Question regarding scooter exhaust By zomgos (Read 1,259 times)

[zomgos]

I saw some posts on various forums talking about drilling holes in mufflers to make them faster. 

Has anyone tried this or have any experience with this?

Will drilling a hole in near the exhaust port on the cat/muffler increase power and top speed? 

My scooter seems to top out at 60kph and want to make it go a little bit faster. 

[jjoshua20213]

On a stock engine all your doing is loosing back pressure. The only time you need to need to let more air out is when you physically change something create more air going out.

Think of your home vacuum cleaner. Its sucks air in and filters it with a bag, and blows it out of its exhaust port. widening the exhaust port will not make air suck in faster or speed up the cleaning process.


Now think about having an upgraded blower motor physically sucking/pushing the air faster. Opening the ports will allow the "extra" air to flow freely, but would just decrease velocity without the added force

[Guitarman]

Some scooters have a restrictor in the muffler pipe near where it connects to the exhaust port. Unless you know how to weld, forget about getting it out. It's much easier to get a muffler that's not restricted.

Also, look in your CVT. There is a washer in there that keeps you from hitting the full top end its capable of. Remove it and you will get maybe 5mph more.

Your best bet is to adjust your weights in your variator, change the air kit and upgrade the carb. That will be the biggest bang for your money.

[drahken]

if its a 4stroke, my best upgrade, was a uni filter and a 90 main jet. and, lighter rollers. but not rollers, sliders. 5 grams. if this is a 4 stroke chinese. and i knocked, on mine, the bung plug that comes out the end on the exhuast, off. it openned the hole up a little bit, and i noticed a little bit more power. this was a 08.

[jjoshua20213]

If you want a quick FREE easy way to let air flow free"er" simply take out the exhaust gasket. It will allow the exhaust to leak a little providing the same effect s drilling only reversible.

[Bashan]

Have to disagree with that one. A leaking exhaust leans out the mixture and can cause backfiring. Second, you expose your exhaust valve to cold air and it can warp the sealing edge of the valve and hurt compression. A certain amount of back pressure is engineered into engines. If you are going to mess with that you must do it correctly. You need a lower resistance exhaust with a good gasket and then upjet to keep the mixture from leaning out. JMO, Rich

[drahken]

i have to disagree, lean doesnt mean backfire, lean is detonation. rich is backfire, and shoots out your exhaust.

[Guitarman]

I have to disagree. I think Rich's avatar is a little too happy with that needle...

[cyborg]

One of the reasons i would not do the gasket removal is long term head seat erosion ,,,or as they say flame cut,,,

[Alleyoop]

WOW!! Then people wonder why I have this saying in my SIG.
WATCH OUT...THOSE WHO DO NOT KNOW, DO NOT KNOW THAT THEY DO NOT KNOW

[Bashan]

The following is from Randy Pozzi, one of the most respected experts scooter mechanics. He has written several service manuals and is considered the ultimate expert on the CH 250 and the 150 Elite. Take it away Randy:


#21 Backfiring On Deceleration
Hello Group,
One of the most asked questions pertaing to the Honda Elite CH 150/250 and CN 250 Helix involves a backfire through the exhaust on sudden deceleration.

An engine is not supposed to backfire and, when it is persistent, It could be harmful to the engine. If there is an explosion in the exhaust pipe when the cylinder is trying to exhaust its spent combustion products, a back pressure is created which interferes with the next cycle. This could result in incomplete extraction, irregular charging with the fresh mixture,
and overheating. In addition, valves are not designed to seal against pressure from their backsides and a broken valve rattling around in the combustion chamber is a real possibility. While the explosion sound ranges from a baby fart to a gunshot, its annoying to say the least. In almost all the situations studied various causes and remedies have been found with none answering the question entirely.

In general, backfiring on deceleration (as opposed to acceleration) is generally caused by a lean condition in the pilot circuit. What happens is that the mixture leans out enough to where it fails to ignite consistently. This, in turn allows some unburned fuel to get into the exhaust pipes. Then, when the engine does fire, these unburned gasses are ignited in the exhaust pipe, causing the backfire. Then, the classic diagnosis is too lean a fuel mixture. The real mystery is where that lean condition is coming from.

Here are the suspects:

1. Low Idle. Some folks cure backfiring by turning up the idle slightly. This is the screw with the spring on it in the right front of the CV carburetor. Recommended rpm is between 1100-1500. Many scooters never run at the low end of the recommended idle rpm range anyway. Turn up the idle first.

2. Exhaust Gasket. This seems to be one of the most frequent causes of backfiring problems and an item that has been seen to fail quite often. Air is sucked back into the combustion chamber causing a lean condition at ignition and the backfire. The exhaust gasket is a small metal rimmed fiber edged part that people seem to forget to inspect when servicing the exhaust or leave it out altogether.

3. Leaking Carburetor Intake Boot. The CV carburetor is fastened to the intake with a stiff rubber intake boot. Sometimes it is not sealed properly or gets a small crack in it enough to allow excess air into the intake. The extra air from the split boot will cause the popping. To check your intake rubber, you can spray WD-40 in the suspected area while the engine is running. Any decrease in engine revs confirms a leak. Another method is
to use an unlit propane torch, just barely cracked open, and see if the idle changes when the gas gets sucked in. Don't get carried away or you will flame the scoot! Less messy than using WD-40.

4. Leaking Vacuum Hose. Any misconnected or leaking carb vacuum hoses can also encourage backfiring. That means the carb fuel mixture is also slightly leaner, more prone to backfiring. Check for cracked or stiff bending hoses and replace if necessary. Putting the hoses back correctly may help with the backfiring without having to adjust the idle screw or pilot screw. The misconnected hoses can also degrade low end response slightly.

5. Pilot Screw. On the back of the Keihin Constant Velocity (CV) carburetor is the pilot adjustment screw also known as the enrichment circuit adjuster. On the CH 150, it is covered with a plug which has to be removed. On the 250 cc motors, this screw should have a limiter cap on it to vary the adjustment. For a rich carburetor adjustment, turn the adjustment screw clockwise (in toward the carb body) a tad to eliminate a lean running
condition. All adjustments should be made with a warm engine. See Tech Tip #7 Pilot Screw Adjustment & Fix for a more detailed service).

6. Air Cut-off Valve. This component is the prime suspect. On the back of the CV carb is the air cut-off valve. The air cut off valve enrichens the pilot circuit on deceleration by means of a rubberized piston. One hose to the valve (nearest air filter) is the air tube connector and the other (nearest the autobystarter) is the vacuum connector port. If air pushed through the cut-off valve by a pressure pump leaks by the vacuum port connector while vacuum is initiated with a vacuum pump, replace the air valve. I replaced a faulty air cut off piston and my backfiring ceased. The air cut-off valve is expensive (about $70) and should be replaced after all other suspects tested have failed.

7. Enrichening The CV Jetting. I solved a similar deceleration backfire on a CH250 by removing the 112 (lean) main jet on a CH 250 and using a 115 or 118. The CN250, which uses a leaner 110 main jet, could also benefit from the main jet enrichment.

Randy Pozzi (Rev. 01/2005)

You can read more of Randy's valuable advice and tips by viewing his website.

[Alleyoop]

You forgot to mention Randy Did indeed attend my class on this subject

[Bashan]

I know pal, I just figured an outside source would put a stop to further debate on the matter. You have said all along the first thing to check with backfiring is the exhaust gasket and then the ACV valve. When the Oopster speaks we lend an ear. Rich

[drahken]

heres what i read on another site.
anything that will raise temperatures in the cylinder will help promote detonation

hence my reason for saying that taking the gasket off will detonate. rich conditions, which isnt from taking the gasket off and thats what we where discussing, will lead to an air leak and detonations, rich conditions arent from lean conditions and they happen when fire comes out of the < made up by me > it happens, of the exhaust port as the exhaust gasses are being, expelled out fo the port or valve, and reignite leftover exhaust build up gasses in the muffler.
.. and where talking about a backfire and not all that stuff.

if somenoe couldnt describe it, noone could of made it to begin with.
the thing your readme, fails to mention, is how lean leads to backfire.

and the thing im confused about in your readme is that, what do you mean by backfire? did you say it backfires when the exhaust valve gets blown? sure i bet a lot of fire gets shot out when that happens.

[Alleyoop]

You missed a couple of sentences:
Alleyoop

What happens is that the mixture leans out enough to where it fails to ignite consistently. This, in turn allows some unburned fuel to get into the exhaust pipes. Then, when the engine does fire, these unburned gasses are ignited in the exhaust pipe, causing the backfire.