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Old 04-10-2006, 12:02 PM
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Low idle vaccum in gear on SBC 350

I have a bit of a tuning issue with the 355 chev in my 48 ford PU. Motor was a crate motor that I bought from a builder. This is kind of long, sorry. Don't know how to shorter it effectively.
dart IE 200cc heads.
Comp 292H cam, 244, 244 .050, 110 LSA
10.5:1 comp ratio
670 holley avenger (from the previous motor), 6.5" PV.
Performer RPM intake, headers, flowmaster .
700R4 with 2500 or so stall. 3.73 gears

My problem is that I have always had a tough time to get this motor to idle slowly but not die when in gear. If i set the idle to 1000RPm, when I put it in gear, it will only idle about 600-650 RPM and pull about 6"Hg. I guess this probably makes the PV open and just makes things worse.

I have fooled and fooled with the timing, and more advance makes it idle better, but at the expense of too much total timing, and its hard to start when hot, neither of which I can live with very well.

So, as a tuning aid, I added a Crance cams adj. vac. adv. on this motor, and have it all in ASAP and a manifold vacuum source like the instructions say to start with. This definetely improves the idles, but when the motor slows down, the vac. adv. lets bunch of the advance go away, and the motor suffers more because of this as well. I have the same setup on my 406 Camaro, and it works amazing, but its a 6 speed manual.

Also, even with a electric choke, the thing won't hardly run after the choke comes partially off and before its warmed up. Runs great when you first start it, then it will just die. Again the camaro has no choke (750 cfm 4160) and i don't have this problem (and its a much hotter motor).

Anyone have any ideas? I sure would appreciate a couple of opinions.

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Old 04-10-2006, 12:22 PM
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Quote:
Comp 292H cam, 244, 244 .050, 110 LSA
I'm going to guess this is the culprate. That's a fairly healthy cam, and requires a fair bit of idle speed to keep it alive at low idle loaded.
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Old 04-10-2006, 01:06 PM
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That's kind of what i was thinking too. But I'm not ready to give up on that cam yet, since it is otherwise excellent.

I could set the idle higher, but then it really kicks when it goes into gear.

I'm going to try a 4.5 PV and see what that does just for reference.
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Old 04-10-2006, 01:22 PM
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idle

You say you messed with the timing,,,give us the numbers..Initial, total, how much in the can???

The PV will do zero to fix the problem... It feeds the booster circut and not the idle circut so it will have no effect on what the carb is doing at idle...

Do you have a pvc valve on this engine???

Are the transfer slots in the carb set correct???


Keith
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Old 04-10-2006, 02:12 PM
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Quote:
Originally Posted by hotrodf1

So, as a tuning aid, I added a Crance cams adj. vac. adv. on this motor, and have it all in ASAP and a manifold vacuum source like the instructions say to start with. This definetely improves the idles, but when the motor slows down, the vac. adv. lets bunch of the advance go away, and the motor suffers more because of this as well.
If you have the vacuum connected to FULL vacuum source it should not go away when the motor slows down. It should only go away when going to WOT.

Try setting your Total Mechanical Timing to 38 degrees with the vacuum hose disconnected and plugged. Re-connect after setting the timing to FULL vacuum source. Set your idle RPM with the trans in Drive to around 650-700.

Adjust the air/idle screws to yield the highest vacuum possible at idle.
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Old 04-11-2006, 06:28 AM
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Thanks for the info!

The timing numbers I have tried are from 16 to 24 deg. initial, with the stock style advance giving about 20 -21 degrees.

I think the can is dialed back to about 16-18 degrees. I might try taking a touch more out, but the thing gets pretty good mileage so I hate to change that.

I do have a PCV valve on this motor and power brakes. I have tried disconnecting both, and no difference.

Thats a good point about the PV. My neighbor actually has gotten that advice directly from Holley a couple of days ago.



As far as the vacuum going away, the can is set as low as it can go, IE 9"Hg (or so) pegs it out. But, when the motor slows down in gear, I have seen 6"Hg - thus some of the timing the can was adding is now removed, which slows the motor down, which takes out more timing, and so on. Pretty much kills it.

But I will try your suggestion Frisco, haven't done it exactly that way.

I am wondering if a Summit cam like the 1106 with 114 lsa would do better. I wonder what desktop dyno would say. Will have to check it out.

Thanks again.
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Old 04-11-2006, 07:12 AM
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The high overlap cam is the real problem. That's a big cam for a power brake car! Check for vacuum leaks around the intake. Your vac advance should be connected to the manifold somewhere, not the carb. If it is connected at the carb, make sure it's BELOW the throttle plates (like on the spacer between manifold and carb). Set the idle with the car in gear and your foot on the brake. You may have to set it at 800 rpm with that cam. Yeah, it will jump in gear, but you may just have to live with it.
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Old 04-11-2006, 09:04 AM
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this is a fix, not a cure,

mount a carb linkage A/C compressor-motor idle speed control solonoid to your brake lights switch,
foot on the brake in gear=solonoid "on"=higher rpms in gear (helps the power brakes too)

check "simple" first?....retorque the intake manifold and carb mount? it's the 6Hg that's killing you,
if you had an open plenum intake and 750 carb and 244 cam, then yeah, 600rpm/6Hg (use the fix above or 1,000rpm idle)

I'm stumped, sounds like you did all correct, plan "B"=less cam?
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Old 04-11-2006, 10:20 AM
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I do have the vac. adv. connected to the below plate source on my carb so I should be okay there.


That's a good thought on retorqe for the intake. Think I'll try that. Haven't checked since I put the intake on a while back. Also have noticed that as the motor gets warmer the idle speed increases. Say at 180 it idles at 1000RPM, when the gage says 200, it will idle at 1200 RPM. Does that sound like a leaky intake to anyone? This is a funny issue that has always plagued this motor.

I know some guys use propane to test for leaky intakes, other carb cleaner. Any other alternatives? I don't like the carb cleaner method, and I don't have any way to spray propane from my grill tank. Besides I would probably blow myself up.

BTW, at 1000RPM it idles around 13-14Hg, which I would think would be okay for power brakes. Guess maybe I will have to set the idle higher to ensure that my vacuum stays up far enough to keep the vac. pot pegged.

Hmmmm more playing to do tonight.
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Old 04-11-2006, 10:40 AM
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Folks have mentioned changing out the cam, since its timing is likely the problem. But the alternative of increasing idle speed should not pose too much of a problem if you change out the converter. It should definitely reduce the kick. If your stall speed was 3000-3200, a 1000 RPM idle should not be an issue. And you'd have very little drag on the motor when you went into gear at about 750, hence you might get away with a slow idle (like you're at) but with less load on the motor, your vacuum would improve.

Pat
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Old 04-11-2006, 10:49 AM
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more likely the idle increase is due to the intake manifold is getting hotter helping keep the gas/air in suspension with the low Hg values =better burn
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Old 04-11-2006, 07:28 PM
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rhoads has some new lifters than will give you up to 5 more inches at idle so they say anyway. something to look at.

i have a new 383 holley 750 double pump, mecanical secondarys. i have 15-16's at 1000 rpm out of gear and pull about 11-12 at 750-800 in gear. if this helps any. my cam is 224 intake @.050 with .520 lift and 232 exhaust and .525 lift. this cam is a roller and the smallest my engine builder has.
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Old 04-11-2006, 07:45 PM
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The propane method that I use, is those 1lb. blue bottles..non refillable...plumbing torch...I take the burner end off and the little jet that's in the brass neck...so it almost looks like a blow gun...I put a rubber hose on the end of the brass tube and slightly crack open the valve, just a small hiss, and locate leaks with it...don't light it...and don't smoke around it...and don't leave it open...and waste the fuel...one can usually lasts about 3 yrs with weekly usage...just my 2 cents worth...Cheers!
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Old 04-12-2006, 06:48 AM
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THanks for the tip on the propane Nevermind.

Just an update - and again thanks to all that give me suggestions!

Last night I started fooling with stuff again. Basically what I ended up figuring out was that I needed to advance my timing about 2deg., turn the idle screws out more, and definetely use the manifold vacuum source (like I had originally thought). One thing I should mention is that I ended up putting a vacuum canister on this thing. This way putting it in gear still drops the vacuum some, but I have a little on reserve and it somehow manages to keep the vacuum at a level that pegs the vac. adv.. Before when the motor would slow down as you put it in gear the vacuum would fall briefly the vac. can would let loose of a little timing, which would tend to stall the motor. I believe the canister helped even the idle out as well. I've never used one before, but they're pretty cool. and it was FREE! (off my old junk 83 chev PU - i knew it would be useful someday).

I also put a 4.5" PV in the carb which seemed to help keep it from loading up when I drop it in gear. Can't really verify that, since I've changed other settings as well, but it seems to be happier.

So now it idles happily about 1000 RPM with like 35deg timing, and only drops to 800 RPM in drive. Sweet!

Now the only problem is it smokes quite a bit while in idle! But the throttle response is very nice and it cruises very smooth. I guess this motor just has to run rich because of the reversion in the intake due to the big overlap cam?????
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Old 04-12-2006, 07:21 AM
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one more "check simple first"....cause it isn't mentioned in the thread....disconnect and plug all the vacuum ports (tranny mod, power brakes, etc) and test in drive to be sure they are "perfect".......every tiny bit of Hg will help

RE:your first post: runs crummy as the choke decreases=same as idle increase=cold manifold and leaner-less rich choke mix,= turn the choke cap "richer" so it will stay on longer =warmer manifold=helps a little?
(again it's the 6Hg that's killing you as the choke decreases)
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