Found out why my truck was running like crap at 4300 rpms

[DonsBolt]

I was going to make a post last night, trying to figure out why my truck has been running like crap about 4300 rpms, but I decided to wait till today and tear into her before I posted for help.

The idle control screws on my main metering block on my 670 truck avenger didn't want to work right. So I called Holley up, and they sent me a new metering block. Put it in, it started awesome, better than it ever did before, idled awesome, and the idle control screws worked better. I knoticed a little stumble at about 4300 rpms, but it felt more like a ignition issue.

A friend had a MSD 6AL he wasn't using anymore, so he gave it to me.(love free stuff) So I swapped it into my truck last week to see if that was the problem, but with the other box it still did the same thing.

The other day I messed around with the carb, played with the accelerator pump, and got rid of the stumble at 4300 in 1st and 2nd gear, but a new stumble satarted in 3rd gear, but at a lower rpm.

This morning I pulled the carb off, and found the replacement metering block had a 25 power valve, instead of the 65 the 670 calls for.

I never even thought to look at the power valve on the part holley sent me. I know I should have looked, but figured they would send me the right one.

Well all is well now, I put the old metering block back on, and the idle screws seem to work now. No idea why they all of a sudden work better. It wasn't untill I took the replacement part off that I even decided to look at the power valve.

I think I am going to crush the 25 power valve, so I can't make the same mistake. Unless someone wants it

 

[oleguy74]

25 power valve is used mostly for higher elevations .6k feet and up.opens up at 2.5 hg.wide open throttle.you need a 5-6.5 valve.

 

[DonsBolt]

I wonder if was effecting my fuel millage at all. I seams to have been worse as of late

 

[oleguy74]

would have been a little lite on power.but with 65 power valve should come alive a lot sooner.might have to get a h.a.n.s. device now...lol. at your elev.should come in about 2800-3000.

 

[7elk]

DonsBolt, I'll drop you a p.m.. I live at 5500 and usually only go up from there.

 

[DonsBolt]

would have been a little lite on power.but with 65 power valve should come alive a lot sooner.might have to get a h.a.n.s. device now...lol. at your elev.should come in about 2800-3000.

hehe I allready have a hans device I use in the race truck

DonsBolt, I'll drop you a p.m.. I live at 5500 and usually only go up from there.

I think I am all set, I have allready hit my 6500 rev limiter since I re installed my old metering block

 

[feitctaj]

you got me thinking, being I have the 670 TA and live at 250 Ft and drive from Sea level to 9000 FT and was really wishing it did not run so rich at elevation

 

[Madgyver]

Micks having problems with his TA now.... Vent tubes are spilling out fuel.. Sounds like a needle and seat that is stuck open.....

We'll find out tomorrow morning..

 

[broncnaz]

Lose them spring loaded needle seats they are more trouble than they are worth unless they have improved them some how. plus normal ones are way cheaper you only need the spring loaded ones if you do a lot of high speed runs in the rough stuff.

A power valve takes the place of 6-8 jet sizes replacing a 2.5 hg power valve with a 6.5 hg power valve should not increase mileage but as you've found out cures your 4300 rpm stumble. Cant imagine youd get good mileage at 4300 rpm anyways. As for your idle screws working now maybe there was some debris in the passages that finally found its way out as the power valve has nothing to do with idle unless its diaphgram is ruptured.
Power valves are a whole different tuning tool on holleys and probably the least understood as there is a lot of conflicting info out there.

 

[ugly74]

I tune the power valve with a vacuum gauge. hook it up where I can see it while I drive, and pay attention to how much vac I have when I'm cruisin through town and down the highway.
you want it to open when you stick your foot in it but not when you're just maintaining speed. I've got a 65

 

[DonsBolt]

well I only drove it like this for a week or so, and it is fine again

A bit of warning, do not hold the carb by the vent tube. Those things can come out and the carb can go flying out of your hand. I got lucky and was able to catch it, but not before it took a gash out of my forarm, hehe

There are some sharp parts on a truck avenger. %)