Technical Revisiting exhaust backfire on decel issue

Also yes o2 sensor shows more lean on misfire but there is actually unburned fuel being pumped out into exhaust, it takes a certain AFR to burn in the pipes that’s why when tuners are trying to get rid of decel pop on fuel injected engines they add fuel on decel to fatten it up so it won’t burn in pipes or they cut all fuel on decel sometimes so it’s too lean to burn

 

I swapped to a whole new distributor, cap, and rotor last fall with no change. Going to check plugs and wires on that side again tomorrow possibly. Leakdown and compression are good so I don't think it's valve related. Will have to look at vacuum leaks for a 50th time after that. All my readings for carb adjustments have been off the other side of the engine so they should at least be accurate.

 

Ok, OT to the HAMB but my daily truck had bad intake gaskets. I did the spray thing too, no dice. It’s a SBC but made in ‘03. The smog guy did a “smoke test “. Told me it was “coming out everywhere”. I put new gaskets on it, issue was solved.
I just spent the day changing the gaskets thinking “no way”.

 

We smoke tested the exhaust on the driver side last fall and aside from the carb butterflies and bowl vents we didn't notice smoke coming from anywhere else up top.

 

Hello,

Without going through the motor or specs from everyone’s ideas, could it be something as simple as a muffler back fire? The cause being a bad muffler or the style and restrictions of your muffler system? Having been behind many “idiots in a mustang” and loud hemi powered Chrysler cars, they do the backfire by simply letting off of their gas pedal at a stop light, slowing down. The motors are usually stock and the identification of the noise is from the mufflers they added to their exhaust systems, catalytic or not.

If one is to be behind or get passed up by those cars, when they let off of the gas pedal at a danger spot or to slow down for traffic or even a stop sign, the muffler sound is loud and backfires. The motors are supposedly tuned and running fine as from the dealers, but for some reason the backfire remains an irritation thing for those that are driving around. The well known muffler systems and muffler companies offer mufflers that are known to cause the backfire issue. Some folks like the tube mufflers with little restrictions, but they are noisy. So, check out the restrictions of the mufflers.

Jnaki

So, without thinking along the lines of a technical issue with heads, valves, etc, why not try a friend’s smooth flow exhaust system and see if it does not change the backfire noise. Those side pipes should not give any restrictions as they go right out to the outside, but if you have a downtube to connect to a muffler system for legality, then there are plenty of restrictions in that set up. It is just a thought.

A friend lives in our neighborhood. He has/had a long line of Corvettes. But, his latest is a new model and when it fires up in the open garage, we can hear it a long distance away. We know when he is going out for a ride, short or long. But, as he goes down the hillside street, he lets off of the gas and sometimes, it starts a short bubbling, backfire effect. So, factory set ups can do that exhaust backfire on deceleration. YRMV

 

Just went for a ride with the opened up IFRs and with the 58 jets it was pretty rich on acceleration....dipping into the low 11s. I threw the 57 jets in and it got better....actually to the point where that 1800-2200 RPM range was going lean on moderate acceleration...like right around 14.5. I guess I need to choose now between this setup or the 58s with smaller IFRs and having the idle screws full out. I'm thinking of putting fuel to the end carbs tomorrow just to see where I'm at during WOT and going from there. I feel like the leaner idle with slightly larger jets is the safer bet right? Although thinking about it I've been seeing the lean condition on accel above half throttle most likely so with end carbs hooked up it'll actually be adding more fuel anyway.

 

I’m not discounting what you did. My issue was lean O2 codes. OT for sure. I don’t know how the shop hooked the machine up, etc as I left the truck with them. But try as I might, carb cleaned detected nothing from the outside.
I only mentioned it because one of the replies mentioned intake gaskets and mine weren’t evident being bad from external checks.
But when you smoke tested the exhaust I assume you kept rolling the engine over to seat the exhaust valves? Would be the right way to do it, but a lot of work/effort.

 

Slightly lean at idle should not be a problem it shouldn't result in detonation, unless you are going to idle for hours like a taxi or police car.

Wide open throttle is where you don't want to go lean, and experience major detonation.

 

Went back to idle fuel restrictors that are around .029 today but kept the 57 jets. I'm pretty happy with the numbers for the center carb operation except I can't get it to essentially sip fuel at cruise. In 4th gear doing about 60 at 2200 the best I'm seeing is about 13.7....was hoping for better. 5th gear at 75 about the same. Idle is right around 14.7 now and under acceleration I'm in the 12s. Plugs look very clean and slight tan on porcelain. Not sure if going down 1 more jet size will help my cruise economy or just hurt my acceleration?

Now on the other hand I made a few WOT pulls and it looks like I'm actually going LEAN and I'm not sure why. When I first opened them up my numbers were ok but then I started seeing into the 14s and as soon as I got near or to 15 I let out. The end carbs still have 62s in them from when the shop went through this setup. With them already being much larger than the 58s in the middle why am I going lean? I was expecting to be super rich. I'm assuming jetting and float height are the only 2 factors here right? Most of the research I've done has shown guys running smaller jets in their end carbs. This is not an original tripower setup so the carbs are mismatched. Would that explain it.

 

Hook the vacuum advance back up to manifold vacuum and try it.

 

Already is

 

With the way the numbers are after all that you've done, I'd be leaning towards that explanation.

 

Just thinking about something @carbking mentioned earlier. Jon it seems like my rich cruise is likely from being in the idle transfer slots even around 2,000 RPMs correct? If I go smaller on the idle pickups the idle goes too lean to run. You mentioned the bypass holes in the throttle plates earlier. Would those effectively keep me out of the transfer slots more at cruise or am I way past the slots at that point anyway? I was under the impression these holes were only necessary if the idle screws made no change to the idle mixture. I'm not sure I fully understand the IAB's but would opening those lean the mixture in that 2000ish RPM cruise range?

 

You should not be in the transfer slots at 2000 RPM.

Any fuel from the transfer should die out at maybe 1200~1300 RPM.

The bypass holes in the throttle plates are to allow the idle circuit to work, maybe 400~800 RPM.

With your cam, the idle air bleeds should be smaller, not larger. Of course, with the wrong size carburetor venturii, very difficult to totally predict what is going to occur.

One more time: check the second line in my signature block!

In general:

The idle circuit should function from maybe 400~800 RPM, and die out totally by about 1000.
The transfer circuit should start to function from maybe 700 RPM, function through maybe 1100, and totally die out by 1200~1300.
The main circuit should begin to function at maybe 1000, and continue to function.
The power circuit should function as directed by the engine vacuum, and add fuel to the main circuit when needed.

The idle air bleeds are a component of the idle circuit.

The idle circuit starts with negative pressure present at the idle jet, allowing fuel to be pushed out of the jet into the idle passage. There the fuel is mixed with air from the idle air bleeds. This mixture is then forced through the restrictors which accelerate the velocity of the air/fuel mix causing more rapid mixing of the fuel air. Following the restrictors, the idle air bypass now adds more air to the air/fuel mix, and then the air/fuel mix is delivered to the idle discharge ports, the VOLUME of which is determined by: (1) the setting of the idle mixture control screws, and (2) the relative position of the throttle plates to the throttle body wall.

The air bypass holes in the throttle plates, if present, allow some of the idle air to pass through the holes, thus allowing the angle of the throttle plates to be reduced, and the edge of the plate be closer to the throttle body wall. By moving the throttle plate edges closer to the throttle body wall, the air velocity past the plates creates a higher negative pressure in the idle circuit, and improves the efficiency of the idle circuit.

Jon

 

@carbking thanks for the clarification on RPM ranges....I couldn't find that info anywhere. As for checking the 2nd link in your signature it could be the fact that I worked 24 hrs yesterday or the fact that I slept 4 hours this morning after but I see no links. I've read through your website several times now however.

I've seen just about everyone state when running tripower the IFRs should be opened up however my truck seems to have the best cruise AFR when they're stock or close to it. Once the throttle is past the transition slots the idle circuit continues to add fuel right? Or does it lose the vacuum signal and essentially shut itself off? I guess based off your post I'm to look at my main jets still if I'm trying to lean my cruise out more as long as I don't go lean anywhere else in the RPM range before the end carbs open? I do have a set of 56 jets coming after the weekend and I can pretty much swap them with my eyes closed at this point in 10 minutes time so I'll see what happens.

Any thoughts on the end carbs seeming to want jetting substantially higher than the center on WOT? Different venturi sizes?

 

Hardcore - you are still asleep The word is line, not link

The idle circuit loses vacuum past maybe 800 RPM, and shuts itself off.

Another "tuning conveyance" in the Rochester 2-G (or the 4-G, for that matter), is the power circuit; and if improperly set-up, it can interfer with the main metering circuit at cruise.

For tuning purposes, the power valve SHOULD BE REMOVED, AND THE THREADED HOLE PLUGGED INITIALLY. Now the main metering jets may be calibrated for cruise. If you can get the main metering jets properly calibrated for cruise, then the plug should be removed and the power valve reinstalled to tune for WOT.

When tuning the power circuit, consider:

There are at least 19 DIFFERENT power valve assemblies. These differ in orifice size, and plunger height; one size does NOT fit all! Typically, tall plungers are used with automatic transmissions, and short plungers are used with manual transmissions.

There are at least 2 different length power valve actuator valves.

There are a number of different power valve actuator valve springs. I have never seen specifications on these these, but decades ago, when still modifying original factory set-ups to run with bigger cams, I produced a set of 3 different performance springs. These were wound by a spring winder, not made by cutting coils.

Really difficult to predict, because of the carbs you are using; but it may be that the power circuit is coming in too soon, due to wrong components. This could cause the mixture to be too rich at certain RPM's, more accurately, certain vacuums.

Once again look at the second LINE in my signature block

Jon

 

Never underestimate adequate sleep hahah. I've been burning both ends of the candle and the middle for over 15 years now. I'll reread your post again shortly and check the line in your signature.

If you could indulge me a bit what's the explanation for my cruise AFR in the 2000-2200 range getting so much better with the narrower factory IFRs?
Also about 2 weeks ago I had tested the truck with the power valve removed and plugged and it made no difference in cruise AFR.

 

I'll try to make it easier for you, here is the second line in his signature!
The most expensive carb you ever buy......is the wrong one you try to modify!

 

If you are reading on a phone, turn the phone 90 degrees. Assuming you have the feature “rotate screen” active on your phone.
Then you’ll see things that are below the post, links, sig lines, etc.

 

Decided to change gears for an hour today and go back after the exhaust backfire but had no luck. Put a vac gauge on it and it still floats about 1hg sporadically. I used a propane torch to go all around the carbs and the intake on that side and saw no change in idle. If I put propane right up to the air cleaner idle would increase so I know it would have worked if I was drawing it in elsewhere. I also tried to put a vac gauge onto the dipstick tube but got no reading positive or negative with the valvecovers both blocked where the pcv and breather go in.
One thing I did notice and this may be my lack of knowledge on the pcv system but if I took the hose off of the pcv valve and plugged it going into the carb my vacuum would drop and idle got rough. I would have assumed plugging it up would make no change in vacuum. What causes this?

I also swapped a new wire onto 1 cylinder at a time on the driver side of the engine just to see if I heard the slight misfire change but it made no difference on any of the driver side cylinders. Last leakdown test I did about 2 weeks ago showed all cylinders basically the same and lash was checked twice then so I really doubt a valve is hanging either.

Went for 1 ride around the block but noticed in say 3rd gear at 3000 my afr was down at 12.5 so maybe my main jets really are too rich still as well. I have a set of 56s coming tomorrow that I'll try out.

 

Got my 56 jets today, installed them, and reset my idle. This may be the correct jetting for the center carb. Idle is around 14.5, acceleration 12.5-13.2. Highway cruise now is right around stoich...14.3-14.8. Only thing I noticed is if I let it wind out a bit in 2nd or 3rd on very mild acceleration, almost cruise it goes to about 14. A little richer right there would maybe be good but I think it's ok. I do have to have the RPMs up past 2500 before I punch it in 3rd or I'll get a bog but that's probably normal for that load? Also it's pretty cool here tonight....truck will mostly be driven in weather 25-35* warmer so I'd see slightly richer numbers then I'd assume.

I do need to jet the outer carbs up for WOT though....despite having 62s in them its going up to 15. I have a few larger sets coming this week. From my research I see most guys actually have smaller jets in the end carbs. I guess this engine just wants all that fuel?

Oh.....and still backfiring!

 

Maybe switch to adding more advance in the ignition...?!
Once read.......most carb issues are solved with ignition adjustments.
You GOTTA BE nearly outta options man
6sally6

 

More timing for what? I've thrown more timing at it before and it didn't solve the backfire. Only thing I can see it helping now is the bog in 3rd maybe? I'm at 14 initial and 32 total plus whatever the vacuum advance adds. I could up the total a bit but GM calls for 32 on this crate engine with no vacuum advance.

 

SBC typically uses 34 to 35 degrees total timing, plus vacuum advance.
You should have enough cam and compression to easily handle it.

 

I'll see what the next size stop bushing for the distributor would bring me to

 

36* total advance seems to be the "magic-bullet" for most engines unless......ya got really $weet flowing/breathing
heads (AFR's fer example.) They have such complete combustion they don't need the extra 'lead-time' most stockers and after market heads need.
One of those ......."gee...wonder when more total don't make no difference" moments.
Seriously....really makes the acceleration snappy. Solves a bog problem a lot of times. Run high test (of course) and LISTEN for spark knock when tuning...But you do that anyway......
6sally6
PS Ya tried almost every thing else.

 

I'll bump the total timing for the potential performance gain. I definitely had it that high during testing at one point and it didn't fix the backfire. At this point if I get a lull in car show season where I have a day or two free I may pull the intake and check it for straightness and regasket it. Only thing I can think of is internal leak or it's because of the short pipes.

 

Been doing some reading and seems like most dyno testing with Vortec heads shows they make the best power in the 29-32* total range. Not saying at 34 I may not squeak out slightly more but it would be marginal at best from what I've read.

My larger jets came today for the end carbs however we are going to have 3 straight days of rain so testing will have to wait it looks like. I'll probably go from the 62s to 65s and fine tune from there. I bought all the way up to 68s so I should be able to get that ratio down into the 12s.

 

Ok gentlemen I got a test drive in today. Largest jets I ordered for my ends was actually 67s....place I got them from didn't have 68s. First with 65s I was still 13.5 or higher at WOT. Went to the 67s and I'm right around 12.5 until about 4000 RPMs then it starts creeping up to the 13 range. Truck doesn't seem to bog at that range so I'm assuming I still need more jet and it's not a float level issue right? Floats are set to 23/32 currently I believe. I may just leave it as is for now and see what it does on a warmer day.

My numbers will be richer most of the season. It's only 50* out tonight here. Most of my show season driving is in the 70-90* range. Saw somewhere that a 20* change in temp is equivalent to about 1 jet size

 

For anyone still following this never ending story I swapped the wideband O2 over to the side with the backfire this afternoon. It's raining here so I didn't get a drive in but at idle once it was nice and warm the O2 reading seems to be the same as the passenger side. Doesn't guarantee I don't have a misfire but there was nothing glaringly obvious. If the weather is better tomorrow I'll go for a ride. Curious if the extra 2' of pipe will make a difference in the backfire as well.

Picked up an old engine analyzer yesterday too with a low RPM scale so I'll also attempt an engine balance/cylinder drop test one day soon.