Tuning Help … Erratic on tick over and running (650 engine, 26IMB10 carb

[Albuzz]

Good evening…help please!

I’ve a 1972 fiat 500 fitted with a 650 engine but with a 26 IMB10 carb and a 123 ignition.

12 months ago I replaced the head gasket on a reconditioned/skimmed head and went through the tuning/set up process followed by a brief road test and as I recall all was good. On coming back to it 3 weeks ago whilst it seemed ok initially, but since then it’s decided to act up for me and I have been desperately trying to get her set up without much success.

When cold the engine starts readily on choke and revs willingly until warmed up, but once warmed up it’s all over the place. I’ve tried adjusting the fuel mixture/tickover and often get it running sweetly (apparently) and then when I try taking her out on the road she can be be good for 100 yards or so then lose power and possibly die. Then I can go back and have another twiddle and maybe it’ll tickover nicely for a while and then suddenly race followed by it dying and then it can be difficult to start.

I’ve been through the tuning process and the following:

- New carb (made in Spain) 18 months ago.. checked the float open/close gaps and no crud in the bowl and the valve closes/opens correctly
- checked and set tappets
- checked and set the timing (123 ignition fitted 10 years ago) as per 123 set up.
- adjusted carb as per many posts on here
- checked compression… cylinder #1, dry 125, wet 142, cylinder#2, dry 107, wet 121. I’m not sure what is good for a 650.
- checked fuel pump is ok (temporary filter between pump and carb shows fuel in the filter) and pump output looks good
-checked for air leaks around carb base with easy start etc.
- Petrol tank cap breather is good.

It seems better/best when the mixture is adjusted all the way in (I’ve seen comments that this is unusual but can happen).

One point to note is that it’s a 650 engine fitted with a 26 IMB carb rather than the 28 IMB. could that be an issue or does it make little difference re: consistency. Also, given the different compression between cylinders is around 15% to 17%, could that be contributing to the problem.

I’m at loss as to what to try next.

Any pointers/ideas would be appreciated

Thanks Alan

 

[smart51]

The 594cc engine was designed to have the 28 IMB carb, but was fitted with the 24 IMB in the 500R. Correctly jetted, it ran fine. The smaller carb allowed good mid range torque but restricted top end air flow, and so power.

A 26 IMB on a 652cc engine will be similar. If correctly jetted, it will run nicely, but with limited top end power. Your problem is that the factory never determined the correct jetting for your combination. What jets do you have? If they're the same as are stated in any of the books, they're likely wrong.

 

[Albuzz]

The 594cc engine was designed to have the 28 IMB carb, but was fitted with the 24 IMB in the 500R. Correctly jetted, it ran fine. The smaller carb allowed good mid range torque but restricted top end air flow, and so power.

A 26 IMB on a 652cc engine will be similar. If correctly jetted, it will run nicely, but with limited top end power. Your problem is that the factory never determined the correct jetting for your combination. What jets do you have? If they're the same as are stated in any of the books, they're likely wrong.

Thanks for the reply… main jet is 112 and the idle jet is 4E .. is that standard? Would you know what jets would be correct for this configuration?

Could incorrect jets cause the erratic behaviour?

Thanks Alan

Ps in case it’s relevant the starter jet is marked 90 F 5

 

[smart51]

Fiat changed the jetting from time to time, so there are variations but this is a starters guide to jet settings:

500s with a 26 IMB had 112 main with 235 air corrector and 0.45mm idle jet.
600s with a 28 IMB had 125 main with either 215 or 225 airs depending on the year, and 0.45 idle jet
600s with a 24 IMB had 090 main with 235 air correctors
650s with a 28 IMB had 115 main, 190 air correctors and 0.50mm idle jet

You can see that the 600cc engines with the "too small" carburettor had a smaller main jet. That's because the engine had to suck harder through a smaller straw. That harder suck meant more vacuum drawing in fuel, so a smaller main jet was needed to hold the fuel back. I'd guess your 650 with a 26 will do something similar.

When Fiat went from 600 to 650 they reduced the main jet, again because the 650 sucked harder drawing in too much fuel and needing a smaller main jet to hold it back.

The idle circuit feeds through the main jet. From my experiments, the main jet does affect the amount of fuel in the idle circuit. My guess is that your car is running rich, which is why you have to screw the idle jet in a lot.

 

[the hobbler]

Fiat changed the jetting from time to time, so there are variations but this is a starters guide to jet settings:

500s with a 26 IMB had 112 main with 235 air corrector and 0.45mm idle jet.
600s with a 28 IMB had 125 main with either 215 or 225 airs depending on the year, and 0.45 idle jet
600s with a 24 IMB had 090 main with 235 air correctors
650s with a 28 IMB had 115 main, 190 air correctors and 0.50mm idle jet

You can see that the 600cc engines with the "too small" carburettor had a smaller main jet. That's because the engine had to suck harder through a smaller straw. That harder suck meant more vacuum drawing in fuel, so a smaller main jet was needed to hold the fuel back. I'd guess your 650 with a 26 will do something similar.

When Fiat went from 600 to 650 they reduced the main jet, again because the 650 sucked harder drawing in too much fuel and needing a smaller main jet to hold it back.

The idle circuit feeds through the main jet. From my experiments, the main jet does affect the amount of fuel in the idle circuit. My guess is that your car is running rich, which is why you have to screw the idle jet in a lot.

As "Smart51" has pointed out, and it is quite a common phenomena, the LARGER engine engine has the SMALLER jet because, as he says, the bigger engine has more 'suck'. You mention that you have had a "123" ignition on the car for 10years----well done! I say this because the original "123" full-distributors (as against the "1/2 distributors") used to have a label on them that pointed out that continued exposure to "in excess of 100C" would cause the unit to wilt (spelt D.I.E.), and the heat of the engine's "cooling" air is often above that figure---and it blows straight over the distributor when the Thermostat is open.
As an experiment, have you tried running the engine on'points', just to see if that makes any difference. I have suffered an 'electronic' ignition'wilting---it would run fine when cold, but as the electronic unit warmed up, the car became undriveable, to the point that I had to call on the services of my AA card---of course, when the AA driver unhitched the car outside my house, it drove perfectly into my garage---doh!
I also think that it would be worthwhile purchasing a "28LMB",jetting that as per the original Fiat specs. for the '650' 126, and I think that the performance will improve.

 

[Albuzz]

Thanks for both extensive replies.. much appreciated.

Your point about the electronic ignition had crossed my mind a. Due to its position and b. I too experienced this with my MGB a couple of years back.. it always ran erratically so I eventually decided to sell it but needed to resolve the issue in order to sell and traced it to the electronic ignition using a cheap spark gap tester.. new dizzie and she purred like she always should have. Sadly I can’t find the original distributor for the 500 so would have to buy one to be able to rule it out. BTW, the spark gap tester at least on tickover shows a healthy spark. I need to get it up to temperature and try again.

The 26IMB is barely 18 months old so would prefer (financially) to try more appropriate jetting first, since the car had a 26IMB fitted when we first had the car 15 years ago and could have had non standard jets fitted. Unfortunately due to working away a lot in those days I used a garage to fix an earlier head gasket and in the process they distorted the carb base (I only discovered later) and the original carb has long since gone

Could anyone suggest a better jetting set up to try?

Also, is the disparity between cylinder compression within tolerance.. I think it is but I have seen 10% difference mentioned as the max but I’ve also seen 20%?

Thanks again
Alan

 

[smart51]

I'd leave cylinder compression for now. Trying another distributor is a good idea. As for jetting, the best bet is to take it to a rolling road that sets up carbs and let them measure it.

 

[the hobbler]

I'd leave cylinder compression for now. Trying another distributor is a good idea. As for jetting, the best bet is to take it to a rolling road that sets up carbs and let them measure it.

Try Lukasz at "L.P.Garage" in Sudbury, Suffolk for a 2nd hand distributor.You need one specific to the '126'engine---they 500 and 126 distributors ARE different. Even a 2nd hand dizzy, with new points and a "competiton" condenser (Swiftune or Shacktune) which are made so that you can mount them up by the coil(away from the heat) would show whether the problem is "fuel" or "spark". After I had my big problem with an electronic ignition, I devised a method of fitting a small "heat-dam" inside the Thermostat housing---it keeps just enough heat away from the distributor to look after the electronic ignition unit.The article that I wrote is on paper---if you would like a copy of the article, contact me direct ( [email protected] ), let me have your address and I will put a copy of the article into the post to you.

 

[Albuzz]

Try Lukasz at "L.P.Garage" in Sudbury, Suffolk for a 2nd hand distributor.You need one specific to the '126'engine---they 500 and 126 distributors ARE different. Even a 2nd hand dizzy, with new points and a "competiton" condenser (Swiftune or Shacktune) which are made so that you can mount them up by the coil(away from the heat) would show whether the problem is "fuel" or "spark". After I had my big problem with an electronic ignition, I devised a method of fitting a small "heat-dam" inside the Thermostat housing---it keeps just enough heat away from the distributor to look after the electronic ignition unit.The article that I wrote is on paper---if you would like a copy of the article, contact me direct ( [email protected] ), let me have your address and I will put a copy of the article into the post to you.

Sincere thanks for the help to you both… really much appreciated.

Regards
Alan