Author Topic: off idle stumble/lean mixture  (Read 4536 times)

Offline forkitman

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off idle stumble/lean mixture
« on: August 26, 2016, 01:52:11 PM »
  I have an off idle stumble until about 2,000rpms.  APT adjustment does very little.  I notice that the power valve is not mov ing at 1500rpm.  Is this right?  It has 14hg. at idle.  Do I need a stronger spring under the piston?  Carb #17084227.  75 jets, 47 rods. XE262h-10 cam.  10/1 compression, big valve angle plug heads.  350/4speed 81 vette.

Offline Cliff Ruggles

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Re: off idle stumble/lean mixture
« Reply #1 on: August 29, 2016, 09:25:52 AM »
What 47 metering rods are  you using?  They should have a secondary stamp on them after the numbers....Cliff

Offline forkitman

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Re: off idle stumble/lean mixture
« Reply #2 on: August 29, 2016, 02:51:48 PM »

   The rods have 51M stamped on the main body.  The taper runs from 43 to 51.  I don't know why I thought they were 47.    This carb originally ran good on the 81 L81 motor with 73 jets. I put 75 jets in and it runs better, but the plugs show it to be lean still.  How do you know if you have the right spring in the power valve?  Thanks----Bob

Offline Cliff Ruggles

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Re: off idle stumble/lean mixture
« Reply #3 on: August 29, 2016, 05:32:36 PM »
M series metering rods are correct. 

We NEVER read plugs to determine rich/lean, impossible with this new fuel.

We tune for results, smooth off idle, and at light heavy throttle, thru transition, etc.

RPMS have NOTHING to do with the movement of the power valve, it moves with throttle angle/engine load, which you can not duplicate without actually driving the vehicle. 

A stronger spring lowers the cut-in point, but seldom the correct course of action to tune for light part throttle issues.  The PP spring is for tuning heavy part throttle and quick movements of the throttle which require increased fuel delivery.  The accl pump also helps these scenarios.

For light/part throttle operation and light load tuning do it with the APT system.  I prefer to determine the correct jet size FIRST, by doing some heavy part throttle driving, then go back and fine tune the part throttle/light load range with the APT.

If you can't come in range and the jet size is fine, they go with smaller metering rods (assuming too lean).  If it's rich, go with larger metering rods.

The camshaft you are using is not a user-friendly cam.  It may be contributing or even causing the low rpm issues.  I've had a number of engines in here with multiple issues, and the XE cams have been a common denominator more times than not.

The are on 110LSA, with short seat timing events and excessive opening/closing velocity for the valves.  "Chop" or irregular engine operation below 2000rpms is not uncommon with them, and is caused by the early intake closing point, combined with the short seat timing/quick valve events (bouncing the valves on the seats and some noticeable "reversion"). 

This certainly produces a nice sounding idle, if you like a little "lope", but I've seen those engine run rough up thru and past 1500rpm's or so until things would smooth out, with the only real contributor being the camshaft.....FWIW.....Cliff

Offline forkitman

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Re: off idle stumble/lean mixture
« Reply #4 on: August 30, 2016, 12:15:01 PM »
 
  Thanks for the info.  I am going to try and richen it up some more with jets and/or rods and see what happens.  I have had a hunch since I started this engine up that this cam was junk, but you would think that Compcams would know that.  WTF?  I appreciate your help.    Thanks
Bob

Offline Cliff Ruggles

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Re: off idle stumble/lean mixture
« Reply #5 on: September 01, 2016, 09:06:06 AM »
It's no secret I'm not a big fan of those camshafts.  Went down that road when they first came out, and once again not that long ago, and both times very disappointed in the results.

Aside from the Thumpa and Mutha-Thumpa super tight LSA grinds, I get more complaints in here about XE cams than any other.

I know it's not good news, and doesn't help the situation, but hopefully with careful tuning you can get past the negative traits thrown into this deal by the XE camshaft.......Cliff

Offline forkitman

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Re: off idle stumble/lean mixture
« Reply #6 on: September 02, 2016, 08:53:16 AM »

    This is all a hobby, a labor of love.  Sometimes I think I need a different hobby,LOL.  Thanks again for your advice.
    I will keep you updated .     Bob

Offline jwilson645

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Re: off idle stumble/lean mixture
« Reply #7 on: November 07, 2016, 01:58:45 PM »
M series metering rods are correct. 

We NEVER read plugs to determine rich/lean, impossible with this new fuel.

We tune for results, smooth off idle, and at light heavy throttle, thru transition, etc.

RPMS have NOTHING to do with the movement of the power valve, it moves with throttle angle/engine load, which you can not duplicate without actually driving the vehicle. 

A stronger spring lowers the cut-in point, but seldom the correct course of action to tune for light part throttle issues.  The PP spring is for tuning heavy part throttle and quick movements of the throttle which require increased fuel delivery.  The accl pump also helps these scenarios.

For light/part throttle operation and light load tuning do it with the APT system.  I prefer to determine the correct jet size FIRST, by doing some heavy part throttle driving, then go back and fine tune the part throttle/light load range with the APT.

If you can't come in range and the jet size is fine, they go with smaller metering rods (assuming too lean).  If it's rich, go with larger metering rods.

The camshaft you are using is not a user-friendly cam.  It may be contributing or even causing the low rpm issues.  I've had a number of engines in here with multiple issues, and the XE cams have been a common denominator more times than not.

The are on 110LSA, with short seat timing events and excessive opening/closing velocity for the valves.  "Chop" or irregular engine operation below 2000rpms is not uncommon with them, and is caused by the early intake closing point, combined with the short seat timing/quick valve events (bouncing the valves on the seats and some noticeable "reversion"). 

This certainly produces a nice sounding idle, if you like a little "lope", but I've seen those engine run rough up thru and past 1500rpm's or so until things would smooth out, with the only real contributor being the camshaft.....FWIW.....Cliff

Cliff, is there any particular brand cam that you recommend? I have the XE cam also and am having a helluva time getting my carb dialed in. I know that isn't the only issue but I don't really like the way my truck idles with it anyway. It's a 350, bored .040 with stock 99 Vortec heads and stock exhaust manifolds.

Offline Cliff Ruggles

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Re: off idle stumble/lean mixture
« Reply #8 on: November 09, 2016, 07:10:13 AM »
What is the compression ratio and which XE cam are you using?.....Cliff

Offline 77cruiser

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Re: off idle stumble/lean mixture
« Reply #9 on: November 09, 2016, 08:48:53 AM »
What's the timing initial & mechanical curve setup?  Any mods done to the carb?
Jim

Offline jwilson645

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Re: off idle stumble/lean mixture
« Reply #10 on: November 10, 2016, 07:24:23 AM »
What is the compression ratio and which XE cam are you using?.....Cliff

Compression is 9.56 and I'm currently using the XE256H

Offline Cliff Ruggles

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Re: off idle stumble/lean mixture
« Reply #11 on: November 11, 2016, 03:50:24 AM »
The XE256 cam is WAY too small for 355cid at 9.56 to 1 compression.  A stock replacement cam has 274 degrees seat timing.  Small cams close the intake very early, especially on a 110LSA.  I'd look at something with more advertised and .050" duration on a 112-114LSA.

The generic 214/224/112 cam from Summit would be a very good choice for what you are doing.....Cliff

Offline jwilson645

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Re: off idle stumble/lean mixture
« Reply #12 on: November 11, 2016, 05:03:45 AM »
The XE256 cam is WAY too small for 355cid at 9.56 to 1 compression.  A stock replacement cam has 274 degrees seat timing.  Small cams close the intake very early, especially on a 110LSA.  I'd look at something with more advertised and .050" duration on a 112-114LSA.

The generic 214/224/112 cam from Summit would be a very good choice for what you are doing.....Cliff

Thanks for the info, Cliff.  The .466 lift of that Summit cam isn't too much for stock Vortec heads is it? I've head the stock springs can only hold up to .450 cam lift.

Offline Cliff Ruggles

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Re: off idle stumble/lean mixture
« Reply #13 on: November 19, 2016, 04:26:57 AM »
The only way to know how much lift a spring can handle is to test it.  Most stock SBC springs are fine with .460" lift or a tad more, but in all cases they should be checked with the retainers being used to see if they have enough room.

You also must check retainer to seal clearance.....Cliff