Playing with carbs: Mod to lean mixture at cruise speed

**johncoder** · October 10th, 2013 05:38 PM

I was thinking a lot lately of how to further lower my FC on an old 13T carbureted engine. I can't afford to buy a new more economical car to boot. Napamana lang itong assembled wrangler sa misis ko. The car runs smooth now after thinkering with the issues and making the valve clearances as close to spec. values making about 11-12km/l highway.

I now present you my plan:

Manifold vacuum at idle is higher then when opening your throttle so i can applying this to my mod which is simply composed of a needle and a vacuum diaphragm. When the engine is at idle, vacuum sucks at the diaphragm making the needle move away from the main jet hole and when the throttle is opened, it retracts back acting as valve (imagine the needle jet in a motorcyle). At cruising speed the mixture will be a bit leaner thus the FC improvement is achieved.

**lowslowbenz** · October 10th, 2013 05:57 PM

How do you plan in placing a needle inside your main jet? A 13T has an Aisin carb IIRC. It has a primary and secondary jet. Where will you install this needle? Primary or secondary? AFAIK, the secondary barrel/jet is activated by vacuum/linkage mix.

You may want to look at this link: http://www.sterlingbritishmotoringso...ech%20tips.pdf

and here: Reopus Ignition | Stromberg carburettors

Seems like you got your idea from this type of carb.

**johncoder** · October 10th, 2013 06:09 PM

the needle will be in my main jet since the secondary is seldom used or not used at all in my case. in the aisin carb, there are service holes on the revservoir (for emergency jet cleaning) which is plugged with a 14mm bolt. i'm planning on drilling a 4mm on the bolt for the needle and mayble orings to seal.

thanks for the link btw! honestly wala akong alam na existing na ganitong mod. well technology has gone way too far and old tech is being forgotten and there's so much room for improvement imho.

**lowslowbenz** · October 10th, 2013 06:36 PM

The 14mm plugs you see on the side of the fuel float reservoir of the carb is not used for emergency cleaning. It is there to plug the access holes when the jets need to be removed for replacement or "off-carb" cleaning (overhaul). A 4mm hole is very small, I do not know how will you be able to fit rubber o-rings into that hole, let alone look for suppliers of 4mm rubber o-rings.

Your "needle" needs to be tapered if you want to restrict/meter the flow of gasoline. But then again, you would also have to control the amount of vacuum going into the dashpot so that the needle does not fully retract and block your jet. Patay makina mo nyan.

Honestly I have no idea if your mod will work or not. Given my experience with carbs (which go a long way), I would rather tune my air/fuel mixtures to near optimum levels, taking into consideration the wear and tear of components, rather than tinker with unproven mods which may end up with a non usable carb.

My thoughts.

**johncoder** · October 10th, 2013 08:04 PM

points well taken sir.

you're right about the service holes, what i mean is emergency cleaning of the jets when it gets clogged hehe, sorry for confusing you

yes the needle is tapered just like the one you'll find in motorcycles.. the brass connected to the throttle linkage. and i'll make sure that even the needle retracts fully there is still a gap to allow gas

i can salvage orings from broken lighters and even spray bottles from small to medium sizes. its from tinkering with airsoft guns too much oh well

if ever my mod fails i can just shut the service hole with another plug