Main jets on an '06 RSTD?

[Flash18]

Okay so here's what happened:

Occasionally, my bike drops a cylinder or two, both on the right side, for no apparent reason. My cute boyfriend, who knows what he's doing, found that the problem was not ignition related and decided to pull the carbs out and clean them up. The carbs were filthy throughout, but what was unusual was that he found that the rear carbs both had the 122.5 jets installed, but the fronts had two different jets, a 117.5 and the other which was visually a different size (couldn't READ the numbers.) Not having any idea why they would be different, but also not having any good reason to wait on an answer to this question (the bike ran fine, aside from the occasional loss of the right side cylinder) he put it all back together and Carbtuned them and the bike runs great. Hasn't been back on the road long enough to discern whether or not the arbitrary loss of cylinder(s) has been cured. But, herein lies the question:

 

WHY WOULD THERE BE 2 DIFFERENT JETS IN THE FRONT CARBS?

We (he) knows why the jets are different from front to rear, but this is the first time he's ever seen a set of carbs with 2 the same jets in the rear and 2 TOTALLY different jets in the front. Has anyone else seen this before?

[V7Goose]

First, this is normal. You can download the shop manual from the tech library and it will have all the specs.

 

I do not know what indication you had for "drops a cylinder or two", but it is not likely that it was caused by dirty carbs. The only way the carbs could have contributed to that was if there were large chunks of stuff floating around in there that just occasionally got stick in the jets, then fell out when the engine was stopped. This is HIGHLY unlikely. Plugged jets are very common, but they do not plug and unplug.

 

A better possibility is a clogged fuel filter, or even a failing fuel pump.

 

Secondly, please do not cross-post the same question or information in multiple threads - this is very irritating for those who either want to help or are interested in following the subject, as you get different conversations and ideas that really should be kept together (I have deleted your other thread to prevent this).

[Flash18]

I do not know what indication you had for "drops a cylinder or two", but it is not likely that it was caused by dirty carbs. The only way the carbs could have contributed to that was if there were large chunks of stuff floating around in there that just occasionally got stick in the jets, then fell out when the engine was stopped.

Haha, funny you should say this. A common house fly (musca domestica) was found INSIDE one carburetor.

 

The indicators were quite decisive. You'd be riding along, all is well, and with no rhyme or reason, the engine would begine to stumble. At one point, we'd got the bike to start, but was barely running, by touching the header pipes right near the heads, it was clearly obvious that the engine was running on the left two cylinders, but not either of the right two. Further diagnosis showed that there was spark at all four plugs.

 

As far as the question about the jets, I don't think it says anything in the manual about this issue. We are confused as to why there are 2 different sizes in the front carbs. Are we missing something in the book??

[gibvel]

Here's the service spec. document. It says what sizes all the jets are:

 

http://www.venturerider.org/forum/showthread.php?t=7839

Actually, lots of good info in this document, right at your finger tips.

 

After re-reading your post I think you may have had the carbs turned incorrectly, or thought the front and back were really the side. As the document says that #1 and #2 carbs have the 122.5 and the # 3 has the 117.5 #4 has a 120. Cylinders 1 and 2 are on the left (left front being #2) and 3 and 4 are on the right.

 

Promise I'll quit editing after this.

 

Here's a link to download the service manual:

 

http://www.venturerider.org/forum/showthread.php?t=1828

Edited October 18, 2012 by gibvel

[Flash18]

Here's the service spec. document. It says what sizes all the jets are

 

Thank you for that. We surely missed that. We have the manual for the '06 and it also shows the very same thing...just not were we'd been looking!

 

SO...does anyone know why Yamaha uses two dissimilar jets in the front carbs? The bike is running great after the carb clean/tune, so there is again no good reason to tear them down again, but I feel like I want to take out the 117.5 and stick another 120 in its place.

[Freebird]

I know that it is normal but am not 100% sure WHY. I wonder if it has something to do with the firing sequence of this engine. Maybe something to do with how much fuel/air mixture is actually pulled into that cylinder based upon the timing? I know that these engines aren't completely balanced in that manner. I'm grasping here because I really don't know but am hoping that my response will trigger the thoughts of somebody who DOES know.

[frankd]

I THINK it has something to do with the air cleaners. First Gens have one air cleaner, so the carbs draw from a common chamber....and the carbs have identical jets.

 

2nd Gens have a left and right cylinder air cleaner. The difference in main jets may be to compensate for different amount of restriction between the left and right air cleaners.

 

Frank D.

[Flash18]

2nd Gens have a left and right cylinder air cleaner. The difference in main jets may be to compensate for different amount of restriction between the left and right air cleaners.

 

This is quite sensible, but you'd think the engineers at Yamaha might have considered this?? WTH is up with that???

[gibvel]

I know that it is normal but am not 100% sure WHY. I wonder if it has something to do with the firing sequence of this engine. Maybe something to do with how much fuel/air mixture is actually pulled into that cylinder based upon the timing? I know that these engines aren't completely balanced in that manner. I'm grasping here because I really don't know but am hoping that my response will trigger the thoughts of somebody who DOES know.

 

I think I've heard Goose discuss this very thing. I believe he said the valve timing is different on some cylinders, SPECULATING that Yamaha wanted to give the bike less of a "even" feel to it and a little bit of a "lope" that made it sound a bit Harleyish. I don't really hear much, if any, lope but then it's not as even as a auto engine either.

 

Now, this is just my memory of what, I believe, I heard Goose speculate at one time. I'm not sure I got that correct or if he even said it or if it was just said in jest.

 

Hopefully, he'll come in and straighten me out if I was incorrect.

[GeorgeS]

This has been discussed a few times over the years here, also applies to the 1st gens.

 

Could the different jet size's be because of different rates of " heat dissipation " for the front cylinders, as compared to the rear cylinders ??

 

ie: there would be more air flow over the front cylinders, to dissipate heat, as compared to the rear cylinders ?? Just poseing that question here , Hmmmm ???

[Flash18]

This has been discussed a few times over the years here, also applies to the 1st gens.

 

Could the different jet size's be because of different rates of " heat dissipation " for the front cylinders, as compared to the rear cylinders ??

 

ie: there would be more air flow over the front cylinders, to dissipate heat, as compared to the rear cylinders ?? Just poseing that question here , Hmmmm ???

 

This is the reason why the rear carbs have bigger jets than the front. What is in question here is why are the jets dissimilar between left and right in the front carbs?

[ACE50]

I THINK it has something to do with the air cleaners. First Gens have one air cleaner, so the carbs draw from a common chamber....and the carbs have identical jets.

 

2nd Gens have a left and right cylinder air cleaner. The difference in main jets may be to compensate for different amount of restriction between the left and right air cleaners.

 

Frank D.

Even though there are two air filters, they join up into one path to the carbs, then split again left and right. I don't think there is the issue of front cyl getting cooler being in front being it's water cooled. They are 122.5 on 1&2 cyl. (left side) 117.5 on #3 and 120 on #4 (rt side) They are all jetted that way stock though.

Edited October 19, 2012 by ACE50

[Flash18]

Even though there are two air filters, they join up into one path to the carbs, then split again left and right. I don't think there is the issue of front cyl getting cooler being in front being it's water cooled. They are 122.5 on 1&2 cyl. (left side) 117.5 on #3 and 120 on #4 (rt side) They are all jetted that way stock though.

 

I just realized that I've been misrepresenting the position of the jets...y'all are right, the two on the left are 122.5 and the two on the right are 117.5 and 120...but my question remains.

 

For most any other bike, you'd want them to be paired equally, whether they have the same jets in all four, or 1 pair larger and 1 pair smaller (usually running the leaner jets in the front carbs cuz ambient airflow to the outside of the cylinder will compensate for the hotter condition of the leaner jets,) but why did Yamaha run two large jets on the left side, and two smaller, and dissimilar jets on the right side?

[BOO]

Could it be the way the coolant flows through the cylinders?

BOO

[Flash18]

Well, I just HAD to know, so I called the Yamaha Corporation, Star Motorcycles division.

While I was unable to speak directly to an engineer, the operator I did speak with relayed for me. What I was told:

 

The resulting turbulence of the air induced through the separated air boxes causes slight vacuum differences which were compensated for by using the different sized jets.