Compression.

67 1/2 1600 · Post by **67 1/2 1600** » Sat Jun 03, 2006 7:44 pm

Whats the average compression on the roadster? I just did mine today and I got 90-100 on all cylinders which i think is awfullllllll. Any ideas?

datsunrides · Post by **datsunrides** » Sat Jun 03, 2006 8:06 pm

That is on the low side. Should be up around 140-150 on an engine in good condition with stock components. First check valve clearance to make sure you have no tight valves. If its still low, plug off ajacent (sp?) cylinders and see if compression goes up. If so, you may have a head, head gasket, or block sealing problem. The final thing you can do is put a little oil in the spark plug hole and see what that does. Compression goes up, rings are bad. It is possible the the compression reading is normal IF engine mod have been done such as dished pistons or a big lopey cam with a bunch of overlap. If you have access to a leak down tester, that is the best way to see where the problem may be. Also, slow cranking speeds can affect reading on a gauge.

Mark

67 1/2 1600 · Post by **67 1/2 1600** » Sat Jun 03, 2006 8:18 pm

my starter cranks slow, but i had it on a charger, a nd i still got 90-100. I can figure out my idle, I have fluid in the domes, balance is even, fuel is a tad rich, will backfire if I lean it out. so im assuming my valves are probably too tight? Will check everything again before I jump to conclusions, or toss the 1600 for a SR20 for that matter...

Post by **dbrick** » Sat Jun 03, 2006 11:44 pm

make sure you check it with all 4 plugs out and carbs wide open, pinch off the fuel line first and let it stall before you start so you don't flood it.
this will make it crank faster and give more accurate numbers. You can use jumper cables too to get the voltage up.

67 1/2 1600 · Post by **67 1/2 1600** » Sun Jun 04, 2006 2:52 am

My valves were very tight. After setting them at .10 i got a better reading of a high 100 compression. not quite as id like it to be but then again my starter is slow. The car runs wayyyyyyyy better, now the carbs need a bit of tunning i think? But it seems to run better now.

Post by **Datrock** » Sun Jun 04, 2006 8:39 am

Joaquin, do you mean .010 inststead of .10... If I remember right a 1600 shoud have around .017 clearance at the rockers once the engine is warmed up. Did you check the clearance hot or cold??? Bill

67 1/2 1600 · Post by **67 1/2 1600** » Sun Jun 04, 2006 5:29 pm

.010, and it was very very hot.

eastmedia · Post by **eastmedia** » Sun Jun 04, 2006 7:43 pm

Joaquin,
Did you remove all the spark plugs at the same time when you did the check? I think that makes it crank easier. If you need the carbs set come see me! Timing?
Ron

67 1/2 1600 · Post by **67 1/2 1600** » Sun Jun 04, 2006 7:46 pm

Yeah, I removed all 4 spark plugs of course... but my starter just cranks real slow.

Whens a good day to stop by? Let me know.

twinight · Post by **twinight** » Mon Jun 05, 2006 8:15 pm

valves that are too tight will cause low compression if they are not closing entirely. It could also explain the backfiring.
In the worst cases, the car won't even idle.
I'd re-adjust to .017 and check again.

Post by **dbrick** » Mon Jun 05, 2006 11:03 pm

Have to agree on the valve adjustment, Maybe even go to .020 and recheck.
Was there a sudden change in the way the car ran that prompted you to do a compression test?

Here's the long shot...Little chance of this being the problem but...
If you don't know the history on the motor and it has always run like this, you might want to verify the valve timing, if the timing chain was installed with the cam and crank not on the marks, it will make it run strangley and all the normal checks will come up wrong, including compression and slow cranking.

67 1/2 1600 · Post by **67 1/2 1600** » Tue Jun 06, 2006 1:12 am

I will re check, I just wanted to make sure that was the problem. I had been driving it like that for about 2 years 4-5k miles? I drove it all over the place, they valves were set by my dad at about 600 miles on the engine on the rebuild. I guess he must have been drinking some good stuff and overtightened them. It didnt run that bad, I just had odd idle and backfiring from the carbs/exhaust once in a while. So I said what the heck ill check compression and I was shocked at the results. The Cam is right on the dot all that is fine. I do however need to check the timing, and probably some more carb adjustments.

Post by **dbrick** » Tue Jun 06, 2006 6:14 pm

That's good, it saves alot of time when you KNOW stuff is right. I've had alot of luck using a vacuum gauge when looking for problems. There is a chart in almost every Chilton book, it points out all kinds of stuff.

Silly question, did you break in the engine on synthetic motor oil? That can cause new piston rings not to seat and compression stays low on all cylinders.

67 1/2 1600 · Post by **67 1/2 1600** » Tue Jun 06, 2006 6:22 pm

Yes, I did use synthetic.....

Post by **dbrick** » Tue Jun 06, 2006 8:58 pm

BINGO...I think it's time for some dinosaur juice. I have no direct experience with this, just read about it. possibly someone else knows more?