Introduction and questions

[OG-Rider]

Hi guys --

I bought a pretty nice '81 XS400 Special last September. It has 9500 miles on it and the over all condition is good to very good. I grew up surrounded by motorcycles, and if I counted correctly, this is my 15th bike. It's a real pleasure to rip around town on, but on the road it has some issues. It is annoyingly under powered and not very stable at freeway speeds. I'd like to change that, so I have a couple questions.

I read somewhere that there is an XS360 swingarm that is a couple inches longer than mine. Which one?

Theoretically, it should be really easy to make horsepower with this engine: 180 degree crank, overhead cam, good quality dual carbs, electronic ignition, et. A guy ought to be able to make an XS400 engine run like a rocket. The problem is parts availibility. Now here's the rub; an internal cumbustion engine does not know anything about brand names or model numbers. It only cares about dimensions, weight, metalurgy, and so forth. So, has anyone here used pistons from some other scooter to increase displacement?

And finally, the combustion chamber volume is part of the formula you use to calculate the amount to mill off a head to achieve a specific compression ratio. Considering the low quality of todays fuel, it is best not to exceed about 10.5:1. So, what is the combustion chamber volume?

Thanks,
Bob

 

[OG-Rider]

I just read a post where someone said you can use Kawasaki GPZ1000 pistons to get 440cc. Anybody know if that's good info?

 

[16VGTIDave]

I think you'd be better off opening up the exhaust and intake, then clean the carbs excessively before jetting them much richer. This will get you a noticeable increase in power.

For increased stability, I've become a fan of Dynabeads. Not only do they balance the tires perfectly, but their mass is distributed as close to the tread as possible so they increase the gyroscopic stability of the wheels. And, of course, make sure that the suspension and wheel bearings are in good order.

Dave

 

[OG-Rider]

I think you'd be better off opening up the exhaust and intake, then clean the carbs excessively before jetting them much richer. This will get you a noticeable increase in power.

For increased stability, I've become a fan of Dynabeads. Not only do they balance the tires perfectly, but their mass is distributed as close to the tread as possible so they increase the gyroscopic stability of the wheels. And, of course, make sure that the suspension and wheel bearings are in good order.

Dave

I don't have any wheel and tire balance issues, and a longer wheel base is inherently more stable than a short one. In addition, the japs always had a bad habit of using front tires that were a bit to small. I'll address that when I buy tires.

Intake and exhaust improvements are a given. Both tend to be very restrictive on old japaneses motorcycles. However, intake and exhaust are only two of the five paths to power. Increasing displacement and compression are inexpensive and effective ways to create horsepower.

As for displacement, I just need to find a pair of 73-74 mm pistons that will work.

If I know the combustion chamber volume, I can calcute how much to mill off the head myself. That means any good machine shop can do the work, and I won't have to take my head to someone who is 90 minutes away.

On the intake side, the stock mikunis are good carbs, and plenty big enough to handle more displacement. Good quality pod filters, approprate jetting, and getting all the stock knick knacks out of the way, should be fine. But that doesn't go far enough. I noticed that Megacycle sells a cam with a bit more performance oriented profile. This combination will work out well.

The stock ignition seems really good. This bike can sit for days, and start on the first kick. I'll index a pair of iridium plugs during the engine build. I think that's the only ignition improvement I'll make.

Exhaust is the one area where I sometimes compromise. What I think sounds good, and what performs best on a dyno, are not always the same thing. The '81 Special has welded on mufflers. I'll use a die grinder and cutting wheel to careful remove the weld, and then put on some short megaphones with removable steel baffles, so I can experiment with sound and performance.

 

[ksqrly]

ANy updates on the engine build?

 

[Nuno]

you can use pistons from the Suzuki GS 500 twins.
Anything with a 16mm pin (if I remember correctly) would do the job with little or no pain at all.
I had a few notes at home about some pistons that would give a 560cc displacement but that would require machining the cases as well.

Today's fuel is bad? Well, my zx6r runs at 13.5 compression ratio with regular fuel.

 

[ksqrly]

If you search "big bore" in the search you will find a thread called the mother of all big bores. aaronxs400 made a diagram and it says the gs 500 pistons have 18mm pins. Is this wrong? He also has a picture of gs500 pistons installed (i think). Do you know anymore about this?