Cobra Exhaust back to Stock

[stobrien]

I just got my first Vmax 2003. The past owner changed the Stock exhaust to a cobra 4-4 Set up. I have read poor things about the set up and loss of horse power and am looking at changing back to the stock system that was supplied with the bike.
The past owner put different jets in the carbs and this is where my concern for changing back begins. I have no idea what jets he put in.
I have a K&N filter and would like to re-install the stock exhaust with Stock exhaust modifications. Do I need to change the jets?

Tom

 

[Traumahawk]

Do you HAVE to, no........BUT, there is no telling what jets the PO put in there. They couldve put in Mikuni 165's and stock are 152.5's. So, if you going thru the trouble of putting stock exhaust back on, at least look at the jets, and if you want a little bit of an increase, go to Mikuni 150s on the mains.

 

[caseyjones955]

Do you HAVE to, no........BUT, there is no telling what jets the PO put in there. They couldve put in Mikuni 165's and stock are 152.5's. So, if you going thru the trouble of putting stock exhaust back on, at least look at the jets, and if you want a little bit of an increase, go to Mikuni 150s on the mains.

X2, also make note of the jets on regards to whether they are DJ (Dynajet) or Mikuni. The sizes are not the same. There is a handy dandy cross reference chart but cant seem to link it on my phone.

Going into the carbs is a great opportunity to have a peek at the jets and the needles & shims. There is a pic somewhere here that illustrates and labels all the common needles, which of course I cant link. I got my vmax with the Dynajet stage 1kit and it was many steps to rich so swapping out the needles and a couple jets made massive differences in performance and rideability. Its fairly quick and simple to pull the carb rack and access main jets through the plugs. Have a toothpic handy. You can access the needles and the pilot air jet with the carbs in place, also only takes a few minutes.

Congrats on your score!

 

[tugla]

Especially being up in Calgary with stock mufflers go to 150 or 147.5 mikuni jets.
Do you have stock mufflers? I got a mint set of 97 black mufflers if you need a set to buy.

 

[Jan Bee]

X2, also make note of the jets on regards to whether they are DJ (Dynajet) or Mikuni. The sizes are not the same. There is a handy dandy cross reference chart but cant seem to link it on my phone.

Going into the carbs is a great opportunity to have a peek at the jets and the needles & shims. There is a pic somewhere here that illustrates and labels all the common needles, which of course I cant link. I got my vmax with the Dynajet stage 1kit and it was many steps to rich so swapping out the needles and a couple jets made massive differences in performance and rideability. Its fairly quick and simple to pull the carb rack and access main jets through the plugs. Have a toothpic handy. You can access the needles and the pilot air jet with the carbs in place, also only takes a few minutes.

Congrats on your score!

Width------Keihin # -- DynoJets # -- Mikuni #

0,0540---- 140--------- 130--------- 121,9
0,0550---- 142,5------- 132--------- 123,8
0,0560---- 145--------- 134--------- 125,6
0,0570---- 147,5------- 136--------- 127,5
0,0580---- 150--------- 138--------- 129,4
0,0590---- 152,5------- 140--------- 131,3
0,0600---- 155--------- 142--------- 133,1
0,0610---- 157,5------- 144--------- 135,0
0,0620---- 160--------- 146--------- 136,9
0,0630---- 162,5------- 148--------- 138,8
0,0640---- 165--------- 150--------- 140,6
0,0650---- 167,5------- 152--------- 142,5
0,0660---- 170--------- 154--------- 144,4
0,0670---- 172,5------- 156--------- 146,3
0,0680---- 175--------- 158--------- 148,1
0,0690---- 177,5------- 160--------- 150,0
0,0700---- 180--------- 162--------- 151,9
0,0710---- 182,5------- 164--------- 153,8
0,0720---- 185--------- 166--------- 155,6
0,0730---- 187,5------- 168--------- 157,5
0,0740---- 190--------- 170--------- 159,4
0,0750---- 192,5------- 172--------- 161,3
0,0760---- 195--------- 174--------- 163,1
0,0770---- 197,5------- 176--------- 165,0
0,0780---- 200--------- 178--------- 166,9
0,0790---- 202,5------- 180--------- 168,8
0,0800---- 205--------- 182--------- 170,6
0,0810---- 207,5------- 184--------- 172,5
0,0820---- 210--------- 186--------- 174,4
0,0830---- 212,5------- 188--------- 176,3
0,0840---- 215--------- 190--------- 178,1
0,0850---- 217,5------- 192--------- 180,0
0,0860---- 220--------- 194--------- 181,9

 

[stobrien]

I drill the Cobra exhaust and was not happy with the increased sound. It did however give me back some of the lost horse power. but I have decided to put the original Stock exhaust back on my VMax. I spent an hour or so fighting with the alignment.
Is there any secret to fitting this back on?:bang head:

 

[[email protected]]

No major secrets to it. Just aint easy.

Make sure you loosen all the bolts on the head pipes. You need the wiggle room.

It helps to support the muffler assembly so you are not trying to lift it and position it all at the same time. A floor jack or even some thin ratchet straps can be very helpful to pick the exhaust up and get it close while you maneuver it in place.

Take care to not damage the gaskets in the muffler assembly. Install one pipe just a little bit at a time. Work left to right or vise versa. Once all are in, push and wiggle till it seats.

Once seated, install that center bolt. Then install the bolts on either side that support the muffler assembly. Get them tightened up.

After that, wiggle the head pipes a bit to make sure they are seated fully against the gasket up by the cylinder head. Once they are, torque those bolts down.

Save the clamps on the muffler assembly for last. The head pipes slip in/out there a bit so it is the most forgiving joint. Tightening these first makes it harder to get the rest of it sealed up.

 

[DMAN999]

^ What he said, there are no real tricks to it other than
loosening the head bolts so you have a little wiggle room and supporting them with something or someone.

 

[carrizog60]

making the jets smaller increase hp?
is the max jetted rich from factory?and what about stock exhaust but drilled?
going small on jetting improve hp?i am at sea level....

sorry for the highjack...

 

[davesax36]

making the jets smaller increase hp?
is the max jetted rich from factory?and what about stock exhaust but drilled?
going small on jetting improve hp?i am at sea level....

sorry for the highjack...

It's the way the carbs work. Opposite of the way most automotive ones do. More air in makes them suck in more fuel so you need to choke that down.

 

[sdt354]

making the jets smaller increase hp?
is the max jetted rich from factory?and what about stock exhaust but drilled?
going small on jetting improve hp?i am at sea level....

sorry for the highjack...

They are a bit on the rich side from the factory,imho. But the carbs are set up to run in all kinds of conditions and altitudes. And run well at that.
Steve-o

 

[Traumahawk]

making the jets smaller increase hp?
is the max jetted rich from factory?and what about stock exhaust but drilled?
going small on jetting improve hp?i am at sea level....

sorry for the highjack...

Yes, the Vmax is jetted rich from the factory, so going leaner will help wit hp, but at the stock 152's, the Vmax runs well, but going leaner will make it run better. Drilling out the stock exhaust doesnt help because the main restriction of the stock system is the size of the head pipes.

 

[carrizog60]

i wasnt expecting that drilling holes helped with HP,just with sound.
i mean,with drilled pipes is still valid to jet a size down to increase hp?

 

[Traumahawk]

i wasnt expecting that drilling holes helped with HP,just with sound.
i mean,with drilled pipes is still valid to jet a size down to increase hp?

Well even stock, the Vmax does better going down 1 jet size.......but I am a HP guy too.

So no, you dont have to, but being leaner will increase the HP, and make the bike a bit more responsive.

I actually think the Vmax was jetted rich A) to decrease the HP a bit, B) as a quick and dirty way for emissions. The leaner you go, C02, and NOX actually increase.

The stock Vmax on the dyno depending on jetting, float level height, is usually in the 12.5 to 13.0 A/F range.

http://www.crypton.co.za/Tto know/Emissions/exhaust emissions.html

 

[stobrien]

So I removed my cobra Pipes and put the stock pipes back on.

Supposedly the past owner re-jetted the carb for the Cobra pipes, seemed to run rich, Now that I have gone back to the stock exhaust is it going to run rich or leaner?

Love the sound of the Stock exhaust.

Thank you for your help.

 

[davesax36]

You should pull your carbs and check the mains. You can do that without splitting the rack. Then we'll know. If they're 150 or 152.5 you'll be fine. Bigger will be worse. Smaller will probably still be ok.