In theory, a single IDF makes more sense than duals on the oval.
1. Less carb noise. So I can put the kids in the back without making them prematurely deaf. Though the thought of IDAs drowning out their noise is an upside.
2. More room for the carb and adding a waterproof air cleaner. I a bit over the duals sucking in water and running like a sack of sh!t. And having to plastic-bag them when parked outdoors.
3. Easy access to idle gets. For some reason, despite constant cleaning and snorkels I regularly found myself lying on the ground, reaching through the access holds in the inner rear wheel arch and fishing out the idle jets to blow them out. This always happens at the worst time.
4. No synchronising and linkage adjustments. Unless the carbs aren't perfectly synced driving on the freeway with a slightly opened throttle is torture.

So I got a new manifold. Carefully fitted it. Ported the end castings to the mini D-port shape of the heads. Bought an elcheapo exhaust with outlets for the manifold pre-heater pipes. An exhausted I hate, hate, hate.

I bought 65 idle jets (up from 55s) to get it to idle properly. Upped the mains to 140 (with 155 airs).

So how does it drive? Sh!t.

It's hard to start.
It's rough up to 2000 rpm.
It pulls like a train from 2000 - 5000 rpm and runs out of puff. It used to run out of puff at 6500 rpm.
It also intermittently backfires under load. To chase this I've switched coils, fixed exhaust leaks at the crappy flange and at the pre-heater pipes.

So, should I keep chasing this tune. Maybe put a bung in the exhaust with an AF gauge to help find out what's happening. Switch to a vacuum advance dizzy and switch to a 44IDF I have with vacuumed pipes?

Or should I give up?

More than a few have a said I'm mad to try a single a single weber. But the stock engine worked fine with a single carb.

Thoughts? Experiences?

I'd love to get this to work while I build up the 78 x 94 which is happening at snails pace.

Cheers
CT

CT
i will be interested in how you go

I have NEVER heard (seen) anyone having real success with a single IDF on a hi-po

I can only assume you DON'T have pre-heat?

That would be the first place I would start

I used a Holley 2BBL instead of a IDF, I used a lot of heat from the exhaust mainly by ending one side only at the collector, and I also used a preheat pipe to the aircleaner using a big air filter assembly off a Clevo no less (also partly coz I drive in dirt a lot).
Compared to duals I lost about 10% power at the top, and it started at about 3500 and got a bigger gap slowly as the revs went upwards. Under that however, it made slightly more grunt, not less.
If you use a cheap shitty exhaust to strangle it, and use big intake runners, then all the heat and tuning in the world won't stop it running like crap (as you know).

I did it because at very low throttle openings, and at under 3000rpm, it works very well indeed. Yes, even better than duals, and the response was instant with a late vac/mech dizzy. One of the big ways to help make it work is to have a fine droplet air-fuel mix spray, which is why I used a Holley. To get your 44 IDF a bit closer maybe try really small venturis, like possibly 30mm, then use small runner diameters around 35mm internal or less.
Airspeed through the runners is the key. If you were doing it again I would recommend a smaller cam, since this type of engine makes less top end anyway, but since you already have the FK8 then use it. It's a lot smaller than the FK87 I used with a single carb for a while.

Yes, you will lose power at the top, but gain all the advantages you mentioned. It's all a trade-off, depends what you want most I guess.

CT

It's rough up to 2000 rpm.

check for inlet leaks get a bit of tubing stick one end in your ear and check around manifolds and joints it might be leaking or sucking air

I forgot a couple of things you might like to try being chucking it in CT.
To speed up the airflow thru the runner jam a piece of exhaust tubing into each end of your manifold (making sure you can fish it out again of course!), most of them are way too big.
Try some stock puny DP end-pieces without porting (or else minimal porting), I know it sounds stoopid, but the airspeed will be huge. Don't worry about the flange mismatch too much, it's just to speed it up.
Connect one side of your manifold heat to the collector using some steel tubing (hydraulic offcuts? fuel lines? bundy tubing?).
Strange as it sounds, if you have a decent set of stock rockers then throw them back on instead of the high-ratio ones. It slows the valve motion down and speeds up the airflow and mixture past the seat.
If you are still using a 009 not a vac/mech dizzy, then set it up at 15* advance at idle and 30* max advance at 3000 as a compromise to get idle manners with your FK8 just for now.

I hope some of these help.

Hey modnrod. Other than tossing the manifold in the ocean, that's exactly the type of advice I was after.

First off I'm going to try bending some tabs in the 009 for more advance at idle, as you suggest. And I needed to restrict the advance a bit anyhow. Then I'm going to try Dave's pipe diagnostic trick. I like the air-speed suggestions, mind you the weber sounds like it's sucking pretty hard now.

Over the weekend, to overcome a cold, I switched coils, redid all the exhaust joints. Replaced the manifold gaskets. Switched 44idfs. Prayed to the Gods of speed (who probably thought what I was doing was sacralidge and didn't answer) and then went for a test drive to warm it up. Set the timing, tweaked the mixtures. And guess what. The same bloody result.

Interestingly Art Thraen, US VW guru (and no doubt one of the above mentioned Gods) says one should drill a 2mm hole in the throttle plates. Fortunately I have a spare, third, 44idf with vacuum ports and I have a spare vacuum advance dizzy (thanks to the National swap meet) in order to try a few things. I think attaching it one end of the pre-heater tubes to a pipe connected to the collector is a better idea. There's a loose one anyway (which could be a culprit). There's also a suggestion that bigger accelerator pump jets may help too.

I really want to take to drive the Oval to Warwick in a month, and even disrupt a Porsche cars and coffee event this weekend with Jak (pending Camira radiator removal) so I think I'll put the IDAs and big exhaust back on. And deal with single weber challenge when I'm full of self loathing and feel like punishing myself again.

Thanks Baz and Dave

CT, although it's tedious, making one change at a time will at least show you where the problem was. There may be a combination of 'faults', but systematically working through it could be the best way in the long run. Hope it ends up going like it should.

Cheers, Greg

And when u get it right, you will love it... No more lying on your back on the cold bitumen pulling idles.. Yeehaa

Wouldn't it be easier to put proper firewall sound deadener in the car (for noise), make rain shields for the filters (no more water in carbs), ultrasonically clean the carbs alongside proper fuel hose that doesn't erode in todays fuels and a decent inline fuel filter (to solve the blocked idle jets)?

As for the single carb I'd try the crappy new exhaust with the twin carbs. I'd say it could be a big issue if it just can't flow well enough.

Pre-heaters, well if the shitty exhaust doesn't pipe the pre-heater properly within the exhaust then it won't give the venturi effect neccesary to suck the hot gasses thru and hence properly warm your inlet manifold.

IMO, a vac advance dizzy will always give a better engine than a 009.

As the shitty exhaust can't clear the cylinders like a good exhaust should then increasing air speed should help with filling the cylinders and making her run smoother.

Your mains and air seem a bit close together. I'd try leaning things out a bit once you have it idling better.

have you checked that your intercooler has no air bubbles in the coolant?

I would start with that first.

CT, once an engine is running,
the next step is to get the sensor in the pipe,
and work out the jets on a dyno,(or careful street driving).

All this assumes there are no 'new' faults(eg float level) and also unrelated to the carb itself.
eg air leaks and basic service stuff,
and the manifold is of the correct design:
Plenum or balanced,
OR long stacks to prevent the stand off in the air cleaner when IR manifold is used.


What you are doing is not new,
FAT performance, and others, have been using single IDF's for 35 or more years on off road engines.

CB's Dellorto book should have the baseline 'trend' on the jet changes required
when using a carb as a single,
(yes, I know it is an IDF and not a Dell, but the 'trend' should tell you that you WILL need to change some things).

Another interesting read is HotVWs magazine May 1984
about the jet changes and manifold design that CB did when running single dells,
and perhaps May and June 1983
for some insight into the circuitry of the Dells when they were first released.

P.S. for your idle jets:
https://www.cbperformance.com/product-p/7557.htm 

http://www.cbperformance.com/product-p/6412.htm 

Scroll down here to compare these jet sizes with yours.