rough idle bad vacuum and assorted gremlins

[dey59]

I just bought a 70 gmc engine number says its a 72 model 350 its auto the guy put a hei dist. in it says it has a mild (rv) cam but don't know what it is idles very rough but not like the lope of a cam its erratic I can get the idle down to about 1000 and it will stay running of course you have to get it up pretty high to put it in gear or it dies he says it has a 3000 stall converter in it for the cam I got it out on the road and once you get it moving at 35 or so it runs and sounds fairly well i'm sure theres carb issues I put a vacuum gauge on it and at 1000 rpms (idle) the needle bounces erraticly between 14 an 16 3 of the bolts where loose on the drivers side intake and when I adjust the mix screws theres no change he's got old auto light plugs in it gapped at about 50 also its a 72 motor but the fireing order is for the 75 and later hei does this make a difference I loosened the dist and turned it almost two plugs counterclockwise and it has no effect when it turn it clockwise the same amount it stumbles and tries to die what would be a good plug and gap ac44st or 44str it has a 1405 600 cfm edlebrock which will either be rebuilt or replaced has edlebrock performer intake not looking to make a hot rod just a good dependable truck right now my two main issues are the firing order thing and plugs haven't had time to put the timing light on it what timing specs should I use 72 or 75 and later yet any helped is appreciated greatly thanks doug

[71K20chevy]

For one thing, with HEI, the plugs should only be gapped at .045. Try that before throwing parts at it

[luvbowties]

Originally Posted by dey59

I just bought a 70 gmc engine number says its a 72 model 350 its auto the guy put a hei dist. in it says it has a mild (rv) cam but don't know what it is idles very rough but not like the lope of a cam its erratic I can get the idle down to about 1000 and it will stay running of course you have to get it up pretty high to put it in gear or it dies he says it has a 3000 stall converter in it for the cam I got it out on the road and once you get it moving at 35 or so it runs and sounds fairly well i'm sure theres carb issues I put a vacuum gauge on it and at 1000 rpms (idle) the needle bounces erraticly between 14 an 16 3 of the bolts where loose on the drivers side intake and when I adjust the mix screws theres no change he's got old auto light plugs in it gapped at about 50 also its a 72 motor but the fireing order is for the 75 and later hei does this make a difference I loosened the dist and turned it almost two plugs counterclockwise and it has no effect when it turn it clockwise the same amount it stumbles and tries to die what would be a good plug and gap ac44st or 44str it has a 1405 600 cfm edlebrock which will either be rebuilt or replaced has edlebrock performer intake not looking to make a hot rod just a good dependable truck right now my two main issues are the firing order thing and plugs haven't had time to put the timing light on it what timing specs should I use 72 or 75 and later yet any helped is appreciated greatly thanks doug

First, fir. orders are the same: 18436572. Second, a "mild r-v cam" should not need a 3000-stall convertor. Third, sounds like a novice installed intake, as evidenced by 3 bolts being loose; so, check behind him further, as int. gaskets run good risk of needing replacement, if leaking.

Start by using a piece of thin cardboard the size of notebook paper; punch a hole in center to slide down over air cleaner stud; with air clnr. OFF, slowly slide it down while engine is idling. If engine speeds up noticeably & idles much more smoothly as the cardboard gets closer to choking the carb top, you have a serious vacuum leak.

If vac. leak is determined, while engine is idling, slowly move around the intake sealing surface spraying wd-40 along seam. If you find a spot where engine speeds up and idles more smoothly, it indicates a vac. leak. If found, fix leak.

If intake is found ok, spray wd-40 around every possible intake leak--carb mounting, vacuum hose connections(incl. pc-valve connectns.), etc. If any leaks are present, eng. idles better and speeds up & levels out somewhat.

Someone else may chime in & tell u about timing: I cannot describe, in words alone, how I use a "feel-of-the-pants" method for setting timing w/out a timing light--before I verify via tim. light that it is then within reason so as to not be dangerous to engine-detonation.

HTH,
Sam

[dey59]

thanks guys i'm starting on the vacuum leaks tonight I hope I don't have to go to deep in it but I like working on them(read as leaning on it drinking beer) as much as driving them. just got off work going to pickup some plugs and gaskets. thanks again i'll post my progress or lack there of.

[GASoline71]

Periods my friend... makes old eyes able to read these posts a little better...

I personally wouldn't use WD40 to check for air leaks... unless you want oily spatter stains all over everything. A little starting fluid around the carb base is all that's needed. You'll know when you have found your vacuum leak.

Gary

[dey59]

yeah I used carb cleaner, but starting fluid would be better. anyone out there know if when you go from point dist. no. 1 one pass. side. to hei, no.1 on drivers side. which way should you wire the dist.. for the original engine no.1 at approx. 7 o'clock, or for the hei no.1 at approx. 5 o'clock thanks guys

[dey59]

update: I did a compression test, all plugs out 3 cranks take reading and then another 3 cranks and take a reading. cylinders 1-3-7-2-4 and 8 150 cylinders 5 and 6 130 the intake bolts above these 2 cyl. were not very tight but if my valves are working right should it matter. or is these 2 being across from each other just a coincidence. vacuum reading erratic bounce at 14 to 16 please comment on compression test thanks

[72 K20]

Number 1 cylinder is always the front cylinder on the drivers side on a chevy v8. I always set the distributor to point at number one cylinder with it at TDC.

[dey59]

my mistake when I said no. 1 on pass. side. I was talking about the position on the dist. cap looking down as shown in engine manuals. no.1 on the cap is at about 7 o'clock left of center from the front for point type. but on hei looking from above no. 1 is at the 5 o'clock position left of center from front. my engine is a 72 point type originally, but now has hei. which wiring diagram do I use.

[WadmalawJoe]

Originally Posted by dey59

my mistake when I said no. 1 on pass. side. I was talking about the position on the dist. cap looking down as shown in engine manuals. no.1 on the cap is at about 7 o'clock left of center from the front for point type. but on hei looking from above no. 1 is at the 5 o'clock position left of center from front. my engine is a 72 point type originally, but now has hei. which wiring diagram do I use.

It doesn't matter. Using the 5 o'clock position seems to make wire routing easier.
Posted via Mobile Device

[Rufton]

Originally Posted by dey59

update: I did a compression test, all plugs out 3 cranks take reading and then another 3 cranks and take a reading. cylinders 1-3-7-2-4 and 8 150 cylinders 5 and 6 130 the intake bolts above these 2 cyl. were not very tight but if my valves are working right should it matter. or is these 2 being across from each other just a coincidence. vacuum reading erratic bounce at 14 to 16 please comment on compression test thanks

Did #5 and #6 plugs read different than other plugs?

Not ideal compression data but I've seen worse on a smooth running engine.
Compression numbers can vary depending on how much oil is on cylinder wall, temperature, etc.
Out of curiosity, I would warm up engine then run compression test again. This time in reverse order.

[dey59]

no they all looked about the same very old and a little gassy like it was running really rich

[Rufton]

I would delay the curiosity compression test till rainy day. I feel confident those numbers are high enough for smooth running engine.

If it is running that rich then some plugs may be fouled and causing your problem.

After doing the leak tests everybody mentioned I would tune carb idle.

I use different test fluid depending on car and what's available. WD40 won't damage paint but is a lil messy. Start fluid and carb cleaner won't make a mess but may damage paint. There are other tests if engine is near show quality.

[dey59]

not show quality for sure, I think your right on the carb. I got ac45ts gonna gap at 45 and see where we're at thanks

[hatred]

Any update on this?

[dey59]

sorry been a crappy week just finished new compression test warm engine about six cranks(thats where it quit going up) dry test-1-160 3-160 5-140 7-160 2-150 4-165 6-135 8-165 wet test 1-175 3-170 5-150 7-175 2-170 4-170 6-140 8-175 cyl. 5 and 6 bother me especially 6. idle is erratic it idles so rough I can't tell much when I spray the intake with starting fluid what do you think . thanks how can you tell stuck valve from bent pushrod or collapsed lift. thanks