Gas cap

  Good day.
Has there been any discussions regarding what seems to be suction under the gas cap? I’m desperately trying to get my project running properly so I can obtain a safety. Engine will run somewhat erratically but only with about a third choke. When engine is shut off it can be heard hissing under the cap. All I can think of is the orientation of the plate that screws to the underside of the cap, which happens to be new but I’ve never worried about that in the past. Stock 900 motor, 29mm Smoothbores and individual filters.
Thanks in advance.
David
 

I have taken a couple apart on a KZ650 (same PN as '76 KZ900) and drilled out the "vent" hole bigger in the steel/aluminum and opened up the corresponding hole in the paper gasket.

My 831cc engine with 33mm flat slides runs hot and if its 34C out you could hear it hiss when you came to a stop. After the cap mods the hissing stopped.

FYI - You do need to be careful when pulling them apart as there were no spare parts ever available except the rubber gasket as they were just sold as assemblies.

 

 

 

On one I took apart there was a small yellow spider dead in the vent "tunnel" basically blocking it from venting!
The first pic is the stock cap - note how there is hardly a opening for air to escape. On the one in the pics the paper gasket was mismatched to the steel hole, so blocking slightly. I also polish the aluminum cast "track" to make it smooth. On the third pic you can see how its "ported".

This was a 40 year old cap that I put a new gasket on. So far I have done this mod to 3 gas caps, one I have ridden with for 12 seasons and the other for 10 seasons with zero issues.

I am not sure if this will alleviate your issues, but it can't hurt.


 

I got fixated on the hissing issue, and forgot to ask about the safety concern with the idling. Smooth bores are a "race" carb, so I find they like to idle a bit higher than a stock Mikuni VM carb (26/28) You may think its erratic but its probably OK.

I need to ask but you have you done a vacuum sync or just a bench sync?

I am in Ontario and got a KZ650 safety checked 5 years ago. I told them up front that I restored the bike and did the work myself. Maybe because of this they were pretty lenient about the way it was running (bike in the pic below - 831cc w RS34's) It had a slight weep on the rear caliper and the idle screws were set a bit rich. Just bench synced the carbs. 

 

They just asked me to fix that stuff and signed off on it.

I think another Ontario member on here (rstnick) also recently got his ELR safety checked. He may be able to shine some light on what they did.

I do know they are more strict on car safety checks now in the past couple of years. Had trouble getting one car to pass recently and failed on second attempt because of a technicality, and they had zero remorse!
 

Thanks for the responses gentlemen. I’m in Ontario also, (Burlington),  and I kinda know Rob. I was CB not aware there was a paper gasket under the cap but I intend to check the holes and alignment. I bench synced the carbs with the prescribed wire under the slides and I did notice some minor differences with the number one carb and as that was the only cylinder off drastically with temp I decided today to go through them and follow the Sudco manual to the letter. Have good spark, air and fuel so hoping I find something in adjustment. Realized Acewell tach was setup for different rpm so will correct that.
Will advise on what’s found.
Thanks.    David