Runaway Revs?

[MaxKev]

Hey all!

I’m having a heck of a time trying to tune my carbs on a used MAX I bought (15k miles). Just as I think I’m getting it synced - I blip the throttle and she runs away on me (goes up to 3k to 4k rpm’s and won’t come down). Throttle cables are fine. Float levels are perfect. Jets are not clogged. New spark plugs. All brand new boots for vboost as well as carb boots. The air box boots look fine. I’ve sprayed wd40 around the joints and nothing changes. It seems to be sucking air somewhere? I’m stumped! Any help would be appreciated.

Also, vacuum is almost non existent on my gauges (not sure if that’s relevant).

 

[Fire-medic]

Any clamps you might have forgotten?

Did you check the manifolds to the cyl heads, where there's a thin O-ring for each?

You say, 'throttle cables are fine.' You have some slack on each side? They're routed properly? Look in the appendix for proper throttle cable routing. A pinched cable housing can impede throttle cable movement. You can have the proper freeplay, but it hangs up the cable.

https://dn790009.ca.archive.org/0/i...ice-manual/yamahavmaxvmx12-service-manual.pdf
What type of gauges do you have? Liquid, analog bourdon tube gauge, electronic? Have you used them before on other motorcycles?

 

[MaxKev]

Any clamps you might have forgotten?

Did you check the manifolds to the cyl heads, where there's a thin O-ring for each?

You say, 'throttle cables are fine.' You have some slack on each side? They're routed properly? Look in the appendix for proper throttle cable routing. A pinched cable housing can impede throttle cable movement. You can have the proper freeplay, but it hangs up the cable.

https://dn790009.ca.archive.org/0/i...ice-manual/yamahavmaxvmx12-service-manual.pdf
What type of gauges do you have? Liquid, analog bourdon tube gauge, electronic? Have you used them before on other motorcycles?

Appreciate the reply - let me re-check those cables. Are the o-rings under the v-boost housing? If they are - I saw them and failed to really inspect when replacing the vboost boots.

Gauges are analog - I bought them on Amazon (I’m sure they’re not the best). I guess I should probably test them on something to make sure they’re drawing vacuum accurately.

 

[Fire-medic]

Part # 7 2007 Yamaha V-MAX (VMX12W1) Intake | Ron Ayers



Since that manifold assembly comes off as a unit replace all 4, even if you find only one is bad. Using starting fluid on the manifold to cyl head area will increase revs if there's a leak. Some people use propane from a hand torch or a gas grille bottle, directed by a hose. If you choose to try that, do it outside and nowhere near a source of combustion. Propane gas fumes can travel dozens of feet to a source of ignition, like a gas hot water heater, and ignite.

O-Ring

93210-45578-00

Retail Price: $6.49

Your Price: $5.32

Gauges of that type usually have restrictors built into them to steady the reading. Do they look like these?


I have a pair of these, Emgo, they must be 40+ years old, and still work for their intended purpose. The 'absolute value' they give isn't as important as the reading from one cyl to another. At rest unhooked, they should all read about the same, one way to check 'em is to hook up each gauge in turn, to the same cyl. They should all read the same.

 

[99YAMAHAVMAX]

You can sync the carbs with the airbox off but rarely will it rev above 5k rpm's with it off---open the choke lever a few times and and the needles or rods on the sync tool will move into the 18-22 zone to try and get a reading--if carbs # 2 & 1 are close go to the other side and start on 3 & 4-turning the sync screws is going to change the rpm.

 

[MaxKev]

Any clamps you might have forgotten?

Did you check the manifolds to the cyl heads, where there's a thin O-ring for each?

You say, 'throttle cables are fine.' You have some slack on each side? They're routed properly? Look in the appendix for proper throttle cable routing. A pinched cable housing can impede throttle cable movement. You can have the proper freeplay, but it hangs up the cable.

https://dn790009.ca.archive.org/0/i...ice-manual/yamahavmaxvmx12-service-manual.pdf
What type of gauges do you have? Liquid, analog bourdon tube gauge, electronic? Have you used them before on other motorcycles?

Appreciate the reply - let me re-check those cables. Are the o-rings under the v-boost housing? If they are - I saw them and failed to really inspect when replacing the vboost boots

Part # 7 2007 Yamaha V-MAX (VMX12W1) Intake | Ron Ayers

View attachment 90429

Since that manifold assembly comes off as a unit replace all 4, even if you find only one is bad. Using starting fluid on the manifold to cyl head area will increase revs if there's a leak. Some people use propane from a hand torch or a gas grille bottle, directed by a hose. If you choose to try that, do it outside and nowhere near a source of combustion. Propane gas fumes can travel dozens of feet to a source of ignition, like a gas hot water heater, and ignite.

O-Ring

93210-45578-00

Retail Price: $6.49

Your Price: $5.32

Gauges of that type usually have restrictors built into them to steady the reading. Do they look like these?

View attachment 90430
I have a pair of these, Emgo, they must be 40+ years old, and still work for their intended purpose. The 'absolute value' they give isn't as important as the reading from one cyl to another. At rest unhooked, they should all read about the same, one way to check 'em is to hook up each gauge in turn, to the same cyl. They should all read the same.

That’s exactly what my set looks like. The throttle cables have play so they’re not getting hung up. I still need to check out those o rings on the cylinder. The only things I’m noticing is the valve cover gaskets are all cracking - can that cause issues? Also, I must have an o-ring letting gas out around the bowl drain screw.

 

[Fire-medic]

Minor valve & cam cover gasket cracking happens, not an issue until it starts to leak. You spotted the float bowl seepage, just replace the O-ring of any which are seeping. If you're close to a Harbor Freight, you might try their O-ring assortment, to see if you can match yours up, or buy some OEM parts from someplace like Ron Ayres Yamaha. Sean Morley [email protected] or dannymax (his email: [email protected] ) will probably have them also, he's a carburetor overhaul specialist.

 

[telebb]

I also had something similar and I found that when someone pried the carbs off they bent the bar that the idle screw is in so check that as well.

 

[MaxKev]

Minor valve & cam cover gasket cracking happens, not an issue until it starts to leak. You spotted the float bowl seepage, just replace the O-ring of any which are seeping. If you're close to a Harbor Freight, you might try their O-ring assortment, to see if you can match yours up, or buy some OEM parts from someplace like Ron Ayres Yamaha. Sean Morley [email protected] or dannymax (his email: [email protected] ) will probably have them also, he's a carburetor overhaul specialist.

Not sure if this could be my issue - any chance you’ve seen this? Both sync screw and the receiver are gummed up. Anyone successfully rethreaded these?

 

[Fire-medic]

Both sync screw and the receiver are gummed up. Anyone successfully rethreaded these?

Any decent carb solvent should clean the M-F threads up from gum/residue. Then see if you have stripped threads anywhere. If you're lucky, no stripped threads, or the stripped screw needs replacement. If the bracket female threads are stripped I might try some flux/solder & re-tapping of the female threads.