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Idle Mixture/Pilot Screws Access?


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Hey Gang,

 

I have a '98 Royal Star Boulevard (at least that's what the title says). A previous owner installed K&N Air filters (4), Barron's 4x4 Nasty Boyz exhaust, and rejetted the carbs, and presumably adjusted the Pilot/Idle mixture screws (as the caps are missing). It does have the 28mm carbs, so it's a completely different set up from the RSV's 32mm carbs.

 

I found the pipes to be far too loud and have installed baffles that bring the exiting diameter of each pipe from 3" down to 1 1/2" or slightly more. This gave me a slightly quieter bike, a better sound note, and lots more low end torque.

 

As you would imagine, the bike is running very rich. It's hard to start (but does!) and smells slightly like fuel when running. I'm not really a carb guy, but I thought at least for now, I could adjust the pilot/idle mixture screws to lean it out a little, at least in the low speed circuit of the throttle. After lots and lots of research, I finally found where the pilot/idle mixture screws are, but I can't readily access them as there is a metal "tab" (for lack of a better word) just like the RSV's have (see pics below).

 

IMG_20171030_141101.jpgIMG_20171030_141132.jpg

 

The second pic is taken at an angle. You can see the hole and I can confirm there is a slotted screw deep inside that hole. Unfortunately, there is not a straight shot at any of these screws so I can't get a screw driver down in there.

 

Does anyone have any advise on how to adjust these? IIRC, those tab things are welded on (this may not be true, as I wasn't specifically looking at them when I had the tank off, but that's kind of what I remember). I had hoped to get a ColorTune and adjust the pilot/idle screws in real time, but I just can't see how that can be done.

 

Any advice would be greatly appreciated.

 

Thanks,

Rich

PhishHead61

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I'm not sure what you have us looking at. I don't know much about the Royal Stars but the pilot screws on the RSV are on the back side of the carbs. You have to reach through from the opposite side with a long screwdriver. If they haven't been removed, there will be a cap on them about the size of a pencil eraser. No tabs of any kind.

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On the RSB, they are on the outside, under the diaphram assembly. It's shown as #8 (the plug) and #9 (the screw) in the carb shown at the bottom of the diagram:

 

https://www.bikebandit.com/oem-parts/1998-yamaha-royal-star-xvz13ak-carburetor/o/m146104sch106609

 

In the first picture, the carbs are behind that "tab thing" pretty much right in the middle of the picture. That "thing" is what's in between me and getting a straight shot at the holes the pilot/idle mixture screws are seated down in.

 

The second pic is from an angle to the right, and it's showing the actual hole the pilot screw is down in. It's that hole that's shown right next to the bottom right corner of the "tab thing". It's actually behind it, but because of the angle, you can see it.

 

Thanks for the quick response!

 

Rich

PhishHead61

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Never had the honor the working on an RSTD but, IMHO, makes no sense why Mom Yam would permanently attach those guards (if that is what you are referring to) over the carbs. Also,, a quick glance at the parts diagram reveals the frame does not have those welded on.. I would put my money on a raising of the tank would reveal those guards being held in place by bolts or screws..

Also, you mentioned that the previous owner did rejet for the mods? Normally this is an indicator that larger main jets were installed and, if this is the case, adjusting the pilots is not going to help much.. IMHO, most of the air/fuel mix as soon as you crack the throttle so the engine is no longer at idle, is controlled by the main jet and the metering rod = this is attached to the slide which is controlled by the diaphram on a CV carb. On most bikes that I have tortured thru the years, those metering rods are adjustable by moving an "E" clip up and down the metering rod.. On all my Ventures those typical "E" clipped metering rods were a horse of a different color = no "E" clips = so I ended up adjusting the metering rods (mid range air/fuel) by shimming them with plastic washers. Their is a carb guru here that sells a kit for doing just this = @skydoc_17, but I have no idea if your scoot is of this uncommon type of carb = maybe "The Doc" knows what your dealing with?. Looking at the carb in the parts diagram I looked it = it appears that your carbs may be the normal E clip type.. For this type of carb need on my Ventures I just go to ACE Hardware with my verniers and by the nylon washer assortment I want and tune my carbs accordinly..

Possible,, depending on how far oversize they went on the rejet, you could drop the needles by raising the e-clips up enough to lean er down adequatly.. Could also be possible that you will end up tearing the carbs out of it and rejetting back to stock.. Have you ever jetted a carb? Jets are sized and the sizes are stamped on the top of them = very easy to read what size your dealing with once you pull the bowls off the carbs.. Also, a simple phone call to a Yam Shop (or even looking on a parts diagram may tell you) should reveal what is stock jet sizes both mains and low speed (idle)..

Edited by cowpuc
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Hey Rich,

I think what you are looking at are the plates and "shields" that connect the front Carbs. to the rear Carbs. They are removable, and are held in place with 2 pan head screws in the top of each Carb. The tank will need to be removed to access those screws. BUT adjusting the Idle Mixture Screws WILL NOT solve the issue you are describing. You WILL need to rejet the mains, and possibly the pilot jets as well. You will also need to factor in the use of the K&N air filter, and any additional exhaust air flow you have over the stock exhausts with the muffler "mod" you did. Another thing to be mentioned here is that the aftermarket jet kits don't use the same number system that Yamaha uses for the stock main jets and pilot jets. In other words, a stock 17.5 pilot jet from Yamaha is NOT going to be the same as a 17.5 Dynojet, or Mikuni pilot jet, I am sorry to say. Your two choices at this point are to replace ALL of the mains and pilots with the stock Yamaha jets, and "live with" the results, or in my opinion, take this bike to a shop that has a Dyno, and have them "set up the Carbs." with the K&N filters and the muffler mod you currently have on the bike, using the Dyno to achieve the best torque and horsepower that engine can make, if you are happy with the sound of the bike currently. There is a possibility I could help you with reshimming the mains, but this would require you measuring the main shims, or sending me a main shim for me to measure, and then me machining a set of shims for you to "try out". The rub here is if we go to lean on the shims, we do damage to YOUR engine. This bike has been set up to run with a modified air filter, and free flow exhaust, a twist of a screw or two is not going to change that.

Earl

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Thanks for the input everyone!

 

I figured I was going to have to rejet, but I'm not going to have time to work on it until after the first of the year. Hope you're still around!

 

My understanding is the sizing of the jets is based on air flow. At this point, with the baffles in, they seem to be the restricting factor in the equation. If the exhaust output size is the same as stock, even with the K&N filters, the jets should be the same as stock. This is my understanding...I'm not a carb (or engine) guy, so I could be, and probably am, missing something. If so, please let me know what I don't know...lol.

 

Having said that, does anyone know the output size for a stock 4x4 exhaust?

 

As I said, I've read a LOT, but have no real hands on experience. I'm sure I could handle this with proper info from you guys, but I'm in the dark as to where to start (sizing wise).

 

On the other hand, does anyone know approximately how much it would cost to do as Earl suggested and send it out to a proper shop?

 

Thanks,

Rich

PhishHead61

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Thanks for the info Gentlemen.

 

You guys are right, I'm going to have to rejet (argh!). I spent an hour and a half trying to find a shop with a Dyno that would work on my bike in my area to no avail. I just wanted to get an estimate as to what it would cost to have them do it, but I guess I will be doing it myself.

 

I'm not going to have time to start working on this until January. I'm thinking the first step is going to be pulling the carbs and seeing what's in there, compare that to what the stock settings should be, and go from there. Is there a video available showing the removal of the carbs on our bikes? It would be helpful.

 

Earl...Is there a chart or something that will convert the size of one brand of jets to the other brands? Also, do you know what size/brand jets are stock for the 28mm carb? Are the jets for all 4 cylinders the same, or are some of them different?

 

Thanks for your time. I'll probably be posting about this again in January. Hope you're still around for advise if needed.

 

 

Rich

PhishHead61

Edited by Phishhead61
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