Jump to content

back-fire on 40 Olds 6 cylinder


1959olds

Recommended Posts

Engine idles great with no miss. When given 1/2 throttle or more, explosion in the air cleaner sounds like a popcorn popper. I have taken off exhaust manifold, and removed the stuck butterfly completely. I installed new exhaust gaskets, plugs, points, rotor, condensor, and wires. Not having a strobe light that night, we rotated the distributor by hand, and found the smoothest sound came from where it presently is located. Any ideas as to where to go now??

Link to post
Share on other sites

Hello,A weak/broken intake valve spring can do exactly what you describe.If there is actually and explosion in the intake manifold due to a leaky intake valve,you can sometimes locate the cylinder by shorting out the spark one at a time and listen for a change in the sound.I usually just pull plug wires from the cap one by one.If the car has sat a long time,pour a small stream of marvel mystery oil or trans fluid down the carb while running at fast idle and see if it loosenes a sticking valve.hope this helps.

Link to post
Share on other sites

forgot to mention that we ran Marvel down the carb, it helped, but didn't fix it. I also removed, and replaced the gaskets around the valve covers on the side. While they were off, I inspected the springs and found no problem. Thanks for your input.

Link to post
Share on other sites

Think about putting that butterfly back in the heat riser and making it work...

(parts available from Antique Auto Parts Cellar)

Might not be directly related to the popping-back, but will be an issue down the road: good service text can be found in most pre-1960 MoToR's manuals, including the whys & wherefores as to its presence on auto & truck engines.

What kind of choke is on this car: automatic or manual? Does it still "pop" if you choke it part way?

Is the popping a new phenomenon, or are you bringing a long-dormant car back to life?

If you're trying to wake "sleeping beauty", I'd suspect sticking valves (intake)...it is also possible that if said valve were stuck open, that rust may have formed on the valve face and seat, and be preventing a good seal, even though the valve is moving freely...(had this issue with my '48 New Yorker, after a 20 year nap in a field.)

Also had sticking valve problem with my '41 De Soto, on #5 exhaust valve, when I first got the car.

I would suggest a compression check...pull all plugs, hold throttle wide open, get three readings off each cylinder...then go back and squirt some oil in each cylinder (about 3 squirts from a pump can - shoot it away from the valves, so that it lands on the piston), then re-test(this seals the rings). Any cylinder that does not show an increase in compression very likely has leaking valves.

Your compression should be at least 100# /cylinder, but more importantly, all cylinders should be within a 10# range between lowest and highest reading...

Is the "popcorn" regular and rhythmic, in sync with a cylinder firing, or is it sporadic ?

Link to post
Share on other sites

Frank, thanks for the input. The car has 16,000 miles on it, and was brought back to life about every 5-7 years then put away again. The popping is very sporadic, and does not seem "timed-out". After all the boken bolts and drilling to get the exhaust manifold off, I'm hoping trying to remove the head bolts scares me to death! I like your idea of compression testing, and I will try to see if one of my buddies has one to borrow. Thanks again!

Link to post
Share on other sites

Try running it with the choke partially closed...cars will tend to pop-back through the carb when not thoroughly warm; I especially remember this with my old Chevy sixes from the '40's & '50s...

( According to my 1935-'42 MoToR's manual, your Olds should have a Carter WA-1 carb; this is very similar to the Carter W-1 that was used on Chevy from 1937 - '49...these will pop like the 4th of July if not choked when the engine is cold or cool.)

Does your Olds have an automatic choke, or is there a manual choke on the dashboard?

On cars that are run infrequently, often times the accelerator pump in the carb dries-out (it has a leather plunger ), and does not squirt the necessary burst of fuel into the carb throat when you open the throttle...the result is that the mixture suddenly leans-out, and the car either dies or "pops-back" until the thottle is closed again or the main jets can catch-up with the fuel requirements .

A compression check would be worthwhile, but you can investigate the carburetion issues I mentioned above at little or no cost...

BTW: that manifold heat riser gizmo is designed to help preheat and vaporize the fuel mixture while the engine is warming-up... engineers found back in the '20s that cars without a manifold -preheat had to be run with the choke partly closed to avoid stumbling or backfiring (this was also due to the very poor quality fuels available at that time, with octane ratings in the 50's or 60's -almost like kerosene- as opposed to our modern 85 or 87 octane cheap grade)...this resulted in a very rich mixture that did not completely burn in the cylinders, unburned fuel washed past the piston rings and into the crankcase, causing the oil to thin out ("Crankcase dilution"; read: shortened engine life.).

They came up with a manifold design that used a dash control at first to operate a butterfly valve placed in the exhaust manifold. When the control was set to "cold" (for starting & warm-up), the valve diverted the hot exhaust gases up around the intake manifold, between the carburetor mounting, and the "runners" to the cylinders...this helped warm the intake manifold, and vaporize the fuel mixture, so that drivers didn't have to "overchoke their cars"...when the engine reached operating temperature, the control knob was pushed in, and the exhaust gases went straight to the exhaust pipe, bypassing the intake heat riser.

Eventually, the whiz-kids came up with an automatic heat riser valve, controlled by a thermostatic spring...it seems that if one forgot to open that heat riser valve, then the manifold would get too hot, and the car would loose power, knock, perhaps overheat, start hard when hot, etc...(lot's of potential for "operator error"...)

Most of the "automatic heat risers" ( these became the "standard" kind by 1935 or so) were designed so that exhaust pressure tended to force the valve open...so if the spring broke or became weak, the "default" position of the riser valve was "open", the lesser of the two evils.

It's not essential for a car that's not driven, but when it's not there, it does affect the performance of the car noticeably; case in point: my 1941 De Soto (flathead 6 ,single throat Carter carb, much like your Olds) has both an automatic heat riser valve and an automatic choke. The thermostatic spring has been gone from my heat riser since I've owned the car, and while I have a rebuild kit on the shelf, I too fear broken bolts and such and just haven't been ready "to go there" yet...

crazy.gif...at any rate, in cooler weather, the car starts and runs fine when first started from dead cold...it runs fine for the next 5 minutes or so, no stumble, no backfire...then, the heat from the exhuast manifold begins to open the choke (normal operation), but because I don't have that additional manifold pre-heat, the intake is too cold for the leaner mixture, and the car tends to stumble and cough on acceleration, until it gets up to operating temperature (180 degrees), which takes about another 10 minutes...

In the case of my De Soto, the auto-choke and the heat riser work together to provide reasonably smooth and consistent performance while the car is coming up to operating temperature. When one or the other is on the fritz, the car doesn't run as well.

Well - that's a lot to digest...play with the choke a bit, and see if you get it to behave...might be time for a carb overhaul...

Have fun !

Link to post
Share on other sites

59 olds

Just in case you might break something - I've got a complete 41 olds 6 and three speed - rusty sitting next to garage - it hasn't be touched in years - its been in weather - might be good - for a part or two - just keep me in mind - down the road - wife wanted to make a flower pot out of it frown.gif

Link to post
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...