Back Pressure, performance, heat and damage.

[AllanB]

I have been reading up about back pressure and get a few conflicting views from people about lowering exhaust back pressure.

Jo Modify took part of the baffle out of my exhaust, thus lowering the back pressure, I actually asked him to tune the system to get rid of the popping on deceleration, but something got lost in the translation and it is still present, possibly worse.

Now the effect of that should mean a leaner mixture and more heat, but since the mod the engine seems to run cooler. The engine is a 400cc v-twin, liquid cooler (hence the temperature gauge) with a carburettor, which he also adjusted in the tuning process.

I would like to hear from others about the effects of altering back pressure as many people do this to get a better engine note, some even going for straight through systems, with no baffles.

What I want is maximum mid-range torque, without engine damage or excessive wear .

Edited December 21, 2014 by AllanB

[seedy]

Popping on decel is caused by a lean mixture

Do your carbs have a dedicated idle circuit ? If so, try adding some fuel.

[guzzi850m2]

Setting up carburetor(s) can be tricky and takes time and not many know how to do it correctly.

As seedy mentioned it looks like your bike is running lean and depending of mods you done on your bike you might need to do a re-jetting (bigger jets).

Have you done anything to the air filters?

[yankee99]

My experience to stop decel popping is to remove the pair valve and block it off. Although i dont know if your bike has one.

[dave_boo]

If the popping on deceleration continues after enriching the mixture, I'd be looking at the carb boots and seals for a leak some place. Wouldn't hurt to check the vacuum lines and exhausts also.

[Shurup]

...

Jo Modify took part of the baffle out of my exhaust, thus lowering the back pressure

...

Now the effect of that should mean a leaner mixture and more heat, but since the mod the engine seems to run cooler.

...

If I understand it correct, modifying the exhaust or intake only, will not affect the AFR very much, but it may not be true for all the bikes.

To really screw the AFR you need to change both, then you would have to adjust the fuel maps or rejet your carbs.

[Shurup]

Not related to the OP, but a funny thing I've learnt in the past is when it's very hard to start a vehicle in the winter time (when it just turns over but won't start), plugging up the exhaust pipe with your hand and letting someone else turn over the key, will normally fire it right up! May even work in the warm climate on older vehicles that are hard to start, but I can't confirm that.

[AllanB]

The carbs are set as rich as they can be, so rejetting would be necessary for the baffle I have now. However, when I bought the bike a year ago, I had the baffle changed to quieten the exhaust for the MOT guy and there was no popping at all then.

So presumably I need jets to match the exhaust, or an exhaust to match the jet, the latter being easier?

[papa al]

Changing a jet would likely be easier than changing the exhaust, methinks.

Allan, you look different.

[RubberSideDown]

You're looking to treat symptoms instead of looking for the cause. You've got a major conflict as far as the assumption it's running lean goes (lower operating temperature after the adjustment). Your mechanic adjusted the carb- what did he actually do? Have you checked your plugs? They will tell the story of a possible lean (or rich- decel pop isn't only caused by running lean) condition- if they're white, you're running lean- if they're black and oily, you're running rich).

Running lean does seem possible, but you've already spent a year riding a poorly-running bike and chasing gremlins- if your plugs look fine, live with a bit of decel pop (I do) rather than mess with a bike that's running more-or-less properly- do the easy, less-invasive thing (that will have no effect on performance) and check the plugs first.

[yankee99]

You're looking to treat symptoms instead of looking for the cause. You've got a major conflict as far as the assumption it's running lean goes (lower operating temperature after the adjustment). Your mechanic adjusted the carb- what did he actually do? Have you checked your plugs? They will tell the story of a possible lean (or rich- decel pop isn't only caused by running lean) condition- if they're white, you're running lean- if they're black and oily, you're running rich).

Running lean does seem possible, but you've already spent a year riding a poorly-running bike and chasing gremlins- if your plugs look fine, live with a bit of decel pop (I do) rather than mess with a bike that's running more-or-less properly- do the easy, less-invasive thing (that will have no effect on performance) and check the plugs first.

Most bikes do it its just muffled by the exhaust.

[AllanB]

Just returned from a 600km round trip of the Loei province mountains...ish and the bike ran like a dream, albeit a noisy one. Apart from both soles falling of my very old boots, the only problem was a knurled screw securing the fan switch working loose, which should be a hex bolt (actually the 2014 Kawa Z800 had similar problems when his wing mirrors worked loose).

Also the seat shape needs a re-sculpture, it really played havoc squeezing my botty, but the torque and power were both ample, making the up-mountain twisties great fun and overtaking pretty safe, even in overdrive. So maybe you are right RSD leave well alone, the bike is doing 65mpg, which what I would expect and no dead finger problems, like with the last bike.

By the way Jo fitted 4 new plugs.......oh I forgot the petcock leaks in "main" position so I ran on reserve and filled up every 200 clicks, must be a repair kit available somewhere.

Edited December 25, 2014 by AllanB

[seedy]

With unleaded fuel you can't do the old brown plug color anymore.

Get one of those lighted magnifying gizmos. Looks like a flashlight with a lens at right angles on the end.

If you look at the plug insulator inside the threaded portion you can still detect a color change due to mixture differences.

I always err on the rich side to more adequately cool exhaust valves and piston crowns. Nice chocolate brown.

New plugs easier to read than old ones.

[papa al]

How about a photo of your ride for the fans?

[AllanB]

Changing a jet would likely be easier than changing the exhaust, methinks.

Allan, you look different.

My bike is rare and even getting information on the jets is difficult, but that is for the new year now, when my next alimony check arrives.

Yes, Assange was looking a bit serious of late, so I thought I would employ the services of Cosmo Smallpiece, "oooo knickers, knackers, knockers."

[BSJ]

It's probably the anti-pollution plumbing.

[Garry]

A bike back firing / popping under acceleration is a telltale of a lean condition. Popping under deceleration is not really a major issue unless it is quite severe.

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[AllanB]

It's probably the anti-pollution plumbing.

There doesn't seem to be much in this bike, which is 31 years old, that may also go someway to explain why my NV400 produces 43bhp and the NV Steed produces only 30bhp..on paper.

This bike has no fuel pump (to go wrong) either, which is great.

What would be ideal would be to get a drawing of the baffle, or even buy a baffle, so that I have a start point, but I need to find a way of communicating with the Japanese to do this.

[Shurup]

^ You can try getting small parts for your bike on your trip to Europe, that what I did for my Dragstar when I go to Canada.

[garryjohns]

A bike back firing / popping under acceleration is a telltale of a lean condition. Popping under deceleration is not really a major issue unless it is quite severe.

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Yes and commonly lean idle setting.

Try enrichening the idle mixture a little.

.Also common, is any leaks at all in the exhaust system can cause backfiring when decelerating.

And de-restricting by baffle removal can obviously effect back pressure which also can cause backfiring.

If these basics don't fix it, it's not really doing much harm as Garry says.

Just live with it and enjoy it, my bike does it and i quite like the sound.

[AllanB]

^ You can try getting small parts for your bike on your trip to Europe, that what I did for my Dragstar when I go to Canada.

I travel to Europe every year, but the bike was only made for the domestic market and rarer in the UK than Thailand, the Dragstar is a very common bike, incidentally one I quite like. Nice long wheelbase, shaft drive and nice engine, two of my friends have them and nothing goes wrong, it seems.

Mine is a 1983 Honda NV400 Custom, rectangular headlamp model made between 1983 and 1985, if anyone has one and could take a photo of the baffle I would appreciate that.

[Garry]

^ You can try getting small parts for your bike on your trip to Europe, that what I did for my Dragstar when I go to Canada.

I travel to Europe every year, but the bike was only made for the domestic market and rarer in the UK than Thailand, the Dragstar is a very common bike, incidentally one I quite like. Nice long wheelbase, shaft drive and nice engine, two of my friends have them and nothing goes wrong, it seems.

Mine is a 1983 Honda NV400 Custom, rectangular headlamp model made between 1983 and 1985, if anyone has one and could take a photo of the baffle I would appreciate that.

I had a (bought new) 2002 Yamaha XVS1100A DragStar in the Middle East. Absolutely faultless road bike. Ran like a Swiss Watch

.

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