Hot Rods Does this look like an exhaust leak that could cause a backfire?

I run this thing lean and hot . And it sure does run good. But no two people tune a car the same.

 

On GM own distributors for sbc, the drive gear is installed with a roll pin that goes all the way through the gear shank.
One side of the shank has a small dimple above the roll pin.
The gear is to be installed on the shaft with the dimple and the rotor contact aligned. The dimple should NOT be on the side opposite the rotor contact.

You might see if your distributor has these characteristics, and if the gear is correctly installed.

 

I'll have to pull it again and check it out. Since the whole unit is aftermarket I'm not optimistic they are marked the same though

 

No time to look at this today but I found a used Mallory Unilite local for a cheap enough price that I will pick it up and throw it on the engine this week to test. I just have to add a ballast I believe as the Pertronix does not require one. Anyone know if I can run it without a ballast for just a few test miles without frying it or if there's a generic ballast I can grab locally. I know they come in several resistances

 

The punch line comes in around the 3:40 mark.

Rotor Phasing Explained | TECHNICALLY SPEAKING - YouTube

With all do respect... I like reading the posts. I glean something from most of them and some are pure entertainment. I try for that with mine.

Rockets crash, brain surgery patients usually come out dumber then when they went in. I'm sure **** your intentions are good but what have we gained for knowledge from your mechanical approach here? I think we are past it friend. I didn't mean to be confusing in my appraoch, but I to am looking for answers.
I'm ***uming, the crowd has now ruled it isn't a spark issue and the focus should be on fuel and air?
Well...good luck with that.

 

I have watched that video and I understand the theory and the process behind it however I'm pretty sure that only works if you have electronically controlled Ignition. If you hear him talk about locking out the timing and setting TDC offset through the computer that is essentially the only way to make that work I still believe the only way to change the rotor facing on facing on a typical old distributor is to physically move the reluctor ring and pick up which involves dis***embly. If I had to do this it would mean the distributor was faulty from the manufacturer and not aligned properly.

As for fuel and air.. Obviously I have plenty of the air getting into the engine but I have not ruled out fuel and carb issues yet. I do however know that I have good compression.

 

Really late to this party. But, after reading all the things said and ideas posited, I come back to post 1. Backfire out the exhaust must be be due to unburned fuel in the exhaust system. So the question is not what lit it, we know the hot pipes will do that, but where it came from. My money is on a ****erfly not sealing when closed. Coast vacuum pulls air and fuel into the cylinder and for some reason it isn't completely burned there.

JM2C

 

If an end carb isn't sealed that could cause vacuum to pull out of the bowls right? If the centers don't seal that wouldn't matter since the idle adjustment opens them slightly anyway right?

Is it also possible if the rotor phase is off and firing late at TDC the fuel is still burning in the exhaust at low speed?

 

So if the rotor phasing is off, and you think the spark is coming after TDC rather than 12 degrees before TDC, can't this be overcome be cranking in more static/initial advance, so it fires sooner?
More advance would likely increase performance.
Try adding 5 degrees at a time to see what happens? Hard startup cranking will be one indication you've gone too far.

 

I was going to say that wouldn't work to change phase but visualizing it makes me believe it should. I've already tried increasing base timing by quite a bit though at one point and the backfire didn't go away. What I do know is that if I decrease base down to about 8* btdc the engine won't even stay running.

 

I'm sure this has already been asked and answered, and I don't remember, so I'll ask it again. How does it run with the vacuum advance hooked up?

 

I have it on ported vacuum and I picked up 1 or 2 mpg ...from 10 to about 12....still terrible for this combo I feel. I had it on manifold vacuum at one point and still had the backfire.

As far as just advancing the initial....thinking about it again the rotor will still fire at the same point on the terminal...just earlier than TDC. If it happens to be missing the terminal or crossfiring this won't help. However thinking about the vac advance question...the manifold vac should have advanced the distributor essentially effecting phase by those few degrees I believe?

Was hoping to grab that new distributor today but was unable to meet up with the seller....hopefully one of the next few days.

 

Here's a question that never even dawned on me.....how would I go about testing the ignition coil to see if it's weak? That's pertronix as well

 

Wouldn't the ignition be breaking up under a load also, if it was the coil?

 

I would think so. Yesterday I had to creep into the throttle to really bring on the power. If I just punch it normally it just felt like forever to make power....yesterday it actually hiccuped a bit and I thought I felt a misfire. I didn't push it much because I didn't want to get stranded far from home.

 

Your on page 7 ********. You know what that means don't you. Long past the 48 hours, now it's almost a cold case file.

Have you heard back from Per Tronix if it was a deemed defective unit? I'm guessing that's either a no or you need to call them again.

I had a brain fart...make the next post #1. As it's been said, it doesn't matter and it doesn't care as long as it's following an order. Then, as I've wanted it approaching the firing pin.

Backing up, and to maybe shut me up...
I know what I'd do. But that's the right way and it's to darn hard.
So... Try moving all the wires clockwise, one terminal. Then put a timing light to it. Or not. Think it's start? Probably.
I'm going to ***ume that alone should paint a picture. Like the picture of how it runs now you painted. Less then perfect and with a back fire still? All we can do it try.

I'm also ***uming that my understanding of the cause and effect to positioning that balancer mark, is as well affecting the open or closing of the valve train.
Backing up... You had a degree wheel on, went piston stop to stop and found center. At that point, I'm ***uming the intake valve is closing at this point of compression.
Was the Valve cover removed to confirm any of this?

Anyways... if you had at this point rotated the crank to position it at 12BTDC, that would translate to "X" movement of the valve train. Now dropping in the distributor and spinning it to 12BTDC isn't from a neutral position, but with an additional 12BTDC of valve movement as if you were to degree a cam for rpm and horse power not drivability or gas mileage.
I think it was mentioned about a timing tape? To see what the advance does as you rev it up? I'd be curious as well to those numbers.
Any cam number or specs to speak of? Not that I know jack **** about cam profiles but...I know enough to ask.
Wouldn't that be something, a EFI cam engine running multiple carbs. Would that even work? lol, probably?

So to wrap up, your running the 12 BTDC distributor timing, just move the wires.

You got 12 at the crank and 12 at the distributor, lose the 12 at the crank and move the wires.

 

I've changed the position of the number 1 wire a few times....doesn't matter. The inner and outer "circles" of the distributor will always be in the same phase once you set ignition timing. Also the engine and the distributor are not at two seperate 12* settings....that would yield base timing of 0 or 24....timing is 12* as per the light.

No word from Pertronix yet despite follow up email and phone message. Trying to pick up another distributor this week just having trouble arranging it with the seller to pick up. I have 50 hours of training for my 1 job this week so the truck has to take a back seat until the weekend. Had it out again for a 5 minute trip to help a buddy last night and performance seems to be degrading which makes me think ignition still.

As for valve timing.....never had an actual degree wheel on the cam/crank just the piston stop to find actual TDC and compare that mark to 0 on timing tab. I did however confirm both valves closed at this position with the valvecover off. I'll try to dig up cam specs on the engine during a break today but it's the GM SP350/357 designed and tested by them with a 4bbl on it.

 

Definitely gone down a worm hole . After the header fix which was probably never the cause of a full on back fire but worth fixing regardless, my next port of call would have been to throw the air cleaners in the bin . That might not fix the back fire either but it will definitely get more air down those carbs . The simple fixes and eliminations are best taken care of first before delving into the tech stuff .

Nothing ,but nothing these days available for the m***es is of any significant quality , its marginal ,thats the way thigs are these days and the best we can expect from the m*** produced aftermarket .. so its safe to consider other issues but not until the visible stuff is eliminated .

The chances of the dizzy being out of phase are slim . these things are ***embled on jigs that you can train a monkey to use ,but its not out with the realms of possibility that a drilling might be out , but unlikely .. Pertronix will blank you, its a product that has its likers and dislikers .

If you are convinced its the dizzy, go full on electronic control and lock off the base plate, throw away the vacuum can and fit an amethyst unit .

The amethyst takes the vacuum signal does what it does by the magic of technology and causes the electronic ignition to fire exactly where you want it .. so much so you can map your own ignition timing with a lap top .. The dizzy is then trully just a vessel for carrying the hardware .

But that aside , I personally think its carburation and adjustment. By throwing away the restrictive air cleaners , and making sure the carbs are all pulling the same vacuum (balanced) that would be a start .

I always set the carbs so they are pulling as much vacuum as they possibly can then adjust the idle speed . but all of that is pointless if the start point on each of the carbs is not balanced , so you have to start by disconnecting the carb linkage, allowing the carbs to find there individual base setting then go into each carb and check that the ****erfly's are all set to the same start point and seating correctly . and take it from there.

If checking for vacuum leaks at gasket joints WD 40 is your friend . It wont stain and is much much less volatile than easy start type products..

If this set up isn't giving you an adrenalin hit then there is a carburettor issue . There could be a false reading from your plugs , they look OK for a tri power but if you took the car for a spirited run then drove back home and parked up before you pulled the plugs the readings wont be accurate , they will be showing your idle /slow running readings, You should do your testing out on the road , cruise it for a bit at speed then switch off, dip the clutch and coast to a stop then check the plugs ..

None of the above will be that helpful but sometimes we cant see the wood for the trees..

 

I know it's not traditional, but an AFR gauge is fairly inexpensive and easy to install. It may help shed some light on your problem. I have one that I use on multiple cars and it has made things a lot easier than reading spark plugs for me.

 

I'll have to try a quick run down the street with the air cleaners removed....I'd know in the first 200 ft. As for balancing the carbs I can understand that as I have an old 4carb Honda bike however this runs off the center carb only until about 75% throttle and it does it when only on the center carb....I've even run it with blockoffs in place if the end carbs.

 

There are some good suggestions here on alternative distributor choices:
https://www.jalopyjournal.com/forum/threads/whats-the-best-hei-aftermarket-distributor.1274118/

 

Pertronix finally got back to me today after having to reach back out several times. Their "tech" also suggested it could be a tooth off which I believe I've already proven not to be the case with having swapped it a few teeth each way and the offset not changing. They did agree however that it seemed quite a bit off but that I'd have to send it in for them to look at and turnaround time is over a month! I have tentative plans to hopefully pick up a used Mallory tomorrow night. Otherwise I may pull the trigger on another brand new one....but I'm not a huge fan of throwing money at a problem if I'm not certain.

 

Your backfire problem is a sticking exhaust valve/not properly seating exhaust valve.

 

Wouldn't a compression or leakdown test show this?

 

********...you remind me of my kid.
As much as I love the boy, some days I'm reminded he's got his mother's 50% in him.

That aside, I still want to help.

No, you haven't proven anything. If you had I would have gone away wiser for my mistake. But I'm not sure I'm mistaken, just that I'm not sure I'm right either.

Ya knows that to fan the flames of controversy isn't such a bad thing if it leads to the results you were hoping for when you lit the fire.

Now I'm going out on a limb here because frankly, it's a vague suggestion statement. But a real possibility. I agree.

I'd ask, are you suggesting that on the basis of...?

Here's the thing...you did this you did that but unless you tell us what this and that entailed, you could have done it wrong.
Yes, you've done lots of stuff and by all accounts, mostly got it working. But you yourself tell us, it should be better.

No harm no foul but...you touched a few things. You will have to be the guy to fix it. Fixing is not throwing money at a new distributor.

That said... it's the weekend. Time to fix the car once and for all.













Or not. But if you put a vacuum gauge, it could show if a valve was sticking or holding up?


A leak down test not so much if it wasn't sticking or holding up at the time.

How to do a leakdown test | Mobil™
cylinder_leak-down_testing_011019.pdf (alldata.com)

Maybe post a video of the big bang? Of the repairs? Or not.
As far as it goes, I'm still following along.

 

With pictures of the distributor in at 3 different teeth mesh locations and the relationship of the rotor to #1 terminal not changing how have we not put that one to bed??

As for compression and leakdown.....engine warmed up and all plugs out for compression test. I cranked the engine with WOT for 8-10 seconds on each cylinder and compression was within 5lbs on all 8.
Leakdown basically how your article described....engine warm. Pulled a plug and inserted tool after turning engine so both valves were closed as verified by pulling valve cover. Regulated leakdown tester to 100psi and plugged it in to get reading....again consistent on all 8 cylinders.

As for vacuum.....that chart is slightly flawed as you're ***uming this engine would run 17-21 hg if all was correct. Everything I've read on the SP350/357 seems to show a normal vac in the 13-15hg range which is where I am. The gauge acts closest to L however as it gently floats up and down by about 1hg. Plugs are set to GM recommended gap.... .040 if I remember off hand. And I don't have points but that is distributor related if there's a sync issue with the pickup though possibly?

 

For laughs maybe one of you guys can help me out another way. If you have easy access to your engine and know your base timing....turn the engine over until your balance line is at whatever mark on your timing tab represents your initial base setting. Then just pop your cap off and see where the rotor points in comparison to the center of the #1 terminal....if you put a little tape on the distributor and make a marker line so you dont have to draw on your actual distributor. I'm curious

 

The reason you can't find the sticking exhaust valve with a compression test or a leak-down test is because it is sticking only when it is functioning, as in when the engine is actually running.

You'll have to dis***emble the engine to find it, down to the last nut and bolt (That will keep you busy for a while, just remember to take pictures).

 

I'd say, forget about rotor to cap position. Properly install the distributor, then set the initial timing 5 degrees greater than you've been running.
I bet you see a performance improvement. If so, then try 5 more and see what happens

 

I have run it with around an additional 10* of advance....didn't get rid of the backfire and led to hard start issues.


As for a sticking valve I may have to eventually look into pulling the heads if I can't find anything else. When it's at idle if I really rev it up good by hand with no load I can get the backfire out of both sides but when driving it only does it on the driver side. What's the chance I've got a sticking valve on each side?