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Hello people, I'm about to take possession of my first standard bike and have a beginner question. So without making my question too generic, I'd like to make it more specific to an 80's Honda Nighthawk CB450SC which has 2 exhausts.

The previous owner replaced the 2 stock baffles with ones that have larger holes. He said that he rejetted the carbs to match the exhaust.

My question is what would be the effect of this modification on power, fuel consumption, and noise?

What would be the effect of replacing the jets with stock jets?

Thanks in advance.
 

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Jets

Stock is a good place to start. What you want is the correct size jets. The Mikuni guide says to start with the main jet for the best power.
Sometimes the online logic does not pan out. Why?
Logic says if we put on better air filters and exhaust the engine will breath better and require bigger jets.
Logic says if we increase the compression or porting, or increase the piston size the engine will breath better and require bigger jets.

BUT: If the engine breaths better, it will suck better, and if it will suck better it will suck in more fuel through whatever jets are in there. To get maximum power you may have to go smaller on the main jet. However if your motor is down on compression it will not suck very well, and may require larger jets.

Chainsaws have adjustable main jets. You hold the throttle wide open. Screw in to lean, until the engine goes pop and basically stops like running out of gas. Just like John Denver. Screw it out until the engine screams, and then starts to gurgle from too much fuel. Because the saw needs power to cut, you set it at a bit out from the screaming position.

So ride the bike at WOT in third and see what it does. Pop, too lean, gurgle, too rich.
4 strokes are nice in that they generally do not seize when they are too lean.

Unkle Crusty*
 

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There is a science to it. Sadly most do one of three things. Listen to someone who they think knows, or they listen to the exhaust manufacturer, or they randomly try different combinations till they either don’t like it no more or they feel something that they do like.

But as for what will you notice is louder and sometimes no change in fuel but more often is more gas consumption. I think yours is air cooled so might make it run hotter? I would think, maybe.
 

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The previous owner replaced the 2 stock baffles with ones that have larger holes. He said that he rejetted the carbs to match the exhaust.

My question is what would be the effect of this modification on power, fuel consumption, and noise?
As a general answer, if he did nothing else, such as a free flowing air cleaner or porting the heads or changing cams, and the correct jets were installed, then there may have been a very slight increase in power, a very slight increase in fuel consumption, and a noticeable increase in sound from the pipes.

What would be the effect of replacing the jets with stock jets?
Assuming the stock jets are smaller then what's in there now, you'll have a very slight decrease in power and fuel consumption, no difference in pipe noise, and an increase in engine heat.
 

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So, put in fresh spark plugs, ride it a while, then take the plugs out and examine them. They will show whether it is rich or lean, on average. To know if the mixture is correct at each operating range, you have to do throttle chops (or have it dyno tested with an exhaust probe).

Opening up the exhaust on the older bikes would often lead to burnt valves, and running rich cools them, so bigger jets were installed, often without determining which was the actual correct size.
 

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I like stock, takes all the guess work out of it.

That makes sense though have to have bigger jets on two strokes, when you are wrapping them up tighter or big bore kitting them. I know complete paradox to what I did with my scooter, but that just confirmed what a PITA it was to Jet something when you don't know nothing about it.
 

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Meh. Have a buddy put one set of jets in and NOT tell you if they're the stock ones, one size larger, or one size smaller. Go ride it and see if you can even tell any difference.

Leave in the ones you THINK are giving you the most of what you want --- either brute, low-end power, best throttle response, smoothest cruise, best fuel mileage....and check the plugs after a few hours of riding
and again a day or so later. Anything but running LEAN is ok.

Zen and the Art of Motorcycle Maintenance describes how the author's bike began running rough as he went up in the Rockies, up to where the air is thin and (these days) dope is legal. He re-jetted on the side of the road.

Carburetors and airflow SENSORS cannot alone detect a change in altitude, it requires a MASS AIRFLOW SENSOR like on a modern EFI bike to detect that the actual amount of air being ingested has decreased. Or a simple "airflow sensor" combined with a bunch of barometric sensors and some software (FORD, perhaps HD).

On a modern EFI bike an oxygen SENSOR detects a LEAN condition in the exhuast and causes the EFI to "adjust" the fuel map to richen things up at the regime (rpm, load...) where it's seeing a lean, and to lean it out where it sees it running rich. It constantly adjusts up and down looking for a sweet spot, and can even tweak the timing a bit -- like advancing or retarding timing AT idle for better emissions AND a smoother, lower idle.

Disconnecting the battery, btw, starts that "learning process" all over again, making fine adjustments to the fuel/ignition maps.

So in the EARLY days of emissions controls, certified "repair stations" shoved a sensor up the tailpipe of a bike/car that gave immediate readings of N0x, C02 HC (hydrocarbsons) etc at idle and under load (on a dyno, typically) and those TOLD you if you were running rich at idle, overly lean at cruise, had plugged up EGR, vacuum leaks, etc.

Those tailpipe sensors are now dirt cheap on Ebay, OR you can fit your bike with a wide-band 02 sensor (welded into the header, it's gotta get VERY HOT to work), and use a simple DVOM or gauge to read rich/lean while out driving, or on a dyno. (Hint, it's a wild ride watching it swing AND trying to stay in yer own lane... I'd prefer just to do it by feel...)

So bottom line is the "good ole boys" who played with carburetors "back in the day" do adjustments, jetting, etc. by feel, by sound, by smell, by the look of the plugs, and the new computer-controlled, oxygen-sensored bikes with EFI do it all by themselves.

(Which is why putting a "Juice Box" or other tuner on a bike is generally unnecessary, a waste of time and $, as well as a good way to run lean and destroy your motor. Or else run rich and waste a lot of gas.)

There are indeed SOME mods (pipes filters, cams...) on SOME bikes with, ahem, more "primitive" EFI systems that need the assistance of a tuner box to run right.

Goal here is to have fun, to play around with jetting, to accept that no carburetor will EVER get it perfect and to add to your skillset by learning how close you can get to perfect with YOUR bike and YOUR carburetor and YOUR collection of jets/needles.

Zen and the Art of Motorcycle Maintenance left out one critical piece of advice...find a MENTOR to teach you about carburetors and tuning. Self taught is the hard way.
 

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Jets

Back in the day during the winter months we would take apart the race bike engines, and do whatever work was required. Usually rings for the 2 strokes, sometimes crank bearings, valve work for the four strokes, check the crank and cam bearings. Plus all the frame bearings and brakes.
We had to take them to the track to fire them. We had a stack of jets, would do many plug checks. If we went too lean on the 2 strokes they seized.
The 2 strokes often had to be started on hotter plugs, a 7 or 8, and switched to a colder plug, about a 12 using the NGK range.
We did all this with out a sniffer.
I have changed the jets in my XS400 at least 10 times. It now runs real sweet.

Unkle Crusty*
 

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A lesson many learn the hard way, with burnt valves, holes in pistons, and other damages. Done properly, it takes a dyno, exhaust sensor, and experience.
Speaking of holes... Perfect example... DT100 and my doing one of the hard lessons of know what you are doing before doing it...


 

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New port

The new Aphrodite tuning port allowed for increased flow of fuel to the combustion chamber, but sadly the compression left town. Quick trip to the bike shop for a can of compression, to get the mighty DT100 running again.

I think I need a scanner, so I can show some old photos.

Unkle Crusty*
 

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Discussion Starter #13
As a general answer, if he did nothing else, such as a free flowing air cleaner or porting the heads or changing cams, and the correct jets were installed, then there may have been a very slight increase in power, a very slight increase in fuel consumption, and a noticeable increase in sound from the pipes.

Assuming the stock jets are smaller then what's in there now, you'll have a very slight decrease in power and fuel consumption, no difference in pipe noise, and an increase in engine heat.
Thank you for all of your replies.
@Eye_m_no_angel, thank you for answering all of my questions. I will be doing some carb work on the bike soon and my concern was that I would only be able to put stock jets. In that case, the bike would be running hot (because it's running lean?) and combined with an air cooled engine, this would be a bad situation. I think I will look for a used pair of stock baffles for the bike if that is the case.
 

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Since this is an 80's bike, start by looking at the plugs in there now; they will tell you about where the engine is, unless it idled for a long time. You may just need to adjust the jet needles a bit, and leave the main jets alone; CV carbs can be tricky that way.
 

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I like upgraded machines when they are race toys, but stock if not. You have to think about ins and outs when getting into jets and pipes. Boring jets, increasing air intake (open filters), and porting / polishing all help with the input of fuel/oxygen. An exhaust system helps with the output that should match the increased input.

Jetting an engine without increasing air can cause for richness in the ratios, just as increasing air intake and not fuel can create lean circumstances.

Having pipes with no jets or air increase is a waste for the most part as the pipes are designed to work with higher inputs. And vice versa, if you up your jets/air and use a stock exhaust, you wont have the right input versus output.

In my opinion, it is all or nothing.
 

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Since this is an 80's bike, start by looking at the plugs in there now; they will tell you about where the engine is, unless it idled for a long time. You may just need to adjust the jet needles a bit, and leave the main jets alone; CV carbs can be tricky that way.
^^^True.

If it has adjustable jet needles try working with them first.

I'm assuming what you're wanting to do is re-baffle the bike to make it quieter?
 

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Plugs

Is that an Aphrodite(R) tuning port or a Pandora Piston (tm)

You said you had to start on a hotter plug. When did you step down as the engine broke in are we talking hours days or weeks?
Minutes.
I am generally referring to 250 Yamaha GP 2 stroke bikes. They made about 45 hp in the early 70s and ran 12,000 revs safely. The port timing was very radical, so much so a rookie could never start one, or get it to take off in first.
To start the engine a number 7 or 8 plug was put in, the bike was pushed in first or second, let the clutch out, gurgle gurgle with zero throttle, then the throttle was squeezed very gently until crackle crackle crackle. Pull the clutch in and gently squeeze some more so the revs build slowly. Once the engine is running clean the throttle can be opened willingly, with the bike in neutral.
You check the cylinder heads for warmth. Once warm enough, you shut it off and put in a 10 11 or 12 depending on the weather. Now the bike should run on the hotter plug, but caution is required to fire it up. All this is timed for the start of a race. At the start line you keep the revs above 3,000 and up to about 8,000. When the flag goes up you run it up to about 10,000 and go WOT as the flag comes down. Let the clutch out too quickly and the motor will bog and stall, and not readily fire on the hot plugs. The clutch is slipped and tortured for quite a while until the bike runs clean. After that you stay above 8,000. I never seized any of my Yamahas.

Break in time was the warm up time. We put the engines together, then ran them at the track during practice, checked the jetting and raced them.
We kept accurate records of all the engine info, and any other related stuff.
The got the 250 2 strokes up to about 85 hp before they switched to 4 stroke singles, which make about 48 hp

First race for the 015 championship starts this weekend.

Unkle Crusty*
 

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Discussion Starter #18
^^^True.

If it has adjustable jet needles try working with them first.

I'm assuming what you're wanting to do is re-baffle the bike to make it quieter?

Guys, I really appreciate all the help. This is all new to me but I'm a fast learner. I will check the plugs and adjust the needles if they are adjustable or not (I don't know what the difference is at the moment but I can do some research). So from the above messages, a dark plug suggests it's too rich, and a white plug suggests too lean.

And yes, my first objective is to make it quieter, and the second objective is to stay away from non-stock jets so that they are easier to replace.
 

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starving or flooding

there are a couple easy tests to see if you need jet work.

1. engine running warm on centerstand, exhaust pumping outside...
thinking like a rocket scientist, grab the throttle and rev it once real fast.
did it stall? was the stall instantaneous? did it stammer like it was choked up? did it hesitate and then rev slowly up, these are signs of flooding, you have big jets in there and they are capable of handling all the fuel you can pump in.
did it take a second for the rev to build? was the rev a slowbuild up to a high rev? was it a slow speed up with no rev? These are signs of small jets in there.

If someone told you it has big jets, you will find all of the first one, none of the last ones. Personally, mechanics increase jet size to compensate for a power shortage, wither because they are fat and heavy and the bike bogs down at startoff, or because they have a steep hill they need to climb regularily. There is always some good reason for jetting, if it stammers, you should be contious of the fact that you have bigger jets, and you need to learn to run them...

"never change the carburetion in an engine, it has been dynamo tested at the factory to maximum horsepower" but if you bore stroke or port, you should, at least, consider jetting, turbocharging or nitrous.
 

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..........."never change the carburation in an engine, it has been dynamo tested at the factory to maximum horsepower" ............
I don't fully agree with the Quoted Statement above. If I followed THAT logic, I'd never move the 87 percent settings on my Modulation Pot of my CB, and never get the maximum output potential.

What does one (the CB) have to do with the other? Only the fact that Factories have Delta settings (Nominals) that they set things to, and there is room for "tweaking."

I don't attempt to do anything that I don't know much about. So I wouldn't delve into this arena if I didn't have a confidence that I could back myself out of it, without damaging the thing I'm attempting to "Tweak." Obviously the previous owner decided to "Tweak" the bike, and you should decide if the work done to it is good enough "as is" or not. Is it worth playing with at this point?

Is it truly giving you THAT much trouble, or are you just looking to "play" with it for the sake of "playing?"

To the folks who post pictures in here: Please download the free "Irfanview" Photo Editing program. It has a feature in there that allows you to reduce the size of pictures to a reasonable useable size for Forums. I myself have let some photos slip thru in the past, that were oversized, so I don't excuse myself in this request. Just .............well.............make em smaller, will ya!?

I guess I'm feeling my Wheaties this morning or something.......maybe it's time for another cup o' Joe!! (lol)

-Soupy
 
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