gpz750 going, going, not going?

Hey guys, sorry its taking so long, Its been a busy week, working 2.5hrs from home doesn't help ether. I'm going to try and pull the jets tonight.

The past owner had some aftermarket 4-1 exhaust on it (looked like a kerker but no name on the can). The header was thinning out and falling apart at the collector so I opted to switch it out. He was running the stock airbox with it.

Tyrell: I believe they are standard for a 550, but they are the same size pipe as the header that came off, so I'm not 100% sure, the cans on them are by F1, so possible that they are too.

Sounds like PO tuned the carbs for that rusted out kerker, now you have changed it you need to re-tune again?

What colour are your plugs? try lifting or lowering the carb needles -even if it gets worst that is good as it means you are narrowing down the problem. I never recommend thrashing a bike from cold, but as a diagnostic maybe see what happens on half or full choke - again narrowing down your problem.

Tyrell: the needles are all the way up I believe (richest setting)

We increased the jet size by 10, the jets in the bike were 117.5, so now I have 127.5 in it. Went for a test ride, and while not perfect, it doesn't fall on its face any more. Instead it coughs a bit and feels like it wants to go but its not there yet. choke out still helps. so I think If I go up another 10 it should be good, if it ends up too rich I can drop the needles back down a bit to make up for it. I need to head over to see a friend soon to do a badly needed alignment on my daily so I'll take it apart and do the jets again tonight. I'll check the plugs again then.

Edit, added pic of plastic (nylon) jet.

127'5s sound way to rich but if they helped ? I don't really know GPZ carbs though. Maybe try a plug chop at speed and read the plugs to get an idea ?

Mr.Cool wrote: Tyrell: the needles are all the way up I believe (richest setting)

We increased the jet size by 10, the jets in the bike were 117.5, so now I have 127.5 in it. Went for a test ride, and while not perfect, it doesn't fall on its face any more. Instead it coughs a bit and feels like it wants to go but its not there yet. choke out still helps. so I think If I go up another 10 it should be good, if it ends up too rich I can drop the needles back down a bit to make up for it. I need to head over to see a friend soon to do a badly needed alignment on my daily so I'll take it apart and do the jets again tonight. I'll check the plugs again then.

Edit, added pic of plastic (nylon) jet.

Attachment DSC_1633.jpg not found

I'd try two steps up on the pilots. 17.5 to 18.5 for example. The mains do have a minor influence on the lower end, it's the pilots that meter all the way up. Too big a main will hide the progress you're making in the bottom end. I'd start there, then work on the mains.
Steve

Sorry Swest, I ignored your help, not really, but the plugs were still white and I wasn't sure if you had my issue reversed, I had great 0-1/4 throttle and could cruse at 140km/h max. it was above 1/4 throttle that I had no power.

I decided to go up 10 more on the main jets making them 137.5 (keeping in mind I can still drop the pins if it ended up being a bit too much) and.... WOW! That's how a 750 should go! Bike rocks now, it still spits a bit around 7-8K rpm at 100% throttle but nothing that's too concerning. Ill just tweak it as I go to try and remove that.

Also to KZB2 650, to me 117.5 seemed tiny, like I have no idea how a 750 could run on those, I think our rz350s we use like 250+ and all the jets we had still sitting around were 350-400.

Try your pilots.
Steve

still think the pilots are an issue? I don't think I have any for these carbs so I would have to buy some.

When I was dialing in my 33's I over jetted meaning I put in larger everything to compensate for what the PO did. Still had a flat spot but could get past it. Still it was too lean in places. When I got the bike up this time I went through the carbs piece by piece. I found the moron had drilled the emulsion tubes. I soldered the ones that didn't belong and was way too rich. I went back to stock and have the suggested 125 mains and 60 pilots. Still a little rich on the bottom but I'm stopping here until I install my 10.25/1's.
What I'm trying to say is, I and maybe you are skipping a stage in the carbs. The pilots make a big difference all the way to 3/4 throttle.
Less and less as it climbs but still a difference.
Steve

My 750 is running 137.5 mains, sounds a big jump from standard I know.

Stock KZ650 mains are 102.5s and most of us run 117.5s with a header and pods ...... that's what was throwing me off (should'nt have commented not knowing the difference in your carbs) ..... Tyrell pretty much along with your now excellent running bike confirmed it ....... 137.5s are whats needed for a GPZ.

I'm going to step up the primary's as well by 1 size, like swest recommended. The 137.5s did the trick but its hard to start when cold. plugs look fairly good. I do assume that Ill be a tad rich after bumping the primary's but I can alleviate that by dropping the pins back to correct on the mains.

Just to be sure, I'm 99% positive the primary's are the ones down the tubes next to the mains? Just want to be sure.

plug for ref: