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Old 03-03-2010, 05:40 PM
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350 popping thru carb above 3k RPM

My Blazer has a 350 with an Edelbrock Performer dual plane intake with a Holley 600cfm vac. sec. Carb. Dual exhaust (stock manifolds)

It pops through the carb starting at aprox. 3k-3.5k, and thus doesn't produce the power it should.

I've tried a variety of timing settings and none make it better or worse.

I've also put on an Edelbrock 750cfm today, and it still pops, same RPM's.

So in conclusion, adjusting timing has no effect, different carb no effect.

The ignition components seem oke, although the truck had sat for 2 years only being driven occasionally before I bought it and I did have to clean up the dis. cap etc.

I'm having trouble setting the timing with my timing light as the damper seems to be on all wrong...

Where to start? Valve clearance? I don't think a worn cam lobe is applicable as it's only in the upper RPM range.

Look forward to hearing thoughts!

Thanks,

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Old 03-03-2010, 05:47 PM
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It could well be a worn cam as when they go they begin to spit back through the carb in the upper rpm area. Remove the rocker covers, check all the rockers, check for broken valve springs. Lastly, check valve movement, if you see one or more where the rocker barely moves you found the problem.
Other things that can cause this is weak ignition, stuck shut mechanical advance, too lean fuel mixture, incorrectly set timing, vacuum leaks.
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Old 03-03-2010, 06:15 PM
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Thanks,

I'm going to rule out fuel mixture as with a 600cfm carb and 750cfm carb the symptom is exactly the same.

I think I'll replace the cap, rotor, wires and plugs first and check for vacuum leaks.

Perhaps check for play in rocker arms while I replace the plugs.

Rockers barely moving indicating worn cam lobes you mean?

I've had a worn cam before and that was a different type of sound, much more of the spitting sound while this is a real pop...

How to check for stuck shut mechanical advance? Vac adv. works.
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Old 03-03-2010, 06:25 PM
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Rockers barely moving indicates worn cam lobes.
Under the rotor button is where the mechanical advance is that is controlled by weights and springs, with the rotor on you should be able to move it, if not it`s frozen. The biggest clue in your case why I believe it`s the cam is due to the vehicle sat. When they sit a extended lenth of time and are restarted, they must be recycled as if the cam were new so lube can splash on the cam to keep it from going flat.
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Old 03-03-2010, 08:10 PM
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My rotor has movement with resistance, so I can feel the springs pushing back, so that rules that out.

Sorry I wasn't clear, it didn't sit but was only driven about 600 miles in 2 years time, probably once every few months a short trip I was told.

I'm hoping it's not the cam but wouldn't be a bad excuse for somethin' lumpy!

Any harm in running with warn lobes for a few thousand miles IF that's what it turns out being?

Thanks,
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Old 03-03-2010, 08:18 PM
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Quote:
Originally Posted by DQ81
Any harm in running with warn lobes for a few thousand miles IF that's what it turns out being?

Thanks,
Yes, all that iron that used to be a cam lobe is now floating around grinding things up.



Larry
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Old 03-03-2010, 08:53 PM
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DQ 81, This is a longshotbut, had a problem like this one time many years ago and found out the cause was a lack of fuel pressure to the carburetor, Hope this might help
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Old 03-04-2010, 05:16 PM
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Thanks,

Fuel filter is see thru and relativly new. Pressure looks good. Although it's not metered. Accel pump on carb is fine.

btw, quickly off topic, I'm not running a PCV as that's how it was when I got it, should I be for a daily?
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Old 03-04-2010, 06:45 PM
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Might as well be ordering a new cam and lifters. I would say with the others on this it's a worn cam. And yes you should be running a pcv.JMO


Cole
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Old 03-05-2010, 12:48 PM
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As I am in no way disagreeing with the worn cam idea, I"m ruling everything else out first obviously.

To run through what I've done so far:

Tried different carb, Edelbrock 750 obviously richer than the Holley, no difference.

Check and doublechecked timing. Set at 10* at idle 730RPM vac adv. disconnected and plugged. Timing peaks w/ vac. adv. hooked up at 44-46* around 3k RPM above it levels out to 30*. Seems like the mechanical advance is falling a little short could this be playing a role?

Pulled the plugs one by one while running engine at full operating temp and no difference or obvious cylinder causing popping.

Checked plugs on cyl. 1 and 4, both nice beige colour.

Check for vacuum leaks, none, only running 2 vacuum lines, vac. adv. brake boost.

Going to check valve lash tomorrow the hot and running way (have valve covers with cutouts) and I guess if that doesn't do it it's time to find a cam and lifters.

Recommendations for a daily driven 4x4 Blazer on big tires? It will never see more than 5k RPM so looking for low end grunt with most even distribution possible. I like Lunati and what I've read about them, and I sure don't mind a bit of lope. Will run stock converter, might tow something once a month and will never be off road, maybe 1 day a year for fun.

What's the best low budget bolt on head option. No machine shops here in NL so it has to be bolt on, no valve grinding/reseating etc. Just altnerates like the 305 or Vortec idea.

Any other ideas on popping?
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Old 03-05-2010, 03:20 PM
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Sounds like you`ve covered all the bases. The last thing to check is voltage to the HEI, it has to be 12 volts or higher. If that checks out okay then that only leaves the cam. The downfall of running it with a bum cam is it`ll wipe out the rest of the engine, so if you want to save what`s left you may want to install a big magnet on the oil pan to help gather up some of the metal particles floating around in the oil. Keep a eye on the oil pressure, if it`s low chances are the engine will need a rebuild. The grit will destroy the bearings.
Your vac advance should be connected to a full time manifold vacuum source, one that pulls vacuum full time even at idle.
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Old 03-05-2010, 04:11 PM
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I'll check the voltage to the HEI, thanks not been mentioned yet.

The oil pressure is very good based on the stock gauge, 2/3 or more...and repsonsive to throttle.

I guess I'll have to park her if nothing magical proves itself as the problem tommorrow, bummer. Love driving this truck.

I just don't have a garage to do the work in, but I refuse to pay some idiot to do a bad job of it...hmmmmm.

Report back tomorrow.
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Old 03-08-2010, 12:18 PM
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So I pulled the drivers side valve cover today to adjust the valve while running.

They seem to be on quite tight. Did the intake and exhaust on cyl 1. and it took almost 720* to get them to 'tik' and then when retightening the advised 1/4 to 1/2 turn it did cause the engine to almost stall, which I thought was strange.

In any case as I moved to the no. 3 cylinder I noticed the exhaust valves rocker is barely moving at all.... Game Over. Warn cam lobe as was called many times. PITA!!!!!!

Cam recommendations for a 350 in a K5 used daily which will never see more than 5k RPM? I hear great things about Lunati, I don't mind a choppy idle but I'm not looking for an Idle speed of 1200 RPM either.

Thanks for all the advice, I haven't even gotten to the passengers side as it's more than obvious what needs to be done...

Thanks,
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Old 03-08-2010, 02:38 PM
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All you need for a cam is the Edelbrock performer plus cam or Summit racing Eqiv. which is cheaper.
It has duration @.050 .204 intake, .214 exhaust. Make sure you replace the timing chain and lifters while your in there.
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Old 03-08-2010, 02:59 PM
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This is why most of us guys won't use a flat tappet cam anymore. Now is the time to pull the motor out, completely disassemble and clean completely, including all oil passages and galleries with proper "rifle" brushes. If you install another flat tappet cam without going through the disassembly and cleaning drill, you're probably gonna be doin' this again.

Think about this. You have to pull the motor anyway to clean it. Why not opt for an L31 roller motor ('96-'00 Chevy truck, 5700 Vortec). I have found roller short blocks here in Phoenix for less than 200 bucks.
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