The Stovebolt Site Forums The Shop Area Engine Shop 216 carb back firing

I had this problem with the 235, the engine would backfire through the carb with enough force to slam the choke shut, bending the cable, even after having the carb rebuilt. Last year, I put in a 216. The 216 wouldn't run with my Carter carb, so I put on the Rochester that came with it. It ran well until a couple of weeks ago when the same thing began to happen.
Just to see, I opened up the Carter and there was some grit in there. I cleaned out all the passages and put it back together. In the shop manual, it shows a linkage between the accelerator pump and the metering rod lever. On mine, the lever just rests on top of the pump.
I put theCarter back on with an inline filter and it started right up and idled nicely. When I started to move the truck, it was the same old same old.
I had two toughest on this; maybe dirt from the fuel system OR could it be related to the manifolds? When I changed the engine, I reused my manifolds because the heat riser is functional and the bolts were free enough to put in new gaskets. Having replaced the whole engine, those are the only things that were reused.
Any thoughts?

Back fire is usually cause by slow ign. timing or sticking or bad valves. Check the plug wires & dist cap. Check the spark plugs for carbon buildup. If the mixture is a little rich with the gas we get now the plugs can quicly build up a layer of carbon. Make sure you don't have vacuum leaks at the intake manifold. A 235 manifold does not fit a 216 just right.

Backfire through the carburetor occurs when an intake valve is open during firing of the same cylinder.

Thus:

Sticky intake valve
Valve timing off
Faulty ignition

Jon.

Incoming charge can also be ignited by a piece of carbon if
the piston or combustion chamber is carboned up.

dg

If it is a very rhythmatic popping it can also be a bent or out of place exhaust push rod. With no place for the exhaust to exit it has to force it's way back past the intake and is under higher pressure than just having an intake valve open.

Today I paid some attention to the vacuum fitting for the HydroVac booster. After sealing the threads with teflon tape, it seemed to run a bit better, but I noticed very little fuel in the see-through filter. I suspect I need to focus on the fuel supply system next.

Today, I drained the gas tank, fuel flowed nicely the whole time. For my own convenience, I replaced the broken shut-off while I had it empty. The gas is getting to the engine mow, but still not running right. It's fine at an idle, but put it under a load and it doesn't run for beans.
While I was under the truck, I noticed a rubber hose connection on the HydroVac unit that looks like it has seen better days. It's on the opposite end from most of the connections and has a steel tube that makes a 180 degree bend and runs over the top to the other end.
The new question: would temporarily disconnecting and plugging the vacuum line to the HydroVac tell me if that's where the problem lies?

Following various suggestions, I started with the easiest things first. I pulled the plugs and checked the gaps. They look good, maybe a bit lean, but I was having fuel issues as well (see yesterday's post). The wires looked like crap, so I put on a better set I had, then took a closer look at the distributer. It was easier to replace it with a known good one I had on hand. I static timed it and the engine started right up, idles OK, but won't rev. When I open the throttle, the vacuum advance sends the timing to the starting position. I plugged the vacuum line to the HydroVac just to see if there was any difference, but there wasn't.
Someone mentioned sticky valves. I had the valve cover off and the oil was running out of the weep holes on most of the rocker arms, but not for #1 or #2.
The big question: should I investigate further or save up my $ for a better engine? This one is borrowed until I can replace the 235 that let loose on me last year.
Just a little background, I'm talking about a '46 Chevy 6400 that I use on my farm. It may see 2,000 miles in a busy year and most of them are easy running, but sometimes she does have to do a bit of real work.
Thanks for your thoughts.

Its easy enough to remove the rocker arm assembly and clean out the inside of the rocker arm shafts to restore oil to all of the rocker arms. While it is apart, you can check for excessive scoring on the rocker shafts.
As far as sticking valves, that can refer to where the valves contact the seats. It can be caused be running the engine on old fuel loaded with varnish.
Don't give up yet. You can't kill these engines. Yours seems to be just temporarily disabled.
Carl

Thanks for the encouragement, Carl. I had the carb apart again for cleaning. It now has a leak, so I have to pull it yet again. I decided to give the rocker arms a closer look. The shaft they pivot on was broken where the mounting bolt goes through. That would explain the lack of oil. I replaced the front rock reassembly with one from another engine and set the valves. Still no change. I just don't get it.

Did you look for crud on the inside of replacement shaft? (Sorry, have to ask.)
Is there excess clearance between the rockers and the shaft? This will prevent oil from maintaining oil pressure all the way to the end of the shaft. When this happens you can usually see excess oil dribbling out at the bottom of the rocker instead of out of the hole in the top.
Try to dial up the RPMs a bit with the idle stop screw to see if the oil comes out of the holes in the top of the rockers.
Carl

The lack of change I was referring to is that it won't run above an idle. No matter what I have done, opening the throttle results in sputtering, coughing and stalling. My first instinct, and the only thing I have not tried, is the manifold, as that is the only part I have used on both engines with similar issues but haven't swapped out for something else. Could there be a pin-hole in the casting causing my issues?

The manifold hot box may be burnt thru . Its a bit of a pain but you will have to strip the manifolds apart to check that .

You have mentioned rocker arm assembly oil flow but did you see if all valves are moving properly. I apparently didn't snug the rocker assembly bolts after replacing bent pushrod from stuck valve. Engine ran fine at first then started popping back thru carb. After trying several of the steps you refer to I pulled the cover and found some valves weren't moving. Tightened the nuts and problem was solved. A very slightly bent push rod may not fully open valve but still stay seated in rocker assembly.

I am thinking along the same lines, Dusty. kb3csw, I've had that problem in the past. I think that's how the shaft got broken. This is a problem that has been building for a while and actually started on a different engine with the same manifolds. I have another manifold I have to clean up and try.

Idles OK, won't run well at higher RPM. I have been there and done that. All of the above posts give good things to look for. A couple of things learned the hard way follow.
1, Use a vacuum gauge to set timing on a running engine. The BB is good to start. With modern fuel and possible flywheel changes, the best way to set timing is with a vacuum gauge. Adjust for maximum vacuum. Idle only. Advance line can stay on. It is ported with the stock carburetor. After you get the best vacuum, dial it back a bit.
2, Recheck the firing order. I know, sounds like dumb stuff, however, I have crossed plug wires and others have done as well. I have also had good starting engines run with the order all messed up. so, check. turn to the BB. with the valve cover off, wiggle the valve on #1. Loose? Good. Tight? Try #6. Loose? 180 degrees out. No problem, just turn engine until the BB shows up again(one complete turn). Where the rotor points is now #1. You can reset the Dizzy if you don't like where the rotor is pointing, but you don't have to. Just start with #1 there and go clockwise 1,5,3,6,2,4. Reference: http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm1230.htm.
Start with these and see what happens. All inline 6 engines, 216,235,261 have the same firing order.