Custom Manifold for 6 Carburetor Setup

[pkz]

Hello again! 

 

I've been back at school so available time for my project has been very limited. I've figured out the new carb spacing, and I'll be cutting tubes this weekend. I've also realized that I'm going to be pushing my welding capabilities a bit- especially where jigging is concerned. Could anyone offer some advice on jigging? My experience is limited.

[Derek]

For me there are two ways to go. Make a fixture, weld it all together. pull it out of the fixture and watch it spring into an unusable mess.

 

Or

 

Use an assembly fixture, tack everything together, remove from fixture and weld. As you're welding keep checking your fit and adjusting either with force or opposing welds.

 

On prototype pieces I've had better success with number 2.

[MidnightCafe]

I was at a friend's motorcycle shop checking out his distributor catalog for mikuni carbs. I've had this idea rattling around my head ever since seeing Eiji's 240z with 6 mikuni motorcycle carbs on youtube. Just found this thread with a photo of Eiji working over his bank of six Mikuni motorcycle carbs

 

PKZ, what carbs did you end up going with? RS smoothbores, HSR? HSR has impressive flow numbers but are pretty massive-they come in 42, 45, and 48mm bore size.

 

I haven't done much research-installing a bank of six motorcycle carbs would come out more expensive than purchasing a complete Weber DCOE 45 triple carb set and manifold (assuming $300 retail for HSR carb), then there's the engineering hours put into the manifold and tuning... I'll put this idea into the 'wouldn't that be nice' category and focus on building an engine for my 280Z first!

 

How are you going to setup for fuel delivery? Depending on the carb they are likely gravity-fed carbs that'll puke gas once you apply fuel pressure.

[Tony D]

And all the while, second-hand set of SK ITB's ($800) and Brand New Intake Manifold ($500) languish in the classifieds here...

[Lurcher]

It must be an American contrivance to backwards engineer, then build something costly - like what the OP is doing - and making it WORSE. 

 

I'm bewildered... 

[djwarner]

I'm a little bit late to the dance, but your original post asked for technical resources. Pat Braden wrote a book on Weber Carburetors. On page 36, there is a chart for single cylinder capacity versus main venture diameter. This chart was supplied by Weber and is calibrated for racing engines. The chart is for independent runner manifold feeding a single cylinder.

 

The chart has three curves based on maximum RPMs of 6,000, 8,000, and 10,000 rpm. For a 2.8 liter engine with 467cc per cylinder, the ventures work out to 36mm, 45mm, and 52mm respectively.

 

For a 2.4 liter engine, the venture diameters work out to 33mm, 42mm, and 48mm respectively.

 

These numbers seem pretty consistent with the advice given by others in this thread.

[MidnightCafe]

And all the while, second-hand set of SK ITB's ($800) and Brand New Intake Manifold ($500) languish in the classifieds here...

It must be an American contrivance to backwards engineer, then build something costly - like what the OP is doing - and making it WORSE.

 

I'm bewildered...

Why build a datsun motor, then? It's way more expensive than building a v8 engine, given the plethora of aftermarket parts. Or why even build a car? We should all be driving japanese minivans, that'll please the gods of common sense!

 

I get the criticism, but you should consider the design intent of this kind of project-to go out and do something different. I've seen holley carb setups, I've seen weber setups, and I've seen fuel injection setups. A bank of six motorcycle carbs has been done before, but it is uncommon. Perhaps for a reason-there are other established practices with a wealth of knowledge behind them. It's the OP's time and money, he can do as he pleases. I'd love to do the same, knowing full well that better options exist for cheaper. Why not? Or should I sell my Datsun and buy a honda minivan instead?

 

Here's a video of a 240Z that runs six mikuni carbs. Tell me that doesn't sound cool-I dare ya!

[Lurcher]

Why build a datsun motor, then? It's way more expensive than building a v8 engine, given the plethora of aftermarket parts. Or why even build a car? We should all be driving japanese minivans, that'll please the gods of common sense!

 

I get the criticism, but you should consider the design intent of this kind of project-to go out and do something different. I've seen holley carb setups, I've seen weber setups, and I've seen fuel injection setups. A bank of six motorcycle carbs has been done before, but it is uncommon. Perhaps for a reason-there are other established practices with a wealth of knowledge behind them. It's the OP's time and money, he can do as he pleases. I'd love to do the same, knowing full well that better options exist for cheaper. Why not? Or should I sell my Datsun and buy a honda minivan instead?

 

Here's a video of a 240Z that runs six mikuni carbs. Tell me that doesn't sound cool-I dare ya!

He can do whatever he likes!

Doesn't make it right though - especially fitting 30mm carbs to a L6 as the OP originally posted... Why expend so much time/money/effort to go slower?

Edited November 2, 2015 by Lurcher

[pkz]

Everyone- thought I'd check back in. Just to clarify I took on this project so I could have something unique and fun to drive- pretty subjective goals I know but it's really all I have in mind. Partway through the process I realized that the 30mm carbs weren't going to cut it and bought 40s. Now, the second-hand 40mm carbs I bought cost me a total of 200$, Steel for the manifold cost me another 60$. I know before I have anything close to driving I'll need to weld everything up, build a throttle linkage, work out fuel delivery, and then work out jetting. That said, I think I've still got a chance of having this project come in at well under the 1000$ mark and be way more fun to drive than my original 73' flattop carbs currently do. I've read up and even TonyD never liked the 73's (save for throwing them on a mill and doing some experimenting, from my reading).  As for my own time spent on the project I see this as more of a fun challenge than actual work, especially if I can learn something in the process. 

 

All that said, I fully understand where all the criticism is coming from, especially given that I was less than clear about what my goals were for this project from the onset.

 

Djwarner, thanks for the info on carb sizes. I think my 40's will work, but another source reinforcing the same things people have been saying here is good to have. 

 

By the way, carbs are Mikuni TM40 flat-slides.

[MidnightCafe]

You purchased a set of 6 Mikuni TM40 carbs for $200? That's cheap! Where did you get them for that price?

 

I'm not familiar with Mikuni TM series carbs, but the slides are operated by a throttle shaft.  (As in, the cable doesn't directly pull the slide-the slide is operated by a shaft which is rotated by the throttle cable). Proof photos here.

Take a look at the image I attached above. That bank of 6 carburetors looks like HSR series carbs. These can be lined up on a single throttle shaft rather than operated by individual linkages. Here's an example of two in series linked by a common throttle shaft.

Four Mikuni RS40 carbs on a common throttle shaft:

Four HSR carbs on a common throttle shaft:

 

Using a similar setup, you only need to run a linkage that turns the common throttle shaft across all six carbs. Not bad.

 

Also, here's a page where the author describes use of Mikuni HSR carbs on a Holden straight six motor:

 

Weber (or Dell'Orto) carbs have for a very long time been considered the ultimate in Holden six carburetion (apart from the ocassional use of Amals), but I feel that there are now much better systems available. Modern flat-slide carbs such as Mikuni's HSR series are aerodynamically much slicker, and flow extremely well while maintaining a high velocity. Originally designed as a performance carb for Harley Davidsons, they cope very well with the severe pulsing of an individual runner layout. Six of these on a 12 port head will perform exceptionally well, while three on a 9 port head will provide a much cleaner flow path than the usual DCOE on each port. A further advantage is that almost any length runner is possible with the Mikunis, whereas with the Webers it may be difficult to get suitably short runners on a high revving engine, at least without having fairly severe kinks in the runners. If you decide to try these be careful with the sizing - a 45mm (for example) HSR will flow much more than a similarly sized throat on a DCOE.

To summarise, IR setups generally give very good results, but are more expensive and require a bit more work to synchronise the butterflies and tune properly.

[Tony D]

Actually, I preferred the 73's over the 74's as they have a larger float bowl capacity.

 

Like many things these days, it's just not posted on the interwebs because of the misinterpretation of context that most reading online seem to have.

 

Kind of like this  comment:
 

"I get the criticism, but you should consider the design intent of this kind of project-to go out and do something different."

 

That being posted right after the 'why build at all' which is a false exaggeration.

 

People adding nothing to the thread but butthurt makes a lot of this stuff go underground. It's not worth the vomit that is induced and splashes everywhere.

 

In fact, I saw someone rail extensively that the DOHC Head Project currently underway because the detractor couldn't understand no compromises performance, "couldn't justify the cost" yet denied he was budget-constrained.

 

The dearth of intellectual dishonesty online and creeping into even this forum is saddening.

 

That it's not making any sense, that it doesn't make any sense many times is more the point of the exercise than anything else.

 

So what? His car, his build. You said your detractive piece now either cork it or advance the project towards it's stated goals. If I can do it, so can everybody else.

[MidnightCafe]

Actually, I preferred the 73's over the 74's as they have a larger float bowl capacity.

 

 

 

People adding nothing to the thread but butthurt makes a lot of this stuff go underground.

 

I'm enthusiastic to see where this project goes. Didn't realize I'd upset anyone, but then again, this is the internet

 

 

So what? His car, his build. You said your detractive piece now either cork it or advance the project towards it's stated goals. If I can do it, so can everybody else.

 

Sigh.

[pkz]

MidnightCafe- I bought the carbs on ebay from a few different sellers (they come in sets of two but all the ski-doo snowmobiles used two TM40s in the early 2000s years). I lined up all the carbs the other day, right before I was about to leave to cut tubes, and realized that they spacing between carbs is WAAAY to much. I did a comparison to triple webers' manifold and determined that I was definitely going to run into intereference problems with the distributor.

Since the carbs come preassembled in sets of two- I'll have to chop down the spacers between them and run smaller T-Fittings than stock when I go to reassemble it all. Here's what the carbs look like before they're separated.

Separated the Carbs (Sorry for the blurry photo, lighting in my room sucks)

Floats and main jet:

Now for the pesky throttle shaft... It's tapped, and the slide mechanism is fastened using a countersunk phillips-head screw. Now, if I'm going to machine my own throttle shaft, I'll have to run 6 tapped holes lined up precisely to ensure the throttle positions are all the same. I have access to a decent drill press, centering bits, and maybe even the cross-hole jig I'd need to do this properly. However given my machine experience is limited- and that having the throttles lined up is highly critical to this functioning properly am I better off having a professional do this?

Something else I hadn't thought about running a single shaft. If I run a single throttle shaft with pre-tapped holes, I'll have no way to adjust the relative flow-rates of the carbs. In theory this shouldn't be a problem, because the throttle position holes should be properly matched up so all the throttle positions are the same. In practice though, I'm worried there will be discrepancies in flow and if this is the case I'm going to kick myself later if I build something that can't be adjusted. Maybe I'm better off trying to join the ends of the throttle shafts using end couplers like this?

http://www.mscdirect.com/product/details/35465731?src=pla&cid=PLA-Google-PLA+-+Test&CS_003=7867724&CS_010=35465731

[Gareth. J.]

Are the fuel 'T' pieces the black plastic ones at he bottom of the carbs? I know Keihin stock different widths to suit different carb spacings perhaps Mikuni are the same? If not I'd look at switching each pair around and try fitting a fuel elbow on each carb, it will eliminate the risk of fuel leaks as the T pieces are only sealed with O rings..... Ask me how I know! Ha.

 

They look in good condition, do they have accelerator pumps?

[Tony D]

Most of these flat-slides have a power-valve setup (you know, like the hated Flat-Tops had to handle WOT enrichment....) Flat-Slides usually do, it's not all needle taper. You don't really need it as long as there is a functioning power enrichment setup and you sized the carbs right for the velocity...

 

The Mikuni HS40 (which I think these are) does  have an accelerator pump for really low-speed, with no power valve. They're two-stroke carbs with a strong inlet pulse so they work well on individual runner engine setups...though piston size is usually 2X of the 2-stroke version to get comparable performance in terms of response.

 

http://www.mikuni.com/pdf/hs40_manual.pdf

[pkz]

The black plastic pieces at the bottom of the carbs are the fuel T fittings. So far I haven't found anything for different carb spacings, but I think it might be worth checking a local auto parts store to see if they have anything (especially since they'll probably let me test fit it against the carbs. If nothing fits then I'll be a little stuck, because simply flipping the carb pairs around won't fix anything in my case (I'll still end up with two fuel inlet ports right next to each other because I have multiple carb pairs). I think regardless I'll try and find a brass fitting and toss the old O-rings, in my mind they're just a leak waiting to happen.

 

Also, does anyone know the spacing center-to-center of triple weber carbs offhand? I know the spacing between carbs in the same set is different between two carbs from different sets- if anyone knows either or could take a measurement for me that would be great. I'll use the total spacing as a reference for my setup. 

 

Tony, Gareth- the TM40s are very similar to the HS40s- HS40s are the smoothbore configuration of the same carb, as was mentioned. You are both right- they don't have power valves but they do have accelerator pumps- needless to say setting these properly is pretty crucial to prevent off-idle lag.

 

Tony, what's your reasoning behind the 2X piston size for a 4 stroke vs 2 stroke? 2 stroke will have the valve open twice as often vs an equivalent four stroke (ie the frequency is double), but where does doubling the piston size come into play, out of curiosity? Is there a functional difference between doubling piston size and doubling the overall displacement by some combination of bore/stroke? Sorry to get off topic- but I'm very curious as to why this is. Thanks.

[Tony D]

Air movement at any given rpm for combustion is twice that of a four stroke. To size the carb right you need almost twice the carb size on any two,stroke as compared to comparable four stroke and vice versa.

 

Sachs 292 runs fine on a Tillotson HR, a Briggs and Stratton 356cc Lawn Mower engine....notsomuch. But an XR500 one lunger? Back up to HR Size again.

 

Carbs are sized based on air movement.

Edited November 10, 2015 by Tony D

[pkz]

I finally made some real progress today. I cut the intake manifold flanges to size and drilled all the holes. Pretty simple compared to some of the stuff I've seen on this forum but it felt good to get back in the shop. The last few days I also had a bit of free time and made an actual 3D model to verify hole spacing/clearances. I spent alot of time double and triple checking my measurements, and made some tube cutting templates as well. Tomorrow I'll be back in the shop to cut tubes and start welding. I hope to have something resembling this by the end of the weekend. Classes are finally over, so I'll have some time to spend in the shop.

[pkz]

More progress. Cut all the tubes and mocked everything up...

My mig welding could use some work...

 

Flange/carburetor adapters came in. Bought 6 for under 50$ on amazon, had to shop around to find them cheap enough. Took a risk on some generic ones but they fit great.

 

Two of the adapters installed:

 

First two carbs mocked up. Matched up well considering all my measurements were marked with a sharpie and a ruler.

 

Side View.

 

Tomorrow I'm going to set up the remaining carbs and set the single throttle shaft in place. Then I'll get to work shaping the carb mounting flange (right now it looks like it belongs in a Mad Max car). 

[pkz]

Today I set the throttle shaft in place and set the rest of the carbs in place. 

 

Side view of the carbs:

 

I used a cutoff wheel to grind down the plastic T-fittings so they would fit with the reduced carburetor spacing, then removed the O-rings and sealed the fittings using silicone- I didn't have room to place fuel elbows in between the carbs.