New Cams or Displacement Increase

So I will probably be doing more work on my bike when the winter comes around (who isn't?). The only complaint I have ever had about my bike (and it might just come down to tuning) is that the low end of my 76 KZ900 is weak. It screamed at the high rpm's but I just wanted a little more oomph from the bottom.

It had been suggested to me once to drop in a set of Web Cams with the 118 grind number. I don't understand what the duration and lift numbers refer to, but I was just looking for some insight as to wether this help or wether I should just increase the displacement of the engine. If I do add displacement, what other considerations do I need to examine? Will new cams be necessary? Carbs? Obviously I would need different jets at the minimum.

Again, this is an area I don't understand very well - but I like to learn.

Post edited by: DoubleDub, at: 2006/06/30 23:36

well id run a compression test and a good stiff tuneup first to figure out if the motor is running right first and also check your valve clearances before you start doing internal stuff to engine .

Forgot to mention - it was suggested to change the sprocket ratios a tooth or so as well. I believe one tooth smaller in front and one larger in the rear? I always get that messed up, though - could be wrong. (sprocket ratios take too much thought for my puny mind)

Yeah - a head rebuild is number one on the priority this winter. There's a shop here in Madison that seems pretty proficient with the older KZ's and I planned on taking it in there to have them do a compelete overhaul...I have a used head on the bike right now that I think will need some work first. I did a compression test myself and was getting about 90psi and up on all cylinders with the throttle wide, but I'm sure some of the loss was from the head and not the cylinder block.

did you check your valve clearances?if you have some valves hanging open your compression will be low also,dump a little oil in cylinders and see if compression comes up any.

Yeah - I got the clearances pretty close - I think a couple of them might be a touch off still but they are all pretty close.

I haven't tried putting any oil in the chambers yet - I saw the Clymer suggested that as well. If the compression increases after adding a little oil does that suggest that the rings are bad? The bike has a ton of miles on it (I think over 40k?) so it's definitely time to do something with it. I don't think I want to make a authentic rebuild of it (it's not like it's a 73) but I do want to make it last as long as I can. And I want it to look nice. I work for a major bicycle company so I can get the body work painted nicely by our painters on the cheap. But the rest is all up to me...

Longer duration cams ( the valves stay open longer) will cause a cylinder pressure drop that will hurt bottom end power while helping top end. As this is not what you are looking for, I would suggest a 1015 piston kit which will raise cylinder pressure ( and low end power).

Have your head guy "cam clearance" it and put in some VS900K valve springs. Then at a later date you will be able to drop a healthy cam in it.

Jay

Will the 1015cc kit require larger carbs or just a rejetting? I'm assuming to get the most out of the larger displacement I would want a "healthier" cam too?

the 26s or 28s whichever you have will be fine,just fatten em up a little.

I agree, cams will reduce the bottom end power as they did with my kz650. Boy does the top end scream though. HELLO WISECO!

if you put a 1000 topend on there and keep the 900 crank youll be plenty occupied without worrying about a big set of cams! :evil:

I put a 1015 in my76 900. Big jets and the timing backed off. Way off. The thing flys. You needed to maintain cylinder pressure to make good power with any cam. Mine has 150psi with 260deg cams. It had 180 with the 1015cc kit and stock cams. More gas after the 1015, will net you the highest power gain. The cams will need more air flow to stuff the cylinder. Anything in the exhuast pipe will kill power. The scavenging goes to sht. Lobe center is the governer of the power band. Three choices here. Power right away, power all across, power up top. I have my centers at 109-111, it seems that the power is everywhere. The bottm is good, the middle is goood and it screams at the top. And all the way to it. Port the heads and dont be shy. Enlarge the ports where they curve to the seat. Make this radius the same diameter as the seat is. And smooth the transition to the seat as much as you are able. Keep compression high and any cam wil do. Give it plenty of gas whether you have 26mm carbs or any other.

Post edited by: nads.com, at: 2006/07/02 18:42