too high compression?

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pishta

I know I'm right....
Joined
Oct 13, 2004
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Location
Tustin, CA
Not slant but four banger related.
82 Mazda 2.0 knocks on acceleration under med load. I dont remember it doing this but its been OOS for a few years. I had the head resurfaced and they had to take like 30 thou off as it was heavily pitted. in doing that it raised the compression x amount of points. They sell a .020 shim to use when the head is milled (oil port is fixed to block and sticks up about .250 so head has to be a fixed distance off block for this to seat) but I cant find it anymore (found one..https://www.carparts.com/cylinder-head-gasket-spacer-shim/dnj/dnjhs402) . Anyone ever use 2 head gaskets on a 4 motor to get the compression down? I suppose I could cut shim out of .020 copper if that is feasible? The timing is set in stone on these as it wont pass smog if the timing is not spot on in CA. No way to degree the cam....any thoughts? Tighten up the EGR valve (its adjustable/possible) so it doesn't open as quickly? will check compression....8.8:1 .2 over stock. balanced..
 
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130psi x3 and 135psi. well balanced....No head gasket issue or valve issue as I can tell. The EGR does move right off throttle so im wondering if I have it too 'loose'? Engine has new std. rings and pistons but cylinders were not bored: no taper on old cylinders, just a dingle ball glaze break.
8.6:1 is stock, under what my psi to ratio calculation is x(14.696)/1=130psi is closer to 8.8:1 (at sea level) does this sound right? And would a .2 point increase do anything in regards to knocking?
 
I figure that the EGR valve being overly active would actually help reduce the pinging. 130/135 seems fairly low on the compression. Is this an EFI motor?
 
Are you 100% certain the camshaft timing is correct? 100% and not 99%.
 
if it knocks at 130/135psi, on pump gas, in Tustin/elevation 138 ft, something is very very strange.
Have you looked at the sparkplugs?
Have you checked the guts in the Distributor?
 
Check the gas, thats probably why...its bad.

Shouldn't ping at 135.

Disconnect egr and run it.l if it continues after tank is empty and refilled with 89 octane.
 
You never said what you were running for fuel. Try some premium. But it sounds like something else might be off.
 
gas is recent gallon of generic CA 87 E10 stuff as it was empty, carbed motor, cam timing is fixed: no way to degree it, just line up the crank dot to a colored link and put the dot on the cam gear between the other 2 colored links. Cam and crank cog are woodruff keyed, cam was never changed, new chain installed to spec. idles fine. runs fine, just a light knock on a slight bogging of the manual transmission motor. Im going to tighten the EGR pintle (fully adjustable as its threaded) to delay onset as maybe that is issue "Engine performance issues: if the valve is stuck open the vehicle’s air-fuel ratio will be disrupted causing engine performance issues such as reduced power, poor acceleration and rough idle" watching the EGR valve unseat right off idle may be the issue (effectively stuck open opening so soon). It will seat at idle but right off idle it starts to open, thats bad as it should open a little later. I lengthened the pintle so it delays the onset and will try it out. I'd disco it but it needs to get smogged one day and wont pass without a functional EGR valve along with its elevated NOx levels. The balancing act of these early 80's passive smog devices was ridiculous!
 
And just think about how lean the carb is set up to run! I might just go through the carb and check all the adjustments/float level to sooth my mind on that.

But maybe the EGR is suspect too....
 
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Thanks, new carb. clear float glass right on the money. Only thing I altered was the curb idle speed chasing a sluggish choke but I got that straightened out. Now it starts up with one pump and warms up fine. Will advise on new EGR setting. Does anyone know offhand when the EGR is supposed to open, ie what RPM? 2000-2500? Or does it depend on other factors. It is venturi velocity vacuum operated as the sense line is above the butterflys on the carb, then goes into a vacuum amplifier, then through a thermo valve that seems to not be functional as it should be closed when cold and only open when up to temp. Another out of production part Ill have to source. Who said the chase was more satisfying than the victory?
 
Not knowing your car nor your system, here are ,I hope, some helpful tips;

On period Mopars;
EGR was limited to Part Throttle.
no EGR was allowed;when cold, at idle obviously, or when accelerating briskly, and not at WOT.
There are multiple controls to limit EGR to more or less, a cruise only situation.
Since Exhaust gasses are inert, they do no harm in the chamber when the system is operating properly. Their job in there, believe it or not, is to reduce the temperature of combustion, to reduce the amount of Nox created; a stinky harmful to the atmosphere,pollutant . Which is also partly why the Scr is in the basement.
But in my estimation, they also serve some other possibly useful functions.Since they are inert, and they occupy space, and they are sucking up heat in the chamber, and they are homogenizing the mixture;
1) the reduced temperature should reduce the propensity toward detonation., and
2) when the fire lights off, they become a bit of a shock-absorber in there, and
3) they should increase the Effective compression ratio.

Having them in there at Part-Throttle is no big deal, except that the throttle has to be opened a little further, to compensate for the power loss caused by the lowered combustion pressure, caused by the reduced temperature of the expanding gasses.

IIRC, in the beginning, on Mopes, they were controlled by venturi vacuum, and a vacuum amplifier system. This made them very sensitive to throttle position. And of course, throttle position is somewhat dependent on ignition timing, and cylinder pressure.
As the engine gets older the pressure drops, and it takes more and more throttle-opening to accelerate and to maintain a given speed.
Also, if the timing goes away, the power also falls, giving the same results.
And the greater throttle opening, increases the venturi-vacuum signal, which the amplifier dutifully magnified and opened the EGR valve further, reducing the power, and you can see where that's gonna end up.
On Mopes there was a Thermo-vacuum switch in the stat house or the rad, that was calibrated to eliminate EGR below a certain coolant temp. If your engine never came up to temp, you never got EGR.
If your throttle opening at idle is too far,say because of engine load, or retarded ignition timing, or retarded cam-timing, then the throttle may be in danger of activating the venturi-port.
You may not be able to check the cam-timing, but if your engine has a valve cover,you can always check split-overlap.
 
Thanks for that. Believe it or not, I understood everything you stated as I became a student of EGR and am no longer afraid of it nor will it automatically go into the bin when I run across one. I'm pretty certain this particular system is all venturi velocity related as the sense line is high in the carb, probably not affected by manifold vacuum. Ill have to test my theory of 'early opening equals knock' on Sunday when I get some down time. Trying to think: there is an idle, 1500 RPM and 2500 RPM load test on the rollers for CA smog. It surely gets opened at 1500 and should not be open at 2500 load test ( IIRC there is an electric engine speed sense valve that closes the vacuum source to amplifier at higher engine speeds) as they set the throttle and then load the rollers to simulate a pull. They watch to see if the motor goes under a certain RPM when loaded at a constant throttle position. I know this as I failed this one when my fuel pump was crapping out and making the carb go lean. Guy could not figure it out so he just stopped the test, took the hit (can only have a certain number of aborts in a month) and told me to go elsewhere as this truck was not testable. Pretty much if its got a carb, they will tell you to go elswehere, too much work and too high a failure rate.
 
The venturi vacuum port should be pretty high up the bore-wall. It should show zero vacuum at idle, and not start signalling until the throttles are quite far open. The vacuum should steadily increase, then start falling off. It should never reach manifold vacuum. That's why it requires an amplifier which is supposed to follow lock step to the venturi signal, just with more power to operate the EGR valve.
Good luck.
 
Update for you following along at home. I tightened the EGR so it would delay the opening (FSM says 1500-2500RPM should open) and then I started investigating other parts of the system Took the EGR pipe off and cleaned it (wasn't dirty) checked all the gaskets, all good. Pulled the therm valve the EGR runs through and tested it opening. It opens at 120F with a little click, so far so good. But I did learn something that could be relevant: The thermo-valve has 2 nipples and a vent. one nipple is closed at lo temp, that goes to the vacuum amp that meters the vacuum to the EGR through this valve. The other nipple is open to the vent on the bottom when cold, that one goes to the EGR itself. Only when the intake water supply is above 120F does the vacuum source nipple open to the other nipple, close the vent and pass the vacuum. Im almost certain I had these reversed so I was getting a vacuum leak until warm 120F and then the valve would close to opertional status. That would explain the rough cold idle and possible the lean knock, ot that could have been part of the early opening of the EGR. So now it idles really good, replaced the leaking rad cap so it holds pressure now and darn it, I couldn't drive it to test but I believe I'm making progress. Next is road test and dialing in that EGR pintle height. Gotta run it, might as well make it work.
 
Ima thinkin' the vent is there to dump the vacuum that would be trapped in the line to the EGR, if the dump-valve was not there,lol.. If the vacuum got trapped in there, the EGR valve, of course , would not close.On Mopes there might be a lil foam breather on the vent, a lil cube as I recall.

I looked in the catalog once and found a myriad of those Thermo-Vacuum valves. I found several for Mopes spec'd about 10* apart. I was gonna use one to give me automatic advance on my cold 367, to act like a fast idle. (no choke). But with the Dial-back timing delay box, my 367 ended up not needing it. Now I just got to remember to dial the advance back out after the engine warms up!

Good luck on your roadtest.
 
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That EGR has ~10lb spring in it. if it took 5" at around 4 square inches to pull it off (youtube vid) at the low setting. Its tighter now, I got to get another mighty-vac to see where 1500-2000 RPM is on the line from the amplifier to dial in the pintel. Those thermo-valves are interesting....there is second down there for evap, and that one is routed correctly.
 
^^ no smoke at all, recent head rebuilt:rings, guides, seats and oil seals. PCV is new. All this stuff is super cheap on Rock during their closeout emails so I just buy stuff when it shows up in the email. PCV was $2.89..and its all metal! Waiting for that big pulse injection check valve to go on sale. Mine works, seals but I had a rust hole in the bottom that I fixed, and that **** wont unscrew off the tube, twisted the tube slightly trying to unscrew it!
 
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