Technical Distributor Rotor / Cap causing backfire?

Don't take this as a negative, but more as an amusing observation.

One of the main reasons for retaining carburetors has always been the argument that they are simpler to install and have none of those "technical" problems that non-carbureted electronic systems have. Lately there have been several threads where there have been difficult to diagnose problems with someones carburetor(s). The threads didn't have obvious (easy) solutions and became quite lengthy. This is the second recent one that I can remember where the problem was a slight backfire.
On top of that, most amatuer tuners then turn to newer electronic tools like an O2 sensor, Air temp sensor, and an Air Flow meter and even exhaust temp (via infra red gun) in attempts to diagnose the problem. Often its hard to determine if the problem is caused by the carb or the equally simple (?) ignition system.

And while I'm typing this, a friend just called to meet for lunch and told me his carb (rebuilt a year ago) is giving him problems again so he's just gonna get a new one to replace it. (Motorcraft)

Now, if I was running a carb on any vehicle, I would do the same thing and arm myself with the electronic dianostic/guage stuff when trying to figure out whats wrong............but I do find it amusing that so many die hard nostalgia buffs believe that carburetors are simple..........

 

Carbs aren’t simple. They are a complicated mechanical device that runs on vacuum and springs, with finely machined passages and holes, attempting to manage several different requirements. At best, they compromise to approach “good enough” for all of the engine’s needs. On a street car, you can’t ignore anything.

Modern tools like a wideband o2 just make it a little bit easier for the less experienced tuner to work with the adjustments. Reading plugs to try to establish what the engine needs is closer to magic than reading a gauge.

It’s possible, maybe even likely, that this is beyond my capabilities. I’m not done yet, though.

 

I think you are good at logically sorting out information, and won't rest until you find the problem and fix it. No doubt in my mind about that. I just think that many people have a carb that operates well for years, pretty much trouble free....
then when they do experience a problem they find out that locating and fixing the problem can become quite perplexing and even frustrating and becomes expensive as well. Best of luck in solving the problem.

 

If your hunting a surge at cruise the vacuum advance can cause that. Unhook it and take it for a spin or use a hand held pump and see if it will hold vacuum.

 

Do you realize that horsepower is a function of rpm? What's moving the car is torque. Isn't it funny under these high gear loads, that if an engine is prone to pinging, thats exactly when an engine will ping or detonate if you start going up a hill or decide to pass someone and try to accelerate? This low rpm, high load scenario is where problems pop up sometimes first...ignition, fuel...etc.

 

Both old and new vacuum advance cans work and hold vacuum when pulled in. Confirmed with vacuum pump.

It’s not a surge at cruise. It’s a surge in transition from cruise to main jets.

Cruise is fine. Runs lean (15:1). Goes down the road, no complaints.

Very slow acceleration from cruise is ok, too. Like when you can tense your foot muscle, but not your ankle, and get 50 to 100 RPM.

Slight acceleration, where you feel your ankle move, but not much. That’s where the surge is. It goes rich here (11:1 or worse). I think I want it less rich here, in the 12:1 range.

Hard acceleration to WOT is also fine.

So idle is ok, I think it’s right. Got good vacuum at idle. Cruise is ok, that’s mostly idle adjustment as well. Transition is not, which is where I’m hunting. Main jets and secondaries are ok, which I think implies that fuel supply (pump, lines, filter) is ok.

 

I spent most of my life troubleshooting and keeping track of complex information. I just don’t have the experience in carb tuning that I’d like to have tackling this.

I’m learning. That’s both fun and frustrating.

 

You were going to go for a drive and watch the vacuum gauge.. good. What is it doing at cruise in od? With the 1.91 gear any additional throttle application with show quite a change in vacuum with maybe starting to open the power valve. It doesn’t take much to open a 105 or even an 85. On every street engine I used a Holley on I ran a 65 as I didn’t want it opening at all in city driving.

 

You may have removed a carbon track when you cleaned the inside of the cap. The modules in those distributors are somewhat unreliable, too. Even new ones. I had to replace two of them on a trip to Alaska. One in Alaska and one in Canada. I had a Dodge 318 that I converted to use these modules and had several just quit in the middle of nowhere. They don't seem to like high heat. Some new ones don't work, out of the box. I started carrying spares.

 

Here is a run showing the a/f and vacuum:

https://youtube.com/shorts/XHQz_K5XT40?feature=share

The sound is mostly wind noise, but you can hear the backfire.

The power valve is a 6.5. I just pulled it out to verify it. The main jets are 65. Looking at Holley’s specs list, my carb (1850-3) should have come with 66 jets.

I noticed that if I keep the vacuum below about 5 on the gauge, it doesn’t do it. So it seems like maybe it’s the power valve or primaries. I don’t like how the gauge is showing rich. On different attempts, I saw it go as fat as 10.x:1. Mostly it’s in the 11.x:1. It doesn’t do it when it’s in the 12.x:1 range. I know it’s better to be rich than lean, but I think I’d like it to be a little less rich.

Looking at the power valve, there is nothing obviously wrong with it. It may be ok. A new one is $15 and a local place says that they have them in store, so I’ll probably just swap in a new one because it’s already apart and I need a new gasket for it anyway.



Plugs look ok to me. Seems to match up with cruise being slightly lean.

A compression test seems to show #8 has worn rings.

1: 152
2: 150
3: 145
4: 150
5: 150
6: 152
7: 150
8: 125 (150 with a shot of oil)

 

Also, I was surprised to see just how little throttle movement drops the vacuum so quickly. That was interesting. I expected mashing the pedal would drop the vacuum, but even the slightest movement and it’s down around 5”.

 

[QUOTE="David Gersic, post: 14884029, member: 261209"

. . . I’m learning. That’s both fun and frustrating.[/QUOTE]

That right there is probably going to get you where you want to go.

 

Actually, most carbs are quite simple! It is the understanding of what the specific engine wants the carb to do that is complex.

With the electronic whizbangs, the electronics remove the necessity of the understanding the engine (assuming the dude that programmed the electronics had the understanding AND wanted the engine to do the same thing YOU want the engine to do!). And EVERYONE knows the definition of "assume"

Remember that the old updraft carburetors from 100 plus years ago gave far less trouble than more modern ones! Why? The old updrafts often were at the bottom edge of the engine, and quite difficult to remove. Much easier to determine the real issue rather than just automatically blaming the carburetor! The more modern downdrafts sit right on top of the engine and are the first item(s) seen when the hood is opened. Of course its the carburetor!

Jon

 

You just keep on believing that, Jon. Just keep on believing.

Ben

 

...or possibly running rich, perhaps because it's the path gravity likes to send any liquid fuel in the manifold down into, so the excess fuel washes the oil off the cylinder walls. But that may be way off, I have no idea how rich or lean that cylinder is running, or the manifold layout.

 

Just in case you might be interested. Holley has a 2 day class on rebuilding its carburetors and tuning them. It held at the Corvette Museum in Bowling Green Ky. Includes a tour of Holley Factory and you can see the Corvette museum on your own time. Also Beech Bend Raceway Park is in Bowling Green as well. They are having two classes this year and the first one is sold out. The second one has a few seats left. Bad news is that its only a week or so away. Costs $425 I think. I assume they will have them again next year.

 

A good point. Intake is an Edlebrock Performer.

Plug says lean / ok.

#8 was 135 PSI a couple years ago going by my old notes.

 

It’s a mechanical analog computer.

 

I picked up a new 6.5 power valve this morning. Compared it to the old one, and there is a difference.

Old one, fully open:


New, fully open:


The power valve feeds through some restrictors in the metering block, so this difference may or may not matter. Holley doesn’t have a published spec on how much the valve should open. I don’t have a way to flow test the valve.

A couple of test drives with no backfires today after changing it. It’s not quite as warm out, mid 80s instead of mid 90s. So, maybe. Maybe not.

I still have a surge under part throttle acceleration. I always have, though, so that’s kinda normal. There’s one spot where it’ll do it. I’m considering trying more mechanical advance. I have three curves to choose from:


I currently have the silver springs, dotted line. Thinking the black ones may be better. There is plenty of fuel, air, and spark, so it may want timing. A/F meter is in the 11.x:1 range, vacuum is 4-5 in Hg when it surges.

 

I believe my hei has 22 degrees of mechanical advance.
Most back yard mechanics seem to advise for typical street cruising the sbc responds well to getting all the advance in before 3000 rpm.

 

Well, more advance got me some detonation, so back to the medium springs I go.

 

Crossfire between bad wires can cause both intake and exhaust pops.

 

Try one black with one silver , yea , you can do that , the spring tension is cumulative .

 

Accelerator pump shooters & cams control the amount of fuel & the timing & duration of the quantity .

 

If your vacuum is dropping below 6.5hg under light throttle , you're opening the PV ,that seems odd to me ?
At cruise rpm I would think your vacuum would be +-18" ? Light throttle shouldn't drop below +-10" ?

 

Can you delay (or slow down the pump activation of) the cam that operates the accelerator pump?

If the accelerator cam is activating the pump when its not yet needed, it could be causing your rich and exhaust backfire.

At 1900 rpm, you are past the idle circuits and into the transition ports on the carb, a too quick accelerator pump action could easily enrichen the fuel mixture in a quick time for long enough to dump too much gas.

One thought about the low compression on #8, my BIL had a 350 where the pcv was sucking the oil out of the cylinder, not directly under it but next to it, turned out the baffle on the valve cover wasn't quite right. The dry cylinder was scuffing the rings too fast.

 

@gene-koning , what is BIL ?

 

I’m probably going to try that, but it’ll be a couple of weeks. Or maybe sooner.

 

I tried going down a size on the pump shooter, but that’s not doing much once the power valve opens.

 

Cruise is ~15 in. Hg. The overlap on this cam isn’t helping my vacuum numbers any.

Light throttle, vacuum is 4-5 in. Hg.