Technical Chevy 235 backfiring on loaded deceleration.

Hi everyone,

So I’ve searched the forums a bit and couldn’t really find an answer to the issue I’m having so I figured I’d make a post about it.

I have a ‘53 Chevy Two-Ten that I recently put a 235 straight six from a ‘61 truck into. I have dual single barrel carbs on a Fenton intake, with dual Fenton cast exhaust headers attached to a dual exhaust. Exhaust has an h-pipe.

The issue I’m having is I’m getting quite a bit of backfiring out of the exhaust when I shift down and use engine braking. When the car is accelerating it’s fine and when it’s idlig it doesn’t have an issue. But when I shift down into 2nd from 3rd I get a lot of backfiring.

I have done the timing on it twice. I think right now it is at 5 degrees BTDC. As far as I know I have both floats adjusted right in each carb. And I don’t have any leaks anywhere that I know of. Any suggestions on what might be causing this? Any help is much appreciated, and I did search the threads but mostly just found topics about back firing out of the carbs.

I’m still very new to working on engines (this is my first project car and was my first engine swap) so I’ll try and provide as many details if I know what you are talking about haha.

 

1st thing that comes to my mind is “ backfire in exhaust “ is to rich effect , “ backfire in intake “ is to lean effect . May not be what your issue is , but many be worth looking into .

 

have you adjusted both carbs for air fuel mixture and idle sync? Why the H pipe?

 

Yeah I’ve read up on that a bit. Right now I have both mixture screws turned all the way in, and then backed out a half turn. I might try and back them out more and see what that does.

 

A vacuum leak can also cause this but if you have dual carbs, I'd lean toward a rich condition.

 

*Check your timing restarted timing can cause this.
*How old is your tune up? Bad wires, cap , coil points condenser can all cause this .
*running rich can cause this double check float levels etc, fuel pump pressure etc.

Have you set base engine timing, then set timing with vacuum advance disconnected , and with the octane selector at 0 then fine tune with octaine selector?

Little different advance on the 235 as the advance moves the entire distributor housing and not just the internals.

 

I’ve got the files synced up pretty well and as far as I know I have the air fuel set for both. I was told there is a tool you should use to measure the vacuum of both carbs, don’t remember the name of it, but I haven’t gotten to that yet.

I was told the h-pipe helps with equalizing the back pressure since I have separate exhaust headers.

 

Should be about 1 1/2 turns to start. You want to use a vacuum gauge to adjust the carbs. When turning the air fuel screws you want to get the highest reading, around 20, that you can. When I did mine, I adjusted each carb until I had a smooth idle and even putt putt out of the exhaust. My front carb would sound from the left pipe and the rear carb would sound out of the right pipe. That is why I ask about the H pipe. Why did you do that?

 

I've never heard of anyone using an H pipe before. I would do my best to get the two carbs adjusted and in sync before going too far. You have a new motor and it might just be a matter of working out the kinks. May be very simple to do. I've been there and got things just right after a little trial and error. When I adjust my idle screws on the two carbs, I can literally listen to the air being sucked into the open carbs and tune them to sound the same pitch and then in sync.

 

Distributor is good. Cap, wires, and spark plugs are all new, and I installed an electronic ignition so no points to deal with. I have the floats set to what the rebuild kits said to set them at, but if running two carbs should I reduce the amount of float since those insurrections are based on using just one carb? Or does that even matter?

I have the vacuum advance hooked up, but we still haven’t done the thing where I’ve read you plug the advance and set the timing. And the advance is plugged into only one carb, the front one, and the back carb advance is plugged.

 

You could put the advance to the center port on your intake. That would be the best place for it. I set my timing the old fashioned way. I advanced until a ping then backed off. I also watched the vacuum gauge when doing this and again got the highest reading I could. Still, there are times the engine will do want it wants and will run better by doing it the old way instead of the high tech way. As for the floats. Leave them as they should be according to the spec when re building them.

 

Is it ported or imported vacuum makes a difference.( manifold vacuum is a little more “ touchy” then ported, ported is more for emissions stuff)
Front or rear carb should not matter if it’s manifold vacuum.
Have you hooked up a vacuum gauge to see what the engine is doing ?


You need a carb synch tool. Or two vacuum gauges one on each carb and get them matched and dialed in

What carbs did you use?
Are they new ,rebuilt , matched?

 

Are you referring to the port that is in the middle right side of the intake manifold The port you hook the vacumm wipers into?

 

Back fire in the exhaust on decel is from cool air being drawn into the exhaust, colliding with unburned hydrocarbons that have collected in the exhaust causing them to ignite. This will occur with any engine, but is accentuated with long stroke undersquare engines like your straight 6. Tighten up any joints in the exhaust to help prevent this.

Re tuning the idle mixture screws, using a vacuum gauge as described above is one way, my preference is to use a tach. Not a tach inside the cab, a low speed tach intended for tuning which you will have setting out next to the engine while you tune it. The first thing you want to do is to set the idle speed, if the carbs are run in unison (not progressive), which I believe they are on your engine, then you really need a vacuum gauge to synchronize the 2 carbs. Once you have the carbs synced and the idle set, adjust the idle mixture screws by choosing 1 carb to start with, adjust the idle mixture screw in/out to achieve the highest idle speed (without touching the idle adjustment of either carb, you're just looking to achieve the highest idle speed that will occur from adjusting the mixture screws), once you've got it idling the highest it will you then want to screw the mixture adjustment screw back In until it achieves ~ 20 rpm drop in idle speed (this is the lean roll).

Now do the same thing with the other carb.

Once you've got both carbs idle mixture adjusted, go back and set the idle speed and sync the carbs one last time.

This should give you the smoothest idle and the best throttle response, and will minimize unburned gasses collecting in the exhaust. At the end of the day, some backfiring on deceleration is going to happen, it's just a matter of how much. The harder the decel, the more it will backfire; the more unburned hydrocarbons that collect in the exhaust, the more it will happen. Proper carb adjustment and tight exhaust joints is the best you can do to minimize it.

 

That is where I have my advance. Of course, I don't have wipers on my car. Still, I think it is the best place to get vacuum for the advance. As for the carb sync tool. I have one and found it inaccurate. It was older and never really gave me a good and steady reading. As I said before, I have perfect adjustments done by listening to the air rush into the carbs and getting the sound of the air to be the same pitch. It worked better on my carbs than the sync tool did.

 

I don’t know what ported/imported means in this context. I have both carbs on the manifold. The vacumm advance is going to the carb closest to the radiator and the other one is plugged. Their is a port on the side of the intake manifold, the vacumm wipers are currently plugged into that.

Have not gotten a vacumm gauge or carb sync tool yet to check these. Starting to realize that I maybe shouldn’t have taken on dual carbs as my first ever project haha.

The carbs are two Carter YF single barrel carbs. I took four different YF carbs to make two that all had matching part numbers. Everything i researched said you needed to make sure they were identical.

 

Blues4U has it nailed down straight. Follow what he said.

 

Don't give up or be discouraged. You'll get there. The YFs are good carbs. I had a matched pair on my motor first. I rebuilt them with kits that were obviously old and had been sitting for a while. They were not ethanol ready. meaning the alcohol ate the rubber parts and caused me major problems. I them swapped them for a matched pair of earlier Pontiac Carter carbs. and they've been fine ever since. If you try doing what has been suggested see what you get. It may just be a big change in the way your carbs work for your engine. Do the air fuel mixture screws and see how the engine reacts. Then try syncing them by ear for now. You can always fine tune if you get the tool.

 

Wow, I didn't realize how hard it is to find a simple analog tach/dwell meter these days. Here, you want something like this. This is a cheep model, but it will do the job just fine. Or go out on the bay and find a vintage tach/dwell meter.

https://www.amazon.com/Actron-CP760...00062YUUS/ref=cm_cr_arp_d_product_top?ie=UTF8



This dual vacuum gauge set up will work for syncing the carbs:

https://www.amazon.com/AlphaMoto-Vacuum-Carburetor-Synchronizer-Cylinder/dp/B004MSJ7C8

 

AND...Stop using your engine for brakes...

 

I thought all Chevy 6's backfired when you decelerated. Every one I ever drove did it, I thought it was just the nature of the beast. Maybe they were just all worn out.....

 

There's also this tool; been around for years:
https://www.amazon.com/Edelbrock-4025-Carburetor-Balancing-Instrument/dp/B00062YC2E

 

The mixture screws you mention are for the idle. I think.

Ben

 

What? you say your 235 backfires on deceleration? COOL! Isn't that what you want? I leaned out the idle mixtures on my 235 to make it backfire. I look for long hills that I can use 2nd gear to make that sound with. To me that's part of the mystic of the old Chevy 6's. Most of them, when tuned to book specs - will do just that.

 

Backfiring on deceleration is something we hoped for on a stovebolt in the 50's and early 60's with dual exhaust. Like waldo53 said it's a common result. I'm not sure you can ever cure it if you have good compression and use down shifting as a braking tool.

 

My 235 did backfire when downshifting. That was a very long time ago. A good 10 years or more. It has been running an HEI dizzy and has a pair of matched and well adjusted Carter carbs on a Tattersfield intake and exhausting through a pair of original Fentons and a true dual exhaust. No H pipe and no backfiring here now.

 

Air leak in the exhaust is the most common cause of backfiring under deceleration that I have heard of. Sometimes it sounds pretty cool though, but if it is a bit too much it can be irritating.

 

This should probably resolve your back firing. Heat your gas up a bit so you aren't sucking cold raw fuel into your hot headers when decelerating.



Header Heat Kit. Fenton Headers To Fenton Intake (235ci Only)
Item # 838573B
Weighs 1.5800lbs.

This is a heat kit. header to fenton intake, will also fit offenhauser intake. 235 cubic inch only.



This part fits most 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 Chevrolet Cars and Trucks.; exceptions may apply. Please read the complete description to see if this part fits your specific vehicle and application.

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Available from Chevs of the 40s and probably other supplers at about $46
This info came from another source.

Warren

 

That is probably more to stop carburettor icing by warming the fuel. For a guess.

By the time fuel has been thru the carb, thru the intake, thru the cylinder, past both valves, survived the combustion process, and arrived in the exhaust, it is probably nice and warm already.