700 double pumper on a road car

Blue · Post by **Blue** » Tue Sep 02, 14 1:25 pm

Assuming the timing is correct then a severe lean bog seems to be the case. Shooters are only for fine tuning, you won't fix a bog of that magnitude with pump shot. I would set up the idle mixture first and then cruise around with the vacuum gauge hooked up to see what the engine is making at the point you nail it and it backfires. My feeling is you might need a higher rated power valve rather than a lower one.
The x amount below idle vacuum rating for power valves doesn't always work, especially if you are dealing with very low vacuum. In a nutshell, a power valve opens when vacuum falls to it's rating, 6.5 for instance. below that it's open, above that it's closed. When it opens the amount of fuel it delivers is the equivelent of going up 10 main jet sizes, so loads basically. So, say your cruise vacuum is 20, you mash the throttle and introduce all that extra air into the mixture but the power valve won't open to compensate until vaccum has fallen to 6.5 and that's your lean bog right there.
Contry to what the books will tell you you can have a power valve open at idle and be able to get the idle mixture right. I've had to do it on a couple of occasions.
Vac. sec carbs are much more forgiving in the "mash the throttle at any old speed" department beacuse the secondarys will only open when there's enough vacuum to overcome the spring that's holding them shut.

Adrian Worman · Post by **Adrian Worman** » Tue Sep 02, 14 2:07 pm

Excellent explanation

GJUK · Post by **GJUK** » Tue Sep 02, 14 2:37 pm

Thanks Blue but the problem of 'very little response, low power and large backfire through the carb and fireball' was had when going from a stand still (850rpm) to medium throttle on the pedal.

I will set the idle mixtures tonight after checking the plugs.

My understanding was if my car runs with a vacuum of around 9 (in drive) the power valve should be lower than an 8.5, more like a 6.5 or as Holley say in the books 1/2 it so a 4.5.

I have a 6.5 though so I will drop this in.

The car runs MEGA rich, it bleeds your eyes to stay in the garage with it running, I had to wear a mask yesterday, even after 1 minute of it running.

The timing had changed since I did a good power run in the car, maybe it is slightly too advanced, 17* idle and 37* at full load 3k rpm.

At least the car did not cut out on the driver of about 3 miles yesterday at all. switching it off and back on it started on the button - that would kind of indicate the timing is not too advanced also, as its not bogging the starter.

So to recap

Check sparkplugs for colour
set idle screws (these could be way out for my car?!)
Drive it.
Then change powervalve to a 6.5? Though Blue you are the only person that says to go higher?

I will be bringing my double pumper to HRD's. Anyone that can sort that out will get a case of beer.

Thank you

Jon

Blue · Post by **Blue** » Tue Sep 02, 14 5:49 pm

My gut instinct says you still need more initial timing, especially with your comments about it stinging your eyes at idle. Yes the books will always tell you the power valve must be closed at idle, and in 99% of cases that is correct. But with some combinations you just can't get rid of that lean bog without going higher, my Dart for instance, Pete's Bee, Jem's Belvedere come to mind, hell I even had to go to an aftermarket metering block to get that thing to work right. In the end it's a case of understanding how the various systems work and then experimenting to get your particular combination to work. Keep at it, you are making progress and you'll be pretty clued up when you've got it right.

GJUK · Post by **GJUK** » Tue Sep 02, 14 6:20 pm

Blue wrote:My gut instinct says you still need more initial timing, especially with your comments about it stinging your eyes at idle. Yes the books will always tell you the power valve must be closed at idle, and in 99% of cases that is correct. But with some combinations you just can't get rid of that lean bog without going higher, my Dart for instance, Pete's Bee, Jem's Belvedere come to mind, hell I even had to go to an aftermarket metering block to get that thing to work right. In the end it's a case of understanding how the various systems work and then experimenting to get your particular combination to work. Keep at it, you are making progress and you'll be pretty clued up when you've got it right.

I'll be able to design a bloody carb myself by the time I'm done

GJUK · Post by **GJUK** » Tue Sep 02, 14 6:21 pm

I've made a start on my design by the way, keep it under your hat.

Adrian Worman · Post by **Adrian Worman** » Tue Sep 02, 14 6:57 pm

You mentioned in an earlier post you had no vac hooked up to the carb? That will give a very rich idle condition, you may need well in excess of 20 deg. A lot of vac advance promotes efficient combustion, if you are gettin an eye watering exhaust the chances are that the carb isn't too rich it's just simply not burning the cylinder charge completely.

GJUK · Post by **GJUK** » Tue Sep 02, 14 8:14 pm

I have no idea.

I'm grateful for the ideas but im running out of energy with this car.

The car runs like a bag of Bananarama! at the moment, idles great but now does not rev at all.

The idle screws are set at 1 and 1/4.
The car idles at 900 rpm and 10 " mercury
In drive it drops to 700 rpm and seems happy and drops to 5 inches of mercury
The spark plugs tell a tale of being spot on, golden brown, nice.
The car starts on the button and idles better than it did on the double pumper
The car pops out of the carb when you boot it from idle

Tried a 6.5 PV in the car, no real change, if anything its now worse.
Set the Accelerator pump, set it to .15" inch with a feeler gauge (it was really tight) still no better.

floats are bang on checked them again.

From what I can tell I'm doing everything by the book.
I even tried retarding and advancing the timing by ear and it is just not happy anywhere else and still pops when you give it some.

Can't help but think that this car needs a special fix due to this long duration cam and low vacuum. In Drive at the moment the power valve is going to be kicking in, perhaps thats an issue?

I have a video, will upload.

MilesnMiles · Post by **MilesnMiles** » Tue Sep 02, 14 8:19 pm

Jon, don't get too down. If two carbs have failed to fix it I imagine the problem is elsewhere. You really do have too much cam for a low comp motor I suspect.
I can't recall, but is this a auto trans or stick shift?

GJUK · Post by **GJUK** » Tue Sep 02, 14 8:24 pm

Hi,

It's an automatic.

The car really struggles to rev now, where as with the double pumper pulled like a bloody train right through the gears then flooded.

Maybe I would have been better sorting that flooding/float issue :-/

It starts to rev now, then there is a gap of nothing then it catches itself and revs again.

More so now with the lower power valve in it.

Maybe I should just put a 200 power valve in there or drill a huge hole for the fuel to flood in?

GJUK · Post by **GJUK** » Tue Sep 02, 14 8:33 pm

View My Video

GJUK · Post by **GJUK** » Tue Sep 02, 14 8:34 pm

Well that video quality is Bananarama! just like the car! There is a pop at the end you can hardly make out the stutter but the car revs like so.

REEEEE- gap EEEEEEEEEEEEEEEEEVVV

GJUK · Post by **GJUK** » Tue Sep 02, 14 9:48 pm

I'm out of ideas.

What do people suggest, the largest power valve, see if that works?

Thanks,

Jon

Les Szabo · Post by **Les Szabo** » Tue Sep 02, 14 9:49 pm

Try another rear bowl on the DP?

Mick · Post by **Mick** » Tue Sep 02, 14 10:02 pm

Les Szabo wrote:Try another rear bowl on the DP?

This Jon, swap the bowl and float mechanism.