Hot Rods Exhaust Valve sticking ?

I have a '69 307 block (stock with a mild cam) in the T-bucket and every so often I get a slight "popping" noise in the drivers side exhaust ONLY. It sounds like one cylinder and goes away under load. It only happens while cruising and lifting my foot off the throttle. I checked the valve springs and stems, even peeked into the exhaust ports with the headers off, all look ok..Checked all the sparkplugs and they look good and no differences between them. .Maybe a bad spark plug wire??? I thought I had a vacuum leak on the intake or maybe a bad exhause gasket...changed both with no results. I'd like to get this resolved but have run out of ideas. Any thoughts?

 

Pull the plugs, they're the quickest way to diagnose an issue.
I had a similar experience, it was a loose plug wire. Worked fine while cruising but at wot it would backfire, I though it was the power valves and chased my tail lol. A compression test is another sure fire way to find issues.

 

I'm dealing with almost the same problem in my daily but a lot worse... I'm pretty sure my problem is traced back to the longblock since i replaced my carb and 1/2 of my mallory unilite(i might break in the old YC points mallory this week since it is staged for going in there) Anyway i'm interested, even tho i'm 1/2 way done with my new 10.5 to 1 327 for it.

 

It runs strong at WOT...pops at idle/light load ONLY...

 

Is it really rich at idle?

 

What ignition are you running and what base timing?

 

Are you running the little mufflers in the side pipes? Before you installed them, did you see if they installed them correctly? Mine were shipped reversed, and they popped and crackled until I fixed them.

 

If it's easy to create the popping condition, how about running it with one plug at a time disconnected on that side of the engine. If it doesn't pop, with say cylinder #3 not firing, it might narrow your search down to that one cylinder. Maybe something mechanical or ignition related on that one cylinder. If it still pops with any given plug disabled I'd lean more toward a fuel or ignition timing problem.

 

I'd tend to believe a fuel or ignition issue would show up as random popping....this seems to be isolated to one cylinder. The plugs look good, tan and clean...not running rich. I'm leaning toward a bent valve now...so hard to tell with the heads on. Gonna remove the keepers and springs on that side and spin the valve stems by hand and see what they look/feel like. I think one exhaust valve is sticking open on low load.

 

Exhaust leaks can do that too.

 

If an exhaust valve hung open and didn't close completely, the next thing that happens in that cylinder is that the intake opens. Then intake, compression, fire, power stroke.
You'd get no popping, you'd have a miss.

 

ok, I checked EVERTHING ...even moved some plug wires around side to side to try to see it it was a bad wire....no change. pulled the plugs and they all look the same. checked lash on the valves and rechecked the springs as well as the valve stems. all valves seem to be free in the bore and appear to be undamaged. Put it all back together and it STILL pops with my foot off the gas while rolling 30-40 mph. Not all the time but every couple minutes...got all new exhaust header gaskets, no leaks there. could a pin-hole in a weld on the header be an issue??? Hmmmm. Maybe tomorrow I'll try a new distributor cap.

 

Lean or exhaust leak. Yeah a pin hole will do it if it's close enough to the head

 

You didn't mention your intake setup, but if a manifold plenum or runners are too big
for engine at low RPMs the fuel comes out of atomization and plates the floor and runners.
This then becomes a random part of the intake charge and will be swept out under wot. Single plane manifolds, blocked heat risers and poor vacuum would be suspect especially on a stock motor. The symptom shows up under light load.

 

That's a good point, 307 does a lot better with smaller intake runners to keep the velocity up and hold the fuel in atomization. Due to some weird casting irregularity it could affect one cylinder more that the others.

Is this a new problem, as in it was perfectly fine before ?
The next question would be before what ?

 

The motor has a Holley Strip Dominator intake with a 650cfm carb, vac. secondaries and electric choke. I'm running a dual-point distributor and timing has never been an issue. the "pop" has been in the motor for as long as i care to remember....every summer I try different stuff to find the problem....and once again it seems to be just ONE cylinder....or so it sounds...someday I'm just gonna pull the heads and have a fresh grind done and put in new valves...I guess there are no easy answers. For now I will be looking a little closer at the header tubes and flange welds.

 

What is your initial and total timing advance? Are you running vacuum advance? If so, where is it hooked up?

Lean fuel mixtures are hard to ignite, and take longer to burn. If you don't have enough initial, or total advance, the mixture can still be burning when the exhaust valve opens.

If you are already on the hairy edge of too lean, this can be exacerbated by uneven fuel distribution, due to insufficient manifold plenum/runner velocity, especially on open plenum manifolds, when operated at part-throttle, at low RPM.

Pull all of the plugs, and post a picture of what the tips look like.

 

Oh, and which Holley Strip Dominator is it?

This:

Or this:?

 

Holly strip dominator -
Open plenum Has and Rpm range of 4500 - 7600 based on a 350 engine

The holly strip dominator
Dual plain has rpm range of off idle - 6000 also based on a 350 engine.

If yours is the open style - you have your problem for sure.

If you really are planning on pulling the heads off that 307, get some HO 305 heads or from a van or truck. ( basically the same higher compression heads) use a stock steel head gasket and something like 262 cam. Bigger if you spend a most of time at high rpm. Small/conservative intake runners (a stock 305 aluminum with a spacer works really well) and small 1-1/2" headers. You'll be really fucking happy and it won't cost much.

 

^^THIS^^

 

I think the carburetor is too big, too. For that, I would run a 500, a 400, or even a 390.

Carburetor calculators always see, to want to ask you what your maximum operating PRM is, as if, on a street vehicle, you operate there a lot.

From my observations, on a properly setup street vehicle, the average operating RPM is just south of 50% of the maximum. Plug that RPM number into the carburetor calculator and see what it says.

At 83% V.E. @ 5500 rpm, your 307 would need 405.5 cfm.

At 83% V.E. 2750 rpm, your 307 would need 202.75 cfm.

At 83% V.E. @ an average cruising rpm of 2100 rpm, your 307 would need 154.83 cfm.

From that, manifold aside, you are not even properly utilizing the front half of that 650.

The air velocity through the venturis will be low, and this may hinder proper fuel metering.

The manifold situation, if you have the open one, could just make it worse.

Who tuned it, and did they use an exhaust gas analyzer, or wide-band oxygen sensor?

 

Ok...you guys need to understand that the "popping" I'm hearing is isolated to JUST ONE CYLINDER....sorry if I seem ungrateful for your help, but this motor has been running in this car without any change in major components SINCE 1972. yes, it's an open plenum intake, yes the carb may be a little oversized, but it runs well with the exception of this one small issue.
It does idle well and has exceptional accelleration, never overheats and the plugs run clean. I always run premium non-ethylol gas and VR-1 oil. This motor has been very good to me.
All of the advice given above is great for putting a new motor together and I understand how the focus can be on fuel and ignition, but my problem is JUST ONE CYLINDER.
Thank you for all the help, sometimes I'm best keeping my mouth shut. I'll go pull the heads and perhaps we can all get an education on valve stem wear....

 

I agree. Stuck valves tend to defeat compression and result in misses. Popping is the result of combustion happening in places it's not supposed to.

 

Together and no change since 72 - check

Popping for longer than you care to remember. - check

One cylinder - check

307 runs well with 4500 - 7600 rpm manifold and too big a carb - check but I don't understand and have reached the end of my knowledge base. So with an intent to help you and strech the things I know I went looking. I found this - maybe it explains some stuff. Keep I'm mind I do realize that its random and only one cylinder.



------------- https://www.hmfracing.com/forums/showthread.php?t=3927

Deceleration Backfire is caused by fuel burning in the exhaust manifold or header.
No ifs ands or buts, that’s what causes it. But the bigger question is how does gas get there in the first place, and that’s a bit more complicated. Generally, there are a variety of ways it gets there, and a variety of things that can make the backfiring worse. But there’s a kicker, and something you should understand before we go any farther:
A motor in perfect tune will exhibit deceleration backfiring.
Therefore, just because your motor is banging it up, doesn’t mean there’s anything wrong. And consequently:
Getting rid of the noise means de-tuning your motor.
Yup. If you’ve just got to eliminate that popping, you’ll have to accept the fact that your motor is going to be forced to run rich to do it, and that isn’t necessarily a good thing. So lets talk about what causes the problem.
Ok, so you’re riding along at some given rpm, and suddenly you decide to decelerate, and you reduce the amount of throttle. This causes an “overrun” – that is, the motors rpm is faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen.
First, when you close the throttle, you are also closing the throttle plates. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. Remember I said earlier, that the A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.
1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.
So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire. In addition, Honda has added a device called a “programmed air injection valve” (Pair Valve) that actually injects some fresh air into the exhaust to help this process along – since fully burning the fuel results in cleaner exhaust. So the backfiring is not only a normal part of the engines operation, it’s also intentionally amplified by Honda! Of course, normally, that massive bazooka pipe Honda hangs on your bike hides most of the noise, but it’s there, even when you can’t hear it.
So the bottom line, is: That backfiring is perfectly normal and expected. If you’ve just got get rid of it, that’s up to you. You’re entitled to set your motor up the way you want, and your goals are your goals. But don’t refer to it as “fixing” the popping. Rather, the correct way to think of it is “de-tuning a bit to get rid of the popping”.

 

It sounds like you've got an open plenum intake with no exhaust crossover heat so I could see the possibility of fuel dropping out of suspension and puddling on the floor of the intake.

Just for my own curiosity I'd try adjusting the idle mixture lean on one side of the carb and rich on the other side. Reverse the rich and lean sides and see if the popping might move to the opposite side of the engine. If the popping exists during closed throttle coast-down it should only be getting fuel thru the idle circuits.

 

What dist is in it?

 

I read "pop has been in the motor for as long as i care to remember....every summer I try different stuff to find the problem....and once again it seems to be just ONE cylinder....or so it sounds" I didn't read that you have determined that the pop is consistently one exact cylinder, However even if always one particular cylinder, this is consistent with plating and puddling given factors like engine angle (even if slight) and the likely port mismatch with that combination....

 

31Vicky...THANK YOU, that explains alot. I guess I just never saw this as a "backfire"...it's so slight and brief. Yes, ClayMart, I have played with the A/F idle settings to see if it made an impact...it's always the drivers side. I'll live with it...I guess things could be worse. sdluck..it's a mallory dual-point, no vac advance, all mech. has a mallory super-coil. Running NGK plugs.

 

- I think I'd dig out the dwell meter and look for steady dwell at all rpms.
- Then hook a timing light to each cylinder in turn and watching for steady timing at idle and higher rpms.
- Also watch a vacuum gauge when idling comparing the results to old school diagnostic charts like this one, making some allowance for how peppy the cam is.
http://www.mtfca.com/discus/messages/331880/389883.jpg

FWIW I think a valve "sticking" in a nice cyclic fashion is less likely than a REALLY worn valve guide messing up the idle mix locally and a somewhat randomly.

Even with a clean "chop" I figure any used plug I look at is reporting on the average mixture condition over the last several minutes or even hours, except when way too much choke can soot them up pretty quick.

 

Can you swap out the dist and test this.