changing to a Weber carb...

[Blujay]

My Max is not getting the mileage I want for long distance touring. I'm getting a 26mpg avg. I've gone completely through the carbs, and instead of spending more $ on them I'm considering a switch to a two barrel downdraft Weber. I was wondering if this type of swap has been done before or if anyone has any suggestions on doing it.? Jay

 

[2stangs69-91]

hmm 26 you have something really wrong with the set up. What all have you done to your bike?

 

[one2dmax]

We are just about ready to buy a set of them to try out. We probably will use 2 sets of 40's. Probably one larger unit would work too.

Sean

 

[Blujay]

My Max is an 85, all stock. It had been sitting since 97. I've had the carbys apart too many times and spent way too many $ on them. The bike runs real strong, smooth and quick but I need to get more than 85 miles on the main tank.

I'm pretty much commited now I'm getting a 38/38 DGAS Weber, I won it on ebay and should have it by weeks end. I've got a sheet metal upper manifold designed (in my head) but I'm still wondering if I should do it dual plane or open plenum. I'm thinking with the V-boost working, it would open plenum above 6K anyway. I've played with a bunch of different Webers on a bunch of different engines, my favorite thing about them is you can change jetting without taking the carb apart.

We'll see what happens. I'll post progress reports w/pics. Jay

 

[Blujay]

Just thought I'd post an update. I got the 38mm two barrel offa ebay then built a a manifold out of 16 gauge mandrel bent exhaust tubing. At first I just removed the Vboost manifold and clamped some short open tubes in it's place, I think that scenario was making the plenum too big and affecting idle quality. Now I've plugged those tubes and idle is way more adjustable and just a little rumpy. I've only run it in the shed but the engine seems to have quicker throttle responce, 1.5K - 8K is almost instantaneous. I'm working out the throttle cable/linkage this weekend. If it came equipped with an umbrella and some pontoons I 'd take it out for a test ride tomorrow but that ride will have to wait till the rain subsides. Here's a couple of progress pics. Jay

 

[KJShover]

Thats some nice high-rise fabricating you have done. keep up the good work.:clapping:
I can't wait to read the ride report.

 

[MaxMidnight]

Surely you don't need the V Boost links on you set-up as each barrel of the Webber is effectively supplying both cylinders already?

Assuming that the Webber's have accelerator pumps then that would account for the instantaneous pick-up You get a similar effect with Mikuni Flat Slides.

My concern would be if they will be able to supply sufficient charge at full throttle?

As the chokes of your set-up are bigger than a single OE carb it shouldn't be a factor up to when V Boost starts to open. However after that the choke cross sectional are will be some 60% LESS than OE.
I appreciate that that may be a bit simplistic so may not reflect the real world.

I'm looking forward to seeing the progress and final outcome. Do you have any plans to put it on a dyno?

Keep up the good work! :clapping: :clapping:

 

[spursmech]

unk: Nice looking fab work. Lets see the Dyno numbers for this upgrade. Good Luck, Spurs

 

[SpecOps13]

When are you going to market a manifold?????:clapping::clapping::clapping:

 

[one2dmax]

The manifold could be shaped a bit better to prevent pooling but I like it none the less. We were planning on doing a similar build using 40IDF Webers and just haven't gotten around to it yet. Might just have to do that in the very near future!

 

[tugla]

nice work man looks great, cant wait to hear what kind of milage you get now.

 

[Saml01]

Worse than before.

EFI would have been a much better path if fuel efficiency was the goal.

 

[Blujay]

Thanks guys. I'd post more pics but for some reason when I plug my camera or it's xD card into my computer it tells me that the disc in drive k needs to be formatted before it can be used. I have to use a friends computer to download from the camera then email the pics to me, isn't technology grand.

As I was hot glueing the manifold together I was already redesigning it, if I build another, the center section will slope down from the middle to both end pieces. I got lucky with the U-bends, they're 1.625 od with a four inch radius, after a little drum sander action to the id of the Vmax manifold boots they slipped right in. The hard part was keeping everything aligned while welding it together.

Saml01, you think I'll get worse mileage? I was only getting about 26mpg before. I like the EFI idea but all the kits/conversions I've seen are way pricey. I have less than $200 and a few enjoyable hours in this experiment so if it flops I 'll probably transplant an existing injection system offa a big cc I4, after all it ain't rocket surgery.

I've got the throttle worked out, using the stock Vmax cables and a fabbed pulley and bracket. If the weather cooperates next weekend, I'll take it out and get some plug readings and a seat of the pants feelup. Jay

 

[one2dmax]

I suspect you'll make more low end torque with a slight loss in peak hp. But, not sure what the fuel mileage will do. It depends on the a/f ratio you got the first time around.

Sean

 

[Saml01]

Saml01, you think I'll get worse mileage? I was only getting about 26mpg before. I like the EFI idea but all the kits/conversions I've seen are way pricey. I have less than $200 and a few enjoyable hours in this experiment so if it flops I 'll probably transplant an existing injection system offa a big cc I4, after all it ain't rocket surgery.

Assuming that the stock carb was tuned to maximize fuel economy in cruise then you will have a hard time doing better. If it was not, then tuning yourself you might be able to increase fuel mileage. But then I wonder why wouldnt you just tune the stock carb, its a much better place to start and someone did most of the work for you.

In the end, if you do see a difference it will be small and IMHO not worth the effort. If you dont, then you wasted your time in any case.

I question the increase in torque Sean says you might gain. Carbs rely on the Bernoulli effect to function properly, if you dont have a properly sized carb you could hurt HP and Torque because the rate of flow wont be the same at a given engine RPM. Have you done the math?

Problem with carbs is the inherent way they work makes it impossible to tune to be good everywhere.

 

[Blujay]

No test ride today, she developed a problem in the clutch hydraulics. The clutch slave is leaking like a stuck pig. Oh well, it's too frigging cold anyway. I need to refine the throttle shaft pulley a bit, the ratio of handle bar twist to throttle plate opening is way too quick, less than a quarter turn from closed to wide open. I'm going to fabricate a larger pulley for the throttle shaft. Come on spring... Jay

 

[adamax]

Very good fabrication. I hope you get the reasonable mileage you look for.
Keep up the good work.

 

[PATMAX]

And people are paying good money for 1/4 turn throttle set ups! WTF? Obviously not practical here. I applaud your gumption to try something else. You clearly got mad skills. Although it probably was not his intent, why hammer the guy with theroretical banter and mumbo jumbo equations on flow and blah blah blah. If it took all the time to do all that calculating, he could have it built and tested real world. Did you catch his comment " a few enjoyable hours welding up the manifold" this cat is living his ideas out. Who knows how many designs were discovered by " let's try this"! Rock on man, hope you pull 120 hp and get 120 mpg!

 

[Blujay]

Thanks for the encouragement. Only actual road testing will tell if I get what I'm aiming for. I 'm pretty sure I'll be able to jet it for good range on a tank of fuel.

This isn't my first unconventional modification. I built this roadster a few years ago, much to the dismay of VW purists and traditional hot rodders alike. It lives in Florida now, it's way fast and gets a disgusting 12mpg.

 

[bazwell]

Interesting project. Well done!

What I would like to see is a couple of 38mm IDA's with little adapters welded straight on the standard manifold with vboost. It's keeping in line with the original "muscle-bike" philosophy, and would stop hot rodders in their tracks. Only worth it if it would work well though!

Is that what you were thinking Sean? If so, do eeeeeet!