Still a miss at 4,000 RPM..., Backfiring at WOT, even with all new ignition components Options

[rhodyguy]

leak down test. i think i mentioned this earlier. you should reconsider my "best offer". round trip would be about $32. running it to 6k with a fi cam (and old everything else in the valve train) is pointless unless you just want to make noise.

k

[ThePaintedMan]

So here's what I did today. New manifold to head and manifold to carburetor gaskets. No difference. Tried richer air correction jets. No difference.

Here's video of the carbs (sorry it's so crappy - my GoPro wasn't charged). It's hard to see, or hear but you'll notice that the backfires are coming from the rear cylinders on each bank. This is reaffirming my belief that that lobe on the cam is flat/going flat.


https://www.youtube.com/watch?v=C7NnZ0cHyvs





https://www.youtube.com/watch?v=9OkSH-2iCGs



Kev, I'm still considering a leakdown test tomorrow, if it's not raining like it is today. If it is a flat lobe though, it wouldn't show up on a leakdown, correct? A leakdown can only help diagnose bad rings or valves, correct?

[rhodyguy]

don't know what to tell ya. what the hell are the wires running down the drain funnel for?

[r_towle]

If the camshaft was getting smaller, it would not open the valve...

It would not open the valve all the way.

It would not backfire.


Adjust those intake valve like I told you, and take your time.

Rich

[monkeyboy]

If the camshaft was getting smaller, it would not open the valve...

It would not open the valve all the way.

It would not backfire.


Adjust those intake valve like I told you, and take your time.

Rich

Yes, it would. I don't know why, but my dad's car did exactly the same thing when his cam went flat. It would idle well, but almost any throttle would cause it to fall on it's face and backfire. But that was a V8.

[ThePaintedMan]

Adjusted the valves per Rich's instructions, and everything was still in spec.

I worked until it was dark so I could try one thing. I pulled the plug wires off of first the #3 cylinder and then the #4 cylinder. Neither one made a difference in the idle, so the car essentially seems to be running on two cylinders. I checked both plug wire ends, and both were emitting good spark, so I know the plugs are at least getting spark.

Neither mixture screw is making much of a difference at idle now either. I cleaned both carbs well before they were installed, and I am at least getting a good squirt out of the accelerator pump jets, so I'm at a loss as to why the passenger bank isn't doing its part to keeping the engine running. It appears to be getting fuel.

[Mblizzard]

If you really think that the spark is there then I would try adding fuel when at idle through the carb. OK I can't tell you how dangerous that is and how much I would tell you to only consider it as a last resort. But you sound like you may be close to that point. If the idle changes then I think you know there is a fuel problem in the idle circuit. That might be faster than swapping carbs from side to side.

Does nothing to solve your problem but it may point you in a direction. Another option to check the valves is to scope them through the spark plug hole. I just got a decent USB scope from Amazon for about $40 bucks. It seems to be ok nothing great but it can also be used to inspect your cam lobes through the push tubes. It was called the Supereyes 7. It has a 7mm head and can take pictures and record video to your Laptop. Might make you feel better about the cam if you can look at it. Just a thought.

[r_towle]

Status?

[ThePaintedMan]

Thanks for checking on me Rich. I went ahead and replaced the carb gaskets as well as the idle mixture o-rings, even though both were new, just to rule out the last variables.

The backfire is still there and as long as the engine is completely in synch, it seems like it's isolated to the #3 cylinder.

However, I did another compression test as well as a leakdown this weekend. Compression still shows around 105-110 on all 4 cylinders and the leakdown was phenomenal - I didn't see really any loss at all on any cylinder.

The only thing I can think of at this point is a weak valve spring, as this is a high RPM/high load problem. What do you guys think?

On that note, is it safe/recommended to replace one, or several valve springs on a older, even "tired" engine? I'd like to give it a shot at least before I pull the engine and install my spare.

[carr914]

4,000 RPM is not High RPM/Load

I still think you have a Bad Flux Capacitor!

[TurboWalt]

If the camshaft was getting smaller, it would not open the valve...

It would not open the valve all the way.

It would not backfire.


Adjust those intake valve like I told you, and take your time.

Rich

Yes, it would. I don't know why, but my dad's car did exactly the same thing when his cam went flat. It would idle well, but almost any throttle would cause it to fall on it's face and backfire. But that was a V8.

Based on my own experience from years ago, a flat cam will cause backfiring. This was in a SBC V8, but a poppet valve engine is an engine, right?

[ThePaintedMan]

Thanks for the reply Walt. It's something I've considered, but in theory, since both sides of the engine share the same lobe, I would have the issue on both sides, and a few people have already weighed in and doesn't think this is the problem.

Here is my logic for the weak valve spring:

Compression test is fine, and leakdown is fine. But on a compression test, we're only rotating the engine very slowly in relative terms, so the springs still have plenty of time to push the valve back closed, and as long as the valves are seating fully, there is no reason that the leakdown shouldn't be fine.

So what I think is happening is that one or both of the springs on #3 isn't supplying adequate force above several thousand RPMs, the symptoms of which simply don't show up on a compression or leakdown test.

I know this probably frustrates the hell of you guys, me asking a ton of questions, but as a student of science, I feel the need to find out the cause of a problem before I move on to other options. I'm searching through this to learn more about troubleshooting/diagnosing engine issues so that one day I'll be able to tackle a rebuild on my own.

Like I said, I have a spare engine, but I'd like to work through this problem to the end before I put it in. Then again, it's probably better to swap engines, and then I could pull the heads off of this one to better diagnose what's happening. What would you do?

[TurboWalt]

A broken or weak valve spring could very well be the culprit. I had a BBC with big hyd roller cam that would chew up outer springs. It would present itself as mild miss or popping (not harsh like a backfire). I chased my tail on that one through the ignition system first. My headers kept burning up my spark plug boots which compounded things. I sorted that with ceramic boots. Try hooking up a vacuum gauge to manifold port and see if the needle is smooth or erratic. erratic would suggest valve spring issue.

[messix]

He's carbed the needle will bounce on the 2 throat carbs. No plenum to smooth out the. Intake pulses.

You could have had the other motor in and running by now........

My bet now is bent valve or broken spring

[Bills914-4]

Thanks for checking on me Rich. I went ahead and replaced the carb gaskets as well as the idle mixture o-rings, even though both were new, just to rule out the last variables.

The backfire is still there and as long as the engine is completely in synch, it seems like it's isolated to the #3 cylinder.

However, I did another compression test as well as a leakdown this weekend. Compression still shows around 105-110 on all 4 cylinders and the leakdown was phenomenal - I didn't see really any loss at all on any cylinder.

The only thing I can think of at this point is a weak valve spring, as this is a high RPM/high load problem. What do you guys think?

On that note, is it safe/recommended to replace one, or several valve springs on a older, even "tired" engine? I'd like to give it a shot at least before I pull the engine and install my spare.

Hello George ,
If your up to it I have a few things that might help you out . I have a Full set of
springs ( low milage off some 1.7 NOS heads I picked up/I dodn't know why I saved
them)

Springs ,Retainers,keepers,& the valve stem seals (still flexiable (IMG:style_emoticons/default/biggrin.gif) ) & the tools
to them while the engines in the car (IMG:style_emoticons/default/biggrin.gif) , just pay for shipping & return the tools
when you finish, please ( no rush ) I hope this might help you Bill D. let me know I'll send it out tommorrow (IMG:style_emoticons/default/smile.gif) ,

[yeahmag]

Interesting... I've never used valve stem seals.

[ThePaintedMan]

The generosity in this place never ceases to amaze me. Bill has already sent me a box of spare goodies before, and now this.

(IMG:style_emoticons/default/pray.gif) (IMG:style_emoticons/default/pray.gif) (IMG:style_emoticons/default/pray.gif)

PM sent... I report back on whether this fixes it or not on an otherwise healthy engine.



I hope you realize what a great thing we have going here. Truly an amazing group of people.

[ThePaintedMan]

(IMG:style_emoticons/default/piratenanner.gif)

Bill's tool and parts came in today and I can't wait to try to swap out springs this weekend. Thank you again Bill! While I'm at it I'm going to try to decipher whether the valve(s) are actually sticking in the guide too. I heard this occasionally does happen, especially with an older engine. This is why I'm curious about the possibility of a sticking valve:


https://www.youtube.com/watch?v=XExnP7wpafo




This was when the car was completely cold, and it happens almost everytime I try to start it when it's been sitting more than a couple of minutes. Does it tell any of you anything else? Again, it runs fine otherwise, until the backfiring issue.

If nothing else, the video is kind of funny...

[rgalla9146]

(IMG:style_emoticons/default/piratenanner.gif)

Bill's tool and parts came in today and I can't wait to try to swap out springs this weekend. Thank you again Bill! While I'm at it I'm going to try to decipher whether the valve(s) are actually sticking in the guide too. I heard this occasionally does happen, especially with an older engine. This is why I'm curious about the possibility of a sticking valve:


https://www.youtube.com/watch?v=XExnP7wpafo




This was when the car was completely cold, and it happens almost everytime I try to start it when it's been sitting more than a couple of minutes. Does it tell any of you anything else? Again, it runs fine otherwise, until the backfiring issue.

If nothing else, the video is kind of funny...

What was the outside temp for that start ? I can't imagine it's too cold outside wherever you are.
I can't see your throttle linkage to see if you are pumping the gas or not.
Just like 356's or any other carbed car with no chokes you would probably get good results by pumping the gas once or twice, then with you foot off the pedal crank the engine.
When it started it sounds pretty good.
This may sound contradictory to my last comment but have you tried changing the #3 spark plug to another cylinder ?
PS please don't kill me if I'm way off, I haven't read the entire thread

[edwin]

If you can pull a plug off and it isn't affecting performance then I'd be really looking at firing order. That start up seems like leads going to wrong cylinders
Have you tried stopping by a mechanic and getting one to take a spin around the block? Surely worth a couple of dollars if they can shed any light at all as to what's happening
My money is on firing order or timing
Edwin