Running rich - low compression

* Apologies - this probably should have gone in the Carburettor forum as that is the main topic. *

So I had my bike over at the shop for a good spring once over. I do my own work but being relatively new, I like to have a professional second opinion from time to time.

The bike:

1978 KZ650
stock airbox, new filter
V&H 4-1 pipes
107.5 main jets
stock pilot jets
needle clipped at 4 (left in factory position)
WiredGeorge coil wiring mod done

First thing that came back was low compression across the board: 115, 120, 105, 125

My guess is this is from riding around with gas in the oil. My big dumb newb mistake two years ago.

The second thing they found after replacing points, retiming and synching the carbs was that it was running way rich. The mechanic said they used to run them a little leaner than stock when they were new.

These guys have been in the business for 25 years or so, they are one of the few shops willing to work on old bikes, and the main guy used to race KZ650's.

But isn't that opposite of what people here tend to think? I thought they were run lean for emissions purposes? The mechanic suggested I go back to stock. what do you all think?

Post edited by: Blake_ae, at: 2006/05/08 14:18

My stock 78 650 has 102.5 main jets, it looks like yours is 2 sizes larger. An 4-1 exhaust pipe may only need 1 size increase. Do a plug chop test and see what you get.

About the compression, did they adjust your valves first? If they didn't, and they tested the compression on a hot engine then that may give low readings if the valves have very low clearance.

Was the throttle open when the compression was checked?

I don't know their process for compression checking but since they've been wrenching and racing KZ's for 25 years, I'm assuming they know their stuff.

I will ask about the valves, I don't think they checked them, at least I didn't specifically ask them to. Thanks for the pointer.

** I live downtown Toronto. There is nowhere around here I could run the bike WOT for half a mile. Even at 3:00am there is enough traffic here that I could be getting into a lot of trouble. It would have to be done hours outside the city. I'll just chat a bit more with the tech and probably swap down stock jets and see how it goes.

Post edited by: Blake_ae, at: 2006/05/08 13:27