[gluntly]

Hi folks,
I recently installed a Comp Cams BMT in my 355 cu. motor, 750holly/vac. 10to1 411 4speed headers .
I'm having a problem getting it to idle without dying, unless I have the idle set up to 1200 or so. I'm what you would call a rough mechanic, good at getting the base set up all together, but lousy on the fine tune.
Any one had a similar experience with these bump sticks.
Sounds really awesome so far ..
Keep America Working
Gluntly

 

[brian oneil]

what part number cam you have??

 

[gluntly]

12-602-4 it shows a grind # of 295th7

 

[brian oneil]

.500/.486
295/313 duration

First thing you should check for is a vaccum leak. Around the base of the carb and your intake gaskets, side and ends. Also check and make sure that you have everything plugged up correctly on your carb, hoses....etc.
Check to see if your float bowls are set at the proper level.

At 1200 rpms you may be getting out of your idle circuit which will kill your milage.

 

[gluntly]

Yeah I've pretty much done all that, I can't seem to get a straight answer from Comp as to what a favorable Idle would be for a cam like that, maybe it's happiest up there above 1000. Thanks for the input Brian, I will go over the base again..

 

[brian oneil]

you wont be able to get them into any specific set up types, because they dont want you to come back and bang them for a set up that dont suit your/there needs. CYA basically.

I run a cam with 306/306 duration and it likes to idle in gear around 1100 rpms and out of gear about 1400 rpms. Of course I really dont care too much about the idle circuit as this engine I use to go to the strip with.

what stall is your converter?

 

[gluntly]

Got an ST-10 and a Super Shifter3,
from what you say I could be right on the money or close if I could just keep it from stalling out.
maybe a little more advance, till it pings, then backa hair

 

[Philip]

It may not idle lower. The sound is produced by having both the intake and exhaust valve opened at the same time. The duration of overlap determines how rough the idle is. If idle speed is dropped to far the overlap will cause it to stall.

 

[brian oneil]

I would assume your timing total advance should be some where between 32-38 degrees, but basically between 34-36. Your engine may want a lil more or a lil less.

do you have a fuel pressure gauge? if so, what is your fuel pressure?
If your float bowls are set correctly your vac. secondary carbs still has air/fuel mixture screws in the front bowls. adjust them until you git the highest rpm and lets see what the rpms is after backing off the idle screw that the engine will hold and idle at

Also on a side note, is your rockers adjusted properly?

 

[Philip]

I have a milder cam in my 406 and it will not idle well below 850 rpm's, at 750 it sounds like it wants to die. With a manual trans you don't want your idle to low anyway. I have a TKO 600 and will set the idle high enough that I can start engaging the clutch at idle and gradually ease into the throttle until the clutch is fully engaged to reduce the time it is slipping (at least for street driving )

 

[gluntly]

I think I hit it on the head. I ran the initial up to 14, turning up the idle as I went, (kept trying to die) until it was running at about 1250 RPM's. Then I plugged in the vacuum line and and it doubled in rpm. I've got it idling at 1100 now with all the torque in the world, and it sounds pretty mean

 

[wildman926]

I think I hit it on the head. I ran the initial up to 14, turning up the idle as I went, (kept trying to die) until it was running at about 1250 RPM's. Then I plugged in the vacuum line and and it doubled in rpm. I've got it idling at 1100 now with all the torque in the world, and it sounds pretty mean

You only have 14* initial timing?! That thing will need at least 20*, if not more. That is your problem, not enough initial timing.

 

[onovakind67]

Earl Parker of Earl Parker Carburetion sent me the following setup memo. We used it on a 4779 Holley used on a 331" road racing motor and it worked very well.


One method of setting up your idle system is a follows:
To start with, invert the carburetor and check the position of the throttle
butterflies. Turn the idle speed setting screw to set the bottom edge of the
primary throttle butterflies about .020" from the bottom edge of the transfer
idle slot. Don't worry about measuring anything - your eyeball is good
enough. Positioning the throttle butterflies near the bottom of the transfer
idle slot at curb idle is absolutely critical for maximum acceleration.
Next, turn the idle mixture needles in until they are lightly seated. Excessive
force here will damage both the needles and metering block and make the
idle fuel mixture difficult to set with any accuracy. After seating them turn
them out 1 1/4 turns, which is a good baseline setting. Now you're ready to
reinstall the carburetor and setup your idle system.
Before you start the engine, examine the fuel bowl side of the throttle body.
Hopefully you'll see a little tube, covered by a rubber plug. This vacuum port
connects with a passage in the throttle body that 'sees' manifold vacuum.
Remove the plug, attach a good vacuum gauge to the port and position the
gauge where you can see it clearly. Don't forget to zero out the gauge.
Without touching the carburetor, turn the engine over until you have
pumped fuel into the bowls. Work the throttle a few times then start the
engine. If it dies, which is likely, you'll have the turn the idle speed setting
screw to increase the RPM to get it to idle while it's cold. Since throttle
butterfly position is critical, count the turns and fractions of turns so you'll
know exactly where you're at. The whole idea is to be able to return the
throttle butterflies to the position you originally set them at. As the engine
warms up it should gain rpm, so you should be able to reduce the throttle
opening at least somewhat without the engine dying. Now the fine tuning
begins.
With the engine idling, pick one of the idle mixture needles and turn it in 1/4
turn while you're watching the vacuum gauge. Give the idle a few seconds
to stabilize. If manifold vacuum increases repeat the process, letting the idle
stabilize each time, until it starts to decrease. If turning it in hurts manifold
vacuum then try turning it out. When you've found the 'sweet spot' (i.e. the
manifold vacuum is as high as you can get it) repeat the process with the
other idle mixture needle.
Presumably you'll be able to pick up enough idle speed by optimizing the idle
fuel mixture that you can close the primary throttle butterflies down to their
original position near the bottom of the transfer idle slots.
As a final check give each idle mixture needle a slight turn in then a slight
turn out. If any motion hurts manifold vacuum, you know that needle is set
properly. At this point if the idle is stable and the engine responds quickly
when you just crack the throttle, you should be good to go.
One final note: Make sure your timing is set correctly before starting this
process.
If your distributor has a mechanical advance system there is a much better,
though more involved, way to setup the idle system.
Position the throttle butterflies and idle mixture needles as described above,
attach the manifold vacuum gauge to the vacuum port and start the engine.
Turn the idle speed screw to increase the RPM, again taking note of exactly
how much you have to turn the screw to open the throttle butterflies
enough for the engine to idle while it's cold. Allow the engine to warm up,
the close the throttle butterflies as much as reasonably possible without the
engine dying. Attach a timing light, check to see how much initial ignition
advance you have and make a note of the figure.
Next, loosen the distributor hold down clamp and turn the distributor so as
to increase the initial ignition advance. When the initial ignition advance is
increased the RPM should rise as well, allowing you to reduce the throttle
butterfly opening. Simply turn the distributor to increase the initial ignition
advance and continue to reduce the throttle butterfly opening until they're
in the original, correct position and the engine is idling at the desired RPM.
Lightly snug the hold down clamp to make sure the distributor can't move,
then adjust the idle mixture needles for best manifold vacuum. Once they're
properly set if the idle RPM is higher than desired, loosen the hold down
clamp and turn the distributor slightly to achieve the desired idle RPM.
Recheck the idle mixture needle position then tighten the hold down clamp.
Once the idle system is setup you'll need to correct the distributor's
advance curve. The first step is to attach a timing light and recheck the
initial ignition advance. Let's say, for example, that it was originally 15° and
now it's 22°, a 7° increase. If your total ignition advance was originally 35°,
in order to keep that figure the advance curve will have to be shortened by
7°. Assuming you have a centrifugal advance system you'll have to limit how
far the advance weights can move outward, which will limit the total
advance. The method required will vary from distributor to distributor, so I
won't get into that here, but any competent technician with a good
distributor machine should be able to do it for you.
If you don't have access to said technician/distributor machine and you can
come up with a way to limit the outward motion of the advance weights,
you can do the same thing using your engine as form of distributor machine.
Limit the motion of the weights somewhat, make sure you have the correct
initial ignition advance then check to see how much total ignition advance
you have. If the total ignition advance is still too high, just continue to limit
the motion of the advance weights until you achieve the desired total figure.


AED has some good tips on carb setup, also.

http://www.aedperformance.com/Tuning Tips.htm

 

[gluntly]

I've printed that info along with the timing advise, now it's time to "Tweek On"
Thanks everybody for your help.

 

[Gatecrasher]

Most unaltered distributors will produce around 20-24 degrees total mechanical advance. The more radical the cam is, the more advance it will like. You're probably going to need somewhere between 36-38 degrees total advance which would translate to 12-18 degrees inital.

My 406 has a 300 degree mechanical roller cam and likes 38 degrees total mechanical advance. To achieve it with my MSD Pro-Billet distributor my initial timing is set at 17 degrees initial.

20 degrees initial will almost always be too much and will give you 40+ degrees total unless you are limiting your mechanical advance with a stop. Mine runs great right where it's at so I opted not to limit my mechanical advance. Idles at 900 RPM (loper).

 

[wildman926]

20 degrees initial will almost always be too much and will give you 40+ degrees total unless you are limiting your mechanical advance with a stop.

Key words there in BOLD.

Very easy to do as in this pic - Use the screw on the left if you want to limit the mechanical advace. Either grind to find the correct advance, or find a screw with the right size head. Use the screw on the right if you want to lock out your timing.

 

[Guest]

this is a little more work but it'll do the job for ya too:

http://www.jollyrodder.com/hot_rod_links/Daves_Vac_Advance/Daves_Vacuum_Advance.htm

 

[gluntly]

Got it to idle at a grand, and it likes it. With great response at 16 deg. initial. Thanks everyone for all the different avenues of approach to get this corrected, and running like it should. And since that is the case, have noticed now that my oil pressure has dumped to the basement.
50 lbs at start up, down to 4 lbs at idle, never going down to 0.(1000rpm)
Long story made short, Installed new cam and kit, great oil pressure, never below 15 lbs; 2 weeks later, I had a senior moment, over revved it (that baby was singing) and blew a push rod through the top of the stamped steel roller rocker, bent the other, both on Cyl.2.
Bought new alum. roller rockers and pushrods, after making sure the cam was ok, installed new hyd. lifters. Will changing lifters make a difference in my oil pressure???
They are Comp Cams Hi Perf Hyd.
Keep America Working
Gluntly

 

[brian oneil]

No, changing out lifters should not effect your oil pressure. It would seem to me that if you have 50lbs at idle and once warmed up in gear going down to 4 psi, I would suspect cam, rod or main bearings only because you have oil pressure when cold.

Have you checked to see when the engine is running, pull the valve cover and see how much oil is coming up through the push rods.

I also hope you bought new valve cover :thumbsup:

 

[Philip]

Is the bypass in the oil filter adaptor still in place? If it gets a piece of debris lodged in it the oil pressure can be affected. Mine was removed and a plug installed to insure all the oil was going through the filter.