Compression on the SIX, what do you think? Options

[Trekkor]

The low cylinder is the one I'm having trouble with... getting it to fire at idle.

KT

[rick 918-S]

QUOTE (trekkor @ Jan 20 2005, 07:50 PM)

A local mechanic said no more than a 25% differance between best and worst. I'm at 14% (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) KT

14% true, but the method you used to achive the readings are not correct. Check them again to be sure.

[Gint]

BTW: (IMG:http://www.914world.com/bbs2/html/emoticons/agree.gif) with Rick

Try new plugs yet? Take those carbs apart and clean out the sealant?

I don't think the 14% difference in compression is the root cause of that cylinder not firing at idle.

This post has been edited by Gint: Jan 20 2005, 10:05 PM

[iiibdsiil]

It's hard to say what the PSI should be at because I don't know the compression for your motor, or the conversion from a ratio to psi...

25% sounds pretty high though to me... When I checked my motor I was within 9 psi on all 6 cylinders. I can't remember for the life of me what PSI it was, but I am sure it is higher than yours.

25% might be acceptable, I guess. I mean, it all depends on what your tolerances are. Same thing with alignments, the factory has a broad spec, and if you don't care, you take it to the alignment shop and say align it, if you do care, you watch over them, and tell them that you want it closer.

I'm not sure on the story of your motor, haven't been paying attention at all, but if it was a freebie or you didn't spend a whole lot, then run it and enjoy it. If you want it to be closer to perfect, then worry about it.

But, you need to do the compression check right first, all plugs out, WOT. If readings are still varying like that, put a couple drops of oil in the cylinder, and see if that helps. If not, get an adapter and do a leak down test. If you hear air coming out of the exaust, you have an exaust valve that is screwed up, coming out the intake, I bet you can figure that one out. And if you hear it out the oil cap, or however you can hear the crank case on a 911, then it is rings whatever else would allow air into the crank case.

[Martin Baker]

Per Factory Manual...Reference 10.2-1/1

128-156 PSI, Oil temperature at least 60 degrees C/140 degress F, pressure uniformly distributed between cylinders. No mention of percentage of variation between cylinders. I have read up to 20% on other engines as well.

[Martin Baker]

With a 160 reading cold I would guess this engine to have higher than original compression...

[Trekkor]

I have not cleaned up yet... (IMG:http://www.914world.com/bbs2/html/emoticons/slap.gif)

[Martin Baker]

Number I have written previous are for 2.0 liter 914-6, 8.6 to 1 compression. Your engine 901/06, 1967-68 911 (all) 9.0 to 1 compression. May be normal reading for 9.0 to 1 compression...my mistake. I own no 911 manuals at the moment for compression data.

[Trekkor]

Plugs are new Bosch Platinum's.

KT

[Gint]

My .02 - Time to take the carbs down again. Get that sealer out. Dip 'em. And get some NGK's. Check the plug wire on that cylinder. This all sounds so familiar. Except for the sealant on the carbs part. Lawrence just did most of that same song and dance.

[IronHillRestorations]

TREKKOR!!! Are you trying to find trouble? I wouldn't worry about it until you get at least 500 or more miles on the engine. A engine that's sat won't have good compression until it gets "re-broke in". The main thing is how the engine runs.

Are the plugs the fine wire platinums or the $$$ platinums? If they are the fine wire, throw them away and get the Bosch Supers or the $$$ platinums. Fine wire platinums are for INJECTED engines. Your carburatted engine will run too rich and foul the fine wires real quick. There is just a little dot of electrode there on the fine wire plugs.

To check the idle circuit on the suspect cylinder you can back out the idle jet, just a little bit, like a half or 3/4 turn or so. Not the mixture screw, but the actual idle jet carrier. If the engine stumbles a little then that jet is working. If nothing happens then that idle jet is probably clogged.

[Trekkor]

$$$- no doubt about it.
They were about $25...each. (IMG:http://www.914world.com/bbs2/html/emoticons/dry.gif)


thanks for the info, all.

KT

[IronHillRestorations]

QUOTE (trekkor @ Jan 20 2005, 08:48 PM)

$$$- no doubt about it. They were about $25...each. (IMG:http://www.914world.com/bbs2/html/emoticons/dry.gif) thanks for the info, all. KT

Well you won't foul them easily! Those are the real deal. (IMG:http://www.914world.com/bbs2/html/emoticons/aktion035.gif) I won't tell your wife how much you shelled out for spark plugs (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

Try the idle jet trick. It can save you all sorts of hassle.

[newdeal2]

QUOTE (trekkor @ Jan 20 2005, 07:19 PM)

So, a warmed up motor and plugs out will give me improved numbers? KT

Try it HOT...after a good workout. I did and the numbers bumped up [allthough still a range]. I am waiting until the rings have about 500 miles then a retest. From all the info I have been given it's crucial to wait for the rings to seat. I have also heard a cam will throw numbers off due to the longer duration.

Is this freshly built? If so relax and see what happens...if it runs strong and doesn't smoke what's the problem (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif)

[Trekkor]

Not a fresh motor? Mystery motor all the way.
Based on a 2.0 case. Could be *built*.

Pulls strong. Scary fast. (IMG:http://www.914world.com/bbs2/html/emoticons/unsure.gif)

Fires up, warms up, doesn't die, runs cool, I have a few little issues.
It'll work out.

two months 'til a/x season. (IMG:http://www.914world.com/bbs2/html/emoticons/mueba.gif)
( JP, come down here )

KT

[Trekkor]

Good news to report:

Went out for a hot up run. Did a recheck on #5 160#'s!

Much better than 140.

I guess I'll break the carbs down and hunt out the clogger. (IMG:http://www.914world.com/bbs2/html/emoticons/smash.gif)

KT

[ein 6er]

trekkor,

congrats on getting your project running, the sound of a 6 is sweeeeet, isn't it!! sorry you are having problems.

i also have a '67 2.0 mystery motor, 901/05. i'm guessing i only have about 135 hp.

i had the same thing, it would only fire on 5 at idle. i tried new plugs/wires/cap/rotor/points, my compression #s were all even, didn't do a leak down. i thought the idle circuit was plugged, i tore them down about 4 times and soaked them. i replace all the intake gaskets. even after that, the one cylinder wouldn't fire at idle. adjusting the air screws had no affect on idle.

never determined exactly what was causing it, but i had my dizzy rebuilt, (i had excess play in the shaft) and went to the crane optical trigger. i also had the throttle shafts rebushed and readjusted my valves. it's been running perfect since.

check your dizzy shaft for play, and lube it while you are in there. also check your throttle shafts for play.

hope you get it figured out!!

doug

[Trekkor]

Oh, yeah.

I tried a set of the NGK's Gint recommended.
Seems like an improvement. No soot on the plugs after a good run.

The #5 cylinder which is not running at idle does get hot like the rest of them. It is firing off idle.

KT