Dellorto misfire

[jasper2347]

1973 Big valve motor. Anyone had an issue with the rear choke on the twin Dellortos running lean or not at all? Been told because the engine is slightly tilted towards the rear, the back choke does not get enough juice if the float level is correct. We had to correct the level due to the rear choke flooding from the trumpet, maybe we have over done the level? Engine is getting fuel, but not enough to fire correctly, causing a 'fluffy' misfire. Number four spark plug shows lean, number three shows perfect. Plenty of comp. on the cylinder and getting a regular spark right up the rev range, so assume carb problem. Set the level on both carbs from the workshop manual. Front two cylinders are fine, number three is fine. All jets are clear. The reason for doubting fuel is getting there correctly is the adjustment screw makes little difference on that choke, other three work as you would expect. Oddly as a rule if a choke is running lean you get the carb spitting back, which we don't have, unless you 'bottom' the screw. Any ideas welcome. Cheers, Rob

[sprintsoft]

Hi Rob,

have you measured the airflow into Nr 4 to see if it’s balanced with 3?.

Additionally the Dellorto has the air balancing screws which often get misused and out of adjustment. I would first reset these and do the balancing from scratch and see if it affects the issue you have.

Regards
Iain

[jasper2347]

Hi Ian, thanks for your thoughts. Airflow is virtually the same on three and four. Compressions are within a pound or two, one through four so that would be expected I suppose? If we set the fuel bleed screw on number three choke to the normal 2-1/2 up from the seat then fine tune it, that works fine. Number four however has little or no effect till around 4 turns out. We have borrowed a Colourtune set and this agrees with the settings on all the cylinders apart from number four. We can only get a flash of orange as the pump jet works when the throttle is opened fast, then it goes back to virtually no pattern at all. The others can be adjusted lean or rich without problem. Think we might have another go at the level and maybe go slightly higher to see what happens. Check the jets again as well. Just don't want fuel all over the place like before. Might see if I can find a flow diagram on the net to see if both chokes are fed by one fuel feed hole from the chamber, or one each. Don't want to leave this as it is or we will be needing valves, or worse, very soon. Thanks again for your input, it's most appreciated. Rob

[mbell]

On webber you can see the fuel level by removing the jets. So you should be able to pull the idle jets from 3 and 4 and compare the fuel level between them. As long as the jet nose is in the fuel I'd think it would be ok. The jets are central and close together so I'd expect minimal difference between them.

You could also raise the rear of the car slightly to level the carbs front to back and see if it improves or not.

[elans3]

First check the float level and get it bang on. Next, do you run a servo ?
If so, is the vacuum taken from adjacent to Number 4 cyl, ie at the back end of the inlet manifold ?
Had a similar issue before when the servo was going bad. Not enough to suck lots of brake fluid into the inlet and burn off as white smoke, but enough to lean out number 4 cylinder.
Common issue with Alfa Twincams.

[sprintsoft]

Hi Rob, Let me clarify, the air bypass adjustment screws are independent from the mixture screws your refer to and are often set wrongly.

I’m attaching a tuning guide that should help you, see item 7 in the diagram.

Bear in mind the compression results you have bear no relation to the airflow through each choke, that is governed by the butterfly angle, possible twisted spindle, possible butterfly loose or misaligned, and finally the air bypass adjustment.

Regards
Iain

[vxah]

Rob, you didn’t say if it is a +2 or not?
If it is then it should have a brake servo take off in number 4 intake runner.
If so pop the hose off, plug the spigot and test again. As has been said an air leak through the servo will lean out only number 4 cylinder!

[rgh0]

The design of Webers and Dellortos is such that the angle of the carb ( within reason) makes no difference to the fuel level as the floats surround centre well where the jets are and control the level at the jets regardless of carb angle. This is required for the fuel level not to be affected by acceleration / braking / cornering which at 1 G is equivalent to tilting the carbs at 45 degrees.

The problem on No4 on a Plus 2 is likely to be an air leak from the booster vacuum tapping or adjustment of or blockage in the idle circuit as others have observed

cheers
Rohan

[jono]

Have you checked the O rings/Misab plates on that barrel?

[jasper2347]

Thanks for that guys. Servo is new and holds vac fine when stood for over an hour, so I think that's OK, also tried taking off and plugging with no difference apparent. Think I'll pull the carb off. That way as suggested I can check the 'o' rings and also fully strip and blow out the jets easier than on the car. Hopefully that will sort the old girl out.
Thanks again for your your input. Rob

[richardcox_lotus]

Have you tried swapping the jets between #3 and #4 just to confirm it’s not a hidden blockage in the jet ?

[jasper2347]

Have a load of spare jets, might not be quite the correct size, but worth a go before ripping the thing apart. cheers.

[Roland]

I have a Sprint on Dellorto DHLA's and have had similar issues which I have solved through reading advice on this forum. It has been a learning curve getting the Dellortos set correctly but two things I have found need to be right other wise you are wasting your time trying to balance and tune them:

1. Float Levels
2. O rings/Misab plates being installed correctly and not leaking.

Regarding float levels the workshop manual will only give you an approx. setting, at least in my case. There is a procedure (look on this form and also Dellorto/sidedraught forums) where you ensure the float bowls are filled, remove the floats and then check the fuel level down from the top flange (where the idle/main jets screw in) to the fuel level. It needs to be 27 mm. Both my carbs needed adjustment from the WSM measurement ( which is 17mm down from top cover for 10 gr floats) to achieve the 27mm. My front carb needed to be readjusted and set to 15.5 mm from the top cover. The rear carb need to be set at 16mm from the top cover. The front carb on the WSM setting was actually very lean.

I find the O rings/misab plates quite tricky to get set properly, access is difficult and small movements on the nuts seem to make quite a difference. In other words the adjustment is very course. Certainly what I find helps is to put new o rings in even if the existing ones look fine.The new ones seem always fractionally larger in section diameter and are 'squishy' because they have not hardened at all. It still takes me ages to get them set properly.

Once I got these two things correct the carburettor balance and idle mixture became very easy to set. It has transformed the car and my under bonnet temperatures and water temperatures have also dropped. My hot starting problems disappeared and the idle mixture screws can be tweaked based on plug colour after a decent run. My engine is a standard sprint and all the Dellorto chokes, jets etc and standard.

It took me a long time to realise how critical actual fuel level in the bowls was.

Regards

Roland

[jasper2347]

Must have been a bit of shh you know what in the jet or jets. Just given the car a good blast up the road and had to back the fuel bleed off to around 2-1/2 turns out to get it to idle as it was too rich. Runs perfect now. Will have a look in the chamber however to see if any more of said stuff is still in there. Will check the level while I'm on. Fuel filter looks nice and clean. I like easy fixes. Makes a pleasant change! Off up the Yorkshire dales to Hawes in the morning for a run out

[alan.barker]

With Dellorto carbs the best thing to buy is a "CarbTune" 4 vacuum columns.
There are already vacuum take offs in Dellorto Carbs. As pour Air Bypass it's not on all Dellorto Carbs.
The good thing with the "CarbTune" you don't even need to remove the Airbox and it's done in 10 minutes spot on.
Alan