Stumble at cruise speed

[edharrison]

The compressor goes 90 psi max so should be ok

 

[edharrison]

Looking a little deeper the holes that the needles go in - emulsion tubes? Are oval!!!


Ed

 

[edharrison]

Dyno result - Stumble at cruise speed

Heres the dyno sheet - all the obvious and more have been looked at including all airbox / carb boots for leaks, PDQ stripped the carbs and cleaned them, I have changed everything bar the coils which are my last hope! The dip in the A/F baffles me but according to PDQ its no issue but it is just a little coincidental thats bang on in the rev range where the issue is....all the normal bits are new, fuel filter / HT leads / plugs / air filter....and so on. Everything except the exhaust is standard, jets, needles, intake - Supertrapps with 8 disks to suit stock setup..

Any ideas?

Ed

 

[gamorg02]

what jets are being used at cruise vs WOT at the same RPMs are completely different. You're almost all mains at that point at WOT whereas at cruise you're barely into the needle (i believe).

 

[edharrison]

All standard jets / needles, only after market parts are Supertrapps with 8 disks / closed caps.


Ed

 

[gamorg02]

so u may need a smaller main and shim your needle or get a sg1?

 

[edharrison]

With everything stock other than slip ons wouldnt smaller mains just lean it all rather than the wierd dip at 3000 rpm?

Ed

 

[Johnwhatsisname]

I have the same problem. What was the final solution on this thread?

 

[Traumahawk]

Typically 0-4000 rpm are governed by the A/F screws (pilot jets). 4000 to 6-7000 is governed by the needles, and past that is governed by the mains. NOW, these numbers arent set in stone, because the carb circuits (the areas that are governed by the various areas of the carb), overlap. Here is an old dyno sheet that I did comparing my 07 when it was stock, vs a muscle jet kit. You can see on that sheet, mine did the same thing. I believe its where the A/F screws are still operating, and the needles are starting to come in, creating a momentary rich condition.

 

[Fire-medic]

Typically 0-4000 rpm are governed by the A/F screws (pilot jets). 4000 to 6-7000 is governed by the needles, and past that is governed by the mains. NOW, these numbers arent set in stone, because the carb circuits (the areas that are governed by the various areas of the carb), overlap. Here is an old dyno sheet that I did comparing my 07 when it was stock, vs a muscle jet kit. You can see on that sheet, mine did the same thing. I believe its where the A/F screws are still operating, and the needles are starting to come in, creating a momentary rich condition.

Hey, don't confuse the poor guy, I know that you meant to call them the pilot screw, and not the "pilot jets."

Pilot screw: (often referred-to as the 'A-F screw') located below the CV carb diaphragm cap
Part #22
Pilot Screw Set
12R-14105-00-00
$6.51

Pilot jet, which is inside the jet block:
Part #42
Jet, Pilot (37.5)
4G0-14142-37-A0
$6.10

https://www.ronayers.com/oemparts/l/yam/50045c0bf8700209bc7942e9/2007-v-max-vmx12w1-parts

 

[Johnwhatsisname]

Thanks guys,
@Traumahawk, if that’s the case should I screw the Pilot Screw in or out to lean up the mixture?
I have noticed it’s smells very rich on idle but I think that’s something else... carbs 3 & 4 float levels are 5mm high

 

[Traumahawk]

Hey, don't confuse the poor guy, I know that you meant to call them the pilot screw, and not the "pilot jets."

Your right

Thanks guys,
@Traumahawk, if that’s the case should I screw the Pilot Screw in or out to lean up the mixture?
I have noticed it’s smells very rich on idle but I think that’s something else... carbs 3 & 4 float levels are 5mm high

Set the floats first, and then make any adjustments thats needed. The floats will make it run rich......and will tend to affect EVERYTHING.

 

[Fire-medic]

Your right

Set the floats first, and then make any adjustments thats needed. The floats will make it run rich......and will tend to affect EVERYTHING.

I agree with my friend Traumahawk, his advice is spot-on. You're gonna have-to re-set the float level. That means carbs off, float bowls off. A pic to help you:



I suspect your "5 mm high" wet float-measurement carburetors are going to look like the one on the right, remember that the picture is upside-down, once you invert it, you'll see that a too-short distance between the float's topside and the large, flat brass screw in the jet block will cause a high gas level, leading to crummy gas mileage, poor throttle response, and overall improper fuel metering. You should be proficient at the disassembly/reassembly, by now! Practice makes perfect.

 

[Johnwhatsisname]

Yes, your correct about the disassembly/reassembly bit
Anyway, I need to look at the gaskets in the crankcase breather assembly as I have some oil in the coolant - This is in another post...
As far as the float levels go, I am going to hook up a fuel source on the bench and do them individually but wet. I think this will be most accurate.
I'm not completely sure of the correct jets for here as we are at about 4500' asl. I will go with what I have at the moment as there are just too many variables otherwise.
When the floats are done and the CC breather fixed I will synchronise the carbs and give her a road test.
There was another very useful thread on jetting and the airscrew. This related to the origin of this thread, "Stumbling at cruise"
Anyway, when its back together I will give some feedback and hopefully provide some resolution to the issue.

 

[Johnwhatsisname]

Could someone assist?
I have removed the carb assembly and am now trying to remove the manifolds which includes the vmax butterflies. I have removed the 8 Allen key bolts but it is not budging. For obvious reasons I don’t want to apply too much force.
have I missed something or should I just try harder?

 

[Fire-medic]

Could someone assist?
I have removed the carb assembly and am now trying to remove the manifolds which includes the vmax butterflies. I have removed the 8 Allen key bolts but it is not budging. For obvious reasons I don’t want to apply too much force.
have I missed something or should I just try harder?

Just tap the casting with a hammer a few times, it should pop-loose. You can use something like a 2x4 scab if you want, just don't do it on the open edge of the casting, do it someplace like in the middle of the casting. You could use the rubber end of your hammer if you want, and give the head of the hammer a couple raps. Why are you removing that? Did you find a vacuum leak at the bottom, where it attaches to the cyl head?

There are large thin O-rings which provide the gasket-seal between the VBoost manifold castings and the intake ports on the cyl heads. You can use a bit of water-soluble gel like KY to hold the O-ring in-place, just remember to wipe-off the greasy, oily fingerprints before you bring it back-inside.

 

[Johnwhatsisname]

I’m removing it in order to remove the crankcase breather(pencil box)

 

[Fire-medic]

Yeah, easy-does it, you're only overcoming a baked-on set of four thin O-rings, assuming that you've removed all the fasteners! A lesson my father taught me about fasteners and stuck-on parts. Life's hard lessons, best-learned by the bitter fruit of experience.

 

[Johnwhatsisname]

I would agree. Is ALL the fasteners 8 off? Ie 2 on each intake?

 

[Fire-medic]

Yes, I believe-so. Your eyes are probably better than mine, I'm old.