NickJ's Lada Niva TwinCam

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With the workshop staff still on summer break I spread Lada/Fiat bits across the main workbench and had a leisurely measure up of vitals


I then spied a lonely looking mill in the corner and gave her something to do


End result was 2.1mm off thickness taken off the 8mm flange to get it flat, was a pig to hold down without warping too much, I now fear it may flex and wobble in use, but hopefully will seal better/easier than it was before.

The engine side is thankfully not as bad as it would be a pain to hold down but if I can work it out it might get similar treatment.


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Much to talk about...

First of all, 


#CleansheetVTNZ #Levelimposibleclocked


Time to set my mind onto more interesting things, hooked up the wideband to the Niva and had a horrible surprise, all nice at idle and up to 50kmh, but then open up the secondaries and she's rich as, so rich no amount of tweaking and miniature jets could alter, I gave the single carb a full half day of messing about but in the end ditched it for the dellortos, even though these are rich, the results were far more consistent and changes to mixture followed changes I made.

With the twins on it was time for some serious thinking, driving around with the wideband on the dash wasn't really the easiest thing to do so I sat down last night with a beer and bashed the keyboard to get python to read throttle position and mixture via arduino, all thats needed then is a tps on the carb, PAP wanted $40 but i'm too cheap for that kind of extravagance, headed down to jaycar and grabbed a $3 10k potentiometer and aluminium knob for $4, mix that with a bit of scrap ali sheet and boom, tps! (well not quite, still had to hack a slot for the shaft to engage in, man I wish I had a mill!)


All that set up I dumped the laptop in the passengers seat and had a play


Worked out far better than I imagined, managed to tune Idle much quicker than listening, by simply looking at the rev counter and O2 reading, awesome!

Pre-drive calibrating of the tps found this wee fella:


The 3 equal peaks are me pushing the throttle pedal, the larger peak is holding full throttle on the carbs, few free ponies if I sort the linkages better!

With that in mind, I went for a drive and things got tricky...


Blue line is O2, red tps, scale is kinda random, but ~500 = good for mixture, tps runs 180->1024 and ignore the green, thats there for when I get engine speed mixed in, but so far its floating along for fun. Oh and graph reads right to left.

At cruise and idle mixture is about right, average acceleration (pulling away from lights etc) is lean as, while planting foot runs mega rich(after a brief lean out), my conclusion is the idle jets are a bit small, and needs a size or two up there, and the mains need to be dropped a similar amount with a little more on the pump jets to see about the lean as the pedal hits the floor.

Fun times, but man I wish I could just go full EFI without having to cert the darn thing!

Discussion about carb tuning tips please:



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Because Anton failed my last submission i've tinkered with the code:


Labels, scale and grid, well kinda

Bench test only, cos its dark out.

Speaking last night with a knowledgeable carb tuner, my initial analysis is off, word is idles sound fine, but mains are lean and the air correctors could be opened up to prevent flooding at high revs, I have a lead for some jets to try and failing that there may be a set of jet drills near by too.

I also brought a LM2917 IC, this has a fancy trick of turning frequency to a linear voltage, ie give it a signal from the points and it will output 0-5V for the arduino, i've put together a small circuit on veroboard, but just need to bench test it along with an isolation circuit (optocouplers) so i don't risk getting the points running via my laptop!

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99% sure the dellortos are now heading for the shelf, had some younger, more attractive twins dropped off last week


Nissan GTiR itbs, complete with all the bits and loom required, i've also got the speeduino assembled and running on the workbench to organise the electrons, small issues still around injector choice, engine sensor locations, fuel pump and regulator, but all in all the pieces are there.

Biggest pain is it will require a re-cert, i've been through it all once before and comfortable it will be able to pass, but am left considering if I should do any other mods while there! At least with christmas running up fast I just don't have the time so will worry about that next year!

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Got the Speeduino running and sat down to give it a bench test, hooked up the tps, fed a simulated crank sensor in and scoped the outputs.IMG_5669.jpg.c7018836b08c97258ded3d3102fef55a.jpg


Pretty darned stoked to have it this far, found I had installed a few parts wrong during initial assembly which caused some searching, I even contemplated ordering a megasquirt if the fault was too hard to find, but thanks to open source and having all the circuit info available, a reversed capacitor was to blame and all is now working as intended. 

Drew up a concept for the manifold, while I can't quite make it as I initially though, it will still work with some extra manual labour, the idea being a single piece aluminium adapter plate won't require welding (I just don't have that much time anymore!) bit of manual machining needed and then sitting down with a die grinder for the final match up shouold do the trick. This will also keep the final package tidy in the engine bay, currently the manifold I made to snake around the distributor and dellortos is as large as the head and looks a little out of proportion (say the guy putting all this effort into a Niva)


Printed out fullsize for a quick check, good thing as I had the itb sketch upside down! i'll fix that and then look to getting the blank cut out.


also need to give them a good clean, a little grimy as well as the token Al swarf from being under a workbench the last few years!

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As normal in life, just as you commit to a project priorities change, sadly the EFI conversion is now on the slow boat as the house needs some funds, i've had the car long enough, whats a few more years!

Having another think about the mixture, even though the wideband says its running lean at cruise, fuel consumption suggested otherwise, most likely cause, the brake booster, so that was disconnected and I went for a drive.

80kph with vacuum hose connected


And with the hose disconnected/blocked off


Green = wideband
Red should be engine speed, but the analogue-digital converter wasn't playing nice so it just floated along with mixture.

While there are a few months between the two comparisons, i'm confident this is worth chasing as there was no change in the brake feel, best be adding a brake booster to the shopping list! 

Another interesting point is the mixture plot has less fluctuations, interestings.... I'm also sitting right where I was told i'd end up, having to choose between cruise economy and wot performance, regardless I will fiddle with the jets and see what happens.

I've also acquired some rear callipers from Bart, plan to graft some front disks onto the rear shafts and finally get rid of the constant wof issues drums are so keen to provide.



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Some good learning today, have been bracketing jet size around what is currently working and found going from 110 main to 100 is like falling off a cliff, having the datalogger hooked up allowed me to see when the idle/primary system moves into the main jets, everything would drive fine up to 40% throttle, hit 45% and nothing but lean, this confirmed the primary circuit is close if not a touch rich and I can focus a bit more on fine tuning the mains. This also showed up a another gem, the lean hole that i've had forever between the two circuits is looking most likely to do with the emulsion tubes, I have increasing confidence now to solder up some ports in the emulsion tubes to see what happens.

Lada themed curtesy image:



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Pulled up my suspenders, slid on some spectacles and fired up the soldering iron, the day has arrived to attack the emulsion tubes!

From the previous post, suspicions of the emulsion tubes not being right was strengthened, spending the day pondering methods to attack I settled on the 4 large holes at the top, from my understanding, the jet at the bottom controls fuel flow to the venturi, via the emulsion tube, which mixes air into the fuel assisting atomisation, this air mix is governed by the air corrector and the series of smaller passages along the length of the tube. roughly speaking, the holes at the top control mix at idle, the holes lower down, WOT. Having already messed with the air corrector diameter and unable find a combination that gave good mixture throughout, I was always left with a gaping hole around 3500rpm, so, dead spot around 40% throttle and 4 large holes around one third of the emulsion tube, reasonable place to start!

Now, one would ask, why not just buy another set? well first of all, i'm cheap, a set of four to try would be a good chunk of efi money, second, i'm a sucker for learning and thirdly, the only difference between the 4 available styles is the body diameter of which I can't find solid data on what this actually changes.

So, remove said items:


Air corrector at the top, emulsion tube center, main jet bottom.

Fill 2 of the 4 upper radial holes with solder


and leave to dry


Being a Friday night, I felt a quick drive was needed, so not expecting much I casually drove out to the open road and wow what a difference! the chasm is now a friendly ditch, I really should have hooked up the data logger as mixture stayed within acceptable limits for the first time throughout the rev range, more testing needed but a step forward in battling against the 50 fudges that constitute mechanical carburettors. 



PS, this is in no way an endorsement of twin carbs, EFI is king!


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Development has slowed, but the Niva is driving well, even completing a few road trips. Fuel economy was a tad crap, but performance has been satisfactory, I had the wideband hooked up the majority of the trips and the results are pleasing, while still a bit rich, it never leaned out under full power, for now that will have to do!

I recently returned from Jackson Bay via the west coast, here's a few pics, such a nice place to travel through.


On the way I called in to catch up with a good mate, TCGC.


Found some long straight (bumpy) roads




Surf was a bit lousy

On the engine fixing side I had a fellow OSer 3d print some intake trumpets, they look the part, I just need to tidy them up a tad, 


After 2000km of worry-free motoring something had to give, most likely picked up some British virus near Timaru and now sports a large oil leak from around the sump, i'll have to explore further, so for now she's having a rest as I focus on work.

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