Flat spot on a new engine

[Grundo]

Sorry Guy,

44 DCNF
55 Idle Jet
70 pump jet

So fundamentally the message is to increase the air corrector?

[Guy Croft]

All things being equal, ie: no faults found either in ignition, exhaust or fuel supply (no blocked jets/flooding etc) yes, definitely.

You don't need such big idle and pump jets. Idle 45/50 and pump 50 is adequate. An overly big pump jet will knock out the rings after a while (the delivery is huge) and a requirement for such a large one masks a problem elsewhere.

If all else fails go down to 30mm chokes. Better still get a 40DNF and do the same, the smaller choke with smaller carb will give a better fuel signal (stronger venturi depression).


G

[Guy Croft]

Adam's response retrieved from the backup:

Hello, well I've found fault with the carburettor - clogged jets etc causing the main stumble as the pump jets would work and then nothing would come out of the Main jets so fixed that problem. I have ordered some smaller pump jets and air correctors. The engine seems to run well - revs well with no hesitation, I am still worried about lean running though as I have a narrow band air/fuel gauge in the car which does tend to drift around the spectrum. I have borrowed a friends wide band meter and sensor to look into it further as I don't trust the narrow band sensor.
Can anyone suggest what tests I should run are on the car to test the Air/fuel mix at different loads?

It may also seem like a very dumb question but I want to re-check the ignition timing and looking through the gap in the gearbox casing to the flywheel/clutch there is no sign of a mark, is there a suggestion on how else I can do it?

Thanks Adam

[Grundo Farb]

Hello,

a slightly newer update, on where the car is and two questions. I've had the AFR checked and the car is definitely running lean at high RPM. So it is not being driven at all until the problem is sorted.

I have rechecked the timing on the car and the timing is quite advanced. 10 deg static and 35 deg centrifugal advance at 3100 rpm compared with the 10 deg static and 25 deg centrifugal advance for the original car. I'm not sure what the effect this will be on how the car will run so some help here would be great. I have another distributor I can put on the car if that is recomended.

I'm also taking Guy's advice and sourcing a 40 DCNF with 30mm chokes to use, I bought one which didn't have any Auxilliary venturis and have now found some for my model of carb. They are 2.5's, is there any advice out there please on whether these are too small for this application?

Thanks,
Adam

[GaryR]

Hello Andy.

For information the Original Equipment Volumex Carburettor was a Weber 36 DCA 5/250. Similar to a DCNF but with a water heated automatic choke.

I will attach a file to show the Carburettor Data of the OE 36 DCA. Perhaps this will help determine to some point the jet sizes you will need using a 40 DCNF with 30mm Chokes.

All the best.

Gary.

[Grundo Farb]

Thank you for that Gary. I have the same book which I guess I should have drawn upon in my post as you are right, the size of the Aux venturi is larger in the original. Given I have gone down the DCNF route and after reading James Bowens SOHC tuning thread (as suggested by Guy) the question of what size Auxilliary venturi is suitable is something I haven't seen discussed anywhere. I notice James mentioned with his DCNF's the Aux venturis are 2.5's as well but they don't seem to be commented on.

So really, is it a suck it and see (pardon the pun), can I drill them out or should I source something larger?

Thanks,
Adam

[Guy Croft]

Leave the aux venturis alone, you never change them,

G

[Grundo Farb]

Hello, an update on progress...

I've taken your advice Guy and changed the Carb over to a 40 DCNF. The set-up is now:

Main venturis 30mm
Idel Jet 55
Pump 50
Main jet 145
Air corrector 175
Emulsion tube F24
Needle valve 300

Since installation I've now got the car running and idling but according to my AFR meter runs lean once the throttle is depressed and the engine is under load - and well basically runs the same as before.

On a previous post I noted that when the static advance is set at 10 degrees the advance under load is measured as 35deg rather than the standard 25deg (I'm going from memory here sorry). Is this likely to cause any issues? I have another distributer which I can change over if this something wrong.

The next step I'm thinking of is to get a full spectrum AFR meter installed and take it to someone who understands webers (and has the weber drills/jets to tune it) as I am out of my depth on what I can do here and I am paranoid about damaging my engine. If I do that though a plan of what to test/look for would be very helpful.

Some thoughts would be extremely appreciated.

Thanks, Adam

[Guy Croft]

10 deg static + 25 deg centrifugal 35 deg total is right

GC

[Brit01]

Hi Adam,

Even though I have an Alfa with Dellorto DRLA's I have a Bosch wideband lambda and digital gauge. Brilliant investment.

I can tune the idling and also see how rich or lean it goes when I accelerate and see when cruising on the mains.
I love it.

Now I have fitted 2 gauges in this spot, the AFR and oil pressure gauge.

My Alfa likes to idle at around 12.7-13.2 (any leaner and the rpm drops) and cruises on the mains at about 14.5.

[Brit01]

Adam:

Any updates on the flat spot?

I'm working on a huge stumble under acceleration after a new rebuild of my flat four. Will be checking all belts/timing/compression first as per Guy's advice, then move onto the carbs if all look ok there.

Chris

[Grundo Farb]

Yep some progress.

Two points.

1) Buy Guy's book and do EXACTLY as it says
2) refer to point 1.

Guy's book arrived last week, and there is a page in there recomending jetting and settings for carb combinations with engines. I did not quite have the same jetting as my carb had 30mm chokes. I installed some 28mm chokes and used the same jetting as in the book. Now I know the forum has the same/similar information but the table in the book had the GC assurance. "If your car does not work and you have used these settings in the carb you can be sure you have a problem somewhere else". He is right.

It works. No lean running, no flat spot. A little less ignorance too...

[WhizzMan]

Good to hear you now at least have a working setup. It's easier to "improve" from that, since you can always go back to the last known good configuration.