Engine running - Power Valve query/nozzle query

GJUK · Post by **GJUK** » Wed Sep 30, 15 6:18 pm

Hi all.

My engine is running better now, though I hope someone can guide me with power valves as I think that is where recent changes are pointing me towards.

The car had a huge backfire issue with .25 nozzle in the primary.
.31 nozzle was fitted and that improved but did not remove the back fire.
.37 was fitted - almost cured the problem
Now it has a .40 in there, with a hollow screw and no backfire. So this problem is cured.

The car is after firing however, through the exhaust, it has done this before a bit but it is more noticed now.

I think the PV might be incorrect though I have not yet looked to see what it might be.

In drive (due to the low comp engine and huge overlap on the cam) I get 10 Ins hg on the vacuum reader (below the carb at the manifold).

The car revs well now, responds well and does not bog. But there is this afterfire through the exhaust I now need to cure.

Float levels set
idle screws all set the same (in D with vac reader) 1+1/4 turns out.
Timing not checked, though was set by John on here (marked up and not moved)
no vacuum leaks
spark plugs all cleans up (were quite black)

Carb is a Holley 750 DP. 4150 model.

Hope you agree with me on the PV?

Thanks,

Jon

Pete · Post by **Pete** » Wed Sep 30, 15 6:42 pm

Out of the box it will probably be a 6.5" Power Valve.

GJUK · Post by **GJUK** » Wed Sep 30, 15 6:47 pm

Thanks, I think I read somewhere that a PV should be 1/2 what you get at idle as a reading on the vacuum gauge?

So unless there is something very low in there that might not explain the issue. Though I could try putting a 8.5 in...

I guess I need to see what is in there... Does it sound like my above theory has legs?

Pete · Post by **Pete** » Wed Sep 30, 15 6:54 pm

Rule of thumb is 2" below your Idle (in "D") Vacuum, but sometimes you may have to change this to get rid of flat spots when driving.

GJUK · Post by **GJUK** » Wed Sep 30, 15 8:08 pm

I might bump that up then a little to 8.5 see if that helps.

Jets wise there are 69's installed.

GJUK · Post by **GJUK** » Fri Oct 02, 15 9:25 am

Theory today is that it could be underfueling still.

Holley 750DP's have 70 or 72 size jets in the primary, this had 66.

So I have bought a range of jets and will try a 72 jet to see if this helps.

Exhaust popping/after firing seems can be caused by overfueling or under fueling. As the car responded much better to larger nozzles and now revs clean from 800rpm tickover to 4500rpm

I need to get an AFR meter

Dave999 · Post by **Dave999** » Fri Oct 02, 15 9:53 am

I know little about holleys

but on a weber, popping as you lift off indicates too small on the idle jets or too big on the air jets for the idle circuit

i.e. lean

it can also be indicative of a air leak at the exhaust manifold or early on in the header. heat + un-burnt fuel + nice oxygen = pop

Dave

MilesnMiles · Post by **MilesnMiles** » Fri Oct 02, 15 12:23 pm

Hey Johnny, this might not be helping...

quote '(due to the low comp engine and huge overlap on the cam)'

I'm with Dave, popping on the over run sounds lean to me.

On my Holley I went from 6.5 to 8.5 with no change and a rich condition. Went to 3.5 and pretty much cured it. However, its a vac sec carb so not apples to apples.

GJUK · Post by **GJUK** » Fri Oct 02, 15 12:48 pm

Interesting. When I thought about it later in the evening, raising the PV would not help an underfuel issue on the primary, I would have to lower it like you mentioned so it kicks in earlier.

10" mercury on tick over so maybe the 6.5 in there is a tad high.

I will try larger jets first of all, out of the box these should have 71's in the main, not 66 like this has. Got some jets here from real steel so I will have a play this evening

MilesnMiles · Post by **MilesnMiles** » Fri Oct 02, 15 12:55 pm

As I understand it the secondary mains simply support the enrichment by the power valve not the other way around.
That's why if you plug the powervalve for instance, you have to up the main jets a significant amount.

People move jts sizes al the time (me included!) but its probably a power valve thing as well.

GJUK · Post by **GJUK** » Fri Oct 02, 15 6:22 pm

Something is not right with my ignition system,

I changed the jets, no real change. put 72's in the front.

I hooked up a timing light and no1 lead was getting no spark, tried no 2 and it was sparking. no3 sparking on and off. others were sparking fine.

reseated all HT leads, king lead. No change.

It could be a faulty timing light but probably not as it is quite new.

Going to just bin this dizzy, coil and ignition unit for a new aftermarket one.
Where and what should I buy?

Mick70RR · Post by **Mick70RR** » Fri Oct 02, 15 6:44 pm

At least check the ignition timing first. Sounds to me like the timing is way out.

GJUK · Post by **GJUK** » Fri Oct 02, 15 7:11 pm

Mick70RR wrote:At least check the ignition timing first. Sounds to me like the timing is way out.

I hear what you are saying, but I should be getting a spark on no1 constantly even if the timing is out.

Its the original dizzy with the car, the king lead is getting a strong regular spark but I'm not convinced that the dizzy is not moving about, spring related or cracked of just ruined.

I'll upload a video it in a second.

MilesnMiles · Post by **MilesnMiles** » Fri Oct 02, 15 9:09 pm

John, step back a minute chap. Huge overlap with cam, low compression so cylinder pressure low etc.. Is this a mismatched parts issue? Have you cranked up as much advance as the starter motor wil allow. Sometimes need baby steps to sort out these things.

GJUK · Post by **GJUK** » Fri Oct 02, 15 9:32 pm

MilesnMiles wrote:John, step back a minute chap. Huge overlap with cam, low compression so cylinder pressure low etc.. Is this a mismatched parts issue? Have you cranked up as much advance as the starter motor wil allow. Sometimes need baby steps to sort out these things.

Yes the parts in the engine do not work well with each other.
Not sure why I'm not getting a good spark...

Do you suggest to by ear advance the timing as much as possible? I've not touch the timing at all...