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Discussion Starter #1
So, a couple things, in making my new exhaust, it seems to backfire a bit. Thinking of an exhaust leak, I don't remeber seeing a gasket in the exhaust. Is it just something that is fitted into the pipe? Or some rubber thing that is compressed between the exhaust and the head? Also, would a hotter spark plug help the backfire? I know in the owners manual it says if there's carbon deposit to switch to a different one. Didn't know if that would do anything. Also, my sputtering is back. I had used the gum out and it seemed to fix it, but a couple tanks with out it, and I'm getting the same problem. Anything I could clean to fix it? It's almost like it's not getting gas, then jumps when it does. This is only at like 3000-3800 rpm though. Didn't know if that fuel strainer could come out easy enough and I could clean it? Would a higher octane gas do it? I'm just sorta out of ideas. Thanks for any input.
 

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On your 400, there is a gasket between the cylinder head,
& the exhaust header pipe. It is metal, but looks sorta
like spirial-wound mesh. They are a use-one-time gasket.
They DO have a tendency to disintergrate, so any time
the exhaust header pipe is disconnected from the cylinder
head, a new gasket needs to be installed. The same gasket
fits both ends of the cast iron exhaust header pipe.
Gasket # 13181-18C00...about $5 US
 

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I had same sputtering issue. Cleaned the PAIR valve,and it got better. Cleaned idle adjstment screw,and there was more improvement,then and used Sea Foam. Problem went away. I think the bad gasket in the exhaust may be contributing to the problem. Also, I found the spark plug was loose, which may have contributed to the sputtering.
 

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FWIW the head pipe on my '03 is a steel weldment...

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GK13, the heat range of the plug will not effect backfiring to any great extent. Hotter running plugs are generally called for when plug fouling occurs from cold climate operation or lost compression, colder plugs can cure pre-ignition problems caused by carbon buildup in the CC.

Backfiring in the exhaust happens when a combustible mixture gets to the exhaust header and is ignited by residual heat in the head pipe, or if a combustible mix is created in the head pipe (though an exhaust leak or the exhaust gas pulse's negative pressure tail pulling in air through the exhaust) and then ignited by residual heat.

A combustible mix can be expelled from the cylinder by a lean or rich intake mixture as either can result in incomplete combustion. With a lean mix a misfire (no ignition) occurs and the lean mix is exhausted unburned, with a rich mix the combustion gasses are only partially burned when exhausted

Air can be pulled in through the relatively short exhaust system on most bikes. This is because an exhaust gas pulse is a sort of tear drop shaped thing with a high pressure head, a median pressure body and a low pressure tail. With a less restrictive exhaust the negative pressure tail can suck air in, all the way back to the header at lower engine speeds. This is assisted at closed throttle by manifold vacuum also pulling in through the exhaust during intake/exhaust valve overlap period. This is called reversion.

I suspect your backfire with the new exhaust may be a combination of a lean mix and reversion.

How does the scoot run at higher engine speeds and loads? I ask because if it runs well at higher speed/loads a fuel supply problem is unlikely as those conditions are also when fuel demand is highest.

In the old days of diaphragm fuel pumps directly driven by a lobe on the camshaft and sucking fuel from the tank, or the gravity feed system used on older bikes, dirty filters and strainers could affect idle and low speed operation. However the advent of tank mounted pumps that push the fuel from the tank at a constant pressure means that there is almost always enough fuel supply for idle and low speed needs.

Sorry for the verbose response, it's been raining here all afternoon...
 

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Discussion Starter #5
Thank you for the explanation. It seems to run better when the ambient air temp is a bit warmer. It also for some reason is running better without the baffle. I had to take the baffle out because the vibrations caused the hole for the bolt to become a slit. And it was just rattling around something bad. Where is the PAIR valve to clean? Is it the thing screwed into the airbox? I cleaned that once and it solved my idle issues permanently. I've done the idle screw many times. Only thing that seems to help it is the Gumout. Sea foam never did much. Would a higher octane gas help it? I'm tempted to try it and see if that would do anything. I looked at the exhaust and saw no type of gasket at all. Is there a reusable kind? Would the revised 08 version fit? And my performance at high speeds is near perfect. Could I use some type of sealant in between the exhaust and header pipe? I would think that would be more air tight than a steel wool type of thing.
 

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Never heard of a reusable gasket. I don't know what the dealer would charge you to put it on if your not mechanically inclined but its about the only way to make the seal airtight.
 

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Thank you for the explanation. It seems to run better when the ambient air temp is a bit warmer. It also for some reason is running better without the baffle. I had to take the baffle out because the vibrations caused the hole for the bolt to become a slit. And it was just rattling around something bad. Where is the PAIR valve to clean? Is it the thing screwed into the airbox? I cleaned that once and it solved my idle issues permanently. I've done the idle screw many times. Only thing that seems to help it is the Gumout. Sea foam never did much. Would a higher octane gas help it? I'm tempted to try it and see if that would do anything. I looked at the exhaust and saw no type of gasket at all. Is there a reusable kind? Would the revised 08 version fit? And my performance at high speeds is near perfect. Could I use some type of sealant in between the exhaust and header pipe? I would think that would be more air tight than a steel wool type of thing.
The PAIR valve can be removed and tossed in the trashbin; make sure you plug the nipples on the airbox and the PAIR reed valve at the front of the cylinder. Removing it will reduced head pipe temperature quite a bit.

Mine came off and when in the bin in June of 2011 just after I got the scoot, it is not a monitored system so removing it will do northing except make the headpipe temperature 100° to 200° F cooler.

The PAIR valve looks like this:



Here is a pictorial of its installation and where your need to cap things off:



And the airflow diagram for the PAIR and PCV systems:



Over the years I have often used muffler cement in lieu of (or to assist aging) gaskets--in fact on my '03 the flanged joint between the head pipe and the mid-pipe is entirely sealed with muffler cement. Let it cure for a couple hours before running the engine.

Any FLAPS will have it...
 

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Discussion Starter #8 (Edited)
So what does the PAIR valve do, or supposed to do, anyway? I think that goes along with the post about how the exhaust overheats the oil. I have already taken the PCV hose and extended it out to do my little mods. And the muffler cement is sorta what I was talking about. Is that stuff removewble though? And it shows in the diagram that there is an actual PCV in the bike. Not just a PCV in the oem hose. Could that be replaced if bad? I know in cars you can usually take a wrench and replace them pretty easy. One side has threads and the other a smooth hose connection.
 

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So what does the PAIR valve do, or supposed to do, anyway? I think that goes along with the post about how the exhaust overheats the oil. I have already taken the PCV hose and extended it out to do my little mods. And the muffler cement is sorta what I was talking about. Is that stuff removewble though? And it shows in the diagram that there is an actual PCV in the bike. Not just a PCV in the oem hose. Could that be replaced if bad? I know in cars you can usually take a wrench and replace them pretty easy. One side has threads and the other a smooth hose connection.
The PAIR (Pulsed AIR injection) system is an emissions control thingy that injects fresh air into the exhaust port/head pipe so that any unburned hydrocarbons in the exhaust can burn up in the exhaust. It reduces levels of hydrocarbons and carbon-monoxide in the exhaust. It also, because combustion continues into the exhaust system, raises the heat of the gasses in the head pipe.

There is no "actual PCV in the bike", however I see your confusion--I should have made those labels read "blowby gasses from crankcase", and "gasses to airbox". PCV is the name of the entire system, Positive Crankcase Ventilation.

Muffler cement is removable, though it can require some chipping at it and a wire brush--I use it on the flanged joint every time I have the muffler off...
 

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Discussion Starter #10
Ok. So it sounds like that muffler cement really is cement. I was thinking of a rubber of something that could be applied and pressure fitted to form a good seal. And how hard is it to get to that pair valve? Is it under the airbox again where everything has to come off? Because if it's not going to give better gas mileage of make it run better, I'm not seeing the point.
 

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There are very few rubber-like products that could withstand 1800+° F, certainly none regularly available at the "happy-homeowner" consumer level; one would need to go to NASA or Lockheed Martin to find them...

Removing the PAIR valve will make the head pipe run cooler and may help (and certainly won't hurt) your backfiring problem. it has no effect on fuel economy, so you decide...
 

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Discussion Starter #12
Lol ok. I guess I'm miles off with the idea of a rubber gasket. I just wish there was a reusable type of sealant that was cement hard that I could use. I have been doing tests with the new exhaust and stock seeing which I like more and why. And after I find those things out, Im modifying the new exhaust to be just perfect. And on that note, what would give me a bit more power on lowend? It handles great once I hit 50, but before that it just feels like it isn't as efficient as I'd like.
 

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Lol ok. I guess I'm miles off with the idea of a rubber gasket. I just wish there was a reusable type of sealant that was cement hard that I could use. I have been doing tests with the new exhaust and stock seeing which I like more and why. And after I find those things out, Im modifying the new exhaust to be just perfect. And on that note, what would give me a bit more power on lowend? It handles great once I hit 50, but before that it just feels like it isn't as efficient as I'd like.
If by "bit more power on lowend", you mean better acceleration then there's not a whole lot to do in the way of tuning (intake or exhaust) to get a lot more low rpm torque out of the 400 engine--Suzuki did a pretty good job on that, and you have to keep in mind it's a thumper so low-end torque is not its thing.

So what needs to be done is get the engine speed at take-off and hard acceleration up into where the engine is making some power, around 6300 rpm. Lighter (18g or less) DRP sliders will do this as they will let the engine get up into that range. and stay there, at take-off and acceleration.

I have tuned my '03 400's variator using four 18 g and four 15 g sliders, and added a 6 mm spacer to increase the pre-load on the torque multiplier spring. With these mods a hard roll-on of the throttle will result in the engine speed climbing to 6400 rpm and staying there until 70 mph if you continue giving it the juice. Roll of the throttle after 50 mph and the revs will drop as the sliders settle in.

At a steady 50 mph the engine runs at 4650 rpm, at 60 around 5550, 70 comes at 6450.

Note: all speeds mentioned above are actual mph, not as would be indicated by the stock speedometer (I have a SpeedoDRD correction device).

Regarding the exhaust: Though it is somewhat counter-intuitive a single cylinder 4-stroke/cycle engine's power will not be greatly affected by exhaust modifications--unless the original exhaust was so restrictive that the engine was taxed just getting the exhaust gasses through it. The OEM exhaust on the early 400s is not that tight. Unlike a multi-cylinder engine header there is not scavenging effect to be had by playing one cylinder's exhaust pulse into another's, and unlike a single cylinder 2-stroke/cycle piston ported engine there is no benefit from exhaust pulse reversion (think "expansion chambers").

What is affected as you have found is the fueling and therefore AFR. This because the OEM fuel maps programmed into the ECU were designed to optimise engine operation WITH the flow characteristics expected of the OEM exhaust system. Altering the exhaust in a manner that changes those flow characteristics , without retuning, can often result in noticeable power losses in some rpm ranges and overall poor/changed performance.

In many vehicles, not just one-lung bikes, the "increased power" perceived by many after installing a custom exhaust (or intake for that matter) is just a very powerful placebo effect brought on by the increased noise--there are threads all over the enthusiast forums surrounding someone's concern because some time after installing "X" exhaust (or intake) they feel that the original "power increase" they were so thrilled with has lessened or gone away.

It hasn't. Thing is it was never there in the first, and what has lessened or gone away is the placebo effect as they have grown accustomed to the increased noise.

I bought a used Leo Vince system for my 400 three years back when the baffles in my OEM muffler started rattling. The only noticeable effect it produced was increased exhaust noise--which I found annoying. Sold it on eBay once I had tracked down an OEM exhaust from an '06...
 

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The PAIR valve can be removed and tossed in the trashbin; make sure you plug the nipples on the airbox and the PAIR reed valve at the front of the cylinder. Removing it will reduced head pipe temperature quite a bit.

Mine came off and when in the bin in June of 2011 just after I got the scoot, it is not a monitored system so removing it will do northing except make the headpipe temperature 100° to 200° F cooler.
Thanks cliffyk

I did your mod so that should help with my LeoVince stainless exhaust system I installed with the DynoJet DFC to adjust the fuel/air ratio.

Hopefully the stainless header pipe will run a bit cooler than before.

Here is a video of the exhaust when I did the first startup.

https://flic.kr/p/nP7N22

Edward
 
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