TOPIC:

Aloha,
So I was fortunate enough to take over the stewardship of a original owners 83 GPz750 ! My question is this, on the day of purchase 33 years ago, the dealer installed a Yoshimura 4-1 exhaust and left the jetting alone but unscrewed the fuel screws out 4 turns. Stock filter in stock airbox.
The bike runs REALLY well once warm and pulls well in all the gears. BUT the plugs are very white !

What was the "hot set-up " for jetting back in the day for Yosh 4-1?



Mahalo,
Kelly

Are the spark plugs the correct heat range? Check against the stock specified spark plug number. 4 turns out on the pilot screws is excessive - try 2.5 to 3 turns out. If you can take the carbs out, I would check and be sure the stock jets and needles are still in there.

I don't think it should take anything more than 3 turns out on the pilots, and jets one or two sizes larger than stock with only an exhaust change.

The spark plugs I initially pulled out were in fact a heat range (or two depending on the spec sheet you were reading) too hot. I put 9's in and pulled 7's out. After a 100 mile ride on the 9's they were still showing a lean condition..

After reviewing the instruction online for the DynoJet Kit, I don't feel uncomfortable with the 4turns out on the fuel screw as it is their spec and their kit doesn't include pilots..... But I've ordered new larger pilot jets and mains anyway and will adjust the fuel screws accordingly. Bike would hardly idle at less than 4 turns out while syncing the carbs and the idle speed screw being manipulated to reach the desired idle rpm.

I was more curious if I was simply missing a common knowledge tip like "use the needles from this year and this pilot and this main"...

I Hope everyone has a great weekend, I'm off to the beach!

Aloha,
Kelly

Pic is of the 7's I pulled out.

4 turns open on the pilot screws is fine. Opening beyond this point is not beneficial since you will be at max flow already.

Your plugs don't look all that bad. I'm not sure how but you may want to lift the needles a little to richen up the mixture.

BTW, your bike looks great! Enjoy!

Thanks for the help and the kind words Ed.

I have indeed shimmed the needles but not withthe desired results. As I mentioned, I've ordered the next coulple of sizes larger mains and pilots. Looks like I get to fiddle....

Mahalo,
Kelly

That plug looks good to me, cone is a little moist, no burn down the threads so not too hot either, also the idle circuit needs to be addressed before any plug chops will reveal a good read, if 4 out your open or close to open, I would work with idle only and increase the pilot two sizes then adjust the fuel screw to 1.5 -2 turns and see if you can get an idle. If it's performing at op temp it would seem to me the mains are good, if it bogs the needles would be my next step. Up the mains you may get some top end but may make the mids soft/rich.

Make small adjustments, I think if you can search the magic numbers are there, but no two alike ya know

Take what you may, but this is a nice read:
precisionmoto.com/2014/01/10/spark-plugs/

Sort of looks lean to me. I rarely jet by plug colour, rather by ride feel, Also depends on how you ride, even though the diaphragm carbs are supposed to compensate for load, you still will need a jet size richer if you ride the bike hard. This will give best power under hard load and feel best, but seem a little rich under light loadings in the same throttle range. Also, jetting by plug colour is only good for the range of throttle opening you were last riding at, and mostof these bikes are a little fast to do wide open plug chops, on the street anyway.

UPDATE

Thanks again for all the help guys !

So I installed 2 sizes larger mains and pilots. (115 &42.5), re-synced the carbs (#4 was just a hair off) adjusted the fuel screws to 1.5 out (.5 in from stock but they were 4 out!), got the idle set and went for a ride.

She doesn't bog when you crank the throttle open super fast (Neutral, no load) where she would almost die before, She pulls nicely from 2000rpm in 5th and rips to redline, really coming on strong from 8-10K. A definite improvement. I only went for a 5 mile or so ride and haven't pulled the plugs yet, but I can feel an improvement.

A side thought.....I wish people wouldn't confuse a "plug-chop" from a general a plug reading. They are very different and used for different areas of throttle opening and general tuning. "Joe Q Public" on a larger displacement motorcycle rides around town or on the hwy at 1/4 throttle-ish most of the time. His plugs are going to indicate what is happening at the pilot jet, needle position and fuel/air screws. "Joe Q Racer" is doing a "plug-chop" to get an indication of what the main jet is doing as he rides at WFO and wants to know if he needs to go up or down a size or two. Sorry that term is a pet peeve of mine....Like when people say "it wont crank"...aaaargh.

Just rode 50 miles.... MUCH BETTER !!

Again, MAHALO for everyones help!

Braaap

KK

Welcome Kelly.
Glad you got the carbs sorted.
That is a great looking GPz!


Would love to see some of your other rides, and the beach you mentioned too.

That plug indicates too much fuel for the engine to burn now. The first plug indicated a false rich look by the carbon ring. That's because the pilot jet was too small and burn was incomplete, leaving carbon deposit. I've seen this many times but could be wrong. It's rare that an engine needs jets 2 sizes larger with an exhaust change except for two reasons. Restricted factory exhaust, lean factory jetting. Anytime you change to a larger pilot and then go2 larger on the main , your asking for trouble. Two reasons again. The pilot size effects main fuel dramatically. Less main fuel is needed when a larger pilot is used. Sounds like you got her ripping pretty good. Maybe one down on the main later if that's warranted. The true test is your 8 to 10

Thanks Nads,
I agree with your pilot/main jet theory and I think I went too large on the pilot. What's weird is she is a bit burbely when cold below 3000 rpm (choke off) but then runs great once warm, and pulls nicely through redline. I cant remember where I put the fuel screws (2 out maybe?), but they are in from the 4 turns previously set.

I went through a series of old documents from the 80's and the general rule of thumb was up two sizes on each when installing a full system. And considering how sketchy white the plugs were I felt it would be a good place to start. These bikes from what I've read were REALLY lean and restricted.

I'll keep meddling with them but I'm focused on my ZRX1100 build more now....

Aloha,
Kelly