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 Post subject: Idle problems
PostPosted: Thu Apr 16, 2009 2:25 pm 
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LUVTruck.com Lifer

Joined: Sun Jun 12, 2005 10:42 am
Posts: 684
I have been fighting this motor from the beginning. Its a 355 10:comp GM 486 heads with major work, Vic Jr intake, Comp cams 292 .501 hyd flat tappet, Factory HEI with moroso recurve kit. Initial timing set at 12* total 36* at 3000 RPM. It has a fatory vac advance but there isnt enough vacuume at idle to make it work. I bought a crane adjustable vac can last night and havent had a chance to put it on yet.

The truck runs good as long as it isnt at idle. I stop at a signal light or stop sign and it wants to die. If you dont give it any throttle and put it in neutral or park it will idle fine, after its warmed up and you take it out of gear while its idling sometimes it will rev up to 1200-1500 RPM on its own and will not idle back down. I dont know if I should be looking into the carb or the ignition system. Could my current factory style Vac advance be doing weird stuff with the timing?


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 Post subject: Re: Idle problems
PostPosted: Thu Apr 16, 2009 5:40 pm 
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lives at LUVTruck.com

Joined: Sat Feb 21, 2009 12:57 am
Posts: 357
Location: Minden, Nevada
That cam may require a little more initial timing to idle. Try setting it at 16* and readjust the carb. Also check the secondary throttle plates to be sure they are closed, and for any vacuum leaks. For vacuum leaks you can get a spray bottle of water and spray around the carb and see if the idle changes. Also check the vacuum line going to the PB brake booster and trans.

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 Post subject: Re: Idle problems
PostPosted: Thu Apr 16, 2009 6:34 pm 
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Joined: Mon Aug 28, 2006 11:54 pm
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Location: Pelahatchie, MS
What is the convertor stall speed?

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 Post subject: Re: Idle problems
PostPosted: Thu Apr 16, 2009 6:50 pm 
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LUVTruck.com Lifer

Joined: Sun Jun 12, 2005 10:42 am
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I have checked for vac leaks and cant find any. I have initial set at 12* and it doesnt want to crank over when it gets hot 17 and it probably wouldnt stat but with the vav advance timing should be around 18-20 at idle. Stall converter is a bit shy of 3000 RPM


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 Post subject: Re: Idle problems
PostPosted: Thu Apr 16, 2009 8:24 pm 
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Location: Hermiston,Oregon
The vac adv is not supposed to be working at idle if it dose then you have it hooked up wrong (it's supposed to be at the ported vac. fitting not manifold vac.)
It sounds like maybe the idle feed circuit in the carb may not be rich enough also check and mke sure the timing marks on the dampner are correct with the #1 piston at TDC.

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 Post subject: Re: Idle problems
PostPosted: Fri Apr 17, 2009 4:55 am 
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I think the convertor is too small...

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 Post subject: Re: Idle problems
PostPosted: Fri Apr 17, 2009 9:18 am 
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LUVTruck.com Lifer
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Location: Lost Causes, NM
mytmouz wrote:
I think the convertor is too small...


I agree.... to much cam to much intake. not enough converter.

A.j.

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 Post subject: Re: Idle problems
PostPosted: Fri Apr 17, 2009 9:31 am 
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LUVTruck.com Lifer

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I think the converter is a big part of a two part problem. As far as vacuume advance goes ported vac is for emissions and works well on daily driven stock motors. ported vac would give me way too much timing. The way I have it set now with an adjustable unit and limiter plate really helps throttle response and runs lightly cooler. I would say it would get better fuel econemy too but I am not really interested in checking that. When I go from Idle or cruise (high vacuume) the timing is is up at about 22* when the throttle opens the vac advance goes away and mechanical kicks in keeping me in the 36* zone. I was skeptical at first and after reading up on it I decided to try it. I will tell you and a swear by this just changing over to manifold vac (limited to 8 crank shaft degrees) and limiting timing to 34-36* it made a huge difference in the buttometer.

I think there is possibly a small vac leak under the intake manifold due to the machinest not milling it right. I drove it last night after changing vac units and it was better I didnt notice any weird fluctuation in idle and it did idle in gear on its own but needs more converter I think.


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 Post subject: Re: Idle problems
PostPosted: Fri Apr 17, 2009 9:41 am 
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LUVTruck.com Lifer

Joined: Sun Jun 12, 2005 10:42 am
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This good info on timing and vac advance it kind of scratches the surface on what I mentioned but not to over bearing.

http://forum.grumpysperformance.com/viewtopic.php?f=70&t=1135


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 Post subject: Re: Idle problems
PostPosted: Fri Apr 17, 2009 11:58 am 
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LUVTruck.com Lifer
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Location: Lost Causes, NM
I personally wouldnt run a vac advance system on a car with that much cam and intake combo, what was your actual vac reading? I bet its bouncing around alot and causing your surging issues. hooking it up to manifold vacuum is just compensating for the excessive overlap and low dynamic compression. The combo is being inefficient at low rpm and wants alot of timing to level out. combining it with too tight of a converter will just make the problem feel worse. Not to mention you have enough cam and intake to easily go past 5000 rpm- which is where a factory GM HEI will scatter. I would almost bet you could go as high as 40* total timing and that engine would really wake up.

i think you have a mis-match of parts.

A.j.

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 Post subject: Re: Idle problems
PostPosted: Fri Apr 17, 2009 1:50 pm 
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Location: Hermiston,Oregon
I run a HEI but i modified the advance curve in it and run a performance module and coil and no problems at 6000 rpms but i also got rid of the vac adv.

The vac adv was for fuel economy only and was designed to work at part throttle.

How was the cam installed because it sounds like it could be advanced to much (6*+)because that cam would be hard difficult to pull 19"s of vac.

This just my input as it is difficult to do diagnostic by proxy.

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 Post subject: Re: Idle problems
PostPosted: Fri Apr 17, 2009 3:39 pm 
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LUVTruck.com Lifer

Joined: Sun Jun 12, 2005 10:42 am
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I have a MSD module and coil in my HEI and it runs up to 6000 RPM with out a problem. If I didnt drive my truck on the street I wouldnt have a vac advance. It really help out at part throttle. I installed the cam strait up which gives it 6* of advance and it only makes 9.5 " of vac not 19.


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 Post subject: Re: Idle problems
PostPosted: Sat May 02, 2009 4:07 am 
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da LUV masta

Joined: Sat Apr 12, 2003 11:50 pm
Posts: 582
Location: San Leon Tx.
Idle fluctuation of 1200 to 1500 rpm's sounds like a vacuum leak. It could also be other things, but I think that's where I would start. If you do have a leak, it might be that the heads were milled enough to where the intake should be milled to match or different head gaskets. The long red plastic tube on WD-40 works really well to get into tight places to spray between the heads and intake. When it makes a quick seal the engine will rev up a significant amount. The end gaskets on the intake slide around easily and you can get a vacuum leak. Been there done that, long hours into the wee hours of the morning, tired but won't stop cause I want to get it running. Ok, my shot in the dark.

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