Technical Revisiting exhaust backfire on decel issue

I'm not saying mine is running in the perfect air fuel mix range 'cause I don't have O2 sensors so I really have no idea what the actual A/F mixture is. That said, it pulls 15mpg around town, and 20ish/+ on the highway. It's not fouling plugs. Exhaust smells like exhaust (not eye watering rich). Starts well, idles great, pulls strong when the dumpers are called in to play. No backfires on deceleration, no lean pops (unless its's cold) etc. No ill manners. It took me a couple months of playing with it to get it where it is...I kept telling myself, hot rodders worth their salt have been able to make them work for the past 70 or so years...as has GM (Olds, Cadilac, Pontiac and Chev) Ford and Mopar to name a few. It may not have the same power as a current 4 barrel with a modern manifold, or a pair of 4 barrels...but I like it.

 

I appreciate the words of encouragement. I'm getting maybe 11mpg highway right now so if I somehow saw 15 I'd be happy.

 

If you change to Canadian gallons(imperial) your mileage will increase 20%

 

OK - you want the "look", and are maximum power is not a concern.

How about pulling the 307 carb from the center, and replacing it with a Chevy 400 Rochester 2-barrel. Now, place block-off plates under both rear carbs, dial it in on the center carb alone, and enjoy while you comptemplate your next move. With all of the time you have spent, I doubt anyone here will accuse you of being a "poser". If they do, who cares, its your truck.

With the cam you have, and the RPM you want to run; it may be necessary to install the idle air bypass holes in the throttle plates, and also to modify the power circuit. The idle air bypass holes won't hurt even if not needed. But test for idle vacuum before modifying the power circuit. If you have 13~14 inches of more, leave it alone. If necessary to modify, DON'T clip the coils; this is an old wives tale (actually, an old-husbands tale, old wives are smarter than this ). Spring kits are available.

Some on this thread have suggested that the carburetor(s) may not be the cause of the backfire. I don't know, but I do know the tripower just adds an extra degree of uncertainty. Once you get the engine running on a single two barrel that is sufficently large to run the engine, you can, if necessary, do other tests.

EDIT: it just occurred to me; I believe you have posted idle vacuum. If you install the larger carburetor, it should change, perhaps significantly. Check it again.

Jon

 

This reminds me of my cousin buying a brand new 63 Galaxie with a 390. When he let off the gas it would backfire, just 1 pop, the ford garage was going crazy working on it, after many visits they finally took the intake manifold off and there it was, bad gasket= intake leak. Take it for what its worth but i remember it well,

 

I've tested all around the outside with carb cleaner and propane with no luck. I need to put a vac gauge on the dipstick tube still to check if it's possibly internal. This engine has the hard plastic FelPro intake gaskets with the rubber inserts molded in so I'd be surprised if it was blown out.

 

Ok I'm back from vacation and back on tuning the center carb today. First thing this morning I installed a set of "factory" IFR's that I opened up by about .001 to .0292. Truck starts and idles fine and the idle adjusting screws offer plenty of adjustment. I have it adjusted to idle right around 14.0 AFR currently. I've tried 3 different jet sizes in it today...62, 60, 58. 58's seem to give me the best general AFRs but create a bog. My cruise AFR on the highway at 65 is right around 13.8-14.1. I could live with that I think. On easy acceleration I'm in the 13-13.5 range. When I'm cruising at about 1500-1800 and get on it a little more aggressive I'm getting a lean bog...AFR goes into the 16 range or so. Once the RPMs reach about 2200 it clears up and goes back to the 13's. The accelerator pump appears to be working fine. Fuel spray starts with the lightest bit of throttle.
I'm assuming this 1800-2200 range may be in the transition between idle circuit and the main jets right? I believe the 2G has a port that feeds the transition slot...does this need to be opened slightly? Will that in turn hurt my cruise RPM though since I'm right in that area at highway speed?

 

You can get larger discharge nozzles and adjust for a larger pump shot, was that cam degreed in? Maybe too far advanced I don’t know, going to scavenge some unturned fuel into those headers but maybe intake valves opening a little soon ? Maybe some experts will chime in

 

Seems like you need a bigger shot from your accelerator pump. I'm not familiar with your carb so I don't know what the fix is. For example, is the AP adequate but delayed?
EDIT: I'm a slow keyboard jockey. So what Heavy said above. ^^^

 

Sounds like good learning and good progress.
Rather than richen the center carb, is it time to add a second carb to see how idle and cruising afr are affected?

 

So I did a bit more playing around. This carb is specd for a 1971 307 when you look up the number. When I adjusted the float last week I set it for a 1971 350 at 25/32. The setting for a 307 calls for 21/32. I just worked my way down to that and my bog decreased substantially....almost not noticeable. It did however bring my cruising AFR from the 13.8-14.1 range down more like 13.5. I know they say roughly a 1/32 change in float height is the same as 1 jet size. I'm wondering if going up to 59s and dropping the float back down would bring my cruise AFR back to the 14 range while helping the bog. Any thoughts?

And I did think about bolting the end carbs back up without fuel and linkage next to see what it does. Just wanted to dial this in a hair more.....before I change it all up hahah

 

Engine is a brand new crate motor from GM so I didn't degree the cam myself. I'd "assume" they did but you know what they say....

As for the AP shot....I know I currently have to slightly move my pump arm up to fit it into the throttle bracket so I'm losing a little bit of the pump stroke. I'll see if I can find the spec and then try tweaking it a little. Maybe a little extra volume will be enough to help that stumble.

 

They did not have ethanol in the fuel in 1971 ,so I am not sure it is the same.is this a 5 spd?

 

Yes it has a modern NV3500 in it.

I just checked the accelerator pump and it definitely wasn't set to spec for a 350. I probably gained another 3/16 or so of pump travel....it had lots of preload in it originally. Still sprays with the lightest movement. Maybe tomorrow night I can take another ride and see how it feels. If the bog is better I'll move the float back again.

 

hm, my brothers chevy does something like that, whenever he lets off it isnt a smooth engine brake, it rumbles and slows way down.

 

What gear does it bog in?

 

I believe 2-5. Off the line I don't really notice it.

 

Back to it again today. I set the float back to the original spec I was at a few days ago...25/32. With the pump arm adjusted it helped a bit. Then I talked to someone who suggested adjusting my timing some. I swapped advance springs to bring timing in a little faster and it helped that bog if I'm cruising in 2nd gear. If I'm in 3rd or 4th at that same RPM range and punch it it still bogs a bit but 2000 RPMs may just be too low for the load in that gear and speed. I tend to chase perfection all the time and I don't think that's possible with this setup. My acceleration AFRs are in the 13s and cruise is right about 14. I'm going to bolt the rear carb back on later without fuel and see what it does to my numbers. Front throttle body needs a little work to seal it up better before I reinstall that one.

Still backfiring though

 

What rear axle ratio are you running?

 

Early Bronco rear with 3.50's

 

Put the rear carb on before and adjusted the idle screws and went for a ride. Everything else seemed to stay the same. May still be a little rich as cruise AFR didn't change at all. I put the front carb on and I have it adjusted to idle around an AFR of 15 now. There's no adjustment left unless I try opening into the transition a bit more and readjusting or drill the IFR's another 1 or 2 thousandths but I feel like that'll change my cruise AFR. Lean idle should be fine....it picks up as soon as you touch the throttle.

It's also like 15* warmer today than 2 days ago when I was testing so could explain the slightly richer readings. I'll go for a ride later with the front carb also bolted up and see what I have. My acceleration numbers are still in the 12.5-13.5 range which I feel like is good. May order 1 jet size smaller just to see what it does but again I'm chasing perfection on an imperfect system.

Of course I still need to address the original backfire problem more. I have an extra plug wire that I'll swap into each spot on the driver side and test drive. Also may try to get a dwell/tach to see which cylinder seems to be the misfire for sure but I'm pretty sure it's #1.

 

Just got a decent drive in with both end carbs bolted up with no fuel. I'm fairly pleased with everything. Idle AFR is right around 14.7-15.2 but screws are maxed out. On normal acceleration I'm in the 13.5 range and on harder it drops to about 12.5 as I get into the main jets more I guess? Right now cruising at 40mph in 3rd gear I'm right at about 14.5 and at highway cruise of 65mph I'm 13.7-14. Still have that slight bog but it's not terrible.

I'm thinking of opening the IFR's just .001 more to bring back the slightest tunability with the idle screws. Does that seem to make sense? After that I'll put fuel back to the end carbs and as long as nothing seems to get out of wack I'll attempt to tune the end carb jets for WOT.

Driver side still backfiring and I swear at idle I can feel/hear a misfire still. Also seems to smell richer at idle on that side...possibly from unburnt fuel. Next week I'll also throw new plugs in all around once my tuning is done and test the plug wires on that side. If that doesn't solve it I'll bolt my test pipe on that side and see what the AFRs look like. If it's misfiring I should see leaner numbers as there will be more unburnt oxygen in the exhaust I believe.

 

You'll see richer number's because of unburned FUEL.

 

You sure? Oxygen sensor measures oxygen, not fuel. I'd assume unburnt fuel and unburnt oxygen shows lean on an AF meter.

 

An oxygen sensor smell's fuel and convert's it to AFR.

 

Apparently there is a ton of debate about this topic online. It does in fact smell the fuel mixture but my understanding is it reads the oxygen level and not hydrocarbons or gasoline....so if there was a misfire there would be oxygen in the exhaust that wasn't used for combustion right? Either way if I have a misfire I expect some sort of AFR swing....whichever direction it may be in haha

 

An 02 sensor monitors oxygen, incomplete combustion (misfire) will send excess oxygen out the exhaust, it will think it’s lean.

 

^^^^^this

 

Been swamped with my regular job, yard work, and a customer job in my resto shop but managed to open the IFR a bit Monday. I went from .0292 to .031 as that was the next size in my pin drill kit. It definitely brought back the adjustment in my idle screws. Before I was 4 full turns out and still idling with an AFR of 14.5-15. Now after letting it warm up good today I'm only about 2 turns out now to get an AFR of 14ish. I didn't get to take a test drive today because I was filthy and didn't want to get into the truck interior. Hopefully the adjustment didn't throw me way rich again on part throttle and cruise....we'll see. I did order a set of jets 1 size smaller that are coming tomorrow just in case for testing. Currently I have 58s in it and it was pretty close. Hopefully by Friday center carb is dialed in as good as its going to get. Then I need to just mess with the jets in the end carbs for WOT. Currently there are 62s in them and I have a feeling I'm going to be pig rich when they open up but we'll see in a few days when I add fuel back into them.

 

When was the last time you checked your distributor cap ? Carbon tracking or another issue causing backfire you been dialing in the carbs and popping still there on one side, I think ignition related.