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

So, was the performance better with 5 degrees additional advance?
Did you read just idle mixture after adding advance?

 

I honestly don't remember. I ran it just long enough to see if it backfired which was the length of a parking lot and it was hard to start after that so I put it right back

 

I just found this fantastic article just now explaining timing, rotor phasing, and why the being a tooth off theory is bogus. Of course the article also goes on to state that at just base timing when it fires the rotor should be on the trailing end at the end of the terminal inside the cap which is kinda where I'm at....soooo back to looking at valves and carb maybe
https://www.w8ji.com/distributor_stabbing.htm

The section about finding the center of your timing range, turning the TDC to that point on the timing tab, and pointing the center of the rotor at that has me confused though. Are they implying that only works for a crank trigger setup? This same method was mentioned in the earlier Haltec link but they stated you had to electronically lock timing and program in TDC offset after that. This method won't change phase in a fully mechanical system right?

 

Vacuum test Junkman8888, that might indicate a sticky valve. Engine is running for that test.

Properly install the distributor? Late for the party glad you could come. Have another drink, I'm sure you'll see a performance improvement. If so, then try 5 more and see what happens. That's a good timing.

I asked a couple of my friends today what they thought. Hey, car's got a back fire, what's the problem. Ted, he say's it's either fuel or spark, I think spark.
Jack, he didn't hesitate, carb problem.
So I said...
New GM ZZ crate motor blah, blah, blah 7 pages. Ted still figured distributor and timing, Jack was now on the fence.

I say good luck, you got this.

 

The chances that you have a sticking valve on both banks is zero, your best course of action is to take it to a shop that has one of the old diagnostic testers for non-computerized engines, one that will show distributor output, engine vacuum, air fuel ratio. Go from there.

 

I think I saw an old Sun machine local on marketplace recently....this may be my excuse to buy it

 

I give up.....get me a gallon of gas and a match! Picked up a used Mallory Unilite yesterday and installed it today only to find out the module in it is no good. Just said screw it and ordered a brand new ready to run MSD distributor that'll be here next week. I'm considering it an upgrade over the Pertronix even if it doesn't solve my issue. If it fixes it I'll have Pertronix fix their unit and then I'll keep it on my shelf as a back up or sell it. Now I have to wait another week to find out if the distributor is my issue. Also have to change the terminals on my plug wires to fit the MSD cap.

 

Well...I'm keeping my mouth shut but to say I have been wondering, and good of you to update us that have been following along.

Ta hell with that. I think I said you remind me of my kid a few replies back. A module would have set you back what? 60 bucks USD? That new MSD, my son, my son. This is when I'd say no wonder your always broke but you could afford that MSD so that doesn't apply here. I'm at a loss for words?

And what's funny as hell is I know how much one of those GM modules costs because my kid was driving my OT old 92 Buick Roadmaster when it gave up the ghost.

Far be it for me to ruin a guy's start to a distributor collection but common, you were trying to confirm or refute if that was a problem causing the backfire.

And again (insert dead horse) I'm still of the impressions it's you not it, no offense.

But by golly, a while back you asked for pictures of timing marks and rotor alignment. Still want to see? I just remembered I had one wrapped in plastic that was carb compliant.


Still want to see? Cause I'll cut the plastic and dollar to doughnut full transparency I will show you where it's pointing.

 

I'd love to see where the rotor is pointing on that. Set the mark on the balancer to whatever the initial timing is set at on the tab and pull the cap so we're comparing apples to apples.

As far as buying a new one vs just the module....I never intended to run the used Mallory full time anyway. I don't like that they require a ballast for starters. Secondly the entire truck is new and has been sponsored by several large parts suppliers including Pete and Jake's, Tanks Inc, Lizard Skin, Cl***ic Tube, and more. When I take it to shows and show off all of their parts I certainly can't have a beat up used distributor right up top .

As for it being me.....not likely. I've probably stabbed 50 or more distributors into old engines in the last 18 years in my shop. Never had a timing issue with one. They were always the originals though and came out of the running engine....unlike the fancy new unit here.

 

Road to hell is paved with good intentions. Can't help you, seems the tab is or was used elsewhere. Oh well. With apologies for having offended you, sound like you have it happening past what I can offer.

 

Thanks for trying to check. I'll update this thread next week whenever the new MSD unit comes and I get it in

 

As I'm waiting for my MSD I'm still researching. Do these electronic distributors need to have a ground between the housing and block besides just the ground wire from the module? Everything on mine is painted or powdercoated so it is definitely not grounded to the block.

 

As per Pertronix just now it needs a good ground. Hopefully tomorrow I can clean all the surfaces and see if it helps. I have powdercoat and paint on everything right now

 

8 pages, discussions, mill the head and manifold, put it together with 1 gasket, and be done

 

I'm ***uming you mean the exhaust manifold? It no longer leaks.

 

Cleaned grounds and reinstalled everything...still backfires but ran "ok" other than that. Thinking the end carbs may need more jetting for my top end however that doesn't solve the backfire. Maybe it really is just the short pipes....but still doesn't explain the poor mileage. Mid range power all feels good which still makes me believe it's not valve related. I'd ***ume I'd feel a miss or backfire somewhere in the RPM range

 

Me to her. Hey babe, was that the best *** you ever had?
Her to me. Ahh, it was "ok". Except for that little fart at the end. lol.

Well, I thought it was funny.

 

Welp.....got the MSD this week and installed it today. To nobody's surprise it didn't fix my backfire. Truck seems to start and idle a little better though and the distributor is just better made all around. Lots less play in everything. So I'll see the Pertronix or keep it as a spare.

I'm ending this witch hunt for now. Once show season is done here I'll run a full exhaust under the truck and see if that stops the backfire. Maybe I'll play with jetting a bit more to see if it helps overall power and/or fuel mileage.

 

The Race is On - YouTube
Old guys will remember it.
I'm not sure why, but this song came to mind. Pride in the back stretch kind of way...?
Then this played after that.
The Race Is On - YouTube
A little more modern version.
Then it was Brooks and Dunn and I said enough of that shat.

Well, I ain't calling you a quitter. Or a giver upper. But funny as it sounds, it does appear to be the case?
LOL. Calling it taking a break works.
Good for you. Don't worry be happy.
We learned something from it. The distributor apparently works. That's repeatable results for you.
Is the full exhaust the answer. Maybe it is, but for arguments sake, I don't hear many race cars popping off when they shut down.

In the end while I get where things are at, I wasn't going to hit "like" because frankly, I found the ending disappointing. But you did give it an ending.
Just not a happy ending.

 

Not giving up by any means. Time to regroup and come at it differently. I don't have time in the next few weeks to design, build, and install a full exhaust. I'm committed to 1 more big car show for this year in 2 weeks and then the truck is due at the upholstery shop for the headliner, door panels, kick panels, etc. Once it's back from there we'll be into winter weather here in NJ and I'll have several months to install the exhaust which I need either way because the truck is honestly too loud for comfortable driving (Can't believe I even just typed that...I might be starting to feel old).

I find it very weird that a general internet search of this particular engine brings up basically no results from anyone dialing one in or reviewing their experiences with it. This is the only real world post I've found regarding one of these engines and it sounds eerily familiar in some ways. Same basic vacuum reading and overall characteristics of it running "ok" but not great. And of course there was never any type of resolution or follow up posted.
https://nastyz28.com/threads/low-engine-vacuum.337623/

 

Well I'll be damned! I reached out to the guy on that other site last night who was having issues with his GM350/357 and he got back to me after it'd been a year since his last post. Check out his reply in this screenshot....he's also having exhaust crackle issues and very similar all around symptoms! Maybe I'm not crazy afterall.

 

I just did a carburetor adjustment on my 51 Coupe. Adjusting the mixture on my 1951 239V8 caused the car to backfire on deceleration. The car seemed to idle better, and the adjustment was set with a vacuum gauge.

I confirmed timing with a light. It was right on the point at idle. The advance was working on acceleration. I was done there.

Over a period of a couple of days of driving the car and adjusting the mixture with the gauge, It's fine, no backfires. It just needed some more fine tuning.

This was a single Ford 94. It was a little tricky to get just right. It was close but it backfired. It was just a little out.

This was on an engine that has one carburetor. You have 3.


^^^^ You need something like this. When you run multiple carbs, you need good single carb set up to establish a base line. Without a baseline you are a wandering zero or in other words...chasing your tail.
This whole set up is less than 700 bucks from Summit. I would suggest just finding an iron intake and a good 2bbl for testing purposes but unless you have one hanging in the wall or another car, that set up may be more than this Summit kit.
You need to run that engine with another induction system (good single carburetor). You have to get a baseline.

Don't get stubborn.
I could have fixated on the distributor, valve timing, carb jetting ect ect ect...No telling where that would have led? It just needed adjustment.
A wise Old Hamber Bruce Lancaster always advocated a single carburetor and intake as the premier diagnostic tool for a multi carb set up.

 

While I don't disagree that keeping it simple for initial dialing in is good....wouldn't having blockoffs for both end carbs serve the same purpose? Maybe I need to reinstall them and look into the center carb more. Just found it odd but also refreshing that I found someone with the same engine experiencing the same symptoms. And he's running a single 4bbl.

 

Block offs would be a good thing to do. This way you are running off of one carburetor. I have no idea of your set up. I don't know if you are running Rochesters, 97s or 94s.
I don't know if you are running a primary with 2 secondaries or if all 3 are in sync. If you are running a primary and 2 secondaries, The primary could be the problem. This could make it harder to get a baseline.
With this said, I would time the engine to spec. Use block offs and see how it does with just the primary. Then I would try to tune that primary and see if the trouble goes away.
It would really be nice if you had a known good primary to do this test.
Still though, running just the primary with the secondaries blocked off may tell you.

 

2G's......primary is the only one with an idle circuit. Had them gone through twice by a reputable shop. First time he found all kinds of issues. Second was a rejet and double check on the primary.

 

Love the suggestion that cam timing's a tooth off! Ever start an engine that was a tooth off @ cam sprocket?
Did it start? Maybe. Likely not, but starter sped up like it was going to start...
If it did, it wouldn't have enough power to pull a greased hat off a bald head.

 

Just for the hell of it this afternoon I pulled the driver valve cover again and took a good look around. Springs all look good, no noticeable bend in any pushrods without pulling them out, and I could see through the heads down to the lifters and none of them appear to be collapsed.
I have some time during the day tomorrow before work to mess with it. Maybe I'll throw the blockoffs back on and really dig down into the primary carb more. On the 2G I know there's obviously the main jets and there are the 2 adjusting screws for idle. ***uming I find no vac leaks again around it what else should I consider?

There has to be thousands of these engines out in the world by now yet I can't find much mention of them anywhere on the web.

 

Make ABSOLUTELY SURE there is NO vacuum leak around primary carb base........ or intake to engine.
(just to totally rule that out)
6sally6

 

I spent 3 or 4 hours messing with it this morning and I believe I picked up some performance but no change in the backfire per say.
I put a vacuum gauge back on it first thing today and at idle it seemed to fluctuate between 15-16...not wildly, just kinda floats up and down. Went around again with spray to check for leaks and found none.
I did a bunch of reading last night on ignition timing on GM crate engines and a general belief was that the recommended base timing is too low. I loosened the distributor and added advance and watched the vac gauge come up with the increased engine speed. Eventually I settled on 18* base with vacuum advance disconnected. Then I reset my idle speed back down to about 700 RPMs and readjusted my idle air screws for what appeared to be max vacuum. Was able to turn the p***enger side in about 1/4 turn before vacuum dropped while the driver side was about 1 full turn in. I also removed the mechanical stop bushing from the distributor and installed the lowest one provided which is 18*...giving me 36* total and set it up to be all in by 2,500.

Took the truck out for a ride and it certainly feels more responsive. Low end power feels like it's up and top end feels a bit better too although I feel like I have to creep into it. If I just punch it in 4th from say 40 or 50 I don't get a huge kick...it almost feels like it's bogging.
Regardless of the increase in power the backfire is still there....primarily on the driver side with the occasional backlash on the p***enger side....and always only when the throttle is shut.
I may bring my base timing back down to 16* to get me to 34* total which is closer to GM's recommended 32* total. However even at my current 36* I don't feel any pinging and there are no hard start issues.

At this point I feel a bit in over my head with timing curve and possibly jet changes so I'm open to advice. Definitely happier with my overall performance now though sans backfire.

 

A lot of people may not like this, but on a new engine I always do a running hot lash valve adjustment. With the engine warmed up at idle, I back off the rocker nut until it clicks. You'll feel it before you hear it. Then I'll tighten down 1/4 to 1/2 turn after the clicking stops.
If the lash is too tight or too loose, it can cause all kinds of issues, cam wear, no power and maybe even excess fuel and spark getting into the pipes causing or exasperating backfires on deacceleration.
I have a special valve cover with the top cut out just for this.