kz650sr runs on only two cylinders

I have a 1981 KZ650SR that I've had for a few months. It's been running fine but today it's only been running on cylinders 1 and 2. The bike starts and warms up to an idle but only those two cylinders get hot. The other two get warm only from the friction and their exhausts stay relatively cool. I appear to have spark on all four cylinders. Before I start taking things apart, starting with the carbs, I thought I'd post something here. There is one thing more I should probably mention. The guy I bought the bike from replaced the carbs. Usually, one of the carbs on a bike of this vintage has a vacuum tube coming out of it that runs to the petcock. None of the carbs on this bike has the tube and the petcock does have a corresponding connection on it. Despite this, as long as the petcock is in the RES position, the bike runs OK.

Thanks

Welcome Mark.

If your getting spark on all four, then either your not getting fuel to the cylinders, or the timing is off.

Are the plugs wet when you pull them out of 3 & 4 ? If they are, we can rule out the no fuel issue.

The timing would need to be checked for all 4 cylinders. I'd expect you'd be getting some backfiring at least though.

Do make sure your plugs are not fouled and the gaps are correct.

Also a commpression check might reveal some answers.

Is the vacuum port on the petcock plugged, since it's not in use? I'm not familiar with the '83 carbs, CV type I believe. Maybe they're not supposed to be a vacuum type?
Anyone?

What I've found is that when the petcock is in the ON or RES positions, fuel isn't getting to cylinders 3 and 4. When I switch it to PRI, fuel flows freely and eventually 3 and finally 4 fire. There is some backfire while the bike is still warming up but once warm, it has full power. I believe I'll just finish up with rebuilding the set of carbs that were original to the bike and set up the fuel system the way it's supposed to be.

Thanks for all the responses!

Yes put the original carbs back on her. Sounds like a good plan.

As for the problem at hand, I think the fuel inlet tube to the 3+4 carbs is partially blocked.

OKC is right. The prime allows enough pressure to push past the blockage in the carbs. There are 2 choices I would take I would keep that existing carb and take the bike on a good hard ride, along with some in the tank carb cleaner or take the carbs off and rebuild them, and replace that vacuum line that goes to the petcock. I am not familiar with the 83 CV's but I should guess they have a vacuum port on any carb on there to hook up to it.

Are you starting it and trying to run it on the side stand? Sometimes my bike starves for fuel on 3&4 when on side stand, stand it up and everything clears up. Just a thought..