db's 66 Belvedere ***FOR SALE***

Dave-R · Post by **Dave-R** » Mon Jul 15, 13 1:32 pm

Bozwell is probably spot on with those mixture ratios. But mopar engine builders these days seem to find 13:1 makes more power on the dyno than the old 12.5:1 did back in the day.

I like an idle mixture to be 14:1 (when in gear on an automatic).

OR (if manifold vacuum level is critical and you need as much as you can get) I would just tune for max manifold vacuum at idle and don't worry about the air/fuel ratio. As long as you are sure you are not on the richer side of max vacuum.

db · Post by db » Mon Jul 15, 13 4:04 pm

Ta Dave. I was tuning for max vac which seems to be 5 at 800rpm, this makes around 14- 14.5 AFR.
If I up the rpm to 1000, vac gets to 7, afr stays about the same.
I need to tear that up and start again though now I've found the leaky shafts.

I'll re-check my timing and get some figures written down.
Then start again setting idle, primary mains, pv, pump cam, etc. Then connect the secondaries and do mains and pump cam.

AM I RIGHT IN THINKING-
PV, shooters, pump, etc have NO effect at idle.
The secondaries, if WIRED SHUT, have NO effect on primaries (aside from tweaking the stop screw to alter the transfer slot openings)?

Pete · Post by **Pete** » Mon Jul 15, 13 4:15 pm

PV WILL have an affect at idle IF it is incorrectly selected.

This is the paradox if you need it to open at cruise and off-idle but you do not have enough vacuum at idle - curse of the big cams..........

You should be ok, avoid having it open at Idle but you need to ensure it is opening as you put your foot down at cruise speeds / vacuum......

Dave-R · Post by **Dave-R** » Mon Jul 15, 13 4:51 pm

When you initially open the throttle you loose manifold vacuum so you need the PV to open as quickly as possible to richen the mixture until engine speed increases and restores vacuum. The pump shot does the same thing but if you have a good PV set-up and TOO MUCH pump shot on top you can cause a TOO RICH bog.

Always best to keep the Power shot small until the rest of the carb is right. Then just small increases in pump shot to smooth out any flat spot on launch.

Pete · Post by **Pete** » Mon Jul 15, 13 4:54 pm

The day I get a "TOO RICH" Bog

Dave-R · Post by **Dave-R** » Mon Jul 15, 13 4:56 pm

Pete wrote:The day I get a "TOO RICH" Bog

Yeah that isn't going to happen on a 500-600 inch monster with intake ports you can get your fist down.

Blue · Post by **Blue** » Mon Jul 15, 13 5:06 pm

"When you initially open the throttle you loose manifold vacuum so you need the PV to open as quickly as possible to richen the mixture until engine speed increases and restores vacuum." Yes, and for that very reason sometimes you have to fit a power valve that is open at idle and then lean the idle circuit off to compensate. I found with 5" at idle and 14" at cruise using a 3.5 PV gave me a big bog if you stood on it at around 30mph. Fitting an 8.5 PV eliminated that straight away. I think the most important thing with a PV is that it's not opening and closing at idle.

Pete · Post by **Pete** » Mon Jul 15, 13 5:15 pm

As Blue says.

I have this issue with the 6-Pak at the moment, so when the car is up and running I will confirm the best option Power Valve for cruise and sort of work backwards.

I have the added advantage that I have a lot more scope to richen up the idle mixture with the Original six-pak; something the "Jenson" set up could not do....

If you found yourself in this situation you MAY have to go 4 corner idle, but then again with a baby engine.......

The Hemi would not idle with 2 x 850 DPs when converted as an experiment from 4 to 2 corneer idle (each)......back to 4 corner idle x 2!!!

Then "BANG"

db · Post by db » Mon Jul 15, 13 7:15 pm

Blue wrote:"When you initially open the throttle you loose manifold vacuum so you need the PV to open as quickly as possible to richen the mixture until engine speed increases and restores vacuum." Yes, and for that very reason sometimes you have to fit a power valve that is open at idle and then lean the idle circuit off to compensate. I found with 5" at idle and 14" at cruise using a 3.5 PV gave me a big bog if you stood on it at around 30mph. Fitting an 8.5 PV eliminated that straight away. I think the most important thing with a PV is that it's not opening and closing at idle.

Of course, I hadn't thought of that!
If I have my 4.5 pv fitted and idle produced just 5" (allowing also for the 'accuracy' of my vac gauge

), it's bound to fluctuate a bit and the valve will open and close at random throwing everything out! At least if I bang a 10.5 in there I KNOW it's open!

Dave-R · Post by **Dave-R** » Mon Jul 15, 13 9:53 pm

I once tested a half dozen PVs of various ratings. I found they were highly inaccurate. I had valves rated at 6.5 that were more loose than a 4.5 I had.

In the end, I rated them in the highly un-scientific order of how hard I felt I had to suck on them to get them to operate. I then picked one based on that.

Pete · Post by **Pete** » Mon Jul 15, 13 9:55 pm

..and Queue Cannonball

Bozwell · Post by **Bozwell** » Mon Jul 15, 13 10:28 pm

i'm wondering if your masking a problem by having the secondaries permantly shut? at what point in the primary throttle position does the secondaries start to open? i suspect at about halfway?. maybe just tune to 13.5 afr up to this point* (put a throttle stop on the cable so you cant go over that point). after that then tune with the secondaries to get the 12.5 to 13 afr at wot.
going richer on the secondaries will compensate for a slight leanness at wot on the primaries giving a correct afr.

* you may then find that the power valve may do it's job properly at wot once both sets of throttles are open? reading some of the links the power valve should put just enough fuel in to compensate for the main jet selection of economy at cruise throttle positions. what happens if you blank the power valve and tune just on the jets? is it worth an experiment?

also it seems that fuel is drawn through the pump jets at high engine speeds making a richer mixture at high rpm wot throttle. this could be another tuning aid if the mixture leans off or richens at high rpm by fitting larger or smaller jets.

mind you tuning at wot on the road with a large engine can be interesting to say the least. it may be worth biting the bullet and taking it to a rolling road.

db · Post by db » Mon Jul 15, 13 10:28 pm

...also I'd been working on the presumption that they ping open in an instant. Which of course they won't- they'll open gradually over probably a couple of inches of vac.

I might just try and fit one in a rubber tube adapted down to fit my gauge, via a T-piece with another hose I can suck on. Then I can watch it open and check the gauge reading.

Dave-R · Post by **Dave-R** » Tue Jul 16, 13 8:40 am

Bozwell wrote:i'm wondering if your masking a problem by having the secondaries permantly shut?

Arrgh!!

You are not trying to tune a bog out with the secondaries closed are you db??

Holding the secondaries closed is ONLY for setting the primary jet size at a steady RPM. You have to have the whole carb working for everything else including acceleration mixture and ironing out a bog. In particular with a DP carb.

Blue · Post by **Blue** » Tue Jul 16, 13 9:35 am

Not a good idea to plug the PV on a street car. you would have to jet up 10 sizes to compensate. Basically you would be disabling an enrichment circuit giving yourself even less tuning options.

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db's 66 Belvedere FOR SALE