Vacuum advance is running of ported vacuum from the carb body. I'm confident distributor advance (mechanical and vacuum) is working well. I've actually slightly increased total mechanical advance (just a few degrees) by filing out distributor weights slightly. It's just the original distributor, but it's in good (however no longer original) condition.

Thanks sss. I'll do as you suggested. I've played around with this before, but only while under the bonnet. I've never driven the car with the acc. pump disengaged. I'm guessing it will suggest acc. pump not fully contributing, but will be worthwhile all the same. I'll post results when I get around to it.

So you think it's worthwhile removing the bridge in the manifold hey? I guess it can't hurt (I don't think..) and it does make sense intuitively.

Wot he said..

Check the timing is correct.

Then also as Benny said, check the idle to progression by disconnecting the accel pump and very slowly opening the throttle, it should just slowly increase in revs with no stumbling.

Removal of the bridge in the manifold will make significant difference straight up. Are you running the vacuum advance off the ported vacuum port in the carb or off the manifold. On an aside, have you checked the condition and operation of the bob weights in the dizzy. Validate they are within spec.

Cheers Benny.....I totally agree with your diagnoses rationale. It struggles with both scenarios really. A quick blip in neutral will definitely stumble/gasp for fuel (clearly shown on A/F guage), however quickly repeated subsequent blips will tend to be smooth. It feels/seems like the liquid fuel from initial throttle opening is just used to line runner walls to allow fuel from subsequent throttle movements to easily flow into cylinder. So increasing fuel delivery from accelerator pump may help, but I can't help feel that it isn't the whole problem. I'll certainly look into the pump, but I know it's working pretty well. Perhaps it could deliver a little quicker though. It's worth noting that pulling away from a stop or from reasonably low down in any gear will hesitate as well (also shown clearly on A/F guage), and this is generally only a gentle accelerator movement, relying more on the progression curcuit than acc. pump.

Haven't removed bridge in manifold.....

Thanks for all your help again.....Stu

I know this isnt exactly going to help alot regarding the issues you posted but if you are after going to be interested in a webber upgrade have a chat with recarb..
http://www.recarb.com.au/advanced_sea ... .php?keywords=a15&x=0&y=0
(link is for A15 manifold and carbs but will bolt onto your A12 - but they have brand new single for A12 and others just give them a call)

Here is the link for the A12 single side draught
http://www.recarb.com.au/advanced_sea ... .php?keywords=a12&x=0&y=0

You can pick up a twin Webber set and also manifold from them.. or single carb and manifold

I got my twin 45mm Webbers and redline manifold for my A15 from them.. great guys with some awesome help, they will also tune the carbies to your engine specs if you provide them..

I had mine tuned and sent, bolted on with no issues and runs a treat starts cold no choke most days and run a treat..

I cant really give an exact idea though on the heat displacement from them but after a long while they still are cold enough to touch by hand

On my DGV-5A on a 1200cc engine. I ran 45 and 50 idle jets feeding the primary and secondary throats. Another method to assit in diagnosing if the problem lies in the pump cct or idle cct is to test the following. a) Does the hesitation happen when you give the throttle a quick stab [this relies heavily on the pump cct to counter the sudden rush of air or b) does the hesitation occur on very slowly applying the thottle which relies more on the idle and progression ccts to counter the increased air moving through he venturi. Set the float level to the correct level as specified for your carby, no if or buts in a NA application. Lastly the pump cct lever has provision for two settings to adjust the arm ratio which alters the time taken to squirt the same volume of fuel.

kind of a side point but there is inlet manifolds that have coolant ports around the bottom of the runners

i dont think you should be needing to heat the inlet though, thats not normally necessary (hence wondering if you lived in cold climate)

with that much work done to the carby, and you sound like you know what your talking about, is it possible the problem is elsewhere? maybe as simple as an air leak in the manifold gasket or carby adaptor

did you remove the bridge on the inlet manifold where the carby bolts down?

Is there a way to increase the rate of fuel delivery from the acceleration pump? If it's lean squirt more fuel in.

Physically what's happening is that the pressure increases in the manifold rapidly when the throttle opens so the quantity of fuel stuck to the walls increases until a new balance is achieved. So, following the lean hesitation on throttle application you get a rich hit when you close the throttle again. I don't know much of the details of your carb but increasing the accel fuel dose would probably fix it.

Thanks for your responses!

1200rallycar: Fair call, but I come from a reasonably mild climate in Newcastle (just north of Sydney…). I’ve recently moved to Melbourne though, so the problem may get worse!
I think I’ll look for the Hitachi or SU option. Depending on what I can source and prices of each. It will probably largely depend on availability of what inlet manifold I can source too. I’d prefer to buy new stuff when it comes to the carb, but I’ll certainly look at second hand for the manifold.

Thanks for the previous post too. I’m pretty confident I’ve eliminated the risk of air leaks through throttle shaft bushes. The primary shaft did have a small amount of play until I got it re bushed recently. There was no change to engine running after the rebuild, but it’s good to know everything is working as it should.

Benny: I hear what you’re saying about the idle jets. Similarly, I’ve tried increasing the float level to further richen the idle circuit. Not the best idea though as it messes all your circuits obviously. I mainly did it just to see if it made a difference. I’ve played around with a few idle jet sizes, but could probably try a larger one. I think I’ve had a 70 in the primary side before. They’re available up to 80 though if I’m not wrong. The only noticeable difference from memory was a poorer idle. I haven’t tried it again with the coolant hot box though, so might be worth a try again.

I recently fitted a new accelerator pump diaphragm and it’s working well. I’m thinking that the relatively large amount of non atomized fuel being delivered by the pump is potentially causing me the most grief, in that I’m not getting the full benefit of all the extra fuel from acc. Pump as it’s struggling to keep up with the air along the inlet runners as it makes its way to the engine. As I said, of all the things I’ve tried messing with as far as jetting/acc. Pump etc, the best improvement by far has been heating the inlet manifold. I’ll definitely try the larger idle jet again though now that I’ve got at least a little bit of heat in the manifold, so thanks for the idea!

Cheers,

Stu

Going by your description, the hesitation as proven by your diagnosis is due to an extreme lean condition. The idle jet of the dgv also regulates fuel to the progression cct also, so maybe look at upping the size of the idle jet and recheck the fual ratio again. This will fatten up the fuel delivery at slight throttle operation and off idle conditions. Secondly, check that the pump jet cct is opperating correctly, check the jet, pump diaphram and pump nozzle. They have been known to to give trouble when dirty fuel has entered the carb.certainly worth checking. The pump diaphram is located external of the carb and can be easily accessed.

cheers
benny