Rejetting carbs

not really interested in more HP.....just want to get it running right, really.

 

Wonderful stuff Aircon. I have a '75 GT4, so I am particularly interested in your results. Did you try 135 mains with different Air Correction jets? (Right now I have 60, 135, 200, F36)
Thanks,
Martin.

 

60 idles? i can't comprehend how rich that must be running....i have 12.5:1 with 55s!

i also have the F36.

i found the 200 a/c leaned it a little up high. as you can see, the 170 a/c richens it a tad. something between it probably spot on, but i'm satisfied with the main circuit. i'm going to play more with the idle circuit though.

 

Hello Aircon

Well, my car came with 60 idles (actually 55s drilled out to 60s) It pays to check the jets with a gage, because lots of "tuners" drill out jets without filing off the numbers.

I now have a set of 55s which I will install at first oppertunity.

The car originally came with 220ACs, which I changed to 200s. A noticeable improvement on top end. Mid range is excellent. I think I will try to buy some 180 ACs.

You dyno chart comfirmed what I have suspected for some time, and that is there is no point in reving beyond 7200 rpm. It is past max power point and considerabley past max torque point.

In fact because centrifugal rod bearing loads go up with the square of the rotational speed, reving beyond 7200 rpm unneccesarily loads and wears engine internals at an increased non linear rate. Particuarly so for 30 year old engines, and that means increased risk of faster rate of wear and or break-down.

Thanks again for sharing your testing data. It is helping me considerably.
Martin

 

i'll let you know how i go with my 50s which i might be drilling out to 52 before settling for the 55s

ohhhh...good thinking!

i also have quite an aggressive advance curve which helps with the flat power curve. it'd be interesting to see if like more than the standard total advance at the higher revs, actually.

 

Aircon:

Would you care to divulge a bit more on that advance curve?

I have great interest in the subject and am always wondering how far folks have pushed it...

Thanks!

Rob Stewart

 

i'll have to get back to you...i don't have the numbers handy.

 

using the 55 idle jets produces and overly rich mixture 12.8:1 apart from slightyly down hill straights, where the throttle is closed almost entirely, and it is very lean there.

mechanic said can't go smaller jets cos that would make it worse. regardless i put in the 50 idles....i'm not sure what mixture i'm getting yet...but it'll obviously be better....and because cruising required more throttle with the leaner mixture, i don't really get the light throttle popping.

not theres juts a stumble when the transition occurs ,,,,i might be able to live with it.

more work to be done.

 

If you get it running right, it will make more power - really!

It won't run right if the exhaust is an inhibitor - if the GT4 is anything like the std 308 exhaust, you are leaving a lot on the table.

 

it's not the standard exhaust. it's an ansa, i think...so it's definitely flowing better than a standard one.

the car is now very smooth with lots of grunt.

just got to fine tune the idle circuit and i'll be laughing.

 

Plus Air Correction jets and chokes.
Your tubes won't be 220, that will be AC jets
ET's are a letter and number - eg. F36

How does this all come together? - Ah, the black art!!!
FIIK

Good luck - you'll enjoy the experience

 

i hope you enjoy it as much as i have been.

unless you have F24 emulsion tubes, your 125/220 mains will be FAR too lean, i reckon.

If you have F36 emulsion tubes, your 55 idles will be too rich....i'm not sure if F24 ETs will make that even worse...probably not.

I had 12.8:1 AF ratio with the 55 idles.

i'm still not sure what I have with the 50s i have in there now.

I found that not having the float levels absolutely identical made the car run like crap. how important it is that they're at exactly the factory height, i can't say for sure...i did mine at 47mm for reasons that i won't bore you all with.

suffice to say that when they weren't identical, the car ran terribly with the 50 idle jets. now it's almost perfect.

 

While my settings on the 3.2 qv do not exactly mirror the 3.0 requirements, I thought I would just post a couple of references test graphs (and I have a lot) from a dyno pull and using an Innovate LM-1 mobile A/F recorder which I use to tune my carb jetting in addition to the dyno. The dyno only really tells you about behavior under full load, which is good as this is where the most damage could happen to the engine.
Just for reference this is with a 3.2 qv with 140/F24/190/53 on 34mm venturis on the dyno run; The street run was an experimental run on 36mm venturis and 135 jets and 60 idles- hope this is interesting for some.
Image Unavailable, Please Login
Image Unavailable, Please Login

 

I think one item thats not been well discussed, is the crossover between each area. The idles work in the area where the throttles are nearly closed. Even at highways speeds, as long as your not accelerating your running pretty much off the idle circuit. When you open the throttle, depending of engine rpm and throttle opening, the mains will come in through higher airflow at the venturi. At higher throttle openings at higher rpm's, the idles will drop out and not be a factor. So not only engine speed, but your foot in unison, control the airflow.

The emulsion tubes are still a great mystery and no one seems to have a great deal of knowledge of how they control things, but obviously they are making a great deal of difference. Back before the emissions craze, cars got very good economy. Then they made the cars run rich and get horrible fuel economy to lower emissions. Catalytic convertors need even more unburned fuel to operate efficiently. Somewhere back in the past there is information to make these engines run like they were meant to. Perhaps we should be looking at the jetting of the late 60's 365 GT engines as well as ignition advance numbers to lead us. I am of the opinion that these engines could give both good economy AND good power, if we could only find the best jetting combination, and I think the emulsion tube holds a large key in that discovery.

 

http://www.fcavic.com.au//308/index3.htm

I installed .50 idle jets in my 308 and cut the engine whilst doing a light cruise.

Haven't stuck the analyser in yet, but this looked pretty good to me.

 

You know, I think that rich idle jets do/can mask a lot of improperly adjusted carbs, particularly on the carb to carb balance, float valve heights and idle adjustment screws.

Conversely, that means if you lean out the idle jets,to get the ideal AF ratio, the carb adjustments need to be spot on.

As discussed previuosly, main jet size really only becomes a factor at higher power settings.

Air correction (AC) jets realy only come into play at high power settings AND HIGH REVS. And that is where you never want a lean mixture. In fact it is better and safer to err on the side of a slightly richer mixture at max load and RPM. This will help prevent pre-ignition, keep combustion and valve temperatures cooler, and keep the engine in one piece. This is particularly important in sustained high speed running, and track conditions.
Martin

 

i'm with you on that. hence my 12.5:1 a/f ratio on full throttle.

i tried the .50 idle jets before and it didn't run very well...i decided to redo the float levels...and that's ALL i changed...and they weren't perfect...now they are (sync was already spot on) and now it's excellent (for a carby)

i'm just waiting for a new sensor for the portable analyser, then i can see what the actual a/f ration is with the .50 idles....but the plugs look quite good.

 

just thought i'd mention....i'm hopeless at setting the idle mixtures!!

i don't know if i'm deaf or what the hell is going on, but between the fans coming on, and whatever else, i reckon i'm lucky if i'm withing 1/2 a turn of where they're meant to be!!grrrrrrrrr

it's running great, though.


how are you going with your experiments, Hans?

 

ok. all testing done today.

Basic setup is .50 idle jets, 135 mains and 170 a/c

cruise and light acceleration is all 13.8-14.2:1

as soon as you reduce the throttle position, like on a slight downhill, it goes even leaner...too lean...but it did that with the .55 too.

The car does not like running leaner than 14.5:1, really, but from 14.8:1 and leaner it's quite fluffy. This is, of course, only downhill and under "slowing down" conditions, so is of no consequence. The main thing is that on steady cruise from 2000-4000rpm it's pretty much on 14:1 average. It's not what i'd set up fuel injection to do, but it's pretty damn good.

There is a slight lean spot at the transition from the idle to the main circuit. It's so minor that sometimes it's actually hard to find, but it goes up to 15.5:1 for a milllisecond....this was masked by the 12.8:1 mixture that the .55 idle jets had.

For me, I'd rather keep the .50 idle jets and put up with that little burble during the transition.

with the 135 mains, as previously stated, it's pretty much 12.5:1 everywhere on the main circuit, all revs and all throttle positions.

I think this is the end of the experimentation, as i don't believe it could be any better than that without major modifications to the carbs,

 

I think you've done a great job - theproof is in how it runs and you have the mixtures solidly in the power band.
Congrats! Would love to see some graphs if available.
I'll be doing the same as you except breaking some new ground changing from 40 to 44 DCNFs over the next 2 - 3 months.
Again, well done!

 

thanks. it's been fun


i only have this graph for the full throttle curve (i forgot to say i have F36 ETs)

the rich dip at 2000rpm was only temporary and from the accel pumps.

http://pless.com.au/cars/gt4/rebuild/images/308dyno.JPG

as far as a graph of cruise, it'd just be a flat line from 2000-4500rpm!