My Alfa Romeo 33 rebuild - FIRST TIMER

[Guy Croft]

The flat spot could easily be inherent with the exhaust system you have! Intereference from one cylinder upsetting another. Bit complicated to demonstrate but if anyone want to try with a good sketch be my guest!

If bigger pump jets doesn't cure it then that is the most likely 'culprit'...

G

[Brit01]

Currently I have emulsion tubes 9164.4, 55 idles, 33 pump jets, 142 mains. 32 chokes.

I have tried increasing the pump jets once but it tends to nosedive indicating too much fuel going in (maybe just too big an increase and I needed something in between). I swiftly changed them to avoid cylinder wash.

One thing that I noticed last time that one of the emulsion tubes was a little loose, the bottom part(emulsion) that joins the top part(air corrector). Could this be significant?
Do they pull apart easily? If so I imagine it needs to be adjusted to be a tight fit.
If 1 emulsion tube is sitting a little lower (due to the loose fit) in the carb than the others what effect can this have?
If I was to change the emulsion tubes what would you recommend?

Guy: the new exhaust manifold is still pending. I need to to get some quotes.
But it used to work fine on this original exhaust system.

I also want to triple check the opening of the 2 carbs to make sure they open at the same time.

Thanks

Chris

[Guy Croft]

40 DRLA on emulsion tubes 9164.4, 55 idles, 33 pump jets, 142 mains. 32 chokes.

What is the cubic capacity of the engine?

Before anyone else comments I would like the chance to go over the jetting with you personally,

G

[Brit01]

1.7 8v engine Guy.


Thanks

Chris

[Guy Croft]

1700cc 8V on 40 DRLA with emulsion tubes 9164.4, 55 idles, 33 pump jets, 142 mains. 32 chokes.

I imagine the number .4 DRLA emulsion tube is standard for that unit (?) in which case it will be likely be right for your spec motor.

a) 55 idle is big for a 1700cc 8V - but OK - provided the adjustment screw for idle mixture will go in far enough. A 45 would ordinarily be plenty big enough. b) 33 pump jet is a bit conservative and could certainly contribute to a flat spot I would try a 40
c) 142 main jet OK I would not go smaller
d) 32 chokes OK. You could run bigger - 34mm but it would give at the 'top' and take away at the 'bottom' of the powerband.
e) You have not told me the air corrector size but it should be 180 or 185 I reckon.

My recommendations may help; thing is with jetting you just have to try it and see what happens. My jetting archive info indicates that Dellortos always seemed to need bigger idle & main jets than Webers. Dunno if this is necesarily the case. I do stress 'my info'. I had a lot of help in the early days from the the (then) Dellorto UK concessionaire, Contact Developments.

G

[Brit01]

Thanks Guy.

Yes I believe the air corrector is 180.

a) 55 idle is big for a 1700cc 8V - but OK - provided the adjustment screw for idle mixture will go in far enough. A 45 would ordinarily be plenty big enough. b) 33 pump jet is a bit conservative and could certainly contribute to a flat spot I would try a 40

We tried the 52 and 53 jets with the result of loud crackling/popping sounds on the overrun/deceleration. 55's seemed to have cleared 90% of this up.
Borderline rich settings are actualy about 4.5 turns out on the mixture screws with the 55.
Dellorto instructions state to start at 5 turns out and go from there.

d) 32 chokes OK. You could run bigger - 34mm but it would give at the 'top' and take away at the 'bottom' of the powerband.

I don't want to lose any bottom powerband here.

b) 33 pump jet is a bit conservative and could certainly contribute to a flat spot I would try a 40
c) 142 main jet OK I would not go smaller

So we'll try slightly large pump jets and possibly larger mains. I have a set of 45 pumps which were too large I felt. So 40's could be made locally here for me and 145 mains possibly.

When the AFR gauge was installed and was cruising fast with an AFR of 14.5-14.7 on the mains.


Many thanks for your guidance Guy.

I know Dellortos can be a bit stubborn to get right but once you hit the sweet spot they can be a joy to use.
I'm intent on getting these set up right.

[Guy Croft]

OK Chris, thanks.

"When the AFR gauge was installed and was cruising fast with an AFR of 14.5-14.7 on the mains"


Cruising is a relative term because it can be very light throttle depending on the load. Cruising up a long slope will need a lot more throttle & gas than the flat. Your AFR is rather lean. 14.5 is chemically correct and best power does require running slightly rich and that keeps the engine cooler too. 14.7 is way too lean.

You should tell me what it says in 3rd, 4th or even 5th gear near full throttle over 4500rpm up a long incline. Long enough to stabilise the Lambda reading. In other words maximum load. The attached plot of an engine on carbs (with usual weird fuelling that carbs give) will give the picture I trust better than words could. Try and write down what you get - yes I know it is not easy hold full throttle on the highway but it is more important to get the throttle (s) wide open than hold the rpm constant. It is the trend in the fuelling in that critical 4500+ range I am interested in. The main jet controls the fuelling pretty-well from 3000 to top end but the air corrector only controls something like the very top 1500 rpm.

Please quote all the jetting when you reply.

G

[Brit01]

Hi Guys,

Thanks for the graph.

Currently I cannot rely on the AFR readings as a few pinholes have appeared on the manifold weld (primary/secondary join) hence the disconnection of the gauge until I get new piping done.
As far as I can remember from the past AFR would range from 12.5 or so to 14.7 under a variety of loads and speeds but we'll need to get the new manifolds done and go from there.

So I'll pay the local machinist a visit and get some new pump jets made (40's) and also a spare set of mains drilled out to 145 to make it a little richer than the factory specs.

Regards

Chris

[Guy Croft]

Hi Chris,

yes - please try and get some accurate Lambda, for the fact is this post is so widely read. Folk will be guided in part by what's written here.

Now it's 7pm and I am goin' home.

G

[Brit01]

Thanks Guy.

Yes I am trying to find time to get a quote for the new manifold so I can get accurate AFR readings again.

In the meantime I was thinking about the emulsion tube height. (been a little sick last few days so haven't been able to get to the garage)

As I mentioned I noticed one was slightly loose in fitting to the top air corrector jet which screws into the top of the carb.
This is in effect will leave the emulsion tube hanging a few mm's lower than the other 3 emulsion tubes.

Aren't the various holes in the e-tube dependent on the height of the fuel in the float chambers?
If one is lower then this barrel would act differently to the others.

Please correct me if I'm wrong.

Many thanks

Chris

[WhizzMan]

I'm a little concerned that a "local manufacture" of a jet might give different result than one coming from the same manufacturer as the original jets. In my tuning motorbikes phase (something not uncommon for teenagers and young adult males here), you could get plenty of different jet sizes from different manufacturers for the same carburetors. They did make the orifice the same diameter, but the flow rates differed due to manufacturing design variances. They differed so much, that going from a 138 from manufacturer to a 142 from another sometimes gave you less fuel, while you wanted more. Nothing you can't solve, but if you don't keep this possibility in mind, you might be searching for a long time what causes your fuel ratio to be not what you expect it to be.

[TomLouwrier]

hi Chris,

I'm having a little trouble getting a good sectioned view of this particular carb. I know the IDF, but there may be differences that matter.
Could you show us a picture of the parts we're talking about and indicate the 'looseness' you found?

regards
Tom

[Brit01]

It is the trend in the fuelling in that critical 4500+ range I am interested in. The main jet controls the fuelling pretty-well from 3000 to top end but the air corrector only controls something like the very top 1500 rpm.

Hi Guy. Re-reading your post I'd like to comment on it more now I'm feeling a little more human after a stomach bug this week.
Over ~3200 rpm the pull from the engine feels good and smooth.
The hesitation/stuttering tends to happen with WOT below this rpm range.
One interesting thing to mention is that one time(about 10 months ago) a main jet got blocked and under half-full throttle even from 1500 rpm the engine felt like it was running on 3 cylinders.

I'm a little concerned that a "local manufacture" of a jet might give different result than one coming from the same manufacturer as the original jets

Currently whizzman the idle jets(53 and 55 - both tested, 53's crackle/pop on overrun as mentioned before and 55's are much smoother on overrun) and mains(142) are from the popular and well known dellorto supplier from Italy, all flow tested. Not sure if I can mention his name here.
I still have the old original dellorto idles(52) and mains(142) which I could give a shot.
Maybe the Italian supplier, as you said, made a glitch on one of the jets but he has a good reputation.

The local suppliers here have the old dying art of building/refurbishing these parts for decades. They can even re-make your old Alfa oil pump from scratch. Each small workshop has a specialization and are all squeezed into about 5 blocks of the downtown area. You'll never see anywhere like it in the 1st world.

Tom:

Here's a pic of the tube and also a schematic of the carb.

[Guy Croft]

Chris - some site issues this morning sorry.

That looseness - don't worry about it. You can give it a bit of a squeeze - it just makes removing the assy harder if it's loose.

As for your kind pms about spray bar conversion - never tried it would not trust it - yes have seen it B4 and said the same thing then.

G

[Brit01]

Chris - some site issues this morning sorry.

No apologies necessary but thanks Guy. Happens to all of us working with software. I work as a systems analyst and having a problematic week. I blame it on the solar storm lingering from last week!

That looseness - don't worry about it. You can give it a bit of a squeeze

Ok - I'll pull them apart and squeeze them back tighter just in case.
I was thinking that the specific height of each hole would have an effect on the air/fuel ratio being pulled in so if one emulsion tube is sitting lower than the others then the large holes of the .4 tube would act differently.

As for your kind pms about spray bar conversion - never tried it would not trust it

ok that's what I was thinking. They were kind/honest enough to tell me they weren't suitable for smaller bore engines like the 1.7 when I inquired about the choke sizes just out of curiosity.

Regards

Chris