GZ250 power increase.

[trykemike]

Hey Quime rider

I did a test yesterday evening. The temp was about 20 c ( 70 ) degrees and almost no wind. I got out on an open stretch of highway
no hills . The bike was running well. I could easily hit 120 k/ph so I got upto 100 k/h then pulled the choke lever about 1/8 forward. I
opened the throttle and pulled easily to 120 k/h then open the rest of the throttle I looked over at the tach it read 8000 rpm ( 5 th gear ) and the speedo was touching 130 k/hr (81 mph ) it was still pulling but I had run out of space so I let off .

Very rare to get ideal conditions but 80 mph is acheivable. ( still have stock 120 main jet ).

I read that the choke pulls a plunger that opens an ericher jet and air jet to feed the engine. I spoke to qa mechanic who did and engine rebuild for me in the past about this phenomenon. He said that the mix had been enrichened enough to allow the engine to reach
its power potential.

Now that it is done I will slow back and drive to enjoy the ride. I just had to see the bike reach it's potential.

BTW I get 85 mpg regularly .

[Quimrider]

Wow! What type of driving do you do to get that high of mileage? I do about 85% freeway @ 105+k/h(65mph+) and the rest in town. I got about 70-75mpg with the stock air filter and stock 120 jet. I take back my earlier statement. I think I'm running rich because my mileage has dropped to ~55mpg after doing the needle shim, 130 jet and K&N filter. Other than being much noisier I do have a little more pep at the high end, but still about the same top speed. I did manage to hit 84mph briefly before the proximity of the bumper in front of me convinced me to slow down I think I'm going to try the 122.5 jet that I have with the K&N and see how that works.

[trykemike]

Hey quimrider

I usually drive on the outskirts of toronto . back roads at speeds varying from 80 -90 km/hr when I get the 85 mpg ( 150 km / 5 litres of gas ).

The mechanic that I spoke to actually suggested a 122.5 main jet for the bike because they are jetted low to acheive great fuel economy at the expense of a slight decrease in performance.

He is a good mechanic he rebuilt the engine on my 25 year old honda 250 3 wheeler.

[themarauderer]

Your doing well with 75+ MPG...I have the 125 and get about 75-78 MPG approx.

This is mostly town riding, doing no more than 40mph most of the time....that said, I sometimes take a wee ride about and can reach, on a good day 60ish MPH...not much more tho . Going down hill, APPARENTLY the GZ125 can touch 70ish .... I, have my doubts.

Its a 4 stroke, 125cc bike. It does have limits. The 250, without doubt but the 125, if its hitting 70ish, God himself must be blowing it along ;-)

[trykemike]

hey Easy I did a test with the air filter. I took it out to have it cleaned at work with compressed air. I took off the diverter and inhaled to check the air flow near the bottom of the filter. I then put the diverter back on and performed the same test I noticed more restance. I drilled a 1/2 inch hole in the centre of the diverter. I reattached the diverter and installed the filter ( which was clean ) I took the bike for a 100 km ride on the main highway ( speed limit 100 k/hr ) the bike hit a flat spot on accelleration and top end stopped @ 110 k/hr so I pulled the choke lever a little bit top end went to 125 k/hr and the flat spot was eliminated.

[Easy Rider]

Originally Posted by trykemike

hey Easy I did a test with the air filter.

What? I have no messages in this thread. What you say is mildly interesting but why did you put it HERE.....if you are responding to something that I said (presumably in another thread) ???

[trykemike]

hey Easy I was just passing on this observation based on a test that I did. You rightly informed me that the "choke " on the carb is not a butterfly but an enricher circuit that opens a secondary jet so I have been experimenting with it because the bike does perform as I had expected when I bought it. I have in the past driven a triumph bonneville and a norton commando so I did not expect a lot of power. I did however drive a honda 305 dream when I was a kid ( 19 ) it could just touch 80 mph ( slight down grad ) and cruise all day @ 55-60 mph it also weighed about 300 lbs. I was looking for cruiser styling and the same functionality as that dream. I have 14000 km on the gz now most at hwy speeds 50-60 mph . So far with maintenance tips from this forum I have been able to keep it reliable.

[Quimrider]

I brought this here from another thread as to not anger there off topic police

Originally Posted by trykemike

hey quimrider did you back out of your power mods ?
I still have 4 7/32 holes in inner ring on exhaust and 1/2 hole in intake hood and use 1/16 enricher circuit with hot idle set to 1500 rpm. Tip of plug is black but rest is light brown.
no starting problems runs good 100-115 kph on hwy cruise mainly 80-100 kph get top of 130 kph and 90-95 mpg (32 k/l ). pipes are blue/gold until 1st bend then chrome the rest some blue at weld where 2 pipes join.

I don't know if you read my other post about my cylinder head warping but that put a damper on things for a while. Still not sure what caused that. If anything I'd have to say my riding style. It would seem overheating, but I'm not fully convinced. I guess maybe it just happens sometimes with the higher running temps of air cooled engines. A couple things I learned from trying to increase power is that at least for me it isn't worth it. I could run rich as hell and gain an extra 5mph sacrificing 10-20mpg. Since I bought the GZ for it's fuel thriftiness, this was unacceptable. When they had my engine tore apart they said there was a "mound of carbon" that they scraped off the top of the piston. I suspect that was from when I was running very rich while playing with the carb. In case you don't remember I was using a K&N air filter, needle shim, have 4 3/8" holes in the exhaust (from previous owner) and experimented with almost every jet up to a I think it was a 150. I came to the conclusion that it's a 250cc and Suzuki has it tweaked pretty closed to as much power as you're going to get out of it with decent fuel economy without some serious re-engineering.

I also read about a reason not to open up the exhaust. Contrary to popular belief back pressure isn't always a bad thing. without back pressure, the momentum of the exhaust gases in the exhaust pipe will pull some of the incoming gas/air from the carb thru the combustion chamber and into the exhaust pipe the brief moment when both the exhaust and intake valves are open. If you have the carb sufficiently enriched you will get some more power, but take a big hit in fuel economy and your chrome pipes will blue due to excess heat from the combustion occurring in the exhaust pipes. since my exhaust has 4 3/8" holes, I verified that there is indeed some overlap in the intake/exhaust valves, and my pipes are very blue, I would tend to believe that this is true and applies to the GZ250.

So to answer your question, Sort of yes. With the weather getting cold and a leaking head gasket last fall, I decided I was done changing things until spring. Plus with the winter formulated gas and more dense cold air it's difficult if not impossible to compare results.
I ended up returning to something closer to stock for now. I didn't want to burn up my pipes anymore than they already are (blue thru the entire first bend and then some). I plugged 3 of the 4 holes had to tap them at 1/2" and use 1/2" bolts as plugs with loctite. one bolt had to be shortened otherwise I couldn't screw it all the way in. I had the 4th hole plugged but can't get anything to keep it plugged permanently (bolt rattles loose and disappears). I do want a little extra breathing, but I don't know if I want an extra 1/2" hole in my exhaust. The increased intake noise got to be annoying after a while so I went back to the stock filter. I removed the diverter (I think the same thing you're calling "intake hood") to get a little more air. The carb has the needle shim mod and 1 size up from stock on the main jet 122.5 IIRC. I wish I knew how you got your mileage. I still only get ~66mpg although 11 of my 16 mile commute is freeway WOT so if I kept closer to your speeds I'd probably get closer to your mileage. When it warms up I'm going to tweak the main jet and needle shim to see if I can get an improvement. I haven't done the plug check at various rev ranges to see how rich or lean I'm running. If we ever get it fixed I could borrow the combustion analyzer we have at work. It shows just about everything you'd want to know, %O2 %CO2 %CO %HC (hydrocarbon aka unburned gas)

Another thing I've considered is fuel injection borrowing parts from the TU250 and improvising other parts. There is an open source fuel injection project called mega squirt. It would be expensive I'd guess in the neighborhood of $1000 for a total conversion which then begs the question of why? The only reason I could think of would be nice to be able to customize and tweak the FI which you wouldn't otherwise be able to do. If I really wanted FI I'd probably just wait till I could get TU250 parts and swap them. I couldn't bring myself to buy a TU250 because it reminds me too much of the moped I had as a kid.

What's this about a "1/16 enricher circuit?" you talking about the choke? Does that really make any difference?

[Sarris]

QR; Great observations. My GeeZee has over 13k miles, all stock mechanicals, no carb or exhaust mods, and no blueing headpipes at all, and runs like a champ. I think that for the price, Suzuki did a great job right out of the box. Remember too, the entire third world gets around (the whole family, chickens, etc.) on GZ's in either the 250cc or the indescribably slow 125cc model.

I guess your motive for owning a GeeZee has a lot to do with your expectations. If it is/was your first bike, it's time to move up to a larger bike when it starts feeling too slow or too small. If it was a financial decision to save some money, you may be unhappy with the performance, perhaps feeling relegated to a fate worse than death by riding one. If you bought it for the sheer joy of riding a lightweight zippy motorcycle, it fills the bill perfectly.

:2tup:

[trykemike]

Hey sarris.

Your last statement sums it up for me . The joy of riding a light zippy motorcycle. It's a hobby
for me but as 1 of my buddies said "Mike you just had to change something !!!!." So I kept it very minimal.

My pipes went bluish/gold from new . Dealer serviced under warranty. Now I maintain it.

I really don't push the bike. I want to enjoy the ride there and BACK.