The V-max carburettor and inlet manifold assembly are a most likely source of problems like an elusive rough idle, hesitation and sputtering when rolling on the throttle at low and high speeds and other seemingly fuel related issues. Before you go tearing into the carburettors it often pays to check the simpler things first.
Something that is often overlooked by many riders (and drivers) of vehicles using carbs or EFI is the disruptive effects of a ‘vacuum leak’ on the smooth operation of your engine.
The downward movement of the piston on the engine’s intake stroke creates a partial vacuum (low pressure) and allows the atmosphere to push (high pressure) air into the carburettors. Fuel is pushed into this airflow by fuel pump pressure, gravity feed, atmospheric pressure or a combination of these forces.
We subjectively perceive this air and fuel to be ‘sucked’ into the engine.
Provided you have no other avenues for air to enter the inlet manifold, all air entering the cylinder must pass through the carburettor throat at a predictable flow rate that will vary only with the rate at which the low pressure gradient can be replenished by the intake stroke of the piston. i.e. the speed of the engine.
When you experience a rough idle or hesitation and sputtering when rolling on the throttle at low and high speeds, your first inclination is to inspect the usual suspects like a faulty plug, plug lead, blocked idle or main jet.
This forum is full of advice on how to service carbs. There are also numerous recollections of riders doing this only to discover a cracked rubber boot, a disconnected V-boost hose, loose air box etc.
You should check these things first. They cost nothing to fix.
If you have an older model V-max, the rubber fittings, seals and hoses associated with the inlet manifold and carburettors will become stiff, and brittle over time. It will be difficult to get a good tight pressure seal once you unseat them unless they are perfectly refitted.
While looking for the cause of a rough idle (after doing the ‘shotgun’ and ‘peashooter’ cleaning of the carb jets) I inspected the rubber end caps that fit over the vacuum line connections on the inlet manifolds. These manifold connections are the brass pipes you attach the hoses of your carb sync tools to. They had hardened and had started to crack. Just removing them to sync the carbs and then putting them back on over the widened pipe end had stretched them and they had no capability to shrink to back to an original fit. So they all had slight leaks. Even with the metal clips in place they can still leak.
In spite of a big fat spark, number three cylinder was not taking part in the engine idle at all and the header remained cool enough to touch. Spark plug was dry.
Inspection of the manifold’s rubber end cap showed that the top of the cap had split on the side facing the manifold wall. This could not be seen without rotating the cap toward the observer and was allowing air to freely enter the inlet.
I placed my finger over the split and number three fired instantly. A bit of silicon affected a temporary repair until a few cents worth of aftermarket end caps solved the problem.
So if you are experiencing some hard to pinpoint ‘fuel related’ problems (especially on older V-maxes), check the rubber connections and hoses on the carbs, air-box, inlet manifold, v-boost unit and pressure sensor before you tear things apart.
You could save yourself a lot of frustration and expense.
Something that is often overlooked by many riders (and drivers) of vehicles using carbs or EFI is the disruptive effects of a ‘vacuum leak’ on the smooth operation of your engine.
The downward movement of the piston on the engine’s intake stroke creates a partial vacuum (low pressure) and allows the atmosphere to push (high pressure) air into the carburettors. Fuel is pushed into this airflow by fuel pump pressure, gravity feed, atmospheric pressure or a combination of these forces.
We subjectively perceive this air and fuel to be ‘sucked’ into the engine.
Provided you have no other avenues for air to enter the inlet manifold, all air entering the cylinder must pass through the carburettor throat at a predictable flow rate that will vary only with the rate at which the low pressure gradient can be replenished by the intake stroke of the piston. i.e. the speed of the engine.
When you experience a rough idle or hesitation and sputtering when rolling on the throttle at low and high speeds, your first inclination is to inspect the usual suspects like a faulty plug, plug lead, blocked idle or main jet.
This forum is full of advice on how to service carbs. There are also numerous recollections of riders doing this only to discover a cracked rubber boot, a disconnected V-boost hose, loose air box etc.
You should check these things first. They cost nothing to fix.
If you have an older model V-max, the rubber fittings, seals and hoses associated with the inlet manifold and carburettors will become stiff, and brittle over time. It will be difficult to get a good tight pressure seal once you unseat them unless they are perfectly refitted.
While looking for the cause of a rough idle (after doing the ‘shotgun’ and ‘peashooter’ cleaning of the carb jets) I inspected the rubber end caps that fit over the vacuum line connections on the inlet manifolds. These manifold connections are the brass pipes you attach the hoses of your carb sync tools to. They had hardened and had started to crack. Just removing them to sync the carbs and then putting them back on over the widened pipe end had stretched them and they had no capability to shrink to back to an original fit. So they all had slight leaks. Even with the metal clips in place they can still leak.
In spite of a big fat spark, number three cylinder was not taking part in the engine idle at all and the header remained cool enough to touch. Spark plug was dry.
Inspection of the manifold’s rubber end cap showed that the top of the cap had split on the side facing the manifold wall. This could not be seen without rotating the cap toward the observer and was allowing air to freely enter the inlet.
I placed my finger over the split and number three fired instantly. A bit of silicon affected a temporary repair until a few cents worth of aftermarket end caps solved the problem.
So if you are experiencing some hard to pinpoint ‘fuel related’ problems (especially on older V-maxes), check the rubber connections and hoses on the carbs, air-box, inlet manifold, v-boost unit and pressure sensor before you tear things apart.
You could save yourself a lot of frustration and expense.
Last edited:
![Aomiker Motorcycle Phone Holder Mount - [Metal Extension Arm] [Dual Vibration Dampener] Motorcycle Cell Phone Mount, Bike Phone Handlebar Clamp for iPhone 16 15 14 13 Pro Max Plus, 4.7-6.7" Smartphone](https://m.media-amazon.com/images/I/51gU2g5VxgL._SL500_.jpg)
![Bovemanx Motorcycle Phone Mount Holder, [150mph Wind Anti-Shake][7.2inch Big Phone Friendly] Bike Phone Holder, Motorcycle Handlebar Cell Phone Clamp, Compatible with iPhone 16 Pro Max Smartphones](https://m.media-amazon.com/images/I/51F+1sontPL._SL500_.jpg)
