Timing and Compression, Are Problems Related?

Road-race engines and ancillaries - general discussion
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KKemerait
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Timing and Compression, Are Problems Related?

Post by KKemerait » July 29th, 2011, 12:41 am

Hello folks,

I am new to the board and this is my first post. I could use some assistance as I am not a mechanic by trade. I own a 1978 Fiat 124 Spider that seems to have two problems, I am wondering if they could be related, and what the best course of action might be.

Problem 1. When timing is set to 0 BTDC, the engine runs very roughly, sputters at the tail pipe and there is significant hesitation when accelerating from a stopped position (up to 4 seconds...then it lunges forward). This problem seems to resolve itself if I advance the timing to 25+ degrees...then it smooths right out, idle speed increases and the hesitation disappears, however I seem to lose power at higher RPMs.

Problem 2. A compression check revealed the two adjacent cylinders were definitely off. To be specific the numbers were:
Cylinder #1: 159
Cylinder #2: 80
Cylinder #3: 75
Cylinder #4: 120
From what I've read that would seem to indicate a bad head gasket (Please correct me if my understanding is wrong).

Other things that I have noticed, are that the spark plugs are all clean and a light grey color without any evidence of oil and when using a timing light the advance seen when the RPMs increase doesn't always happen at the same RPM and it takes 5-10 seconds or more to return to baseline once the throttle is released. I am not sure if either of those items is significant.

The car does not have fuel injection it is using a Weber 32 ADFA carburator and was converted, prior to my purchase, to use only a single set of points.

Is there a connection between the poor compression and the need to advance the timing so far out of spec to get the engine to idle smoothly? Should I leave the timing where it is, or is that problematic?

Thank you in advance for any assistance you can offer.

WhizzMan
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Re: Timing and Compression, Are Problems Related?

Post by WhizzMan » July 29th, 2011, 9:18 am

Make sure your springs/weights inside the distributor that control RPM-based advance move freely. If those get stuck, you get unpredictable results like you are describing. Once you have that under control, set timing to factory spec. Moving the distributor so far out of spec is not what you should be doing to any normal engine. I don't know the exact (if any) vacuum advance mechanism on your car, but if there is any, it should be functioning properly. You can check functionality of any vacuum advances by simply sucking on the hose and watch the base plate of the timing mechanism move.

Regarding your compression, was that measured with a warm engine, full throttle and all spark plugs removed? Have you tried with a little oil squirted through the spark plug holes? By doing that you can rule out bad seal from worn/broken piston rings or severe scratching of the cylinder walls.
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KKemerait
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Re: Timing and Compression, Are Problems Related?

Post by KKemerait » July 29th, 2011, 1:03 pm

Whizzman,

Thanks for your advice. To answer your questions. When testing compression the engine was cold, all plugs were removed, I did not do anything with the throttle, simply turned the key and allowed engine to turn over 5 or 6 times, then read the gauge, released the pressure and moved on to the next one.

I will do it again, this time with a few drops of oil and check the distributor springs. To my knowledge there is no vacuum advance on that distributor.

LanciaNut69
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Re: Timing and Compression, Are Problems Related?

Post by LanciaNut69 » July 29th, 2011, 2:53 pm

KKemerait,

Do the test first as Whizzman recommends, ie without the oil in the bores first. At least then you will have a baseline to work from. What you are looking for is a similar reading on each cylinder, taken over approximately 10 revolutions of the engine. You should see three quarters or so of the final reading after about 3 revolutions.

Once you have completed that, try adding oil through the plug holes if the reading is still low.
__________
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WhizzMan
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Re: Timing and Compression, Are Problems Related?

Post by WhizzMan » July 29th, 2011, 6:25 pm

You should test with a warmed up engine as well. The difference between the cylinders is not promising much good, but chances are that with your engine warm, all 4 cylinders will seal properly and you will have good compression on all four. Also, Test with full throttle. You don't want a closed throttle to inhibit compression to build up. Besides that, it's the standard way to test. If you want to compare to others, you should follow the same procedure as everyone else.

Make sure you either take the power off your ignition coil, or you have 4 spark plugs sparking away at a harmless location. You don't want to blow up your coil just because you are testing compression.
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Brit01
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Re: Timing and Compression, Are Problems Related?

Post by Brit01 » July 29th, 2011, 9:56 pm

yep follow whizzman's advice.

I did the same test - hot engine, plugs out, coil plugs/ignition removed, full throttle for 10 secs or so until full reading is reached on gauge.

On my Alfa flat four I got 183-193 psi over the 4 cylinders.

Guy Croft
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Re: Timing and Compression, Are Problems Related?

Post by Guy Croft » July 30th, 2011, 12:39 pm

8V TC Ignition timing:

1. With conventional distr you need 10 deg static (ie: at idle 850rpm before centrifugal cuts in around 1000rpm) +/- 2deg
2. The max advance measured with a timing light should be 34 deg +/- 2 at 3500 or 5500 depending on distr type (I think the 3500 setting only applies to the 124 CSA - Fiat-Abarth version). That means 10 static + 24 centrifugal.
3. The contact breaker setting controls the 'dwell' angle which is the period for which the breakers are closed & recharging the coil. Using a feeler gauge is OK but a dwell meter is better. If dwell is wrong so will also the advance. A mismatched coil and condenser/CB setup can burn the points (and even weld them together) and throw everything 'haywire'.
4. If the timing 'hops about' when the strobe is applied it can be due to the things Whizzman has mentioned also worn distr bearings or faulty ignition amplifier unit (Marelli electronic only - never seen it with the Bosch one).
5. Vacuum advance must always disconnected from the distr when doing any timing checks. It is a part-throttle device that brings in more advance at, well, part-throttle, to improve economy and no more than that. At full throttle there is no vacuum hence no vac adv!

Comp tests as already stated must be done on a hot engine (that has actually been driven around to clean off the valves and seats) with all plugs out, wide open throttle. You must, as said, crank till the gauge stops_going_up. How fast it climbs to peak value is an indication of ring and valve sealing. A 'wet test' should always be conducted. This means squirting about a teaspoon of oil into the cylinders via the plug hole (in turn of course..) and repeating the comp test; that is a great way to 'wet-up' the rings and make them seal (better/properly!). Thus a wet test really can indicate whether rings or valves are the problem. If the wet test gives much higher values than the 'dry' then the rings are worn out.

Ignore the wet test for now. Apart from the cyl-cyl peak readings the variance is important. More than 10% down compared with the others on any cyl indicates a ring or valve problem. Because you're only cranking (and let's face it all engines crank at different speeds) you might only see 120-140psi hot on an engine that is still capable of reasonable power (albeit high oil consumption eg: 800 miles to the pint). That is from my personal exp. If you have 180psi or more then the engine is in fine fettle.

There is always much debate about how much comp test readings relate to CR. They don't. The comp test is affected by engine condition, CR, cam timing, valve size and the cranking speed itself - so no direct correlation is ever possible.

G

KKemerait
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Re: Timing and Compression, Are Problems Related?

Post by KKemerait » August 1st, 2011, 12:55 am

Gentlemen,

First of all let me offer my gratitude for your assistance, it is much appreciated.

After following your advice concerning proper compression testing technique, I arrived at the following set of values, which represents both good and bad news. The compression numbers are better, but that leaves me more in the dark as to the nature of the problem. The compression numbers are:

dry/wet

1. 150/160
2. 130/150
3. 150/148
4. 150/155

I took a look at the distributor and discovered inside that there is what appears to be a thin, paper like gasket which is under two "wings" (incorrect term I'm sure!). This gasket had torn and was in several pieces. I removed the remaining bits, cleaned out the inside, applied a little WD-40...the wings open and close with some pressure applied, but the springs attached to each one are of no use in pulling them together. I am not certain if this is by design or indicative of a problem.

I have attached photos of inside of Distributor and remains of the gasket.
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Guy Croft
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Re: Timing and Compression, Are Problems Related?

Post by Guy Croft » August 1st, 2011, 9:50 am

Good work.

Definitely doesn't look like a core engine problem with those compression readings, little variance or evidence of unserviceable rings, so I wouldn't go pulling the head.

Likely ignition to blame here.

G

TomLouwrier
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Re: Timing and Compression, Are Problems Related?

Post by TomLouwrier » August 1st, 2011, 10:18 am

hi

That gasket is not paper but phenolic resin on a cotton base. An early form of composite plastic, just like GRP. Very good for electric and thermal insulation.
You should replace it of course, it has a function.

As you notice that the springs don't return the counterweights ('wings') by themselves, the next step is to remove that circlip at the end of the shaft, pull the advancing mechanism and fix the sticking. Clean, oil, take off any burrs, etc. They should move freely and smoothly.
Sticking weights will give you very erratic ignition timing with running problems as you described (and as Whizzman indicated this in the first reply).

Also, the wire to your points has too much copper core exposed for my liking. You should correct that too. (left hand lower corner of the pictures; between the black insulation and the red terminal sleeve)

regards
Tom
GC_29

Guy Croft
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Re: Timing and Compression, Are Problems Related?

Post by Guy Croft » August 1st, 2011, 12:02 pm

I myself would do no more than see that the weights can fly out (see pic) and are then pulled back by the springs. The springs are always a loose fit so don't assume the thing is faulty because they are a loose fit. I would do that check and give the weights a tiny bit of oil and leave it at that.

Of course you may find that when trying to twist the 4-lobe rotor that it's tight - which might point to an advance problem.

It is important to have matching cap and rotor, not easy. And new points and condenser will certainly rule out any switching problems. There are numerous good suppliers for these things Stateside among them - Autoricambi, Bayless, Allinsons - all friends here. There are few things that can go wrong (and you do seem to have an ign problem) but ignition fault-finding is by no means easy and I did it professionally with Crypton equipment some years ago, a diagnostic system with oscilloscope system will pick up 90% of faults without having to 'shop & swap'. The unstable advance curve does kindof point to a worn or stuck distributor though.

G
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KKemerait
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Re: Timing and Compression, Are Problems Related?

Post by KKemerait » August 1st, 2011, 5:46 pm

Guy,

Thanks for the help. I took apart the distributor, cleaned out the excess gasket pieces, cleaned up the weights and springs so that they move very freely now. Unfortunately I found yet another free floating item in the distributor along with the gasket pieces. This one worries me more, you can see in the photo there was a plastic spacer that is broken, but more importantly there was a tiny ball bearing floating around in there. I have no idea from whence it came or where it belongs. Secondly the circ-clip flew off and into the grass... reminded me I need to cut the grass more often...In any case, I reassembled the distributor with the parts I had left :) .... the car runs, I'll put a strobe on it later and see what improvements, if any, are noticeable.


Tom,

I see what you mean about the exposed copper...good catch. As you can see, problem fixed. Thanks!

Here are the photos...again, any suggestions or guidance is very welcome.
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Guy Croft
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Re: Timing and Compression, Are Problems Related?

Post by Guy Croft » August 2nd, 2011, 10:45 am

After so much fun it would be a shame to junk it - but you might have to - you need that circlip as it holds the lobed bit in place on the shaft. As for the errant ball-bearing it doesn't say much for the bearing pack and stability of the advance curve does it?!

G

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