RE-jetting "The revisit"

Ok, this one is for the high end knowledge guys. I installed a dyno-jet kit in a 1981 Seca 750 this weekend. The first thing I found out was it already had a jet kit in it. 120 mains vice 110 stockers. The needles had been changed to Y-13 vice the stock Y-10’s. So the kit came with fours sets of main jets. 120 & 124 for stage one and 132 & 136 for stage three. The bike shop said it was lean from mid to high RPM with the 120’s in it. I skipped the 124’s and went to the 132’s. The kit also included new needles. I installed everything according to the instructions with one minor exception. The instructions wanted me to drill out two air holes in the bottom of the slide on either slide of the needle. The difference in size was very small and I was worried about screwing them up, so i skipped it. Now to the trouble. When I got the bike running. Everything was fine, until I got it out on the road. It runs better than it ever has, It was strong and smooth at lower RPM, but it has a problem at higher RPM. It starts to hesitate around 6 thousand and up.

Now the questions

One: Is this because I over jetted it?

Two: Is this because I didn’t drill out the air passages in the slides?

Three: Both?

The sooner you can get back to be would be great. My reputation at work is riding on this.

 

Welcome to the jet kit nightmares!

120 mains and Y13 needles are stock for the 750. The 110 mains and Y10 needles are stock for the 650...

Now, the hesitation... First, determine whether it's a lean or rich condition.. One quick/ugly way to do this is to add a bit of "choke" as soon as it starts hesitating. If the hesitation gets worse, you're too rich. If it smooths out, you're too lean.

If the needles are set too high, you will run richer at mid RPM. If they're too low, you will run lean mid to high RPM.

I'd say that 132 mains might be too hot. Try the 124's first.

Drilling the passages in the slides is a whole another can of worms. The slides will come up quicker, supposedly improving throttle reaction time... But they may also create a lean spot if they come up too quick.

 

thanks stereomind

I kind of figured it out on my while the forum was down. I took the slides out and drilled them, put it back together and no difference. So I took the carbs off a third time and put the new #120’s that came with the jet kit it to see if it would drive normal. Which it did! Better than ever! I’ll drive it like this a short while then try the #124’s. If it drives the same or better then the #124’s I’ll leave them in and call it done.

81 Seca 750