Technical carb size for 595cc engine

[Seabeehenry]

what is the stock carb size for this engine?

 

[Bambino]

In what state of tune?

Chris

 

[jjacob]

I believe that the 500R came out with a smaller carburetor than the 126 even though they had the same engine. Fiat was trying to make differences to get people to buy the new 126.

The 500R had either a Weber 24IMB or 26IMB. The carburetor that you should install (as stock) is the Weber 28IMB. This is really the proper carburetor for the 595cc stock engine. The smaller carbs are really too small.
John

 

[Bambino]

The Abarth 595 was equipped with a Solex C28PBJ. The engine had a bore and stroke of 73.5mm x 70.0mm, a 9.2:1 CR and produced 27bhp @ 5000rpm.

The Fiat 500R (1972 - 1975) also had a 595cc engine (same B&S) though in a lesser state of tune. It was equipped with a Weber 28IMB3 and produced 23bhp at 4800rpm.

The final version was the 652cc engine fitted to the 126. It used a Weber 28IMB1.

The Weber tuning manual suggests a main venturi diameter of ~28mm for a single throat carburettor feeding a multiple cylinder petrol engine with a single cylinder capacity of 300cc at 5000rpm.

As John has suggested, a 28mm carby would suit, though engine performance is dependent on many things other than just venturi diameter. Some carbies are constructed better than others and the individual features also differ (eg: accelerator pumps etc.)

Sorry this is so long-winded,

Chris

 

[Ian Stanford]

Hi everyone. I'm a newbie to this forum, been browsing for some months, and I've been struck by the friendliness and the keeness to help fellow Classic 500 owners. I have a 500R, which I bought in 2009, and have carried out a rolling restoration to the bodywork and mechanicals, which is finished - not that you ever finish a restoration! I have kept it standard, except for installing electronic ignition, an Abarth exhaust and a 28 IMB carb, and it's a great little car, huge fun to drive.

In answer to the stock carb size for the 594cc engine, the handbook states the 24 IMB as standard. Apparently, the outgoing 500R was originally fitted with the same engine and the same 28 IMB as the incoming 126, but the older car, being lighter, was faster than the new one - not a good marketing point! So Fiat simply detuned the 500R by fitted the 24 IMB car, dropping the claimed 23 bhp to 18 bhp, more in line with the 500F!

Cheers
Ian

 

[Bambino]

Thanks Ian and welcome to the forum.

Chris

 

[Seabeehenry]

As far as I know the engine has a stock tune to it. I haven't had a lot of money to put in to this rebuild. But have still somehow found myself at the parts store more often than I like. I have a weber 26 and a weber 28 but the engine seems held back by the 26.

 

[jjacob]

The Weber 28IMB is the way to go. The 26IMB just does not flow enough for the 600cc engine.
You can buy rebuild kits cheaply. Easy carburetor to rebuild.
John

 

[tbiggham]

Henry speak with Mario I left a weber 28 there it should be rebuilt and good to go he should seep it to you for pretty cheap

 

[Seabeehenry]

Hey Troy. I picked one up at the breakers market in Agira last weekend for 25 euro. Had it checked by one of my guys and they said it was good. I was just wondering if I needed to make any adjustments before putting it on. I was originally running a 26.

 

[tbiggham]

Nope should fit right on

 

[jjacob]

The carburetor will fit easily, but you will have to make both idle speed and mixture adjustments. You will also have to make sure that both the throttle cable and "choke" cables are set so they open and close completely.

Here are the factory pages for adjusting the carb. The 28IMB is the same as the 26IMB
https://www.fiatforum.com/500-classic/326428-fiat-factory-manual-carburetor-adjustment.html

See post #5 in the link below for how to adjust the mixture and idle speed.
https://www.fiatforum.com/500-classic/297444-weber-26-carb-rebuild-tuning.html

The above assume that your timing and valve clearances have been set correctly. If not, you need to do this first.

John

 

[Seabeehenry]

My timing is correct but I'm not too sure about the valve clearances. I've seen too many different measurements and got confused. I definitely have to check out the choke and mixture because I can't keep the engine running unless I have the choke lever up a bit.