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Chevy 350 spitting/coughing

 
bigdee2
V-6 Camaros rule!
Posts: 55
Joined: 12/10
Posted: 12/31/12 05:25 PM

1969 Chevelle  

This car has a GM Crate Engine with the Hencho in Mexico Heads made around 1991.  My understanding was this was a crate engine to replace in Chevy Trucks - had 76cc heads 1.94/1.50  8.5:1 compression, around a .397/.405 flat tappet cam, 260 HP and 304 TQ. It has an Edelbrock RPM Performer Dual Plane Air Gap Manifold and a Eddie Performer 750 cfm

The car has a 3.73 Moser Posi-trac and a turbo 350 trannie.

I decided to do the following upgrade:

Edelbrock E-Street Heads  2.02/1.60  70cc combustion
Edelbrock RPM Performer Dual Plane Air Gap Intake - same as before
Summit 1.5 roller rockers
1 5/8 headers - 2.5 exhaust - dual
Lunati Voodoo Cam  10120702LK  1,400 - 5,700   .468/.489 at .0500   219 duration
Performer 750 cfm carb - same as before


Basically, wanted a little more compression and low/mid end power/torque.  The voodoo cam came highly recommended on this forum.


The tune on that performer 750 as before obviously won't work on this new set-up.  It ran fine, but it spits/ coughs/hick-ups/stalls/misses when you gun it or get on it.  

The guys that did the work are fuel injection guys.  The car did not have this issue with the old set-up.  They switched to a 600 cfm performer and said that cleared up the problem.  But it did not.

They did not degree the cam - they say the timing marks were lined up and that was all that was necessary, so I am afraid the engine is missing the power band completely.

Before I have someone go in and sink a lot more $ into this thing, should I first try to get the local shop here to do a chasis dyno tune?

Any other suggestions other than Edelbrock heads suck as well as Lunati cams and pretty much all the parts I have do too - I've already been through all of that.

I just want a well tuned street car and has a little get up and go.  Thanks.  

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joe-
My first time was in a Chevy
Posts: 143
Joined: 11/11
Posted: 12/31/12 06:57 PM

sounds like it could be an ignition timing issue?  
joe-

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Dave632
I mow my lawn and find Chevys
Posts: 2253
Joined: 07/08
Posted: 12/31/12 07:34 PM

I agree, I would look at all ignition components first, cap, wires, plugs.
Look at any area where the wires are near the headers.
Lift your hood on a dark night with no lights around and have someone rev the engine a bit while you look for arcing. You may find more than one place where your spark is jumping.
You do not have to degree a cam in to make an engine run decent, that is just for fine tuning and max power. Lining up the marks as stated should be fine and usually it is within a few degrees of 0 either + or -.
Sounds like you have a decent combo and it should run great, by all means don't give up. I have had some crazy problems with my engines in the past and always eventually figured out the solution. Some times it can be a real pain, just keep at it. By switching carbs it pretty much rules out that area. I would look at the ignition system hard.  
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pepsi1
I mow my lawn and find Chevys
Posts: 1718
Joined: 09/11
Posted: 12/31/12 11:50 PM

bigdee2 these aren't the happy hour guys?

  Like Dave said you don't have to degree a cam to make it work. Even Cams with .650 lift are coming in to what the cam makers are advertising. If fact I had someone a year ago this time that thought knew more then I did. Went to a chassis dyno and blew the engine up. "The engine had over $12,000 in it." His mother PM'd me. HAPPY HOUR GUYS TOLD HIM TO USE A BIGGER CAM!

Remove the coil and see if the GROUND STRAP is in there. Many people throw them away.

  Remove the Distributor cap and look in the cap from any carbon tracking, look at the distributor rotor cap for cracks and carbon tracking.

  You could take a spray bottle with water only, spray the ignition wires while the engine is running, and if it the wires it will really start missing.

  If its not an ignition problem hope they broke the cam in correctly.
Also you could have a valve lifter thats not adjusted correctly.
  Lastly do a compression test or a leak down test.
The chassis DYNO is the last place I would go to have a problem corrected.
Get the engine running correctly first.

SOME FYI: I am working with a guy on the Car Craft forum that went to a chassis dyno for a tune with a simple 318 engine bebuild and upgrade. (I'm saying we) he is doing things he can do.

(He) We are undoing everything they did. (I must add this guy spent a lot of money on the best parts). Every change he did has made gains in the engine performance. I'm wating to hear from him. He is waiting for his mechanic to bring the cam back to straight up. The guy also told him not to go to a chassis dyno.
Good Luck

Bob  

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BuzzLOL
I have an SS396 tatoo
Posts: 380
Joined: 12/10
Posted: 01/01/13 10:45 PM

.. Possibly likely is the distributor was stabbed back in incorrectly and/or the lifters/rocker arms were incorrectly adjusted...

.. The 750 carb. should actually run better with your new setup than the original setup which only needed about a 500 carb...

.. As Pepsi suggested, a compression test on all 8 cylinders could tell you a helluva lot... do it with ignition disconnected from coil, all sparkplugs out, and throttle wide open...

.. Even before that, set engine at 20 degrees before TDC (Top Dead Center) and see if the rotor in distributor is centered on #1 sparkplug terminal... if it appears to be about 180 degrees off, turn engine 1 full turn and look again...  engine turns twice for each one turn of camshaft/distributor...

.. This crate engine, if you have the whole thing, had dished pistons and big 76cc chamber heads and poor quench distance... nominal "8.5:1" compression ratio, but actually about 7.6:1... very low... by adding a bigger 219/229 cam in place of the stock 194/202 cam, you've almost introduced a vacuum leak to an already too low compression ratio... your too low vacuum is prolly causing the carb. vacuum secondaries to open way way too soon... resulting in wheezing by this asthmatic engine... the carb. can be adjusted to work, but this combo will always be sluggish, weak, and a gas hog... not really familiar with that carb. internally, but if it has a power valve or similar enrichment device, it's also opening way too soon...

.. The cam you have wants to make about 425HP... the engine about 295HP... a mismatch of parts...

.. A vastly better combo would have been at least 64cc heads... and, even better, flat top pistons as well...  

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xFourLeaf
V-6 Camaros rule!
Posts: 62
Joined: 01/13
Posted: 01/03/13 12:55 AM

Assuming that it only spits and burps when you nail it, I would guess that you need to adjust the accelerator pump on your carb. It sounds like it is getting to much gas to fast and doesn't have the air flow to match it so it burps the gas back up.  
xFourLeafxFourLeaf

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68scott385
I mow my lawn and find Chevys
Posts: 1994
Joined: 10/09
Posted: 01/03/13 09:57 AM

Icon QuotexFourLeaf:
Assuming that it only spits and burps when you nail it, I would guess that you need to adjust the accelerator pump on your carb. It sounds like it is getting to much gas to fast and doesn't have the air flow to match it so it burps the gas back up.

I have a tendency to agree with this except it's an Edelbrock carb. In my experience these don't give enough gas when you crack the throttle.

I was waiting to hear a report from the OP stating what he found after doing the aforementioned checks before mentioning the carb settings, don't want to overwhelm a guy with possibilities.  
68scott385 68scott385 68scott385

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