131mirafiori forum

One of the annoying characteristics from our twin cams is that initial hesitation when pressing the throttle.

Does anyone know of the best solution for this ie I want instant response.

I have heard that changing the jets in the carbuerretor can fix this...any thoughts?

Thanks.

Are you claiming that all twin cams have this hesitation? I don't agree and mine certainly doesn't have it. I'd say you need to have your engine tuned properly and the most likely cause for the hesitation will be ignition advance. Most likely it's been retarded to accommodate the low octane fuels we have these days. Incorrect carburation can also cause hesitation. But a well tuned twin cam should not have hesitation so make sure all is as it should be before you start changing things away from standard.

Hi Theo.....I've had this experience in more than one Fiat twin cam but couldn't claim that all perform the same way as I haven't driven them all ...yet!  ;D

Do you think your lack of hesitation could be down to your electric fuel pump...?

In any event thanks for the suggestions.... :)

No, the electric fuel pump has nothing to do with it. Initial pick-up, which is where you presumably have your hesitation, is fed fuel from the carburetor's bowl so the pump is out of the equation. What makes a huge difference in my Sport is the twin 45 webbers and the Guy Croft ported head  ;) ;D That engine has the most instantaneous throttle response than any car I've ever driven  :D But I also have the same twin cam in my Trevi with its standard twin-choke carburetor and even though the auto gearbox makes the car quite sluggish, there's no throttle hesitation. Don't forget these engines were designed to run on higher octane fuel so retarding the ignition nowadays so they run on 95 octane is not doing the engine any favours. Reset your ignition to 10 degrees and keep filling up with 98 octane fuel and you'll probably not have any hesitation  ;)

Higher octane fuel isn't readily avaiable here - Any other suggestions with that restriction in mind?

Don't you have Shell V-Power? That's 98 Octane in the UK and my Trevi only runs well when filled with it  :D If you can't get that then you can try one of the many available Octane booster additives.

Ok thanks Theo......😊

You are most welcome  ;)  But if your ignition timing has been retarded, then filling up with high octane fuel alone won't have the desired effect. You need to reset the ignition timing to 10 degrees BTDC (I actually have it set to about 12 on my Sport and Trevi - I like to live on the edge!  ;D ;D ;D) Of course that's just one thing that might be causing the hesitation and your problem might be something else  :-\ :-[

Check the accelerator pump jet.

Check your accelerator pump diaphragm, and jet output. There is a measurement, can't remember how much, but a certain number of CCs for 10 pumps.

I agree with all that has been said so far & will add; If your exhaust is to big for the carb jetting or your gearing too high you will get this hesitation.
As concerns the contemporary low grade paint thinners fuel; it is not only the initial ignition timing that is a problem, it is the whole advance curve.
My twin cam 8 valve Toyota is from the 1970's when we had super fuel & it did not like un leaded poop of any type till I had the distributor sent off to get it re curved to match what else is going on with the engine now running on ULP .
From my results I will very much recommend the distributor gets a re work to suit the current fuel. That goes for all a ya all.
My 131 Super Brava is all original & runs the ULP OK but lost its very sharp edge a bit & has no hesitation. It did have a slight hesitation at one time but only after a rear tyre change up to 185 /75 - 14"
I went to a 195/65 - 14" & it disappeared.
O.