Carbs at idle: p*ssing fuel?

[Loco]

Hi all, two questions, I suppose.
This is on a 62T superjet bored to 760 with a fair few mods: b pipe, ignition, carbon reeds, spacer, etc. Carb setup is 2.0 135 75, shiny spring. Carbs have each had full rebuilds within 2 months, and not much use since. Standard 38mm Mikuni carbs.

1) I've been trying to get a rough ballpark tune on the low-speed adjusters before taking it out and tuning under load. I believe you can achieve this by setting them to the stock settings (top adjuster out 1.5, and the lower out 1.25 turns), then dial in the lowers until it leans out and there's an increase in RPM, then open out a 1/4 turn and that's a good starting position...

However, I screwed both in gradually, and heard no difference, even when they were both fully in! Then I took a close look what was going on in the carbs and this brings up point 2:

2) It looks like there's loads of fuel going in there at idle, in that the fuel is dripping off the carbs into it. I only had my poopty phone cam with me, but I think you can make it out here. I don't think that's right, and am thinking maybe the pulse feed is too strong, so it's running super-rich or something, which would explain the above. Video:

When you increase the revs, it does then start to work properly, with the fuel diffusing as a gas rather than pissing in there as a fluid, but to me it doesn't seem right.

Thanks in advance.

 

[Dartman]

did you use popoff springs from the kit or reuse the stock oem ones?

 

[Harbor]

^^^^, and how's the needle and seat ? Particularly the needle tip, and the arm, Bent any ?

 

[Loco]

I used springs I specifically bought, rather than those from the rebuild kit.

The needles seem dead straight. Maybe I need a popoff pressure gauge and to measure it.

That fuel going in the carbs looks wrong though, doesn't it?!

Cheers

 

[john zigler]

Your needle and seats are "dribbling." This is from either bad leaking needle and seat, or too low pop off.

 

[Loco]

OK, sounds exactly like the 'fuel dripping from the inner venturi', second from bottom, here: http://www.factorypipe.com/t_carbtuning.php

I'll try replacing the needle & seat in case it's worn out,and failing that, I've got the shiny springs, so I'll go for the next stiffer - the black - and see if this resolves it.

If I've gone for a stiffer spring, do I need to make any changes to the needle, etc?

Cheers

 

[Little_Franny]

I am gonna say your popoff is too low, my suggestion is 75 low with adjuster 3/4 open, 135 high with adjuster 1 turn open, 1.5 needle and seat with a 115g spring. also make sure your needle is actually sealing on your seat. Popoff tester woudl be very handy.

 

[as22js550]

Make sure your matching your spring to your needle and seat. a 2.0 needle will need a stiffer spring than a 1.5.

 

[Fro Diesel]

A popoff tester is the only way to make sure your carbs are set the same. You cant just put in needles and seats and springs until you think its right, get a popoff tester. Sometimes right out of the package the same color spring will achieve drastically different popoff in each carb.

 

[Loco]

A popoff tester is the only way to make sure your carbs are set the same. You cant just put in needles and seats and springs until you think its right, get a popoff tester. Sometimes right out of the package the same color spring will achieve drastically different popoff in each carb.

Very much seems that way:
Not got my hands on a pop-off gauge just yet, but I have replaced the needle and seats in both carbs with new 2.0 units (rubber in old needles had a ring worn in them).

I went to a shiny 65gm spring and tried it out. Front carb wasn't dripping, but the rear was still dripping as it did before.

Went up to the next weight of spring - 80gm - and still the same outcome. Didn't have time to go up to 95, but seems very odd indeed that the front is OK, but the rear is still dripping badly.

I have rather stiff uprated engine mounts, so the engine is subject to quite a lot of vibration, especially at idle.

Any suggestions very much welcome!! I guess I can't do much until I get a pop-off gauge.

If you have different pop-offs achieved in each carb, presumably you could use different springs and bend the hinge until they are equal?

 

[Loco]

Another thought: previous owner has got rid of the dual fual feeds, i.e. main & reserve: there couldn't be some issue with too high a fuel pressure because of this?. Clutching at straws now!!

 

[Fro Diesel]

Yes, bend the arm a tiny bit to acheive the correct popoff....once you have the tester.

Can you post a few pics so we can see what you are seeing

 

[Loco]

What sort of pop-off should I be aiming for?

Thanks

 

[Little_Franny]

What sort of pop-off should I be aiming for?

Thanks

I have really good luck with a very high pop off, like 50-55 with oem 38's on engines without porting. I have used the specs in my previous post as a really good starting point for about half a dozen skis

 

[Loco]

OK, update:

Went up to the 90g springs, and was getting about 26-28psi pop-off. Pulled the carbs apart a couple of times and went through them, with no obvious issues.

Still getting carb dribble at idle, though this time from the front carb, and there's a lot less of it. The dribble is from the main jet venturi, each time.

I think we're going to test with 115g springs, and might have to move to a 1.8 or 1.5 needle and seat. I think we're expecting 32psi pop-off from them.

Vibration still not eliminated as a possible cause. I'm running Cold Fusion engine mounts - surely it would be a common problem with non OEM mounts?!

Cheers.

 

[Dartman]

always use a 115 gram spring on 38's with 2.0 n&s
yes mounts can aggravate this as well
i only use oem yamaha mounts

 

[JetManiac]

There is a tiny hole in slave(front) carb rear cover to restrict return fuel flow to regulate fuel pressure. If this hole is clogged or restricted, then fuel pressure goes too high and will cause fuel to force out of whichever carb has lowest popoff.

 

[Loco]

There is a tiny hole in slave(front) carb rear cover to restrict return fuel flow to regulate fuel pressure. If this hole is clogged or restricted, then fuel pressure goes too high and will cause fuel to force out of whichever carb has lowest popoff.

Hmm, this could be it. I just put the 115g springs in last night, and I'm still seeing the same issue.

Does anyone have a picture of what I'm looking for? I've had the carbs apart, front and back, several times, so clearly must have missed it!

Thanks in advance.