General Fiat 500 28 imb carb issues

[Flipyoung]

Hi there, have not posted on here before but reading the treads there is much knowledge and help available so would really appreciate some help.

I am currently restoring a 1973 500R and adding some abarth styling but my issue is with the carb.

The car has a 650 126 engine which I understand is pretty common and it also had a 24 imb weber carb. I picked up a cheap 28imb carb refurbished it and sorted out the fuel return pipes and installed it.

It does run and start but if you rev it hard it losses power, so you can drive the car but only if you accelerate lightly. I have swoped back to the old carb and it works fine so I am not sure what is wrong? Is there a particular jet or adjustment that could be wrong....any help very great fully appreciated

 

[Toshi 975]

The 500R had a 594ccs engine the same as the early 126 except the 500R was detuned by fitting that 24 carb to make the 126 look good as it had the 28 IMB. If your engine has been replaced with a 652ccs unit then the 28IMB is the correct carb. The only adjustment on the carb is the idle mixture screw or the throttle tick over screw so if the carb does not pick up then neither of these come into play. Might be worth checking the flatness of base flange and for air leaks.

 

[Paolo66]

I would go along with Toshi and say it sound's like you have a leak at the base. This can either be from carb itself or the bakelite base

 

[the hobbler]

Hi Philip;
The fact that the engine runs OK with the original '24' carb would indicate that the fault lies with the '28'. I would reiterate Paolo's and Toshi's advice and check the flatness of the base of the carb (and just for good measure, check the spacer for flatness as well). It is not unknown for the bases of these carbs to get warped. When you run the engine with the '28' carb, does the engine behaviour improve if, when revving it hard, you have the choke pulled on slightly? If it does, this would indicate that the carb is running weak, possibly due to an air-leak at the spacer/carb base. It could also be caused by an incorrectly set float in the float-chamber. Check this (6mm between the top of the carb and the float--with the gasket in place, when the top of the carb is held vertically). Also take the float out, and give it a light shake (in a QUIET room). It may be that the float has a pin-hole crack in it which has allowed it to fill with fuel---you will need to get another float.
The jetting for the '28IMB' is:----Main jet:--------------115
Idle jet:---------------50
starter:----------------F5/90
Air-correction jet:----90
Emulsion tube:-------F8
Needle valve:--------125
I hope that the above helps--I will be in contact re. your message.

 

[Flipyoung]

Hey many thanks for the comments really helpful, I have to say when I started the 500 project I thought it would be very difficult as I have only done air cooled Porsches before and knew nothing about the 500 gem but reading this site is so amazingly helpful, this is very contrary to many porsche sites where people very much only have half facts..

Anyway thanks for the advice the air leak makes sense, the more you rev the more the air is sucked in, I will check this out and in fact there was dirty splatters on the engine cowl so this may be from this....(the car is perfectly clean after an anal. Rebuild

I very much wanted to get it running well before I take it apart again and rebuild the engine to a slightly advanced spec.