Per Sempre Alfa is the offical magazine of the Alfa Romeo Owners Club of Australia (Queensland Division). This is an on-line version of an article originally printed in the magazine and is subject to the Australian Copyright Act. Other than for the purposes of and subject to the conditions prescribed under the Copyright Act, no part of this work may in any form or by any means (electronic, mechanical, photocopying, or otherwise) be reproduced, stored in a retrieval system or transmitted without prior written permission.
Well known AROCA (Qld) Alfisti, Ken Percival, tracks downs a misfire in his very original 1.5 litre Sud. (Per Sempre Alfa November 1998)
It all began on the Montville Run, when my Sud Ti Twin-carb started running on three cylinders on trailing throttle. It would rev clean up to the redline (6,500 rpm), but try to do 60kph in fourth gear and it would just about shake your fillings out!
Having owned this Alfa for 14½ years now, I was familiar with their habit of suddenly dropping a spark plug. I was therefore nonchalant when other members asked what was wrong after hearing a decidedly uneven exhaust note while following me. "It's just a plug," I said, and promptly ordered a new set of genuine plugs (Golden Lodge 25 HL). These plugs are the only ones I'll use because of their unique four earth electrode design.
Now, in true mechanic style, I had faith in my diagnosis of the misfire and didn't bother fitting the new spark plugs until the Friday afternoon before the car's next outing to Stanthorpe/Leyburn. It wasn't running well from the start of our trip and by the time we got to Stanthorpe the misfire was bad. As it was mainly apparent on trailing throttle and low revs I simply used a lower gear and the car was not as bothersome to drive. On our return the Sud was parked and the misfire put on my list of things to do. The next outing, for the Queensland Classic Rally was in four weeks so it was not urgent. About two days before the Rally I took it to my workshop, for some serious tests.
The Sud has travelled only 65,000 kilometers, so I was confident the problem couldn't be too serious. I cleaned and tested the plugs and they were okay. I then checked the H.T. leads for resistance and corrosion and the state of the distributor cap and rotor button. The leads, cap and rotor were showing there age with corrosion and pitting. I dutifully replaced them with genuine Marrelli items.
I also checked the compression of each cylinder. This is done by first removing all the spark plugs and coil lead from the distributor. Then have an assistant hold the throttle wide open and crank the engine several times, while a compression gauge is held in the spark plug hole. On the Sud all cylinders were between 150-170 PSI and acceptable. Another test you can perform with a compression gauge is to add a few drops of oil into the cylinder after initial testing and run the test again. If you had an initial low reading and it rises significantly after adding the oil, your compression rings are worn. If there is no rise, but the reading is still low, you may have a valve burnt out. These problems display symptoms similar to the uneven exhaust note my Sud had but thankfully my valves and rings tested okay.
I'd finished working on the car by the Friday before the Rally (between all those other distracting, paying transmission jobs) and thought I'd sorted the problem. Imagine my surprise then when the misfire was still there. The car ran the same as before, okay on power but rough on the over run. With no time to do anything about it before the Rally, I decided not to run that day and just putt (literally) over to the finish. That was no fun, so I decided to get really serious.
I carefully examined all the things I'd previously done then decided to check and adjust the tappets as these also affect engine tune greatly. As valves and valve seats wear on these overhead cam engines, the clearances reduce and in extreme cases can hold the valve off its seat. As they heat up they expand so it's a problem that can sometimes only be apparent when the motor is hot. For this reason, you perform tappet adjustment on a cold engine.
First remove the cam covers that are on either side of the motor. Be careful with the gasket and wipe off any dirt around the openings. You will then have to drain the oil from the cavities - I use my oil hand pump to transfer it to a cannister. Keep the oil for later on. You will see that Alfa's brilliant and simple design employs a pair of cam lobes for each tappet bucket with a hole drilled through the cam between them. The clearance is measured between the lobes on either side of the tappet bucket when valves are fully closed. Do one side and one cylinder at a time.
Most manuals show you the genuine tools to use but as these are out of the reach of most of us, I'll tell you the K.P. method. Your adjustment tools consist of a 5mm Allen key cut down to size and a 8mm pin punch. A normal Allen key has a long and a short leg and as the short leg won't reach you'll have to cut down the long side to 40mm. You can do this by using a grinder carefully, not forgetting those safety goggles.
Back to the car. Put it in first gear and move slowly back or forwards until one cylinder's valves are closed (cam lobes not touching the tappets and cam cross holes pointing to tappets - you should be able to rotate tappets freely by hand). As you rotate the tappet buckets, you will see slots on each one. This is where you insert the pin punch to lock them and stop them rotating. Check the clearance first before inserting the pin punch and always extract the punch after adjustment before rechecking the clearance.
Check the clearance by inserting a feeler gauge between the cam and bucket. It must be a firm sliding fit. The two outermost tappets are exhausts (clearance 0.45 - 0.50mm/0.018 - 0.020" cold). The two tappets/cam lobes in the centre beside each other are inlets (clearance 0.35 - 0.40mm/0.014 - 0.016" cold). If you decide they need adjustment, insert your modified Allen key through cross drilled hole and gently rock car back or forward to turn the motor and cams until the Allen key enters the tappet bucket screw. Turn clockwise to decrease clearance and anti-clockwise to increase clearance. Make sure to remove all tools before going onto the next cylinder.
When finished with the tappet adjustments, pour back the oil you removed and refit cam covers, fitting new gaskets if necessary and remembering not to over tighten the cam cover bolts. Well my tappets were now done and as they were a little tight I was feeling that this could have been my problem. I hit the starter and let the engine warm up a little. Time for a test drive - and yes folks my misfire was still there!
The only other thing that could affect tune up was the carburettor. In the Sud and 33 the 1.5 litre engine was fitted with twin Weber 36-IDF carburettors. These like most Alfa components are first rate items fitted as standard to our Italian stallions. My trusty manual told me all I had to know about the carbys in the form of a great exploded-view diagram. First, remove the air filter by extracting six nuts and screws and two engine breather pipes from the centre plastic pipe union. In case you disturb the other pipes going into the union, the four small hoses join to the back of each carby barrel (one to each) and the large hose connects to the oil filler tube. Four metal screws and one 10mm bolt holds the top of the carby. Remove these and gently lift straight up (checking gasket is not sticking) until floats clear carby top. Place the assembly to one side. Don't let fuel leak onto the exhaust if it's hot and don't forget to remove all fuel spills. Check all hoses for cracks and replace if necessary. Any replacements should be done only if the motor is cold and tighten the hose clamps firmly.
The two large protruding brass screws in the centre of the fuel bowl contain the emulsion tube and air-jet and the main jet. Remove and check one at a time for any blockages, then replace and tighten securely. Check for blockages by visual inspection and blowing and sucking air through the jets. Jet holders and jets are in one piece when extracted and can simply be pulled apart, checked, cleaned and then firmly reassembled for refitting.
The next jets to check are the starting jets, which are the flush brass jets towards rear of carbys. Remove, check for debris and refit tightly. Always use correct size screwdrivers to avoid damaging jet slots. So far no problems with the Sud. Two smaller protruding brass screws contain the idle jets and upon removing the left rear jet I found the cause of my misfire. The jet was partially clogged and after a clean out and reassemble I refitted the carby top, taking care not to snare the float or gasket. This sort of work is the most you should attempt on your carby without specialist vacuum gauges to set mixture and idle.
It was a large smile that covered my face when I started the Sud and heard that sweet, smooth exhaust sound again. A test drive around the block and more smiles confirmed that my misfire was gone. Next time I see you on an Alfa run, it will be will all cylinders firing!