34 replies to this topic

[rexy]

I am attempting to improve the tune in the hatch. The above carb is on the engine and I am trying to sort out the apparent rich state.
Unfortunately I don't have access to any A/F measuring devices. Besides it's interesting to try to do this the old fashioned way.
The engine is a 383 holden, 11:3 comp, approx 240@50thou hyd roller cam, come alloy heads, redline hiwinder single plane manifold, 1.75 inch primary Tri Y pipes. Usually run BP Ultimate. Engine made an honest 480 plus HP and torque by 5500 on the engine dyno.

The plugs were very sooty.
Standard jetting in the carb as delivered is 75 primary, 80 secondary with pink cam.
Went to 73 primary, 79 secondary. Still very sooty.
Went from NGK heat range 6 to a hotter NGK 5 plug. Bosch HEI with 1.2 mm gaps. While changing the plugs I also fixed the exhaust manifold leaks.
Plug still looks fairly sooty today after a good run. The exhaust tips on the pipes look a little lighter in colour.
Irritatingly has now developed occasional staggers on acceleration and have had a couple of backfires. I did fiddle with the pathway pattern of the ignition wires while doing the plugs. Thought maybe I was getting crossfire from them running to hard against each other so put them back to where they were. No change.

Questions.
Am I heading in the right direction? What sort of jetting have people run in their DP hollers on similar size engines?
Suggestions other than taking it to the pro tuners?

[71Ranger]

Hi Rexy,

Don't think your cross firing.

I'm going to stick my neck out here and suggest that you are still running rich. May I suggest removing the plugs and cleaning them and readjust.

That stagger reaks of over fuelling ( excuse the pun)

Hope this helps

Cheers Brian

[warrenm]

You main jets may be ok, but the transition circuit may be causing you "sooty" plugs

[mick_in_oz]

Agree with above, what are the IAR and IFR sizes front and rear?

 

These will give the best indication to what might be going on, I suspect that they will indicate its an overly rich setting out of the box.

[rexy]

Thanks for the help so far.
I had a bit of a play with it yesterday. Went back to scratch with the idle screws which cleaned up its low speed manners. Interestingly the holley site suggested they should be screwed out 1.5 turns from seat as a baseline. This of course is incorrect as I discovered and the old 3/4 of a turn out was pretty much spot on.
Fiddled with the pump cam and activatior to make the rear shot smaller and later. This seems to have cleaned up a lot of its other problems. No more staggers and backfires.

Here is a pic showing how dark the plug was. The slightly lighter plug is from the old engine (early headed 383) that just came out of the hatch. The carb was new on that engine with its original jetting/setup.
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Maybe the come alloys better efficiency accounts for some of the extra fouling?

For the life of me I cannot find a parts schematic for this carb anywhere online. Are the IFR and IAR jets the little brass jets visible on the front of the primary and the back of the secondary venturis in this pic?
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If so the primary side has two that are much larger than the secondary side. These are the outside ones.

Edited by rexy, 09 November 2014 - 09:20 AM.

[mick_in_oz]

In the above pic the outside bleeds are the IAR's (idle air restrictors) and the inside ones are the high speed bleeds for the main circuit.

 

The IFR's (idle fuel restrictors) are in the metering plates, being a HP series carb they should be located up high near the top of a well.

 

The relationship of these two sizes set the mixture of the idle ad transition circuits and its quite common for them to be on the fat or very fat side being a HP series carb.

[rexy]

So should the idle air restrictiors on the secondary side be so much smaller than on the primary side? When I get a chance I will pull the carb off and see if I can find the sizes.
On a happy note, driving it today it is well mannered. Idles well good part throttle and pulls cleanly from low rpm in 3rd and 4th gear.

Did relearn an old lesson today. The starter suffers from heat soak. Trying to restart at one point it taunted me by almost coughing enough for me to keep it keyed on for too long. Melted the thin part of the battery cable to battery connector (lead one). Replaced with a new brass one and was good to go.

[warrenm]

As mick_in_oz mentioned they are usually pretty fat out of the box. These pics might be of assistance, but your idle jets may not be screw in.

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[rexy]

So I finally got to give the car a good drive today and pull a plug. Still very sooty so pulled the carb again.
Was jetted at 73 primaries, 79 secondaries (out of box was 75/80).
Have changed these today to 71 primary, 76 secondary.
Checked the IFR set up. They are press in not screw in and I couldn't find a visible size indicator.
The IAR sizes are 71 primary side, 28 secondary side.
Has a 4.5 power valve.
Squirters are 28 primary, 31 secondary.

Sadly the IAR screw in jets are a different thread size to the fuel jets so I don't have anything at hand to play with.
If I change them how big a step do you suggest? 69/26? Do the primary side only?

Thanks for your help so far.
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Primary side pics.

[rexy]

Secondary side pics.

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[rexy]

So I bought myself a Christmas present to have a play with.
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It took about an hour to install. Very easy to follow instructions for install and calibration.
A drive showd the idle, low throttle and cruise were at the upper limit of ok tending to lean. Apparently very lean when given more than light throttle.
Was at 71 primary, 76 secondary. Changed to 78 secondary jets without much change to the readings.
Changed to 73 primary, 82 secondary and reset the pump cam (pink) to the most aggressive position. Idle, light throttle and cruise AFR are now mostly 11.5-12.5. Still going to very lean on anything more than light throttle.
Went to 86 then 90 secondary jets with at best mild improvement in the AFR on fuller throttle.
These readings are under load while the car is moving. With the car stationary and no load the readings are ok.

I feel like I have missed something obvious to do with the fuller throttle AFR under load.
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[rexy]

$229 including postage off eBay.

[axistr]

I used a VDO simular unit to set up my 383, I got the best results with playing around with the power valve. I use a 780cfm street avenger Holley Vacuum secondary. I also found that the factory jets were very close. Its hard to get a fantastic AFR in every aspect of engine R.P.M, load, acceleration, cruise and idle. I got my plugs colours a nice light grey after running it at all aspect of the driving spectrum. I just have to be carful not to foul a plug on warm up due to the auto choke.

 

Keep in mind If your engine backfires up the intake manifold, nine times out of ten its running lean in the affected area. 

 

E.F.I generally doesn't give much power increase over a well tuned carby, but you can get the A.F.R ratio spot on through out the entire spectrum and that's the only reason the OE boys went to E.F.I due to pollution rules. 

[SA EH]

Best thing money can buy, now playing has become much more fun.

[mick_in_oz]

Do you have some way of measuring the Idle Fuel Restrictors? They have a small drilling at the bottom of the larger entry hole, a typical ranger would be .028" to .035" and also the Idle Air Restrictors range .065" to .080"

 

There is also a max Jet size you can use with a standard flow power valve before you need to be using the High Flow unit, somewhere around the 85 mark, but I cant remember right now.

 

As mentioned above the standard jetting is often close unless the engine has lots of cam, is quite a wild build, or the carb is a very poor choice size wise for the engine. The larger the cam gets in a small cube engine the more difficult it gets to sort the carb properly.

[rexy]

$229 including postage off eBay.

Now less than $200.

[rexy]

So as expected I made a couple of beginner mistakes.
Being inherently in favour of the path of least resistance I had installed the O2 sensor in the bung that was already fitted in the passenger side pipes (std fitment from the manufacturer.) This bung is in the secondary pipe for the front two cylinders. The AFR readings were very labile. Re-reading the instructions they recommend the sensor be fitted at least 24 inches AFTER the collector.
So I got out the welder and was reminded how crap a welder I am. Got the supplied bung fitted after a bit of farnarkling around and found the AFRs were much steadier.

My second error was I thought I had fixed all the exhaust leaks. A good close listen with a piece of heater hose in the good ear and the free end right up close to the exhaust manifold revealed I was wrong. Defective installation when I had swapped the engines over a little while back. I re-sealed it before I left for the Nats, giving it plenty of time to cure before my return.

Drove it around yesterday and today and the AFR seems generally ok. 12 give or take at idle and light to medium throttle. Same at cruise at all speeds up to 115km/h. Finally gave it the berries and it's around the 11 mark at WOT. It still seems a bit lean if you change up at half throttle as you hit the next gear, going to 14 odd with throttle position unchanged.
Pulled a couple of plugs and they look pretty good.
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I am reasonably happy with where I am at so I think I will rack up a couple of hundred kays and recheck the plugs.
It's noticeably nicer to drive. Starts first crank now the plugs are clean.
Nice light throttle behaviour. Noticeable improvement in mid to upper rpm grunt.

As a side note, it was interesting watching the dynos at the Nats. I don't remember them showing the AFR on the screen in previous years. It was noticeable that once the AFR dipped into the tens it produced quite a dip in the power curve.

[mick_in_oz]

Oops, brain fart earlier, the high flow power valve is for when you enlarge the power valve restrictors to add extra enrichment, the rest is right re jet sizing etc.

 

Plugs look a bit fat still, but its probably fine as long as its drives about without fouling plugs, good job.

[rexy]

Thanks for your assistance so far.

[rexy]

Thanks for the reminder, I have been meaning to retrim the idle mixture screws with the vac gauge.
Did this today. Best idle vac achieved is 12 inches of mercury.
This brought the idle rpm up a bit so readjusted the idle stop screw to a stable 900 rpm.
Idle and light throttle AFR now mostly 12.5 - 13.2.

[rexy]

Thanks. What is the usual relationship between idle vac and preferred power valve setting?