screen and into the carb bowls and immediately cause problems.

If you have clear fuel lines you can see where the fuel is going. As I already mentioned, fuel delivery to the lower cylinders can be disrupted by poor fuel line routing.

Poor or restricted fuel flow through the petcock should be eliminate as a cause. Vacuum issues, dirt, age, etc.

If it was mine I'd be running fuel from an aux bottle directly to the carbs, completely bypassing the petcock. That eliminates fuel delivery as a cause for the lower cylinder issue, assuming the fuel line routing is correct.

We are along way from the weak spark issued this started as.

[MadDogRoger]

Hi guys,

"You didn't mention, if you bought original Mikuni 22.5 idle jets. So I assume you're still using the parts of unknown size?"
I did mention but I think you missed it. Here's the whole story. I bought the bike as a non runner in 2014. The guy had had the bike running but drained the tank. It took me 5 years to do the patching up (wouldn't call it a restoration). When I bought the bike the guy had already bought carb revision sets for it. I thought these were original Yamaha. Turns out they weren't. So when I did the rebuild of the carbs, I used these sets effectively replacing original Mikuni parts with aftermarket parts. Now with all the problems and testing done this is the status.
- old idle jets are back in the carbs, so these are original Mikuni, 22.5;
- new pilot air jets are in the carbs, size 1.3 original Mikuni;
- new float valve seats, needles and O-rings, original size 2.8, original Mikuni;
Still in the carbs from the aftermarket set:
- main jets, size 165;
- needles.
I still have the original ones. There's some gumming on the needles but I reckon this would polish right off.
Also still in the carbs are the original floats. They were tested in petrol during the rebuild because they looked a bit scruffy but proved leak proof. They were however bent all over the place. It took a gentle hand at tapping the metal straight again. When I had them back straight also the little lid you mentioned became apparent. Before this they were so badly bent that the little lid didn't work at all so the float would "sink" deeper than it should and it would get stuck.

I've heard about polishing the brass floats but since they are new, original from Mikuni with needles, I don't want to do that.

BTW where do you have the cir clip on the needle, currently I have them in the third groove from the top, so in the middle.

"Is the bike's tank spotless inside and has no fine particles?"
Well, after I had the bike finished (repainted and all), the tank decided to leak at the base at the front (where you're nuts are so to speak). I reckon this was due to the addition of Ethanol to the fuel and the RD not being ridden daily. Allowing the ethanol to attract water at the lowest point and start to rust. So the freshly painted tank would now have freshly bubbled paint, as fuel is a paint stripper. I solved this by coating the inside of the tank with an epoxy coating and respraying the tank. So yes the tank is spotless now. I had the carbs running with a fuel filter on both lines for over half a year. Couldn't see any dirt or debris in the filter and since the filters were actually too big to be mounted (they were also kind of in the way/sticking out) I removed them. I filtered the fuel from the carbs when I drained them to work on them and it was clean. In the float bowls there were only some very small and very few particles.

About the fuel lines and the routing. I replaced all the fuel lines when I did the rebuild. I routed all the new lines exactly the same as the old ones I took out. Of course if someone had changed them and they were not original, I have no way of knowing. They looked pretty original to me.
Does it help when I say that I had the fuel tank connected the whole time I worked on the carbs, also when I pulled out the float seats and there wasn't any fuel leakage? Doesn't that mean the fuel tap is OK?

I can start by swapping some of the internals of the carbs between front and rear carbs, see if there is a component that takes the problem with it.

You see, again if I think about this logically: I did a rebuild of the carbs, used non original parts, switched back some of the parts, still have problems. Maybe I'm the problem and I don't know what I'm doing. In that case it doesn't make sense to do it all over again since the result will still be bad.

"We are along way from the weak spark issued this started as."
Yes we certainly are and I don't feel like being close to the finish line

Greetz,
Roger

[MadDogRoger]

Hi all,

Had a good night's sleep so woke up with a clear head. Thinking about the overflowing carbs, there is only one logical explanation: the floats.
There's the petcock, if that were the problem, all four carbs would suffer.
Float valve seats and needles looked wrecked (with mangled O-ring), were replaced with original Mikuni's (that slid right into place without hassle), but no change. So it's not the needle in the seat.
Only option left is the float itself.
As I mentioned, the floats were pretty crooked when I opened the carbs for the first time. Obviously someone had had a go at them previously.
I straightened them out as good as I could and they do look straight, however the one thing that is very hard for me to check is the hole for the float axes. The hole consists of two pieces that have to be perfectly aligned. Bending on the float won't exactly help in the aligning department.
It only takes a tiny bit of misalignment to have the float stick. I don't think this can be detected by hand.
So I ordered 4 new floats and float axes today from Topham. If that doesn't solve the overflowing, then I don't know what will.

Once that is fixed I can set about adjusting the float height.

Fingers crossed this works.

Greetz,
Roger

[theredbarron]

Wow, you are having some fun and games with this one. If you were in the UK i would lend you all the parts, carbs, cdi, coils etc. to try and eliminate the possible defective ones you have.