Can't get it to idle right! (Part 2)

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Valkman

Valkman

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I posted the in the electrical & ignition section a while back you can go here for more info:
Can't get it to idle right!

Basically what have is a '73 340 with J heads, a seemingly wild cam solid lift cam, RPM intake, Eddy AFB 750 (I also used an AVS 625 with no improvement) with an electric choke, a Mr gasket light spring replacing the big one in the stock electronic distributor, Crane 1.5 roller rockers, a Stock mechanical fuel pump (pumping a consistent 5-6 psi)

I haven't had any luck since getting it to stay at a consistent idle!

It seems to do best at when I set the timing 35 deg. total advance.
I've done a compression test and it has 150 psi in all cylinders
I tried to measure the valve lift with dial indicator, but I could never get a consistent reading so I use a ruler to get a somewhat of a ballpark reading (see pictures). @.647 and intake and .653 exhaust did I calculate this right? If it is how does this engine even run?


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What is the timing at idle. That will want to be locked out is my guess. If it has any curve in it, it's likely not going to like it.
 
It surely appears to need longer pushrods....
 
do you have an adjustable fuel regulator? If so try to lower fuel pressure, edelbrock carbs don't like much over 5 psi and most gauges are not very accurate. Or watch down the carb at idle for fuel dripping to see if it's pushing past the needle and seat. Also that is not an accurate way to measure lift, if the cam has over .600 lift, it most likely has lots of duration, none of that works well with cast iron exhaust manifolds. Also where are you measuring manifold vacuum? Needs to be under the carb throttle blades, you said 12 inches at idle and 22 revving it up? Your vacuum should drop as you rev up the engine.
 
Also solid or hydraulic cam? As said, pushrods are too short if the rockers are adjusted correctly, and that also brings up a good point, as asked by AJ, how did you adjust them?
 
gtsdude said:
Also where are you measuring manifold vacuum? Needs to be under the carb throttle blades, you said 12 inches at idle and 22 revving it up? Your vacuum should drop as you rev up the engine.
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GTS
this is only true of very small camshafts.Like factory teener cams, or slantys.
All performance cams I have tuned (under 250*@050), will idle with low vacuum(the bigger the cam,the lower the idle vacuum) and increase as rpm goes up. Depending on the size of the cam, they will vacuum peak from 1500 to around 2400, or perhaps a little higher.
The spark-port will follow this almost exactly, being a little sluggish at the start, but usually catching up by 1500ish.
This lowest rpm at which the vacuum peaks is the lowest rpm that the engine first becomes efficient. Reversion in the intake has stopped and the fuel-charge is finally being trapped in the cylinder.
From here the vacuum peak usually hangs on another 1000rpm or so (no-load/neutral), before beginning a long slow decline.
For best fuel mileage, I gear the car to run at or a little higher than this vacuum peak.Then if I have to,or as may be desired,I modify the spark-port to pull the Vcan all in, at that cruise rpm.
 
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AJ/FormS said:
but not make it run bad.
In post #1, you said solid-lifter cam, but in the linked thread you say hydraulic.
In either case;has the lash been correctly set?
Click to expand...

The guy that sold me the motor thought it was hydraulic, but I've since found out (when changed out the intake gasket ) its a solid lifter.
 
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I know this was a grude way to measure a cams lift, but is this cam as wild as it apears? I want this car to be a driver not a track star, if this is the case i think I'm going to switch out the cam. Again I knew very little about this motor when I got it because it was a good deal and I knew it wasn't perfect, it was in a beat up Duster and I think the person who built the motor was going to bracket race it.
 
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AJ are you using ported or manifold vacuum for vacuum advance? That makes total sense, I was envisioning a vacuum gauge in the car going up as the op went to full throttle.
 
Valkman said:
I know this was a grude way to measure a cams lift, but is this cam as wild as it apears? I want this car to be a driver not a track star, if this is the case i think I'm going to switch out the cam. Again I knew very little about this motor when I got it it was a good deal, it was in a beat up Duster and I think the person who built the motor was going to bracket race it.
Click to expand...

If you could get a dial indicator on the rocker or take the rockers off and get it on the pushrod you could get an idea on lift. I have used 2 solid cams in my 340, I have the .557/296 MP solid in my 340 now, it will idle down to 500rpm if I wanted it to. The other solid was a .508/280 MP cam and would idle down the same.
 
gtsdude said:
AJ are you using ported or manifold vacuum for vacuum advance? That makes total sense, I was envisioning a vacuum gauge in the car going up as the op went to full throttle.
Click to expand...
Yeah I thought you mighta bin thinking that, which is why I jumped in to clarify.
I always run vacuum advance from the sparkport, on everything I have tuned. I only tune streeters, mostly sbms, and the largest cam I have seen was the 292/508 Mopar.
 
Buy the right pushrods, could not get my small block 340 to idle worth a crap without welding up the mechanical advance slots and filling them to make the slot around 1/8 smaller, then set your total advance to 35 degree and you should be OK. If your not sure what size pushrods you need you can buy a comp cam adjustable pushrod to get the geometry right, had to do it on my motor also.
 
Valkman said:
I posted the in the electrical & ignition section a while back you can go here for more info:
Can't get it to idle right!

Basically what have is a '73 340 with J heads, a seemingly wild cam solid lift cam, RPM intake, Eddy AFB 750 (I also used an AVS 625 with no improvement) with an electric choke, a Mr gasket light spring replacing the big one in the stock electronic distributor, Crane 1.5 roller rockers, a Stock mechanical fuel pump (pumping a consistent 5-6 psi)

I haven't had any luck since getting it to stay at a consistent idle!

It seems to do best at when I set the timing 35 deg. total advance.
I've done a compression test and it has 150 psi in all cylinders
I tried to measure the valve lift with dial indicator, but I could never get a consistent reading so I use a ruler to get a somewhat of a ballpark reading (see pictures). @.647 and intake and .653 exhaust did I calculate this right? If it is how does this engine even run?


View attachment 1714955030 View attachment 1714955031
Click to expand...
Where you took your measurement already has the rocker ratio in it. So a mid .440 cam
 
push rod moves the same as the cam. measuring on the spring side would already have the ratio figured in.
 
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