Running rich due to exhaust mod?

[Jaoteay]

My dad recently did the stock exhaust mod - we drilled 5 3/8" holes and then spaced it about 1/2" before putting the cover plate back on. I recently received this email:

The dealer called me this morning (Cheyenne). They put my bike up on their dyno and said I had a couple problems. One of them is my T-boost is defective, which I believe. The other problem is my bike is running too rich. They are going to adjust the needles, and swap out some jets. The mechanic said this may be due to the muffler modification, but can't say for sure. It might end up costing me $240 to get the bike back to running well after drilling the holes :-(.

Does this seem likely??

 

[one2dmax]

it is going to be richer. You can run it like it is or change the jets out for a size or two smaller. It's not that hard if you want to do it yourself. There is a sticky on here to show you how to do it. The t-boost will make the midrange rich if stuck in the open position. If you have a U.S. bike then you won't have adjustable needles (unless you have stage 1 needles then you are going to be rich everywhere most likely).

Sean

 

[Thevmaxrider]

Yep, that is very possible. You have to think of how air flows through the engine, which is an air pump after all. The stock airbox has a limited in flow capacity through the air inlets, you opened up the outlet which increased flow/velocity out. The fuel system is pressurized and will provide more fuel to compensate for the increased out flow of exhaust. This also happens when people put on an aftermarket exhaust. I dropped 2 main jet sizes when I installed my HMF to get the tune right.

 

[Jaoteay]

Yep, that is very possible. You have to think of how air flows through the engine, which is an air pump after all. The stock airbox has a limited in flow capacity through the air inlets, you opened up the outlet which increased flow/velocity out. The fuel system is pressurized and will provide more fuel to compensate for the increased out flow of exhaust. This also happens when people put on an aftermarket exhaust. I dropped 2 main jet sizes when I installed my HMF to get the tune right.

Ahhh.. Thank you! That explanation made perfect sense to me... So I suppose this is a good time to get the carb kit... Already have to work on it anyways!

 

[Shuriken]

jaoteay,

How did you put the end caps back on? You had to drill out the rivets, right? Do you have exhaust airflow coming around the perimiter of the baffle plate or does it all come through the middle hole?

I have this mod. In general, I've gotten used to the sound - it's like a mean stock car growl kind of sound.... But I may revert. So I'm wondering how you remounted the end caps and so forth.

 

[Mitchell]

Ahhh.. Thank you! That explanation made perfect sense to me... So I suppose this is a good time to get the carb kit... Already have to work on it anyways!

Here's what Mark did several months back. Just scroll down the thread to view it all. It's excellent instuctions with color pictures and even some comedy included :biglaugh:

http://www.vmaxforum.net/showthread.php?t=3570&page=2

 

[lankeeyankee]

I wouldnt buy a carb kit with a stock header.

I would just drop the main jet down a size. Mikuni Jets are very cheap!
Got to Jetsrus.com I just found this site and awesome for buying jets. Great prices. I just ordered some pilots for my Fortune Cookie Scooter

Ahhh.. Thank you! That explanation made perfect sense to me... So I suppose this is a good time to get the carb kit... Already have to work on it anyways!

 

[Thevmaxrider]

I agree with Lankee, don't waste your money on a jet kit (especially if they try to sell you a Dynojet Stage 1!!). Drop the mains a size and have them re-dyno if you so desire. When/if you decide to go to an aftermarket exhaust, then you can start talking jet kits (I will highly recommend Sean Morley's Muscle Jet kit), otherwise just change jetting and tune from there.

 

[naughtyG]

So I'm a little confused by what's happening here:

- by increasing the exhaust output velocity, but with air intake flow not increasing, mixture's become rich because fuel input has increased (compensating for exhaust flow)
- by decreasing jet size, fuel flow is brought down, to get back in tune with air flow
- so now you have same air flow, less fuel flow and more exhaust velocity

How does this affect performance?

 

[Thevmaxrider]

Think of air/fuel/exhaust as one big balancing act when tuned properly. Everything will be in the correct proportions flow wise when in balance, ergo tuned properly. By drilling the exhaust he upset the balance to the rich side of things, thereby reducing his performance. If he rejets the carbs down a size or two (depending on the bike as each is its own critter) it will get the balance back and restore performance back to stock. Adding an aftermarket header doesn't really get you much performance wise, just changes the sound. What Sean (and Dynojet, just like Sean's kit best) does with his kit is open up the intake too so you get better flow througout the system and thereby get the performance enhancement. A stock bike, with stock (even if drilled it is still essentially stock) exhaust will not benefit much from the kit as the inlet flow will be too high for the engine to use adequately. Hope this helps clear this up a bit.

 

[Jaoteay]

Think of air/fuel/exhaust as one big balancing act when tuned properly. Everything will be in the correct proportions flow wise when in balance, ergo tuned properly. By drilling the exhaust he upset the balance to the rich side of things, thereby reducing his performance. If he rejets the carbs down a size or two (depending on the bike as each is its own critter) it will get the balance back and restore performance back to stock. Adding an aftermarket header doesn't really get you much performance wise, just changes the sound. What Sean (and Dynojet, just like Sean's kit best) does with his kit is open up the intake too so you get better flow througout the system and thereby get the performance enhancement. A stock bike, with stock (even if drilled it is still essentially stock) exhaust will not benefit much from the kit as the inlet flow will be too high for the engine to use adequately. Hope this helps clear this up a bit.

Ahhh!! Very clear.. thank you!

 

[naughtyG]

So to recap, all that Jaoteay has done by drilling his exhaust is change the sound, not performance.

But surely, by dropping jets 2 sizes down, once everything's back in tune, he will have improved mpg since less fuel is now flowing through, right?

So what I don't get is, how can you get the same performance with less fuel?

 

[Thevmaxrider]

You don't have less air, remember your inlet flow is limited by the airbox inlet and is a constant, just higher exhaust velocity. What you have done is created, for lack of a better term, a higher vacuum (or lower inlet pressure take your pick) which in effect "pulls" more fuel through the main (I know that someone will have the proper terminology out there and correct me, but the idea is essentially correct) jets. By going down in jet sizes, reducing orifice size, less fuel can be moved through the jet (same as what is experienced by the airbox top), your A/F ratio gets back to a proper one, performance returns to a well tuned stocker.

 

[naughtyG]

yes I understood what you're saying, but the bottom line is that to get back to the appropriate A/F ratio, the jet sizes had to be decreased. So now there's the same air flow (unchanged), less fuel going (smaller jets) but still the same performance.

Something's still not adding up. If you can get the same performance with less fuel, why don't manufacturers make it so to start with? Just because of noise?

 

[Jaoteay]

You don't have less air, remember your inlet flow is limited by the airbox inlet and is a constant, just higher exhaust velocity. What you have done is created, for lack of a better term, a higher vacuum (or lower inlet pressure take your pick) which in effect "pulls" more fuel through the main (I know that someone will have the proper terminology out there and correct me, but the idea is essentially correct) jets. By going down in jet sizes, reducing orifice size, less fuel can be moved through the jet (same as what is experienced by the airbox top), your A/F ratio gets back to a proper one, performance returns to a well tuned stocker.

Since we live up in the 5000ft range then what we are experiencing by more air coming through is actually not as bad as someone who would be lower and have more oxygen in their air, right? So the lower elevations would be more affected?

 

[maleko89]

Good explanation Jim. Air flow is the same in this case. The engine, being a pump, is pulling more fuel through the carbs/jets. Another solution would be to shim the air box "Y" a bit.

 

[Thevmaxrider]

Actually there is the same amount of fuel that would be there with stock jetting/stock exhaust. Remember he is rich now and trying to get back to the proper tuning. His performance is decreased by running too rich, he will get a very slight enhancement when getting his carbs tuned. The bike is only using less fuel when compared to running rich, not when properly tuned. We are also not talking a massive amount of fuel here.

 

[maleko89]

Go to Jetsrus.com I just found this site and awesome for buying jets. Great prices. I just ordered some pilots for my Fortune Cookie Scooter

Thanks Lankee! Best online prices I've seen! Does anybody know the pilot fuel jet type for a Vmax? To me N151.067 looks correct by looking at the picture.

 

[naughtyG]

Actually there is the same amount of fuel that would be there with stock jetting/stock exhaust. Remember he is rich now and trying to get back to the proper tuning. His performance is decreased by running too rich, he will get a very slight enhancement when getting his carbs tuned. The bike is only using less fuel when compared to running rich, not when properly tuned. We are also not talking a massive amount of fuel here.

OK so now it makes sense. Smaller jets just allow the same amount of fuel through as stock because it's being pumped harder by the more open exhaust.

Blimey that took some working out - thanks for humoring me. :clapping:

 

[Thevmaxrider]

Since we live up in the 5000ft range then what we are experiencing by more air coming through is actually not as bad as someone who would be lower and have more oxygen in their air, right? So the lower elevations would be more affected?

It's all relative, the air at higher elevations is not as "dense" due to lower air pressure, not oxygen content. What you will find is that the higher up in elevation you go, the smaller the main jets as the bike would tend to run richer all things being equal than the same bike at a lower elevation.