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Old 12-11-2011, 06:29 PM
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Timing Question

I've got a a new 496. Still in the break in period new! Here are the spec's on the motor.
Rod length 6.135
Bore 4.310
Stroke 4.250
Compression ratio 10.4:1
Intake lift .510
Intake Duration 287 degrees
Intake valves 2.25"
Exhaust lift .495
Exhaust Duration 304 degrees
Exaust valves 1.880"
cam lobe center 110.0
Ignition 6500 HEI
carb 850 holley hp
Intake elde air gap
320cc Heads
When she's cold she wants to stumble with some back fire before I can get it fired up. The timing was set at initial advance 20 degrees and 38 final degrees set when it was ran on the dyno. Since I got it home and put in the car it's been a pain getting it to fire up. Once it's fired up it runs great with no hesitation. I have it set at 12 intial and 38 total. Valve's are set right. new distributor and carb supposed to be tuned when dynoed. All that I can think is the timing being off making it so hard to get it to fire up. Any suggestions would be greatly appreciated. She's mean and runs great, but dang it should be so hard to get her going once its warm it will fire up easy everytime. It's all when its cold.Maybe 10 miles on the motor since dynoed. Thanks,

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Old 12-11-2011, 06:45 PM
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No choke on that HP. Id try 2 sizes up on the primary jets and see if that helps. I always found if the carb spit and sputtered for the first 30 seconds with a Holley and no choke, you were jetted pretty close, but if you are having that much trouble go up a couple sizes. Floats djusted right? Mixture right and accell pumps working properly?
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Old 12-11-2011, 08:51 PM
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You had it running good on the dyno is looks like, why did you change the initial timing back to 12*?
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Old 12-11-2011, 10:26 PM
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[QUOTE=68NovaSS]You had it running good on the dyno is looks like, why did you change the initial timing back to 12*?[/QUOTE

I changed the timing just to see if I needed a little less initial timing. Since it is acting up so bad I was just trying diff timing settings. It's so hard to start. It turns over stumble, stumble then the flame thrower kicks in with your occasional backfire that blows your eardrums out. Acell pumps are set right. Mixture I need to double check. Floats are right. Guess I may need to check the jets. I'm just trying to stepback from it and see what I'm missing. It's one of those times were I could take a hammer to it. Probably going to set the timing back were it was tomorrow. Does'nt want to start, but it sure runs good once it does. It's driving me crazy!!
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Old 12-11-2011, 10:30 PM
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Quote:
Originally Posted by Richiehd
No choke on that HP. Id try 2 sizes up on the primary jets and see if that helps. I always found if the carb spit and sputtered for the first 30 seconds with a Holley and no choke, you were jetted pretty close, but if you are having that much trouble go up a couple sizes. Floats djusted right? Mixture right and accell pumps working properly?
The jet sizes are front left 78, front right 78, back left 86 and back right 86. I'll order some more jets for it and give it a shot.
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Old 12-11-2011, 10:45 PM
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The dyno run should have given you some A/F ratio readings. It sounds like your timing is way out of whack or you have a crossed plug wire, or you're severely lean or bad accelerator pumps, like Richiehd is saying. Does it stumble on quick throttle opening when warm?
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Old 12-11-2011, 11:01 PM
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Look at the idle feed jets and the pump cam profile / nozzle size.
Try a cam with a quicker ramp and up on the nozzle size, you will likely see an improvement in the cold start up, regardless of dyno results, the accelerator pump circuits need to be tuned to the application.
Overlooked more often than not are the idle feed restrictors in the metering blocks. These need to match the application as well.
If the floats and the pump linkage are adjusted correctly, the pumps and idle feed circuits are the remaining two items to fine tune the driveability issues.
If the engine runs rough at light throttle low RPM cruise the idle feeds are very likely lean. No more than .001" at a time on the idle feeds if needed.
Lots of cubes there, could easily be very lean on these circuits. IMHO leave the main jets as is, the dyno has proven them.

Last edited by boss68; 12-11-2011 at 11:07 PM.
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Old 12-12-2011, 11:17 AM
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Quote:
Originally Posted by 68NovaSS
The dyno run should have given you some A/F ratio readings. It sounds like your timing is way out of whack or you have a crossed plug wire, or you're severely lean or bad accelerator pumps, like Richiehd is saying. Does it stumble on quick throttle opening when warm?
I triple checked the firing order on the plug wires, so I'm good there. Once it fire's up it runs smooth and throttle is crisp cold or warmed up. It's just a pain to get it to fire with out backfire through the carb. It only stumbles while trying to fire it up. while turning it over it will try to crank and then puff puff this will go on for awhile and then all of a sudden she'll fire up. Makes no sense
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Old 12-12-2011, 11:23 AM
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Quote:
Originally Posted by boss68
Look at the idle feed jets and the pump cam profile / nozzle size.
Try a cam with a quicker ramp and up on the nozzle size, you will likely see an improvement in the cold start up, regardless of dyno results, the accelerator pump circuits need to be tuned to the application.
Overlooked more often than not are the idle feed restrictors in the metering blocks. These need to match the application as well.
If the floats and the pump linkage are adjusted correctly, the pumps and idle feed circuits are the remaining two items to fine tune the driveability issues.
If the engine runs rough at light throttle low RPM cruise the idle feeds are very likely lean. No more than .001" at a time on the idle feeds if needed.
Lots of cubes there, could easily be very lean on these circuits. IMHO leave the main jets as is, the dyno has proven them.
Thanks for the list of thing to check for on the carb. I never really went to deep into the carb spectrum. Seems pretty straight forward though. I'll look into each thing. Dang! I would have thought when it was on the dyno they would have done all this though.
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Old 12-13-2011, 08:34 PM
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Yeah, that would be nice!
The variations in application prevent them from fully dialing it in, especially for cold starts at various temps, fuel quality..etc.
One consideration among many is a timing retard unit such as MSD makes. I personally found on my own vehicle that an initial timing of about 10 degree's makes starting with no cold enrichment much easier. Needing 18 or so to run on the street with a decent idle ..etc presents a problem, the adjustable timing unit gave the option of choosing any initial timing and then adding what you need after it starts.
Everything aside though, i believe if all the basic carb settings are right, a little pump and intermediate circuit tuning from an experienced local "carb guru" will do the job, better overall performance too.
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Old 12-14-2011, 12:04 AM
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Quote:
Originally Posted by 396chevelle
I've got a a new 496. Still in the break in period new! Here are the spec's on the motor.
Rod length 6.135
Bore 4.310
Stroke 4.250
Compression ratio 10.4:1
Intake lift .510
Intake Duration 287 degrees
Intake valves 2.25"
Exhaust lift .495
Exhaust Duration 304 degrees
Exaust valves 1.880"
cam lobe center 110.0
Ignition 6500 HEI
carb 850 holley hp
Intake elde air gap
320cc Heads
When she's cold she wants to stumble with some back fire before I can get it fired up. The timing was set at initial advance 20 degrees and 38 final degrees set when it was ran on the dyno. Since I got it home and put in the car it's been a pain getting it to fire up. Once it's fired up it runs great with no hesitation. I have it set at 12 intial and 38 total. Valve's are set right. new distributor and carb supposed to be tuned when dynoed. All that I can think is the timing being off making it so hard to get it to fire up. Any suggestions would be greatly appreciated. She's mean and runs great, but dang it should be so hard to get her going once its warm it will fire up easy everytime. It's all when its cold.Maybe 10 miles on the motor since dynoed. Thanks,
It would have helped if you'd have said you had a thumpr cam from the get-go.

Are you using a vacuum advance and if so what vacuum source is it hooked to (ported or manifold vacuum)?

For now, put the initial back to 20 BTDC. When you moved the initial down to 12 BTDC, you also changed the total to 30. Neither of these figures is near enough timing and especially true w/a hhumpr cam. Overly retarded timing can cause hard starts and miserable idle/low RPM performance, especially when the engine is cold.

Save the tuning of the main jets until you're done dialing in the idle and transition circuits. When you get to the main jets (if they need anything) use the plugs and the performance as your guide- not how well or poorly it starts up cold.

Is the carb stock or set to the stock baseline? In my experience the HP 850 Holley has plenty of idle fuel as-is. Doesn't mean that you won't need to change the IFR or idle air bleeds, but for now leave them alone.

The first thing you should do after putting the timing back where it belongs is to set the idle mixture screws to give the highest vacuum at idle. Only make changes that you need to make. Make one change at a time. Keep notes. You think you'll remember, but trust me- you won't. Write it down!

After you adjust the initial timing back to 20, you will notice the idle RPM is higher. Adjust it back down where it belongs w/the curb idle screw. When you first retarded the initial timing, chances are you had to raise the idle w/the curb idle screw. This can over expose the transfer slot, and that, too, can contribute to a poor idle and lower than normal idle vacuum.
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Old 12-14-2011, 07:30 PM
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Quote:
Originally Posted by cobalt327
It would have helped if you'd have said you had a thumpr cam from the get-go.

Are you using a vacuum advance and if so what vacuum source is it hooked to (ported or manifold vacuum)?

For now, put the initial back to 20 BTDC. When you moved the initial down to 12 BTDC, you also changed the total to 30. Neither of these figures is near enough timing and especially true w/a hhumpr cam. Overly retarded timing can cause hard starts and miserable idle/low RPM performance, especially when the engine is cold.

Save the tuning of the main jets until you're done dialing in the idle and transition circuits. When you get to the main jets (if they need anything) use the plugs and the performance as your guide- not how well or poorly it starts up cold.

Is the carb stock or set to the stock baseline? In my experience the HP 850 Holley has plenty of idle fuel as-is. Doesn't mean that you won't need to change the IFR or idle air bleeds, but for now leave them alone.

The first thing you should do after putting the timing back where it belongs is to set the idle mixture screws to give the highest vacuum at idle. Only make changes that you need to make. Make one change at a time. Keep notes. You think you'll remember, but trust me- you won't. Write it down!

After you adjust the initial timing back to 20, you will notice the idle RPM is higher. Adjust it back down where it belongs w/the curb idle screw. When you first retarded the initial timing, chances are you had to raise the idle w/the curb idle screw. This can over expose the transfer slot, and that, too, can contribute to a poor idle and lower than normal idle vacuum.
Looks like I got it the issue straight! Thanks guys for all the replies. Long story shortI ended up getting me a holley manual and studied it until my brain bled. I got the carb pretty much dialed in. Before I did that I did as cobalt said and put the initial timing back at 20 degress which was better, but not there yet still some notas bad blow back and popping. I then bumped it up to 24 inital with 14 in a 3,000 rpm. I let it sit over night went out that morning, and she started with no problems. That evening after I got home from work I started it back up with no problems adjusted the carb. Runs and sounds Great! knock on wood! I still need to take it out and see how she does on the road.
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Old 12-15-2011, 06:52 AM
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Glad it is better, with 24 deg BTDC initial it should buck the starter big time when warmed up or possibly even when its cold unless you have a kill switch for the HEI?

Do you have a high torx starter? kill switch? if it starts good at 24 deg BTDC base timing without kick back then I would say the timing marks may be off, better check TDC/damper marks again IMO.
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Old 12-15-2011, 07:23 AM
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I was going to say it sounded like the ballast resisiter was going bad or the ignition switch wasn't making good contact in the "Start" position.. My brothers 68 Goat was doing the same thing a couple weeks ago... I ran a jumper from the battery positive to the coil positive and it fired right up..

Good luck!
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Old 12-15-2011, 11:46 AM
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Good suggestion but a GM-type HEI typically doesn't use a ballast resistor or resistor wire, it needs full current to work best.

But this DOES bring up the point that if the engine is installed in a vehicle that originally had points or an ignition system that used a ballast/resistor wire to supply the "run" current to the coil, it has to be changed to a full current supply wire, about 12-14 gauge, w/o any added resistance.
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