TOPIC:

Here is the motor:
79 KZ1000
1075 kit with 10:1 cr
V&H 4 into 1 pipe
Also tried a 4 into 1 pipe that was longer from the 80's (Kerker maybe?)
K410 cams set at 108* lobe separation
Minuni BS34 carbs hooked to the factory airbox, K&N filter.
Head had only 4K miles, so I just lapped the stock valves.
Head was ported by an amateur-me. The runners taper from 34MM to 29mm and are equal volumes.
Timing is factory specs and the advance works.
Spark looks slightly weak, voltage at the coils is 11.5V.

Here is the problem. The motor flutters between 5K rpm to 6K rpm at WOT. In high gear I can go WOT at 2500 rpm and the bike pulls clean to 5k stutters to 6K then pulls clean to 11000 rpm. It does not stutter if I accelerate using 1/2 throttle opening. The jetting seems good and I hooked an A/F monitor to it and got 12.5 mixture throughout the rpm band. However if I shut it down during the stutter the plugs look rich. I machined grooves in the carb needles and tried raising and lowering them. Lowering them (leaner) made the flutter much better but did not eliminate it, but I get ping if I snap the throttle open while cruising.

So I wonder if I have a reversion problem at 5K rpm that causes the mixture to be rich just for a short 1000 rpm band? I have tried with and without airbox or filter, same problem. I am now considering reducing the runner size with epoxy to a "D" shape to try to increase intake velocity. Is this a reasonable next step? Should I be looking at something else?

does it feel like its surging, that may be when your crazy 11,000rpm cams start to kick in ;D

No, it blubbers like its rich for the few seconds it takes to go from 5K to 6K causing a big lag in acceleration, then runs smooth. I wonder if the air is double carbed from reversion? The intake ports are clean but the intake valves are sooty black with build up. Anyway I am remodeling the ports with epoxy to try to increase intake charge velocity and see if that makes any difference. This has stumped me for 2 years now. The bike just has not performed up to my expectations. But its all fun anyway. I'll post the results since everyone is steering clear of this post.

Exitpupil wrote:

No, it blubbers like its rich for the few seconds it takes to go from 5K to 6K causing a big lag in acceleration, then runs smooth. I wonder if the air is double carbed from reversion? The intake ports are clean but the intake valves are sooty black with build up. Anyway I am remodeling the ports with epoxy to try to increase intake charge velocity and see if that makes any difference. This has stumped me for 2 years now. The bike just has not performed up to my expectations. But its all fun anyway. I'll post the results since everyone is steering clear of this post.

Sit tight a little while on this. I'll give you some tips on your head work to help it run strong. For starters, leave the epoxy to those with ports that actually are too big. Yours are not..

Larry, I used a set of calculations you graciously worked up for me couple years ago where I gave you the motor specs. I followed your 2 pages of instructions carefully except I did get the ports 1mm too large at the pinch point because I got one too big and had to equalize their volumes. I avoided the SS radius as directed. I was quite proud of the job I did as I had only ported a handful of heads in my life and this looked good to me. If I don't open the throttle up WOT it will accelerate from 2500 to redline nice and smooth. But at WOT 5-6K rpm it just blubbers, but only at that rpm????

Exitpupil wrote:

Larry, I used a set of calculations you graciously worked up for me couple years ago where I gave you the motor specs. I followed your 2 pages of instructions carefully except I did get the ports 1mm too large at the pinch point because I got one too big and had to equalize their volumes. I avoided the SS radius as directed. I was quite proud of the job I did as I had only ported a handful of heads in my life and this looked good to me. If I don't open the throttle up WOT it will accelerate from 2500 to redline nice and smooth. But at WOT 5-6K rpm it just blubbers, but only at that rpm????

I don't remember that.

Don't epoxy up the port. You should instead give it bowl volume.

The throats should be at least 88% of valve size and that area should be the smallest section in the port. No section upstream from there should be smaller.

Larry, I pulled my file and you did the calculations and instructions posted 10/10/2006 under a thread titled "professional porting". I printed out 7 pages with everything I needed but a rotary tool LOL. You recommended larger valves or bigger cam to reach my goals, but what the motor should have yielded with my combination would have been fine. I did increase bowl volume slightly by widening it slightly around the valve stem area. If you don't mind sharing, what do you suspect may be happening? I did not suspect the ports were too large as the bike pulled smoothly in high gear from about 2500 rpm. I did wonder if the port flowed too well backwards?
Thanks for all the help.
Will Piatt in NC

Exitpupil wrote:

Larry, I pulled my file and you did the calculations and instructions posted 10/10/2006 under a thread titled "professional porting". I printed out 7 pages with everything I needed but a rotary tool LOL. You recommended larger valves or bigger cam to reach my goals, but what the motor should have yielded with my combination would have been fine. I did increase bowl volume slightly by widening it slightly around the valve stem area. If you don't mind sharing, what do you suspect may be happening? I did not suspect the ports were too large as the bike pulled smoothly in high gear from about 2500 rpm. I did wonder if the port flowed too well backwards?
Thanks for all the help.
Will Piatt in NC

Will,

You've got an unusual setup. Sounds to me like the slide lift hole may be too small.

I still don't recall but that's not all that important anyway. The CV carbs shouldn't be trying to feed the motor through a 29mm choke point.

Those CV carbs are designed to work with a larger port runner than you've created. Also, you didn't say what needles you have in the carbs or any other jetting specs.

I don't know how you're measuring the port dimensions because a stock port is larger than 29mm in width, everywhere in the port. Throats should not be smaller than 88% of the valve.


Back to the port size & shape.

If it's 34mm at the entrance, go back and make the rest of the runner at least 32mm, 33 would be better.

IF - your valves are sealing properly and you don't have carbon build up in the bowls and back down the runners, you don't have a reversion issue.

The valve job means everything. Hand lapping valves to worn seats is a complete waste of time. Every aspect of the valve job, port taper, bowl size, etc., has to be done correctly to yield best results.

If you go stuffing epoxy in there, you're going to loose top end performance.

Something like this is what you should be doing.

Porting is Larry's specialty, not mine, so I'm not going there.

I have a few other questions though

The jetting seems good and I hooked an A/F monitor to it and got 12.5 mixture throughout the rpm band.

This was with a wideband O2 sensor and controller, right?

12.5:1 AFR is at WOT. and not cruise?

I've run into situations where overall, the carburetion was on the rich side but the stumble in a narrow RPM range was caused by a lean condition. Plug readings can lead you in the wrong direction sometimes when you're trying to overcome a stumble situation.

Unless you have your fuel levels accurately set, then comparable jetting for your bike with what other guys have had success with, can be different.

Fuel level is critical and so often overlooked.

At the RPM indicated, that's about the RPM where those particular cams start to find the sweet spot. The motor may well want an extra shot of fuel right there.

What's tricky is the motor is controlling the slide, not the rider.

IMO, if needle clip positon changes of a full clip position in each direction have yielded no difference in the condition, then the slide is probably not lifting properly at that point, under those conditions.

How do you get it to life quicker?

Lighter springs or larger lift hole.

How do you test without springs or drilling the slide?

You can apply a temporary band-aid by positioning the needle to where it can deliver extra fuel. If it helps the stumble, you're going the right direction.

I have a set of Dyno Jet needles somewhere for those carbs. They were custom made many years ago by Dyno Jet for an ELR that we put 83 GPz11 cams in.

IF....and that may be a big IF.....I can find them, you can have them if you like. No Charge. I'll mail them to you to try out.

Have fun and good luck sorting out the stumble.

Larry C

Larry,
I machined grooves in the needles so I could move the clips up and down and higher/richer= worse, the studder started at 4K instead of 5K and lasted until about 6K. Leaner was much (75%)improved but it would ping when I snapped the throttle open while cruising. I appreciate the jet offer and may take you up on it if my other experiments don't cure it. Maybe EFI is where I need to go. I need to learn about FI anyway as I am weak in that area. The carbs will be worn out soon from disassembly/reassembly

Exitpupil wrote:

Larry,
I machined grooves in the needles so I could move the clips up and down and higher/richer= worse, the studder started at 4K instead of 5K and lasted until about 6K. Leaner was much (75%)improved but it would ping when I snapped the throttle open while cruising. I appreciate the jet offer and may take you up on it if my other experiments don't cure it. Maybe EFI is where I need to go. I need to learn about FI anyway as I am weak in that area. The carbs will be worn out soon from disassembly/reassembly

I'm not clear as to what you mean by "higher/richer" but that's often a miscommunicated event with carb needles.

Clip Moved Higher on the needle is leaner. Top groove is leanest position, bottom groove is richest positon.

I suspect you're referring to the needle being higher in relationship to where it was previously and not talking about the clip location. Is that right?