trying to tune with new mufflers

[twist]

I recently read with much interest the thread about mufflers and exhaust. I read up a little and now am taking a stab at it myself, (having conquered the wheel bearings has made me cocky).
My engine is a 77 R100/7. The carbonation is managed by two Mikuni TM 38 flat slide carburetors using stock 40mm intake tubes to a stock clam shell air box that is perforated, (not sure if that is a stock item, the holes that is) and a K&N filter. No leaks that I can detect in that part if the system. The exhaust is managed with a stock 38mm header with a single crossover. The part I installed is a new Dunstall reverse cone mufflers. I was running Mac Whisper tones. With the installation I increased the main jet size in the carburetors 1 size. So fa so good. I took the bike out for a test and it feels different, as expected. Low end torque seems good but the acceleration seems a little flat after about 4000 RPM. I have a broader range in 3rd and 4th gear. Before changing the mufflers the bike was very responsive in every gear but the RPM range was narrower. New mufflers are 26.5 inches and the whispertones were about 28 inches long. I changed mufflers to complete the system by attempting to help it breath better. I need to check the plugs to see if I can go back to a leaner main, I think that may help the response at higher RPM?

[vanzen]

Not sure why you would choose to increase main jet size –
assuming the engine ran well with the previous smaller jet
and only the cans were changed ...

To determine how well the main is delivering:
Install a set of fresh plugs & run a throttle-chop at WOT ...

then check the plugs ...

[twist]

I used a larger jet because the new mufflers will cause the engine run lean. However, I think it was fine the way it was. Will the power band be changed by the jet size or is that more an effect of pipe length?

[R85/8]

Unless the manufacturer (Dunstall) has tested these mufflers on a R100 and provided jetting and performance data, you're on your own. I'd recommend you find somewhere with a dyno and get the bike tuned on that.

My previous experience with Dunstall products leads me to regard them as fashion accessories rather than performance items, but that was back in the days when I was running Norton Atlases and Commandos.

[vanzen]

"... seems a little flat after about 4000 RPM"
Sounds to me like a rich condition – but I'm only guessing.

After-market muffler and exhaust system manufacturers are fond of suggesting a larger main ...
Without additional changes to the "SYSTEM" –
often (at least in my experience) the change will be unnecessary.
i.e. your assumption, "the new mufflers will cause the engine run lean"
may very well be incorrect.
There is very little room for "assumption" when tuning carbs !

"Throttle-chop Tuning" will be the old school method
to determine how each jet, the needle, and each circuit is performing.
More tedious and time consuming than an O2 sniffer or a dyno
and perhaps even frustrating at times for the novice, the method IS effective and works.
To your advantage, tuning a slide-carb using this method
will be much easier vs a CD carb as the (original) Bing.

Get a batch of fresh plugs,
mark the throttle barrel to indicate it's position at each circuit,
Run each circuit under load and use the kill switch to shut-down the engine,
THEN read the plugs to assess how each circuit is performing.

[Major Softie]

The carbonation is managed by two Mikuni TM 38 flat slide carburetors using stock 40mm intake tubes to a stock clam shell air box that is perforated, (not sure if that is a stock item, the holes that is) and a K&N filter.

I'm still enjoying this.

I'm guessing it was "attack of the Spell-Check."

[Garnet]

The carbonation is managed by two Mikuni TM 38 flat slide carburetors using stock 40mm intake tubes to a stock clam shell air box that is perforated, (not sure if that is a stock item, the holes that is) and a K&N filter.

I'm still enjoying this.

I'm guessing it was "attack of the Spell-Check."

That's why I don't use spel cheque.

Edit, I would never know wear to where my gloves.

[twist]

The carbonation is managed by two Mikuni TM 38 flat slide carburetors using stock 40mm intake tubes to a stock clam shell air box that is perforated, (not sure if that is a stock item, the holes that is) and a K&N filter.

I'm still enjoying this.

I'm guessing it was "attack of the Spell-Check."

That's why I don't use spel cheque.

Edit, I would never know wear to where my gloves.

now that's funny! The first carbonated bike. I was trying for caffeinated.
I suppose it's true that the dunstall mufflers are all looks/sound. I really was trying to get a low cost set of replacement silencers for the whispertones that would breath a little better. Maybe a good point to back up and re evaluate what I'm trying to accomplish. I've read up on the chop method and that will certainly be my next trial to narrrow down the jetting a little better. I did replace the larger jet with the size that was there and got rid of the flat spot at 4000RPM. I went from a 200 to a 210 and then back down to the 200. So far the bike runs pretty much the same as it did with the whispertones. The dunstalls are a little shorter but there is a reverse cone outlet, not a bell end. The dunstalls are almost the same loudness as the whispertone, (not much caring about the loud pipes) I want the best performance for the way the bike is set up. I'm close but I think it can be tweaked a bit more. Right now I have crisp throttle response, great acceleration and a broad power band with a lot of low end torque. Highway speeds are a bit buzzy but good pick up when the throttle is opened in 5th gear at 5000RPM. Thanks for the input everyone. I'm learning a lot here.

[boxertwinjeff]

I noticed that the Emgo Norton Megaphones are 27" long,. considerably shorter
than my Staintunes. What's the Physics behind the Megaphones being Short?

[Roy Gavin]

Get yourself a copy of Phil Irvings book, Tuning for Speed.

He covers the subject well, and the rest of the book isn't bad either bad either.