slacker.cam Posted July 5, 2011 Share Posted July 5, 2011 So I'm looking at buying a car - actually, shit, I've actually bought the car already... It isn't starting at the moment apparently due to a ignition fault. It's a 4AGE on carbs with a link G4 running the ignition. The ECU has only recently been installed and one day the motor decided it didn't want to start anymore and now it pops and farts and refuses to start. So I made a deal with the owner to buy the car as is it seemed reasonable that a ground could have come loose or something like that. A simple electrical fault will be easy for me to fix. The deal was if it compression tested OK then I'd buy it without seeing the car running. So I went and compression tested it over the weekend - cold, as the motor won't start. 2 and 4 measured well down (50psi), 1 and 3 were fine. Should I believe anything that the compression tester told me? Or if the motor is cold is it all just a waste of time? The other thing is the car has been sitting 6 weeks without having been started. Could that provide some insight into the low figures? Anyway, I've bought the car (I'm fucking amping) so it doesn't really matter. I'm just thinking ahead and wondering if I may not have to pull it apart after all. Quote Link to comment Share on other sites More sharing options...
Yowzer Posted July 5, 2011 Share Posted July 5, 2011 Cold compression test should still give consistent results (if everything is all fine and dandy), even if the readings might differ from hot. Drop a spot of oil down the bores and try again. Rings may be shot or something. Quote Link to comment Share on other sites More sharing options...
Yowzer Posted July 5, 2011 Share Posted July 5, 2011 Also also also If the ignition and everything is still working, it should still be able to fire up and run on those numbers without too big a worry. Won't run too flash and will lose a lot of power, but there's no reason it should be coughing and shit. Double check ignition and make sure it's firing when it should be, and perhaps check valves for stickyness / bendyness. Could explain both low compression and coughing stz. Quote Link to comment Share on other sites More sharing options...
slacker.cam Posted July 5, 2011 Author Share Posted July 5, 2011 Yeah that makes sense. It's strange that both cylinders are down to almost exactly the same point. I'm hoping if it is actually a legitimate fault that it won't be anything too major. It went from running consistent lap times one meeting to not starting the next so I don't think anything catastrophic has happened. It didn't die while on the track (so I have been told). Could skipping a tooth (or more) on the cam belt make two cylinders have low compression? Quote Link to comment Share on other sites More sharing options...
Yowzer Posted July 5, 2011 Share Posted July 5, 2011 Would have the same effect on all cylinders.. Initially I was gonna it's popped the gasket between the cylinders, but then noticed that they are not adjacent. Makes things a bit funny.. Quote Link to comment Share on other sites More sharing options...
slacker.cam Posted July 5, 2011 Author Share Posted July 5, 2011 Would have the same effect on all cylinders.. Of course it would. Brain is not running at full speed tonight it would seem. The first job is to get a timing light and see what's actually happening with the ignition. If it's all good then I'll pull the head and go from there. Cheers Quote Link to comment Share on other sites More sharing options...
peteretep Posted July 5, 2011 Share Posted July 5, 2011 if the throttle/s are closed then compression will be way down, maybe foot was on the floor for some of the tests and not others Quote Link to comment Share on other sites More sharing options...
KKtrips Posted July 5, 2011 Share Posted July 5, 2011 I'm pretty sure I've written a tech thing on how to do a compression test. However the way you are doing it will most likely give you inaccurate results and I would not use a cold compression test to sway my purchase $$$. The only diagnostic you could use a cold compression test is to see if 2 cylinders next to each other are down or 1 cylinder has very low compression.. With the results you have, even if they are actually down when tested properly then I doubt its that hard to find a decent 4age bottom end and tidy up the head? I'm assuing the price you paid is most probably worth it for the G4 and car?? edit - found it and I've edited it slightly and reposted it below. Quote Link to comment Share on other sites More sharing options...
slacker.cam Posted July 5, 2011 Author Share Posted July 5, 2011 if the throttle/s are closed then compression will be way down, maybe foot was on the floor for some of the tests and not others Hrmmm. The throttle was completely shut for all the tests Amateur... How low would the compression read on a healthy engine with the throttle closed? As low as 50? Quote Link to comment Share on other sites More sharing options...
KKtrips Posted July 5, 2011 Share Posted July 5, 2011 From Feb last year.. My technique for compression test is this.1 - Get engine up to operating temp 2 - Crack off spark plugs a quarter turn each (to loosen any carbon deposits around the spark plug threads) 3 - Run engine for 30 seconds at around 2-3000rpm (to burn off any carbon deposits from stage 2 so they don't get stuck under a valve) 4 - Remove the plugs 5 - Disconnect the distributor to eliminate the spark 6 - Jam the throttle wide open (have someone sit in the car or put a brick on the gas) and make sure the throttle stays WOT until the entire test is complete (opening and closing the throttle between compression checks causes bore wash and can alter the compression from first to last) 7 - Start on number 1 and thread the compression tester in till it is firmly seated 8 - Crank Engine over so the engine turns over 20 times (10 compression strokes) 9 - Write down the result from the gauge (generally between 100-180psi) for each cylinder 10 - Back to stage 7 but move to next cylinder and continue 11 - Compressions shouldn't alter more than 10% between worst and last (up to 20% at most) Quote Link to comment Share on other sites More sharing options...
slacker.cam Posted July 5, 2011 Author Share Posted July 5, 2011 I'm pretty sure I've written a tech thing on how to do a compression test.With the results you have, even if they are actually down when tested properly then I doubt its that hard to find a decent 4age bottom end and tidy up the head? I'm assuing the price you paid is most probably worth it for the G4 and car?? You'd think I'd never done a compression test before huh? I think it was the arkwardness of doing it on another persons car which made me forget to open the throttle and stuff. I got a pretty sweet deal on the car - yeah. At the end of the day a rebuild is on the cards regardless of the current condition of the motor as it's been in the car since 2004. I'd like to do it after the trackday in August if I can get away with it though. The motor is fairly well built (wiseco pistons, balanced crank, etc) so unfortunately it's not as easy/cheap as just swapping in a healthy bottom end. Quote Link to comment Share on other sites More sharing options...
KKtrips Posted July 5, 2011 Share Posted July 5, 2011 Sorry - I didn't mean to sound condescending in my first post.. I reread it just now and the first line sounds like I am.. oops bitches love smilies? Quote Link to comment Share on other sites More sharing options...
slacker.cam Posted July 5, 2011 Author Share Posted July 5, 2011 Sorry - I didn't mean to sound condescending in my first post..I reread it just now and the first line sounds like I am.. oops bitches love smilies? 'sall good man... Didnt really seem condescending at all. Your point about carbon buildup getting on the valve seats is interesting. It could be another explanation. I'll cross my fingers and hope I just scored a bargain Quote Link to comment Share on other sites More sharing options...
peteretep Posted July 5, 2011 Share Posted July 5, 2011 if the throttle/s are closed then compression will be way down, maybe foot was on the floor for some of the tests and not others Hrmmm. The throttle was completely shut for all the tests Amateur... How low would the compression read on a healthy engine with the throttle closed? As low as 50? yeah I think thats possible Quote Link to comment Share on other sites More sharing options...
KKtrips Posted July 5, 2011 Share Posted July 5, 2011 All GEEE! 50psi is low but could be explainable if it has forged pistons and the engine is stone cold - but TBH I would not be taking much notice of a cold compression test. Something as simple as a low battery voltage can give you a false reading too (cos the engine turns over too slow to make a consistent compression stroke) Quote Link to comment Share on other sites More sharing options...
Unclejake Posted July 5, 2011 Share Posted July 5, 2011 If the car has been sitting for a while the compressions could well be down on two cylinders due to rust on the valve faces. The valves that were open when the engine stopped will be the ones that are rusty. With a single carb it matters little if the throttle is open or closed IMHO, as what you are looking for is an even set of numbers. A multicarb/trottle engine with bad butterfly balance and closed throttles will give you less reliable readings. I think a cold engine is fine to do a compression test on if you can't start it. Anyway: It is too late now as you own it ......... and it will be choice. Congratulations Quote Link to comment Share on other sites More sharing options...
slacker.cam Posted July 5, 2011 Author Share Posted July 5, 2011 Anyway: It is too late now as you own it ......... and it will be choice. Congratulations Cheers. Yep, bugger all I can do about it now. I have 6 weeks to rebuild the motor if need be. Rust on the valve faces also makes sense and could explain why two cylinders are off. Although, it has stainless valves so it would have to be corrosion on the valve seat instead. Still possible I guess. It has twin 45 sideys so the butterfly balance is still a possibility too. Quote Link to comment Share on other sites More sharing options...
Unclejake Posted July 5, 2011 Share Posted July 5, 2011 Out of balance twin sidedraft carbs would not give you the results you saw, unless both of them had bent throttle spindles (very unlikely). Get it home and get it running or borrow a leakdown tester. Then you will know more. given it has a Link on it I am very glad it isn't only one cylinder that is down If you do have to pull it down I hope you do find all that good gear inside the motor Quote Link to comment Share on other sites More sharing options...
KKtrips Posted July 5, 2011 Share Posted July 5, 2011 Sorry Jase - but you are wrong, the throttle IS best to be held open - it shows up the difference between cylinders more clearly as there is more air in the cylinder. ie: 10% of 2000ml is easier to read than 10% of 500ml. Also I don't think that 6 weeks would be long enough for rust to build up any more than the lightest coating of rust. Shouldn't affect compression enough to measure. (Unless its been sitting in salt water) Quote Link to comment Share on other sites More sharing options...
slacker.cam Posted July 5, 2011 Author Share Posted July 5, 2011 Thanks guys. I'll let you know how it goes. Quote Link to comment Share on other sites More sharing options...
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