• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

No To Low Compression

Share this page

jhorton3

jhorton3

Jedi Warrior
Country flag
Offline
I'm trying to start this engine for the first time since being rebuilt two years ago, but I can't get it to fire once. I've got a fresh battery installed, the starter turns the motor over OK. The fuel pump is working. I pulled the fuel hose going from the first carb to the 2nd and fuel squirted out. New spark plugs, rotor, cap, and wires. I pulled the spark plug leads to test for spark and spark looks to be OK. I've got oil pressure registering on the gauge.

I knew before I got on here that I'd probably be asked about compression readings, so I pulled out my compression gauge. I screwed it into the first cylinder, turn the engine over, no compression. Tried #4 and the needle on the gauge barely registers. Stuck my thumb over the hole for #1 and there wasn't enough pressure to [censored] past. Number 4 gives a little more. I pulled the rocker cover off to see if anything was sticking, and as far as I can tell the springs and valves are moving up and down as they should. I made an attempt to adjust the valves, but the holding nut on most is on so tight I can't budge it. Busted my hand trying.

What would cause the engine to have such low compression after the rebuild? I'm assuming that until I can get the compression up much higher I'm not going to see this engine run.
 
It most probably is either (A) the rings have stuck themselves into the pistons, or (B) the valves have corrosion built up on the faces after sitting for two years.
If you really MUST let an engine sit for a period of time after a build, it's best to back off all the valve adjusters to allow all of the valves to seat fully for storage, as well as liberally oil the cylinders.
I prefer to do a dry build to verify clearances, etc, and then tear it down and leave the engine disassembled until I'm ready for it.
Jeff
 
Maybe the cam timing is off?
 
Bugeye58 said:
It most probably is either (A) the rings have stuck themselves into the pistons, or (B) the valves have corrosion built up on the faces after sitting for two years.
Jeff
Click to expand...

That's my guess too.
 
You absolutely *must* adjust your valves properly....otherwise, a compression test is of little value.
And remember, they are better to be a little too loose rather than too tight.

Also: I'd pour a little light oil into the spark plug holes. Let it sit for a while and then crank the engine a bit with the plugs out. If your rings are stuck, this *might* help. (put a towel over the plugs holes to prevent oil from spraying everywhere)

After that, it's time to check the cam timing.

If cam timing is OK, then you may have to look at the condition of the valves and rings.
 
I tried adjusting the valves again, but the locking nuts around the adjusting screws were too tight. Rather than risking rounding these off, and that nothing else was working, I took the cylinder head off. Those nuts still wouldn't budge. I had to take the air ratchet to them to get them loose!

With the idea the valves were stuck open I flipped the head over. The larger valves all sit flush, but the smaller valves sit just a tiny bit proud. When I had the head upside down and level I decided to put the spark plugs back in and pour oil in the combustion chamber to see if it would leak down. That's been an hour ago, and so far all the oil is still in the chambers.

While I've got the head off is there anything else I can check while I'm in this far? The block was bored .020 using AE pistons and rings.
 
Were the valves adjusted after the rebuild? If the rebuild included a valve job and/or the block or head were surfaced and the rocker assembly bolted on without resetting the valve clearances, good chance you have zero or worse clearance and some valves are being held slightly open. This would explain the lack of compression.

Marv James
 
Valves were set I with a .012 feeler gauge after the block and cylinder head were decked. I just backed off all the adjusting screws on the rocker as far as they would go. I'm almost ready to put it back together. What clearances should I use this time so that they're a little loose and I've got some room to play with?
 
I just put the head back on the car, torqued all the nuts to spec, and performed compression checks on all four cylinders...180 compression across all four! The adjusters were so far out they were keeping the valves from closing all the way.

Now, I can start see if the timing is closer to correct, put the intake/exhaust manifolds back on, and see if she starts.
 
I would set them 0.014" (cold) to be sure you have adequate clearance.
If they are a little noisy it won't hurt anything and you can always reset them to 0.012" later. It's a good idea to check them after a few hundred miles anyway.

Remember to use the "adds up to nine" rule.
Adjust #1 valve when #8 valve is fully open, adjust #2 valve when #7 valve is open, etc.
 
If you set them to .012, that blows my theory. .012 cold is what my book says. That should be a good place to start. The leak test you did suggests tha the valves are closing and you don't have a gross valve problem. At this point, I would suspect cam timing. You said you had little or no compression on 1 and 4. Did you check 2 and 3?

Marv James
 
I missed your latest post while I was stumblig around. Sounds a lot better. I agree no harm in starting at .014. My late pappy always said "if you can hear them, they aren't burning".

Marv James
 
"Noisey tappets are happy tappets"
 
Taking the head off and getting the valves free was the right thing to do. Much better compression now! I put the exhaust and intake manifolds back on. I've got the static timing (I think) around 10-12 degrees below TDC. Who was it that thought it was a grand idea to place the timing markers on the bottom of the timing chain cover where you can't see them?!!!

I made a couple more attempts to start it up. It tried to catch two or three times. Issue is when that happens the start lets go too soon from the flywheel and sets off in a spin. Battery running low on power, so I decided now is a good time to clean up and head to dinner.
 
Sounds like you so very close.

A fully charged battery just may do the trick now.
 
'k Jim.... we're waitin' for th' "VROOM!!!" post! /bcforum/images/%%GRAEMLIN_URL%%/wink.gif
 
What he said.... /bcforum/images/%%GRAEMLIN_URL%%/iagree.gif /bcforum/images/%%GRAEMLIN_URL%%/grin.gif
 
May the vroom gods be with you../bcforum/images/%%GRAEMLIN_URL%%/bow.gif
 
After many battles today I did get her running, although it's pretty rough. Major problem with timing. Pulled dizzy out and checked it over. Set point gap to .014. Also noticed low tension wire inside is ratty and needs to be replaced. Couldn't get TDC bumping the starter using the key or remote starter. Brought out the floor jack, unbolted engine mounts and was able to attack the crank pulley nut.

Put the dizzy back in, cap, wires, plugs. Tried to crank, motor turned, no fire. I didn't put the low tension lead back on the dizzy. Tried again, still nothing. Look again, and I'd knocked a lead off the coil. Move the dizzy around a bit, turn the key and she fired! It took me by surprise so much I turned it off, then couldn't get it to fire again. Worked the dizzy a little more, and she started and ran for 2 minutes.

Here's the problems I'm facing now. She runs, but only with full choke on. My cable isn't pushing the choke off. I can rev the engine and it smooths out a bit, but when it comes back down to what's suppose to be idle it lopes and dies unless I move the gas pedal. Second problem is the tach is dead as a doornail. Third problem is she's puking oil from somewhere

This is more progress than I've made in 8 months combined. I have the next two days off, and excited about what I can possibly accomplish next.
 
Back
Top