Ralphbf

Tried to have my 75 Dyno Tuned
The guy made a list of MSD pieces he wanted me to get.
List came to $600.00. He claimed HEI's were worthless.

I replaced my HEI with a known good spare, one with the Pertronix pieces. The Kit was $125.00. It ran a little smoother but stumbles bad at 4000rpm, replaced the plug wires, no difference.

The graph had large Horizontal lines in it.
Still pushed out about 240hp at 5000rpm, Cam good to 5500rpm. There was no valve float, I Have ZZ4 heads on it.
He said my Edelbrock 750 was a little lean in the middle but good on both ends.
Any guesses? I'd like to try to fix it before I bring it if possible.

Ralph

Ralphbf

Can someone throw me a bone on this one?
Good spark delivery
Carb. working okay
Stupid thing really stumbles at 4000rpm

Son-in Law thinking maybe cam is going flat.
Engine has less than 4000 miles on it.

Ralph

Crepitus

If the cam is going flat you should see grit in the filter.

The HEI that worked OK in your motor home might not be up to the task on dyno tune. How often did you run the motor home at WOT 5500rpm? My favorite on something like this is the performance distributers(edit; Davis ignition) cap, coil, controller. be sure the vac and mech advance is working with about 34* total in at about 3300 rpm. And a good set of plugs. While the plugs are out do a compression check or better yet a leakdown test.

good luck.

69427

It would be unusual if an ignition system had a problem at 4000, but ran fine at 5000. Possible, but unusual. You might be hitting your torque peak at 4000, which is a reasonably demanding point as the cylinder pressure is high, and the dwell time is getting shorter. But, the hp and RPM you're talking about shouldn't be that demanding on an HEI (or an aftermarket clone).
Just curious, what are your plug gaps, and what was the a/f ratio at the "middle" RPM?

Boswell

Ralph,
Don't know if this will be of help but had similar experience last summer with my 69 L46 350. Would hit 4000 rpm and it would stumble. Changed, plugs, cap, rotor, wires, etc. etc. The problem ended up being the fuel pump. Now runs flawless.
Good luck,
Norm

Ralphbf

I have a fuel pressure / Vacuum gauge I'll check it now.

Thanks

Ralph

63mako

Fuel sock in the tank or filter. Did you see a rise in A/F ratio (lean)in that range on the dyno?

Ralphbf

I'm still trying to get the dyno read out from the shop.
Actually what I need is a .sfd viewer.
I've looked for an hour and made several calls.

If it's the sock I could back flush the line to clear the screen.
That would work long enough for a test run.

63mako: "Did you see a rise in A/F ratio (lean)in that range on the dyno?"
I believe it was lean lower than than but improved. But I just don't remember the numbers.

I should of taken a picture of the screen.

Thanks

Ralph

mad pad house

Had the same problem old fuel pump.Fuel supply was fine at idle but as the car reved up at speed it would miss and stumble.The pump seals were shot.

Ralphbf

I installed an inline fuel pressure gauge today.

The thing pulsed wildly between 6 an zero psi.
Pinching the line it looks like 2 1/2 to 3lbs at 700 rpm.
revving it up did not stabilize the gauge at all.

With that kind of movement the fuel filter, right in front of the gauge, probally isn't plugged.

I have not backed flushed the screen in the tank yet.
Perhaps tomorrow.

Ralph

Mr.Green

Make sure you didn't get any air trapped in the pressure gauge line, it could be causing your oscillations.

toddalin

The pressure is too low and the pump probably needs attention.

I also had this problem except for me it was at the big end of the 1/4. I put a gauge on the line and only had ~3# of pressure. Replacing the pump totally cured the problem.

You stated that the engine had a problem at 4K RPM, but indicated that it pulled OK further up in the band. A bad fuel pump could cause starvation at 4K, but the starvation should continue from there up. (There may be hp lurking in the higher RPM band that you have been missing out on.)

Either way, 3# is not enough and it would probably be even less under a real load (i.e., not just winging the engine).

Ralphbf

I'm thinking electric fuel pump.
But I have the 3 lines on my pump and I'm not sure how to plumb it.

Ralph

scottyp99

They didn't give you a print-out? That's odd......


Keep the shiny side up!
Scott

Ralphbf

Tell me about it.

It took them 3 days to email me the results and they could not for some reason send me the viewer so I could look at them.

I called and spoke with his supervisor so now he's really mad at me.
But I still can not view the results.

There is a place in Fairfield, Ca. They are reasonably close4, EQ Tuning. Perhaps they will be able to help.


Ralph

63mako

A factory type pump will give you plenty of fuel delivery. Let the FI guys have the electrics. They require a regulator, bypass plumbing, wiring and are putting out way more pressure than you need. Check for a soft, collapsing hose from the fuel tank to steel line or steel line into the pump. It is that, fuel pump or tank sock.

billla

really good advice!

Electric pumps are a PITA and expense you don't need...and it's absolutely, positively not a "fix" for the problem you're experiencing.

Ralphbf

So here is the dyno results, well the one he sent me. No fuel air ratio?

toddalin

Did the guy put his wand up your tailpipe? If not, no.

BTW, it would have been better if he reported from ~2K rpm in 4th.

bluedawg

Unless Im wrong, I beleave ralph has a 700r4, 4th gear would be over drive.

Ive never seen a dyno sheet looking like that.