1983 Yamaha XJ900 pilot jet black rubber plugs

Hi, I am currently working on a 1983 Yamaha XJ900. Cleaning the Mikuni's, My question is with the pilot jets do they require the black rubber plugs?? Some say yes others no. Some on YouTube swear by them and say if not used the bike will run too rich.... an opinion would be appreciated. cheers Rodney

 

Exactly what rubber plugs are referring to?

the ones that come in some kits to replace the original brass plug that had to be drilled out to access the mixture screws? No... they aren’t necessary

black rubber plugs like some carbs have over the idle jet? They don't go on an XJ900rk.

 

The pilot jet plug is a Suzuki carb thing you see it in the rebuild manual that was once on liine

 

The pilot jet plug was also used on "early" (1978 to early 1980) BS34 series Mikuni carbs, featured on the XS1100 models. This was done away with on mid/late 1980 carb sets, which then use a standard Mikuni pilot fuel jet, no rubber plug, and life was once again good and simple..........

 

The rubber plug was standard in Canada on xjbikes

 

On which models?

 

cheers thanks for that.

 

All of them, we never had brass or aluminum blanks in pilot jet hole.

 

I think we're all talking about different things............what I was referring to (and what I think the original poster was asking about....he asked about the pilot JETS) was a rubber plug which was used on some models to block off the pilot fuel jet entry passage in the bottom of the carb (where the pilot fuel jet is fitted). My response (some early BS34 carb sets used these) is correct as long as we are referring to the pilot fuel JET port/cavity in the base of the carb body.

I believe Toomanybikes is referring to the metal "anti-tamper" disc that was fitted over the pilot mixture SCREW on the top side (forward) of the carb body. Which brings up an interesting question in regards to what he said, which is that all Canadian models were fitted with rubber plugs to cover the pilot mixture SCREW (if I'm understanding this correctly)..........funny, about 10 years ago I cam across a set of carbs that had such rubber plugs in the mixture screw ports, and they were obviously perfectly form-fitted to those holes, so I supposed that they were factory parts, but I could never find a part number for them. I guess I should have tried to find a Canadian market parts list......

 

My UK new 1986 XJ900F had rubber plugs from new . I thought they were there to stop water collecting more than anything else

 

chacal is correct. The( external) brass/aluminum plugs are a manufacturers way of (Anti- Tamper) covers to keep people from messing with the adjustment of these highly sensitive pilot air screws. As far as any plugs inside the bowls is a redundant idea as fuel will break down the seals and introduce microscopic material into the carb circuits, eventually clogging them. Your pilot air screws are different from pilot jet. Air screw limits the air into the pilot circuit, where as the pilot jet limits the amount of fuel. And by me saying (highly sensitive) meaning the air and fuel mixtures on pilot circuit is for low rpm/idling. Any mis adjustment will alter the engines ability to idle after choke disengagement, as well effect the carbs ability to switch from pilot circuit fuel consumption to main jet circuit consumption. Carbs essentially operate on engine vacuum which draws air and fuel thorough multiple circuits based on vacuum pressure. Pilot on low to mid rpm, and main on mid to high rpm.

Plugs are to stop first buyer from messing with the adjustment, not for 4th 5th owners down the road after any warranties have expired. So no they are not required to function. But a pro bike shop will remove them to adjust and fit rubber insert plugs after a service. Brass is oem plug, aluminum is aftermarket (rebuild kit) plugs and rubber is from professional service that has been done at some point. But not all pro shops will plug them back up.

 

Pictures of mine

 

Those are just as (dust covers) and may be factory for Canadian/Australian markets, cant say for sure. But no they are not required, but I do recommend leaving them in.

 

Yep, those are those ones that I had a while ago, need a sample of one if anyone has any extras!

 

I also believe the OP was referring to the plugs in the bowl that cover the pilot jet as well, I've had two bikes with them. The pilot will pull fuel through a small passage from the main jet. Bike will not run right without them and they can be a PIA to find and expensive. I think they get left out when PO's rebuild them. I had a KZ750 4cyl where the o ring was $17 each. Could never find the right size elsewhere and had to bite the bullet on those. As @chacal mentioned some carbs of the same model had the passage and plug, some did not.

I like those rubber mixture covers! Seems I always have water in mine. Need to get some

Part number 35 in this pic: