Hi all,

This is my first ever post after many years of viewing the site and I'm hoping someone can assist me. I apologise about the length of the post, but it’s about as concisely as I can describe my situation. Hopefully some of you find it interesting!

I'm looking for an alternative carburettor/inlet manifold set up for my a12. For some years, I've been running a weber 32/36 DGV using an adaptor to connect to the stock inlet manifold. Certainly nothing unusual. The motor has been rebuilt in the last few years and has a mild cam. I also have set of custom headers. The motor has been dyno tuned fairly successfully to provide a good amount of top end poke.

However, a problem I have been plagued with for some time is a distinct hesitation off idle or from low speed. I'm fairly certain that I've successfully diagnosed the problem to be a lack of heat supply to the underside of the inlet manifold as the original 'hot box' heat source was obviously removed when the headers were installed. I’ve recently installed an air/fuel ratio gauge which clearly identifies an extreme lean condition coinciding with engine hesitation. I can be fairly certain the carby is not malfunctioning, and in particular, I can confirm the accelerator pump to be spot on which I know would be an otherwise likely source of problem in this regard. What is particularly annoying, is that there is no hesitation when the engine is cold and the choke is on. In fact, the car is at its most responsive in this state, and it’s frustrating that drivability worsens as the engine warms up.

In an attempt to recreate the hot box effect, I’ve fabricated a water heated hot box that bolts to the underside of inlet manifold. Engine coolant now flows through new hot box and is in direct contact with the fins on underside of manifold. To my delight, this improved engine drivability significantly. Understandably though, it did not solve the problem completely, and I’m guessing it is due to the cooler temperature of the coolant (80-90 degrees depending on thermostat) compared to exhaust gases.

Anyway, as I can’t easily provide any more heat to inlet manifold (I do not want to use heat risers from headers for a number of reasons) I thought the next option would be to use a side draught carb. The theory being that the mixture will not lean out from having to make a right angle bend as it passes from the carb to the manifold.

So, my question is: what side draught carburetor/inlet manifold combination is most readily available and also reasonably cheap? I guess fuel injection would be another option, but I know least about this method of induction.


That’s all I’ve got to say for now. I hope someone can help!

Cheers,

Stu

Hi, welcome to the club

where are you located? perhaps in a particularly cold climate?

i've never been a fan of webers, had them for years and even when you get them running perfect they only seem to last 6months before falling out of tune to the point of coughing and spluttering, and i've never pulled one apart that those little diaphragms wern't perished (even brand new replacements)

all that said, you do get great top end power with them, but at the sacrafice of low end

even on the rally car we went back to the twin SU style GX induction, and never looked back, so reliable and such even power delivery, you just need to find an old mechanic capable of getting the right needles to set them up

i just posted this is the other carby thread:

Quote:

perhaps your 32/36 has throttle play that is not being corrected when you rebuild?

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

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

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.

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?

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.

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

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