How much fatter on the mains should I go?? GT550

jaybob · Post by **jaybob** » Tue Jun 16, 2009 2:42 pm

Ok here are a few pictures of my plugs after some testing last night.

I went out on the highway and found a really long hill and did a WOT plug chop. The pictures are of the plugs after riding home. When I pulled them on the side of the road after chopping them they looked much the same but maybe slightly leaner. No ash or aluminum deposits though.

Looks like I could go up a few main jet sizes, but not sure how much to start.

The bike runs crisp right now and I have no complaints. It runs clean from 1100 til 5000 rpms. Then it starts to pull really hard from 5000 until about 6700 and then pretty much signs off at 7000. I think It may do better with more fuel.

here are the specs:
1975 GT550 .5mm overbore a few thousand miles ago
I see some signs that the previous owner cleaned up the intake ports and transfers
Uni Filter pods
Jemco Chambers
Stock VM 28's
30 pilot (up from stock 25)
stock needle jet
stock Jet Needle full rich in 5th clip position (stock is 4th position)
112.5 main jets in all cylinders (stock is 97.5)

So it looks like I could bump up the main jets. How far should I go? 117.5? 120? I don't really know why this bike wants so much gas...but it sure does. Maybe I need a richer needle jet or jet needle. Not really sure.

Jason

ja-moo · Post by **ja-moo** » Tue Jun 16, 2009 8:47 pm

I personally go 120's. It's really better to go too rich and work down, less melted parts that way........

chainsaw · Post by **chainsaw** » Tue Jun 16, 2009 9:30 pm

unbelievably, I agree with Moo

JK

better safe than sorry. get em black and wet and work down.

Buffalo-guy · Post by **Buffalo-guy** » Tue Jun 16, 2009 9:59 pm

I would tend to investigate getting some richer needles, to increase your fuel flow throughout the range. The mains can only do so much, and most of your riding will be involving pilots and needle jetting. If those plugs were pulled after a ride home, then the needle jetting is too lean. IMHO.
Fred

jaybob · Post by **jaybob** » Thu Jun 18, 2009 12:31 pm

Ok here is an update. I put in 115 mains with no other changes and started getting some 4 stroking. So I am going to go back to the 112.5 where it ran clean and then start concentrating on the needle to see what it needs.

I was thinking the the needles were too lean but again I got the 4 stroking in the mid and full throttle.

So if it started 4 stroking on the mid throttle with the larger mains then the needles should be ok in size just needs adjustment.

I am getting pretty good at pulling the rack by now. I've doen it at least 6 or 7 times LOL!!

tz375 · Post by **tz375** » Thu Jun 18, 2009 5:05 pm

Word to teh wise. Needles have a taper, yes? So if the tip of the needle is "right" but the shaft is too large or the needle jet is too small, there's not enough fuel at low to midrange even though it's rich at WFO.

Test at 1/4, 1/2 and 3/4 throttle to see what happens as you progress up the needle. It may change and catch you out.

jaybob · Post by **jaybob** » Thu Jun 18, 2009 6:47 pm

Richard, Thanks for jumping in again.

I have not forgotten the research you did for me previously. it would seem that you are trying to maybe gently suggest it again and lead me towards the correct path through growing my understanding. Specifically this quote:

tz375 wrote:
"the 5DH21 is an unusual needle. It starts to taper at 1/4 throttle with a shallow 1 degree taper and at exactly half throttle it changes to a steeper 2 degree taper.

That makes it a lean needle but particularly lean at half throttle. Closest to that is a 5DI20 and it's slightly leaner at half, which we don't want.

5FL14 is really close to the same shape but slightly richer at the very top end and is available from SUDCO. The taper is 1.5 degrees at the start and changes to 3 degrees towards the tip so it's richer at the very tip/top end.

Combine that with a P-4 series nozzle and you get back to an almost identical fuel slope but 8-11% richer all the way through, and one clip down would fine tune it if necessary."

Going back and reading this it would seem that you are saying I could try the 5FL14 in the third or fourth clip position combined with the larger P-4 series nozzle to try to avoid the aformentioned lean at half throttle which is a characteristic of the stock needle. Then I may not have to run what would seem to be an overly large main jet since the tip of the new needle would be richer also?

The largest obstacles going against me in getting this thing tuned right is:
I am an amatuer tuner.
Time available for testing.

I only get bursts of tuning time since I have 4 young children and I work long hours. I know not an excuse for not doing it the right way!!!

I have been studying and filing away the experienced information provided. So gently prod me some more. Do I spend more time trying to tune to the stock needle and nozzle? Or would I be more wise to go for the richer fuel slope all of the way through?

Post by **Suzsmokeyallan** » Thu Jun 18, 2009 7:58 pm

Richard usually agrees with me on many topics so ill say this and see if we are still agreeing to the record.
What id do is go for the 5FL14 needle with a P4 nozzle to richen the total fuel slope all the way through.
Then leave the original mains and try the notches on the needle to see what sort of plug reading can be achieved first.

tz375 · Post by **tz375** » Thu Jun 18, 2009 9:30 pm

Err. to be honest, I forgot all about that prior post. All I was really saying is that when you are trying to get the fuel slope right across the range, sometimes the results are not what we expect because we forget about needle shape.

That's why I suggested trying it at different throttle settings to confirmwhere it's lean and where it's rich.

If it were me tuning that bike, I'd agree with Allan and try that combo, but there is no guarantee with any of this stuff. It's 10% inspiration and 90% perspiration.

There is really no right or wrong way to work out what the motor needs. they are all different and as long as you are approaching it scientifically, which you are, you'll get to the right place.

My concern is that most of the pistons I melted were done riding along on the needle at say 1/2 throttle or less with big main jets. A few jets are cheaper than pistons and rebores and I hate to see people make the same mistakes as I.

That's why I'd go rich on the needle jet and needle and start there and work down. Just take your time and keep your ears open for detonation.

jaybob · Post by **jaybob** » Fri Jun 19, 2009 9:07 am

Well the good news is that I can't stand to own a bike that does not run right. So I'm willing to put in the perspiration.

I have another solution though. Since the 550 powerband has a fairly low redline I'll just gear the bike way down, put on a drag tire and ride WFO everywhere I go.