Popping back through carb (Dellorto)

[dickie14]

Finally got myself some gaskets and time to fiddle...

Alas new gaskets haven't made a difference. I did find the mixture screw on #3 was about 1/4 turn further out than #4, so wound it in to match and that has also made no difference.

I might be imagining things but after that it seems like the pop is louder/sharper than before and a little more frequent.

Where should I look next - would it be worth pulling the idle jets and giving them a good clean? What else can I strip and clean without requiring a re-balance (haven't got a snail gauge yet)?

Waltraud - I have 60 idle jets already

[kevtherev]

These carbs are balanced aren't they?

[dickie14]

They were when they were properly set up 18 months ago on a rolling road. I guess I couldn't say if they're balanced now or not - do they drift our of balance? I haven't adjusted anything until this weekend. It runs well apart from this popping at idle, and on the overrun. Drinks fuel (17-18mpg at a steady 60 on the motorway), but it is an auto.

I need to get a mate to watch the engine whilst I'm driving - it's quite likely the overrun popping is an exhaust leak somewhere, and I can't tell from the cab if the pop is coming from the carb or the exhaust!

Anyway, a snail gauge is on order to help.

It also has a slow oil leak from the top of the case when running - right in the middle of the split between the two case halves. I assume this isn't related but if it's a sign of high oil pressure, could that happen if fuel was leaking into the oil (dellorto drip...)?

[kevtherev]

popping is a result of un balanced carbs
not sayin this is the reason, but just thought I would say it

[T25Convert]

Hi,

Was having a similar problem with mine (see here http://forum.club8090.co.uk/viewtopic.php?f=37&t=132444) but found it was timing related.

Worth having a quick check of you HT leads / dizzy cap etc if you haven't replaced them recently.

Cheers,

Alex

[dickie14]

Thanks for that Alex - hadn't considered the electrics. Will give them a thorough going over when I get the chance

Cheers,
Dickie

[dickie14]

I've done a few more things:

Swapped the dizzy cap for a new one - no difference
Swapped the rotor arm for a new one - no difference
Waiting on Brickwerks to get some stock in of their HT leads so haven't tried swapping them out yet

BUT

I did get a snail gauge and have just found that cylinders 1, 2 and 4 are all pulling 6kg/h whilst the offending carb throat for cylinder 3 is only pulling 4kg/h... All at idle on a warm engine.

So, does this mean I need to get serious with the air bypass screw on #3? Or is this data telling me something else is wrong?

[sarran1955]

Hello,

Please......

Valve timing.. pre load
then
Ignition timing..dwell angle

then

carbs..idle mixture..then air balance..

Avoid the 'shot gun method..

Cordialement,

[dickie14]

Your advice is warmly welcomed and heeded Sarran, I've checked the timing and points gap (don't have a dwell meter unfortunately) and all is in order.

Not sure how to check the valve timing - isn't this controlled by the gears on the camshaft? If it was out, wouldn't I have a whole world of issues, rather than just carb #3 popping on idle? Similarly, wouldn't timing/dwell issues tend to have effects on all cylinders?

The idle mixture screws on #3 and #4 are both wound out the same number of turns, though I guess this no proof they're correct.

Anyway, the air bypass screw was tightened fully home, so I can always return it there if need be. Winding it out bit by bit until the airflow was equal between barrels seems to have done the trick - though I haven't driven the van anywhere since so I'm loathe to declare a success just yet...

But I can definitely now hear a slight popping through the exhaust - this may have always been there (see earlier comments about popping on the overrun), but could the air bypass screw adjustment had an effect? Is this the kind of thing I could expect if the idle mixture was set up for the lower airflow and is now incorrect? When I next get a chance to play I'll wind the bypass screw shut again and see if it goes away...

Sorry for all the questions!

[kevtherev]

Not sure how to check the valve timing - isn't this controlled by the gears on the camshaft?

No... he wants you to check the pre-load on the lifters... particularly No. 3, I would say.

[dickie14]

No... he wants you to check the pre-load on the lifters... particularly No. 3, I would say.

Well that makes a lot more sense! Er, exactly how do I do it?

[toomanytoys]

Points......
I presume an 009 is fitted??

What timing are you running?? static, idle and whats max advance over 3000???

I

[kevtherev]

grief! talk about can of worms

[dickie14]

Points......
I presume an 009 is fitted??

What timing are you running?? static, idle and whats max advance over 3000???I

Points win prizes

It's not an 009 - I've got an SVDA running 5deg BTDC at idle and max advance is 32deg (hose off).

I've screwed the bypass screw shut again and the exhaust pop has stopped. I'm going to leave it that way for now as I'd rather have a huffing carb than popping exhaust.

Where to look next? Is it possible the idle mixture screws on the two barrels on the same carb should be set differently (they're the same and #4 is fine)? Any pointers on how to check the lifter preload? Do I just look for a broken spring? For the record, I checked the spark plugs on 3 & 4 and they're in good condition and both look the same.

Thanks for the continued input, it's appreciated

[sarran1955]

Hello,

5 degrees BTDC might be a bit low.. try 7 1/2 to 8 1/2.. if you are using 98 RON petrol.


This will alter your idle speed.. so adjust the idle air flows to suit.. keep them mirrored.

To get this engine running right you will need to get the airflow the same on both carbs at low and high regime

..and have the idle mixture (air to fuel ratiio..13.2 to 1) right..

Then it may be useful to look at the size of main jet.

In fuel savings terms it may be worth visiting a rolling road setup...

Hope this helps.

Cordialement,