question

have you set the idle with the trans in gear ? Somtimes with big overlap cams when the engine is loaded and slows the timeing retards and stalls the motor! high lift cams have low vacume! and need to idle higher. MAKE SURE SOMEONE HOLDS THE BRAKE AND HAVE IT IN REVERSE WHEN YOUR IN FRONT OF CAR DOING ADJUSTMENTS.

 

Hmmm havent tried that....I did try idling the engine up to about 1500 and putting it in gear ( I was by myself ) and got the same result.

 

question

what are cam specs? sorry i type slow 1 finger!

 

tech

if your alone jack up the rear of car jack stand it and make sure its secure and do your adjustments use a vacume gauge in gear ideling step on brake slow eng with brake see what vacume reading is just before it stalls it wont run if vacume is too low.

 

It was a friend of mine that put the cam in and he naturally lost the cam card....all I know is its a 530 lift. The thing chops some wood so its quite large wish I could give you more info on it i know the idle issue could be any number of things

 

tech

you may have a bad converter locking up early ive installed bran new ones did that and aspecialy used ones

 

teck

3000 stall conveter should have plenty of slipage at 1000 rpm if it still stalls at higher speed when brakes held in gear say 2500 rpm, but bellow stall speed your coverters locked when in nuetral your converter in free wheeling

 

What is your ignition timing curve?? Large cams will require mods to a typical HEI distributor, you will need to achieve a high initial timing setting like 20-24° but limit the total timing to 36-38°.

The higher initial timing will allow the engine to idle much better. What you have happening is typical of big cams and unmodified distributors.

 

tech

hes right and vacume advance isnt much good on big over lap cams

 

I believe you may have answered my question ericnova72. I am in the process of having the distributor modified (i dont have a welder or proper tools or know what to do to it for that matter). Everything but the distributor and the alternator is either new or rebuilt so I'm assuming that's atleast where most of the problem is. I set the timing around 35 degrees and it still does it so hopefully the mods will help when I get the distributor back. Thanks to everyone for all your input I'll let ya know how it goes and please if you can think of anything else dont hesitate to let me know. Thanks again!!

 

tech

is the cam advanced , retarded or straight up advanced would increase vacume and idle better but you would lose some of the radicle sound

 

tech

if you set your anitial timing high with stock distributor already its not the distributor initial timing is initial no mater what distributor you have in it

 

tech

good night & good luck

 

good night

total timeing is at speed usualy coming in at 3000 rpm on hi po engs initial timeing is at idle

 

in park I idled the engine to 3000rpm and set the timing to 35 degrees and tightened down the distributor. Then I idled it back down to 1000rpm...it still wouldnt stay running idled any lower and it still died when i tried to put it in gear

 

You need to have a lot of initial timing w/o going over the 36º or so of total advance.

A vacuum advance can work in you favor in this case- possibly. The 'possibly' is reliant upon the specs of the vacuum advance can. It needs to give you advance at whatever vacuum the engine makes at idle, in gear.

This will allow you to get "extra" advance from the vacuum can- this allows the idle to come up, which in turn allows you to close down the primary throttle blades to decrease the idle speed.

THIS, in turn, lessens the amount of transition slot exposure the carb has. That cleans up the idle and gives back idle mixture screw control of the idle- which may have been lost if the throttle blades were opened too far before using the vacuum advance.

From an earlier thread:

You should verify that what you are seeing at the damper and timing tab for TDC is actually TDC.
DETERMINING TDC will allow you to be sure the timing tab and damper are correctly indicating TDC.

MAKE A TIMING TAPE to see what the total timing is, w/o needing to use a dial back timing light. You can also buy a timing tape, they're not very expensive. The problem is if you have a different diameter damper than what the timing tape is made for.

Be sure the advance plate is free to move, w/o binding or friction. HERE'S an exploded view of an HEI distributor. THIS is a description of an HEI rebuild.

If you find the idle mixture screws have little or no effect on the idle, or in cases like yours where the idle speed in gear drops too much, I would recommend that you use the vacuum advance, hooked to manifold vacuum. It should be limited to around 10°-15° maximum of added advance. This is key to getting a good idle quality!

Using vacuum advance w/manifold vacuum will usually increase the idle speed- this then allows the primary throttle blades to be closed some, which in turn allows less exposure of the transition slot of the carb. That's desired because the carb will then be running on the idle circuit at idle w/o using too much of the transition slot to provide idle mixture. If the transition slot is over exposed, the idle can be erratic, rougher than it should be, overly rich, and there can be a flat spot transitioning from idle to main circuits.

You may well need an adjustable vacuum advance to get the advance at a low enough vacuum, given the lack of idle vacuum you now have. Using the vacuum advance should help bring up the idle vacuum as well.

If the vacuum advance you have comes in OK but gives too much advance, you will need to physically limit the vacuum advance can's travel w/a vacuum advance limiter plate like the Crane #99619-1, #99619 Instructions. Or you can easily make one.

Crane has an adjustable vacuum advance can kit- Crane #99600-1, #99600-1 Instructions.

The Accel #31035, #31035 Instructions is an adjustable vacuum advance can for GM HEI that is said to allow infinite adjustment to both the amount and rate of advance. Comes w/instructions and tool.

You will need a vacuum gage to see when the existing vacuum advance starts, to see what the idle vacuum is now and how it changes when you use the vacuum advance hooked to manifold vacuum, and to see that the power valve is not opening at idle- this can create problems w/the idle quality as well.

This may seem like a lot of BS'ing around, but I truly believe it's the best way to get a good idle w/a crisp off idle transition. Without knowing the specs of the cam, there's no way to say for sure what the exact timing curve should be, but generally speaking, you want as much timing as early as it can be used- without causing detonation.

This could look something like:

• 18°-24° initial timing
• 12° mechanical advance
• 36° total timing
• timing all in by 2500 RPM
• vacuum advance adding 10°-15°

If the cam is VERY radical, you may even benefit from using a locked distributor, set to the total timing. But this is better reserved to cases where there is absolutely no way to use a curve.

 

What is your ignition timing curve?? Large cams will require mods to a typical HEI distributor, you will need to achieve a high initial timing setting like 20-24° but limit the total timing to 36-38°.

The higher initial timing will allow the engine to idle much better. What you have happening is typical of big cams and unmodified distributors.

X2 on the timing, And what carb are you running on it also ? Re- adjust the mixture and re-check the float levels also. JMO


Cole

 

Yep. Knowing the cam specs and the vacuum @ idle, particularly in gear as the engine appraches death, would be helpful. But, the timing info above sounds good to me (so many experts here, and Cobalt is one of the best) and I'm betting the final death knell to your idle comes when the power valve dumps a ton of fuel into the intake. Assuming you have a Holley-type carb.

Pat

 

I'm betting the final death knell to your idle comes when the power valve dumps a ton of fuel into the intake. Assuming you have a Holley-type carb.

Pat

I believe we're on the same wavelength here, Pat.

Even a Q-jet will go terminally rich under these conditions, as will the Edelbrock/Carter, etc.

Actually any carb that has a 'power/economy' enrichment system that's vacuum dependant will tend to go too rich under low vacuum idle conditions, unless the problem is addressed in some way.

 

vacume at idle on a stock eng is around 17 to 25 in. hg. of murcury [ reading on gauge] LOL the bigger the valve overlap and narrower the lobe center the lower your vacum is at lower idle speeds. fast bleed down lifters help bring up vacume at idle [like rodes lifters]. anti pump up lifters act like solids and are lashed at .001 to .002 and drop vacume. its a big mistake to run stock lifters on a highlift and narrow lobe center cam ! idle quality doesn't matter on the track!!!!!! but on the street if you pull up to a light idling at 3000 rpm women would pass out & men would smile!!! .large over lap cams drop static compression lamons dont know this [when valves overlap compression goes out the exhaust ] example [ i run run a .580 lift 312 degree duratin 108 degree lob seperation cam, 4500to5000.stall converter car idles at 2800 rpm i have jahns forged pistons 12.5 to 1 with .004 deck hight and .021 shim head gaskets to bring my compression ratio to 10.5 to 1] take a vacume reading!!!! { power valves dump gas when vacume drops } that was typed with one finger !!! sorry if i preached but i dont know if your a novis or a veteran . anyone can learn from anyone this spell check is goofy

 

HEY DOES ANY ONE REMEMBER JAHNS PISTONS? HAD EM RAPPED IN PAPER SINCE 69. if you lashed your lifters at 0 you can take them down 1/4 turn see if idle picks up Unless thats a sollid lifter cam!!! i dont remember reading if it was or not I just took for granted it was hydrolic. i just joined yesterday first site i ever joined i keep getting lost

 

Good point on the carb tuning....its a Holley 650 double pumper (which I'm still learning how to tune/ yes I'm a novice with much to learn) so i'm sure its probly dumping too much fuel and not helping my idle at all.

 

tech

35 degrees anitial timing is way too high 35 degrees is closer too total advance at a high rpm like 2500 or higher set it at 12degrees & leave it there unless your running 100 octane or higher fuel then bring it up , but not past 18 degrees enitial remember total timeing is anitial timing pluss cintrifacle pluss vacume advance all added together! if you are trying to set total timeing, plug vacume advance in hold throttle at 2500 to3000 where ever you want it to top out [make sure u marked damper at 35 degrees] and set it . somtimes we try and forse it to be the easy solution becuse well its easy especialy if your on a tight budget. i dont know your situation. im retired and on a tight budget now so now i know what people feel . see if someone can loan you an old or newer distributor and elimenate your guessing on that point remember at idle it doesnt matter what distributor you have, initial timeing does not change ! if rpm is constant ,cintrifacle and vacume advance will not ingage till throttle is pressed and rpm and vacume increases i dont know if you under stand what im saying sorry myself or one of these other whernt close enouph to help you in person its a ***** trying to typing with one finger

 

in park I idled the engine to 3000rpm and set the timing to 35 degrees and tightened down the distributor. Then I idled it back down to 1000rpm...it still wouldnt stay running idled any lower and it still died when i tried to put it in gear

if you set it to 36 at 3000 all your mechanical advance was in most likely so when you went back to 1000 the timing would have dropped off. Did you watch the timing as you backed down the idle and was the vacuum advance connected.
If it was then you probably have more vacuum at 3000 than 1000 with a radical cam.
You theory was good but your implementation needs work. You would need to lock the distributor at 36* so the timing cant change and see what happens.
You most likely need a different power valve as well. You will need to get a vacuum reading at idle and cruise to determine the proper value.
Most Holleys come stock with a 6 power valve, if you vacuum is lower than that at idle is is going to open and flood the engine to th epoint of a stall.

 

tech

the power valve on idle circuit will not have a vacume signal till secondary venturies start to open, pull carb off eng , pour fuel out and hold up to light [carfull dont spill gas on you] secondaries should be closed if you see a crack of light they need to be closed, theres a small screw acsessed from underneath passenge side of carb you can adjust it to close rear throttle plat . if you cant close it! check for binding. [ this is the fist thing to do right out of the new box ]