L20B backfire

[NickD_510]

Hello Everyone, 

 

I recently swapped my L16 for an L20B in my ‘72 510. 

When the engine was bought, it was running and being pulled form an 620. I was told the engine was rebuilt about 1000 miles ago. 
I drop the L20B in and it won’t fire up. It backfires like crazy when I crank it over. I already looked at the timing (TDC on compression stroke, first two lobes at 10-2, pulley lines up with tab on engine, distributor spindle at 11:25, and firing order 1342 counter close wise from where dizzy was pointed under cap). 
 

anyone have any thought on what is going on???

[datzenmike]

Check valve lash. A tight, open valve will pop back through the intake and carb.

 

Is the choke on? It's harder to start by just pumping the gas several times to richen it up. As soon as it fires up it goes lean and backfires.

 

Look at the round sight glass on the front of carburetor, do you see the gas level??

 

[NickD_510]

Datzenmike,

 

I haven’t checked the valve rash but I will. 
also, manual choke isn’t on..it has a 32/36 Weber with the auto choke on the side. 
 

when it backfires, it spits a flame out of the exhaust manifold. (haven’t taken it to be fitted properly for 510 exhaust). 

[datzenmike]

Out the exhaust? Well check the valves anyway.

 

Put an old spark plug in each of the plug wires one at a time, and lay on the grounded valve cover and turn the engine over with the starter. Do all four show a spark?

[wayno]

The last few times I had back firing out the exhaust was because my exhaust valve needed adjustment on my L block, the time before was on my 1979 Ford Mustang2, that one had a cracked valve(burnt valve?) that likely happened when I lost the cam timing belt the year/several months before, that one I had to remove the spark plug wire in the cylinder as I was 300+ miles away from home, I had a 3 cylinder after that, it actually wasn't that bad of a drive home, otherwise the car would have been abandoned because it really wasn't worth that much but it had all my hanggliding equipment in it/on it and I was not going to leave that on the side of the freeway, I don't have a photo of it with the hangglider on top of it.

[datzenmike]

There has to be unburned gas and air in the exhaust for it to explode. Now this could just be electrical failure on one or more plugs that are not firing. One of the other cylinders sets it off in the manifold. Bad plug or wire to it, rotor, distributor cap? Could be one or more tight exhaust valves that don't close properly and compression is pushed out into the exhaust manifold. Valve could be bent, or the valve to seat worn badly. As mentioned, the valve could be burned from running it too long with it set too tight. Hell the seat may have fallen out from over heating.

 

Definitely check/set the lash 0.009" intakes and 0.010" exhaust. This is a cold setting as the engine can't be run and should be 0.010" and 0.012" when HOT. Set engine to TDC on #1 cylinder and adjust both valves on first cylinder, exhaust on two, intake on three. Turn engine ONE full turn up to TDC and adjust all the rest. This method is fast and saves bumping the engine repeatedly. 

[datzenmike]

If there is spark to all the spark plugs and the valve lash is set and still no change, do a compression test. A low compression on one or more cylinders could be anything related to the valves (bent, broken/burned, bad seat and face) the rings may be toast or the head gasket blown. 

 

Do you know the person you got the engine from? because everyone says it was running when removed and that it was rebuilt. Did you see it running?? If running and rebuilt why was it removed? If you can't confirm it's running properly you only have trust in the seller.

[NickD_510]

Datzenmike,

 

going to check the spark plugs today. 
it was pulled because they were putting a Ka in the truck instead of the l20b. It was running, had video of it the day it was pulled. 
 

but I will check all the points you made in your posts! Thanks so much! I will update when I start these checks

 

-nick

[datzenmike]

OK if you're comfortable with the sale then probably something out of whack.

[NickD_510]

Datzenmike, 

checked the valve rash and 6 outta 8 were so tight I could not fit a tab through. 
corrected them to the .009 and .010 gaps like instructed. 
Tried again, no luck...still backfired in exhaust. 
I tested the spark plugs, they were giving off an Orange spark not blue. Idk if that has anything to do with it but I ordered all new spark plugs, spark plug wires, distributor cap and rotor. 
I will replace those tomorrow and try again. 

[datzenmike]

Sounds good.

 

Next is a compression check. I'm hoping that the exhaust valves are not damaged.

[NickD_510]

Mike, 

 

tried again with new parts, still no luck. 

doing compression test today. 

[NickD_510]

Compression Test
           First check.   Second check

cyl 1 -   135.                   135

cyl 2 -   125.                   130

cyl 3 -   125.                   125

cyl 4 -   130.                   130

 

does this sound right? 

[datzenmike]

As long as they are within 10% it's considered good enough. A pressure above 150 would be better.

 

Lets revisit electrical.....

 

 

Mark where your ignition timing is, so you can put it back, then loosen the hold down and turn the distributor clockwise to advance it. Just a bit, then a bit more...

 

 

 

Check all electrical connections to the coil, make sure nothing loose.

 

Inspect the top of the coil for cracks around the high tension wire where it plugs in. Try rubbing some dielectric grease inside the rubber boot or nipple. What may be happening is a random spark jumps from the output coil wire to the negative post. You could try firing it up after dark and look for any visible blue sparks jumping when you rev it up. I don't suggest replacing the coil, too expensive just to see, but maybe borrow one and see if this removes the problem.

 

 

I forgot to ask if this was a points distributor, so I'll assume it is. Clean, (means to file them down straight) and gap the points to 0.020" or replace them.

 

If no change, disconnect the condenser (or called capacitors) and if it runs better now, replace them...... if no change, connect the old ones back up.

 

 

 

[Papada]

I vote for crappy points. 

[NickD_510]

It’s got pertronix on the distributor.