Some Interesting Observations on Our Carburetors

[V7Goose]

Just posting this info for general information - no particular reason (all comments below apply to the US49 models, I did not look at the CA models):

 

I spent some time comparing the Yamaha parts breakdown for the carburetors on various years and models of the Royal Stars. The most interesting and puzzling fact I found is that ALL Royal Star VENTURES have three different sized main jets between the four carburetors! Cylinders 1 & 2 use a #122.5 jet, Cylinder 3 uses a #117.5 jet, and Cylinder 4 uses a #120 jet. To complicate this explanation a bit further, each carb shows three different jets that are labeled as "Jet, main," of which two are always the same in all four carbs, but the third one is different!

 

For example, the parts breakdown for an 05 RSV shows item 26 as Jet, main #85, item 27 as Jet, main #140, and item 29 as Jet, main #120 (UR #4), #117.5 (UR #3), and #122.5 (UR #1,2). Confused yet? I am. The maintenance specifications in the shop manual only shows one main jet (with the different sizes for each cylinder), so the other two that the parts breakdown lists as main jets are misnomers.

 

The part numbers for the carburetors on the Venture have changed several times (99-04 same part number, 05-07 same part number, and 08 another new part number), but the jets are identical on all of them. I did not try to look close enough to find out what parts did change with the new carb part numbers. The RSTDs have a different carb part number than the Ventures, but the jets are identical. The other Royal Star models BEFORE the Venture all used the same main jet size in all four carburetors. In 1999, the Royal Star Boulevard, Tour Classic, AND Tour Deluxe all used 4 #117.5 main jets, but the 1999 Venture was the first one to use the three different jet sizes.

 

For the life of me, I don't have a clue why the two left cylinders should use the same jet size, but each of the right side cylinders has a different size (one larger and one smaller)??? But they do. Wish I had an exhaust gas analyzer; if I did, I would play around with seeing what happened if I set them all the same size.

Goose

[pegscraper]

It is a little strange, isn't it? My only guess is that is has something to do with the coolant path through the engine, with the last ones in line being jetted richer to compensate for running a little hotter. (That would be similar to why air cooled V twins typically have the rear cylinder jetted richer.) I haven't traced out the coolant path though, to see if this might be why. It didn't matter enough to me. Yes, the early Royal Stars used all the same jets for every cylinder. Mine was that way stock. So when I swapped the 32mm carbs on, I stayed with all the same size jets all around. I figured I could watch the plugs and mufflers to see if I thought I might need to stagger the sizes. With four mufflers it's real easy to see what each cylinder is doing. After two years of watching things, I haven't seen a need to stagger the jets. All four plugs and all four mufflers look the same. The main jets are only for WOT operation anyway. At least the pilots were the same from the factory. The pilot jets are where the bike runs most of the time.

 

Did the 1st gens have staggered main jetting like this? I have no idea.

Edited June 23, 2008 by pegscraper

[Redneck]

One possible theory is they are using the different jet sizes to help balance the engine. On some power stroke diesel engines they run a different injector in #8 cyl to dampen vibration so they don't beat the springs out of the dual mass flywheel. this is just a guess.

[V7Goose]

That's an interesting guess - I have long thought that Yamaha has deliberately de-tuned this engine to introduce a bit of a vibration and uneven sound at idle to somewhat mimic a V-twin - there really isn't much reason a V-four like ours shouldn't purr completely smooth, but it does not. I imagined that they did this with a different cam, but maybe it is something else? The main jet shouldn't affect idle, should it?

Goose

[BOO]

I agree with pegscraper and his thoughts on the coolant. I have studied the carburettor a bit and I know less than I did before.

I knew the HD twins ran 2 different jets because of the heating issue and the rear cylinder/piston is usually the one that causes troubles from overheating.

The people racing the 3 cylinder Polaris snowmobiles always ran different main jet because of the coolant path, some would run the same in the outside pistons. Some ran three different ones because as I recall the coolant was pumped into one side and flowed from one side through the center and then the other out side, big difference in coolant temps in the cylinders.

And Goose I do believe the Main jet has some affect on the idle of the bike, the needle is out of the main just a bid and the slide is letting a little air in. And the rough idle is probably something to do with the cam?

It is an interesting bike to say the least. I have noticed something with the Venture I will share but there is nothing scientific about the information.

My bike has the standard Main jets, but is seems when the humidity is up above about 50% the bikes seems to pop quite a bit on deceleration, below that it hardly ever pops.

What does that mean? I wouldn't have a clue.

Jerry

[flb_78]

Rough Idle???

 

I don't think mine idles rough.

[az1103]

I may be wrong on this, but I believe it has to do with getting good continuity of torque from a 70degree V engine. If you look at the firing sequence and dwell diagram (3-10) you will see that the firing order alternates sides 1-3-2-4 and that there is a 70deg. interval between the 3 and 2 cycles and then there is an overlap in the 4 and 1 cycles .Thus you may want a bit more or a bit less from the cylinders to get good torque continuity and maximize the power.

 

Does that make any sense?

[Yammer Dan]

I'm really lost now.

[pegscraper]

Main jets don't come into operation until about 1/2 throttle or more. Main jets have no effect on idle. The main jets don't have needles in them.

 

az's explanation is a little unclear. Let's see if I can muddy if up further. Cylinder #1 fires at 0*, #3 at 180*, #2 at 430*, and #4 at 610*, then back to #1 again at 720*. The 430* comes from 360* plus 70* (the V angle of the engine). That is the gap between combustion strokes. Then 430* plus another 180* is 610*. That leaves only another 110* until #1 fires again (610* plus 110* = 720*), and the combustion strokes of #4 and #1 overlap by 70* (180* - 110*).

 

The thing of it is, the V Max will have this same type of uneven firing spacing. So if staggered jetting makes more power, why doesn't the V Max use staggered jetting? I say staggered jetting doesn't have anything to do with uneven firing. And I think that staggering the jetting like Y did on the 2nd gen engine is a little silly.

[chollahan]

I might have missed it cause some of these explanations made my ears bleed, but do the v-max's have the same jetting?

[GigaWhiskey]

I think I remember a thread I posted to about extremely detailed members being perfectionists in this group.

[Squeeze]

I might have missed it cause some of these explanations made my ears bleed, but do the v-max's have the same jetting?

 

Nope, they have different Jetting, Main Jets are bigger (152.5) but the Engine basically is the same. Lynn's Thoughts are right, if any Bike of the Family should have different Main Jets, then it's the Max, because it has much more Output and therefore much more Heat will be created.

 

I think this wierd Jetting could have something to do with the AIS ??

 

Other than this, it wouldn't make any Sense to my small Brain.

 

 

On the other Hand, i never explored that AIS, but it shouldn't work with different Jets either.

[az1103]

Well, here is a good opportunity for a worthwhile project for someone who has too much time on their hands; Change all the main jets to a uniform configuration and report on the resulting changes!

I, for one, would be very interested in the results. So please document and report.

[V7Goose]

Well, here is a good opportunity for a worthwhile project for someone who has too much time on their hands; Change all the main jets to a uniform configuration and report on the resulting changes!

I, for one, would be very interested in the results. So please document and report.

I would love to do that, but I don't think I am willing to pay for a shop to adjust all the carbs with an exhaust gas analyzer twice just to know! I guess I'll have to go find a shop I can do a deal with to see how reasonable I might be able to do this. I'd sure like to know what difference it makes.

Goose

[Guest tx2sturgis]

I sure hope you guys dont piss-off the carburetor gods.

 

Dont be messin around in the magical wizards relm of jets and exhaust gas...Thats the stuff that gets the gremlins mad, and the witches and warlocks unsettled.

 

Take my advice. If it aint broke, dont fix it!

 

[GigaWhiskey]

I would love to do that, but I don't think I am willing to pay for a shop to adjust all the carbs with an exhaust gas analyzer twice just to know! I guess I'll have to go find a shop I can do a deal with to see how reasonable I might be able to do this. I'd sure like to know what difference it makes.

Goose

 

I think squeeze and greg_In_London would have a field day with your results, in a good way, lol.

[Freebird]

I really wouldn't mind doing it and would just check it with the Colortune but I wouldn't know which way to go. Change them all to 117.6 or 120 or ?????

 

Besides, I don't even want to TOUCH mine right this moment after finally getting it running good again.

[V7Goose]

Since cylinders 1 and 2 are both using 122.5 jets, that is the size I would try in the other two also. But I can certainly understand why you wouldn't want to mess with yours right now!

Goose

[juggler]

on a second gen which cylinder is which?

 

I don't know and I can not find it in the manuals or on the site.

[Guest cwil2988]

lol shut up and ride em, lmao

just joking around people:stickpoke:

[Squeeze]

on a second gen which cylinder is which?

 

I don't know and I can not find it in the manuals or on the site.

 

They are all the same ...

 

#1 left rear

#2 left front

#3 right rear

#4 right front

 

 

Seen as when sitting on the Bike

[Neil86]

V7Goose...

Keep in mind some jets meter fuel, some air. The 2 others you mentioned are called main air jet and pilot air jet in the 1st gen service manual. The 2nd gen service manual carb section doesn't have a good cutaway diagram as the 1st gen manual. You can see that they are mounted in the air inlet stream of the carbs in the parts diagrams.

The 83 parts fiche I looked at had #117.5 main fuel jets.....the 93 had #125 main fuel jets.

The 1st gen had common airbox....2nd gen has two intakes...so air flow could be different left vs right requiring different jetting.

[greg_in_london]

I think squeeze and greg_In_London would have a field day with your results, in a good way, lol.

 

Speak of the devil .....

 

I'd certainly be interested in the results, but I certainly wouldn't go upsetting the carburettor gremlins without good reason. I'd ONLY try it if I could take out the carbs I was happy with, get a set of second hand carbs that I'd cleaned up, run them to try them out and then make changes to the spare/second hand set, before trying them out again.

 

There just seem to be too many variables and it's so easy to make things worse, even when setting everything according to spec. Hold on to your old carbs and do nothing to them so you can go back - that way the replacements are bound to be an improvement !

 

Yamaha seem to bend over backwards to make life awkward - the first gens have the same main jet in each carb, as well as the same nozzle and jet, but change those across each model (sometimes mdels in the same year) but have the same part number regardless of the model's spec. Go figure.

 

If you do get carbs off another model, there may be nothing on the carbs to tell you what they're from, so the information would probably be interesting, but academic - ie no practical use.

 

Only bother if you get different plug colours and want to fix it.

 

[Mind you, carb changes get pretty quick with practise ....)

[juggler]

They are all the same ...

 

#1 left rear

#2 left front

#3 right rear

#4 right front

 

 

Seen as when sitting on the Bike

 

Thanks, Black Owl was over and carbtuned my carbs and for the life of us we could not remember the order. I think I'll re-post this in the Carbtuning VR Tech thread.

[az1103]

on a second gen which cylinder is which?

 

I don't know and I can not find it in the manuals or on the site.

 

 

 

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