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Old 07-03-2011, 05:17 PM
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Good idle, bad in gear

Hey all;

I swapped the Demon carb that was on my 400 off to an Edelbrock and have been getting much better results with it. It's no longer stupid rich and running decently well. My biggest problem however, is that at idle in park, I have her about 1000rpm. When she's in gear (auto) idle drops really low. I've raised the idle on the carb up some so it's higher in park and higher in gear, but I know that's wrong and need to get this figured out.

Engine is sbc400, stock bottom end. Aluminum heads, headers, Edelbrock 4bbl carb (I believe the 650 or so but could be wrong), high rise intake, 282.0 / 290.0 cam, and 2300-2700 torque converter.

Couple things to add. I was setting timing with the HEI hooked up to the vac advance, and after doing some searching here... you're not supposed to do that. When car was at 12btdc, she ran like crap. I changed around some and with the vac still on, she's running great at about 25btdc. Does this sound about right? Also I'm reading that the dizzy should be hooked to the constant, same as the trans instead of the advance. Is this right as well?

So the question really is, should I keep playing with the timing to get her to idle in gear, more so than the carb? Thanks

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Old 07-05-2011, 01:08 PM
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Quote:
Originally Posted by F-BIRD'88
read my posts on recurving your distributor for a big cam.

You may have the wrong converter. Get a 10" 3000+ stall.

You want a minimum of 24deg base initial at idle (not including any that vac advance adds at idle) You must modify the distributor advance stop to get this. HEI is simple, search my posts.
Ok, I've been reading a lot and searching a lot. Just still confused. From what it seems, I will need to get different springs for the dizzy to allow for more/less vacuum at idle, correct? Vacuum does drop quite a bit in gear, I want to say it's at about 6" in gear, about 12 to 15" in park. I also basically lose my brakes when driving and rpms drop to idle. Also, there's no change in vac when using a gauge and adjusting the carb.

I also tried hooking the distribuator to the manifold vac, and there was no change in anything.

Again, thanks for any help.
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Old 07-05-2011, 04:11 PM
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As mentioned ultimately you may want a bigger stall so that in gear your converter is not loading up until then you may need to live with the drop in RPM from park to drive. It will also help the cam reach peak torque properly

For timing,,,When you raise the base timing up you need to limit the mechanical advance mechanism in the HEI. Most HEI's will add 18-22 deg of mechanical advance in out of the box form.

So if you set your base at 20 lets say for sake of round numbers then add your mechanical advance once at 3000RPM you are at approx 40-42 deg advance already, then add another 16-18 from vacuum advance and you are at 56-60 Deg total,,,too much.

Find out what you have using a timing light and then post the numbers. First disconnect the vacuum advance and plug the hose, what is the base timing? at say 900RPM. Then rev the motor slowly up in RPM with the timing light on it (wheels blocked, park brake on) note the RPM that it stops advancing (should be done by 3500) and record the timing BTDC it should have increased. Then finally let the motor idle and hook up the vacuum advance to full manifold vac and take a reading. This will tell you/us what you have.

As mentioned you will most likely need to limit the mechanical advance inside the HEI as well as the vacuum advance. The springs do not limit the amount of advance they simply allow the mech advance to come in faster (lighter springs) as compared to engine RPM or slower (heavier springs). You can use bushings or a mechanical stop to set the limit depends on the HEI. This plate is used to limit the vacuum advance or fashion one up, but for 4 bucks?

http://www.summitracing.com/parts/CRN-99619-1/

At idle you could live with 30-32 deg BTDC. This would include the base timing of say 20 Deg and then another 12 added via vacuum advance on manifold vac,,,try it. You want to prevent any mechanical advance at idle so try and keep the RPM below 900 at idle.

You need allot a patients when you piss around with distributors, it really is trial and error until you have done it a few times,,,take your time it pays off
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Old 07-05-2011, 09:44 PM
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Got to get to the garage today for a little.

When disconnecting the vaccum line from the distrib, the car doesn't want to idle. Was showing about 14btdc at around 1000rpm. I couldn't go up since the parking brake is disabled right now and my bro wasn't going the best job holding the brake down.

Motor with vac hooked up at idle in park is about 25btdc. I tried advancing some to see if it would hold idle better in gear, and that came back negative. Died prettty quickly.

I ended up just ordering the plate... just easier since it'll be here before the next time I get out to the garage and had an order I needed to place anyways. Going to try that. So basically, I just installed that under the weights, it looks like? And it'll block the vacuum from being there at idle? Then I just raise the idle on the carb?

This is very aggrevating as I just want it to run already with no issues. Computer control FTW!
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Old 07-06-2011, 10:02 AM
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You would be best advised to set the base and mechanical timing first, then set/limit the vacuum advance to about 12-14 with the plate. The engine idle vacuum when connected finally to the vacuum advance canister will add advance at idle this is OK and should bring your idle BTDC to over 30 once you set the base or initial at 20ish.

The plate does not block the vac advance at idle it simply limits the maximum amount of advance the vacuum canister can add.

Look for about 20 BTDC on the base timing then add another 14-16 with mechanical for a total of about 34-36 to start. The vacuum advance will pull in and out based upon engine load.

Remember once the mechanical advance limit is set it can not be changed unless you get back in there so it is important to know how much base timing the motor wants then set the mechanical limit to get 34-36 total at approx 3000RPM.

ie if your motor likes 16 deg BTDC base then you can limit the mech to add 18-20, if the motor wants 22 deg BTDC base then you will need to limit the mech to 12-14 etc etc.

You rev the car in park to check the mechanical advance, not in drive (might be reading this wrong).

What kind of HEI? stock GM or after market?
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Old 07-06-2011, 10:33 AM
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HEI is a basic aftermarket. No brand on it, and got the car with it already there.

Maybe I'm not understanding how this is going to help my idle. I understand the plate is going to stop vac from coming in. But how is that going to help the vac at idle?

The example of 16btdc is without the vac, correct?

Also, is there any changes that need to be made to the allen key inside the vac advance? Lastly, would you recommend full time vac or ported for this?

Again, thanks for your help.
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Old 07-06-2011, 02:18 PM
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I've seen the block out screw picture before but didn't get too far into it. After doing some searching, it seems like that's what I want, no? Not the entire lockout since I'm not boosted, but just limit it, correct?

I'm trying not to have a switch on the dash to get her started. Hopefully she'll be fine with some limiting.
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Old 07-06-2011, 03:20 PM
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OK I will try to explain one more time, it is difficult to describe in detail all the in and outs, as we all know. Yes just limit the mech advance don't lock it out.

3 amigo's of timing

1) Base timing -this is set by turning the distributor CCW or CW and then it does not change once you lock down the dist hold down clamp

2) Mechanical advance - this is controlled by the moving spring plate and weights inside the dist. You can set or limit how much advance is added by using bushings over top the pins that slide in the top plate slots or as Fbird shows use a mechanical stop to limit the spring plate travel. The use of a screw to prevent any mechanical advance all together is also shown in the pic as well but you don't need to go there. The side kick to this mech advance is the tension of the springs used on the spring plate posts. Heavier springs hold back the mech advance as compared to engine RPM since the spring plate moves via centrifugal force, lighter springs allow the mech advance to come sooner.

3) Vacuum advance - this is controlled by the vacuum canister which is activated by the engine vacuum levels. The engine vac works upon the canister diapahram to pull the actuator rod which in turn rotates the magnetic pickup and adds advance. At high engine loads like when you rug it the vacuum level drops and the vacuum advance is all but gone. At light engine load like when cruising the engine vacuum is high and the advance is added. The canister can be hooked to full manifold vac or ported carburetor vacuum but we won't get into that just yet I don't have all day and guitar joe is on holidays lol. Some cans have an adjusting screw to set the spring range of the canister, that is a topic that we can get to once you master the mechanical advance settings...hopefully

For your application:

First work on setting up the mechanical advance, never mind the vacuum advance. When the HEI's are un modified they work with un modified engines. This is because the base timing of these engines is low say 10 Deg BTDC. When the engine revs up the mechanical advance will add say 22 deg in stock form, this is OK cause the motor ends up with 32 Deg BTDC total and is happy. Now take that stock HEI and increase the base timing to 20 and don't limit the mech adv, then add the same 22 deg adv and the motor is seeing 44 total not good. So do you see why you need to limit the mechanical advance now? We are telling you to run 20-24 deg base timing, so if you don't limit the mech advance your hooped.

I asked before what the dist was adding for mechanical advance, did you test it out?
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Old 07-06-2011, 03:35 PM
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Quote:
Originally Posted by Custom10
For your application:

First work on setting up the mechanical advance, never mind the vacuum advance. When the HEI's are un modified they work with un modified engines. This is because the base timing of these engines is low say 10 Deg BTDC. When the engine revs up the mechanical advance will add say 22 deg in stock form, this is OK cause the motor ends up with 32 Deg BTDC total and is happy. Now take that stock HEI and increase the base timing to 20 and don't limit the mech adv, then add the same 22 deg adv and the motor is seeing 44 total not good. So do you see why you need to limit the mechanical advance now? We are telling you to run 20-24 deg base timing, so if you don't limit the mech advance your hooped.

I asked before what the dist was adding for mechanical advance, did you test it out?
Thank you for the breakdown. I think I understand it now. Very useful and paints a clearer picture.

Base idle no vac was at 14btdc @ 1000rpm in gear. I couldn't get it any higher since I have no parking break right now and brother was holding the brakes.
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Old 07-06-2011, 03:48 PM
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you can get it higher just loosen the hold down clap of the dist and turn the dist CCW then recheck it you don't need brakes for this , tell your brother to go get beer and ice for the cooler it may be a long day lol
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Old 07-06-2011, 08:12 PM
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You do realize that you are also going to have to adjust the idle speed as you take timing out of the engine. As the engine gets more timing, the idle speed will go up, you will have to dial it down at the carb. An automatic should usually idle around 700-800 in gear.
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Old 07-07-2011, 08:19 AM
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Quote:
Originally Posted by Custom10
you can get it higher just loosen the hold down clap of the dist and turn the dist CCW then recheck it you don't need brakes for this , tell your brother to go get beer and ice for the cooler it may be a long day lol
But in order to do this test properly it has to be in gear, no? That's what I was talking about.

Cam is Erson Cam E110822
Intake/Exhaust Duration
282.0 / 290.0

I've been playing with the idle speed each time I made adjustments to see if anything would change. It was pretty much always the same in idle, give or take a couple hundred rpm, UNLESS at park idle was 2000+
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Old 07-07-2011, 09:18 AM
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No it does not need to be in gear yet,,,advancing the timing will increase idle speed, you may need to look at the carb settings for base idle adjustment and over all condition, thats another story so for now,,,

back up take it one step at a time, if your idle is at 2000 in park you need to focus on getting it at 800-900 in park and the base timing at about 20 Deg BTDC at the same time. Never mind the other two aspects of mech and vac advance for now. Put a vacuum gage on manifold vacuum while you are trying to set the base timing and base idle, disconnect the vacuum advance at the dist and use that hose to connect to the gage. Vacuum leaks are going to through the entire tuning procedure off, best to check what its pulling for vacuum, if you have a leak then your idle speed typically increases and in general your tuning efforts are for not. Another good thing to do would be to verify TDC on the damper/balancer to make sure you are setting the timing correctly: it all helps

http://www.crankshaftcoalition.com/w...op_dead_center

back to the tuning without having to remove the carb,,,

Turn the carb adjustment screw all the way out then back in 2 turns once it the screw makes contact with the throttle plate linkage. Put a timing light on it, leave the dist hold down snug but not tight, fire it up and check the timing in park, turn the dist to get 20 deg BTDC while adjusting the idle screw to keep the RPM at 8-900. Keep the light on it, connect the vacuum advance, tee off and leave the vacuum gage connected to the vacuum line note the timing now it should have advanced when you connected the vacuum line to the dist, the idle should have also gone up, back off the idle adjustment screw so that you have 8-900 rpm at idle with the vacuum advance connected.,,,crack a beer and tell us that its now running fine at idle in park, remember that with the vacuum advance connected at idle this will add advance so you could be seeing over 30 deg timing at idle, that is the whole idea as it will help keep the idle steady when you drop it in gear. What is the vacuum gage reading at this point? what is the timing reading?

hope this helps I am out if ink for now,,,if you can get through this part you can look forward to doing it all over again after you pull the distributor out to modify the vacuum advance limits and set the mech adv limit and curve which is much easier on the bench IMO
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Old 07-07-2011, 09:40 AM
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TDC is perfect. I've checked this out numerous time. Even pulled intake and a head in the past to double check my work. I'm 100% for sure, TDC is TDC on the dampner.

Idle isn't at 2000rpm @ idle. I've gotten it up there previously though in order to see if when in gear, if the idle in gear would be higher then. Idle in park I try to keep at just under 1000rpm right now.

So right now, it's about 9000rpm at idle @ 25btdc. Vac (last time I checked) was at about 12" in park, and about 6 or 7" in gear if i recall correctly.

I didn't get a chance yesterday to work on her, but will be out there tonight.
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Old 07-10-2011, 05:35 PM
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Sorry it took so long, but finally got some answers for you guys. Again, thank you for all the help, tips and advice you've given me.

Vacuum at idle in park at 800-900rpm @ 25btdc is about 10-12". Playing with the adjustment screws on the carb, there is little to no change. They're about 2.5 or so pulled out bottom. When dropped in gear, vac goes to about 5" and idle drops to 300-500rpm and is very shakey and sucky.

I installed a screw on the mechanical advance to limit is (not entire block), but has very little movement. This did help some, as it's not dying as often, but still sucks. Taken the Monte for a spin a couple times, and she drives like a champ until coming up to a stop sign. Brakes basically go away and she wants to die, or does.

I really am thinking the converter is being the culprit here now. Even with more timing set, she still doesn't want to idle well. Question, is the 34-36btdc in gear or park and with vac or no?

Also, is there anything they make to add more power to the brakes? Like a booster for the booster? Something for when the rpms get low, it still gets vac from somewhere to pump in?
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