Show HN: Digital Carburetor Synchronizer (C, Swift) | Hacker News Reader

Show HN: Digital Carburetor Synchronizer (C, Swift)

github.com

Just wanted to say the real interesting content is in the writing here: https://treatwell.engineering/how-i-made-a-digital-carburett...

I see you mentioned the Honda CB750 which also had 4 independent carbs.

I once had a CB750C and while grateful that the throttles all open with one shaft, the carbs still had to be synced. I actually never got around to it and ended up selling the bike without tuning it up properly.

I love your commitment to getting your bike working properly. A very fun read. Thank you and congratulations.

As far as I understand, the only thing you can adjust on CB750 carbs is idle mixture on each carb. Yes, they have to be tuned, but that has nothing to do with syncing. If you had problems with your cabs - you had clothed jets or passages most probably.

I’m glad you found my post helpful, tho. Let’s keep wrenching!

They made the CB750 for 35 years with many different types of carbs. I had a 1971 or 72 for a while and you could perform a standard sync of the carbs. There were screws on the shaft or something like that.

Syncing carbs is an art. This is pretty interest to see how to inject a bit of science into it.

When syncing the dual SU carbs on my MG, the "traditional" way is to take a length of tube, remove the air cleaners, fire up the engine, then stick the tube inside the end of the carb. Take the other end and put it near your ear; you'll hear the airflow. Now stick the tube into the other carb. If it sounds the same, you're pretty close. Turns out the ear is pretty sensitive to detecting the changes.

I also have a "bubble-type" or "SU type" tool[0] that you put over the ends, and the vacuum moves a little ball in a tube.

[0] Similar to this: https://www.jbugs.com/product/5746.html

Oh, that’s VERY interesting. I never thought about ‘hearing’ the vacuum in the intake.

It's kinda wild. You can hear the different pitch in each carb.

I did the same on my MG. Buddy of mine used to so it on his Jaguar V-12 XKE with Zenith-Stromberg carbs...took a couple of hours to get it right. :-)

Those Jag V12s are so cool, though. Carbs as far as the eye can see....

> Syncing carbs is an art.

I fail to see how it's an art at all.

This is perfectly and trivially measurable, it's rudimentary science.

You can construct a manometer from a U of clear vinyl tubing with some water inside, affix one per throttle. If there isn't an existing vacuum port per throttle provided for this purpose, you improvise extensions on the inlets for your hose barbs to tap into.

At that point you just adjust things until the water levels are even.

I have a vacuum gauge I used to tune the twin SU's on my 72 MGB. It combines art and science. Worked on my 440 sixpack as well.

backfire and you'll go deaf!

There's also the rods some people use, stick them in the top where the dashpot covers belong and watch how they go up and down.

I use the speed-up -> stumble technique and you don't need the tool for the fast idle, I just look into the throats of the carbs to see if the pistons rise and fall together.

https://youtu.be/4nvGLgO6pj0

Question on your RPM: You should see one pulse for every 2 crank revolutions, right? So if you measure 600 pulses per minute, that's really 1200 RPM, right?

Yes, I thought that this might be the case. But actually, one revolution of the crankshaft should make one pulse - piston going down and up creates one sin wave. And I’m counting only peaks at the top.

But the piston going down and up doesn't make a pulse. On a four-stroke engine, the pulse is generated when the intake valve opens, which happens once per two crank revolutions. When the piston goes down and up for the power and exhaust strokes, no pulse is generated, since the intake valve is closed. (On a two-stroke engine, you would have one pulse per crank revolution.)

Ahh, you’re right! Well, that’s an easy fix. Thank you.

I was just about to write, "where the hell was this when I had my GL1000?" and then I read the story about how the author developed it for a GL1000.

Great bike.

Great bike indeed.

Now you'll need to track down one of those superchargers.

Have you ever seen the Lars Nielsen build?

Insert old-school mechanics grumbling about how now there's a computer involved in tuning carburetors.

Seriously though, neat project. Personally I'm a "carbs are for weed whackers" guy and would have just seen having to sync four of them as a good reason to swap to EFI, but for those who insist on using vacuum-powered voodoo to fuel their engines this is pretty cool and seems like it'd drastically simplify a very tedious process.

Fuel injected cars with ITBs still need to be sync'd up too!

Originally I had something about it possibly being useful for that in my post, but I'm only loosely familiar with ITBs through the E46 BMW community so I wasn't sure enough. I think those ones use a single shaft straight through so they don't need synchronization.

This reminds me of my recent quest for an affordable after-market ECU. I really expected to find a bustling open source community of DIY ECU units with the advent of affordable micro-boards. But I think the work required and real risk of damage to expensive property might keep the barrier to entry high enough that it's yet to happen.

Have you come across MegaSquirt[1]? The most recent iterations are neither inexpensive nor DIY but the earlier versions are still made as well.

[1] http://www.msextra.com/product-range/traditional-megasquirt/

The Speeduino project is similar but more DIY focused, and newer, and open source. https://speeduino.com/wiki/index.php/Speeduino

Those look pretty good, I'll have to dig in a bit deeper! At first glance, it looks like it might be a bit of a run around getting one to work with MAF and CAS sensors, but pretty much every other ECU wants a MAP sensor anyway so it wouldn't be a bad time to switch.

I had heard of them but I thought they were a Miata specific ECU replacement not a general purpose one.

I think that a lot of it it also due to the fact that the existing modern ECUs in most cars can just be reprogrammed in place. Why swap out the ECU, when you can just plug in to the OBD-II port and tune it?

The use scope for replacement ECUs is pretty limited. Limited to a handful of 80s and 80s cars with extremely primitive ECUs (by today's standards), old cars that are being retrofitted to run fuel injection, and specialty one-off builds (where even most of these people buy something like a Motec, AEM, Holley, etc, etc ECU that plugs in and goes).

The possibility of damage is the same whether or not the ECU is DIY or not. It's all in the tune whether you will blow up your engine or not.

> I think that a lot of it it also due to the fact that the existing modern ECUs in most cars can just be reprogrammed in place.

I think you're right. In fact the only reason I'm digging into this is because I have one of very few 90s Nissan ECUs that hasn't been piggy backed (R33 S2 RB25DET). Because all others have a piggy back board available and aftermarket standalones are so common here, no one does flashing for that ECU either. You can use an R32 RB25DET ECU and piggy back that, but they are scarce here now and too expensive.

> The possibility of damage is the same whether or not the ECU is DIY or not. It's all in the tune whether you will blow up your engine or not.

I more meant from failure of the board or the code. You can be relatively confident with a modern commercial board, but I am worried about board failure on the street with DIY boards. If this were a track only car I wouldn't be as bothered, but it will be a street/track day car.

That is amazing. I have '80 CX500 myself which I have neglected to sync the carbs on. I just bench sync the two when I need to pull the carbs off.

Oh yeah, I miss CX500. I never had a problem with the carbs on it tho.

Nifty. But i do wonder why goto the trouble when megasquirt is viable?

:) for fun only!

Need it for my 4-cyl Yamaha outboard.

Now I need to find those sensors...

The software works on any bike. Buy the hardware parts (super cheap) and you’re ready to go!

Cool project! Might have to try this when I get ready to tune my dual Weber DCOEs in a few months!

Nice to hear that. What engine you have?

It's a 1987 Honda Prelude engine, 1.8 liter, 12-valve engine. It came from the factory with dual Keihin CV carburetors, very similar to the GL1000 carbs (except only 2 of them). I had a very hard time finding parts to rebuild them, so I found a conversion manifold and purchased some new DCOEs to bolt onto it.

Cool! I heard about Weber conversion kits for GL1000 also. But since I’m pretty happy with the original Keihen setup, and currently found spare carbs (with original Keihen jets inside!) - I’ll try to stick to those.

Finally some real HN content! Has anyone else noticed a drop in the quality of submissions recently?

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