Topic: CB750 Racing

[Haybus]

Uh? Hmm. Lots of changes, many dyno runs, head is spinning. The increase is good, but a bit puzzling. I had increased the mains from 128 to 130 at home in anticipation of needing a bit more fuel, but I still had that stumble off idle, which I thought was probable the needles. Sure enough, first dyno run showed it quite lean on the needle, so up they went. However, it also seemed like we needed more main jet, so in when the 132's. First run was 80 hp., so I'm already pretty stoked. Well the needle change corrected the stumble and now it's extremely responsive off idle, very nice. But then it had a bad bog at 6k, clearly from being too rich when we were just too lean. WTH? We ended up going back down to 128 mains, where I had to leave it because I still don't have any 125 mains (did not think I'd need them, for sure!!). Changed back to the short velocity stacks, and you could really see on the runs that they were better, and what actually gave the extra 2 hp. The bog was still there on the dyno, but only about 500 rpm wide. Might get through it quickly and not really notice it. I'll get some 125 mains and perhaps try them at the track if it's an issue. Overall I'm pretty happy with the outcome. I had my long time work friend there with me. He's not into bikes, but he's been to a race with me. He was so excited after the first pull, and he was super helpful with all the changes.

Here's both runs, 4-2 pipes first, then 4-1.

Alan 

[bwaller]

Mine has that rich dip at around the same spot, however I think in my case it's largecarbitis. I never notice on the track, never let the revs fall that low!

Good job, hopefully the dyno work holds true on the race track. Sometimes at speed airflow creates new challenges, but at least you're starting from a good place. 

[Haybus]

You can see I had a smaller rich dip between 5500 and 6000 last year, and I only noticed it occasionally if I got fully on the mains coming out of a corner. It would produce a slight bog. I didn't attribute it to the rich dip, because I never looked back at my dyno run. Not sure I would have connected the dots anyways, until yesterday. I'll certainly be paying attention to throttle position and rpm's this year.

I forgot to tell my friend what RPM to stop at, so he pulled all the way to 12k on the first full pull. I almost had a heart attack. Man was she sounding awesome!!! Last year it didn't want to pull much past 10k. Seems like she was corked up a bit compared to now. I too hope it translates to something nice on the track.

Alan

[scottly]

81+ HP from only 760 cc is pretty damn good!! Your porting job must be working! So you gained 7+ HP  mostly by switching exhaust??!! The torque and HP peaks also moved up a bunch of RPMs; I would have expected a cam change would be required to have such an effect?

[Haybus]

So the rich bog is real, and can't be ridden through. I dropped down to 125 mains and it helped maybe a little, but if I'm pulling through 6k, in 4th gear, the only way through the bog is to shift down. I can definitely pull through it in 3rd, but not in 4th. Some sort of reversion going on with the intake tract at that rpm I guess. Not really sure what to do, other than ride it like a 2 stroke. I don't think going further down on the mains is the answer as I'll just lean out the top end too much. Suggestions?? These are Keihin CR31's.

Alan

[bwaller]

What do those CR's use for air jets in the throat?

[Haybus]

Nothing fancy I believe. A hex shaped jet similar looking to the main. I know all circuits contribute a little, but besides the main air jet are there others worth having on hand to play with? I'm definitely heading back to the dyno to solve this and want to make sure I have any jets necessary.

Alan

[Sam Green Racing]

Alan, The CR31s are fine carbs for tune ability and a good carb company should stock all the parts. There are God knows how many different needle combinations to go at as well as just jets. However, there is the possibility that 31s are a little large for a 760 but that's just my thinking.

[bwaller]

I think Sam is correct to a point, and the 31's may be showing that effect right at the range you're having issues. However I think you'll be able to improve that spot. I'm a babe in the woods with CR's so here's some simple thought. Would a leaner needle taper help? If the plug color is good and the mains work otherwise, a different jet needle might be worth a try. You said elsewhere the exhaust note was good and that can be telling.

I also wonder about trying a larger airjet. Downside to that will be needed changes to two circuits after to compensate.

Good luck.

[Haybus]

I was thinking leaner taper too Brent. I have a richer needle in there than what came with stock and I was wondering if that's part of the problem. The manual shows the main air jet in play at WOT. Since it happens at a specific rpm it's likely I won't be able to get rid of it, just hoping to move it lower.

Alan

[Haybus]

Additionally, lots of folks have used the CR31's with success, or so I've read 

[scottly]

I've noticed a rich condition in the same RPM range on my bike while on the dyno, but not so rich that it flat-lines at 10:1. What is the throttle opening when the bog occurs? If it's WFO, it's a main jet issue, not a needle issue. Even at higher RPMs, your chart shows the mix is on the rich side; you might try another size down on the mains?

[Haybus]

Finally got some free time to tear down the motor today. For those that may not have read, I grenaded my motor at Utah Motorsports Campus a couple weeks ago. First thing obviously not right was the oil pump being in about 5 pieces. The chain tensioner was still attached, and mostly where it's supposed to be, and the wheel spins freely, but the arm is pretty mangled. Doesn't seem like it was the cause. The cases are tore up all over the place, like the chains were whipping around for some time. The left chain ring has some chipped teeth. Worst of all, my lightened and balanced crank is done for with many damaged teeth. Drilled out the rivets on the cush drive and it certainly looks like the cause. The center star hub is cracked in three places on one side and all the holes on that side are a bit oblong. Don't think I'm saving any of those parts. An expensive lesson learned about abusing old cush rubbers. See picture of the aftermath below.

Alan 

[scottly]

The cases are tore up all over the place, like the chains were whipping around for some time.

Is there damage from both chains, or just one? If both were whipping around the same, it may be the loose chains caused the cush failure, as well as the damage to the teeth on the crank? Hard to tell which caused which. More pictures, please.

[Haybus]

No, the left chain saw a lot more damage than the right. The left chain wheels are missing teeth, where the right ones are just beat up a bit with some sharp edges. Both chains were off, so I couldn't tell which was which, but one had a single break while the other was in 5-6 pieces. Not sure what other pictures you want. Kind of want to just call it a cush failure and move on. I had signs of the failure now in hind sight, just didn't recognize them at the time. I got three racing seasons out of a 42 year old cush drive. Impressive, stupid, and costly.

Alan

[bwaller]

This sucks Alan, sorry for your trouble. At least it's not the beginning of the season.

[Haybus]

No worries Brent, seems it's all part of racing. I too am glad it happened at the end of the season.

Alan