Tuning Selecting the right Engine Timing Curve for Hotrod 700cc engine

[Nowuries]

Hey fellow Fiat 500 friends!
First off, thanks to theHobbler for all his help in my build :wave:, and for all the great info on this site. I am in the process of rebuilding my wife's 1970 500F, and she said she wanted more power, so I took that to mean I could put in a monster...
ENGINE SPECS: 700ccm
stroke: 70mm
bore: 79.5mm
compression: 9.5
cam: 40/80
carb: Weber 40 DCOE
modified con rods
rebuilt alternator, etc.
(NOTE: I am upgrading all the other bits too as a result --brakes, suspension, axles, etc, but didn't want to go down that rabbit hole)

To optimize the engine setup, I also purchased a 123 ignition distributor and electronic ignition coil. I have installed the 123 ignition distributor, but it comes with the base curve for a stock 500, which will not work right... so I am looking for input on what you think the best advance curve would be based on the specs. Attached is their curve table info found on the main website and the downloadable manual: http://www.123ignition.nl/files/manuals/123manual_FIAT2R.pdf

I appreciate any assistance you can provide. :worship:

Another tidbit: I currently live at 7500 feet altitude and have heard that may affect timing a bit due to the change to the air fuel mix.

BTW, if you see/know of any major issues, please let me know!

Thanks in advance! Dax

 

[fiat500]

I've found it impossible to get any information about this. But my random thought is that touch could take clues from the 126 BUS which is the same capacity. So the Group 2 and 3 tuning curves all have similar values to that. Trial and error might get it running well.

 

[Nowuries]

Does anyone else have any input? Ideally someone who is running an ignition 123 distributor that also has a 695 or higher output engine? :shrug:

If not, I will try a group 2 tune and see how it goes, and will provide feedback for future users.

 

[adrianh]

any updates with testing different curves? just joined the forum to ask for the same advice.


I have rebuilt an engine that looks similar to yours, the main difference mine uses a dual port cylinder head, compression ratio is a bit higher at 10.5.

I'm using the 123ingition, the R2 model.


Currently the 123 runs on default curve 0. Pulls great, idles nicely.

But, when left to idle for a few minutes, it chokes when I try to rev it up.
The carb was thoroughly checked and runs fine, so the timing it's next on the list.


Planning to play with ignition curves, so any advice will be appreciated, thanks!

 

[the hobbler]

Your scenario Adrian would lead one to look at the (accelerator) 'pump-jet'. If this is too big it will give the engine too big a 'squirt' of fuel upon initial use of the accelerator pump (when the accelerator pedal is depressed). Being that the engine pulls well and idles well, initially would indicate that the timing is not too far out. Initially, try a smaller'pump' jet and see how it goes from there.

 

[adrianh]

thanks for the answer, two things I forgot to mention:
- longer it idles, harder it is to rev up. if left for one minute, it will choke shortly but will rev up after that. if left for five minutes it will be impossible to rev it up at all, need to restart it.
- when it chokes, the best way to make it rev is to keep the gas pedal pressed half-way for few seconds.

Initially I thought it's fuel-starving on idle, replaced the mechanical pump with an electric one and did a check on the carb.

adding pics with the little troublemaker, cheers






cheers

 

[Hutbed]

I somehow missed this post, but I have a similar build and am having the same problem (chokes on throttle after idling). Waiting on some offset screwdrivers to take off the carb top without removing the whole thing. What was the result of your electric pump and carb check?

 

[adrianh]

I'll do the work in 2 weeks, post the findings here.

cheers,
Adrian

 

[the hobbler]

The best tool for removing the carb top when still in-situ is a small ratchet into which you can fit the small screwdriver hex-shank bits.

 

[adrianh]

Promised update: changed the pump jet, downgrading from 40 to 30. 30 is the smallest size I could find.
Unfortunately didn't change anything. Still unable to rev up after idling for a few minutes.

Next step: put a pressure regulator. Current setup is:
- fuel pump: electric Hardi (used to have a mechanical one, changed to electric trying to solve the issue; it didn't)
- carb: weber 40 DCOE
Both sourced from dangelomotori.it

I want to put this regulator in between: MALPASSI FILTER KING CARBURETTOR FUEL PRESSURE REGULATOR & GAUGE KIT, found it on classiccarbs.co.uk


The (new) theory being that the pump floods the carb on revving up.

Experimenting with new ignition curves has to wait for this problem to be solved. Sorry.

cheers

 

[Toshi 975]

If you get too much vibration at the carb it will stop the inlet needle valve from seating and flood the carb with an electric pump as they are pressure sensitive rather than revs