'75 suzuki gt250...Buzzing around the block

Re: '75 suzuki gt250

I picked up some larger pilot jets to try and mess with my low end a little. I'm having a problem where, once fully warmed up, I go to shift and with the clutch in and the throttle off, the rpms raise slightly. Its enough to make shifting awkward and doesn't seem right. With a little research it seems that the low end(pilot system) may be a little lean. The originals are 25's so I got 30 and 35"s to test out. It doesn't bog off the line, but it does take a little bit to idle down. I did test for air leaks. I also chopped off 3 inches off of the center spacer of the exhaust. In theory, it raises my peak rpm hp to 8250 rpm, my old length was around 7550 rpm. I'll have to check tomorrow if this pushes my powerband higher where I want it. Heres a pic to show the new look:

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Re: '75 suzuki gt250

Good news. The larger pilot jets smoothed out my low end and made me able to idle much lower at 1,000rpm instead of 1,500 rpm before. It also cured the wierd reving after letting off the clutch and throttle. The shorter tuned length feels much better. My low end suffered slightly, but the power just keeps coming at the rpms go up. Its no dirtbike, but it feels smooth and powerful. Thanks, Joe
 
Re: '75 suzuki gt250

So, I've been spending a little bit of time trying to tune in the new pipes. I thought I was getting close. It turns out that my throttle slides weren't opening up all the way. the oil cable was limiting the movement. Now that they open up all the way, my main jet is way off. It runs great at low to mid throttle, running 45-55 all day("on the needle"). However, when it is fully warmed up and I go through the gears, it surges, indicating a lean condion. I upjetting from a 122.5 to a 130 main jet and it actually got worse! Instead of jetting by the seat-of-the-pants method, I'm gonna do it right and use a wideband o2 sensor. I already purchased the full kit and it should be in by the middle of next week. The general concensus is that the lifespan will be shortened, but I'm only going to be using it for tuning, not all the time. This should give me all the info I need to tune all of the throttle positions. Thanks, Joe
 
Re: '75 suzuki gt250

I drive a '75 GT 250 by myself (But with the later motor without the ram air-system), to be more precise I own it, still not finishes to drive around.

Your posts are very inspiring, especially your selfmade seat, I wish I've had seen this before I got my new seat!
 
Re: '75 suzuki gt250

Thank you for your compliments. I got the kit early, and did a quick write up. The instructions are straightforward, indicating all the info you need to intall it. For simplicity, I took all of the power wires and crimped them into one power wire, and did the same for the ground. It came with a threaded bung, but no cap, something I’ll need to pick up at ace later tomorrow. It has a built in heater, so just turn on the power(switch I installed) and wait about 15 seconds till the light goes off, then your good to go. It only took me about 3 hours to install the whole kit, welding included. The video shows how quick it reacts to a towel soaked in carb cleaner. On a side note, I fixed the bad surging problem. After messing with the carbs, the fuel flow out of the lines was only a trickle. There was some particles in the fuel valve, and I cleaned it all up. Same flow. I put in a couple gallons of gas, thinking I was pretty low on gas. Same flow. Turns out the diaphram that suzuki used to control the fuel flow finally crapped out, giving me barely any fuel flow in any position. I took out the diaphram and put in a solid gasket. Problem solved. I put in 120 mainjets and drove it home, and it actually seemed close to what it should be. I’ll be able to put in some serious tuning time this week and see what the small adjusments can do for my f/a ratio and power output. Thanks, Joe

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Re: '75 suzuki gt250

Well, after a couple hundred miles and some good tuning sessions, I ended up with 125s for the mains, giving me around a 1:11.5 f/a ratio at full throttle. This proves to me that I won't blow the engine up from being lean, and that I'm also getting about the most power I can without being too conservative on the jetting. Its getting cold, and I'm going to take it to the next step. The cylinders are comming off and getting ported. I'm going to raise the exhaust port up a couple mm. Unfortunetly, I can't widen the port because it already has auxilary exhaust ports, but I may be able to do something for it. I've already polished the exhaust port, and has no buildup of carbon whatsoever when I remove the exhaust. Heres a top view of what I'm looking at down below. After I port it and run it, I'll decide if I want to shorten the exhaust again. I'm not sure if its the cylinder porting or the exhaust holding me back from the higher rpm. From the generous info that Teazer gave me, it could be the small exhaust duration keeping the rpms down along with the long tuned length. I think another chop of 2 inches should do the trick. Thanks, Joe

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Re: '75 suzuki gt250

The Aem doesn't have data logging, but it does have the wires that have the 0-5 volt spread, that can be plugged in to any logging system. If I could get to a dyno, I would, but of the few that are close to me, are Harley only. I understand the 11.5:1 ratio is rich, but after an 80 mile round trip yesterday, the plugs were a dark brown. I think I can go down to 122.5 mains without much of a problem, but I'm very close, and I love how I can ring it out without risking a meltdown. After determining that the engine is in a good enough state of tune, I decided to see exactly what my port durations were, and if I could modify them to get a better high end. The stock exhaust duration is 170 degrees, and was raised to 180 degrees. The intake had a lip where the aluminum met the cast iron sleeve, and was ground and smoothed out. The inlet duration stock was 140 degrees and was raised to 155 degrees. Heres a pic of the piston getting shaved. I've got new gaskets cut out and everything is cleaned and should be all back together tomorrow. Thanks, Joe

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Re: '75 suzuki gt250

Good stuff man, those pipes are looking great and the a/f ratio gear is pretty cool too. Like how this whole thing is coming together.

On the porting, sounds like you've done a nice sensible, calculated job. Nothing worse than hearing about how somone 'only took 3mm off the top' and then had a catastrophic engine failure. Glad to hear you're doing it properly.

I'm still stripping my Kawasaki 100, as a piece of piston ring detached and found its way into the cases. Should be a simple take-down & build-up affair, although I may retrofit a high-comp head onto my now mildly ported barrel...

Anyway, let us know how it goes. Haven't heard a proper two-stroke on song in ages!

Cheers - boingk
 
Re: '75 suzuki gt250

love the project! i just picked up a 74 gt250 and was happy to stumble upon this thread. i really liked the scooter-styled pipes, too bad having the dimensions correct is so critical.
 
Re: '75 suzuki gt250

I've been debating the performance of the exhaust, and I think I can do better. I know theres a definate advantage of having a 2 stage diffuser, so I'm going with it. I focused more on Gordon Jennings work, and was really nice to see the dyno sheets and how the exhaust affected it. I already have the dimensions down, and I'll be picking up material tomorrow. As another bonus point, I'll be designing them to go under the bike, and sprout out around the swingarm, just like the yamaha rd pipes do. On top of that, it will free up space on the sides of the engine, so I'll be able to finish up and use the rear sets I started on. Expect big things this winter. Thanks, Joe

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Re: '75 suzuki gt250

I finished up how the exhaust is going to look. Their not going to go under the frame much, but with the new design there should be ample room for the linkages without getting in the way. Material is in, and should be able to start out the templates tomorrow. Thanks, Joe

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Re: '75 suzuki gt250

I've made some progress...The templates were made using a simple online cone program, and were transfered to the 22 gauge steel. The last pic as you can see is my bench, messy I know. I should be able to get most of the work done this week, next week tops. Thanks, Joe

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Re: '75 suzuki gt250

I finished up the exhaust today. Compared to the first exhaust, the welds turned out much better, and I think the layout on the bike is much more pleasing. As a bonus, they sound a little different than the last ones, a little louder and angrier. I'll be painting them gloss black with the silencers gold. As far as testing them performance wise, I didn't do much, just driving from the shop to home. It did seem to get moving much better than the other exhaust. I'll be able to get numbers at some point before the snow falls. Thanks, Joe

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Re: '75 suzuki gt250

Killer pipes man! Look much better than the first set IMO. Do you think that there will be a problem with oil puddling up in the chambers? Or just rev the piss out of it to clear em? Sorry if you already specified, but where did you get the plans for the chambers? I have a GT380 build and would love to fab my own pipes.

Thanks,
Joe
 
Re: '75 suzuki gt250

When I cut up the first chambers, they were clean on the inside. It's pretty smokey when you first start it up, but wind it out in the first few gears and it clears out. I got the dimensions from a few sources. First was this: http://suzukidave.myphotos.cc/suzukidave/X6-T20%20performance/scantx7.jpg which gave me an idea as to what this engine likes, then I read through Gordan Jennings and Graham Bell's books and linked engine performance to the angles and lengths of the cones. Theres also a free chamber design software that can get you close, along with a very nice one that you can pay about $20 for. For me, its not really the building of the pipes that is difficult, but routing them around the bike itself. Another bonus with these new pipes, is that that I can now restart my rearset idea because I have clearance on the left side for the linkage. After the rearsets will probably be a 1/4 fairing. Unfortunatly, I'm running out of ideas. New bike maybe? Thanks, Joe
 
Re: '75 suzuki gt250

Thanks for those links man. Do you plan on a tear down for paint after its all mocked up? Bike looks bad ass either way.

Joe
 
Re: '75 suzuki gt250

If you mean tear down the exhausts for paint, then yes. The bike itself is already painted. The pipes got some free decals from the 2 stroke oil I excusively use. I also went ahead and did some chopping. The original exhaust mounts are gone, "buddy" pegs are gone, center stand mounts are gone, and front tabs that I think could be used for crash bars. It cleaned up the bike a lot. Unfortunetly I can't ride it till I get a good sprocket and chain. I will be picking up some rearsets tomorrow that I'll have working by the end of the week if everything pans out. Thanks, Joe


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Re: '75 suzuki gt250

looking awesome!! love that you made your own expansion chambers, props on that! can't wait to see her with rear-sets and on the road.
 
Re: '75 suzuki gt250

I hope you post a vid with the new pipe setup, im really looking forward to hearing those new pipes poppin!
 
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