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Tuning Problems. chevy 350

  
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Tuning Problems. chevy 350

 
coachramos1 coachramos1
New User | Posts: 9 | Joined: 01/14
Posted: 01/24/14
02:55 PM

Okay guys I need some help on getting my new motor tuned correctly. I purchased a rebuilt 350 with a 1991 mild cam upgrade for my 1967 el camino. I bolted on headers, a summit hei distibutor, edlebrock performer rpm intake, and a edlebrock 1406 (600cfm) carberator. Stock rebuilt turbo 350 tranny, and stock rearend with basically highway gears. This was meant to be a simple turn key cruiser. .... BUT I can't seem to get the timing right or the idle mixtures right. The motor has what seems to be a vacuum leak skip going on, but it's all sealed up tight. When I put it into gear the rpms drop 500-600 and it goes from a smooth idle to a horrible lurching idle, and  when driving it and getting on the gas even a little it feels as if there is a miss. Double checked the plug wires were connected right, set the timing multiple positions, and played with the carb some. 4 hours of toying with it later and I'm here asking for some advice. Want to give it one last shot on my own before taking it to a shop.

Any and all advice is welcome!!!  

76Skylark 76Skylark
Guru | Posts: 854 | Joined: 12/11
Posted: 01/24/14
03:19 PM

Two things Come to mind.bring the Base RPM to700-800 in gear at Idle, That's a touch High but may allow you to play with the Timing. How Much Vacuum are you Pulling?  Have you Sprayed Carb cleaner At ALL the Spots that can leak? I'm Not to Good with An Eddy Carb so I don't know the Adjustments, You seem like someone that Knows the Basics to a carb tune.  

idrivejunk idrivejunk
Addict | Posts: 5117 | Joined: 12/09
Posted: 01/24/14
08:18 PM

I've run a carb like that for a long time, theres not many places it could leak vacuum. Maybe double check the imprints on the carb to intake gasket to make sure theres a good seal all around. A vacuum gauge might help you. But since its a "rebuild" which means sort of unknown, and with a cam upgrade, it might be wise to pull the timing cover and double check the timing chain installation. Because if it quacks and waddles its probably a duck, as I like to say. Meaning if everything else is right but theres a general craziness to it that you can't pin down, it could be that the valve timing is messed up somehow. I suppose a cranking pressure test might reveal something. Also, double check that Summit distributor. Maybe try a known good borrowed HEI to eliminate a possibly faulty new unit, I got one of those from them.  
idrivejunk

shyrgfuh70 shyrgfuh70
New User | Posts: 32 | Joined: 01/14
Posted: 01/24/14
11:56 PM

do not take it to a shop.

remove the vacuum to the dist, plug it and set your idle timing to 10 deg btdc.

cover the carb top with your hands. if the rpm increases you likely have an air leak.

either way, unplug ALL vacuum at carb and plug vacuum at carb including p/b and pcv.

try it now, if there is still a prob, take a can of FLAMMABLE brake cleaner [not wd40, not carb cleaner]attach the long nozzle and gently spray everywhere including intake at the heads.

you may have an intake leak on the valley side, these are hard to detect.


if you have another coil try that.

look at your plug wires at night with the engine running, move them around and look for sparks/arcing etc.

your carb will be next.  

Dave632 Dave632
Addict | Posts: 2225 | Joined: 07/08
Posted: 01/25/14
08:57 AM

How rough is the cam at idle? Will it idle in the 6-700 rpm range, or do you have to run it in the 1000-1100 range?  
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coachramos1 coachramos1
New User | Posts: 9 | Joined: 01/14
Posted: 01/27/14
12:39 PM

At 600-700 it is very rough. I have to feather the gas to keep it from stalling. Right now to get it running without stalling and smooth we have it set at 1100rpm.  

coachramos1 coachramos1
New User | Posts: 9 | Joined: 01/14
Posted: 01/27/14
12:45 PM

Man you guys are awesome!! Thank you for all of the helpful advice. I got busy over the weekend, so I haven't gotten to the car again yet, but I will be trying these things out soon. My timing light is 10+ yrs old and has been sitting in storage for 4yrs, so I'm going to start the day off by getting a new one. Just not trusting that old one anymore. I'll have an update on here tomorrow sometime as to if the problem was found and fixed. I've got a list of things to try now, so hopefully one or a combination of these things will do the trick.

Thanks again guys!!  

coachramos1 coachramos1
New User | Posts: 9 | Joined: 01/14
Posted: 01/27/14
12:56 PM

one more question I almost forgot. Do you guys recommend using the full time vacuum port or not? I have never used full time vacuum on any of my cars, but while putting this one back together I ran across a video on YouTube that recommended using the full time on any vehicle not equipped with smog or air conditioning. Of course now I have it plugged onto the full time, but I am second guessing that decision too.

Thanks!  

joe- joe-
User | Posts: 133 | Joined: 11/11
Posted: 01/27/14
03:04 PM

i made the switch to full manifold vacuum and like it.  

heres a good video on setting timing.
http://www.youtube.com/watch?v=UYGU7mTwsZc

i use a vacuum gauge to set the mixture screws.   shoot for the highest reading.  
joe-

coachramos1 coachramos1
New User | Posts: 9 | Joined: 01/14
Posted: 01/27/14
04:19 PM

Great link.

Thanks for the info!  

shyrgfuh70 shyrgfuh70
New User | Posts: 32 | Joined: 01/14
Posted: 01/28/14
12:05 AM

hi

do NOT use ANY vacuum to your distributor until your prob is fixed. it will complicate the issue.

very few people run the dist vac to manifild. it can cause problems. i stronglt advise against it.

your old timing lite is probably quite happy as long as its the clamp on type.

put white out on your timing marks at 0, 10 and 30 degrees. it will be much easier to see.  

wayne712222 wayne712222
User | Posts: 208 | Joined: 10/13
Posted: 01/28/14
04:42 AM

first.. is that a brand new carb..   if so.. take it off and check that the throttle plates are centered in the bores when you hold the choke wide open so you are not having the throttle lever against the fast idle cam.. you will need to back off on the idle speed screw to get the throttle plates far enough closed to hold the carb toward the light to see if they are evenly centered. sometimes the carb gets set down and the throttle blades are driven off center..

set the idle speed screw.. so the bottom edge of the primary throttle blade just barely exposes the idle transition slot..

lets look at this diagram...

at idle.. all the fuel is being pulled thru the idle circuit that is controlled by the idle mixture screws...

if you have the idle speed up too far.. you will expose the idle transition slots..

these are not controlled by the idle mixture screws.. they are controlled by the idle feed restriction and the pair of idle air jets.. this is richer than idle and if you try to idle on the idle transition circuit you will be way RICH and it will run horrible..

90C08742 E8aa 4197 8E7a 68837E4e7447 Zpscd75c134

at idle in the image below is all the way to the left .

Idletransitiondiagram Zpsb9fc2fb9

part throttle is the middle image..  see how the fuel is feeding thru the idle and the idle transition also.. there should be another between b and c when both the idle transition circuit and the primary mains are flowing..


you can see the circuits in this.. but the throttle plate is too far closed in the image below..  a fraction of an inch of the idle transition slot should be exposed .. this allows you to open the throttle just slightly and bring in the idle transition circuit to keep the air fuel ratio in the 12.5 to 14.to 1 air fuel ratio.

Idlecircuitdiagram Zpsd255d76d

2100Idle Zps0085c49b

Transferslot

AFB AVS Primaryboosterdiagram Zpse535929c  

joe- joe-
User | Posts: 133 | Joined: 11/11
Posted: 01/28/14
10:29 AM

shyrgfuh, before you put manifold vacuum down, you should learn more about it.
heres an exscript from a well written article by  Lars Grimsrud;


Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

read more;
http://www.460ford.com/forum/showthread.php?t=117504  
joe-

shyrgfuh70 shyrgfuh70
New User | Posts: 32 | Joined: 01/14
Posted: 01/28/14
10:49 AM

Hello joe-

First of all thanks for the info, however you are assuming I know little about the diff between the two however I can assure you, this is an inaccurate assumption.

I actually know quite a lot [but not all] about manifold vs ported and understand the differences in operation.

I know about the infamous engineering report on it. I simply don't have time to go into detail on the differences, besides I did not say it cannot have any benefit.

If you are happy with it, then that's a good thing.


The statement/claim below is generalized, and therefore, is at the very least, misleading.

"Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone."

This is a generalized statement, and therefore, is, at the very least, misleading.


FYI - Above around 1/8th throttle, ported vac and manifold vac are virtually identical.

Vacuum advance units only operate at a vacuum level that is at or above their set initial operating level.

When you accelerate above around 1/8th throttle, they are closed.


Contrary to some peoples belief, just because it's on the internet, it does NOT make it true.


The link below contains an actual "live" graph of ported vs manifold vacuum in action.

http://thisoldjeep.fr.yuku.com/reply/7110


My primary objective is to help him get his car idling properly, after which it will hopefully run without missing etc. What he does after that is not really a concern to me.  

coachramos1 coachramos1
New User | Posts: 9 | Joined: 01/14
Posted: 02/06/14
10:13 AM

****UPDATE*****

I took all of this info into consideration and worked through most of them like a check list. After 4 hrs of fine tooth comb'ing it I am happy to say that IT IS RUNNING GREAT NOW!! Smile

I started with the spray and checking for leaks. Had a small one on the front of carburetor gasket. Loosened the carburetor, re-positioned it, re-tightened it and the leak was fixed. while I had the carburetor off I checked that the butterflies  were centered and they were. I did move the vacuum hose back to the ported vacuum connector and then began messing with the timing again. After 2 cans of carburetor cleaner spray and 4 more failed attempts at setting the timing correctly with my old timing light I decided to give buying a new light a shot. I purchased a $69 dial adjustable timing light from the local parts store and while I was there I talked to the ever knowledgeable "old man" behind the counter and he gave me this bit of advice.

"Do you know the overall advance on your distributor? If not then setting it at idle isn't going to fix anything. Rev it up to 3500 rpm and set the timing to 32-35 BTDC depending on the fuel you are going to run".

When I got home that's exactly what I did because I have no idea the total advance that this summit hei distributor provides. Warmed the engine up for 10 min, adjusted the idle to 3500 rpm, and set the timing to 33 BTDC using my new dial adjustable timing light. I slowed the in gear idle back down to 700 rpm, which has me at 1100 rpm in park and now this thing is running perfect.

THANK YOU ALL for tips and advice on solving this issue. I hope that this thread will help others with similar issues in the future!  

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