Classic Alfa Forums

Hi All,

Anyone ever suffered a Carb vacuum leak and know what the symptoms were?.
The Net isn't proving any real feesable answers via carbs.

My car has developed a problem whilst under acceleration which is the same symptom I suffered at Shelsey Walsh a few years back which I thought I'd cured via carb strip down and re-build with new Weber carb parts.

The car after hard acceleration is fine through to 3rd gear then starts or feels like a misfire and will barely accelerate. The videos to my symptoms starts on the Video run 2 and again on run 3, I thought it is fuel related so have replaced all the jetting, fuel float needles and accelerator pump diafram. If I back off the gass for a few seconds all is okay for a few seconds then the symptom reappears.

Run 3 Video Link.
https://www.youtube.com/watch?v=Sy-rCwvM5as

Any help to diagnose would be useful.

Thanks

If you've got a vacuum leak you'll probably hear a sucking sound at idle.
Try spraying brake cleaner or carb cleaner over all the breather/vac pipes and anything connected, if the idle suddenly shoots up or surges you've found your vac leak as it will draw in the cleaner and enjoy the richness!

If you keep going on and off accelerator and things are ok sounds like the accelerator jet is overcoming a fuelling problem?
I have had it where although the jets themselves are clear the galleries have debris in them.
Used to be standard pre event prep to strip and clean carbs.

Rsfruitbat

Maybe worth checking the Fuel pressure. are you running std mech pump or regulated electric ?

Hi all,

I took the tank out last night and its clean with very little debris, breather pipes didn't seem to be kinked but the one way vent/valve was in effective that in you could blow either way both times with resistance so for now I have left it off and run the breather up around the filler neck.

I have replaced the fuel line to the pump which incidentally recommends a pre filter?, the filter is a new un-used run of the mill unit I had laying around but I only fitted this after the car started to have the symptoms.

Now waiting on idle mixture screw o-rings to turn up as one has snapped trying to put it on the new idle screw Rolling Eyes. The only other thing I may have to change is the fuel line from the pump outlet to the fuel pressure regulator as I have a suspicion it may be too soft even though it was rated as a decent braided fuel line. It may be the weak point if the tank couldn't vent properly. Fuel pressure is set to just under 3 psi which is fine for the IDF carbs.

Hopefully once back together I can check again at the weekend.
_________________

That's almost identical to the problems I had with mine (16v on carbs) when I rebuilt the car. I thought it was a ignition problem so changed the everything and after all that it wasn't much better.

I eventually traced into fuel flow, rather than fuel pressure. Swapped the fuel pump and beefed up the electric supply to it and the problem went away. Seems I was draining the carbs on full throttle, there was enough in the bowls to feed it through 1st and 2nd but not 3rd.

I had the similar issue as fluffyduff - fuel flow restriction draining carbs on a 16v - car was laid up for a couple of years and crud in tank eventually worked its way to the fuel pump gauze filter. Whilst waiting for ATP to reduce their tank price to a decent level I removed the old tank - cleared a couple of kg of rusty gunk out - replaced the fuel lines and filters and now the motor runs like clockwork.

Spent a few hours on it today, replaced fuel pump which runs better than the last unit, removed pre-pump filter , new clips and replaced the fuel pressure gauge. Ran the car up and gave it a quick tune/balance and the problem is still there under load. The fuel pressure drops quite quickly under full throttle and i suspect its either the 12v pump supply or now the fuel line from the pump to the regulator , at least I know whats causing the problem. I'll replace the wiring to the pump or at least run a direct feed from the battery to start with before I rule the current harness out. I'll keep you all posted.

I had to give the pump a feed directly off the battery satisfy it's appetite.

Interesting thread. Got me thinking about where a fuel filter should be placed......before the pump or after?

I’m running a standard pump into twin 40 dellortos on a standard 1.7 8v engine and mine is between the pump and the carbs.

I read somewhere that it should before the pump.....any thoughts?

Sprint88qv wrote:Interesting thread. Got me thinking about where a fuel filter should be placed......before the pump or after?

I’m running a standard pump into twin 40 dellortos on a standard 1.7 8v engine and mine is between the pump and the carbs.

I read somewhere that it should before the pump.....any thoughts?

Just do both

Like it, cover both angles

Right, got a sneaky few hours in this afternoon, replaced the 12v feed and went direct from the battery isolate switch rather than the ignition and sure enough it seems to keep pressure at full chat so hopefully problem solved, i'll re-wire via a relay or at least a fuse as the fuel pump switch I fitted is ineffective and have some new braided PTFD fuel line and a new regulator to go in on Saturday. Thanks for the suggestions they all help.

Hi Oggie
Unless you have an over specced pump i wouldn't suggest running 2 filters. Reason being is every time you do something like that you will restrict the flow.
It's position could also have an effect depending on the type of pump you have. Generally they work best in one direction. Either sucking the fuel from the tank or blowing the fuel to the carbs.
Rsfruitbat

Oggie wrote:Right, got a sneaky few hours in this afternoon, replaced the 12v feed and went direct from the battery isolate switch rather than the ignition and sure enough it seems to keep pressure at full chat so hopefully problem solved, i'll re-wire via a relay or at least a fuse as the fuel pump switch I fitted is ineffective and have some new braided PTFD fuel line and a new regulator to go in on Saturday. Thanks for the suggestions they all help.

Are you not going to fit an inertia switch?

Hi RS,

I moved the pump close to the fuel tank as recommended by the manufacturer, before it was in the engine bay and I wasn't particularly happy with the positioning which was on the firewall 250mm above the Manifold!. The pump does have a built in replaceable filter. I have taken the pre-pump filter out now so all good in the hood!.

Dave,
I think i'll fit one although never really looked into one before, I originally had it wired from the ignition via the immobiliser and a second dedicated pump switch so it wouldn't trigger unless I disarmed the alarm/immobilizer, food for thought though and certainly an important safety point.

Well I seem to have spoken too soon, took the car out again yesterday and although the fuel pressure is now stable the car still bogs down when I give it the beans around 3rd gear to forth. Now starting to think its down to cam timing , could a belt slip a tooth and cause this?. Rev counter only reaches 5K when it starts. I also don't have any timing gear to check the belts either. Think I'm going to have to ask AHM to look at it.

Hi Oggie. Whereabouts are you?
Have you thought of trying Dave Young at Mechmotorsport in Cheltenham?
Rsfruitbat

Hi Rs,

No sadly I'm in Hants, AHM are really busy but has recommended a guy near me with a rolling road that could help. Just need to book an appointment.

Spent a few hours on it yesterday morning methodically going through the complete tank /fuel lines/pump/ new pressure reg to the carbs and eventually taking out the main jets using a gadget similar to what surgeons use to look around peoples bodies (cheap item from Lidl of all places) and found quite a bit of dirt sat where the main jet sits flush to the idf carb housing.
Gave all 4 a thorough clean re-assembled and new spark plugs now changed and reverted back to the F16 emulsion tubes from the F11 versions, went down a few sizes on the air jets to 180 from 200 so all I can guess from this was the car started to lean out under load, also temperature checked the manifold outlets on idle and piston 1 was showing signs of being 10 to 15 degree's C higher anyway took the car out, no misfire under load , rev range back to normal in all gears and with no hesitation. I'm one relived owner at the moment for the time being but would like to change the engine for a 16v unit (anyone have a complete one for sale?). and keep the 1.8 8v for a complete re-build.

That was a marathon problem solver, glad you got it all sorted...............for now.