Carburetor jetting

[Viterbo]

I'm running a carburetor from a 4K engine in my 5K and since I'm running a bit of compression (12,5 bar in each cylinder) I'm afraid of running lean.

Did anybody drill the jets of the standard carb? Is it worth doing? What sizes should I run?

It's a standard 5K with just the compression and slightly ported on the intake side.

[Banjo]

Quote

I'm afraid of running lean.

There is a lot of difference between thinking you are possibly running lean, & actually knowing you are running lean.

My advice is . . . .

Take the car for a good 30 minute run, on the open road.  (not in stop start traffic)   As soon as you get back home, remove all four spark plugs, in order, and inspect them carefully. The colour of the spark plug centre insulators will give you a reasonable idea, whether you are running too rich, or too lean.  Bear in mind, this only works, if your engine is not burning a lot of oil.

There are plenty of good pictures on the net of spark plugs, in different states, operating under different states of A/F ratio.

If the test is not conclusive, because the plugs are pretty dirty, then clean well, or replace, & do the exercise again, I have suggested above.

If you really want to know the real A/F ratio, then I suggest you search Altezzaclub's posts on this forum, for his setup with two (2) sensors & a switched A/F ratio meter between the two.

If you are only using a single carby, you don't need two (2) sensors.

This is where I have mounted my O2 Sensor

That way, you will really know where you sit, in regard to A/F ratio.

If you put the A/F ration meter on the dash where you can glance at its readings, under different driving conditions, it will provide you with great insight into how your engine works, & what part A/F ratio plays.

Let's know what you find with your engine.

Cheers Banjo 

 

Edited October 22, 2017 by Banjo

[Viterbo]

I already have a narrowband O2 sensor and a cheap ebay digital gauge to install on the car, but it's not yet installed.

I want to fully rebuild and tune up the engine at the beginning of Spring and it'll run on bike carbs, but in the meantime I want to run the engine safe. One thing I notice is that the engine struggles a lot in higher revs, like there's not enough amount of fuel mixture going in...

[Banjo]

For the general overall performance, in regard to A/F ratio, a wide band A/F O2 sensor is really required.

There are lots of things in an engine, that can limit revs, besides the A/F ratio being too lean.  Have you experimented slightly with ignition advance ?

Unless your engine already has some mods carried out to improve its breathing & ignition, remember that these engines were detuned from birth, so that they couldn't over rev., to limit warranty claims in the early days of Toyota development.   Experiment with different aspects, for sure, to see what makes a difference, but only change one parameter at a time.  Do too many things at once, or you won't know which parameter had the effect, you are seeing.

Cheers Banjo

 

Edited October 23, 2017 by Banjo

[ke70dave]

Just remember that even AFR can be misleading. For instance a misfire due to too much fuel will show up as lean. 

[peterd]

With regards to running lean, I think you can do that with any carby if it's not adjusted right. It sounds like you need to do the basic tuneup from the start, as per factory specs. I'm guessing that your timing is out.

My jets are resized so I can give you a bit of advise on that one. 

I'm running a 4k block, 5k head with extractors. The compression is very high. It's a 3 speed trimatic auto box so not known for being too responsive with a 4k. I have a standard Aisan carby. With the standard jets, it's foot to the floor all the time, and a bit sluggish. With the jets resized (one size up from memory but maybe a bit more) my foot is hardly touching the accelerator,  and the off the mark acceleration is excellent. Once I get to about 80kmh, it takes a while to get to 100kmh though.  The fuel economy is crap.

For the auto, I was advised against bike carbs/weber etc. If you've got a manual box, I'm not sure how rejetting would compare to swapping the carby for a weber or bike carbs though. 

[rebuilder86]

I like that your first idea of the problem is the jets, and i would agree with you.
I drilled mine (just the secondary throat main jet) with a fucking cordless drill and a 1.5mm drill bit because I'm running a stock carb whose secondary jet didnt match the specs for whatever reason. My dodgy drill thing worked and i have cured a bog down when climbing over the hills, so it can be done DIY in my opinion.
Start by pulling out ur secondary and seeing if a 1.5mm bit will fit though it.
If not, i guarantee its too small for a 5k.
the largest size for any 4k is 1.68mm and that is on the secondary for the 4k flat top piston with the flappy controlled carby (the one without the crazy number of tubes, and the hot idle compensator inside the coverplate on the side)
so if its smaller than that, it can't be right.
someone on answers has posted that the stock jets are 1.3 and 1.6
http://www.answers.com/Q/What_is_the_stock_jet_size_of_a_Toyota_5k_engine_carburetor?#slide=3
Now i know the carby throat size is different, but only marginally, you will have to trial and error it, without making an error haha
start at 1.5 and see if there is improvement

don't go past 1.6 unless u can get some spares.

[rebuilder86]

look at that i was rihgt at a guess
 

http://www.club-k.co.nz/Forums/viewthread.php?tid=27396
Check the jet sizes should be 123 primary and 159-162 secondary from memory make sure the air....
just get a 1.5mm hss bit and work it around a bit. 

[Viterbo]

I think my primary is 0.9 or 1.0 and not sure about the secondary.

I'm not having any trouble on revving honestly. My timing is set to 5 degree to avoid detonation (which I think still occurs on 95 RON). I can rev it to 7k easily. The issue is when rolling in highway for example, 4th and 5th at higher speedsjust seems the engine struggles a lot. I can floor it and it doesn't go any faster. That's why I thought the carb just doesn't give enough for it to go further.

 

I know these engines aren't really performance oriented, but..

[rebuilder86]

get that shit out and drill it

only the secondary. leave the primary alone for now

[Viterbo]

Also my carb is a 28/28, some of them are 28/32. I'm going to try to get a bottom with the 32 butterfly

[Teemutus]

Mixing 28/28 and 28/32 is possible, but requires both lower and upper bodies. It also isn't an straight up bolt-on thing to do... I just assembled mine with 28/32 throats and butterflys and swapped the mechanical linkage so the secondary is also mechanical instead of the vacuum actuator. I used 170 secondary, 116 main, 51 idle and 60 size power valve. The venturis are also different in the 28/32 version. Don't know yet how this will run because the engine is still in parts.

I had to modify the secondary shaft to adapt the bigger butterfly and also shorten it, because the carb body differs how the shaft is going through the other side. Had to make a fixture for the secondary return spring also. There's also no spot for the velocity flap in the secondary side in the bigger throat carb body, but at least the previous carb ran perfectly with the flap locked open all the time.

 

Unfortunately it's winter time here in Finland, so can't test it out until late April. I did this upgrade since my 4K is being worked on. Honed, new pistons, 270deg cam with 10.5mm valve lift, 4-2-1 extractors, 11:1 compression to suit the cam, lightened flywheel, balanced everything, extensive head work done, pistons protruding block 0,3mm (~1mm quench clearance) etc.

[parrot]

Welcome Teemutus

 

I think we might enjoy having you on board! 

[rebuilder86]

hmmm. ive tried locking up the velocity flap and it just bogs down like crazy if i kick it in the guts (give it some quick power). 

this week I'm going to film it and see when exactly it is opening in preperation gor another project.

I'm interested in how urs ran so well with it locked open.

[Teemutus]

I have to admit that the engine I had was pretty much odd ass F. Have to see and fix once I get this one running with the mixed-up-carb. I have a bit of a worry with the velocity flap missing, but let's see and if in need - let's drill a shaft hole for one

Somebody had milled 3,4mm of the stock D-dished pistons of this engine and lowered the twin-squish head to 15cc... I do not know the purpose of these witcheries... The carb in this engine was mech secondary 28/28 with 168 secondary, 105 main, 50 idle and 60 power valve. Ran pretty much decent to my feeling. Haven't been in a K-engined car ever before. Didn't bog down though even when booted, so that's why I'm saying the above... Might be a bag of differences when I get the new engine together and the carb sees a whole different perspective of life

Anyways... I felt the need to share some knowledge I learnt while surfing the internet and trying to dig in to it myself with the gathered knowledge and with my clumsy hands :S I'm more of an engineer (major's in mech engineering), so I might have good knowledge of what to do, but the outcome might be something close to crap Hope not on this case... I'm trying to outsource the tough parts to the handy people

Oh and thanks parrot! I sure enjoy these kinds of forums with active people with shared interests, although they might be in the opposite parts of the world :S