TOPIC:

1976 kz 900 (modified, see below).

When I first got the bike, running stock 903cc, I decided to use 92 or 93 octane gas. Ran ok but now, through education gained on this site, runs better on 87.

But... have now fitted new block/pistons taking bore to 70.25mm so increasing displacement from 903cc to 1,024cc. But, still using the original kz900 903cc head. Pistons are also somewhat more "domed" than the original 66mm pistons.

Bottom line is that I MUST now be looking at a somewhat increased compression ratio and should I be using a higher octane gas?

Thanks.

run some 89-91 in there .stock compression is around 8:1 most big bore kits start out around 10.25:1 thats probably about where youre at now try some 89 and see if it rattles on you and adjust from there .running too much octane in a low compression engine will just carbon up the pistons and valves over a period of time.goodluck,happy wrenching!

Generally, use the lowest octane possible, increasing to prevent detonation.
Scrupulously measuring your combustion chamber at full extension and full compression to determine your exact compression ratio will give you a good guess as to what anti-knock index gasoline you might require.

I'm going from stock (8.5 I think?) to 10.25:1 in mine; I plan to fill it with high-test and see how it runs. If I'm lucky it will not ping. if I'm not lucky, I'll working on retarding the spark by modifying the advance plate or buying a Dyna 2000. If I am very lucky, it will run beautiful, and I'll give it a go on midgrade.

The Gasoline FAQ is your teacher.

"Gasoline FAQ" I think someone had a little too much time on his hands! I think that 3.7% in line 4653 should more correctly have been 3.7500376%.

Thanks for the input(s). I'm going to try 89-91 and see what the results are.

Just did a little rough & dirty math, trying to allow for the 66mm dia chambers on top of the 70.25mm pots and further allow for the more steeply domed pistons. I think this is probably going to result in my compression ratio going from 8:1 (stock) to 9.96:1, call it 10:1.

My current bike is 9.3 to 1 and it runs well on regular 87 octane in the cooler months. In the summer it gets blazing hot (Phoenix) and intake air temps get close to 130 degrees. Then I have to use a mid-grade 89 octane to keep it from detonating under a hard load.

When i put a big bore piston kit in my 900, the timing had to be retarded severely bfore it would run right. The increase in compression speeds up the burn time. Use the same octane and back off the timing. It will run just fine and will have a very proper power band. Leave the plugs alone. If you go colder you will carbon the cylinders terrificly. If u have a dyna, you may have to elongate the slots.

I would give it a go on 87 and see how it does...go up from there as needed.

Do a compression check. That will tell you what octane to use. You can use 89 octane all the way up too 195 psi. I dought you have more than 190 psi on cranking compression.

nads.com wrote:

If u have a dyna, you may have to elongate the slots.

Huh? I thought that was an either/or situation, either the dyna controls the timing, or you make the timing plate adjustable. what's up, nads?

Any suggestions on the amount of retard required or should I maybe loosten the backing plate screws hold it at 2,500 RPM and manually rotate the plate until I get the best result?