Race Engine Forum UK

Interesting. So you disconnected the choke cables to the non-existent choke mechanism in the carb because the choke is actually the venturi restriction in the main body of the carb. And the cold enrichment cable is dis-connected because that was what Weber used for race cars before they realized that closing a door above the carb works better for street cars. :)

I have a set of IDF's sitting in my shop attached to the Fiat I am building a rollcage for. The cold enrichment cables are rusted in place.

Tim

Interesting. So you disconnected the choke cables to the non-existent choke mechanism in the carb because the choke is actually the venturi restriction in the main body of the carb. And the cold enrichment cable is dis-connected because that was what Weber used for race cars before they realized that closing a door above the carb works better for street cars. :)

Well not sure if we can call the cold start mechanism as a choke (not familiar with other carbs). The car has a manual choke lever to start in cold weather. Once running you push it back in and it closes the enriching jets but often causes idling issues after frequent use.

Urbancamo hit the nail on the head. Clear explanation. Thanks.

Pump jets needed only and to nurse the first few seconds. The unsuspected driver who attempts to use the cold start mechanism(choke cables) can easily flood and foul up the system, plugs etc.To avoid these incidents, as mentioned, choke cables have been disconnected as they are a very harsh device.

Next topic please with all due respect as I was trying to find out the issue with the fuel delivery system.

I'm taking the car into the garage for them to inspect the fuel lines and possibly remove the tank to check the internal sock filter. Nasty job that I don't have the time (or health) for right now.

Regards

Chris

Brit01 wrote:

Next topic please with all due respect as I was trying to find out the issue with the fuel delivery system.

Sorry, yes indeed next topic.

Reluctantly I took my Alfa to my local mechanic this morning to look into the pump issue.
If it's a job that requires lowering the fuel tank and cleaning the internal filter then that is something I'm not keen on doing myself.

Lets hope he has good news in a day or 2.

I just pray that it's not a worn lobe on the oil pump shaft that actions the rod to the pump.

Chris (anxiously waiting for his phone call)

Great car is ready to collect.

New pump on and working.
But he said the timing was too advanced - very strange as I used my timing light and set it at 8 degrees, maybe a little more on the mark.
no pinging under load.

Maybe he has more accurate equipment or even listens to any minute detonation with equipment. My vocab in Spanish isn't that advanced!:lol:

I'll pick it up after work today.

Timing was spot on - he jumped the gun without looking at it first. His first impression when starting the engine but when he got his timing light out it was 8 degrees exactly. no detonation.

He noticed the starter motor felt a bit weak though.


New pump works a charm and sounds clean and crisp. Beautiful. Both carbs open the same time.
(He congratulated me on a superb job of rebuilding it). He worked for a UK rally team some time ago. Works on old classics and modern cars. He's currently restoring an E-type for himself and has a lovely white softtop MG.

BUT we're still getting a slight hesitation between 2000 and 2700 rpm.

What next? a step up in idle jet size? Floats raised slightly? (or as mentioned check the pump jet settings - but this feels more like a transition issue )

Not sure. Any further advice would be appreciated.

Chris

Just drive it harder Chris

G

Just drive it harder Chris

Lol. Yes that does help when you floor it but with mid-throttle at 2000 rpm?

But you know me being a perfectionist. Determined to get these Dellortos working well under all driving styles/conditions.

Out of my powerband that one!

You'll need 'Tommi the Finn' for this!

G

Tommi:

The pump jets discharge was mentioned earlier. I still haven't measured the discharge yet.
But the flat spot tends to disappear when accelerating hard.

Seems to be a transition phase issue around the 2-3000 rpm mark.
Going a little lean here. How can I richen up this phase?

Chris

Couple things to realize:

- Dual carbs are never fuel injection (after this called only FI) when it comes to accuracy and mixture quality
- There is always some compromises with dual carbs

Many production cars, even FI ones, are adjusted pretty lean in normal cruising or light acceleration phase...even my friend's -99 Civic with 4-point injection has very lean feeling when accelerating lightly. It's on the edge where it almost jerks or stutters, but it doesn't. But you feel that fuel mixture is very lean.
But over 95% of average drivers won't feel this at all. I feel it because I have fiddled with cars a lot.
These things are adjusted like this to save petrol. It's the only thing average user cares about.

Seems that your A-R has situation that main circuit doesn't come in soon enough and that causes the problems. It's so typical in many applications.
My own Dell'ortos run 15:1 AFR or even leaner when I accelerate lightly in 2000-3000 region. It just on the edge it doesn't stutter. Feels very "thin" though. But this might explain why my MPG is the same or even better than stock engine had. I'm very happy with that.
You can run lean mixtures safely when engine load is low.

Only reasonable way is to beef up the idle jets. If you have possibility to have them within 0.01 increments, just increase one step at the time and find a compromise you are happy with.
You'll find that when transition phase is reasonable/good enough, mixture in low speed might be bit too rich. But these are the compromises I stated earlier.
Also remember to test various idle mixture screw adjustments, they affect fuel amount in idle jet phase.

Second thing to try is increase air correctors 5-10 numbers. Some cases this might help main circuit to come in earlier. Of course fuel mixture has to checked in high rpms after this.

And oh - when running lean mixtures, good and strong spark is A MUST. Lean mixture burns very slowly. I have no personal experience on Iridium spark plugs. But some say they can improve things.
If using NGK, BPR7EIX or BPR6EIX is what you want.

Last thing to notice is this. I have no clue about A-R ignition curve, but there could be place to steep up the curve in low rpm region. Usually stock cars have very modest ignition curve which usually peaks in 4000-5000 rpm range. Usually there is very modest ignition advance in low rpm region.
My own modified engine peaks it maximum advance in 3000 rpm and pretty much stays there. I once tested OE curve and its hideous. Ignition advance is so low in small rpm's that engine will only terribly hesitate and stutter because it needs more advance to run properly.
But I have more radical cam and other mods wich always needs modified ignition curve.

(Took almost 2 hours to wrote this, this is certainly the best place to practice technical English language)

-T

TOMMI - MODEL POST

THANKS VERY MUCH FOR YOUR OUTSTANDING CONTRIBUTION HERE AND ON MANY OTHER TOPICS..

GC

Thank you very much for your time on this subject tommi. Greatly appreciated and well written.

Only reasonable way is to beef up the idle jets. If you have possibility to have them within 0.01 increments, just increase one step at the time and find a compromise you are happy with.
You'll find that when transition phase is reasonable/good enough, mixture in low speed might be bit too rich. But these are the compromises I stated earlier.
Also remember to test various idle mixture screw adjustments, they affect

Currently I have 52 stock idles. I have 53 and 55 idles spare, I will try the 53's first. From previous trials I found the 55's too rich. Maybe a 54 may have to be ordered if the 53 doesn't cut it.

Regarding the idle mixture screws I had them set with the car idling so nicely without any vibration also. The mechanic said he made them leaner, But I think he turned them the wrong way which I find hard to believe though as he works with carburettor engines (works on old jags and Mg's often). I noticed it nosedived when I stopped quickly and then accelerated indicating flooding. Never did this before he adjusted them. So first thing is to fix them again and go for borderline rich.
Then try the new 53 idles, 0.01 increment increase.

The air corrector jets are 180 now. These are unfortunately expensive to change. All 4 including shipping I'm looking at 50 USD. I will try all other changes first before considering this.

And oh - when running lean mixtures, good and strong spark is A MUST. Lean mixture burns very slowly. I have no personal experience on Iridium spark plugs. But some say they can improve things.
If using NGK, BPR7EIX or BPR6EIX is what you want.

Currently I'm running B7ES but I do have B6ES spare. On the highway the 6's turned out quite white. Too hot I believe
EIX aren't available here most of the time but I will keep looking and most likely order them. They are always waiting for stock.
Thanks for the tip.

Last thing to notice is this. I have no clue about A-R ignition curve, but there could be place to steep up the curve in low rpm region. Usually stock cars have very modest ignition curve which usually peaks in 4000-5000 rpm range. Usually there is very modest ignition advance in low rpm region.
My own modified engine peaks it maximum advance in 3000 rpm and pretty much stays there. I once tested OE curve and its hideous. Ignition advance is so low in small rpm's that engine will only terribly hesitate and stutter because it needs more advance to run properly.

This is a tricky one. 8 degrees TDC at the moment. I could probably advance it more as winter is here and cooler air is coming into the combustion chamber.
In the 40 degree summers if I don't have it at 8 or less I do get detonation under load due to the very hot air coming in. (from outside off the tarmac!!) We measured 55 in the sun.
Shame I don't have one of those adjustable distributor's with the advance curve knob on the outside.
I will advance it again by a degree or so as it is on the modest side after the summer heat.

Best regards

Chris

A friend here recently purchased the same car but with Weber IDF's. Now he has a smooth transition, issues, no hesitation.

40/40 (32 chokes)

He has:

55 idles
150 mains
190 air correctors
F11 emulsions. (F7 stock)

How would this compare with the Dellorto DRLA 40/40 (32 chokes)? If they can at all can be compared.

Mine:
52 idles
145 mains
180 air correctors
.4 emulsions

Chris

You cannot really compare two different carbs, even they are the same type.

I have no experience on A-R's and carbs, but I might believe that your friends A-R is running richer than yours. IDF's aren't known about their progression properties as I have stated here in forum earlier.
If running Fiat TC with 55 idles (no matter if carbs are IDF's or DCOE's), no hesitation but pig rich.

55 idle in A-R with IDF's...without lambda measurement you don't really know.