I just put my 575 Speed Demon carb on and it seems to be running rich. I bought this carb from another member that had the carb sent to Lars to be setup. So I know it is setup correctly.

I'm just wondering how I would lean the mixture out on the carb? The total timing is set to 36 degrees. Seems to run good but burns my eyes.

Which screws do I turn to lean it out?

 

hey,
I am the guy how sold it to you.
Is it rich at idle or WOT?
Thanks,
Joe

 

Rich at idle.

 

Hey capt.please post if you solve your problem I've got the same ,holley says to change power valve but my 750 did the same thing at idle I swapped down to a 650 no luck

 

where are the idle screws at?
what is your vac. reading at idle?

 

Try this:
If you have a vac. gauge, hook it up to the second vac. port on the passengers side. It am pretty sure that is direct vacuum. Next, turn one idle mixture scre in 1/2 turn. If the vacuum goes up, then turn it back out to the orginal setting and turn them all in 1/8 of a turn. Next, turn the scre in again 1/2 turn, if the vacuum goes up, then turn it back out to the original spot and then turn them all 1/8 of a turn in. Keep doing this until the vacuum goes down. They should be about 3/4 of a turn out, but do it by vacuum. I f you do not have a vacuum gauge do it by RPM. Whatever your idle speed is, make sure you keep that. If after you make the changes to the idle mixture, the idle speed goes up, then turn BOTH the primary andsecondary idle speed screws the same amount to reduce the idle.
Make sure your car is at operating temp, atleast 180.
If you have an automatic, which I think you do, then make sure it is in drive with the ebrake on and something in front of the wheels.
Let me know about results.
We will get it!!
Thanks,
Joe

 

I have a manual trans. I don't have a vacuum gauge but I turned all mixture scews in 1/8 turn and it seems to be better. I will go get a gauge this afternoon and try that.

Thanks for the help Joe. I will let you know how it goes.

 

My 650 BG Speed Demon also ran rich at idle when I first got it. I couldn't get it lean enough using the idle screws, so I started reading up on Holley/Demon type carbs. Found out that the idle system has its own set of jets called the IFR (most are machined holes and are not replacable). It determines the amount of fuel the idle circuit gets. I had to reduce the IFR's quite a bit to get the idle quality the way I wanted it. On carb's with seperate metering block construction like the Speed Demons and Holley 1450 double pumpers, there is an ideal place to put a V shaped piece of wire thru the IFR to reduce it size (cheap fix).... or you could drill and tap for removable IFR jets, but thats alot of trouble and $$$.

Not sure if you have the same problem, but just thought I'd pass along my experience.

 

let me know how things go.
we will get this!
Thanks,
Joe

 

I have not had a chance to try the vacuum gauge step yet. I will try to get to it tonight and let you know what I find.

 

OK I put a vacuum gauge on and set the mixture scews until vacuum dropped.

I also had to play with the timing and idle settings. How do I check the total timing. Do I set my light to 36 degree advance? Where should the timing mark be with the light set to 36 degrees?

 

timing mark on the balancer should be at 0 when the light is at 36.
Thanks,

Joe

 

So when the timing mark reaches 0 when I rev the motor, is the RPM that it is at when it reaches 0 the RPM that the total timing comes in? What RPM should it be at when the timing mark first gets to 0?

 

 

If you can't get your carb lean enough at idle with the idle mixture screws, your primarys are open too far and you're drawing fuel from the transition slot, which doesn't get regulated by the idle mixture screws, but DOES get regulated by the idle feed restrictors. Close the primaries so that only .020-.040 of the transition slot shows when looking from underneath. Then adjust idle speed with the secondaries.

 

For performance you want it all in by 2800 rpm's, check out Lars' tech papers in the tech section for setting up a performance timing curve

 

 

Chris,
You think his transfer slots are too far open?
Couple of related and unrelated questions:
1. Why adjust the speed with the secondaries? Why not both?
2. Does that cause an off-idle stumble or is that caused by the transfer slot being closed too much.
I am hoping to learn something.
Thanks,
Joe

 

Adjusting the secondaries will allow you get the necessary air into the engine without disturbing the relationship of the primary butterflys to the transfer slot. The transfer slots pour fuel into the engine compared to the idle discharge holes and they are much more difficult to tune. The relationship of the slot to the primary throttle opening also affects off-idle characteristics and can contribute to a lean/rich stumble.

So, I guess to answer directly...

1. You can adjust both, but try to avoid other tuning problems try to keep the primarys adjusted so that no more than about .040" of the transfer slot is exposed to prevent flooding the engine with fuel at idle.

2. Yes, it can. Of course, it also depends on your accelerator pump cam, shooters, jetting, ignition timing, etc. It's a system and all the parts have to be timed correctly for it to work in harmony. You can close the primaries to the point where no slot shows, which would probably give some off-idle, around town driving stumble when the throttle movement is slow and small. I can't see it being a real problem during spirited throttle motion, since the accel. pump and power valve dump SOO much more fuel into the engine than the transfer slot. Honestly, I've never tried closing the primaries that far...


I'm far from an expert in this, but if I'm off base, some of the others will certainly correct me.

 

The support team at Holley suggested that I leave .015 of the slot exposed on both the primary and secondary side. Then adjust the idle speed only with the primary. I am sticking with Lar's suggestion, but I thought that was interesting.
THanks,
Joe