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1970Chevy 11-02-2007 08:57 PM

305 sbc good idle, Dies when in gear
Hey fellas, Ive gotten the best help and advice anyone could ask for from the guys on this site. I have a 305 sbc, RPM air Gap manifold, 750 cfm carburator, hedman headers, stock stall converter, and stock 2.56 rear gears, and most likely a mild cam (all from previous owner).

She sounds great in idle as of today. The problem is that when shifting (th350) from park to reverse or first gear She stalls, or comes close to it.

Currently, When In Park:
The initial timing is set to 12* BTDC.
With vacuum advance plugged in shes at 24* (but havent checked total timing yet while engaging throttle)
The idle is set at 1500 RPM.
The Idle mixture screws are set to 1/2 of a turn from fully closed
Vacuum reads exactly 17" Hg

When I shift into to Reverse:
Timing retards to 9 degrees
Idle drops to 600-700 Rpm
Vacuum is 8" Hg
a noticable amount of smoke comes from the breather of the PCV unit
and Stalls if throttle is engaged

I recently changed the man Rods to one stage Leaner in the carburator. I also have in the lightest springs. I have a feeling that changing jets a stage leaner will also help a great deal. The symptoms of losing that amount of vacuum from park to reverse makes me think that the rods are lifting because not enough vacuum is holding them down, causing too much feul in the A/F ratio and bogging the engine. But I dont get why it would do this just by shifting into gear from park or neutral?

I just checked pretty extensivly for a vacuum leak but didnt find anything. Im researching the different parts of a transmission (modulator maybe?) to see if I can figure something out.


66GMC 11-02-2007 09:08 PM


Originally Posted by 1970Chevy

The Idle mixture screws are set to 1/2 of a turn from fully closed


I'm hoping that is a typing error ... I am sure the advice that you were given was 1.5 turns out from closed ... wasn't it?

That is a "starting point" by the way, and should be fine-tune adjusted from there in order to obtain the highest vacuum reading.

1970Chevy 11-02-2007 09:16 PM

As I stand in front of the engine bay and read the vacuum gauge and the tachometer, this setting seemed the best to me. I must have been wrong. Any more (even 1/4 of a turn more) sounded like it does when its bogging. its a glug glug sound rather than a rakitah-gackita rakita-gackita sound. I guess I'll try it again....

79C10 11-02-2007 09:17 PM

MHO , but a 750cfm is a bit big for a mild 305 ..... might get a better response w/a 600 .

1970Chevy 11-02-2007 09:19 PM

what do you think I am. do I come off as an amature or what!!! ha ha ha

Heres a video. This was a week or so ago. Its not what the current settings are, but it is basically the same problem that Im still having (although I have done quite a bit since then, Ill post another video tommorrow)

1drwgn 11-02-2007 10:02 PM

sounds way too rich when putting throttle into it. out of gear does it bog too when putting throttle into it? if it does thats way too much accelerator pump and primary ,metering and idle, or a combination of sorts. i would pick up an edelbrock or carter competition(same carb) 625 or so cfm. easier to adjust on car, and a lot more responsive to that engine than 750. also stock converter will drag a 305 down to that rpm no matter what, and the cam doesnt help. loss of rpm will make vacuum drop, causing your problem. also if your 750 is an edelbrock, they come rich to fit an array of engines, all of which are larger than the 305.

DHMag 11-02-2007 10:50 PM


Originally Posted by 66GMC
I'm hoping that is a typing error ... I am sure the advice that you were given was 1.5 turns out from closed ... wasn't it?

That is a "starting point" by the way, and should be fine-tune adjusted from there in order to obtain the highest vacuum reading.


1970Chevy 11-02-2007 11:28 PM

When Its in park, and at 1500Rpms it doesnt bog.

I am tight on money and odnt get paid until the 9th. Would it serve me to get a higher stall converter (2400?-3000?) as well as a smaller cfm carburator. I think I might be able to afford those two items (I need a seat too!) to get her going.


Because of a lack of funds would I be able to install smaller Jets ($7 vs. $200) and get the higher stall converter.

would someone think that the cam would be okay with one of these set ups? (i know it'd be better to know the lift and duration)

Then Later down the road I'll get 3.73-4.11 rear gears.

I just need to get this thing out of storage (220 per month!) and on the Road for the smallest amount of money. But It needs to be a cost effective route so I dont end up paying more than I need to in the end.

Im hoping These are the last few steps to my seven month marathon with this el camino. The wiring is mostly complete, The brakes are redone, the steering is redone, The rust is gone (OH the RUST), the tires are great, the chrome is complete (all there), the windows are in great condition, the paint is stripped, the body is clean, Im currently painting with black epoxy, the Dash is re-painted and "set up custom" to my liking, the carpet kit is about to go in, the doors and hood are going back on, and Im outta there in two weeks. This is the plan anyway. Thanks

TRLBLAZR60 11-03-2007 03:24 AM

Hey dumb question have you tried settig your timing by ear? also have youchecked #3 and #7 sparkplug wires to make sure that there not crossed ? brother just had that problem 3 & 7 crossed. No back fires and not much noticeable problems other than a dog. Not trying to insult you but been there. please check the basics and dont believe your harmonic balancer/dampner even if it is knew. Good luck Tom.

P.S. Imost certainly could be wrong but that sounds more like a major timing isuue to me. I know that you checked the cam to crank timing. Its no the first time that Ihave watched one of your videos. :D Just please check the basics to make sure that something else isnt wrong. Ive been there though Im not proud to say. but it more than makes up for it when the SOB runs like its supposed to :evil:

curtis73 11-03-2007 04:19 AM

First of all, a few observations: A stock (or mild) cam 305 doesn't need an RPM, let alone an RPM air gap. You'll probably run into some problems getting it perfect with that intake. Secondly, 1500 RPM is WAAAY too much idle. That thing should idle at 400 all day. That engine is done by 5000, so idling it at 1500 is not logical. Of course, as others have pointed out, the carb is way too big which doesn't help.

It sounds like a classic mismatch of timing, idle mixture, and idle setting. First of all, for now, skip the jets and rods. They are strictly for throttles OTHER than idle. They won't change a thing at idle. It sounds to me like you have the idle mix too lean and have compensated by opening the throttles, then you set your timing. Here's what I suggest:

Set your idle screws to 1 turn out. Start and idle. Advance the timing... doesn't matter how much, just a good advance to make it idle. Back off the idle speed screw a bit, then re-advance. Its a common problem for newer tuners, especially with manifold vacuum on the ignition. What ends up happening is they set idle timing first. What needs to happen is tuning the carb first. Forget about advance at idle while tuning the carb. Throw a buttload of advance in, tune the carb, then back things off while adjusting the idle speed until you get to the proper 12* and the idle you need.

LupinusPectus 11-03-2007 05:57 AM

wanna play with the math?

(CID)x(RPM)/3456 will get you the cfm needs of your engine at a given rpm at 100% volumetric efficiency. So take your cubes, 305, multiply that by the rpm you want to run at, devide by 3456 and you have the cfm needs of your engine. So for 4500 rpm it would look like this- 305x4500/3456=398(rounded)

To get a little more precise I have heard some suggest modifying that slightly to this-(CID)x(RPM)x(V.E)/3456

V.E. standing for volumetric efficiency, in a street or lightly modified engine use 0.9, for a high performance or racing engine use 1.1. Makes a huge difference, using the same rpm and a v.e. of 0.9 you now get 357 and a v.e of 1.1 now gets you 436

kruzr 11-03-2007 07:29 AM

Well '70 that video is better than first one....After watching first one I got up from my chair n fell over I was so friggin sea sick.....haha :sweat:
IMHO....Forget the big cam you aint got one,,forget the converter you dont need one.... :nono: save your pennies for other things you need

You have a lot of good advice here... :D
"if you aint got the funds pay attention" :welcome:
Get rid o that 750.... :spank: somebody said "do the math".1/2 turn no way :evil:

You wont be able to time it,you wont be able to get proper vacuum,youll just keep loading engine with raw gas.Maybe that's why the blow by,your washing the rings.....I wondering what the oil looks or feel like.
Not to mention the fouled plugs youre going thru$$$$$$$$$. :nono:
The last video sounds like engine is "lazy" which means timing has to be "advanced".IMO dont touch anymore till you get right carb,try to borrow one smaller to try out.I ya think the modulator is bad ,,,change that.

I ain't pickin on ya bud... all good experience for you.I not gonna answer this post anymore till I see video of it painted,and running right....
Sorry for long post and babbling on,hope other guys dont mind,just tryin to help.
Have a good day..........

:pimp: RIP doc

1970Chevy 11-03-2007 11:23 AM

Thanks for all of the replies guys! Im about to go down and work on the el camino, and Im taking all of your advice with me. Kruzr, you may be back on the post sooner than you think, ha ha, I have my work cut out for me today.

I'll be back tonight with pictures and video. Hopefully good ones!

kwopper07 11-03-2007 11:45 AM

WAY TO MUCH CARB a305 doesent need that much a 600 is to much but will work you say you have the idle screw out a half turn that means that carb is dumping in to much gas at idle i have a 305 with a 600 edelbrock and i feel thats to muc carb for it.

1970Chevy 11-03-2007 09:29 PM

3 Attachment(s)
Today was a good day. I just got the el camino painted. Most of the other parts of the car have at least their first coat of paint. Interior stuff is mostly ready to go in so I have that to do tommorrow :thumbup: .

Im getting the elco ready for the street while I wait for my paycheck. I get paid on friday. I want to be able to get my check, buy the carb. and bolt it on, and fire her up and drive away! :nono: Wouldnt that be nice. Its a nice thought but after working on this car, Ive at least learned that even with all the planning in the world, always expect the unexpected.

Here is a video of some of my day. I know this post isnt really related to "forum talk" but have a look if you'd like. You'll be able to hear what I'm talking about with the huge difference when shifting from park to reverse.

In case you miss the gauge readings, its the same as what I have posted up above in a previous post.

Here are some pictures for a before and after with the epoxy sealer. Enjoy! I sure do! :thumbup:

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