Slight backfire and unable to preform under load

[Jaybird]

Good morning Everyone,

I bought a 1949 Plymouth deluxe business coupe that the owner couldn't get to start after driving it all summer. So after some work I was able to get the car to run but has a slight backfire and unable to preform under load. Went through timing, plug gap, breaker, and carb adjustment and can assume to rule those out.  I used a piece of paper on the tail pipe and it sucked in so from reading some topics on this forum I pulled the head to verify movement of the valves and they are moving properly. after getting everything back together I preformed a compression test, I get around 75-85PSI among the 6 cylinders. I did a tablespoon of oil down cylinder one and did another compression test and it went to 120PSI. So I guess what I'm asking is do you think I'm in need of rings or start somewhere else? All inputs appreciated. Thanks

 

SPECS:

217.8CI-6cylinder/ 6 volt system

cast iron block and heads with solid lifters

Carter BB1 carburetor 

[FarmerJon]

Did you adjust your valve clearance?

Did you inspect the valve faces/seats while you had the head off?

Exhaust getting vacuum makes me think that one or more exhaust valves are not sealing during the intake stroke.

 

Your rings may be weak too, but not likely to be introducing suction to your tail pipe. 

 

[soth122003]

So if I got this right, you got this car from some one else and it wouldn't start. You checked some things out and got it running. Now it has a slight backfire and will not run under load. By under load, I assume you mean it won't go past the idle speed when revving or driving? Your compression is good so leave the rings alone. 

 

From your comments it runs. If it won't rev, I'm inclined to think fuel. Carb or fuel supply. Check fuel pressure to the carb. It should be 3-5 psi constant. If it drops during revs I am willing to bet you have a going bad fuel pump or the line to the fuel pump has a pin hole or the 6 inch rubber line to the fuel pump is bad. I'd check the rubber line first. You also said carb adjustment, not cleaning or rebuild. Since it is running, this is the second thing I would look at.

 

You said you did the paper check at the tail pipe and it sucked in. That says that one or more of exhuast valves aren't seating properly. Though with those compression readings can't really be sure. Pull the plugs and use compressed air with each cylinder at TDC  and do a leak down check and listen for air leaks. Use about 20 psi.

 

That should get you started, but I am willing to bet it is fuel related.

 

Joe Lee

 

 

 

 

[Jaybird]

FamerJon- i did inspect them and didn't see any issues to the eye. I did purchase a service manual so I will go through and adjust the valves.

 

soth122003- Thought a possible issue with fuel related cleaned filter bowl and what not. Also tried adding some fuel while doing a high rev and still had a sputtering issue. I'm going to go through the valves to knock that out hopefully not to difficult of a procedure on these motors.

[vintage6t]

Sounds like a bad ignition condenser.  

[Jaybird]

Vintage6t- I replaced it with 3 others with same results. But would this also cause an intake at the tail pipe?

[Sniper]

3 hours ago, Jaybird said:

timing, plug gap, breaker, and carb adjustment and can assume to rule those out

 

Since you do not tell us what those are, then no we cannot assume to rule them out. 

 

I'm leaning towards needed a good carb cleaning and rebuild, but that assumes you did indeed set the above correctly.

 

Your compression readings tell me you don't have valve issues, either sealing or adjustment. 

 

Not sure why you felt the need to pull the head off, hopefully you put it back together right, including the proper torquing and retorqueing of the head bolts.

 

 

 

 

[Dave72dt]

Check the spring tension on the points and the rotor fitting snugly.  I've seen clips missing and the point's spring assembled wrong.

[Booger]

all great advice and all systematically going down the line... I think backfire I think timing, Its crazy but disrtibutor parts can be very specific

[soth122003]

Reading the new comments I'm inclined to go with the carb cleaning/rebuild. You can clean the fuel filter and float bowl and still have crud in the jets or tubes in the carb. If the carb does not deliver enough fuel, adding to it can lead to a flooding condition unless the fuel is atomized properly. I would start with the carb cleaning and if that don't work check the rest of the fuel system. The hit the distributor.

 

Joe Lee

[D35 Torpedo]

I think you need to perform proper preformed diagnostic measurements, and get back to us.....

[DJK]

Did you use a dwell meter while adjusting the points? Verify the wires within the distributor are not shorting to ground.

[greg g]

Sounds like me in the morning.  Have you verified the operation of your vacuum advance?  Is the distributor hooked up to the correct vacuum source? Did you do a vacuum gauge reading?  Did you verify proper fuel quantity test as outlined in the service manual?   What do the spark plugs look like? What spark plugs are installed.   Are your spark plug wires in good condition. Is the step up jet in the carb working as it should?

[Tom Skinner]

Just a thought. How about hooking up a Vacumm Gauge Test and determining what actually it going on?

[burrizwo]

Did carb rebuild, installed new firing cables, spark plugs, coil, distributor cap and rotor. Rebuild the ingnition distributor, installed Pertronix electronic ignition, adjusted timing. Did also measure the cyl. compression, adjusted the valves. Everything fine. Started the car and have/had exactely the same problem. Checked the fuel pressure , 0.35 bar (5psi). Removed air filter, opened the choke throttle and checked the function for the acceleration pump, by fast moving the throttle........no fuel. Removed the carb top, checked the acceleraton pump stamp (new), didn't move well, extended the spring for more pressure, installed it again and.....problem is gone.
In my case, missing fuel during acceleration created backfire and poor engine response.
Have to say, I never drove this car on the road before, still don't, is still in progress, bought it in Denmark 3 years ago and at this time the engine was running like sh**.

So I have also a question to all you guys. With the recent fuel (ROC 95) what is, I am sure, all of you are experts and very experienced, the best timing for a 251 cu Flathead 1948 Chrysler business coupe?

[desoto1939]

When you rebuilt the carb and I am assuming it was a Ball&Ball carb. when you replaced the two small ball bearing in the base of bowel did you put the correct one back into the correct corresponding hole.  These are small ball bearing but are different sizes. So this also might be the problem. When rebuilding a carb I always suggest tht you have several egg cardboard cartoons ready and then number each hole and then put each part as you take it off the carb into the next corresponding slot. So that when you go to reassemble the carb you are selecting the appropriate parts in reverse order.

 

Just a thought on your situation.  Do you know anyone that might have a condensers testing gage to determine if the condensers are holding the correct microdiodes. Een brand new condensers can sometime be bad.  I prefer the good old NOS Autolite condensers.

 

Also might spay some WD 40 along the intake and exhaust to see if the car runs better under load. If the engine runs better when doing this then you have a vacuum leak in that area.

 

Rich Hartung

[Tom Skinner]

Once again - Loosen your Distributor Bolt. Static Time it with the engine off. Start your engine. Set your timing using a Vacuum Gauge. 

While its running turn your Distributor until you obtain your highest Vacuum setting.

Tighten the Distributor Bolt. 

Regardless of whether you have ethanol gas or non ethanol gas this method will achieve your best timed setting.

You can also set your carb using the same method.

As for other problems that may exist - using a Vacuum Gauge Chart the Vacuum Gauge Hand should be steady.

If not read the Chart to determine what exactly the problem would be.

Good Luck!

Tom

[Tom Skinner]

Usually Advance Timing to 4-8 DBTDC Book Calls for 2 but new gas changes that somewhat, I use 87 Octane Non Ethanol Gas

[Tom Skinner]

One must bear in mind that gas was 76 - 78 Octane (with Lead) when these Buses rolled off the assembly line.

Somewhere in the range of 10 - 11 cents a gallon. I haven't really heard the coveted ping upon heavy acceleration

after setting my timing since gas was still leaded. Of course that was with my first 1948 Chrysler back in the 1970's.

Back then I timed my 250.6 cu. in. Flat Head by ear. Today I use a Vacuum Gauge and a Timing Light.

 

[greg g]

Did the vacuum gauge timing process as I have decked my block (,010)and milled my head (.040) for more compression.  After I did the preliminary  set, the timing light showed 6 degrees before TDC at idle.  Real life had some ping climbing long grades.  Rolled (retarded) it back to about 3 degrees,BTDC. Idle pulls 19 inches at apx.600rpm, ping is gone.  My vacuum is lower than factory due to dual intake.  Should be 20 to 22 inches with single intake, assuming a healthy engine.  This on 87 with ethanol.

[burrizwo]

On 11/17/2023 at 8:56 PM, desoto1939 said:

When you rebuilt the carb and I am assuming it was a Ball&Ball carb. when you replaced the two small ball bearing in the base of bowel did you put the correct one back into the correct corresponding hole.  These are small ball bearing but are different sizes. So this also might be the problem. When rebuilding a carb I always suggest tht you have several egg cardboard cartoons ready and then number each hole and then put each part as you take it off the carb into the next corresponding slot. So that when you go to reassemble the carb you are selecting the appropriate parts in reverse order.

During disassembling and assembling , I did it as you said. Additional, this is the big advance from new technologie, I made a video clip to remember. By disassembling I was wondering about the differnt sizes of the balls. In you tube you will also find video clips for Carter BB carburetor, made by Mike Carburetor.

Leaks on carburetor (throttle shaft) did I also checked with Break cleaner, but here you have to be careful, that the exhaust manifold is still "cold".
On intake manifold I never did, you are right, same proceedure, good idea, thanks.

[burrizwo]

Tom Skinner, Greg G, thank you for the input. My recent problem is, that I can't verify the engine speed, especially the idle speed. Ordered a tool to measure it, should be here in the next days. This will be the starting point for further adjustments. At the moment timing point is at 8° BTDC (timing light). What you call the "ping", if I understand right, is the knocking noise from combustion. With higher octane, the less knocking noise, or let's say, the range for knocking noise is extended. In Switzerland we have 95, 98 or 100 (Shell) octane.
Vacuum test, as I understand, is a must, but first........adjusting idle speed.
Vacuum, if I am right, has to be set (highest vacuum) at the idle needle from the carb when idling, next step, timing with timing light and vacuum tester, correct?

[Hickory]

Manifold vacuum, highest steady vacuum means the timing is good, on my car i marked 6 degrees in red 10 degrees 15 degrees with blue. Once i set rhe idle vacuum which came out around 6 with the gauge then i accelerate under hood and watch the vacuum advance and compared it the the book compared to the rpm and that may tell you how the mechanical advance and vacuum advance is working.

[burrizwo]

Hickory, you mentioned "on my car i marked 6 degrees in red 10 degrees 15 degrees with blue". The numbers and colors are given by/through the tester you are using?

[Hickory]

I did that so it was easier to compare base timing through advance timing with a timing light. I disconnect the vacuum advance and plugged it, going through different rpms compared the mechanical advance to the manual. Flyweights could be seized. Then at idle using a vacuum pump checked the vacuum advance for timing against manual and it gave me an idea if the dizzy is working close to what it should. You should start with static timing and the adjust with a vacuum gauge to get idle correct then work towards the advancing after.