Straight through exhausts (cautionary tale)

[jjdugen]

Now I'm retired I take on rebuilds and servicing from my bijou garage. A guy turned up with a ZR-7 (Zephyr 750 engine in a mono shock frame) complaining of misfires/backfires. He'd just bought the bike complete with a set of shiny pipes on it, two into one each side (clearly marked 'not for road use') . It certainly made a nice noise but as soon as any throttle under load was applied it popped and banged as it stumbled forwards. Plugs were snow white, lean to the point of dangerous.
Tried upping the main jets until I had them the size of drainpipes, just about ran OK but the fuel consumption was astronomical.
In the end I made up some internal baffles, no more raspy exhaust, but it now runs smoothly, with a good mixture and no more first name terms with the petrol station attendant.
If you must fit these straight throughs be prepared for days spent trying to sort out your carbs or paying for a dyno, its mostly not worth it, save your time and money.
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The CBR900RR has been sold. Aprilia Falco worms its way into my heart.
Try Soi 23 on Amazon for a good read.... Self promotion? Moi?

[MickC]

And on top of all those other problems there is the little extra problem of the decibel restriction placed on bikes registered after 1982/86 decibel ( https://thames-valley-region.mag-uk.org/Pages/legisla_chronology_of_motorcycles.htm ). ( lucky me 81 and fitted with CV carbs which i have been told adjust rather well to straight through pipes - but not to pod filters! )

So crappy gas millage, running issues, a noise loud enough to wake the dead/deafen the rider and they give any bored PC plod an excuse to but fuck you!!!!
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Previous: CG125,XS250 (converted to 400),Z250A, GPZ550, Norton Commando, XS500, 78 Laverda Jota, GPZ1100,Harris bonneville, XV750 (mono shock),TR1

[stinkwheel]

Which is why a dynojet kit, while seeming an extortionate price for a small bag of jets, springs and throttle needles, is actually worth the money. They do the dyno work so you don't have to.

The throttle needles and throttle spring in a dynojet kit are often radically different to the standard ones and the needles are something of a black art, tricky to tweak without a lot of knowledge and skill (and availability of needles).

Fitting a straight through pipe, buy a stage 1 kit, fit it, then tweak on the dyno. One run will probably do it.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[jjdugen]

A DJ kit... £80... £100? A dyno run... £150? A set of pipes... £350... £400? So, to make a bike noisy and actually run worse... circa £500!!!!
____________________
The CBR900RR has been sold. Aprilia Falco worms its way into my heart.
Try Soi 23 on Amazon for a good read.... Self promotion? Moi?

[stinkwheel]

Or with a well designed pipe, a free flowing air filter and a properly set up dynojet kit. More power, smoother fuelling across the rev range and better fuel economy during normal riding.

That's been my experience (VFR 750, K&N filter, straight through Micron end can and stage 1 kit). Smoother off the throttle, better acceleration and better fuel economy. Can't comment on top speed because I've only ever had it fully flat-out twice.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[instigator]

[Kickstart]

Hi

Main jet is just the ultimate limit for fuel flow. Going to have little effect most of the time (ie, when you are not using full throttle / revs). Need to play with the needles for most of the throttle range.

Not surprised some straight pipes really screwed up the fueling.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[jjdugen]

Agreed that most of the time it generally only takes a twiddle with the airscrew and a washer under the needle.... Thats what I thought this time. This engine is the aircooled two valve lump out of the Zephyr (and further back in time..... Looking at its specs for a bit of inspiration, I noticed it has quite significant valve overlaps, as did most early engines. Looks to me as though the exhaust valve, staying open as long as it does, was just allowing mixture straight out the pipe. With no back pressure to contain the mixture it was leaning out the charge to the point of danger.
____________________
The CBR900RR has been sold. Aprilia Falco worms its way into my heart.
Try Soi 23 on Amazon for a good read.... Self promotion? Moi?

[Kickstart]

Hi

Don't think the actual pressure is the issue as it is the pressure waves bouncing back that cause it (actual pressure would really screw things up, in the same way that you get a large flat spot at half the rpm the exhaust is working at). Longer or shorter pipes change this dramatically. Larger bore pipes destroy this effect.

Possible that with a long valve overlap it is pushing mixture back through the engine into the carb and increasing the pressure around the choke. Stops the carb working to draw mixture from the float bowl while also stopping the "slide" in the carb from lifting (as I am sure you know but for others info, on a CV carb the slide is lifted by the lower pressure caused by the air speed through the carb).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[jjdugen]

Interesting take, Keith, not thought of it in that way..... Will digest your theory.
____________________
The CBR900RR has been sold. Aprilia Falco worms its way into my heart.
Try Soi 23 on Amazon for a good read.... Self promotion? Moi?