Timing and cam belt (3 articles)
EXACT tension !Today, I got an authoritative definition for the proper tension of cam belts on all 4-hole motors, from 907 on.
110 Hz (cycles per second)
Mr. Alan Burrows, from Lotus Cars, Ltd. is over here in the states for six months (at Lotus Cars, USA) on some business. His work includes engineering. Alan told me that they did some testing on the 4-bangers after developing the new electronic tension checker. The result was 110Hz (cold), indicating proper tension. He's pretty sure this is constant for all 4-hole 9xx's.
This turns out to be very convenient. It is two octaves below an A-440 (A-440 is the A just above middle C). Frequency is cut 1/2 each octave drop. So, I use an A-440 tuning fork and my mechanic's stethoscope. That fork is as short as a fountain pen, and common. $20 USD will fetch a cheap fork and stethoscope.
PS: In xxx's earlier experiment, the reading was taken after snapping the belt with the Burroughs. That tends to loosen it a bit. xxx's readings were about 10% lower, which is what I would expect
How to....If you put the engine to TDC you will see some dots on the cam pulleys, these will be in line and in line with the centres of the cam pulleys.
Expert tip: there is an idler pulley which drives the distributor; I didn't see any marks on this one so I painted one on. If it moves, the ign timing will be wrong; and the distributor is such a tight fit you might not be able to move it to get the timing right. So ensure it stays in the right position. IIRC the crank pulley has to come off; anyway, it's fairly obvious. And - a rad hose has to come off! So 'roll' this job in with some of the cooling system work to save effort and antifreeze. Getting the belt tension right is to me a 'black art'. Feel what the old one is like before you take it off, similar tension will be a starting point. The belt is in tension, not like a fan belt which is not. The tension should just allow you to twist it, at its longest run, about 45 degrees; so I'm told. These approximations should make it OK to run gently until it can be checked properly. Tension is set by the small idler pulley which is eccentrically mounted and clamped by a lock nut. It's easy to see, not quite so easy to see how it works. After lining up all the marks, with engine at TDC, slacken the lock nut and turn the bigger nut behind it, it *should* go anticlock to slacken but dont rely on this, do it gently and check .... the belt will go slack. When you get the new one on, tighten the belt to what you think is the same tension and do the lock nut up. Then turn the engine by hand (spanner on the crank pulley) a few times, get it to TDC again and check the marks again. Also check the tension and tighten it if necessary. That's what I did. I *think* it's OK. But treat it gently until you can get the tension checked; even if it's only by feeling other peoples timing belts!
Timing belt snubberThe Elite does not have a arm to keep the belt from slipping. That did not come out till the Esprit. There is an extra section in the casting of the head to drill and bolt the arm on. The head on the Elite does not have this casting. I was hoping to retro fit the arm on mine, but I am not sure how to make it work. I purchased the arm and the hardware some time ago.
The snubber was added to all 907 engines, not just to one car model. All 907 applications got the snubber at the about same time. I don't recall just when the change happened, but I want to say it was late 1976. A friend's 1978 Eclat has a snubber, as does my '79 Eclat and '80 Esprit.
The mounting bosses for the snubber are cast into the front of the cam carrier, not the cylinder head.
Belt testersGates make a cheapo tension measuring device called the "Cricket" (sp?). Lots cheaper than a Burroughs gauge and probably close enough. However I recommend finding someone with a Burroughs gauge and setting the tension correctly, then measure with the Cricket to give you an indication of what "right" looks like.
Correct belt tension99 Hz (cycles per second) on the longest topside span, when struck gently with a small spanner (near one end works best).
Find someone with a frequency counter and a pickup device, or ask your favorite piano tuner to stop by for a look-see... Of course you must offer a ride as a suitable inducement ;-)
My own choice is a mechanic's stethoscope and a tuning fork.
PS: As a home-brew pickup device, use a contact microphone applied near the intake sproket, or a "magnetic" pickup near the middle. If the mag. pickup isn't sensitive enough, slip a small paper-clip over the belt (not enough mass to affect it).
Correct belt tension 2I have replaced my timming belt using a Krikit gauge and got readings of approx 55 lbs on the longest span of the belt.
I took readings on my old belt before dissasembling and duplicated the readings with the new belt. My own simple test using two krikits suggests that the readings between two guages is fairly consistent so I would think if you aim for the same reading for yours you'd be pretty close.
I did hear though there might be two types of krikit gauge and if so, it seems the one I used might be called a Krikit one (vice krikit two).
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