The Stovebolt Site Forums The Shop Area Engine Shop Help -- 305 V-6 backfires through carburetor

Same issue as in the fall -- Everything started when the fuel pump died. After cleaning the carburetor and replacing the fuel filter, points, coil, plugs, plug wires, adjusting the points and the timing, the engine is still running a little rough at idle and will cough, stumble and backfire through the carb at high revs or under a load.

I can not guarantee we have the timing or points perfect. And we may not have gotten all the pieces of the fuel pump diaphram out of the carburetor.

Any suggestions or ideas?

Thanks,
John

Hy John Milliman, it sounds to me like you have a sticking, burned, or too tightly adjusted intake valve. It may also be a broken valve spring on an intake valve, also extremely worn valve guides can cause this, or it could be as simple as a chunk of carbon holding a valve slightly off its seat, hope that helps.

check the lead to the condenser as it is also psooible to pull apart when the vacuum advance works causeing misfire and it settles back in place. not saying that this is it but have seen it before

John,
Run a compression test. The only way to get compression back through the carb is through the valves. Either one is burnt, not seating some how or the timing is way off (like 3B says),
Scott

Check your valves. I had something similiar happenin and two off my push rods had jumped off the rocker arm. Hope this helps

I had a distributor / oil pump gear on a 305 get loose on the shaft and give me similar symptoms untill it finally got cock-eyed against the cam and jammed and died for good. At the time I was thinking I needed to rebuild the mech. advance in the distributor because I couldnn't get consistent timing. Before I dug into it the gear failed and revealed the problem.

Put a vacuum gauge on it, and check for any fluctuation at idle, and as you accelerate. If the vacuum has a rapid fluctuation at idle, you've got an intake valve adjusted too tight, or sticking, or- - -worst case- - -burned.

Do compression test, "dry" first, then with about a tablespoon of oil in the cylinder. Last thing to do is a cylinder leakdown test, where copnpressed air is introduced into the cylinder through the spark plug hole and you listen for leakage at the oil filler, the carb, and the tailpipe. Bubbles in the radiator icdicate a leaky or blown head gasket, or cracks into the water jacket. You really don't want to go there!

The vacuum gauge is also useful for timing and carb adjustment. Lean out the mixture screw(s) until vacuum drops, then back them out for the best vacuum reading. Timing is the same basic maneuver- - -retard the timing to get a drop, then advance for the best vacuum number. Good luck!

Backfiring that's not caused by mechanical problems is either lean mixture, or late timing, or both.
Jerry

This is probably a stupid comment but Wynns has a product to run through your engine that frees sticky valves. Ok now I,m demoted back to sweeping the shop floor.

If it is a constant sharp barking noise through the carb under load I would suspect a flat cam. Pull the valve covers, start it up and let it idle and watch the rocker arms. If one or more move less than the others you've got a flat cam. I've replaced several of them and the symptom is always the same.

John

john1,
He's working on a V-6 305 not a V-8, LOL,
Scott

Scott,


What's the difference? Why wouldn't John1's diagnosis apply to a V-6 as well as a V-8. I must be missing something here.

Stuart

305 V-8's are notorious for flat cams, poor hardening process. It's not out of the question on the V-6 but since it's a firetruck, I assume it's a low mileage engine. Until John diagnoses it, all bets are off,
Scott

Great ideas -- Thanks! Actually, the only ones I like are the ones regarding the timing. The other ones are scaring me

I'll let you know!

Thanks,
John

Did it run OK before the fuel pump died?

Can you choke it and get any different response from it?

Did you change anything other then the carb clean out and fuel pump rebuild?

A few things to check,

first, pull the fuel line at the carb, and put a hose on it down to a bucket. Crank it over and see how much fuel its pumping.

If its just a bit, your pump is bad or rebuilt wrong, if its a lot, go back into the carb and check for trash in the jet or a passage ways.

Set the dwell and timing, by ear, by light, or by vacuum. Get it close, where it sounds the best.

Check for fouled out spark plugs or crossed plug wires.
Even if a plug wire is on the right plug, it can still cross fire if the wire is questionable.

Check for vacuum leaks at the carb base since you had it off, and check all the lines going or coming from it.

Its unlikely anything major has gone wrong, cam wear will creep up on you and bent or bad push rods make noisy.

Joe

Don't forget to check the fuel outlet on the tank. Had that similar problem once. It was plugged with rust.

It's a funny little engine. Did you get the firing order and distributor rotation correct. 165432-clockwise.


Stuart

A flat cam that will not let the exhast valve open will cause it to back fire when the intake opens because it doesnt clear out the exhast pressure also a burnt intake valve will back fire through the carb because it doesnt seal,but I would check compression ,recheck your condensor and points real close, mabey try a new set if the compression is good ,I have gotten bad parts new before , also put a vacum guage on it if a valve is not sealing it will jump up and down rapidly .also a spark will miss fire when its under a load Good luck

and last but not least..........the good ole cam ....might have a worn lobe.........

but this deal gives me a concern.. your quote " And we may not have gotten all the pieces of the fuel pump diaphram out of the carburetor."

and if not..........wonder where they might go..

and on the other hand it may just be old varnished up gas as it does look like it does'nt get driven much.'basically if it gets proper gas and electicity it should run. a compreeion test should tell you pretty much about the rest. going to be interesting to see what the answer is.
there should be a pool on who has the correect theory
ron

Try pulling off the distributor cap and with the ignition on, open and close the points making a spark. If the spark is a crisp "snap" and white/silvery in color it's probably OK. If the spark is blue and flashy the condensor may be defective.

To me it sounds more like timing or a valve sticking or not seating properly.

Ran the truck yesterday -- it appears the issue is fixed. Compression, timing and gap all checked out right in spec. Jimmy rebuilt the carburetor and, in the process, removed a goodly amount of mechanical pump diaphram bits out of the high speed jet -- amazing that enough pieces made it through the stone filter to be a problem like that.

Freshly rebuilt carb, new mechanical pump (keeping the new electrical one in line as a standby) and she seems as right as rain.

Winchester Fire Parade, here we come!


John

Thanks for all the advice and guidance!