"Poor Kid" 1981 CB750K - first build
- Thread starter kraptastic k
- Start date
kraptastic k
ask me how i got here
Too true. No ice is nice for one more year. The plan is to keep tuning and enjoy Texan mild winters of 40-50 degrees, and bundle up for short test runs.
But it has to be done soon... Moving to WA state this next summer!
But it has to be done soon... Moving to WA state this next summer!
Hoosier Daddy
Earache my eye...
That'll be quite a change for you!
kraptastic k
ask me how i got here
Don't I know it. Trust me, I'll be much happier.
So I quit my job as a bar manager and got one as an entry level CNC machinist for the months leading up to the move. The experience should be paramount.
Also, have you guys ever seen inverted levers? What about classic basket handles on swords? I have a cool idea...
So I quit my job as a bar manager and got one as an entry level CNC machinist for the months leading up to the move. The experience should be paramount.
Also, have you guys ever seen inverted levers? What about classic basket handles on swords? I have a cool idea...
kraptastic k
ask me how i got here
Don't I know it. Trust me, I'll be much happier.
So I quit my job as a bar manager and got one as an entry level CNC machinist for the months leading up to the move. The experience should be paramount.
Also, have you guys ever seen inverted levers? What about classic basket handles on swords? I have a cool idea...
So I quit my job as a bar manager and got one as an entry level CNC machinist for the months leading up to the move. The experience should be paramount.
Also, have you guys ever seen inverted levers? What about classic basket handles on swords? I have a cool idea...
kraptastic k
ask me how i got here
This machinist job has really taken my interest. The CB died with float trash a few days ago and I dropped it on the five mile uphill push home in the dark. No leaks or permanent damage... Glad I didn't paint it!
It needs a fuel filter and the rear wheel needs truing, but it rides otherwise. I diagnosed my CX500 after weeks of testing- bad CDI and a bad "known good" CDI. Wish I had known it wasn't. I need to get on a group buy for one now!
Most importantly... I'm really into this machining. I have been working with a designer to find parts from old bikes that could be recreated and sold, for my commercial benefit and that of the collector. If any followers are looking for something like that I'm certain we could take the case, as our shop serves many large clients in the Dallas/FTW area. If you have a no longer available part (in decent shape) such as a brake piston, or maybe one considerably larger and more complex, send me a PM and we can talk about reverse engineering it. You would get the first article for your troubles, and we would continue to sell a short run if it were lucrative.
By the way, machining beats the hell out of restaurants!
It needs a fuel filter and the rear wheel needs truing, but it rides otherwise. I diagnosed my CX500 after weeks of testing- bad CDI and a bad "known good" CDI. Wish I had known it wasn't. I need to get on a group buy for one now!
Most importantly... I'm really into this machining. I have been working with a designer to find parts from old bikes that could be recreated and sold, for my commercial benefit and that of the collector. If any followers are looking for something like that I'm certain we could take the case, as our shop serves many large clients in the Dallas/FTW area. If you have a no longer available part (in decent shape) such as a brake piston, or maybe one considerably larger and more complex, send me a PM and we can talk about reverse engineering it. You would get the first article for your troubles, and we would continue to sell a short run if it were lucrative.
By the way, machining beats the hell out of restaurants!
Attachments
Chuck78
Been Around the Block
Any carb progress??? Are you familiar with true plug chops? Throw in a new set of plugs and ride around for about 20-25 minutes. Find a safe and proper location to do wide open throttle for 10 seconds preferably uphill on a straight and deserted road, hit kill switch, pull in clutch, and close throttle all simultaneously. pull plug(s), put in spare(s), ride home.
now at home, clamp the plug hex in a vice, and use a hacksaw,sawzall, or portaband saw to chop the threaded portion of the plug off down to the porcelain,cutting, rotating plug, cutting, rotating plug, then eventually you can slide the threaded portion with the ground strap and all off the end and expose the entire porcelain insulator. for Wide open throttle main jet sizing, you should see just a light tan ring at the bottom of the exposed portion of the insulator. If it's too lean, then the thing will be really light colored with very little coloration, and you may see little speckles of vaporized aluminum (from your pistons!) on the porcelain. This is dangerously lean. Best to start on the rich side to be safe. If way too rich, it will be black and sooty. A little too rich and you will have the bottom half of the insulator looking pretty dark, but more tan towards the tip and electrode.
Plug chop info here:
http://jetsrus.com/FAQs/FAQ_spark_plugs.htm
http://www.xs11.com/xs11-info/tech-tips/maintenance/carburators/68-tuning-carbs-by-reading-spark-plugsthrottle-chop.html
http://jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm
http://www.strappe.com/plugs.html
http://www.dragstuff.com/techarticles/reading-spark-plugs.html
You do have to have the bike in a remotely decent state of tune to start doing that. First things first, you need to make sure the valves are adjusted and that you have even compression across all the cylinders. you'll need a compression gauge, feeler gauges, and probably a very handy Motion Pro valve shim changing tool. On Suzuki;s, they come in .05mm increments, and I think .03-.08mm is the cold tolerance. Has to be done when the engine has sat for at least 6 hours not running. The valve to cam clearance will get too tight over time with wear, and the valves will not seal tightly against the seat, or sometimes the seats get worn or burnt and then the valves get too loose where you have a big gap under the cam lobe. You are rotating the engine until the lobe of the cam is pointing up at the sky, and then check the clearance under the cam to the shim.
Then you need to get the idle set up right and the carbs at least bench synched, if not vacuum synched with a differential manometer setup. ride around at only 1/8 throttle or less for 20 minutes, and then pull all the plugs and note the color. No need to "chop" them for low speed throttle, you look at the top of the plug for low speed. mid range, you are looking at the middle portion that is under the threads, which can sort of be seen without cutting them. Full throttle is where you need to read the very bottom of the insulator that is exposed to the combustion. Play with the idle screw (it's an idle air screw on these carbs I think? Some are idle fuel, some carbs have both).
Here's a rebuild manual for this style of GSXR carbs
http://www.gixxer.com/forums/showthread.php?p=2374074
The main jet sizing does somewhat affect the needle height, so don't spend too much time with the needle adjustments after getting it to idle, just get it to run decent at 1/4 to half throttle playing with the needle height. Then go to the mains, get the WOT jet size dialed in the ballpark first just by riding it and getting a feel for what makes power and what bogs down. Start big & go smaller, too rich is safe, just dirties the plugs. Too lean melts pistons and burns valves! I think people are saying that 120-130 (Mikuni sizing) is in the ballpark. Was it 122.5-125? JetsRus.com sells affordable "OEM Equivalent" jets that are Mikuni copies at half the cost, so you can play around with the sizing without dropping a ton of cash. Start big.
After you get it feeling in the ballpark where it is making power at wide open throttle at higher rpm's, and once you are feeling comfortable riding the bike at potentially very high speeds, then you can do some true WOT plug chops on a safe deserted environment... With an out of true back wheel, it may not be so safe to do yet. Keep dripping penetrating oil on the threads of the spoke nipples for a while and then after a few weeks, deflate the tire to almost nothing and get a spoke wrench and start trueing. If the wheel veers out of true to the right, tighten spokes in that area that come off the left side of the hub, maybe 1/4 turn at a time. Also note where the dull sounding spokes (tap on them with the spoke wrench of something metal) are in relation to the bend. Unless the rim is physically bent, spokes loosen up and will let the rim go out of true to the opposite direction since the spokes on the other side of the hub are still pulling tight.
Be careful trying to do WOT plug chops, especially on an uncertain bike that you are just learning how to ride. Not trying to diss you, this is your first bike, right? In time it will all come together, mechanics and riding...
I'm really curious as to what jets you end up with to make it run good, as I have some 88-90 GSX600F BST31's and some 91-97 GSF600F BST33 Slingshot carbs for a 673cc GS550 project, and an 844cc GS750 project (track bike? building it from all my spare parts), and mainly because my buddy is looking for a set of BST33's or 34's for his 82 CB750 with the p.o.s. stock 3 jet aluminum slide piston carbs. He is constantly fiddling with the stockers and wants me to bring my BST33's over to try to get his bike to run right. I think he's ready to swap the GSXR carbs on...
Best of luck.
now at home, clamp the plug hex in a vice, and use a hacksaw,sawzall, or portaband saw to chop the threaded portion of the plug off down to the porcelain,cutting, rotating plug, cutting, rotating plug, then eventually you can slide the threaded portion with the ground strap and all off the end and expose the entire porcelain insulator. for Wide open throttle main jet sizing, you should see just a light tan ring at the bottom of the exposed portion of the insulator. If it's too lean, then the thing will be really light colored with very little coloration, and you may see little speckles of vaporized aluminum (from your pistons!) on the porcelain. This is dangerously lean. Best to start on the rich side to be safe. If way too rich, it will be black and sooty. A little too rich and you will have the bottom half of the insulator looking pretty dark, but more tan towards the tip and electrode.
Plug chop info here:
http://jetsrus.com/FAQs/FAQ_spark_plugs.htm
http://www.xs11.com/xs11-info/tech-tips/maintenance/carburators/68-tuning-carbs-by-reading-spark-plugsthrottle-chop.html
http://jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm
http://www.strappe.com/plugs.html
http://www.dragstuff.com/techarticles/reading-spark-plugs.html
You do have to have the bike in a remotely decent state of tune to start doing that. First things first, you need to make sure the valves are adjusted and that you have even compression across all the cylinders. you'll need a compression gauge, feeler gauges, and probably a very handy Motion Pro valve shim changing tool. On Suzuki;s, they come in .05mm increments, and I think .03-.08mm is the cold tolerance. Has to be done when the engine has sat for at least 6 hours not running. The valve to cam clearance will get too tight over time with wear, and the valves will not seal tightly against the seat, or sometimes the seats get worn or burnt and then the valves get too loose where you have a big gap under the cam lobe. You are rotating the engine until the lobe of the cam is pointing up at the sky, and then check the clearance under the cam to the shim.
Then you need to get the idle set up right and the carbs at least bench synched, if not vacuum synched with a differential manometer setup. ride around at only 1/8 throttle or less for 20 minutes, and then pull all the plugs and note the color. No need to "chop" them for low speed throttle, you look at the top of the plug for low speed. mid range, you are looking at the middle portion that is under the threads, which can sort of be seen without cutting them. Full throttle is where you need to read the very bottom of the insulator that is exposed to the combustion. Play with the idle screw (it's an idle air screw on these carbs I think? Some are idle fuel, some carbs have both).
Here's a rebuild manual for this style of GSXR carbs
http://www.gixxer.com/forums/showthread.php?p=2374074
The main jet sizing does somewhat affect the needle height, so don't spend too much time with the needle adjustments after getting it to idle, just get it to run decent at 1/4 to half throttle playing with the needle height. Then go to the mains, get the WOT jet size dialed in the ballpark first just by riding it and getting a feel for what makes power and what bogs down. Start big & go smaller, too rich is safe, just dirties the plugs. Too lean melts pistons and burns valves! I think people are saying that 120-130 (Mikuni sizing) is in the ballpark. Was it 122.5-125? JetsRus.com sells affordable "OEM Equivalent" jets that are Mikuni copies at half the cost, so you can play around with the sizing without dropping a ton of cash. Start big.
After you get it feeling in the ballpark where it is making power at wide open throttle at higher rpm's, and once you are feeling comfortable riding the bike at potentially very high speeds, then you can do some true WOT plug chops on a safe deserted environment... With an out of true back wheel, it may not be so safe to do yet. Keep dripping penetrating oil on the threads of the spoke nipples for a while and then after a few weeks, deflate the tire to almost nothing and get a spoke wrench and start trueing. If the wheel veers out of true to the right, tighten spokes in that area that come off the left side of the hub, maybe 1/4 turn at a time. Also note where the dull sounding spokes (tap on them with the spoke wrench of something metal) are in relation to the bend. Unless the rim is physically bent, spokes loosen up and will let the rim go out of true to the opposite direction since the spokes on the other side of the hub are still pulling tight.
Be careful trying to do WOT plug chops, especially on an uncertain bike that you are just learning how to ride. Not trying to diss you, this is your first bike, right? In time it will all come together, mechanics and riding...
I'm really curious as to what jets you end up with to make it run good, as I have some 88-90 GSX600F BST31's and some 91-97 GSF600F BST33 Slingshot carbs for a 673cc GS550 project, and an 844cc GS750 project (track bike? building it from all my spare parts), and mainly because my buddy is looking for a set of BST33's or 34's for his 82 CB750 with the p.o.s. stock 3 jet aluminum slide piston carbs. He is constantly fiddling with the stockers and wants me to bring my BST33's over to try to get his bike to run right. I think he's ready to swap the GSXR carbs on...
Best of luck.
Chuck78
Been Around the Block
I just read this over, it's really helpful. They don't mention plug chops, which is very important, because if you are a jet size or two too small, the bike will still make TONS of power, but you will be risking pre-ignition "ping" melting your pistons and or burning valves/seats. Get it in the ballpark with their method and do plug chops to fine tune.
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
kraptastic k
ask me how i got here
Wow, huge write up, chuck! I'll try to respond to you tonight when I get to read through this. I'm at work currently. Maybe I'll document my carb settings as of now to see how much things change as well... Few hours
kraptastic k
ask me how i got here
I've read extensively through the Factory Pro carb tuning pages, but I haven't checked valve clearance at all, nor compression. The bike is running well enough but isn't legal or near a dead street, so I may need to move it closer to my home in the country if I want to try plug chops.
Really though, what is it about chopping the plug that will give a more accurate reading?
edit: now I see that the insulator can be read up past the ceramic and I understand. This is super helpful stuff either way!
Really though, what is it about chopping the plug that will give a more accurate reading?
edit: now I see that the insulator can be read up past the ceramic and I understand. This is super helpful stuff either way!
Hoosier Daddy
Earache my eye...
Dude!
Check you valve clearance... it is the number one thing that killed these DOHC 750,
Let me know if you need help, really not bad once you figure it out. I got a nifty .xls spreadsheet but can not load it. Maybe e-mail it to you... PM me if you want it.
Check you valve clearance... it is the number one thing that killed these DOHC 750,
Let me know if you need help, really not bad once you figure it out. I got a nifty .xls spreadsheet but can not load it. Maybe e-mail it to you... PM me if you want it.
Hey Hoosier, that spreadsheet sounds pretty interesting. Will it post here?
Hoosier Daddy
Earache my eye...
Grrrr. I tried!
It's a xcell spreadsheet. (.xls) and not supported.
P.M. Me your e-mail and I will send it.
rhosco@ blah blah blah yadda yadda ydda
It's a xcell spreadsheet. (.xls) and not supported.
P.M. Me your e-mail and I will send it.
rhosco@ blah blah blah yadda yadda ydda
Chuck78
Been Around the Block
For the mains, you really have to do a wide open throttle high engine speed plug chop. It seems as with having CV carbs, you need to do it like you’re drag racing up a big long straight deserted hill, gunning it out of first, shifting at no lower than redline, and hoping that 7-9 seconds comes sooner that 100+mph does. Every jet change I made going smaller, the harder 10+second plug chop runs became, as the GS750's w/pods/pipe/850 pistons get incredible power gains... Hold on! And try to first quickly narrow down the main jet sizing by the quick seat of the pants feel, but don't ride it hard except when testing jets, until AFTER you are sure you’re not too lean on the mains.
Also, go by that gixxer forum link for your carbs. Some of the factory pro stuff I am skeptical of, as adjusting the floats and physically checking the fuel level has always been first on the list for ALL old carb bikes I’ve wrenched on, then idle mix second, mains third, needle height/needle taper selection/needle jet emulsion tubes last. And they don’t mention that you MUST do either plug chops or exhaust gas analyzer / AFR meter to know for sure. There is still good info on there besides what I just pointed out though, but refer to the specific carb instructions like on the gixxer forums first.
Also, reading a true WOT/kill switch cut in half plug tells you exactly what is going on as far as the amount of air&fuel vs the engine's requirements for a perfect mixture. If there is a black/dark brown ring at the very bottom of the insulator which is the furthest reaching point in the combustion chamber for the flame to travel to, then there is a surplus of fuel going to waste that is unable to be burned completely during the power stroke/combustion cycle because there was not enough oxygen to make a near perfect mixture. Aldo, this can tell you if you are running dangerously lean, which will burn very rapidly & very hot, leading to serious engine damage.
From what I gather, once you adjust your valve clearances & make sure you have acceptable compression, you will probably want to start with a 125 main. 127.5 would be safer. Best to talk to other people who have properly jetted the same size Mikuni BST carbs for the same engine you have to get a better baseline starting point to be safe.
Also, go by that gixxer forum link for your carbs. Some of the factory pro stuff I am skeptical of, as adjusting the floats and physically checking the fuel level has always been first on the list for ALL old carb bikes I’ve wrenched on, then idle mix second, mains third, needle height/needle taper selection/needle jet emulsion tubes last. And they don’t mention that you MUST do either plug chops or exhaust gas analyzer / AFR meter to know for sure. There is still good info on there besides what I just pointed out though, but refer to the specific carb instructions like on the gixxer forums first.
Also, reading a true WOT/kill switch cut in half plug tells you exactly what is going on as far as the amount of air&fuel vs the engine's requirements for a perfect mixture. If there is a black/dark brown ring at the very bottom of the insulator which is the furthest reaching point in the combustion chamber for the flame to travel to, then there is a surplus of fuel going to waste that is unable to be burned completely during the power stroke/combustion cycle because there was not enough oxygen to make a near perfect mixture. Aldo, this can tell you if you are running dangerously lean, which will burn very rapidly & very hot, leading to serious engine damage.
From what I gather, once you adjust your valve clearances & make sure you have acceptable compression, you will probably want to start with a 125 main. 127.5 would be safer. Best to talk to other people who have properly jetted the same size Mikuni BST carbs for the same engine you have to get a better baseline starting point to be safe.
Hoosier Daddy
Earache my eye...
Dude... they typing instead of speaking if you think your spell check sucks!
That is key right there.
Chuck78
Been Around the Block
Hoosier Daddy said:
HAHA...EXACTLY! I gave up on voice to text translators for anything more than 1 sentence because it isnt even close half the time! I guess I need to get tapatalk or something to get better form spellcheck on forums.
If you have a compression problem after adjusting the valves, you will not be able to optimally tune the engine to the ballpark of what joe schmoe's good compression identical bike jet specs are... Every bike has slightly different tolerances, even on identical freshly rebuilt engines.
Nice Lemmy Kilmister themed bike collection, btw!
Hoosier Daddy
Earache my eye...
Back to the basics. Intake - Compression - Ignition - Exhaust (Suck - Squeeze - Bang - Blow).
kraptastic k
ask me how i got here
I'm very whelmed by all this. Not over or under, just regular whelmed. I knew there were more challenges!
So... Now that the science and reasoning are clearly outlined above, can y'all help me make a to-do list?
Step one: compression
Step two: valve clearance,
Etc!
I'm trying to figure out what I have to buy and borrow versus what I can work on
So... Now that the science and reasoning are clearly outlined above, can y'all help me make a to-do list?
Step one: compression
Step two: valve clearance,
Etc!
I'm trying to figure out what I have to buy and borrow versus what I can work on
