Need carb help please

[Barbarosa]

I just recently had the opposite problem...

Have stock mikuni jets but I could not make mine run richer!
I tried raising the needles, playing around with float levels, turning mixture screws out as far as possible. The solution was bigger main and pilot jets, actually 4 sizes bigger on the main and one size bigger on the pilot. Then I was able to effectively adjust the fuel air mixture with colortune.

Soooo, my point being, maybe you just need to go with smaller jet sizes? who knows what the PO did inside those carbs! Just a thought

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Good point. Going to order some 105 mains and try that. It's got to be getting close to right because when it has fresh plugs it purrs like a kitten and pulls like a freight train.

[3Phase]

I think you've done everything right and it's still not working. I'll have to think about it while I put my bike back together, maybe something will bust loose and roll out into the light where I can see it, then use it or stomp on it.

Are you sure the Intake valve clearances are set correctly? With tight valves it'll go like stink when it's cold, when it warms up the valves can't close and it'll start dropping cylinders and eventually burn the valves.

[dbeardslee]

Might be worth a look just to make sure you're not dealing with a set of bastardized carbs. Seen 'em before without the rubber plug installed over the pilot tower and that will make 'em run real rich. JAT

[Barbarosa]

I think you've done everything right and it's still not working. I'll have to think about it while I put my bike back together, maybe something will bust loose and roll out into the light where I can see it, then use it or stomp on it.

Are you sure the Intake valve clearances are set correctly? With tight valves it'll go like stink when it's cold, when it warms up the valves can't close and it'll start dropping cylinders and eventually burn the valves.

Checked the valves back in May and the only one even close to being out of spec was #3 exhaust so I swapped the shim. I understand incomplete combustion from a leaky cylinder and that's why I recently did a leak down. Ran 60 psi to the cylinders and they all held at 58 so I assume no major leaking. The valves might have went through some kind of "run in" and tightened up since I last checked them lol. If the 105 mains don't help I will check the clearances again though.

[bikerphil]

I would make sure the pilot jets are the BS30/96 type and not the VM type which have the wrong spray pattern for out carbs. Scroll down this page and check the diagram...

https://www.mikesxs.net/products-39.html#products

[Barbarosa]

Might be worth a look just to make sure you're not dealing with a set of bastardized carbs. Seen 'em before without the rubber plug installed over the pilot tower and that will make 'em run real rich. JAT

No passage between the towers on this set. Since no plugs will run rich, if I did install a set would these lean out lol? Just kidding, but this is not as fun as it used to be.

[mack]

Timing

Hows the timing?

[mack]

advances

Next thing I'd be checking are the advances working properly. You say your pushing it up to 7K between lights. If the advances aren't functioning properly you won't get an early spark to burn all the fuel. Check your governor(centrifical) advance and clean lube as required. When you checked your timing did you check the function of the vacuum pot and that the mechanism rotates freely. That bearing can get sticky over time!
I know 3 phase is following this and I know he has some very good pictures on cleaning/servicing that unit.

[Barbarosa]

I think you've done everything right and it's still not working. I'll have to think about it while I put my bike back together, maybe something will bust loose and roll out into the light where I can see it, then use it or stomp on it.

Are you sure the Intake valve clearances are set correctly? With tight valves it'll go like stink when it's cold, when it warms up the valves can't close and it'll start dropping cylinders and eventually burn the valves.

How about this? I pulled the 185 pilot air jets from 1 & 4 and was able to poke a 2mm drill bit through them with ample wiggle room. Got out the calipers and measured the bit just to double check it and read .199mm. I’m guessing someone got really aggressive during cleaning lol. Anyway, would that much oversize cause the rich condition? Haven’t tried checking the main air jets because carbs are still on the bike, but I sure hope they aren’t oversized too.

[3Phase]

I believe Mikuni jet sizes are based on flow, not the diameter of the hole. That said, if the air jet was drilled out oversize that would make it lean, not rich. I was thinking more along the line of someone sticking in weird-Harold jets and trying to drill them to a #185. As you have seen that would be to small and would run rich.

As Mack mentioned, a sticking/broken mechanical advance or vacuum advance would mess up ignition timing and over time and distance it could foul the spark plugs. Stranger things have happened but that shouldn't cause dropped cylinders or be temperature dependent unless someone used heavy bearing grease, then it would start to work better as it warmed up.

Oh, another wild guess but the vacuum advance hose is supposed to be connected to a vacuum fitting on the #2 carburetor, not one of the intake manifolds.

Does the vacuum advance work? Does the vacuum pot leak?

Is the advance plate sticking? With the vacuum advance pot arm unclipped and removed from the advance plate, the plate and its bearing should rotate smoothly and easily like a Lazy-Susan.


That's all for now, the daily stupidity is over for today and it's time to get back to fixing my bike.

[Barbarosa]

I believe Mikuni jet sizes are based on flow, not the diameter of the hole. That said, if the air jet was drilled out oversize that would make it lean, not rich. I was thinking more along the line of someone sticking in weird-Harold jets and trying to drill them to a #185. As you have seen that would be to small and would run rich.

As Mack mentioned, a sticking/broken mechanical advance or vacuum advance would mess up ignition timing and over time and distance it could foul the spark plugs. Stranger things have happened but that shouldn't cause dropped cylinders or be temperature dependent unless someone used heavy bearing grease, then it would start to work better as it warmed up.

Oh, another wild guess but the vacuum advance hose is supposed to be connected to a vacuum fitting on the #2 carburetor, not one of the intake manifolds.



Does the vacuum advance work? Does the vacuum pot leak?

Is the advance plate sticking? With the vacuum advance pot arm unclipped and removed from the advance plate, the plate and its bearing should rotate smoothly and easily like a Lazy-Susan.


That's all for now, the daily stupidity is over for today and it's time to get back to fixing my bike.

As per Mack and the timing the advance moved when I put the timing light on it. And the vac line is to the carb. With fresh plugs it snaps to really well when cracking the throttle. Have a set of 185 jets on the way even if it don't make sense since nothing else does yet anyway lol.

[Bonz]

To make a long story short, a to low a fuel level(which is your carbs case) does, and will create a rich running cond.. Re-set those floats at a spot-on 23mm, warm up, and with a cooling box fan in front of motor, correctly re-set idle mixture screws and re-sync carbs.

Hold on... A lower fuel level in the bowls is a leaner condition. Keep in mind float height and fuel level are opposite. The higher the float height the lower the fuel level. The lower the float height the higher the fuel level.

Maybe you were meaning a lower float height causes rich and you said "fuel level" instead of saying float height.

[skids]

Hold on... A lower fuel level in the bowls is a leaner condition. Keep in mind float height and fuel level are opposite. The higher the float height the lower the fuel level. The lower the float height the higher the fuel level.

Maybe you were meaning a lower float height causes rich and you said "fuel level" instead of saying float height.

Yeah, I was scratching my head with his comment also, but assumed it was semantics...

[skids]

Don't forget about the pressed-in main air jet. It is a pretty small hole that can get plugged. I didn't read through all of the posts again, but if you are using pods and have older carbs, you can't plug the breather hoses - you must let the vent hoses breathe...

[motoman]

Hold on... A lower fuel level in the bowls is a leaner condition. Keep in mind float height and fuel level are opposite. The higher the float height the lower the fuel level. The lower the float height the higher the fuel level.

Maybe you were meaning a lower float height causes rich and you said "fuel level" instead of saying float height.

Got cha' on that one Bonz.........BTW, having proof that if you remember correctly were litterly involved in two Colo. rallies ago.
Correct in your fuel flow interpentation, but unfortunately levels and venturi velocity affect fuel flow by same principle as full or half glass of water with straw submerged and blowing across top of straw. That routine with a full glass of water gets way more control of flow and volume of flow.........now try same stunt with a half glass of water......when it rises over top of straw, it comes all at once.