Carbs and exhaust

[jeffschnase]

Hey guys. So I finally got the 95 vmax done enough to take it out on the test drive. Changes made with the build were loosing the ugly canister muffler and going with a little hot rod muffler that I found online that basically was just a tip with some step downs inside to create some sort of baffle. The truth is its really just straight exhaust that's insanely loud. I plan to add an aftermarket baffle to quiet it down. The other main part was purchasing a set of carbs from a running bike on ebay. They were super clean and as soon as I installed them the bike fired right up. For some reason though my stock air cleaner wont fit the new carbs. I suspect that its a set from a year of gen 1 that was slightly different.

Here's the problem. On my test drive I was basically forced to ride it with out an air cleaner. (i have 4 little K & N style ones coming) Above about 3500 rpms the bike just kind of falls flat and starts missing and sputtering. I'm guessing I'll need to rejet the bike? The plan is to test drive it again with the air cleaners and the baffle but I'm guessing it won't make much of a difference as the new air cleaners will be much less restrictive that the new ones.

Any help and guidance here would be greatly appreciated!

Sincerely, Jeff in Denver

 

[Fire-medic]

If the carbs don't fit they may be from a Venture. The airhorn to which the airbox donuts would mount is a bit smaller on ea. carb. The Venture first used Mikuni BDS34 carbs. Your '95 uses BDS35 as OEM.

Engines benefit from a certain amount of backpressure. Open exhausts don't provide it. Same for intakes, jetting works with a designed resistance. It affects the air/fuel ratio. I'm no carburetion expert, but having an experienced person and equipment like an exhaust 'sniffer' can help tune things more-easily.

Here's a thread about different treatments for quieting down exhausts.
https://www.vmaxforum.net/threads/ufo-4-1-dragstar-shhhh.31804/

 

[dannymax]

Are the headers stock or part of an aftermarket system? The old carbs may have been stg 7 with air correctors to compensate for richness. You may be mix/matching aftermarket headers with stock carbs.

 

[jeffschnase]

If the carbs don't fit they may be from a Venture. The airhorn to which the airbox donuts would mount is a bit smaller on ea. carb. The Venture first used Mikuni BDS34 carbs. Your '95 uses BDS35 as OEM.

Engines benefit from a certain amount of backpressure. Open exhausts don't provide it. Same for intakes, jetting works with a designed resistance. It affects the air/fuel ratio. I'm no carburetion expert, but having an experienced person and equipment like an exhaust 'sniffer' can help tune things more-easily.

Here's a thread about different treatments for quieting down exhausts.
https://www.vmaxforum.net/threads/ufo-4-1-dragstar-shhhh.31804/

Thanks Phil. That must be it. Makes sense since the venturi covers are chrome. Hopin that the baffle and the air cleaners make a big difference.

 

[jeffschnase]

Are the headers stock or part of an aftermarket system? The old carbs may have been stg 7 with air correctors to compensate for richness. You may be mix/matching aftermarket headers with stock carbs.

Pretty sure the carbs are off a Venture. Headers are definately not stock. A 4 into one configuration

 

[dannymax]

What are the numbers on the bottom of the throttle plates.

 

[3-Max]

This is just what I've learned from youtube while doing my own carbs, airbox & exhaust:

Airbox is designed as a resonant chamber. It means it's set up to provide positive (increased) air pressure to the velocity stacks (the long rubber things) at certain rpm (usually low/mid range). The velocity stacks are also designed to give the correct fuel/air ratio at the intake. Gen 2 Vmax has a mechanism that shortens the velocity stack length to provide better high rpm performance.
Swapping out the air box for slip on air filters/socks (or nothing) usually results in weak mixture (not enough fuel) & a loss of power esp at lower/mid range rpm if you don't re-jet.

Exhausts are designed to provide negative pressure (vacuum) at the port at high rpm but this usually results in positive (back) pressure at mid range & you get the choking/sputtering. Exhaust are then designed to mitigate this (I can't remember how - this might be the resonant airbox & the cross balance pipe on the front headers, now I think about it). I do recall 4 into 1 exhausts are notoriously bad for this mid range choking on performance bikes which is why race bikes swapped to 4 to 2 to 1. Header length affects this too. It's a resonant tube, like a musical instrument.
Mine was doing this from about 4.5K until Vboost. I've assumed this as the cause. It had the front headers & cans from a yammy TDM 850 & the length had been altered. I've just finished fitting secondhand OEM headers & a Marving back box (similar to OEM) designed for the Vmax. It's sounding much healthier under no load but I've yet to try it on the road.
After the engine the most development time on a bike is the exhaust system. OEMs may not be the prettiest but they're designed to give the best performance with the OEM jets and airbox. Replacing any of these components with affect how the others work (& engine performance) so you really need to be careful in choosing stuff designed to go on your specific bike model & check if you need to change jets.

Replacing all the rubber carb joints & getting the airbox nice & tight on the carbs also improved things, as did cleaning all the crap out of the carbs. All the above is why I've gone back to as much stock as I can get. The Marvings are designed to work with the Vmax without changing anything else.

I can prob find the vid's again if you want view them yourself. They were experienced professionals & not your home fixer. Hope that helps.