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Engine Compression/ earthing


JamesR

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I've been testing the compression of my engine and all the cylinders appear to read around the 80 psi mark. Hmm.
Firstly the engine was cold. Is that a problem?
I had the throttle fully open.
The pointed on the dial moved up to its maximum in about 4-5 increments.
I tried oil in 1 and 4 and there was a slight improvement, but I'm not convinced about if it was enough to indicate worn rings.
First question is there any advice on what I've done so far?
Second question concerns doing a leak down test. I do have an air compressor, but not a leak down guage, there maybe some guages about, I doubt it though.
As basically the wonderful thing about doing a leakdown test is that you have to feel where air is leaking out of is it really necessary to use a guage? Secondly, what PSi should go into the engine (the compressor has its own guage) and is there a maximum volume? Where should the air go and is there a limit b4 you actually damage it.

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Compresson testers are not precision instruments and are not calibrated.   You cannot compare one against another, so that one's 150psi may be another's 80psi. Far, far more important is the range across the block.   You have an even range, and no significant change with the oil test (Sig= >10-15psi), so I'd say, no probs.  Check with oil in all four to be sure, then move on.
John

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Hmm, ok.
I reckon I'll test in on another car!

Apropos the leakdown test, has anyone made something up for it. I'm wondering about using something like this:
http://cgi.ebay.co.uk/TYRE-INFLATOR-WITH-GAUGE-BLUESPOT-PRO-SERIES-INFLATER_W0QQitemZ160199545908QQihZ006QQcategoryZ30917QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
Connected to the sir compressor and fit an adapter to the spark plug holder.

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Hi James
i did a mates a couple of weeks ago, and got 170psi on no. 1 and the 145 on 2,3,4
Did you take all the plugs out and get the starter to turn the engine over fast if not that might be why because as my mates battery was duff it slow ran down and dropped the readings, but it only did this becuase they were knackered valves anyway !
I agree with JohnD it sounds more like the gauge if off unless your reason for testing was very poor performance or lots of smoke.
Try it on another car seems like a good idea.
Laurence

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I've got a horrible problem with the car lurching about when you drive it. To be honest I was working on the principle that my plugs kept getting wet and the car couldn't be started so I decided to test it whilst charging the battery.
The battery was fully charged, I bought the guage after decoking the engine. I'm just not really sure what the problem is.

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sounds like the only way to try to check the gauge is test a good one, however juerking engine sounds more like timing than compresssion, do you have a problem with your coil i have heard of them braking down when getting hot ? or may just a bad earth on the engine has been a fault i have found in the past .
Any body out there any other ideas?

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Hmm. Interesting because I did get a big spark off the earth braid from the engine to the negative of the battery, but this is where it was touching the brake pipe.
I'm having a though of alterting the dashpot springs and oil.

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JamesR wrote:
Hmm. Interesting because I did get a big spark off the earth braid from the engine to the negative of the battery, but this is where it was touching the brake pipe.
I'm having a though of alterting the dashpot springs and oil.


You need to check the connections on this earth strap as any difference in potential sufficient to generate a spark to another earthed object (the brake pipe should certainly be at earth potential) indicates a VERY poor connection!

Nick

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Ok. Since the spark occurred I replace the negative connection to the battery. I'm not sure that its a good enough connection or whether the braid should go all the way to the battery unbroken. You can see what I have done: (pic1)

Since then I have blown a fuse which goes to the ignition switch, so I'm not sure what state the coil is in now. However some of my other wiring is wrong...
But is this an adequate earthing for the coil. I assme the bracket earths it. (pic2)

Thirdly is the engine earth connection. This should be pretty normal I would think.
What I want to know is how sufficient are these connections, but I know everyone is gonna verge on safety, so what would be the best way to replace the earth braid, or what connections should it use? I'm happy with with stuff from vehicle wiring products as I'm gonna order stuff off there anyway. Oh and what thickness braid should I use?

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That's exactly what mine did do and now I have replaced the section between the battery box upright and the battery. So there really shouldn't be any problems then with the way its earthed?
That's a theorectical question as trying to work out what size the terminal on the bellhousing is, it appeared to be slightly loose. Likely to be the cause of my problems.

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Some time back was working on the carbs of my Spit and ran out of daylight.
Tried starting the engine using the button on the solenoid and was surprised to see a fireworks display throughout the length of the braided earth strap.
Apparently lots of the individual strands in the braid had fatigued and fractured.
The car was only about 18 yrs old at the time, the strap looked OK and the starter worked satis.
Ran a cable from battery to frame and one from there to bellhousing and the starter turned the engine over more quickly.
I wonder how many of the "starter problems" mentioned on this forum could be caused by similar strap problems?

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Yeah, I'm kind of still waiting to hear whether cutting the braid and replacing the connection to the battery is a good or bad idea. Similarly I had a google and its really difficult to tell what amp braid to replace the one I've got with.

I've put in my order to vehicle wiring products now not including a new braid, the spark was on the brake line, so I even wonder how much of a problem the weak connection to the bell housing is.

The other problem I had was that I connected an electronic rev counter to the negative of the coil when the fuse went. That has 4 wires, one to the coil, one 12v which I connected to the voltage regulator and two negatives, both of which are on the dash instrument earths. I did wonder if the negaitive from the -ve coil should be earthed more locally to the coil, but I don't see why. Mine is a MK3 with a mech drive rev counter so the connections are alien to me, but I think its correct.

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Seems like the earth is your problem !
Just remmber that , not just the earth lead has to be good but also the area around the bolt, i suggest cleaning under bolt area with fine emery until it shines then applying so vasaline ( not grease to inhibit corrossion / oxidisation) then retightening the bolts,  from your photos this looks like it was done a long time ago so i would do that too as well as replacing the brades.
regards Laurence

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Bought mine two months ago from rimmers,  good fit, well made, but not cheap ?
Just so happens to be to bloody convienent for me so i end up getting loads of stuff from there. cant remmber the price but less that 15 quid or i would not have bought it.
laurence

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hmm
Rimmers. Negative -  All models - (1) - UKC1361 �22.50

Originally i thought it was this though:..
Engine Earth Lead to Body - (1) - 511485, but the price was not available.
James Paddock has the same item no. for 8.50, I've sent them an email about it.

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