Dizzy advance / retard highway backfire

[Boaf 32]

Fellow LC'ers,

I've had fuel and distributor threads , and reviewed 5 or 6 others on this topic of backfiring at high way speeds. I have an issue that has developed and would like your thoughts (@Dizzy @Pin_Head , @brian among others).

Symptoms:

1. At highway speeds or going up long hills at highway speed, the truck looses power and eventually backfires out the carb
2. Pulls fine at slower speeds without a hitch

Originally from the Previous Owner, the distributor had the Advance / Retard Octane Adjuster all the way forward. When stepping on the gas, it felt like a rev limiter was kicking in. Eventually when up to speed, that rev limiter went away. No problems on highway other than that.

Facts:

1. Backlash adjusted to spec
2. New fuel pump from SOR
3. 38 / 38 Weber
4. 150 PSI across the cylinders
5. Weber has proper jets per my altitude of 650 ft

It almost sounds that the timing is "retarding" when at highway speed for some reason and I am loosing spark. Perhaps that's why the PO upped the dial on the Octane Adjuster to maximum when I first re-installed the distributor. We've been through the fuel starvation issue as well and that does not seem to be the problem.

Ask:
Read in the threads that many run their LC's with only Mechanical Advance and plug the vacuum advance. I am trying that today and see what happens. Can someone help me to understand why a distributor needs to have a retard Vacuum and what happens if it is eliminated?

Thanks all.

Boaf

 

[Dizzy]

Vacuum retard allows the emission system to adjust spark timing based on manifold vacuum, engine temperature, throttle position, and speed of travel. Retard and advance systems are relative, they only have value in regard to the overall configuration (VSV, etc.), and function of the emission system, and, static timing and centrifugal advance of the distributor.

For an example of when vacuum retard would come into play is when your cylinder's flame front is rapid and efficient: a hot engine, low altitude and high compression, low octane gas, low engine load, and greater rpms creating more centrifugal advance. Ideal spark timing produces peak cylinder pressure when the piston has the greatest leverage relative to the crank angle, but not to the point where compression braking becomes diminished. Slightly retarded timing is employed by manufactures in their attempt to reduce NOx emissions, and keep peak combustion chamber temperatures from excessively wearing plugs and exhaust valves.

Advance and retard functions are deleted in our attempts to simplify the operation of a stock emissions configuration.

 

[Pin_Head]

Timing is deterministic, so it is not likely to cause your engine to spit back through the carb every now and then. Timing doesn’t change suddenly.

One of the main purposes of vacuum advance of ignition timing is to retard the timing at high engine load, low RPM conditions to avoid pre-ignition, knock, pinging.

The symptoms are more likely that fuel deliver can’t quite keep up with demand. Under the high load conditions, grab a gear and run at higher RPM, lower throttle conditions.

 

[Boaf 32]

Timing is deterministic, so it is not likely to cause your engine to spit back through the carb every now and then. Timing doesn’t change suddenly.

One of the main purposes of vacuum advance of ignition timing is to retard the timing at high engine load, low RPM conditions to avoid pre-ignition, knock, pinging.

The symptoms are more likely that fuel deliver can’t quite keep up with demand. Under the high load conditions, grab a gear and run at higher RPM, lower throttle conditions.

Thanks gents for the explanation. I am able to replicate a backfire at will with a lower gear and heavy throttle. Long suspected two potential sources of this issue of no power on the highway and backfire: Carb/fuel and Distributor (The only 2 original pieces on the motor) other than exhaust, rad and Water Pump.

From what I'm reading, it sounds like the distributor is doing it's job; the diaphram holds vacuum. The truck will now not go over 50. It just starts to drag then I have to let up. Come on fuel, where are you? I am getting the 8oz in 15 seconds of cranking.

The carb, the Weber 38 / 38 had issues when I got the truck. Incorrect Jets, broken off fuel adjuster.

The only last thing I can do to the carb before replacing it is replace the Passenger Side Idle Jet. It is a 45, the other is a 60. The seat that houses the Passenger side jet is smaller than the Drive side. See picture below. Not sure how to get a 60 that fits the Passenger side. @Dizzy suggested this. Phone call on Monday. I cannot tell by the photo if it will fit.

If that doesn't solve the problem, I may just go ahead and replace the Carb. If the needle was broken off, who knows where the carb was thrown or dropped. @Weber Sarge believes that the Weber is a knock-off and that may be. Also thinks that the rod on the throttles (base of the carb) may be allowing air in. Sprayed Carb cleaner at high RPM, and at idle, and no change in RPM.

Any other tests I can check for fuel delivery? All my lines are 5/16 ID with the exception of the hard-line from the tank to the In-line fuel filter.

Boaf

 

[RustyRanger]

Does it have points?

I had a situation on a old chevy v8 that was sort of similar. Drove more or less fine below 3000rpm. Flooring it or Trying to rev it over 2900 3000 resulted in pops and backfires and it was like I hit a wall at that rpm. I fiddled with everything and eventualy traced it to the cam that acutated the points was worn. My solution was to replace the whole unit. Problem completely cleared up. It was just worn out and couldn't keep timing correct at high rpm.

 

[Boaf 32]

Does it have points?

I had a situation on a old chevy v8 that was sort of similar. Drove more or less fine below 3000rpm. Flooring it or Trying to rev it over 2900 3000 resulted in pops and backfires and it was like I hit a wall at that rpm. I fiddled with everything and eventualy traced it to the cam that acutated the points was worn. My solution was to replace the whole unit. Problem completely cleared up. It was just worn out and couldn't keep timing correct at high rpm.

Eyep, that is a distinct possibility. At one point, I was able to drive on the highway (only 600 miles ago).

I believe that the only reason I was able to do that was because the Idle jet was two (2) turns out of the carb, and it was suppose to be snug, and also had a leaking gasket.

The distributor is on my radar also, but expensive. It is an old points distributor with a single vacuum for retarding and mechanical advance. One step at a time...

Thanks Rusty.

 

[samatulich]

Have you searched about vapor lock?
Backfiring through the carb is a sign of a lean mixture. You may try venting the fuel filler cap? Just crack it open and try to replicate the symptoms. I was having a similar issue and when I cracked open my fuel cap, I could hear the air getting sucked in to the fuel tank. I drilled a 1/16” hole in the cap and the problem has gone away.

 

[Boaf 32]

Have you searched about vapor lock?
Backfiring through the carb is a sign of a lean mixture. You may try venting the fuel filler cap? Just crack it open and try to replicate the symptoms. I was having a similar issue and when I cracked open my fuel cap, I could hear the air getting sucked in to the fuel tank. I drilled a 1/16” hole in the cap and the problem has gone away.

Thanks, I have a vent cap so even when removing it, no explosion of pressure.

Boaf

 

[Boaf 32]

Thanks gents for the explanation. I am able to replicate a backfire at will with a lower gear and heavy throttle. Long suspected two potential sources of this issue of no power on the highway and backfire: Carb/fuel and Distributor (The only 2 original pieces on the motor) other than exhaust, rad and Water Pump.

From what I'm reading, it sounds like the distributor is doing it's job; the diaphram holds vacuum. The truck will now not go over 50. It just starts to drag then I have to let up. Come on fuel, where are you? I am getting the 8oz in 15 seconds of cranking.

The carb, the Weber 38 / 38 had issues when I got the truck. Incorrect Jets, broken off fuel adjuster.

The only last thing I can do to the carb before replacing it is replace the Passenger Side Idle Jet. It is a 45, the other is a 60. The seat that houses the Passenger side jet is smaller than the Drive side. See picture below. Not sure how to get a 60 that fits the Passenger side. @Dizzy suggested this. Phone call on Monday. I cannot tell by the photo if it will fit.

If that doesn't solve the problem, I may just go ahead and replace the Carb. If the needle was broken off, who knows where the carb was thrown or dropped. @Weber Sarge believes that the Weber is a knock-off and that may be. Also thinks that the rod on the throttles (base of the carb) may be allowing air in. Sprayed Carb cleaner at high RPM, and at idle, and no change in RPM.

Any other tests I can check for fuel delivery? All my lines are 5/16 ID with the exception of the hard-line from the tank to the In-line fuel filter.

Boaf

View attachment 1901425

Took the carb off and I am coming to realize how right many folks are @Weber Sarge ! As I removed the carb, the gaskets were soaked with gas. As I wiggled the studs after the Weber 38/38 was off, I could see air bubbles between the first plate (resting on manifold) and the 2nd plate (where the PVC is attached). How the squirting of carb cleaner, soapy water did not reveal this is one of great mystery. You are right @Weber Sarge about the design and quality of those adapter plates. I'm sure there are other issues contributing like the Idle Jets.

Replaced the gaskets, High-heat gasket sealer applied liberally, and snugged down the carb being careful not to rotate the studs that can separate the plates. Will take a drive tomorrow and see what happens. Not feeling confident though. Will keep you posted.

Boaf

 

[Dizzy]

A carb rebuild might get crud from some of those passages clear if that is a problem. You had to replace the inlet gas filter, right? In addition to the accelerator pump, you have the power valve, which if the passages failed, could contribute to leanness.

Keep up the good work Boaf32.

On my pickup, it was the adapter plate studs that are a problem. The thread engagement was sloppy, you could wiggle the studs, so they got a heavy application of PTFE tape.

 

[Boaf 32]

Thanks Dizzy

 

[Martyt]

Wow, @Dizzy and @Pin_Head — good detailed info. I don’t have the issues that the OP has but love MUD for expanding knowledge about our beloved 40s

 

[Trollhole]

You're running out of fuel. Might check fuel flow into a bucket. Filter? Also a restriction in fuel line or in tank.

 

[Boaf 32]

Running out of fuel is correct at highway speeds @Trollhole . Pulled the carb, sealed the Weber plates with gasket sealer and did a short test run this morning, ran much better. Did a highway test and was limited to 50 MPH, like running on one barrel. It would backfire if I tried to accelerate beyond that. But there is a marked improvement so we are on to something.

I still have a 45 Idle Jet in the Passenger Side of the Weber 38 / 38. I ordered a 60 to match the driver side. This is the definition of function I fond on Idle Jet. Can the experienced guys chime in and give a definition here? I hope that when the new jet goes in with the larger opening, I can drive on a highway again. It sure looks it from what I read below.

Thanks,

Boaf

From Top End Performance:
Featured snippet from the web


The function of the idle jet is to control the progression between closed throttle and the main jet circuit. As such it is important to smooth progression between closed throttle and acceleration and for part throttle driving. ... Diagram of idle jet assembly for a Weber DCOE.

One more:
38 DGES:

If you have to open the mixture screw more than 2 turns on a 32/36 DGV or 38 DGES your idle jets are too small...If you have to shut them below 1/2 turn they are too big....Also the 38 DGES can be a little tricky because you are idling on both barrels at the same time. You have 2 mixture screws and they will not be set the same on most cars. This is because the plenum type manifold that they are on distributes fuel unevenly. By having 2 mixture screws you are delivering fuel from 2 places in the intake manifold. The Mixture screw closest to the engine will no doubt need to be in much farther than the outer mixture screw. This is Okay. I do not recommend Stagger jetting the idle circuit on a 38 DGES (In other words, do not use 2 different size idle jets even though you need to adjust the screws differently. This can cause part throttle dirveability issues.)


The carb is suppose to be synchronous.

Thanks T

Boaf

 

[Boaf 32]

Update:

Finally time to put this thread to rest. In summary:

1. Had a piece of Fuel sender unit gasket in the tank blocking the line. Removed and cleaned
2. Leaking Weber carb gaskets between the adapters. Sealed and gaskets replaced.
3. PO installed .45 Idle Jets, 145 Main and 170 Air's. All wrong for my altitude. 60's Idle, 155's Main and 180's Air installed
4. Test drive:

Thanks to everyone including among others: @Dizzy , @Coolerman , @Weber Sarge , this was an education in a humbling way. You think you are on top of it, and nope, need to step back!!!

Boaf!

 

[Dizzy]

Awesome!
If you have a sense of your before and after mpg, I'd like to know. It is nice to have accomplished something that many say isn't really possible, or worth doing.

 

[4x4veteran]

Had a distributor wear out on me years ago.
I was in our little tourist town when the dist started going out of time.
It was a points type and started back firing as I was going up a steep hill.
Got so bad it wouldn't pull the grade so got out and loosened the bolt and twisted the dizzy around until the engine smoothed out and drove a ways before it started back firing again so would move the dist around drive a bit more.
Did this for about six miles to home.
Later on looking at the police report in the weekly paper there were several reports of gun fire on the day I had the timing problem.

Symptoms:

1. At highway speeds or going up long hills at highway speed, the truck looses power and eventually backfires out the carb
2. Pulls fine at slower speeds without a hitch

Had the same thing happen on a trip.
Turned out to be a leaky fuel line allowing air to be sucked into line and starving engine when under load or at speed.

 

[Boaf 32]

Awesome!
If you have a sense of your before and after mpg, I'd like to know. It is nice to have accomplished something that many say isn't really possible, or worth doing.

At one point I was getting 7-9 with the 38 / 38. But that was with the leaking rubber Idle jet gasket. Will check but I'm thinking 12 - 15 max. Anything better than that is gravy.

Boaf

 

[Boaf 32]

Had a distributor wear out on me years ago.
I was in our little tourist town when the dist started going out of time.
It was a points type and started back firing as I was going up a steep hill.
Got so bad it wouldn't pull the grade so got out and loosened the bolt and twisted the dizzy around until the engine smoothed out and drove a ways before it started back firing again so would move the dist around drive a bit more.
Did this for about six miles to home.
Later on looking at the police report in the weekly paper there were several reports of gun fire on the day I had the timing problem.

Symptoms:

1. At highway speeds or going up long hills at highway speed, the truck looses power and eventually backfires out the carb
2. Pulls fine at slower speeds without a hitch

Had the same thing happen on a trip.
Turned out to be a leaky fuel line allowing air to be sucked into line and starving engine when under load or at speed.

I always thought backfire out the carb, timing. out the exhaust, ignition. Now I know better. Fuel delivery is also a possibility. Glad you worked it out.

 

[samatulich]

I’d say 12-15mpg is gravy. Good for you on figuring this out, have fun driving around.