
Show HN: Digital Carburetor Synchronizer (C, Swift) - zilvinassebeika
https://github.com/zilberas/CarbSync
======
joncrane
Just wanted to say the real interesting content is in the writing here:
[https://treatwell.engineering/how-i-made-a-digital-
carburett...](https://treatwell.engineering/how-i-made-a-digital-carburettor-
synchroniser-9ece7fa571fb)

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.

~~~
zilvinassebeika
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!

~~~
dwater
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.

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taborj
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](https://www.jbugs.com/product/5746.html)

~~~
kjs3
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. :-)

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

------
biggieshellz
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?

~~~
zilvinassebeika
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.

~~~
biggieshellz
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.)

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

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halbritt
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.

~~~
zilvinassebeika
Great bike indeed.

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

Have you ever seen the Lars Nielsen build?

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wolrah
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.

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

~~~
wolrah
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.

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ehnto
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.

~~~
schwap
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/](http://www.msextra.com/product-range/traditional-megasquirt/)

~~~
ehnto
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.

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varikin
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.

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

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

~~~
zilvinassebeika
:) for fun only!

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creeble
Need it for my 4-cyl Yamaha outboard.

Now I need to find those sensors...

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

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

~~~
zilvinassebeika
Nice to hear that. What engine you have?

~~~
olyjohn
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.

~~~
zilvinassebeika
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.

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

