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Although tight, you should be able pull it off with your fingers. To get adequate access, you might have to remove the air cleaner. Then, pull the belt forward, a little at a time along the circumference of the pulley.
 

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What model year car do you have. Does the injection pump pulley have a lip on the front?

Before you change anything are you absolutely SURE that it's mis-timed???
 

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I don't know that 'd trust a belt that had been taken off and then replaced, especially with the small expense involved in using a fresh belt. In a nutshell, I'd double check the timing, and if it's off, I'd just cut the belt.

My trick to install a new belt is as follows: first you'll need to get the new belt over the crank pulley, which takes a bit of a "touch". I actually start with the belt inside-out, and after my gyrations (and cursing) it ends up OK on the crank pulley. Then I'll line up the timing marks by rolling the car with the gearbox in 5th gear. next, I'll use my right hand to start the belt on the top side of the pump pulley (holding it in place with my thumb), and with my free left hand I'll grab the top of the right front tire and begin to roll the car forwards (or backwards - I forget), until the belt is fully on the pump pulley. Once you've done it a few times, this method works like a charm. Other guys will work the belt on with WD-40 as a lube, some will remove the pump pulley, and there are probably other tricks, too. As long as you don't force the belt on with metal tools, whatever else works is likely OK.
 

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Here's the scoop on Spica injection pump timing, the abridged version . . . . .

1. The timing belt does NOT stretch. It's not meant to, so don't use a crowbar to try and force it on. If you do, you can break the reinforcing cords inlaid inside the rubber and seriously weaken the belt. Oh . . . . and when the belt breaks, the engine stops, like immediately . . . right now.

a. On a pulley that does not have a lip, thread the belt on the crankshaft pulley. It will slide in behind the crankshaft pulley no problem. Line the up marks and slip the belt smoothly over the rounded front of the injection pump pulley. A little WD-40 will help and won't damage the belt.

b. On an injection pump with a lip on the pulley (Alfettas with air pumps, etc), you're going to have to remove the pulley from the injection pump drive shaft, put the belt on over the lip, then reinstall the pulley on the injection pump. That's easy to say, and might be hard to do. The injection pump drive shaft is tapered and it's likely the last guy that put it on didn't use any anti-seize compound. It could be a real bugger to get off. I have a junk pump here at the house that the pulley will not budge despite pulling about 2 million pounds on with a puller.

c. The belt is pretty robust and isn't subject to much stress, so should last a long time. Even still check for cracking and age deterioration. Belt breakages are rare and usually involve abuse such as pry bars or catching a rock in the works while the engine is running.

d. Pulley cover. I recommend leaving it off as does Wes Ingram. I think the cover is more of a rock/gravel/crud catcher than protecting the belt.

2. Timing the injection pump.

a. The injection pump should be timed at 70 deg BTDC of the INTAKE stroke of the #1 cylinder. To get that you can time the pump off the POWER stroke of the #4 cylinder. This is exactly the same point. It's just easier to get TDC of the power stroke on #4 (both valves closed as seen through the spark plug hole and the rotor pointing to the #4 distributor cap terminal) rather than trying to figure out TDC of the intake stroke on #1.

b. Remove the spark plugs. Rotate the engine CW (as you view it standing at the front of the car looking rearward) until #4 is on TDC of the POWER stroke (both valves closed and ignition rotor pointing towards the #4 distributor cap terminal). The crankshaft pointer should be pointing at the "P" mark. The "P" stands for "Punto," which essentially equates to TDC in Italian. Next rotate the engine CCW about 70 deg until the pointer lines up with the "I" mark, which stands for "ineizione" (injection in Italian). The engine is now timed at 70 deg BTDC of the INTAKE stroke of the #1 cylinder, even though you used the #4 cylinder to do it. Line up the marks on the injection pump pulley and front casting of the pump body and slip the belt on. You should be able to get within 1/2 a tooth of perfect.

3. Symptoms of a mis-timed pump. Even mis-timed, the engine will probably run seemingly ok and there have been owners that have driven around for years with a mis-timed pump. Some symptoms of a mis-timed pump may be:

a. Sluggish engine response
b. Flat spots in performance
c. Lots of throttle to maintain speed
d. Poor fuel mileage
e. Black tailpipe
f. Difficult to get mixture adjustment "right"
 

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In 30 years of driving Spica-injected Alfa's, I've only had one belt break, and that was on a car that had been sitting for ~7 years (just after I got it running, the belt snapped).
 
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