AVS2 Dripping Fuel

[NigelK]

Hi All

I took 115/3484 to Jim Smith for engine tuning a few weeks back, as it wasn't running right. He identified and resolved an issue with the timing, but did not have the time / parts on the day to resolve an issue with the carburettor. Specifically, at anything above 1500 rpm there is fuel dribbling from one or both annular boosters into the intake manifold. This should not be happening - the carburettor is new (Edelbrock AVS2 800 cfm), the mechanical fuel pump is new, the pipework is new, the fuel tank is new.

Some Googling would suggest possible causes are a) excessive fuel pressure, b) incorrect float heights, c) grit / dirt preventing the needles from seating properly, d) heat soak. My money is on the latter.

Jim wants to replace the carburettor altogether (with a 750 cfm Holley), fit a phenolic spacer (we had one originally but had to remove it because the air cleaner trumpet was fouling the bonnet, this may no longer be an issue after we bent the trumpets downwards), block the exhaust gas recirculation ports by fitting a different valley pan gasket.

Any thoughts?

Best wishes,
Nigel

[johnw]

If this was me, I would fit my spare carb (setup and tested known good on 383s) and see if it still happened. It most likely wouldn't is my guess.

If it happened still I think you ought to check fuel pressure with a meter at idle and 1500rpm and then again when hot enough so you believe you have heat soak. I have never had that myself with a mechanical pump where the relief valve is not operating properly above 1500rpm but who knows what China is making these days. Fuel pressure testing Kits from China are very cheap on ebay!

My view on heat soak:
Heat soak would not cause this to only happen above 1500rpm. I would say heat soak could only cause this at lower rpm like tickover, then it would disappear at higher revs as greater fuel consumption reduced the temporarily high pressure caused by heat soak.

800cfm sounds a bit big (not really relevant), I have never had an AVS2. Perhaps you have a non standard manifold and other goodies? Jim knows more than me about fixing problems like this. I am a novice in comparison and I would check the basics above like that first off.

[NigelK]

Thanks John. The original 383 has been stroked to 432 cu in, but I would agree 800 cfm is too much for a street car. The intake manifold is an Edelbrock Performer. Other than the carb, stroker kit, intake manifold, and ported 906 heads, everything is stock.

Best wishes,
Nigel

[Martin R]

I have an Edelbrock (not sure of the model) fitted to my MK3 by a previous owner and a few years ago, whilst experiencing poor running, I sent the carb to Dave Barnet to be checked over.
He commented that the carb, although new, was badly made and somewhat rough. He might have used stronger words.
I still have the carb fitted and it runs fine (turned out to be an electrical problem as well as a fuelling problem due to a blocked brand new in-line fuel filter) but new Edelbrocks are apparently pretty rubbish.

[johnw]

I bought in total 3 Carter AFB Performance carbs for my FFs between 1995 and 2000. I was running two Vignale FFs at the time and had one on each, plus a new spare in a box. They literally came straight out of the box and were perfectly setup for the early 383 FF. I fitted the last one, now 20 years old but new in box, 4 years ago after modern fuel screwed up the other. The Edelbrock Performers you mention Martin look identical to the 1990s production Carters I bought. There were then mergers/buyouts/sales between Carter Edelbrock and Weber from about 2002 onwards. All those carbs look the same to me and layout wise are easily confused with a Carter AVS. Rebuild kits for these carbs are available from Ali Express. The carbs are probably made in China these days. Parts probably interchange between all versions to some extent. The original carbs do look more nicely made than even the Carters, which were made in the USA back then I believe.

[Martin R]

I think you're right to suspect cheap repro, possibly China built carbs....

[Steve Payne]

The Edelbrocks are about the cheapest carbs, I have fitted several and some work perfectly out of the box and others need some work. You only need to look on places like Summit racing to see what else is available for these engines.

Steve

[RockyUSA]

Here’s a slightly different option…

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IMG_4486.jpeg (231.34 KiB) Viewed 258 times

[johnw]

So that is what Jim meant when he said fit a Holley. All falling in to place now. So obviously the right way to go!

Here’s a slightly different option…



IMG_4485.jpeg


IMG_4486.jpeg

[NigelK]

You lot are a bad influence

[88V8]

....Specifically, at anything above 1500 rpm there is fuel dribbling from one or both annular boosters into the intake manifold. ...
Some Googling would suggest possible causes are a) excessive fuel pressure, b) incorrect float heights, c) grit / dirt preventing the needles from seating properly, d) heat soak. My money is on the latter.
Jim wants to replace the carburettor altogether (with a 750 cfm Holley), fit a phenolic spacer (we had one originally but had to remove it because the air cleaner trumpet was fouling the bonnet, this may no longer be an issue after we bent the trumpets downwards), block the exhaust gas recirculation ports by fitting a different valley pan gasket.

Correct fuel pressure is a foundation of 4-barrel tuning.
If you don't have one, fit a Filter King with gauge, and set it to 5psi running.

Heat soak will show up at idle, or on a hot day may lead to stalling if the fuel gasifies in the pipes.
A phenolic spacer certainly a good idea, but this is not the cause of your problem.

Also ensure the fuel pipe is routed away from heat sources, as ethanol evaporates much more easily than proper petrol.

The (genuine) AVS 2 is a good carb with very sharp throttle response and readily tunable providing that one has an AFR meter and works systematically through the settings. An advantage over the Holley is that the Edelbrock has one float chamber for primaries and secondaries, so the fuel in the float chamber does not get old as can happen if one rarely gets into the secondaries.

[johnw]

The (genuine) AVS 2 is a good carb with very sharp throttle response and readily tunable providing that one has an AFR meter and works systematically through the settings. An advantage over the Holley is that the Edelbrock has one float chamber for primaries and secondaries, so the fuel in the float chamber does not get old as can happen if one rarely gets into the secondaries.

I don't think anyone is making fake AVS2 carbs. Edelbrock say their carbs are made in the US (just googled it), which could just mean assembled who knows really.

If your fuel pump is a tin can sealed for life version and not a nice expensive rebuildable one, then if you check it with a fuel pressure gauge, you can assume that pressure is not causing the dribble. In that case it is most likely the carb. 800cfm is irrelevant to the issue of dribble at 1500 rpm on a 383 and this one is almost 440ci. Given an AVS2 is £400, I would just fit another identical one make sure it is brand new in a sealed box. Surely it would cost £200 to get the old one cleaned (including transport etc), it typically won't be tested on a car after cleaning, and the cleaning might not be perfect. Once your car is running you can clean the old one yourself and test it. Paying people to work on your car is expensive. The old one is worth 200 I would have thought.

This filter king is a good idea with an electric pump, but with a mechanical pump a bad idea as you can't have an uninterrupted metal pipe from the pump to the carb as in the stock setup so you have a fire risk. I think these AFR meters are a great idea for teaching the mechanic how to setup a car properly. I could do with one. A bluetooth one that reports to a phone, a bung on both downpipes so you can check both side and remove it once the carb is set up, and move it to your other classic. A carb pro will feel the exhaust temp with the hand, listen, drive the car and set the mixture, check the plugs at the next service. Unless you are going to be tweaking the mixture over the weeks as you drive and aiming for better than injection mixture control (brilliant idea, yes I want to do that!), maybe that is overkill for now, as this dribble is a blindingly obvious issue that needs fixing first.

[NigelK]

Thanks for the further thoughts.

I bought the fuel pump new from Robeys in May 2021. It's a Carter mechanical pump https://www.martinrobey.com/part/14387. I've seen some reports online of high-spec Carter mechanical pumps being fitted with overly strong springs from the factory and delivering too much fuel pressure as a result, but nothing specific to this pump. Is it common for mechanical fuel pumps to produce overpressure?

I bought the carburetor new from Summit Racing in the USA in August 2020. Here's the website link https://www.summitracing.com/parts/edl-1913. When Jim inspected the metering rods, he said that someone had been at the step-up piston cover screws already - I doubt this was done either during engine dyno testing (which was limited to a single run because of a main seal leak) or by the restorers. So perhaps Summit sent out a unit that had previously been got at and returned. Even if it was cost effective to send the carburetor back to Summit, it's now way past their warranty period.

I went with the Edelbrock AVS2 originally because in a way it's a natural successor to the Carter AVS 4682S that was fitted from factory. But I'm happy to go with Jim's suggestion to fit a Holley carb, as these were fitted to the higher CR H-series 440 engines, and my engine is now pretty similar to one of those in terms of cc and CR.

Best wishes,
Nigel

[johnw]

I bought 2 Carter fuel pumps from Summit, similar to yours, but with screw in connectors. They are non adjustable like yours and were both fine. I suggested getting exactly the same carb again as you can simply swap it out yourself in a few minutes, no changes to pipework, and it is good to have 2 things the same to compare when one needs fixing. No other reason. Could be an inexpensive fix especially if it then helped you fix the carb you have. If you are paying Jim to sort the car it makes no sense to ignore his advice. It does sound like your carb was got at. A carb is a simple device, and works off airflow. It is irrelevant what is causing the airflow, engines or electric fans even. It is a faulty carb assuming fuel pressure is OK.

[VFK44]

I am curious why no-one has suggested a fuel pressure regulator? Less than £100 all in for the V8 version. (The gauge detaches during driving).

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