A112 70HP motor with 316 deg cam won't idle below 2000 rpm

Pantdino

Pantdino

Low Mileage
I bought a Coppa Mille racer in 2007, which is powered by an A112A2 1050 motor. The seller said it has a 316 deg cam, and the carb is a 36DCD. It is an 850TC clone, with the early front race radiator, A-arm front suspension, partial roll cage, and Scuderia Topolino adjustable rear shocks and starter motor.

I'm out of town at the moment and I'll post some pics when I get home

The issue is that until it is warm it is almost undriveable, refusing to idle and run smoothly, no matter how you adjust the choke. Getting away from a stop without killing it is a challenge.

Once it is warmed up it is fine and pulls strongly from 2000 rpm, but it won't idle below 2000 and tends to die when you come to a stop, requiring you to pull the choke lever a bit to keep it running

The carb was rebuilt by Paul Vanderheiden and again by me, but it makes little difference. I also got jet advice from Pierce Manifolds and changed the jets accordingly, so I don't think the carb is the problem.

Is this just how these engines are, or is there something I can do?? (Yes, I know it is a race car, but still... :)

Thanks,
Jim
 
Jim,

little motor, long duration cam...

Long duration and lots of overlap will mean an engine will generate lower manifold vacuum numbers than it would with less overlap (general rule of thumb) ... that's likely what's causing your poor (well lack of under 2000) idle issue.

With the long duration / long overlap cam once the engine revs build and manifold air speed come up the engine pulls and behaves like "normal"

I think it's going to be one of those things you can "tune out" to some degree, but (again a rule of thumb) that improving low speed response will be at the cost of some high speed performance.

I would be looking at perhaps a smaller primary choke in the carb to improve low speed response...

SteveC
 
I think your problem is in the carb. No matter how aggressive the cam you would be able to get a 1000 rpm idle. On a weber carb the transition from idle circuit to main is about 1500 rpm. My guess is there is a problem with the idle circuit of the carb.
 
NM850 said:
I think your problem is in the carb. No matter how aggressive the cam you would be able to get a 1000 rpm idle. On a weber carb the transition from idle circuit to main is about 1500 rpm. My guess is there is a problem with the idle circuit of the carb.
Click to expand...

There was a problem with the idle circuit when I got it- the idle mixture screw was all the way in and broken off. Paul V fixed that, but it made little difference. Yes, you are right. The engine is running on the cruise circuit. Turning the idle mixture screw does nothing until it comes all the way out, at which point there is a vacuum leak and the engine dies.
 
fiatfactory said:
Jim,

little motor, long duration cam...

Long duration and lots of overlap will mean an engine will generate lower manifold vacuum numbers than it would with less overlap (general rule of thumb) ... that's likely what's causing your poor (well lack of under 2000) idle issue.

With the long duration / long overlap cam once the engine revs build and manifold air speed come up the engine pulls and behaves like "normal"

I think it's going to be one of those things you can "tune out" to some degree, but (again a rule of thumb) that improving low speed response will be at the cost of some high speed performance.

I would be looking at perhaps a smaller primary choke in the carb to improve low speed response...

SteveC
Click to expand...
I got a smaller choke to use from Pierce Manifolds but I couldn’t get the old one out.
I can sacrifice some high rev power. With fiberglass doors and hood the car doesn’t weigh much so I rarely rev it above 4500 rpm and still leave traffic in the dust.
 
High idle with very hot cam and single carb is normal.
Individual runners (2 dual carbs) would cure that.
Overall, I would say your cam is way to racy for your expectations on low rpm behavior.
You probably need to change your cam or your expectations. :)
 
Make it +2 from here also. My understanding was/is the PBS full race stagger timing cam was dead BELOW 4,000rpm. My warmed over 1050 (PBS 8-port & dual 40DCOE's, carbon fiber pushrods etc.) would pull from 1500 in top gear with gentle pedal use. That was in a 750GT so weight is similar. BTW 4500 max seems a little below the sweet spot on your camshaft.
 
Jeff Stich said:
+1 on each of these.
Click to expand...

I thought this was the case. I guess I’m not too keen on making it drive like a “normal “ car either.

In case any of you didn’t know, if you search under pantdino on YouTube you can see some visits I made to Abarth collections and vendors in 2007- Aumueller, Moell, Berni and the Coppa Mille and Silver Flag races that year
 
Hi, Very nice car! My street car with 300 cam idles at 1300rpm on good gas.
I would check the distributor also. What kind do you have? A112 electronic?
Perhaps the advance weight springs are not retarding at slower speed.
Detune a race engine by back off the timing advance some, smaller choke in the carb (23mm,22mm? first barrel),
Small idle jet gets clogged and causes engine to die at low speed. Use a bigger idle jet size 55,60,?
Don't use the choke lever if you can help it. Might cause faster cylinder/ring wear.
Do you have a thermostat? These cars warm up fast and a warm intake manifold makes the fuel mixture better.
Check spark plugs for rich condition. Maybe a little hotter sparkplug temp.
Weber DCD carbs wear the carb body wall at primary shaft under the float bowl.
Oversize shafts are available if ready bad.
Good luck!
Kent
 
There is a huge difference between “dead” and not idling. I stand by my statement that the engine should idle near 1000 rpm. Idling and having useable power are two different things. Some F1 Engines idle as high as 7000 rpm but they are not production based.
 
Thanks, Kent. Those are some great ideas.
I tried just letting it run at fast idle until I had 90C water and 60C oil temps and it did fine. So maybe I have been expecting a race engine to run when it is cold and it didn’t agree

Yes, it has a thermostat and I could see when it opened on the water temp gauge.

Yesterday it wasn’t dying when coming to a stop either- seems to run better some days than others

On another topic, I had to remove the oil cooler for street use because on cool days the oil temp never got to where it should be. I installed a Moroso preoiler to use when the oil cooler is installed because it takes too long to build up oil pressure after starting with that big circuit hooked up

Jim
 
You must log in or register to reply here.
Back
Top