Cafe Racer Forum banner
1 - 20 of 43 Posts

·
Registered
Joined
·
22 Posts
Discussion Starter · #1 ·
I have a 1982 suzuki gs750t that I built. The stock jet sizes were 115 main and 42.2 pilot. It has a 4-1 Delkevic straight exhaust, I acknowledge the need for some sort of muffler tip for back pressure. Pods with UNI brand filter foam inserted have also been installed. The bike currently runs on 140 mains and the og 42.2 pilots, but the idle is in the 2,500-3,000 range. What size pilots are best for my setup, and should I adjust the mains accordingly? Any information would be outstanding.
 

·
Registered
Joined
·
7,874 Posts
If it has CV carbs, you absolutely need some velocity stack on there, then you won't have as much problem jetting it or using stock jetting. Oiled filter pods should have a flame screen inside them, dry foam it might not matter as much and pods can go after the stack if you really want them, but otherwise, pods simply do not work good stuck right onto the mouth of a CV carburetor. Look at your original air intake and you will find it has a velocity stack of some sort.
Idle should be able to take in down to any speed you want until it stalls or something is mechanically hanging up.
 

·
Registered
Joined
·
2,767 Posts
So the interesting thing about the stock airbox is that it has velocity stacks incorporated into it, AND you can run an air filter.

If you run aftermarket velocity stacks on the street you'll risk the engine eating something big... or even with some sort of mesh filter, the dust that gets ingested by the motor will kill it.
 

·
Registered
Joined
·
7,874 Posts
You can run the stack inside a big foam air filter, the stack just has to be there for air flow, pods right on the mouth of the carb create too much turbulence. It's a 4-cycle engine so if it has an oiled foam or felt filter element, it needs flame screens or you might well have a fire some day, 2-stroke engine does't need the flame screen only because you will never get flames back out through the carb.

If you rip apart even a Honda generator, roto-tiller, outboard motor or lawn mower, you're going to find a stack inlet of some shape on the air intakes.

Airbox is the perfect place to vent your crankcase too, so if it has no airbox, where does your crankcase breathe from now? Just sticking a tiny paper pod filter on the crankcase vent is a very popular but poor solution.
 

·
Registered
Joined
·
7,874 Posts
Any specs on velocity stacks? I have 52mm inlets.
Suzuki already engineered it for you, the length and shape of the stack that was on it must have worked reasonably well. Straight exhaust will kill your performance and fuel economy but bad intake is way worse and trying to compensate for bad airflow by pounding more fuel into it doesn't work.
 

·
Administrator
Joined
·
43 Posts
I have the same bike, and ran into the same issues. It had pods on it when I bought it, and never ran correctly. Finding an airbox for an 81-82 750T is impossible. SO... I took an airbox off an 80 1100L. This is a 2 part airbox. I ditched the filter box, and attached 1 big K&N to it. It gets perfect velocity from the primary "plenum" box, and all the air flow from the open K&N. I run 140's and stock pilots. Pilots do not need to be changed, as the air flow at idle does not change. I have a couple pictures of this setup I can dig up, and post later.
 

·
Registered
Joined
·
22 Posts
Discussion Starter · #10 ·
I have the same bike, and ran into the same issues. It had pods on it when I bought it, and never ran correctly. Finding an airbox for an 81-82 750T is impossible. SO... I took an airbox off an 80 1100L. This is a 2 part airbox. I ditched the filter box, and attached 1 big K&N to it. It gets perfect velocity from the primary "plenum" box, and all the air flow from the open K&N. I run 140's and stock pilots. Pilots do not need to be changed, as the air flow at idle does not change. I have a couple pictures of this setup I can dig up, and post later.
I had the og air box, but I could not for the life of me get it on the carbs. Also, when you said you kept stock pilots, what size are you talking? I read something that said stock pilots were 22.2, and mine are 42.2. I don't know if the previous owner changed them to twice over, or that is stock, but something is making it stall out under 2500.
 

·
Registered
Joined
·
7,874 Posts
:unsure: if you "had" the original airbox then you had the 4 original rubber velocity stacks, you might want to get them back and put them on there, same with the original carburetor parts if you can get them.

Nobody changed the engine displacement or the size of the valves, so no change should have ever been made to the carburetor jetting. The notion that you need to put more fuel into the engine because you have put more air into the engine after installing pod filters is a complete misconception, your engine moves the same amount of air through it as it did before and the previous owner was an idiot to put cheap pod air filters on the mouth of a CV carburetor. If that worked Suzuki, Yamaha, Honda plus every other engine maker would have done it that way to start with and profited a whole bunch more money.
 

·
Registered
Joined
·
22 Posts
Discussion Starter · #12 ·
:unsure: if you "had" the original airbox then you had the 4 original rubber velocity stacks, you might want to get them back and put them on there, same with the original carburetor parts if you can get them.

Nobody changed the engine displacement or the size of the valves, so no change should have ever been made to the carburetor jetting. The notion that you need to put more fuel into the engine because you have put more air into the engine after installing pod filters is a complete misconception, your engine moves the same amount of air through it as it did before and the previous owner was an idiot to put cheap pod air filters on the mouth of a CV carburetor. If that worked Suzuki, Yamaha, Honda plus every other engine maker would have done it that way to start with and profited a whole bunch more money.
I was the idiot who put those on as I didn't know about velocity stacks at the time. Also, I still have the airbox, but the stacks were hard old rubber and would not go on the carbs. If I could get some new stacks, from Z1 maybe, I could try to put the box on again. The dealer that sold the bike 40yrs ago said that the motor had to be off to put the airbox on, and I'm not taking the beast back off. Also, the 42.2 pilots were in the carbs when I got the bike (which was weird to me because I thought stock was 22.2. Could be wrong), and the bike wouldn't run at all until I put the 140s in.
Backstory: Bought the bike bone stock, tore it down to bare frame, repaired, repainted and/or replaced everything on the build up to a cafe racer, substituted a 4-1 Delkevic straight pipe setup, substituted pods with filter foam inserted (per a forum suggestion), and adjusted the mains until it ran, didn't touch the pilots because they were already big enough as it seemed to me.

Remember this though; I just got started in motorcycle building eight months ago when I started this build. Most of what I know about motorcycles, I have learned in that time through trial and error. Anything else, I learned from forums. If you could tell me how you got the airbox back in, fantastic. If you could tell me what size your pilots are, I will try that size. I'm still learning here, so bear with me.
Tire Wheel Land vehicle Vehicle Automotive lighting
Tire Wheel Land vehicle Fuel tank Vehicle
 

·
Registered
Joined
·
2,767 Posts
So generally you can get the airbox in by removing the carbs first, then sliding/jamming the box into place that way.
 
  • Like
Reactions: TrialsRider

·
Registered
Joined
·
7,874 Posts
Re: "The dealer that sold the bike 40yrs ago said that the motor had to be off to put the airbox on"
Dealer lied or didn't know any better. It's never easy to fit an airbox onto a traverse inline 4 cylinder motorcycle but if you had to pull the engine to assemble it then the service manual would tell you that.
 

·
Administrator
Joined
·
43 Posts
The problem isn't "more air" needs more fuel in the case of pod filters. The problem is the lack of vacuum because of pod filters. Less vacuum means less fuel getting sucked up out of the bowls through the jets. So, the solution to that (is really not using pods but...) is to install bigger jets to ease the effort to get the proper amount of fuel into the combustion chamber. It is also recommended to drill out the relief holes in the slides a bit so they can be pulled up fast enough with the low vacuum issue as well.
There is a slight performance gain to be had with opening up the air intake, as well as exhaust. But, as trials said, you are still hitting a max velocity with the heads capability to move it. So the gains are monetary, until you do some head work...
 

·
Registered
Joined
·
7,874 Posts
Laminar Flow :cool:
It not only sounds real cool, it's what your carburetor needs to perform the task of atomizing fuel into the air intake path efficiently. Laminar air flow is the antithesis of air turbulence.
 

·
Registered
Joined
·
22 Posts
Discussion Starter · #17 ·
'The problem is the lack of vacuum because of pod filters.'
That is why one of the forums I read told me to put filter foam in the pods, to slow intake speed. I didn't know about the velocity stacks in the plenum box, but now that I do, I'll try to find some new rubbers and try fitting the box again.
 

·
Registered
Joined
·
1,771 Posts
Those specs aren’t for your year bike. They are for the 77-79 model wit VM26 mechanical slide carbs. Yours would be BS model carbs.
 

·
Administrator
Joined
·
43 Posts
^^^What he said^^^

Also don't have a Kick start, LOL. I am 99% sure I still have a digital copy of the manual for the '82/'83 GS750T. PM me your email, and I can try to get it to you if you don't already have one...
 
1 - 20 of 43 Posts
Top