Need help.........

OK guys, here's the deal. I sold my 521 to another member that's up here in Montana also. I changed the cam, heads, and intake to better suit his useage. We're having an issue with it that I just can't quite get a handle on, so I'll run it by you guys and see if there are any other ideas. Here's the set-up:

'78 F150 4x4 coming in @ 7000 Lbs. +

521 C.I.

Stage II ported D3's with big valves. (2.25/1.76)

Lunati 61605 Hyd. Flat Tappet cam. We had the Lobe Sep changed from 110 to 112 for a little better idle quality.

Edelbrock RPM Airgap Intake.

Timing is at 34*, which seems to be the best for this situation.

We've tried 4 different carbs on it, to try and get rid of this issue. 770 Truck Avenger, Holley 800 Dbl Pumper, 830 Holley Dbl. Pumper Race Carb, 1150 Dominator. I'll get to the carbs shortly.

So here it is:

Brad is plum tickled with the brute power this engine has, but it keeps getting a mid-range "flutter" when the engine is under heavy load. Meaning, if he's on pavement, and has alot of traction, it has this little "flutter". But, if he's on dirt, and has some wheel speed, it doesn't do it. The "flutter" comes in about 4500 RPM, then it cleans itsself out and keeps accelerating. We've tried everything, from changing ignition boxes, distributors, timing, plugs & plug gaps, and CARBS. I talked to Blake, and he had us do a cranking compression test to make sure we weren't possibly getting some detonation under the heavier load. The cranking compression averages 170 Lbs.

One thing that seemed to help the most so far, was to shorten the plug gap from 45 to 35, and to put the bigger carbs on it. It got rid of most of it, but it still has a "flutter". With the carbs, the 770 is just too small, the 800 works very good, but still has this "flutter". As does the 770, but worse. The 830 is not quite as good as the 800. The ONLY other carb he could find is this 1150 Dominator from his friend's BBC. He said the 1150 makes more power than all the others, and the "flutter". He says, it makes crazy power up to this "flutter" then it keeps accelerating.??? The 1150 also got rid of most of what was left from the gap change, but is still present. The main problem I have with helping figure it out, is I'm like 180-200 miles away, so I can't physically check it out. So, I'm asking you guys to do the same thing.? It's probably something simple that we're just overlooking. I hope.

Any other ideas to look at???

My question is what rpm does this flutter occur at? Is it always around the same rpm? Is it at part throttle or full throttle?

Charlie

I put it in the post, but it comes in pretty consistently around 4500'ish. It doesn't seem to matter if he's on it hard or gradual acceleration. It hits about 4500, flutters a little bit, clears up and takes off again. I did forget to add, it has 1/2" fuel line, with a Holley Blue Pump, and regulator. But it also did it before upgrading the fuel system...?

Doug,

It seems like you have really focused on the carbs and fuel system, and haven't found the problem yet. Therefore I'm inclined to think it's not in the fuel system/carbs.

I'm going to suggest a couple of options.
1. This happened to me once several years ago, but I had a cracked flexplate in my muscle car and when the engine reached a certain rpm it had a torsional vibration that could be felt and it seemed like a flutter and then went on and cleared up at rpms either side of the "bad" harmonic.
2. Check your centrifugal advance springs in the distributor. I've seen some that wanted to "hunt" when they were too light. Maybe have your distributor spun up on an old style Sun distributor machine and carefully check the advance curve.
3. Check all of the valve springs and see if you don't have a broken spring or dampner. Again this happened to me and the engine seemed like it had a flutter.

Hope this helps,

Charlie

Doug,

Those are really good suggestions from Charlie, I'll add my small contribution LOL.

I had one do the flutter thing much lower at 2500rpm or so and it turned out to be a weak spark plug wire. Checked them all with an ohm meter and one was way out. Something else to check.

FWIW Mark

like Charlie said,the advance should be checked and if it has a vac advance unhook it and try it.also check for a vac leak,then check again with the carb swaps making some change i'd bet it's vac related.

Thanks guys. The flexplate is an SFI unit, but that don't mean anything in this line of fun.

On the dizzy, it "seems" to be ok as far as timing. I had him check it at RPM with the light, and he said it doesn't move a lick. But, that was only at about 3000 RPM or so. I'll have him double check the springs too.

The valve springs had to be changed, because even though the seat and open pressures were good for this cam, (115/335) for some reason it went into valve float before 4000 RPM. Spring rate maybe??? He put a heavier spring on it (135/405) and cleared that up. What has us baffled, is the fact that it only does it when he doesn't have good wheel speed. Like accelerating on pavement vs. dirt.

I'll also have him check the wires with an ohm meter. Even though he says they're new, there could still be a bad one.

He has checked for vacuum leaks, and came up with none & the vac. advance is not hooked up. It shouldn't even come into play except for off idle performance. Of which there is no problems there. He says it will almost pull the left front tire on launch, and it just annihalates those 44" tires. Until it hits 4500 RPM that is.

Diff wore out? Trans slipping?

Does it have any kind of RPM limiter that may be getting activated unintentionally. I had a downtrack miss once rom this.

u-joint starting to go bad somewhere in the driveline?

I had the exact same problem years ago and it was the cam , i forget what it is called but the lobes have miniscule steps in them and if there not right the springs with go into valve float for a second , the cam I had did it at 4800 rpm ,the motor would rev like crazy to 47-4800 rpm sit there for a second and slowly rpm threw the band to 5000 rpm and rev to 7200 like crazy , putting heavier springs on it would just move the flutter up 200rpm , crane replaced the cam with a new improved grind and the flutter disappeared
Frank

dfree383 wrote:Diff wore out? Trans slipping?

Nope, and nope.

BigDave65 wrote:Does it have any kind of RPM limiter that may be getting activated unintentionally. I had a downtrack miss once rom this.

Yes it does Dave, but he took the chip out completely.

lghting94 wrote:u-joint starting to go bad somewhere in the driveline?

It's not in the drivetrain, it's definitely an engine problem.

Frank Merkl wrote:I had the exact same problem years ago and it was the cam , i forget what it is called but the lobes have miniscule steps in them and if there not right the springs with go into valve float for a second , the cam I had did it at 4800 rpm ,the motor would rev like crazy to 47-4800 rpm sit there for a second and slowly rpm threw the band to 5000 rpm and rev to 7200 like crazy , putting heavier springs on it would just move the flutter up 200rpm , crane replaced the cam with a new improved grind and the flutter disappeared
Frank

Wow Frank, that's kinda creepy. We were just discussing this very same possiblity. I'm gonna call the cam guys today. We done changed/checked everything else. That's about all we haven't dived into. YET!

i had a stock shortblock i stuck a cam in with my dove heads from my good,apart at the time,motor.it run like hell at low rpm but just would'nt make power after 4000rpm.the springs were too strong for the hyd cam.i'd ran the same setup before with better oil psi and didnt notice the problem.i found it checking rockers,turned it over a little to make sure all were tight and found some loose.cranked it up w/o valve cover and all sounded fine cut it off and all the rockers "on a lobe"would bottom out the lifter when the oil psi stopped.i know that sounds f/u but it was the problem.

Doug,

Any update on this?

Yes. Brad put it in a seperate post... Turns out the cam appearently had an issue. He swapped the cam, and it cured the problem.

Thanks,

https://www.429-460.com/t10033-engine-missing-solution

wow...very interesting...glad it got figured out..first time I've heard of that.