XZ400 carbs on an XZ550?

[mapek]

"dellorto" (VHB 29AD) on XZ 550!!!
easy and fast :)          mikuni bay,bay
http://rocket39.blogspot.com/p/yamaha-xz-550-stainless-steel.html

[pinholenz]

And here it is on its first start with the DellOrto carb. Single side draft carb feeding into both cylinders via a common manifold. Happy camper! Congratulations. How does it drive?
https://www.youtube.com/watch?feature=player_embedded&v=4NHwex2gZMc

[mapek]

The first ride was soon...... :)
First comments soon on you tube....... ::)

[The Prophet of Doom]

So I pulled out my XZ400 carbs and measured the venturi best I could.  Can't see any difference between those and the set on the XZ550 (16R model)

Just for interest - Here are the velocity stacks off a XZ400D - my XZ400D by the way had no flapper of any sort (and no sign of there ever having been one) - just a square-ish plastic pipe.


[edit]  Seems (now I've got my old bike back) that the flapper was missing rather than absent by design.  Sorry for the mis-info. 

[Rikugun]

...by the way had no flapper of any sort (and no sign of there ever having been one) - just a square-ish plastic pipe.

Thanks for posting the  v-sack pic but now I need one of the square-ish pipe. What's that all about??  ???  :) Some sort of restrictor for the airbox inlet?

[The Prophet of Doom]

It should still be in one of my spares boxes.  I'll dig it out.
Could it be that the XZ400 was never destined for a country with rigid noise laws, so they didn't need a flapper?

[QBS]

Purpose of flapper is to give small engine with big carbs higher vacume (carbs live on it) at low rpm/low vacume operation.

[Rikugun]

Could it be that the XZ400 was never destined for a country with rigid noise laws, so they didn't need a flapper?

I am intrigued that the 400 doesn't have the flapper.  ???  I never considered noise and I guess that could have something to do with it. Admittedly, I've always been led to believe it had more to do with "fueling" and "driveability".  That is to say the 550 wouldn't carburate smoothly or without substantial bogging unless the flapper is in place. Yes? No?

Did the 400 have as long of a production run as the 550 in those markets? Where I'm going with that question is did they not have the opportunity to adapt a flapper due to a short run OR, was there a several year run but the model just didn't require the flapper to run properly? That's again assuming it has something to do with fueling rather than noise. Didn't Treedragon run his 550 at one point with no flapper? Of course it may have been so heavily modified by then the comparison would be irrelevant..  :)

Purpose of flapper is to give small engine with big carbs higher vacume (carbs live on it) at low rpm/low vacume operation

I've heard this before and I think it infers the motor is over-carburated? If that's the implication, I'm not so sure I'd agree.... The '82 with one 34mm carb per cylinder is feeding 8.1cc of displacement per mm of carb venturi size (276/34=8.1) My '82 GPz with a 26mm carb per cylinder feeds 5.3cc of displacement per mm of carb venturi size (138/26=5.3). In other words, it's feeding less displacement with more carb and has no flapper. The A models went to bigger carbs (27mm) and the later Zephyr model to bigger yet at 30mm carbs - all using the same 553cc displacement.

Based on just the numbers, I'd say the engine is not over carburated relatively speaking. Maybe it has more to do with the type of carb and where the fuel is held rather than just displacement to venturi ratio.  ???

[pinholenz]

When you are looking at Roro's pics - for those who have never seen an xz400 (like me) - check his earlier post.His pic is of both stacks from an XZ400. The bent venturi stack is off the front carb and, when mounted, Roro explains that the bent neck faces towards the rear carb. The smaller venturi stack is off the rear carb. (Imagine the pic with both circlip screws in the centre next to the ruler then they will be oriented as on the bike.)

[jefferson]

I taped open the door on the airbox on my 83 to see what would happen and the thing was unrideable. Went a block and turned around to remove the tape and let the door work as intended.

I just can't believe that a carb with the issues and bandaids it has on the 550 would work on a 400. Maybe they spent more time on the 400 carb with the tuning and such, but then it appears that the 400 had no flapper door. I don't know what to think.

Jeff

[QBS]

Are 400 air horns the same diameter size as the 550 version.  If they're smaller, perhaps they have basically the same effect as the flapper does, namely to create increased vacume after the air horn, and in the venturi area of the intake track.

From what I recall from the Cycle Magazine article on the bike, the mag basically said that the bike was marginally over carburetted in the interest of top end power.  In order to overcome the lower end drivability problems this presented, and thus have  the best of both worlds, the bike was fitted with what is in effect an early version of variable intake.  Considering the bikes' great top end rush and super low end torque, I think the case can be made the Yamaha got it right except for the off idle flat spot (which may be tuneable) and the 4-5k transition zone going from low to high speed fuel circuitry.

An interesting experiment might be the selective application of duct tape around the edge of the flapper to decrease the gap that air passes through at low rpm/vacume operation. The purpose being to see how low speed drivability would be effected.

[Rikugun]

The bent venturi stack is off the front carb and, when mounted, Roro explains that the bent neck faces towards the rear carb. The smaller venturi stack is off the rear carb.

Two different length and shaped stacks - yet another another odity.  :o  :) Is the air filter different than on a 550 or does the longer stack not interfere with the air filter?

[kiwibum]

There was a mention earlier in this thread about the 550 carbs having the fuel pump and 400 not having the pump. I've probably mentioned this else where, the 550 carbs run just fine without the pump. I took the pump off mine, probably only disadvantage would be getting the last litre of fuel out of the tank.

Also another vote for having, clean, balanced carbs with no air leaks (main problem being the YICS) to have a sweet running motor.

[pinholenz]

I have just got the carbs off a 400 and pulled the airbox as well. No fuel pump as you say, and the same bent venturi as Roro has shown, What intrigued me was the airbox. It has a different shaped flapper inlet and the flapper is a balanced type, relying on the vacuum from the venturis rather than a separate vacuum tube like my 82.5 XZ550.

It will be interesting to try the 400 airbox on the 550 to see if it affects my off throttle stumble.

[The Prophet of Doom]

Interesting that my 400D had nothing - unfortunately I no longer have it.  The flapper you describe sounds like standard early 82 issue, though I would expect smaller - it has a different part number.

Common wisdom is that going to the vacuum flapper on the 550 makes things better rather than worse.  That was certainly my experience, but I did a whole bunch of other maintenance at the same time.

[Rikugun]

It has a different shaped flapper inlet and the flapper is a balanced type...

Well now, that makes more sense.  :) I just couldn't imagine there was no flapper and the thing would still accept throttle without falling flat on it's face.  :laugh:  I've not seen an early 550 box but that does sound like what I've heard described.

pinholenz, what makes yours an 82.5?

[pinholenz]

It was actually first registered in 1984. Probably hung around the shop for a long time before being heavily discounted. The Australasian Certification Plate for the 16R is dated 6/82 which would mean that it (probably) came into the country after that date. (Haven't heard of an '83 in New Zealand so far)

The airbox has the vacuum actuated flapper but this could have been a later mod. More significantly though, there is a screw stop on the accelerator pump lever which I understand is a mid '82 factory modification.

BTW, I now have 3 carbs, two from 550's and one from a 400. They all have accelerator pump rods with right angle bends in them, quite unlike the straight ones on the "accelerator pump setting" discussion thread.

[pinholenz]

Had a happy evening pottering looking more closely at the difference between the 550 carbs and the 400 carbs.

Did 400's suffer from the off-idle stumble that plagued the 550"s?