74 620 backfires out the carb

[sanyantho]

So, I recently drove my truck 25 miles into town to get an exhaust leak fix and also to plug a hole in EGR passage. I have a new Weber 32/36 with the smog stuff blocked off, but I have a hole on my EGR passage. So there would be a small exhaust leak from the metal line that comes out the header and into the EGR passage. After fixing both the exhaust leak and the hole, my truck backfires out the carb, bucks, and stalls a little when I give it gas. It idle with no problem. This would happen on occasion before I got the exhaust leak fixed, but now it is a lot worse than before. As soon as I drive off, it would start backfiring from the carb. I was hoping by fixing the exhaust and plugging the hole in the EGR passage it would fix this backfire issue, but It is ten times worst than before. Does anybody know what I should check next? Do I have to readjust the idle speed and fuel mixture after fixing the exhaust leak?

 

 

L18

Weber 32/36

Timing good

new spark plugs, wires, cap, rotor, ans points

Valve adjusted

Firing order correct, but I will check it again if its counter-clockwise 1342

[soundboi]

my l24 occasionally does this, usually when i stomp on the x. I slowly get into it now and it works fine.

carbs are that way i guess, but urs def seems to have something wrong. Hows ur spark like?

[datzenmike]

EGR should add inert exhaust gasses, these are gasses that should be without free oxygen in them. The inert gasses do nothing in the combustion chambers except take up space. This then should reduce slightly the combustion temperatures depending on the amount added.

 

If you have a leak in the pipe, oxygen might get in and this would cause and imbalance of the carefully mixed gas and air ratio of the carburetor. More oxygen means running leaner. Sealing the pipe would not cause what you describe, if anything it would run better or no change.

 

 

 

Can I assume that the backfire is out of the carburetor? If so two things...

 

1/ With the engine off and air filter removed, look down the carb and have someone pump the gas pedal. What you should see and hear is a strong squirt of fuel into the valve cover side of the carburetor. If week or not there the accelerator pump is worn.

 

The carburetor has an idle circuit with it's own fuel supply and mixture adjustments. It's only for idle and is much too small for driving on. For this there is the primary barrel which can handle much more gas and air mixing, but to transition from slow speed to primary use the the air is moving too slowly in the primary barrel and won't suck the gas in properly. To remove this suddenly lean condition, a mechanical 'accelerator pump' is used to force a rich fuel air mix to get past this position. 

 

Without it working properly the engine will go lean and stutter and struggle until the air speed does increase sufficiently. Naturally driving away slowly works best and flooring the pedal works the worst. You might also notice that going around a slow corner in town in second and stepping on the gas too fast does the same thing.

 

The backfire out the carb is likely a 'lean backfire'. What happens is the gas and air mixture is so lean that when compressed and lit it takes a long long time to burn and is still lit when the intake opens. This ignites the fuel in the intake. A proper mixture burns faster and is finished burning almost instantly. 

 

2/ The accelerator pump may work properly but if the primary jet is part blocked the mixture will be leaner than it should be. This will lead to severe lack of power and can backfire if the gas pedal is pushed down. Here you will need to remove the jet and physically clean it.

[sanyantho]

Hi Mike,

       The Weber carb is brand new, would the accelerator pump go bad that quick? I also installed a new fuel filter, would something still get into the carb to block the primary jet? It was running pretty good until I fixed the exhaust leak and plug the hole in the EGR passage. Thanks

[datzenmike]

Pull the hose off the EGR valve. This will disable it and eliminate it as the cause of anything. (shouldn't matter)

 

 

Weber carb? Generally it would be unlikely that the accelerator pump be bad or jet would be plugged on a new carb. Possibly something got in or was flushed loose between pump and carb during the change. Then there is an adapter plate and two (at least) gaskets that are possible vacuum leaks. Perhaps remove and check that they are sealing properly.

[banzai510(hainz)]

idle jets can get plugged if you have rust dirt in tank. had this happen clean and in 5 mins it was plugged with rust again.

 

On my 521 I run the stock filter and a inline one before the fuel pump to prevent this.  main jet is harder to plug but possible.

 

accle pump just cycle the linkage and see if gas is squirting on main barrel side. if not diapram is ripped and usually leaks gas on outside(stinks)  if not leak maybe the bowl doesn't have gas in there and that's why the accel pump isant working. the needle valve has a issue

[sanyantho]

Hi Guys,

      Thanks for you advice, I will follow your directions when I work on her tomorrow. I think I may have found the problem tho. When I removed the EGR passage unit yesterday and plugged the hole, I forgot to add a gasket maker sealant when I reinstalled it back. There is a chance that I may have some vacuum leak from that unit. That has to be it, because that is the only thing I messed with before it started backfiring so bad and so often.

[sanyantho]

Hi guys, so was correct. After cleaning the gasket and applying new sealant to the EGR passage unit, I was able to drive off without backfiring and bucking, Thanks guys!

[sanyantho]

Hi Guys,

        My truck stared backfiring and bucking again this morning. Found a crack in the the brand new dizzy cap, can this be the problem?

[banzai510(hainz)]

yes maybe. or condensation also.

wrong coil installed in still running a points ignition. with no ballast resisitor* the white block next to coil

[sanyantho]

Hi Hainz,

       I running points with stock coil. I will take a picture. Thanks

[banzai510(hainz)]

also cheack for waer in the dist shaft if really loose. side to side can cause the points to open up went you dont want it to

[sanyantho]

Hi Guys,

       So I replaced the cap and checked that there is not water in the dizzy. She is still backfiring and bucking still, now mostly on first gear, when I start to drive off. I inspected the fuel filter, I can see that there is dry grass from my farm stuck in there. I also see air bubbles in the fuel filter when the truck is on and idling. Is this normal? Could it be that i have a hole in my tank. I did a quick visual and i could not find anything. I have not drop the gas tank yet to check further. Please help, I am dying here. :crying:

[datzenmike]

 

1/ With the engine off and air filter removed, look down the carb and have someone pump the gas pedal. What you should see and hear is a strong squirt of fuel into the valve cover side of the carburetor. If week or not there the accelerator pump is worn.

 

The carburetor has an idle circuit with it's own fuel supply and mixture adjustments. It's only for idle and is much too small for driving on. For this there is the primary barrel which can handle much more gas and air mixing, but to transition from slow speed to primary use the the air is moving too slowly in the primary barrel and won't suck the gas in properly. To remove this suddenly lean condition, a mechanical 'accelerator pump' is used to force a rich fuel air mix to get past this position. 

 

Without it working properly the engine will go lean and stutter and struggle until the air speed does increase sufficiently. Naturally driving away slowly works best and flooring the pedal works the worst. You might also notice that going around a slow corner in town in second and stepping on the gas too fast does the same thing.

 

The backfire out the carb is likely a 'lean backfire'. What happens is the gas and air mixture is so lean that when compressed and lit it takes a long long time to burn and is still lit when the intake opens. This ignites the fuel in the intake. A proper mixture burns faster and is finished burning almost instantly. 

 

2/ The accelerator pump may work properly but if the primary jet is part blocked the mixture will be leaner than it should be. This will lead to severe lack of power and can backfire if the gas pedal is pushed down. Here you will need to remove the jet and physically clean it.

 

I know it's a new carb but these two are easy to check. You likely have a lean backfire.

 

Check the valve lash again. A tight intake will backfire.

[sanyantho]

Hi Mike,

          I did check that also today. With the engine off, the squirt is there but weak. Is the accelerator pump worn? But with the engine running, pulling the throttle with my hand and looking down the carb, the squirt is strong. What are the steps to fix this? Air bubbles in the fuel filter, is that normal?

[datzenmike]

There can be a large air bubble in the filter. Normal.

 

If it's bubbling... that's different. Try tightening the clamps it may be sucking air in. This shouldn't cause a backfire unless the carb has run dry.

 

To find out if the carb is running dry take it for a drive and hammer it good. When it begins backfiring you want to stop the engine as fast as possible so it doesn't refill the carb. So get the ignition OFF and the clutch IN and out of gear. Naturally do this is a safe location and coast safely to a stop. Now go look... just realized you have a weber carb. Well if you can remove the top cover on the carb. Is the carb empty? Maybe the float is set too high and you're using the gas faster than it can get in.

[sanyantho]

Hi Mike,

     So you dont think its a worn accelerator pump? How do I adjust the float?

[datzenmike]

If it backfires when you step on the gas. If when driving along under load, no.

[sanyantho]

Thank you very much Mike! I will take the top cover of the carb out tomorrow and check and adjust the float. :thumbup:

[banzai510(hainz)]

you replace the condensor. Its just a guess.

 

this is a simple fix but if you dont hav th extra parts its harder.

I always replace the dist first as I have spares. then look at the carb. which i also have a spare.

 

otherwise I ck the mechanical and eletrical timming first. if good I check above.

[sanyantho]

Hi Hainz,

     I have not replace the condenser, I don't have extra parts. I may have to buy a new distributor,

 

Hey Mike,

    I took the cover off the top of the carb to check and it was running dry. So I adjusted the float level, it running correct now. But that did not stop the backfiring out the carb. In fact, it seem to have gotten worst. Now while I am idling, and I push on the gas, it starts to backfire. Before it would only backfiring as I take off on first gear.

 

I am going to check the valve lashes again, as you suggested. Anything else you can think off? Thanks so much for your help everybody!

[banzai510(hainz)]

if distributor is tight then its fine.

sometimes the condenser isn't held tight by the screw grounding it or wire loose or broke.

 

sometimes esp if a stock carb they just about wore out by 35yrs in use

[sanyantho]

if distributor is tight then its fine.

sometimes the condenser isn't held tight by the screw grounding it or wire loose or broke.

 

sometimes esp if a stock carb they just about wore out by 35yrs in use

The distributor is tight, no side to side movement. I will check the condenser. I have a brand new weber 32/36 installed. Thanks

[sanyantho]

Then there is an adapter plate and two (at least) gaskets that are possible vacuum leaks. Perhaps remove and check that they are sealing properly.

Hi Mike,

     I also checked this real good and tighten them with red thread lock gel. There should not be any vacuum leaks from here.

[sanyantho]

yes maybe. or condensation also.

wrong coil installed in still running a points ignition. with no ballast resisitor* the white block next to coil

Hi Hainz, I have a stock setup, with points dizzy and stock coil and resister.