GT550 jetting with chambers

[ctluba]

Hey Guys, New member, very cool site.

I have a question concerning carb jetting. Background info. I bought this 1975 GT550 and knew nothing about 2 strokes really. I installed jemco chambers and didnt change the carb jetting at all. I holed the center piston within 20 miles!!!!!!! The center cylinder was pinging before the holed piston, I just didnt know what I was hearing and how bad it could be. Also the hole occured at mid to upper range riding, about 6000rpm, 50 mph. It was explained to me by several people that the holed piston was definately caused by running way too lean. Installing the chambers made the fuel situation way more efficient and therefore the carbs should have been richened up. That is the theory anyway.

I had the cylinders bored to the first over size piston. I removed the stock main jets which I think were 97.5s on the outside carbs and a 95 in the middle. Installed 110 main jets on all three carbs. The needles are raised all the way up (clips all the way down on bottom notch) and left idle mixture screw at stock setting. The plan was to run rich and work my way down. Well the bike runs pretty well at these settings but it is running pretty rich also. The bike then started running WAY rich when started and was fouling plugs and not running right at all. I bought new plugs and turned the idle mix screws in 1/2 turn each. Runnin like a champ again!!! I was also running the stock air box. I recently put on some pod filters and seems to be running fine with them, although I hear many people reccomend to stay with the stock air box. I also had a guy go through and check the timing. He confirmed everything was set to stock settings.

So, I was thinking of backin down to 105 main jets. I cant help but think the bike would gain a ton more performance by leaning it out a tad. I think at 105s I would still be pretty safe as far as risking another holed piston.

Any info appreciated. What main jets are you running with your chambers?? Sorry if this has been asked 1000 times. I did some reading on here but couldnt find what I was lookin for.

The monster in question. GSXR750 front end, SV650 rear end, jemco pipes, super bike bars.



How she currently runs, sweet music!!

http://www.youtube.com/watch?v=QdQINhv2 ... lpage#t=0s" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

[Craig380]

Welcome onboard. Man, that machine looks MEAN

If you turned the pilot screws IN by 1/2 a turn, that richens the idle and off-idle mixture. Turning them OUT leans it off. They control airflow, not fuel.

So it still might be a little lean ... I would make 100% sure that she's fine with the 110 mains in before anything else. Keep checking the plugs to make sure!

The bike will not gain much more power by going down from a 110 to 105 ... you may make a couple of extra BHP by leaning out ... but you might not. It might also be too near the knuckle and seize / hole another piston.

Good luck with it

[ctluba]

Thanx Craig. It was definately fouling plugs, one or two cylinders would die out, very "boggy", put your hand behind a pipe and it would get soaked from all the fuel blowing through. I am almost positive I turned the screws in 1/2 turn just figurin I would see what happens. That is why I joined up here, I dont know what the hell I am doing!!!! Sure I can bolt on a swingarm and forks, build a mono shock, but try tuning a carb and forget it. I did manage to change the top end when I holed that piston, but that was pretty straight forward. First kick fired up and is running like you see in the vid. I have to order a clymer manual also. I have been doing all my adjustments from forum advise.

Anyways, the reason I was askin was because of the amount of fuel blowin out the pipes. It is runnin pretty good, just seems like it needes to be fine tuned and jumpin up to 110s was a huge jump.

Thanx for the response bro.

[tz375]

Welcome.

The place to start with jetting is with a copy of the Mikuni VM carb manual which you can probably download. We have to think of the jets as controlling different parts of the throttle range, even though they do overlap somewhat.

For example, the main jet is working from say 1/4 throttle upwards, but the needle and needle jet are smaller than the main jet up to say 3/4 throttle, so they effectively control that lower part of the throttle range.

In general terms it's something like this:

Pilot jet and air screw - 0 - 1/4 throttle
Slide cutaway 1/8 to 1/4
Needle jet 1/4 to 3/4
Needle clip 1/4 to 3/4
Main jet 3/4 to full

Increasing the needle jet size makes it richer all through that range but more so at smaller openings because the increase in jet area is proportionately larger at fully closed than when it's fully open. Let's take your GT550 with 172 series P-0 needle jet. It's 2.650mm diameter which is 5.515 sq mm in area. The 5DH21 needle is 2.515mm diameter at the top (4.968sq mm), so the annular ring between the needle and jet is only 0.548sq mm. Wide open the needle is much thinner at 1.86mm or 2.717sq mm, so the gap at wide open is 2.798sq mm. Changing the needle jet to say P-4 with 2.670mm diameter increases flow by 15% ay the bottom end and 3% at the top. Although that change 2.65 to 2.67 sounds like nothing, it is a large difference in area relatively at the bottom end.

So a change in needle jet makes a bigger difference at 1/4 throttle and less difference at the top end and changes to clip position have major effect though most of the range and most effect at 1/2 throttle.

Raising the needle by one clip position has a similar effect.

Think of the needle and needle jet as modulating the flow from the main jet. So the main jet needs to be correct for the very top end first and then set the needle position for mid range throttle.

I like to get the slow jets right first so that the bike starts and idles nicely and is clean as I crack open the throttle. That's all in the slow jet and air screw. Then I get the main jet right by riding it flat out at speed up a slight hill and do a plug chop. Form that I first look for signs of overheating on the electrodes and if that's OK, I try to get the main jets right- partly by reading the plugs and partly by feel.

Once the mains are right, I mark the throttle at 1/4, 1/2 and 3/4 and try the bike at different throttle positions to see where it's lumpy or rough and adjust the needles to suit.

In your case, it sounds as if the needles are too high, but check it wide open to confirm main jet sizes first. Don't try changing main jets to clean up 1/2 throttle running. Get the mains right first and then fine tune the mid range.

Another book that is very good was Motorcycle Carburettor Manual by Pete Shoemark published in 1980 by Haynes, or the book by Olav Aaen, but the VM manual is good enough.

[ctluba]

Thanks for the detailed info TZ. I have read that tuning two strokes is somewhere between black magic and mechanical knowledge and you seem to have confirmed that!!!

At this point I am pretty happy that it is idleing smooth and running pretty good. I actually took a rip yesterday before this rain / sleet mix moved in and it ran pretty strong. I also understand the concept of different parts of the carb running different stages of the RMP range. That is why I went up to the 110s. I holed the piston at upper RPM range and figured it was running on mostly the main jets there. I am going to tinker with it and try and find the best balance. As I have said, it seems a little rich now, but that is not a bad thing, I prefer to replace plugs rather than pistons!!!

If anybody wants to chime in with your current settings, knock yourself out!!!! Just curious what combos others are running. How about the stock air box vs pods. I know many prefer and reccomend to stay with the box, but I also see many running pods. What is your guys experiences etc??? Thanx

I am thinkin that the idle and low range are set correctly right now. You can hear it in the vid I posted above.
Maybe I will drop the needles by one clip and change to the 105 mains. Or just drop the needle one clip and stay with the 110s. I will get it figured out.

[Madbuffalo]

I also understand the concept of different parts of the carb running different stages of the RMP range. That is why I went up to the 110s. I holed the piston at upper RPM range and figured it was running on mostly the main jets there.

When tuning carbs it's more about 'throttle opening' than RPM although that can be a hard concept to grasp (it was for me). +12.5 mains might be a little rich but it's a good starting point. Richard's (TZ's) comments should get you on the right track. He's our resident carb expert and I'd trust anything he has to say.

For what it's worth, I have a 1975 GT750, all stock except Higgspeed 3 into 3 chambers and after a lot of fine tuning I ended up with +7.5 mains, then pilot jet, air screw, needle position all stock. This information is of little to no value for you though since our bikes/riding styles are surely different and you need to find what works for you. Unfortunately there's no easy answer, ya gotta experiment. As far as the stock airbox, personally, I've had trouble with pods in the rain.

Sweet bike, and welcome to the forum!
Josh

[ctluba]

Thanx Josh. With this info I feel I am on the right track as you said. Just gonna have to tweek and ride, tweek and ride and experiment that way. It is also very helpful just reading what others have experienced with different settings etc. Gets ya in the ball park anyway.

Some pics for ya.

When she got home. If anybody is lookin for nice set of saddle bags, give ma call!!!



Crash diet



GSXR front end arrives



SV 650 swinger



The UPS girl comes around alot



First set up, cafe fighter!!



decided to go mono





New tail and seat





Also, I have a huge pile of stock parts if anybody is lookin for anything (both wheels, seat, swingarm, shocks, front end, levers, pipes). Just give me a shout.

Stable mate. 81 CB750F, gsxr front and rear end, 823 kit, 900 cams, gsxr carbs, oil cooler and bunch of other stuff.

[jabcb]

Might be interested in the pipes & seat. What shape are they in? Any thoughts on how much you want?

I'm in southwestern Pennsylvania.

[ctluba]

PM on the way

[Craig380]

I have a few questions about the conversion, if I may

How easy was it to fit the USD forks? Did you need to fabricate a new steering stem etc? Do the forks weigh more than the stockers?

cheers, Craig

[ctluba]

The front end was simple actually. You go to "All Balls Racing" web site. They have a conversion chart for front ends. You enter your year, make and model and they list all the front ends that will fit. You then buy the conversion bearings from them and bolt the front end on. I prefer to use a front end that someone else has already used. Then you can talk to that guy and see if there are any hidden problems that may arise or any inside tips.

On my CB I had to have the CB stem pressed in the 636 triple clamp. But the GSXR front end bolt straight on the GT.

Any time you bolt on modern parts the old stuff seems to weigh 2 metric tons more!!!!! I didnt weigh them, but the new stuff weighs less. Also the modern brakes are 100 times better. You may have to fab up steering stops if you really want them. I didnt bother. I plan to install the GSXR steering damper, just have to weld a tab off the front of the steering stem.

Later.

[ctluba]

Installed my new LED tail light w/plate bracket.

Also did a little "Friday night sheet metal work" and added an under side cover to my tail cowl. Jeeze I am a friggin hack!!!! Come on now, didnt come out that bad!!

Just gotta make that bend at the bottom edge a little crisper / sharpe. Have to use a peice of wood and square it off nice.

[ctluba]

Here is a possible set up to keep the battery and carbs and everything else from being sprayed with sand and crap.

Not sure if I like the look. Maybe paint it black, maybe cover it with stickers, maybe pull it out. Opinions????

[Craig380]

Looks sweet. I'd keep the sheet, spray it black and get a sticker saying "Yes, I KNOW it's smoking"

Thanks for the input on the front ends. I had heard that pretty much ANY Suzi front end from the mid-70s is interchangeable, but you wouldn't think Gixxer forks would fit an old smoker

[Madbuffalo]

Those gold gsxr forks look fantastic with the stock black/gold color scheme. Hadn't realized you had done all the mods yourself! Keep up the good work!