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Old 07-21-2010, 06:21 PM
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350 surging at 1800 rpm

Hi, this is my first post with HR's

I have a 71 350, 4 bolt, 8:1 compression
TH350 up to a NP205 turning 3.83 on 33's
750 Demon, mechanical 2nds, no choke, stock jets (I know, I know, but new for $200 you'd use it too)
Edelbrock performer intake
Summit billet HEI vacuum, new module, new condensor - set to 14degrees
Stock heads,
Edelbrock 280/290 430/440 @112 hyd flat cam
Roller rockers - 1/2 turn past lash adjusted

I burned a lobe about 1000 miles ago. Put in the new Ed cam kit and ran fine. Noticed that as time went on a surge has developed around 1800 - 2000 rpms. No notice at all at idle or in park and reved up to 1800 range. As soon as I put it in drive and have load the car will cruise down the road then subtle surge will kick in around 1800 rpms. The car will make a very slight acceleration (maybe 1-2 mph) then cut back and start over again. Get it above 2000 and the surge goes away. Problem is I'm always cruising around 1800-2000 and notice it. I checked the vacuum off manifold, solid. Checked the booster valve, fine. Warmed up and sprayed carb cleaner around engine, no change in idle.

Then I was traveling up a grade on the freeway, threw it in 2nd and climbed about a mile distance at 3500 rpm. Got her warm as soon as I pulled off the freeway and came to idle she stalled. Would start back up with some kicking, idle fine, rev fine, put it in D with some load and died. Kept doing it until she cooled back down. Thought maybe the control module was going so I just replaced it. However the surge is still there.

What in the world is it? The carb? It's less than 1500 miles old... Is it a bad ground? But then why would it rev over 2000 fine? Is it a bad lifter messing the valve adjustment?

Any help would be great!

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Old 07-21-2010, 07:14 PM
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Check your power valves probably not matched to your vacume
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Old 07-21-2010, 08:46 PM
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Nope, ran it again tonight, stall issue happened again. Died, would start, idle won't run under load, still had the surge issue around 1800 rpm. Valves were fine with previuos blueracer cam at same specs.

Help pleas
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Old 07-22-2010, 04:37 AM
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Do you have a insulator plate under the carb or is the carb bolted directly to the intake? It could be that you are getting some fuel boil in the bowls. Also, make sure you are running the correct heat range plugs and re-check the plug gap and check that the carb floats are adjusted correctly.
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Old 07-22-2010, 07:11 AM
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A "surge" at the RPM range you are feeling it could be the mechanical advance being right at the cusp of wanting to come in more, and is waffling back and forth. For testing purposes, replace one spring that's now on the mechanical advance mechanism w/a stronger spring- this will increase the RPM where advance occurs and will tell you if that's the problem.

An over advanced ignition will cause a surge, also- so the spring swap will show if there's any issue w/the ignition advance- and if it is, you can then diagnose whether it's the advance coming in too soon or if it's just too much advance at that RPM. If it's the latter, there is an inexpensive cure to limit the vac can's travel w/a VACUUM ADVANCE LIMITER PLATE- Crane #99619-1, #99619-1 INSTRUCTIONS, if your unit is not adjustable.

The carb float level should be verified, and be sure the float hasn't taken on any gas. Usually just shaking the float (if brass) will tell you if it has a pinhole leak, but obviously this requires pulling the bowl. And has been said, heat soak into the carb can cause problems, even possible signs of a vapor lock are being shown by the stalling and hard re-start.

There's always the possibility of too much fuel pressure overwhelming the float's ability to control the flow. What do the plugs look like to you? At this point, you should try to determine if the cause of any carb problems are due to a rich or lean condition. Not knowing if there even is a carb problem complicates diagnosing things. But generally speaking, I tend to want the timing properly sorted out BEFORE I go after the carb tune.

You want to check the vacuum at idle, w/the trans in gear. Take that reading (the engine has to be in tune) and divide it in half to arrive at the power valve you need to use.

But how the ignition timing is set up has a LOT to do w/how much vacuum you will see at idle. There have been many posts on setting up a timing curve for performance, even one today. I'd check it and others out for an idea of what's needed in that regard, or give the details of the vehicle (weight, use, gears, tranny, fuel, head casting number, etc.) and maybe someone here can make some suggestions.

You can DETERMINE TDC if you have any doubt if the damper line and the timing tab are accurate.

Also, you can MAKE A TIMING TAPE. This temporary tape will allow you to read the advance beyond what the tab allows- if you don't have a degreed damper or a dial back timing light.

Good luck.
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Old 07-22-2010, 11:14 AM
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OK,

On the power valves, all i'm looking at is the PCV and Booster Check Valve, correct? The Demon carb doesn't have a master valve like the Holley does.

Then on the carb, I have a 2 inch aluminum spacer. And I think I can rule out boil, cuz the surge happens when running cold. Also the bowls do not get hot when touched. When I got the carb I set it to Berry Grant specs and it ran GREAT with the old cam. That however was during winter and the weather is quite warm now. The floats are fine, there's two filters 1 at the tank, other in front of carb and both run clear.

I'll change the springs, from what I understand the "pink" springs are probably my best bet...

Coming home last night after a cool down, the surge was still there and maybe I'm missinterpreting it as a cuttout... Reason being I got a couple pop's coming from the exhaust when trying to accelerate and there's definitely a loss in power. Am I looking at the begining of a burned valve? Could the previous burned cam lobe, which was on +4 exhaust have started a blowbuy and that is now grown? I'm pulling the plugs and will run a leak down test. Would a burnt valve cause these conditions? Would make sense that when cold the valve is still seating enough to not throw the Vacuum gauge off, but after warmed up and expanded it would start to loose more blowbuy. Would that cause the stall and hard start?
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Old 07-22-2010, 11:36 AM
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Quote:
On the power valves, all i'm looking at is the PCV and Booster Check Valve, correct? The Demon carb doesn't have a master valve like the Holley does.
I'm not too sure I'm following you.

The power valve (aka "PV") I'm referring to, is an internal carb valve that allows a richer a/f mixture depending on the engine load. It is "sized" by what vacuum reading allows the enrichment to occur. A "65" PV will start to add fuel when the vacuum drops to 6.5 in/Hg of vacuum, for instance. Usually anything over 12 in/Hg will use a 65 PV, but lower and higher rated valves are available depending on the idle vacuum (in "D" for AT, in neutral for a stick trans).

When you say "pink springs", are you talking about the secondary spring? There are purple, but no pink- or are you by some chance talking about an Edelbrock or Carter carb, and the pink springs are step up springs, used on the primary metering rods?? This carb doesn't have a PV...

The previous cam lobe wipe-out wouldn't in and of itself cause a valve seal problem.

If the valve was adjusted too tight, that could set into motion a burned valve. The valve needs to be on the seat long enough to cool itself, if this wasn't allowed to happen- for whatever reason- it can overheat the valve and this, in turn, can erode and/or warp the valve.

But I still think the problem is more ignition timing related. But even if it isn't, the tining curve needs to be sorted out to optimize it, and to exclude it as being a cause to the problems you're having. Once that's done, move on to the carb. At least that's how I've usually done it.
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Old 08-03-2010, 06:46 PM
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OK, sorry for the delay, took me awhile to get around to making the changes.

I've changed the following:

1. New ignition wires
2. jets on the demon to 64 mains, 76 secondaries
3. Set Demon idle mix 1-1/2 turns out, EZ idle mix screw 2 turns out
4. "had" 14 psi vacuum, put in a 6.5 PV
5. swapped out mechanical advance springs to Mr Gasket black/lightest
6. loosened the rockers, let the hyd lifters rise and re-adjusted the valves 1/2 turn pass lash
7. Set initial timing at 8 deg, given new spings hit 24 deg by 3K rpm + vacuum adv.

It runs worse...

Current issues:

1. @8deg initial, I can't get the idle to drop down below 900rpm
2. Current vacuum is now at 16.6!!!
3. When I took #1 exh rocker off, there was a mar on the valve head. Looks like the rocker slipped over and the roller was digginn in at an angle. It's a very small gouge, not deep - but noticable, maybe 2mm by 5mm in size. Took a small file to the head to clean up the bur. Came out pretty clean.
4. There's a rocker clicking. Coming from drivers valve cover and damn if it doesn't sound like #1 exh.
5. When running, idles fine, throtles fine in park, nice exhaust, little rich. Accelerate and it studders and is rough from 1600 - 1900 rpm. Fine after getting up over 2K.
6. Bogs when stomping the trottle (probably cleaned up with idle mix tune)

Haven't ran the leak down test on all cylinders yet. Is it a bad valve that's causing all this?

any help would be great
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Old 08-03-2010, 07:56 PM
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OH!!! the surge did go away though

I'm guessing it was the spring, thanks cobalt!
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