Popping back through carb (Dellorto)

[dickie14]

So I have had twin Dells for abut 18 months now (BRILLIANT), but ever since I've installed them I've had popping through back through the carb throat that feeds cylinder 3. It is pretty minor, once every 10 seconds or so when idling, but it also does it more regularly when coming off the throttle at speed ('on the overrun'?).

I've watched it closely with the air filter off and it seems to come from below the butterfly, blowing it fractionally ajar and spraying a bit of fuel up when it happens (into my eye first time - always wear goggles kids!). It's more of a 'Bhup' sound than a sharp POP, but it causes the revs to drop briefly, and after a long run with the van in 'Drive' (it's an auto) but on the brakes at a standstill it can cause a stall.

The carbs came off for an engine rebuild and were stripped and set up on a rolling road a year ago, but still this minor popping has persisted. Is it any coincidence that the manifold that is ported for the vacuum take-off is the one on this cylinder/carb? Could my 33yr-old brake servo valve be leaking? Or are these symptoms nothing to do with an airleak?

Any tips for investigation would be appreciated - got a 2 week European tour planned in August and really want the van in tiptop condition for that!

Cheers,
Dickie

[jimpainter]

Popping back through the carb is lean running/air leak, maybe a gasket not sealing quite 100%.

[kevtherev]

Google "Delorto drip" this also might be your problem.

[dickie14]

Wow Kev, just spent the last hour reading about 'Dellorto Drip' which I'd never heard of before.

It sounds like I have all the symptoms - I can see fuel down the throat of #3 (when the rest look dry), my float chamber seems to drain within an hour, and the other day I noticed my oil smelt slightly of fuel...

[From the web: ''This is a condition that is caused by worn carbs where Fuel will "Drip" past the butterfly and down the intake when the carb is not in use. This will cause a mis-matched running condition at idle (one side gets more fuel than the other and the carbs can't be sync'd. The other condition is that fuel will run down the intake into the heads, past the valves and into the motor oil."

"If gas is appearing down on the throttle plate, then the drip is coming out of the “progression holes” at the bottom. Lead plug is leaking."]

So, is this a problem I can just live with for now? And does anyone know if anyone in the UK can refit the tiny little internal lead plugs?

[dickie14]

...will also do a more thorough check for air leaks first, ta Jim

[kevtherev]

My pal daz had it with his dells... No 4
Although he spent a small fortune finding that out

I thought I'd tell you now before you start tearing the engine apart..like he did

[shepster]

And does anyone know if anyone in the UK can refit the tiny little internal lead plugs?

Daz couldn't find anywhere and when he spoke to Eurocarb they reckoned they couldn't do it either.

Keep an eye out for any secondhand carb bodies, that's what he did and transfered all the good bits over.

[ermie571]

its number 3....its not the servo hose again is it

(was another thread on hear about cylinders 3 and 4 not running properly)

Em
xx

[gsmoke]

Long before you condemn your carb, there are many factors that need to be
discounted.
Are you certain that there is no air leak on the servo fitting, carb to manifold gasket, or manifold to head gasket?
This is far more likely than the dreaded (but rare) dellorto 'drip'.
Are the mixture screws for no 3 and 4 pots turned out exactly the same amount? Number 3 could be running lean with the screw turned out further. Your problem could even be as simple as this.
Do you have a carb balancer? If not then you need one. Many rolling road companys will balance the carbs but not the individual barrels. Your left hand carb might well be out of balance with itself! Simply put the carb balancer onto No 3 the number 4 pots and adjust the air bypass screws until they are the same (five minute job) If this is the case then you should then balance the left to the right carb.
Good luck.

[dickie14]

Thanks for the input guys, I think I've got a bit of a list of things to try this weekend:

1. Blank off brake servo port to eliminate air leaks on the vac line, see if anything improves
2. Spray carb cleaner all round the gaskets again to check for possible leaks there
3. Closer examination of #3 throat to check if it really is leaking fuel, if so where from
4. Check air bypass screws between #3 & #4, consider buying snail gauge...

I've read that damaged orings on the 'needle and seat valves' can be a cause of similar symptoms - I assume this is likely to be referring to the Idle Mixture screw?

And there also seems to be a lot of chat out there about floats being incorrectly set, but surely this would affect both sides of the carb, i.e. #3 and #4?

I hasten to add that, unless someone scares me with advice on just how quickly my motor is going to implode, I consider this minor huffing at idle a mere irritation - I'd rather not get into full carb rebuild territory until winter when the van's being used less.

[gsmoke]

I could be wrong but I think that the needle and seat valve refers to the valve which opens and closes when the floats go up and down. They are easy to replace. I agree that logic dictates that if this was a float problem, both 3 and 4 would be affected. Mind you, iirc, there is a little o ring on the mixture screw so they could be refering to that.

First port of call is inlet leaks. I've run dellortos for six or seven years and every few years I've had leaks in the manfold to head gasket. What doesnt help is the commonly used empi manifolds aern't finished very well on the gasket faces. This is remedied by rubbing them with fine wet and dry paper mounted on glass with a touch of oil.

With regards to your point no 4, The mixture screws need to be exactly the same between 3 and 4 (and obviously 1 and 2) but the air bypass screws will vary depending on the snail balancer figure.

I thoroughy recommend you buy the dellorto tuning book, it takes all the guesswork out of it and a simple carb strip, rebuild and balance can be done in a few hours. It also has an excellent troubleshooting section.
Shame you dont live closer, I'd take a gander for you.
Good luck

[dickie14]

Cheers Mr Smoke, I do have the Superformance Tech Book but there's a lot to take in... Thanks for correcting me on the mixture screws, would've got myself into trouble on that otherwise!

Will be paying close attention to the manifold gasket and brake servo port, I'm hoping it is just an air leak.

I'll let you know what I find in a few days

Cheers,
Dickie

[waltraud]

Dellorto book is lesscomplex than it looks if reading a fettling at same time. In practice its less intimidating than it sounds. The guys at reading Eurocarb?? Are v good on advice re jetting if you have any doubts. Is it at idle or load or both?

[dickie14]

It's only at idle or on the overrun.

Managed to eliminate the servo take-off, and just about convinced myself there was a slight increase in revs with carb cleaner liberally sprayed on the lower portion of the head-manifold gasket. Will order a replacement and try that first.

[waltraud]

Im guessing that ur running 60 idle jets? If less it coukd be an area to explore. I tried 56 or 58 before and got little success until fitting 60s I think.....while ago.....