MTFCA Forum

I am having some carb backfiring and wanted to see if anyone had insights before I start “trying” things to fix it.

Here is what I have: 26 engine,rajo BB head, Winfield SR down draft, AB distributor. Fuel is hand pump, set to 1.5 psi with an hidden regulator. Engine was built in the 70s, I rebuilt the Winfield and distributor this winter getting it back on the road. It’s timed properly, starts easy and idles nice but every once and awhile it will back fire through the carb. It might be one or two fires or it’s a series of them. Took it out for a long ride this weekend and it seemed to back fire when:
- if I forgot to pump the fuel and recharge the pressure while driving. Happened once and once I caught it, it stopped.
-going too fast in ford low
-40-45ish mph in ford high which is pushing it
- a few times it was just random

Once I was in ruckstell high doing 55+ (running buffalos), car is set up for high speed, it’s very happy going fast it seems and there was no back firing from the carb at high speeds.

My initial thoughts are the carb is running lean, maybe it needs more fuel psi 2-3 psi and a reset on the Winfield maybe?

Open to suggestions before I start making changes. Thanks!

It sounds like a lean condition.

First thing is to due, is to get a reading on the spark plugs. A lean running engine will show blistering in the spark plug center electrode. Check for intake system leaks causing your lean condition. The Rajo BB doesn’t run so well at lower engine speeds because the intake passages are so big and the mixture is moving to slow in the large passages. Another problem causing your intermittent miss could be a nothing more than a bad ground , a short, or a bad connection at your coil or at the points.

When I first read this I missed the part that you had a fuel pressure regulator . You could try upping the fuel pressure in one or two 1/2 pound bumps in fuel pressure to see if it helps. Does the Winfield SR accelerator work ok, or does it misfire when you rev the engine some, a slight fuel pressure increase could help.

Thanks, the carb does have a slight surge when idling. I could recheck that seal. Regarding pressure, I backed off on it last year from 2.5 to 1.5 due to a flooding issue but since fixed the carb flooding issue by mating the needle and seat better . Might be time to bump up the pressure. SR pump works well, doesn’t back fire when reving at least not lately. Regarding plugs, haven’t pulled for some time. Good idea to check them out again. I have old champion x plugs now but a NOS motorcraft set is waiting to go on once I get everything tuned in. Interesting about the rajo liking higher speeds, i suspected this but don’t have anything to compare it to. Regarding grounds, the electrical system was 50 yrs old and i replaced the harness with original style looms, cables and have redundant ground on the distributor, trans, and starter, along with a new coil. Condenser is 70s era but it worked better than the new one I bought. I think I am very close here and suspect there is more power to gain through adjustment. Are Winfields hard to tune? I have the manual for set up but any “tips” you have on setting up a Winfield I am open to. Pics of engine and car are attached. Thanks!

I am waking up a BBr Rajo touring that hadn't been run in 11 years.
I have found & fixed several issues, one matches your description closely.

"-going too fast in ford low
-40-45ish mph in ford high which is pushing it
- a few times it was just random"

After draining the old varnish, I drained 5 gals from another car. Noticed is was a bit cloudy.
Suspected bad gas, but I thought it was less than 2 months old.
I put in 5 gals of premium to bring up the octane. As soon as the fresh gas hit the carb she ran great.
So, as simple as it seems, is your gas fresh?

Valid point, yes I should be “good” on fuel quality. It’s a modified period tank (late teens/20s, if anyone knows the year/brand I would love to know) tank that is below the spare. The tanks was cleaned and recon inside. New filter and I have a gauge after filter and regulator to confirm pressure matches my dash gauge too. Running fresh ethanol free 89 octane filled in June maybe. I could double check the pressure on the gauge after the filter to confirm none of the recon coating has flaked off and clogged the filter. Good point!

An old condenser can act up when warmed up, causing bogs and misfires.

RecklessKelly wrote: ↑

Wed Sep 11, 2024 2:06 pm

An old condenser can act up when warmed up, causing bogs and misfires.

Yup. See if your spark is yellow. That's what bad cans do. Should be bright blue.

Great info, didn’t know that!

Hi Drew,
I write this information for what it may be worth in hopes it may help you resolve your issue. By the way, I live in Mechanicsburg, PA. I had exact issue of backfiring, sputtering, etc. on throttle acceleration with my Model T speedster. My car runs the earlier Type 31 Rajo crossflow head. Same type head Noel Bullock used to win the Pikes Peak race back in the day. I run a Winfield S, size C carburetor and high-tension magneto. My car has the same style stubby intake manifold as yours. Anyhow, I resolved this backfiring issue by using a heat riser from the exhaust header to the carburetor via overtop the valve cover. It worked for me.

Good luck,
John

katjack78 wrote: ↑

Thu Sep 12, 2024 4:01 pm

Hi Drew,
I write this information for what it may be worth in hopes it may help you resolve your issue. By the way, I live in Mechanicsburg, PA. I had exact issue of backfiring, sputtering, etc. on throttle acceleration with my Model T speedster. My car runs the earlier Type 31 Rajo crossflow head. Same type head Noel Bullock used to win the Pikes Peak race back in the day. I run a Winfield S, size C carburetor and high-tension magneto. My car has the same style stubby intake manifold as yours. Anyhow, I resolved this backfiring issue by using a heat riser from the exhaust header to the carburetor via overtop the valve cover. It worked for me.

Good luck,
John

John, I thought that was part of my problems. I bought all the pipe & tubing to add a similar heat pick up. After the motor gets hot, the backfiring & sputtering goes away. At least for now this summer. Winter driving might change my mind.
When I acquired this T it had a 97 on it. It was very prone to backfiring until hot & if giving a full throttle stomp. I figured I was over carburated & put an 81 on. Since, there is no issues with a full throttle stomp or any other throttle position when warmed up.(with good gas)
The plugs are an even tan.

This post woke my memory on a warning about intake leaks made by passed friend Hank Becker, cohort of Joe Gemsa.
Each intake port has a head mount bolt centered thru it. A leak can occur from under the bolt head. The fix, or idiot proofing, is to sleeve the bolt hole with a press fit of thin wall brass hobby tubing with flared ends to seal the potential leak. Thats one thing I did.
I little dab of ultra black under the bolt head would be good enough.

Winfields are finicky to moist weather - I run a BB RAJO w/Winfield SR B D.D. - would run like crap until I installed a heat pipe.

Before you make any changes, count and record the number of “clicks” each jet is from seated. This way you can put it back if you make things worse.

My experience is that they all miss and stumble a bit in low gear… unless you hammer it! Tool little load at too low an rpm with too much fuel. Heat riser may help to correct this. Popping out the carb is generally a lean condition, make sure the high screw is open about 1/2 turn more than the int screw. I have also experienced poor intake valve sealing causing popping out the carb.

Remember that the idle and pump screws work in the reverse of the int and high screws. Int and high screws turn counterclockwise to add more fuel, and the pump and idle screws turn clockwise to add more fuel. Opening the pump screw about 3 turns will essentially disable the pump.