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Old 04-30-2012, 09:32 PM
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Strange issue with Chevy 350

Hey all this is gonna be a long post so please bear with me i really need some help. I have a chevy r10 with a mild built 350, 882 heads, edelbrock intake and 1408 carb, hei distributer mild cam not sure of specs. Truck ran great last year but gave me some trouble this year. I rebuilt the carb because it was very dirty and causing the motor to not want to run. Well now she will start and run but is acting very weird. It feels like i have lost power alot of power. The truck idles fine but bucks on takeoff and bucks at part trottle when cruising almost like a vibration but goes away when i give it gas or push in the clutch. Its backfireing on deceleration and revs slowly. I have messed with the timing and the only odd thing is it seems to want alot of inisial timing like 20 degress. Has anybody had a similar problem i am out of ideas. Thanks in advance.

CJ

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Old 04-30-2012, 10:02 PM
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The backfiring on deceleration can mean the timing is retarded. If you've found it wants a lot of timing all of a sudden, I'd check to see if has jumped time. That's when the timing chain skips across the teeth of the cam gear, retarding the camshaft timing, and the ignition timing along w/it. The need for more timing than usual can be an indication of this- the extra timing somewhat makes up for at least part of the large power loss caused by the jumped timing.

To check for jumped timing:
Set the distributor back to where it was when it first gave you trouble.
Take the plugs out so the engine is easy to turn over by using a socket on the damper bolt (5/8").
Bring the line on the damper to TDC on the timing tab.
Remove the distributor cap and note where the rotor is pointing- it should be close to either #1 or #6. If it isn't, the timing has jumped, providing the timing marks are correct.

You can check for timing chain slop/wear by following the directions HERE.

DETERMINING TDC will allow you to be sure the timing tab and damper are correctly indicating TDC.

There were three different combinations of damper lines and timing tabs that go together. I doubt this is going to be a problem on your engine but if you think it might be, THIS page describes them.
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Old 05-01-2012, 06:17 AM
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I have never check how much slack the timing chain has but i did check to see if it skiped a tooth when i first started running badly before i rebuilt the carb. I followed that same procedure and the rotor was pointing at number 1 cylinder at tdc and the mark on the balancer was also at 0.
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Old 05-01-2012, 10:17 AM
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Quote:
Originally Posted by 87ChevyR10
I have never check how much slack the timing chain has but i did check to see if it skiped a tooth when i first started running badly before i rebuilt the carb. I followed that same procedure and the rotor was pointing at number 1 cylinder at tdc and the mark on the balancer was also at 0.
OK, that's good.

It does sound timing related. If the timing was set w/the vacuum advance hooked up to a full time vacuum source (aka "manifold vacuum") the timing will be retarded. I would want to verify the timing using a timing light. Be sure to disconnect the vacuum advance while timing it (and plug the hose going to the carb or intake), reconnect when you're done.

Set the timing to 12-14 degrees BTDC as a starting point. You should always verify the total timing (initial timing plus mechanical timing, w/o vacuum advance added in) to be sure it is not over 36-38 degrees and that the mechanical advance is all in by around 3000 rpm.

You can read more on the GM HEI distributor including how to set up a performance timing curve, here.
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Old 05-01-2012, 05:28 PM
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Ok thanks i will read up on that and see what my timing is. The timing was set to ported vacuum with 14 initail and 33 total is that too little full advance? After i set that it still didnt run right so i kept advancing little by little. I know at the moment the motor is advanced about as far as i can get it any further and it will desiel and be hard to start but it still does this.
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Old 05-01-2012, 05:53 PM
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Quote:
Originally Posted by 87ChevyR10
Ok thanks i will read up on that and see what my timing is. The timing was set to ported vacuum with 14 initail and 33 total is that too little full advance? After i set that it still didnt run right so i kept advancing little by little. I know at the moment the motor is advanced about as far as i can get it any further and it will desiel and be hard to start but it still does this.
33 degrees may not be the ideal or optimum timing but it's close enough that the engine should run "normally". I should have been clearer on this before- ported vacuum shouldn't be present at idle, but vacuum can be present at idle if the primary throttle blades are open far enough.

So if the timing was checked and set w/the vacuum advance hooked up- even to ported vacuum- the timing may be retarded too much. It is always better to disconnect and plug the line to the vacuum advance when setting the initial and total timing.

BTW, you need more initial timing the bigger the cam is. A mild or RV-type cam can use 12-14 degrees and run fine as long as the total is around 36 degrees, give or take, depending on the fuel, compression, load on the vehicle, etc. A radical cam might need as much as 24-26 degrees- or more- to run right. But both cams will still use the same amount of total timing.
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Old 05-02-2012, 02:24 PM
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Check for foul or dead plug or bad ignition wires, carbon tracking in the dist cap. make sure you are getting the max voltage to the HEI.
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Old 05-03-2012, 06:18 AM
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Im gonna have to check the plugs and cap this weekend. I dont have that radical of a cam so i should need this much advance. The shudder is not the clutch its the motor. Going down the road at cruising speed you can feel it miss and if i retartd the timing any it pops at steady trottle at 55. I hope its not the carb because i had trouble with that earlier this year but im not sure if that would cause this or not. Thanks you guys for your help so far.
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Old 05-04-2012, 05:57 PM
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Ok guys i think i might have figured it out and i feel a little stupid about it. I checked everything again but dicided to pull all the plugs to look at them. 4 of the were really fouwled and the others were close to as bad. A new set of AC delco plugs and bam it runs great. Brought the timing down a bit and she runs great. Still have a slight bog sometimes but that might be the fuel filter i have not replaced that yet. Thanks for all the help guys and i will keep you posted on how it runs.
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