My 72 chevelle (307cu. in.) engine will not start. When I turn the key, the engine trys to start, but it just won't fire up. Here is what I know:
1. The starter is good, it cranks the engine strongly.
2. I have spark to all the cylinders.
3. The engine is generally timed correctly.
4. The #2 and #4 cylinders do not have any compression.
5. When I crank the engine smoke blows back out thru the carb.
6. If I crank the engine long enough it will backfire.

If the timing of the engine is off a small amount, will the engine start?

To check the time, I marked the location of the #1 cylinder spark plug wire from the distributer cap on the block and checked the location of the rotor with the #1 cylinder at T.D.C. The rotor appears to point toward the mark on the block but may be slightly off of the mark...How important is it that the rotor points Exactly at the mark?

If I have smoke blowing back out of the carb, does that mean that my intake valve is stuck open? I took the valve covers off and the rocker arms and valve springs are moving up and down on the cylinders that have no compression.

What is causing my engine not to start??

If you remove the distributor cap and have some one bump the starter does the rotor turn? If not or if it does not turn smoothly the timing chain or gear has broke. If this is a high mileage engine that has not been rebuilt it probably has the fiber gear on the crank. They eventually just wear out and strip teeth off. This is my first guess. It can cause all the symptoms you described.

Ditto on Shep 77's reply
Sounds like either gear spun
or the mentioned 2 cylinders with no compression dropped their valves or broke the cam

Is it possible with a broken chain the cylinders with no compression are on the cam lobe and staying open?

shep_77,
What you discribed actually happend a few days before the car would not start. I had to have the cam gear replaced because all the teeth were stripped off. This problem is subsequent to that.

I do not have a broken chain, cam or stripped gears. My rotor turns when I turn the crank.

What else could it be?
What does "dropping valves" mean and how can I check this w/o taking the heads off???

If you check compression on all cylinders and do not have any on 2 and 4, then I would pull valve covers to check the rockers, etc... If everything looks ok, then off come the heads.

Good luck.

Are you 180* out on the distributor? You said TDC, but are you at TDC on the compression stroke?

When I pulled the timing chain cover, I rotated the crank until the dot on the cam gear and the dot on the crank gear were in-line with each other. Then I checked to see if the #6 cylinder was at TDC on the compression stroke. (it was) then I checked to see if the rotor was lined up with the #6 plug wire on the distributer cap. (it was). All these thing suggest to me that the car was in time.

I took the valve covers off. I visually checked the #2 and #4 cylinder rocker arms, pushrods and valve springs while rotating the crank and all seem to be in working order.

I don't understand how if my valves are moving up and down why I do not have compression. Can the valves only be closing part way...If I tighten the rocker arms down some (they were somewhat loose) could that help the cylinders get compression? Also if my timing is off by a small fraction, could that be the problem?

What do you mean by tightening the rockers down? Did you set the valve lash correctly and in sequence, or did you just turn the rocker arm nuts?

When I pulled the timing chain cover, I rotated the crank until the dot on the cam gear and the dot on the crank gear were in-line with each other. Then I checked to see if the #6 cylinder was at TDC on the compression stroke


May be a silly question, or it may be relevant

You say the dots were lined up - but was the dot on the bottom, or up top?

I remember back in my drunken days - Putting a timing set in 180 out, motor backfired & had no compression in 2 cyclinders, but I did get it to run.
Back in your first post, you didn't mention you had worked on timing gears already, or I would've suggested 180 out, right in the beginning.

Checking for bad/dropped valves answer:

On questioned cyclinder make sure rockers are in up position, take plug out, and shoot some compressed air in cylinder - if you hear air escaping through either intake or exhaust - you got valve issues. If the air tries rotating the engine, valves are good. Hear air blowing into oil pan, you got ring issues. Easier than pulling heads off too check into it.

Zeke,
I have not tightned any of the rocker arm nut yet, but if that could help, I will. I have heard you tighten them down until you hear the pushrods knock, then you back them off until the knock is gone...is this an acceptable way to do it?

What is valve lash, and how do you set it correctly?

gwizit,
Sorry I didn't mention that I had the gears worked on in my previous posts. The dot on the cam gear was at the bottom and the dot on the crank gear was at the top. At that point the #6 cylinder was at TDC on the compression stroke and the rotor was pointing to the #6 plug wire on the distributer cap. All these things indicate to me that the car is in-time...correct?

Great idea with the compressed air...I will absolutely try that.

In theory it sounds correct; however, I have never tried setting gears up using #6 cylinder.

So my questions would be:

When you rotate the motor too #1 TDC, where does the rotor point?
(should be just left of the carb. – not towards the firewall, where #6 is)

If your still running a point distributor, are the points set correctly – are they burnt?
(semi-closed points will still give spark, yet cause engine not to run & backfire)

Have you already messed with adjusting valves?
(If you adjust them too tight, you will have valves not seating properly)

Side Note: Your reply to Zeke is backwards

On hydraulic Lifters: Loosen the adjusters until lifters clatter, (on older non-performance engine) I would only re-tighten maybe a 1/4 turn or so after lifter clatter is eliminated.

But first you have to get the motor running before worrying about proper adjustments, loosen them up a bit if valves are not seating, so it can run.

These are just thoughts of mine, hard too say what’s going on – not seeing for myself.

If you haven't adjusted your rockers, then don't. Let's assume they are fine for now.

The easiest way to find TDC on compression stroke: Turn the motor over by hand with a wrench. Remove the #1 spark plug and get a buddy to put his thumb across the hole. As the line on the balancer comes around to your timing tab, if the pressure in the cylinder will blow against his thumb if you are on the compression stroke. Line up your marks on the timing tab and drop in the distributor.

Zeke,

Sorry I didn't mention that I had the gears worked on in my previous posts. The dot on the cam gear was at the bottom and the dot on the crank gear was at the top. At that point the #6 cylinder was at TDC on the compression stroke and the rotor was pointing to the #6 plug wire on the distributer cap. All these things indicate to me that the car is in-time...correct?



I didn't see your other post.

To replace the timing chain and gears is wasn't necessary to remove the distributor nor would it be necessary to change valve adjustment. Did you?

#1 and #6 cylinders are at TDC at the same time.

#1 is at TDC on the compression stroke when both cam and crank gear dots are at 12 o'clock.

#6 cylinder is at TDC on the compression stroke when the crank gear dot is at 12 o'clock and the cam gear dot is at 6 o'clock.

(align dots, not crank key way.)

Questions--- What necessitated the timing chain set change?

Was it worn and jumped teeth? If so, its possible that when the chain jumped teeth that valves were bent.

Preventive maintenance--- Possibly dots aren't aligned up correctly.

Was the crank rotated with the timing chain set removed? Bent valves.

Hey Chris, just found your other thread & read through it (Loud tick in engine/cylinders not firing.) - sounds like you have been having fun.

Big question: If you had a shop replace the timing set, why did they let the vehicle go without getting it running?

Still raises the question of whether it's 180 off, I don't care if a shop did it or not, maybe they didn't wanna eat the cost of re-doing it.

If you were running higher RPM's when it let go, probably bent a bunch of pushrods, or smacked a few valves.

and YEP, a $15 valve can wipe out a whole lotta stuff when it goes.

Sounds like its time to swap the tired 307 out for something else.

The whole story would've been nice to know, so I wouldn't have been scritchin my head in the wrong direction.

you might have the timing off 180 degrees
did anyone think of that you may have top dead
center but not on the right stroke

Was the engine at speed when the teflon sripped of the gears and jumped? Would not be a bad idea to pull the pan and clean the teflon out.

If it were me I'd pull it apart and make sure the valve timing is correct. It should not take that long just resealing the front cover at the pan is a pain.

Could it be that your spark plug wires are crossed? Just wondering since you've been working on the distributor. If you've had them off, check to see if you have them on the right plug. Smoke coming from the carb indicates that a cylinder is firing with an intake valve open, thus those gasses have no where to go but thru the carb opening.