Technical Correct jetting for 594cc engine

[smart51]

What is the correct carb jetting for a 594cc engine? I think mine might be a mix of 594 and 652 jets.

My carb is stamped 28IMB5/250 which will have come off a 652cc engine, so originally jetted for bigger air flow. Mine currently has:
idle 0.45
main 115
air corrector 190

I believe some cars had 125 main jets, some had 220 or 225 air corrector jets and some had 0.5mm idle jets. I suspect I might need a bigger air corrector.

 

[fiat500]

What is the correct carb jetting for a 594cc engine? I think mine might be a mix of 594 and 652 jets.

My carb is stamped 28IMB5/250 which will have come off a 652cc engine, so originally jetted for bigger air flow. Mine currently has:
idle 0.45
main 115
air corrector 190

I believe some cars had 125 main jets, some had 220 or 225 air corrector jets and some had 0.5mm idle jets. I suspect I might need a bigger air corrector.

It sounds like you might be onto something. My 126 manual says that the compensation jet changed to 195 in May 1985, but that's for a 652 engine.

 

[smart51]

It sounds like you might be onto something. My 126 manual says that the compensation jet changed to 195 in May 1985, but that's for a 652 engine.

What does your manual say the jet sizes should be on early 126s?

 

[fiat500]

What does your manual say the jet sizes should be on early 126s?

Up to Nov 1974.
Main: 1.25
Idle: 0.45
Starting: 0.90
Air correction: up to 1974: 2.15 after 1974: 2.25
I'll look at the 500 manual which is supposed to cover the models with a 594 engine; I'm not sure how throughly it does that.

 

[smart51]

Up to Nov 1974.
Main: 1.25
Idle: 0.45
Starting: 0.90
Air correction: up to 1974: 2.15 after 1974: 2.25
I'll look at the 500 manual which is supposed to cover the models with a 594 engine; I'm not sure how throughly it does that.

Thanks Peter. It looks like I should have both a larger main jet and a larger air correction jet.

 

[the hobbler]

What does your manual say the jet sizes should be on early 126s?

According to my 'Haynes' for the "594" engine with a Weber 28 IMB 3 or 1, it is as Peter says, but the problem is that for the '594' engine it gives the jet in what looks like "orifice size in mm" but for the 652 engine it gives the jet size in 'numbers'----mighty confusing!

 

[fiat500]

According to my 'Haynes' for the "594" engine with a Weber 28 IMB 3 or 1, it is as Peter says, but the problem is that for the '594' engine it gives the jet in what looks like "orifice size in mm" but for the 652 engine it gives the jet size in 'numbers'----mighty confusing!

The numbers are just the same measurement in hundredths of a millimetre.

 

[fiat500]

Up to Nov 1974.
Main: 1.25
Idle: 0.45
Starting: 0.90
Air correction: up to 1974: 2.15 after 1974: 2.25
I'll look at the 500 manual which is supposed to cover the models with a 594 engine; I'm not sure how throughly it does that.

I can't find anything in Haynes that covers the 594. I suspect that Haynes just added to the cover of the manual the fact that the 594 variation is covered since the car looks virtually identical; in the same way that it doesn't seem to describe any differences with the gearbox.

 

[smart51]

Google has translated a few foreign language sites for me. This is what I've found, though some sites say Italian and Polish carbs were jetted differently.

main	idle	air	carb	engine
125​	0.45​	215​	28 IMB 3 (pre 1974)	594cc
125​	0.45​	225​	28 IMB 1 (1974 - 1977)	594cc
115​	0.5​	190​	28 IMB 5 (1977 - 1985)	652cc
115​	0.5​	195​	28 IMB 12 (post 1985)	652cc
115​	0.5​	190​	28 IMB5/250	126A1.076
115​	0.5​	190​	28 IMB10/250	126A1.076/E
115​	0.5​	195​	28 IMB12/250	126A1.076/E
125​	0.5​	190​	28 IMB15/250	126A1.072
125​	0.5​	190​	28 S1A2	126A1.072
115​	0.5​	190​	28 S1A1/300	126A1.076/K
125​	0.5​	190​	28 S1A2	126A1.072
115​	0.5​	190​	28 S1A1	126A1.076

Mine has the main jet and air corrector for a 650 engine and the idle jet of a 594. Its probably running a bit lean at wide open throttle, which would explain why it is down on top end power.

What change to the engine was made in 1974 that needed the air correction jet changing? My car is from 1975, so I guess I need the largest air correction jet.

 

[fiat500]

Google has translated a few foreign language sites for me. This is what I've found, though some sites say Italian and Polish carbs were jetted differently.

main	idle	air	carb	engine
125​	0.45​	215​	28 IMB 3 (pre 1974)	594cc
125​	0.45​	225​	28 IMB 1 (1974 - 1977)	594cc
115​	0.5​	190​	28 IMB 5 (1977 - 1985)	652cc
115​	0.5​	195​	28 IMB 12 (post 1985)	652cc
115​	0.5​	190​	28 IMB5/250	126A1.076
115​	0.5​	190​	28 IMB10/250	126A1.076/E
115​	0.5​	195​	28 IMB12/250	126A1.076/E
125​	0.5​	190​	28 IMB15/250	126A1.072
125​	0.5​	190​	28 S1A2	126A1.072
115​	0.5​	190​	28 S1A1/300	126A1.076/K
125​	0.5​	190​	28 S1A2	126A1.072
115​	0.5​	190​	28 S1A1	126A1.076

Mine has the main jet and air corrector for a 650 engine and the idle jet of a 594. Its probably running a bit lean at wide open throttle, which would explain why it is down on top end power.

What change to the engine was made in 1974 that needed the air correction jet changing? My car is from 1975, so I guess I need the largest air correction jet.

Some of the engine types in that list would be special variations for other countries and their specific emission legislations. There is also the variation in fuel to consider, where some countries appear to have adopted ethanol petrols a long time before the UK. They even used E85, which I think is 15% ethanol. Whatever the specifics, all of these would have probably caused a need for different jetting.

'74-'75 was when the 500 other than the Giardiniera was being phased out, so whatever the changes, they probably relate to something about the Fiat 126. Bear in mind, Fiat weren't allowing the 500R to outpace the 126 and had effectively put a limiter on it in the form of the carburettor, and that would have affected the jetting.

I think you're right to head in the direction you suggest. There hardly seems to be a car or engine that isn't now a mongrel and we are all using much different petrol than the cars were originally setup for, so whatever was right for a pure, pedigree 500 may not be right for the cars now

 

[Toshi 975]

Interestingly the jet combination on the 499cc 500 carb remained the same throughout the production run despite the 26IMB carb going through a number of changes.
The 126 28IMB carb however went through a number of changes normally in line with other engine changes such as capacity, compression ratio, valve sizes and distributor advance. One thing remained fairly constant, the ratio between air and main jets with a field of 56 to 60% main to air. The higher figure being the richer mixture.
I used this as a guide when configuring a Weber 30DIC twin choke carb on a tuned 650 engine with some success to get the jetting in the right zone then fine tuning with a gas analyser.
I tended to decide on an air jet setting then work back from there as main jets are readily available and cheap to buy.

 

[the hobbler]

Interestingly the jet combination on the 499cc 500 carb remained the same throughout the production run despite the 26IMB carb going through a number of changes.
The 126 28IMB carb however went through a number of changes normally in line with other engine changes such as capacity, compression ratio, valve sizes and distributor advance. One thing remained fairly constant, the ratio between air and main jets with a field of 56 to 60% main to air. The higher figure being the richer mixture.
I used this as a guide when configuring a Weber 30DIC twin choke carb on a tuned 650 engine with some success to get the jetting in the right zone then fine tuning with a gas analyser.
I tended to decide on an air jet setting then work back from there as main jets are readily available and cheap to buy.

I have 2 'Haynes'----one is JUST for the '594' engine with an ISBN of:---0-85696-305-4. The other manyal covers primarily the '594'engine, with a '652' supplement at the back for "things" which are different. The ISBN number for this manual is:---1-85010-164-7

 

[smart51]

Interestingly the jet combination on the 499cc 500 carb remained the same throughout the production run despite the 26IMB carb going through a number of changes.
The 126 28IMB carb however went through a number of changes normally in line with other engine changes such as capacity, compression ratio, valve sizes and distributor advance. One thing remained fairly constant, the ratio between air and main jets with a field of 56 to 60% main to air. The higher figure being the richer mixture.
I used this as a guide when configuring a Weber 30DIC twin choke carb on a tuned 650 engine with some success to get the jetting in the right zone then fine tuning with a gas analyser.
I tended to decide on an air jet setting then work back from there as main jets are readily available and cheap to buy.

That's an interesting observation. The only exception in my list is for the 126A1.072 engine variant. Both the 28 IMB15 and the 28 S1A2 equivalent have a 66% main jet to air jet ratio.

I've gone for the 125 / 215 combination from the earlier 594cc engines and I'll see how it goes. I can try a 225 air jet later if needed.

 

[Toshi 975]

That's an interesting observation. The only exception in my list is for the 126A1.072 engine variant. Both the 28 IMB15 and the 28 S1A2 equivalent have a 66% main jet to air jet ratio.

I've gone for the 125 / 215 combination from the earlier 594cc engines and I'll see how it goes. I can try a 225 air jet later if needed.

Your observations and calculations are also interesting. Those particular carbs do not appear in any Haynes Manuals which I used for my source material so well done.
Now the interesting part to me at least is that I looked at the ratios for the very successful Dellorto FZD carbs and they came out at about 66%.

 

[smart51]

The polish language Wikipedia site threw up an interesting nugget. The 126A1.072 engine was a special designed to run on 78 octane fuel. It made 21.7 BHP rather than the usual 24.1. It has the odd combination of a big main jet and a small air jet, presumably to take in huge quantities of low grade fuel.

 

[smart51]

Well this is disappointing. I've fitted the new jets, but it hasn't given the results I expected.

The car starts much better. It catches right away and flares a bit before idling. Before it would take half a second of cranking from first catching to build up to a steady idle. On the move, it runs smoothly at all revs but has less top end power. At 30MPH in 4th it runs just as well at light throttle, so on the slow running circuit. It sounds less happy with an open throttle though. It tolerates but doesn't like the new jets.

I went for a 125 main and a 215 air corrector. The other option for the 594cc was 125 and 225. Would one size of air corrector make that much difference?

The carb had a 115 main and 190 air corrector before. 125 is two sizes bigger than 115. A bigger jet must mean mean more fuel, so less power must mean it is running very rich now. Is there a way I can measure what's going on, so I can pick the right size jets?

 

[fiat500]

Have you considered using a Colortune? It's a simplistic device, that probably achieves nothing that an experienced ear could do, but I feel that gives some definitive information that an engine is butning its fuel properly.

 

[smart51]

Have you considered using a Colortune? It's a simplistic device, that probably achieves nothing that an experienced ear could do, but I feel that gives some definitive information that an engine is butning its fuel properly.

That would let me set the idle mixture. How could you use it when the engine is under load?

 

[Goldnrust]

Under load you'd need a Wideband o2 sensor, with appropriate bung welded into the exhaust. That will give you a constant live readout of your air fuel mixture. I've use the AEM UEGO range successfully for many years.

But that's ~£200. There are other ways to monitor air fuel mixtures, the most simple and old fashioned being to read the spark plugs. To do that properly though you need to run the car for some time at the desired throttle opening/engine speed where you want to asses the mixture and then cut the engine and put the car in neutral then bring it to a stop and remove the spark plugs and inspect them. Any time spent idling or slowing down will give you false readings.

An even more simple way to do a rough check is to use the choke. Slowly bring up the choke, and if the engine starts to run better then the mixture is lean, and if it immediately gets worse then you're rich. If you can raise the choke a small amount without the car seeming to run worse, but then it gets worse as you get to half choke or more, then you're in the ball park.

Regarding the jetting changes you've made, you've gone up jet size yet, but you'd also gone up on air corrector jet. Larger air corrector jets lean out the mixture at high revs/large throttle openings. So there's no telling from that whether you're now richer or leaner than before.

 

[fiat500]

Have you considered using a Colortune? It's a simplistic device, that probably achieves nothing that an experienced ear could do, but I feel that gives some definitive information that an engine is butning its fuel properly.

Good point.....at least @Goldnrust has some decent suggestions.