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Timing and carb tuning |
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torkair 
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Joined: 01-May-2010 Location: Tucson, Az Status: Offline Points: 430 |
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 Topic: Timing and carb tuningPosted: 23-January-2012 at 4:56PM |
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Well I've got the 73 up and running but I'm having some trouble with the
fine tuning of things now. The carb is a 650 Speed Demon DP with quick
fuel metering blocks, power valve delete, and is jetted 66/70 right
now. The dizzy is a fully mechanical Mallory Unilite with a hyfire
coil. The problem that I am having is that the engine stumbles off idle
and will backfire through the carb quite frequently. I think I may
have the a/f mixture going pretty well but I think the timing is coming
in way too late and causing it to fire retarded (which is why I am glad
the carb has the power valve delete). Looking for any input that can
get this beast running properly so that it can become my daily driver.
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72 Gran Torino "sport"- in progress
73 Ranchero GT- Q code-460/C6/3.25 open rear |
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ilyes
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Joined: 09-January-2010 Location: Philadelphia PA Status: Offline Points: 3782 |
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Posted: 24-January-2012 at 12:58AM |
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How much advance is set on right now? I never worked with a mechanical unit, but I have one on my cleveland too. I can go open mine out to figure out how it works, and maybe I can help you then
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Regul8r
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Moderator Joined: 26-December-2007 Location: Sarasota FL Status: Offline Points: 6624 |
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Posted: 24-January-2012 at 2:34AM |
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Trial and error right now.
Check the timing and set it according to the CAM specs(if it is an aftermarket cam or by the sticker if it is a stock motor) at the stock idle. Then see how far advanced it is at the RPM the dizzy paperwork says it is at full time. That is just for info. tells you how many degrees the dizzy adds. You could take notes at different RPMs to show you the dizzy curve. Idle, 1500, 2000, 2500, 3000 etc... Then, adjust the timing 2 degrees, drive and check. Repeat as needed. I can usually set it by ear at idle and tell where the motor WANTS the idle timing and that is usually within 5 degrees of the final setting. you WILL know when you go too far!
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Rockatansky
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Joined: 30-July-2010 Location: On The Road Status: Offline Points: 6398 |
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Posted: 24-January-2012 at 7:34AM |
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66/70 sounds about right maybe a little lean for a 650 WITH a power valve, without a power valve the carb needs to be jetted 8-10 sizes larger
do you have a timing light? WITHOUT one you can GUESS that maybe "the timing is coming in way too late and causing it to fire retarded". WITH a timing light you can set the timing to exactly what you want it to be and KNOW that it's set, i would start by setting a mechanical only dizzy at 16* and use the timing light and a timing tape on the balancer to determine what the dizzy is providing for total advance by running the rpm up slowly while watching with the light until the marks stop moving. with mech only you need to run as much initial as the engine will allow by hot re-start and set the advance to stop around 36*-38*... some engines will continue to make power until 45* or more, some with very good combustion chambers quit after as low as 26*
that's where the trial & error comes in, finding what your combo likes for initial / total
Edited by Rockatansky - 24-January-2012 at 7:36AM |
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72 GT Ute
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mule323
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Posted: 24-January-2012 at 3:01PM |
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Not sure about a demon carb but on my summit I had to increase the size of my accelerator pump discharge nozzles to get the off idle stumble cleared up. Still got some tweaking to do but all in all it made a huuuge difference.
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(Steve)
73 G.T.S. Fastback, 351C, C6 |
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73 GT Sport 429
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Posted: 24-January-2012 at 3:17PM |
All engines should have some fuel enrichment under acceleration. Set the timing with a light as suggested and you will probaly still have a backfire under acceleration. If the engine is mechanically sound (no vacuum leaks, ign timing set, valves all sealing propery, etc) then a backfire though the intake is nomally caused by a lean mixture. A power valve prevents this. Personally I would not run a carb without one as the mixture will have to be jetted richer and the fuel milage will suffer. |
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Mark
original owner-93 Lightning 73 Gran Torino Sport 429 (prodigal son) |
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torkair
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Posted: 24-January-2012 at 7:34PM |
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I got a new vacuum gauge tonight after work and will be checking the to see what it pulls in the morning. Now both metering blocks have provisions for power valves so should I set them to the same size or should I try having them staggered so that the secondaries be a little bump when the RPMs climb a bit higher?
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72 Gran Torino "sport"- in progress
73 Ranchero GT- Q code-460/C6/3.25 open rear |
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MarkGubinski
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Posted: 25-January-2012 at 1:14AM |
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Street driven cars should run power valves.
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72 Gran Torino Sport 557
10.12 @136 @3600lbs. 2 Time Milan Dragway Track Champion. 2012 IHRA Div. 5 No-Box Champion. 97 career wins. |
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Rockatansky
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Posted: 25-January-2012 at 8:08AM |
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the needed 8-10 additional jet sizes really fattens up the mid-range, causes an un-necessary rich condition in a 2 circuit carb...
unless you have a 3 circuit carb?
3 circuit carbs have 3 sets of air bleeds
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72 GT Ute
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torkair
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Posted: 25-January-2012 at 1:30PM |
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Got the readings and I'm pulling 16" of vacuum at idle and between 10"
and 11" in gear. I'm thinking that I should go with a #6 or perhaps a
5.5 power valve, I am also gonna increase the primary jet size to 68. I
realized that the idle screw on the secondaries was turned town a few
turns and making it hard to tell whenever I changed anything. I still
can't see my timing marks on the balancer with the timing light but I
seem to have found a "happy spot" in the timing. With all of the
changes I have made the fireball has been reduced to a slight burp and
on occasion a very small jet of blue flame.
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72 Gran Torino "sport"- in progress
73 Ranchero GT- Q code-460/C6/3.25 open rear |
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Rockatansky
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Posted: 25-January-2012 at 8:38PM |
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does it idle below 1000 rpm?
can you turn it down till you can hear each cylinder fire?
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72 GT Ute
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torkair
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Posted: 25-January-2012 at 10:03PM |
I can now set the idle to just about any RPM I want (within reason). My first clue that something was amiss was when I turned the primary idle screw all the way out and there was almost no positive change. The really strange clue came when I turned the idle mixture screws all the way in and the engine actually sped up and was vibrating less. I now have it set to 1.5 turns on the main idle screw, 1/2 a turn on the secondary idle screw, and the idle mix screws are all set to 1/2 a turn out. tomorrow, time permitting, I will take out the questionable primary accelerator pump squirter and replace it with a new #31 squirter and I am hoping to finally be rid of the lean out on the primaries and add the new 6.5 PV that I also picked up today. |
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72 Gran Torino "sport"- in progress
73 Ranchero GT- Q code-460/C6/3.25 open rear |
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Rockatansky
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Posted: 25-January-2012 at 10:30PM |
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if you even out the primary to secondary idle speed screws it might end up idleing down lower, or at least be able to. if the primary (or secondary) throttle blades are open too much you won't be running off the idle circuit at all, it'll be into the intermediate circuit and the idle mix screws won't do jack
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72 GT Ute
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torkair wrote: