Diagnostic Help Backfire through carb

[Green67Alpine]

Hi Folks,
Series V 1725, coughs and chokes intermittently under load (going up an incline on interstate) it's done this a few times, 3-4 times on the way to and from Columbia,SC from Atlanta and did it twice today on the interstate coming home from Roswell,Ga.
Heres what I have stock dizzy (although rebuilt by advance distributors) original coil, stromberg 150's, stock fuel pump,fuel pressure regulator.
Other than this problem the engine runs like a champ although it's "slow off the line" .
Any thoughts would be helpful.

Tom j

 

[Alpine66]

That's a haul from Columbia to Atlanta. My best swag from your description would be the coil if its original.
How old are the plug and coil wires?

Does anything else happen at the same time? My original ignition switch had a loose rivet on the back, lead to intermittent problems.

 

[Bill Blue]

Ignition.

Check out all the ignition wiring. Next, in no particular order, swap out capacitor, coil, ballast resistor (internal connections become corroded, leading to high resistance and weak spark), rotor (it can develop hairline cracks that allow the spark to ground through the dizzy shaft) and cap.

Bill

 

[65beam]

carb

why do you have a fuel pressure regulator with a stock fuel pump? carbs need fuel to get up hills.

 

[Green67Alpine]

why do you have a fuel pressure regulator with a stock fuel pump? carbs need fuel to get up hills.

After the carbs were rebuilt I had fuel spilling onto the exhaust, sent them back twice. Added the fuel regulator and that worked. I had similar symptoms of the backfire just before a valve dropped causing a head reworking that also has been a few years ago, I'm not looking for that again for sure.
Tom j

 

[Series3Scott]

Tom, I had this happen to me years ago before switching to Pertronix.
I limped the car home and went through everything I could think of, but finally noticed the hairline cracks on the rotor. Put on a new rotor and bingo, problem solved. Hopefully it will be that simple for you.

 

[chazza]

I had similar symptoms of the backfire just before a valve dropped causing a head reworking that also has been a few years ago, I'm not looking for that again for sure.
Tom j

I was going to suggest a valve problem, possibly a valve not closing properly.

The reason I was going to suggest it, is that I have the same problem on my S1. I have replaced the ignition system with a Megajolt one, which improved it slightly. Re-jetting the idle jets on my DCOE's made it better as well, but I can't get the valve clearances quiet.

Might be time to inspect the camshaft and followers!

Let us know how you get on,

Cheers Charlie

 

[Acollin]

Backfiring through the carb can be caused by many things-- some of which have already been mentioned.

What you don't want to do is chase a problem or throw parts at it , you want to diagnose it.

This type of backfire can also be cause by fuel/car related issues. This will often leave a telltale signature on the spark plugs. Pull you plugs and see what the look like. Reading the residue will reveal a lot. Most manuals related to just about any car or motorcycle will give you a series of pictures that will suggest causes for the way your plugs burn fuel.

Also: check your timing. Back firing through the carb can be the result of the spark not happening at the correct time.

There are a few simple tests that can reveal a bunch.

Good hunting

 

[husky drvr]

Just a thought, check to make sure the vacuum advance is working properly. If the diaphragm is ruptured, you have no ignition advance and a vacuum leak occurring just as you start pressing on the throttle.
It tends to cause fuel economy to suffer as well.

HTH

 

[RootesRooter]

After the carbs were rebuilt I had fuel spilling onto the exhaust, sent them back twice. Added the fuel regulator and that worked. I had similar symptoms of the backfire just before a valve dropped causing a head reworking that also has been a few years ago, I'm not looking for that again for sure.
Tom j

What psi is the fuel regulator set to? As 65beam wrote, Strombergs shouldn't need one. You might be running too lean - not good for the engine.

 

[beamdream]

Whilst all of the points above are valid I too am curious as to the need for a regulator with a standard fuel pump fitted.

You explain that there was an over abundance of fuel and the regulator cured that, I wonder if in fact the problem relates to the spacer between the pump flange mount and block, if it is too thin then the stroke of the pump is increased and flow becomes too great, so it might be worth looking at.

With this in mind is it possible that your regulator is restricting fuel flow and the engine is simply starving, this might also account for it being slow off the line.

As mentioned elsewhere, pulling the plugs particularly after a hard run can tell you a lot, if they are a pale white color this would indicate that the engine is running lean.

 

[Green67Alpine]

Ok, Thanks Guys I've got a lot of work to do in checking these ideas out. I will report back with any results , although my problem is very intermittent, not every time it's driven.

Thanks again, I'm on to start checkn' it out.

Tom j

 

[Green67Alpine]

starting with the easy items here's my plugs. these are fairly new, less than 2000 miles is a guesstimate

 

[chazza]

They look to be too lean to me - mid-tan colour is ideal,

Cheers Charlie

 

[tom o]

From your picture, it looks like you are running to lean.
The plugs should be a tan color. If you are using Stroms, give each mixture screw a 1/8 to 1/4 turn out. They are under the float bowl.

 

[Acollin]

I agree that the plugs suggest lean--- which is one of a number of causes for backfiring through the carbs. I would consider beamdream's notion that the regulator may be restricting the flow. Many in this discussion believe that a factory stromberg set up should not need a regulator.

If I were sorting this problem on my own car I would reassess the issues relating to why I thought the regulator was a good idea.

The regulator may have gotten you back on the road, but may also be masking the real issue/problem.

Can we get some more detailed information as to why the regulator made sense?

Be well all

 

[Acollin]

I went back through the thread and read that fuel was dumping onto your hot exhaust.

From where was the fuel leaking? One carb or both? A line? the union?

Is there a fuel filter between the pump and the carb splitter?

Be well
Andrew

 

[beamdream]

starting with the easy items here's my plugs. these are fairly new, less than 2000 miles is a guesstimate

Great photo, a prime example of using the plugs as a diagnostic tool, and yes, that is exactly what I expect to see with lean running.

Leave it running lean long enough and you will have some burnt exhaust valves

 

[Green67Alpine]

Thanks for all the input.
The story on the fuel regulator, I had been having a few tuning issues with the carbs so while doing some other work on the car I sent them off to be rebuilt, upon the completion of whatever I was working on(don't remember now but it had nothing to do with the fuel pump)) we installed the newly rebuilt carbs and got it running, great, except now we have fuel coming out the vent holes onto the exhaust, it took a while to figure out were it was coming from, readjusted the floats,sealed the bowls up (overkill) and it still leaked, sent them back to the rebuilder he reworked and tested them asked what the fuel pressure was I told him the carbs were fine before and we did nothing to change or fiddle with the fuel pump,got them installed and same thing, sent them back (rebuilder was great ) he tested and retested them again and could not find any problems, told me I needed to check the fuel pressure, we did and some how it had become too high for the rebuilt carbs, added the regulator a Holley 1-4 psi output can't remember what we set it at,(1.5 I think) the problem was solved. Since adding the regulator I've driven to the Invasion in Winona, Mn, Columbia, SC quite a few times, Greenville ,SC quite a few times, Hilton Head, SC once and countless trips around the North Georgia Mountains.
Thanks for taking the time to read our saga. I'm checking all possibilities, a little slow and starting with the easiest first.
Did I mention how intermittent this was ? Kinda like the car was sneezing.

Thanks again, please don't stop suggesting things.

Tom j

My fuel filter is between the pump and the regulator, the filter in the pump is clean and has no "black stringies"

 

[RootesRooter]

I've dealt with several Strombergs where the inlet valve was worn. When running you can see not just the normal spray of fuel when blipping the throttle, but a small river of fuel pushing past the jet. But I've never had one fill up the bowls so much that it came out the overflow. Could your shop have been working with the wrong float height specs? Did a float develop a leak?

I'm glad your Alpine has run well over those many trips, but that's no guarantee that you're not on the verge of burning a valve or even melting a piston.