Keihin Cr´s breaks-up at full throttle

leitz wrote: ...Bike pulls clean and hard at all the revs and gears but starts to break-up the mix at 4th gear full throttle (100 mph) and on 5th don´t pulls to more speed. I´ve to roll off the throttle because the bike deads....

Weak spark is one of the suspects that might possibly cause the reported symptoms.
Inadequate voltage to the coils is one of the suspects for weak spark.

Good Fortune!

When coil mod was installed the voltage meseaured at each ignition coil primary terminal was the same that in the battery, about 12 volts at idle and about 15 at 6000 rpm. Now i have to check it again

"YY8 needles at 2nd clip."... this leans it out, sounds as though running lean @ 3/4 throttle to WOP thus you'd want to richen it!

You'd want to lower the clip position down to raise the needle for more gas flow....





Possibly different needle... In case you need...


Plug conditions...?

I'm leaning more towards ignition like turboguzzi and motorhead is suggesting. Way more load on the engine in 4th and 5th. This is where your ignition problems will show up more. Make sure everything has a good ground and have fresh plugs.
Since you gained mph with smaller mains, you know it's too rich for your ignition to fire.
One thing you need to be aware of is the quality of fuel you have in the tank now. Bad gas could be part of the problem. Try fresh fuel, or maybe a good splash of av gas to see how it pulls. Av gas has a bit lighter specific gravity, so keep an eye on your plugs at WOT.

There has been a big drop in temperatures where I live and had yesterday a long riding on mountain roads: voila! stutter and break up disappeared at 5th, while get worse 1/2 throttle. Anyway don´t pull beyond 105-110 mph. so i think carbs are rich at mains and i´ll try with 105. About the erratic 1/2 throotle (leaness i suppose) i´ll try with clip at 3erd or 4rd clip to rich the mix, if not improves then i need the next richer needle: YY7. Also i need to check the ignition, coils, sparks ...
Have a good start, idles well for a racing carbs on street, less hp at 4000-7000 revs than originals VM´s but get an enjoy fast ridind from 7000 to 10000 rpm where bike likes to be. Of course engine mods like porting and cams would make the diference, but as we say in Spain "todo se andará": everything will come.
Thanks to all you guys for the info, I will keep you informed of progress

If you're running stock valve springs, connecting rods, etc. I would not take it up beyond 9500 rpm. Others have run these engines up to 10k rpm (or beyond) and ended up with scrap metal. I believe if you put it on a dyno you will find your engine makes less power above 9k rpm than at 8500 rpm. If you can, give that a try as you may be surprised at how much the power drops off when you reach the upper limits. Ed

hola chico

no sabia che hablas español.... con ese nick aleman ni me lo hubiera imaginado! y el filmado en pista se me escapo antes.

Sure, it could be temperatures, but honestly, even in my very highly tuned 750/810 that runs with CR29's, i dont have to change jetting from 8C in the morning to 22C in the afternoon (like i had yesterday on the track at my last race of the season).

If anything, that proves even more that you still have ignition problems as with bad ignition the motor becomes much more sensitive to small mixture changes. a good ignition burns it all even when not on the sweet spot

Ed, I constantly run my 750/810 (essentially a very big bore 650) to 11K without much problems with only HD springs to help against the race cams.

I read this entire thread How is it that nobody even questioned the #80 Slow Jet? I can't see how it could possibly want that much pilot jet. Where are your air screws? 4 turns out I'll wager.

The first thing you want to do is address that situation. It is the single number that immediately jumps out as being out in left field. Regardless of the fact that you're having top end issues, that's got to be way too much fuel being dumped in through the pilot circuit.

It appears you have electrical issues also but you need to start with a reasonable jetting spec. Something around a #60 slow jet is much more reasonable.

turboguzzi wrote: "Ed, I constantly run my 750/810 (essentially a very big bore 650) to 11K without much problems with only HD springs to help against the race cam."

I don't doubt it, but there has been at least guy on another forum who has blown his motor using stock valve springs and con rods at high rpm. I don't know whether his rods stretched or the valves didn't close quickly enough, but there was contact between piston & valve. So over revving a stock engine introduces a risk of blowing it up. I do not hesitate to take mine up to the 9k rpm redline, but I don't go beyond that (even though the engine is very willing), and I have not had any problems since buying the bike new (53,000 miles).

The other issue is that the stock KZ650 motor, perhaps unlike your modified motor, has a power drop-off after 8500 rpm. By the time his stock engine gets to 10k rpm it's making about the same or less power than it was at around 7k rpm - so revving that high is not only risky to the engine it also provides less power. The power curve below is for the 1977 KZ650, several later models lose power at even lower rpm. This is why I recommended he run it on a dyno so he can see his engine's actual performance curve. I think he would be surprised at how little power the engine makes at 10k rpm. Ed

650ed wrote: turboguzzi wrote: "Ed, I constantly run my 750/810 (essentially a very big bore 650) to 11K without much problems with only HD springs to help against the race cam."

I don't doubt it, but there has been at least guy on another forum who has blown his motor using stock valve springs and con rods at high rpm. I don't know whether his rods stretched or the valves didn't close quickly enough, but there was contact between piston & valve. So over revving a stock engine introduces a risk of blowing it up. I do not hesitate to take mine up to the 9k rpm redline, but I don't go beyond that (even though the engine is very willing), and I have not had any problems since buying the bike new (53,000 miles).

The other issue is that the stock KZ650 motor, perhaps unlike your modified motor, has a power drop-off after 8500 rpm. By the time his stock engine gets to 10k rpm it's making about the same or less power than it was at around 7k rpm - so revving that high is not only risky to the engine it also provides less power. The power curve below is for the 1977 KZ650, several later models lose power at even lower rpm. This is why I recommended he run it on a dyno so he can see his engine's actual performance curve. I think he would be surprised at how little power the engine makes at 10k rpm. Ed

Attachment hp0001.jpg not found

Shouldn't need a dyno to feel it going flat Ed...but I know where you're coming from. The chart clearly shows the power falling like a stone on a stock engine. A well built engine with the correct cams, carbs and porting will hang on better after the peak.

LarryC wrote:

"I read this entire thread How is it that nobody even questioned the #80 Slow Jet? I can't see how it could possibly want that much pilot jet. Where are your air screws? 4 turns out I'll wager.

The first thing you want to do is address that situation. It is the single number that immediately jumps out as being out in left field. Regardless of the fact that you're having top end issues, that's got to be way too much fuel being dumped in through the pilot circuit.

It appears you have electrical issues also but you need to start with a reasonable jetting spec. Something around a #60 slow jet is much more reasonable."

When i bought the carbs from new they were jetting for my bike: 65 sj, 1 turn out air screw and YY8 needle, but idle "hangs" after blipping the throttle and the engine be slow to retun to idle after running. I couldn´t solve it turning the air screw, even Colortune indicated leaness whith all the turns of air screw. I tried with several sj: 70, 75 and 80 and always 1 or 3/4 turn out but less than 1 turn out because when blipping the throttle, the rpm fall back down below the set idle speed and then recover, and turning out the screw the engine "hangs". I know 80 sj is so high but it´s the only one that let the idle stable and almost perfect at 1250-1500 rpm. Maybe with richer needle diameter i can try with smaller slow jets. I knew installing these carbs would be a little adventure because i haven´t found any info in all Internet about cr29´s on a Kz650. I´ll post a video at idle
Anyway I'm not an expert and probably doing something wrong.

650ed wrote:
"By the time his stock engine gets to 10k rpm it's making about the same or less power than it was at around 7k rpm - so revving that high is not only risky to the engine it also provides less power."
I feel my engine have a lot of more power at 9000-10000 than at 7000 rpm. It´s difficult for me not to reach 10000 on track and mountain roads without revs limiter, revs up fast, but i try not to overcome 9500 rpm.

Interesting article about carb mods on dyno

www.mediafire.com/?7ttgomzmmjz