Sunday, August 06, 2006

Mixture tests

So the meter came.

I ran £20 of 97 Texaco through the car today, not my favoured fuel it doesn't run that well in my engine, needs booster added.

The meter is no doubt useful but doesn't really tell me anything I cannot feel, when its good on the gauge it feels good!

It did enable me to make some firm jet decisions and see my crossovers in my detail.

I had the main circuit setup 125main - 7 tube- 170 corrector, the meter told me this had a decent cruise mixture and mid range but leaned off a bit at the top and high end WOT, I setup like this as it was a bit stodgy at mid range cruise on 125-7-160 I was using at the sprint on the way home...I find air correctors "air brakes" also slow the start point on the mains, hense my movement to 170, but at the same time they lean the top end also. For whatever reason I choose to leave the main at 125 that day.

I tested numerous combo's 120-7-170, 128-7-170, 128-5-160, 125-7-160, 120-5-160, 120-5-150, 125-5-150, 118-5-140, 115-5-140, 115-7-140, 118-7-140, 118-7-150, 120-7-150, 125-7-150 using a combination of high speed motorway testing and low speed A-Road work. None of them ideal.

Was a touch hot today for the sweetest running, the car was much nicer on my final evening run with 20C air.

I ended up on 122main-7tube-160 corrector, this gives me 13(ish) air fuel ratio 80-110mph on the motorway with a partial throttle opening and gives me 12.2-13 at wide open throttle from 4500-7500. Oddly when I reduce the throttle opening at these speeds it goes 11.1 rich till I get to 3500rpm then goes back to 13ish, not quite sure what that is about, maybe the progression is adding fuel again as the plates pass the progression holes and the load is low and the fuel isn't being burnt just ejected...Its these C carbs I think!! It doesnt have a downside its just a happening..

The meter reads 14.7-15.1 at a closed throttle from any speed and read 16-17 on the 48 idle jets. In the low end-the car doesn't like anything less than 13.5-14 in any situation bar closed throttle, can't expect a race cam to enjoy stiometric fuelling..It likes max rich in progression when you open it up or accelerate gently.

I noted the idle circuit was running a bit rich and long on 50idle - 7850.1 holder, which confirmed by reason for using the 170 corrector...I couldn't detune the progression as its starts to miss off idle.... I tested 48idle and 7850.3 holder, holder a stage richer than my usual 1 but the 48 is too lean at 1000-1750rpm then the mix was good, better than the 50 and 1 holder I think. Missing forced me back to the 50 and 1 holder.

So I drilled the air hole on the 1 holder out by 0.1mm (I think 8-) which seemed to do the trick but possibly another .1mm will be even better. I'll stew on that before wasting a set of holders if its no good. I can't use anything less than 50 idle I just wanted the high end of progression to lean out a bit. Oddly people NEVER use holders leaner than 7850.1...This helped an occassional miss when you decked the throttle from low rpm.

The engine likes the gauge to read max rich during transitions from cruise to power, also possiblty the pump jets adding their fuel making it read rich for a short time and any attempts to lean this out a bit results in a loose of smoothness and the odd hickup, so I left it rich.

All in all worth £60 but I have been on these jets before. Bar modifying the idle holders. The proof of any pudding is what it feels like on the next day, so we will see!

Fitted some new lockheed/delphi brake shoes and adjustment my handbrake as well...Its been a minor pain in the bum on this super low rear susp as when it bumps it shortens the cable as at static its inclined.

I increased the stiffness on my rear shocks too and gave the drivers side 1 more click than the pass - gave the tyres a touch more pressure, think where not balanced to eachother(shocks)...I was doing 6500rpm in 4th gear and hit some manic yumps on this road and the tyre bottomed out on the arch! not the outer arch but the bloody top of the wheel arch! I smelt rubber! the arse end waggled about and it wasn't pleasant at all 8-) When you run out of suspension movement the load has to go somewhere then be released - not quite a brown pants job, would have been for a passenger me thinks , it lurched left and right with a mind of its own!


VitesseEFi said...


Interesting to hear about your O2 sensor experiences - quite difficult to use on the road without a passenger and laptop I would think.

One way to do this (bit late now maybe) would be to use the megasquirt ECU for ignition only as this allows you to datalog using a laptop while driving which is quite useful.

The Megasquirt ECU then means you can readily move on to EFI (Hah!) when you are ready, then you can fiddle with the mix (and timing) while still on the move!

Presume you have narrowband o2 sensor as I do. Pretty useful but wideband is even better as it reads properly over a much wider mix range allowing you to set up power (rich) and cruise (lean much more accurately - I'm trying to convince myself it would be money well spent.....



David Powell said...

You just watch the gauge its quite easy really I only test on quiet A-Road, stop rejet watch some more.

Actually the gauge isn't that useful as with a race-cam it seems to be wanting to be run rich alot. Also I can feel when the engine is running right without any tools I have never used any before.

As said in my post I have used all these jet settings before so there is little to delibrate over, I just couldn't decide which of my favoured 2-3 combo's was best I was happy with them all and still even with the gauge I get good readings from my top 3 (118-7-150/120-7-160/122-7-160)...I only bothered to get this meter thing to check WOT fuelling at 5000-7000+ in top gear for the Ring trip and it saved paying the same figure for a few power runs on the rolling road...I didn't need it to help me tune the carbs, thats my business!

Also running a race-cam needs a rich idle and if you setup for MOT levels of hydrocarbons etc it will run like a DOG! so appears without leaning it out to the point it won't run at idle I will NEVER be able to get the over-run fuelling down to below 15.1 or need to tune this, as you cannot tune the over-run and cruise mix without killing off the idle or progression, I am on carbs!....As the idle jets are feeding it with fuel still on over-run your stuck with what is best at idle...The idle is about 11.8A/F-max rich for best idle, idles like a clock, plugs clean but cam throws most of the fuel out the exhaust? this means closed plate over-run is stoimetric or a tad less...15.1 absolute min. Its not tuneable anyway.

I had some 17-16.1 readings when I went lean on my idle jets without re-tuning the screws, sadly it wouldn't idle at all!

My last engine on 30mm chokes and fast road89 ran so well you could have bluffed anyone into thinking it was fuel injected, this is cause the engine was tuned for streetability in all areas so no comphremises were present in ANY areas, at any TP in any gear, it was a doddle to setup by ear. I bet a rolling road couldn't have got another horsepower from the fuelling....

This engine is setup for top-end power with mad cam etc, as such it always gonna stretch the limits of things...would benefit from fuel injection ...The last engine setup very differently would have been a complete waste of time injecting as it was faultless!

The next engine will run a different cam profile I came by, gives the torque of a TH5 and power between that of the TH7 and FR89....Its a give KENT a load of numbers job! I may inject it, for now it'll all have to do as the damn car has eaten up all my motivation this year! After the Ring and trackday I am gonna cover it with a sheet and get a life 8-)