Thread: SB Chevy backfiring on startup

Tweet

I have had a few problems with my 327 chevy backfiring on startup. This time it blew the chrome rocker covers off, bending the rocker cover bolts and the rocker gaskets flew completely apart! Lots of smelly smoke too.

It has a 600cfm Holley and a edelbrock manifold and headers. I did a compression test recently and it ranges 140 - 150 between cylinders. I have just serviced it with new filters oil and plugs. Oil and water look fine.

The engine was running really sweet when I took her out yesterday??

Please help

Thanks DennyW
It is set to 8 degrees BTDC static. Is that OK ?

However I have noticed that as soon as I connect the vacuum advance pipe to the Mallory electronic distributor, it seems to advance a lot immediately?

I have the vacuum pipe attached to the side of the holley near the idle screw.

Thanks DennyW
Yes I recently changed to a PCV valve as the original aluminium M/T rockers did not have one. The engine runs a lot better with it. so the PCV valve is new.

I don't know about the cam, the engine is really sweet and revs very quickly to 6000rpm? I did not build the engine but compared to other SB chevy's I have had, I guess it must have been balanced or something.

I do just press on the gas pedal a little, I do not pump it. It usually cranks for a few turns (manual fuel pump) and then fires up on a few then all cylinders just fine. sometime is backfires and then the next time it starts fine.

How would gas get in the oil? could it come from the intake manifold?

I'm with Denny, there must be gas within the crankcase. I would check the needle and seats of the carb. If they are leaking, the gas would run through the intake and into the pistons and seep past the rings. That still wouldn't answer as to how it ignites the crankcase mixture. Perhaps this happens through the pipe that comes from the distributor to the pc valve.

I've never heard of some valve covers being blown off like that. As denny and Typhoon says there got to be gas in the crankcase but I would have not idea on what would make it explode like that.

If you have had a severe back fire check your power valve.
If you are getting gas in the oil you may have to much fuel pressure and it may be pushing right by the needle and seat assembly.
I would try and keep it between 6-8 lbs.
Have you checked your compression yet???

I think the gas in the crankcase idea seems right.

I have noticed that the fuel filter has bubbles coming through it occasionally after the engine has been run accompanied by a distinct kind of Phizzing drip noise on the inside the intake manifold but not always. I guess previous backfies did some damage to the carb? I have heard the powervalves can get blown out by backfires?

I can't think how the gas vapour ignited either?

There was a really big bang and lots of smoke. There was so much force the rocker cover holes were enlarged over the cap screws and the covers twisted off a good 2". even the Moroso Wire clamps holes were enlarged over the cap screws. The dip-stick also blew out.

Do you think anything else will have got damaged ? it has a paper oil filter element.

The tube from the PCV valve goes straight to a little "tree" takeoff on the back of the intake manifold behind the carb, the other branch goes to the the brake booster.

I have just recently changed the oil and filter. It was very clean and almost to the full mark now it is a little dirty and on the botom mark. It does not smell of gas. The last time I checked it before now was a week ago during a 350 mile trip and it was fine then.

I was going to replace the carb with an Edlebrock in any case.

Thanks DennyW

where should I put the PCV valve tube? At the moment it is gettng positive vacuum from the manifold?

I don't use the car very much, covering maybe 15 miles a week in the summer and then the occasional long trip. It uses some oil, but the level stayed the same over 120 miles and it doesn't smoke or anything?

Thanks DennyW

I am afraid I have misled you. The Brake booster is not linked to the PCV valve "tree", the other branch goes to the auto transmission I think?

I have attached 2 pictures that I have just taken. The air cleaner is off so the only pipe missing is from the rear engine breather (unique to the 327) to the air cleaner.

Is that gas sitting on your valve covers. If so then maybe your carb is leaking gas on to you intake and being sucked into you lifter valley.

Thanks

The engine has been in 13 years and is a little dirty.

The Manifold is leaking on the middle 2 cylinders and is on the lists of jobs to do (with the carb) this winter. The engine is not leaking any gas externally.

The distributor is electonic without points.

The choke is getting 11.11v, the flap is all the way over and the mark on the housing is set to the central point.

I still would like to know how the gas in the oil possibly got ignited and whether any further damage could have been caused and what to check for before I fire her up again?

Denny Thanks for that info. Learn something every day.

Thanks DennyW

A very interresting article.

As the 327 has the engine breather, should I have a PCV valve as well? I think they were fitted as standard (it is a very early camaro by the way October 66)?

The pipe is just 3/8" but supplied with the valve by my local Motor Factor, What should I replace it with?

I haven't had valve covers blown off before, but this scenario would be possible: No breather cap installed => your crankcase vapors stay in there, just a little bit gets sucked away by the PCV => crankcase full of blow-by vapors, which are often enriched with fuel => vapors ignite with an ever so small spark (steel rockers hitting top of valve stem?)

That is possible, quite apart from the fact, that for example a cylinder which is dead for a short time will really press unburnt mixture in the correct ratio for burning past the rings. So if your engine cycles a couple of times without firing you WILL have the correct mixture for explosions in your crankcase (this is not a "may have" issue!). Rings not sealing properly (you said it uses a bit of oil) will accelerate this fact.

A leaking carb will cause gas to be present on the base of the intake manifold. This will be drawn into the cylinders on the first couple of turns wich won't fire because the mixture is much too rich. But this enrichend mixture gets past your rings even more easily. Rich mixture in cold starts washes down the cylinder walls and causes more friction and more oil to be used.

As you see, there are very many possibilities as to what might be causing this. But blowing off valve covers is certainly something I would look into more deeply: For all you know there might be gas sitting somewhere in your engine in large quantities and could actually cause a considerable fire.

I would certainly install a breather on one valve cover and the PCV on the other, breather on the driver side, PCV on the passenger side (that's because of the rotation of the crankshaft in the crankcase: it acts as a pump and pulls down on the gases on the driver side and pushes them up on the passenger side).
I would also check the carb for leaks internally, maybe just get a gasket set and rebuild it, done in half a day.
I would take off the valve covers and check for faults in the castings. Sometimes valve guides crack without hurting the heads themselves, in which case a spark which goes off too soon or too late can cause the igniting mixture to creep up a worn or cracked valveguide.
I would double recheck my timing all along the power curve und also recheck my valve setting, it's very easy to get one of them wrong and to be holding one valve open just ever so slightly, which would force mixture up into the bowls, from there it can get into the valve covers by way of cracks easily.

As you see, many "I woulds", but what you do is entirely up to you of course. I cannot promise that this hasn't been happening in small bits along the way, when driving you might not notice a small leak somewhere, but your engine parts may. This can certainly hurt other parts, too.

Hope it's of any help,
Max

A small note: gas won't stay in the oil for long, as soon as the oil is warm it is hotter than the boiling point of gasoline, so the gasoline will evaporate out of it and leave clean oil.
And: a faulty spark wire might cause a spark to go directly to the valve cover, which is insulated from the cylinder head by an eightth inch of gasket. The spark can jump that gap inside the valve cover. Combine that with the mixture in your crankcase due to the above mentioned and you're done (The more I think about it, the more likely this version seems to me... Check your wiring)