Yam RD "Blue Dream"- Ride,Maintain,Tune

I wouldn't say the Dave f mod was "useless", more that the improvement is small and could be better achieved with other jetting changes.
 
Re: Yam RD "Blue Dream"- Ride,Maintain,Tune

Would there be any benefit to just changing the jet needle and needle jet but not drilling the air jet and jetting the main and pilot appropriately? Sorry don't mean to thread Jack I am just following this build closely since there are similarities to mine. Thanks.


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Re: Yam RD "Blue Dream"- Ride,Maintain,Tune

Ply318ci said:
Would there be any benefit to just changing the jet needle and needle jet but not drilling the air jet and jetting the main and pilot appropriately? Sorry don't mean to thread Jack I am just following this build closely since there are similarities to mine. Thanks.


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Dont worry man.i like were this is going.
 
well certainly if you have an issue you are trying to fix and are not needing the extra air, but if you are intending to use the needle and emulsion tube in the Dave F mod, then the answer is no - the needle jet/emulsion tube is a completely different style and the combo needs a lot more air.

Ryan, US carbs (early) don't have the block off cap. I am assuming the later ones have caps to prevent tampering with the air jet. There must be an alternative source of the air to get to that passageway, or the jet would be useless. Restoring the cap probably is not needed - just leave it open. The air jet has to have an air supply, where it comes from doesn't matter.
 
Re: Yam RD "Blue Dream"- Ride,Maintain,Tune


jpmobius said:
well certainly if you have an issue you are trying to fix and are not needing the extra air, but if you are intending to use the needle and emulsion tube in the Dave F mod, then the answer is no - the needle jet/emulsion tube is a completely different style and the combo needs a lot more air.

Awesome thanks.


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Thanks mobius!i think i didnt quite understand what you said about the air bore which is now capped in my carb.
This are the parts i ordered for the mod.

R5 needles and emulsion tubes,bigger mains (120,125,130) and idles (which i guess i wont need)
2 MM air jets for the capped air

Did i understand correct that you would leave the air vent open after removing the cap instead of closing it with a cap?
Wouldnt this make the stock carb mixture to lean?

Ooh and i got lucky and got this nice littly scooter RD 50 DX Tank maybe i will use it on my RD...lets see :)
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Better to answer this yourself. Consider the pilot system. Air enters the system through the open port at the 7:30 position in your pic. That air then encounters a variable jet - the low speed/idle air mixture screw. Adjusting this screw controls how much air can get to the pilot circuit. That regulated air travels down a passageway and into a small chamber. In that chamber is the emulsion tube for the pilot system. The emulsion tube is part of the pilot jet. When you look at your pilot jet, you are actually looking at two parts combined into one. The pilot jet itself is just the diameter of the hole drilled into the end of the component. The emulsion tube is the perforated extention attached to it past the threads. Fuel does not flow through the perforations. It flows through the jet itself and straight through and out the other end. The air from the pilot air screw flows into the chamber surrounding the pilot emulsion tube, and gets sucked into fuel flowing through the emulsion tube. So you have the fuel volume being regulated by the available pressure and the diameter of the jet, and air regulated by the air screw combining inside the emulsion tube. This mixture flowsout of the end of the pilot jet/emulsion tube into another passageway into the main bore of the carburetor right at the edge of where the slide is trying to close off the bore. So it is not just fuel spraying out of that tiny hole, but a predetermined mixture of fuel AND air. Of course THAT mix gets further diluted to the correct mixture for the engine with the air zooming under the slide.

The main system is identical, except it has a fixed air jet instead of a screw, has an independently changeable emulsion tube, and a tapered needle to vary the volume at different throttle positions. But the system is the same. Air enters the main system through a port at the 6:00 o-clock position and instead of being adjust-ably regulated by a screw, it is regulated by a fixed jet. That air travels down a passageway to a chamber surrounding the main emulsion tube. Fuel is regulated by the main jet and is sucked up through the center of the emulsion tube, where it mixes with the air regulated by the air jet. The resulting mixture flows up through the needle jet at the top of the emulsion tube where it is restricted by the needle. As the throttles are opened allowing more air, the taper of the needle provides an ever increasing port size for the mix to flow through to combine with the air flowing through the main bore.

So if you think about it, it does not matter where the passageway in front of the air jet gets air, as long as it does.
 
Re: Yam RD "Blue Dream"- Ride,Maintain,Tune

Thanks mobius!

I have questions concerning your words:
Is the main jet permanently submerged in fuel?
This is probably why float levels are so important...

So i guess this mod tries to 'mimik' the R5 carbs,by using the needles and needle jets and the bore which needs to get an air jet.
If i understand correctly the mix will get way leaner then before trough adding the new passageway to the stock air circuit.
So tuning the mains right will get really important (jets are cheaper then pistons :))
 
Well I have never really analyzed the ins and outs of comparing the two, but I wouldn't say one was "way leaner" just because ultimately the fuel to air ratio needs to be always pretty close to the same thing regardless. The difference is in what the engine can consume at any given throttle opening and rpm, and achieving THAT is often a bit tricky. the mod does indeed (as far as I know) mimic the piston port( R5-DS7) carbs in that the needle jet/emulsion tubes and needles are near matches to the oem parts. You need the bigger air jet to go with the different style emulsion tube - not just to add more air from a tuning perspective.

Indeed, all the fuel jets are submerged in fuel. This is not actually required as a pipe could be attached that IS submerged in fuel, it is just convenient to have the interchangeable jets be at the end of the "straw" that gets the fuel. The jets are just a restrictor in the end of that straw. The fuel level inside the straw (emulsion tube) is the same as that in the bowl (well, for the sake of discussion anyway), so the higher the fuel is in the bowl, the higher it is in the straw, and the easier it is for whatever vacuum is available to drag fuel into the venturi (main bore). You can also see that higher fuel levels will cover more holes in the emulsion tubes reducing the air that percolates in. Both things tend to richen the mixture until you are running at the max flow of the jet, at which time it makes much less of a difference. Low fuel level of course will do the opposite and tend to make a leaner mix. Generally, this is not all that profound an effect especially on your thirsty two stroke. Fuel level in the bowls seems to have a more profound effect on four stroke engines, at least in my experience.

The piston port motors are rather different than your RD - more so than you might think considering how similar they are. The reed valves make a big difference in what is happening to the poor carburetors. Both rely (as do all engines) on the effect of sonic waves traveling through the air (and of course fuel) as it goes through the intake tract. Sonic waves are in the main unaffected by the material flow, so they travel backwards through the intake tract just as carelessly as they do toward the engine. (the exhaust works the same way). The waves occur because of sharp change in pressure that happens when the piston mechanically opens and closes the passageway leading to the combustion chamber. When the piston "opens the door" suddenly (or closes it), a shock wave is created that travels down the passageway - in this case out the intake tract through the carburetor. When it reaches the end- at the carb bell mouth - it sees another sudden change and another wave is created that travels back toward the engine. These pressure waves conspire with the gas (air) in the system and its mechanical motion through the passageway to either increase the pressure or reduce the pressure. Much additional power can be had by figuring out how when these events will occur and causing their effect to occur at an opportune time - say 8000 rpm for example, by adjusting the diameter and length of the passageway so you add pressure when the intake port opens for example. The extra pressure will add substantially to the charge in the combustion chamber resulting in a corresponding increase in power. The problem is that the timing of events that work great at 8 grand usually work like shit everywhere else In combination with the lack of the reeds one way valve action, the piston port engines can suffer a lot of "recursion", where air that has already passed through the carburetor and sucked up fuel is actually pushed backward through the carb, picking up more fuel, before being sucked back for a second (third?!) time toward the engine picking up even MORE fuel! In my example, the recursion would (presumably designed to) be non existent at 8K so carb tuning can be perfect there - meaning it might be horrifically bad at other rpm. Reed valve don't eliminate this phenomenon, but the mechanical motion of the air stream is substantially better controlled. All this means is that there is nothing magical about the Dave f mod. It works great on some engine combinations because all the complex nuances of that particular combination result in that carb combo working well. Some do and some do not. Always worth some experimentation though!

Recursion can be very pronounced. I had a drag bike based on an R5 motor back when that was still a fairly new bike. Very fast, but exceptionally unusable for anything else. I remember it was quite hard to start because it wouldn't run hardly at all below 2500 rpm or so. At idle (3000 rpm) you could plainly see a dense fog of fuel standing suspended in the air right in front of the 36 mm carb intake bells. If you stuck your fingers in there they came out dripping like you stuck them in a cup of liquid gasoline!

Yes tuning the mains. Getting them right can not be over stressed. NEVER adjust them for drivability. Wide open throttle at max power only. Drivability get fixed by adjusting everything else. Reducing the mains "because it drives so much better" is an invitation for disaster next time you open her up.
 
Re: Yam RD "Blue Dream"- Ride,Maintain,Tune

Thanks mobius really interesting infos you provide here!
The more i understand motos the more interesting they get.

Today i took the gap and removed the caps from the air passages.oh my i sweated hard.
I used a dremel to make a tiny hole in the middle then took a wooden screw and levered it out.

Then i threaded the outer holes with M5 in order to be able to open and close whenever i want...i will use a worm screw (are they so called in your country?)with a little loctite to keep them in place.
I used my drill press with the threading tool and turned it by hand carefully,worked out nicely!

Whats interesting is that the passageway vents out trough a drilling in float bowl (see pics) its closed again on the side of the float bowl with a brass cap.

The next step would be drilling a 3MM hole and then threading M4 for the air jets (2MM) but probably i'm gonna do this end of this season.
Am i getting this right that i need to drill straight trough till i surpass the wall of the needle jet.
 

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Long time no hear guys!

the season is nearly over, this a pic of me at the viennese distiguished gentlemans ride!
thank you for your support this season!

as far as the bike is concerned i managed to only break my tach :) replaced both tach and rev "bowdens" with new ones....so i can rule that one out.
it looks like something in the tach broke,it constantly stays at 200 kmh when i stop or move back and forth but never where i´m actually at.
did anyone of you guys open up one of these nippon densos and repair them, i heard they are pretty hard to open up....
 

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Re: Yam RD "Blue Dream"- Ride,Maintain,Tune

Looking good.


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Hey guys!

Finally i had time to buy some more parts for my 350 conversion for this winter.
Since there was only a RD400 model in my year and all the parts fit my 250 I bought a nearly complete 400 motor.
No crank and pistons.

I will replace primary gear,clutch basket and oil pump gear with the 400 parts.the 400 has the same gearing as the 350 of the models before so the clutch basket has 66 teeth the primary has 23 teeth.

I even got a 1A0 oil pump,which i'm not sure to be a 400 pump.is there a way to find out if the pump is from a 400 or do i need to measure the pump stroke?

Will it be sufficient to use my stock 250 pump and use the plastic gear cog of the 400 or do i need to change the pump?

Is the 400 oil pump stroke different than the 250 oil p stroke?
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Re: Yam RD "Blue Dream"- Ride,Maintain,Tune

Can't answer your questions but looking good none the less.


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I believe the pumps to be the same, but I can't say with absolute certainty. Possibly the initial setup is shimmed for a longer stroke on the bigger motor but I would bet they are the same. There is no absolute requirement to change out the primary drive aside from wanting the different ratio - used to be a "trick" mod to a 350 to run the 250 gears. Does the 400 not have a longer stroke than your 250? Presumably you will not be able to use the 400 cylinders on your existing engine as the cranks and cases are different - I don't think I have ever worked on a 1976 or later 250 - at least not with respect to the parts interchangeability. AFAIK, you need a 350 top end.
 
Hey mobius!

The 400 has a longer stroke than the 350,this is why i didnt buy pistons and crank from the 400.

The primary gear setup and gears are the same as the 350 though.

I will run 350 LC pistons on my 250 crank,which is already the crank with no lead weights which was later used in the LC RD.
The bolts that hold the cylinder are longer on the 350,this is also why i bought 400 cases,i will need to shorten them a little.

Interestingly the 400 was modded by the PO to fit a 350 crank (he shimmed the area where the crank touches the case).
I will not use the new 400 case,i will run my 250 case since its 'matching numbers'with the frame.

I will need to buy 350 piston bearings ,they are wider then the 250 ones.
I already have 350 top end parts,cylinder and heads (which were modded by the PO,smaller squish and modded combustion chamber,will post pics soon)

I will need to modify the piston skirts of the LC pistons when i remember correctly maybe you have info on this?

My mod list so far:

-Carb mod with R5 needles,needle seat and air jet
-Y Boot and KN filter
-DG Chambers
-Modified squish and heads
-350 top end with 350 gearing (400 parts),oil pump with 400 gearing
-400 oil pump (needed?)
 
You will need to remove the section of piston skirt in the center portion of the piston. It kinda hangs there like a tang so to speak. In the air cooled engines this "tang" will get caught on the port edges and eventually go south.
 
clem said:
You will need to remove the section of piston skirt in the center portion of the piston. It kinda hangs there like a tang so to speak. In the air cooled engines this "tang" will get caught on the port edges and eventually go south.

Right. Very simple - bottom of intake skirt gets gut straight exactly like the exhaust side. Hack saw, die grinder, file, etc. Very easy.
 
Thanks guys!

Just out of interest:
What you guys guess that I will be squeezing out with my setup on the rear wheel?
Since my weight will possibly drop due to my DG chambers it will be a fine ride.
What you guys say?
40-45 true h.p. on the rear wheel?
 
Re: Yam RD "Blue Dream"- Ride,Maintain,Tune

Finally had time to take pics of my 350 heads/cylinders.

Cylinder ports look perfectly stock,as they will stay with my mod.
The heads have been modded.
Looks like someone 'domed out' the combustion chamber and flattened the sqish.
I read measuring the volume of the head is the trick.
I will put a piece of paper on a piece of window glass and fill it up with water till i hit the first plug thread.repeat 3 times,then take the middle number.

What you guys say?
 

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