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Got spark, got compression, got fuel.....

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Monark192

Monark192

Jedi Warrior
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So this evening my neighbor (who is more mechanically inclined with a couple of porsche rebuilds to his name) and I spent 2 hours trying to get the 75 to fire. The PO had installed a new distributer with Pertronix ignition. The Prior PO had fitted a Weber Carb. First problem was that the timing was off, way off. Rotor was 45 degrees off. Got that reset by measuring tdc on one and going from there. Fuel is pumping to the carb. Compression seems good. Still won't fire. Hooked up the jumper cables to his GMC truck to make sure we had plenty of cranking amps and the most we ever got was some nice loud backfires through the muffler. How do I trouble shoot from here????
 
Still sounds like timing. Or you have the plug wires wrong. I still mix them up now and then. Remember the firing order 1-3-4-2. It's easy to connect them 1-2-3-4 without thinking about it.
 
Are you sure it was TDC on the "Compression" stroke?
 
Jack - took the fuel line off the carb and put it in a container, then turned the fuel pump on for a couple of seconds. Good fuel to the carb (and no fire)!
 
Tony, I lined up the timing marks to what would indicate tdc and then checked through the plug hole that the piston was at the top.
 
Both of the cylinder 1 rockers should have clearance if you are on the compression stroke. You can also bump the engine to the marks with your thumb sealing the sparkplug hole and feel for compression. Do not put your thumb in the hole. Pistons hurt!! If this doesn't get her started look in the float bowls for gas. The inlet needle/s could be stuck shut. Then check the valve timing to see if the cam is properly aligned with the crankshaft. Phil
 
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Tony, I lined up the timing marks to what would indicate tdc and then checked through the plug hole that the piston was at the top.

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Not necessarily TDC on the suck/squeeze strokes. Intake (bottom), compression (top), ignition (bottom again), exhaust(top again). You may have the dizzy finger pointing to #1 on the exhaust stroke and be 180deg off.

Remove plugs. Place thumb tightly on plug hole. Crank engine. The compression stroke will blow your thumb off the plug hole.

Or switch #1 and #4 wires and #2 and #3 at the plug ends and see what you get.
 
Dr, it seems to me that the rotor was pointing more to the 7 o'clock position. I did look at the number one plug lead on my other B and that was in the same place. Of course, that one doesn't run either.
 
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Jack - took the fuel line off the carb and put it in a container, then turned the fuel pump on for a couple of seconds. Good fuel to the carb (and no fire)!

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I assume the fuel is fresh? If so, sniff the plug hole after a few cranks with WOT/choke. If is smells like gas, fuel is getting thru the carbs and all bets point to a timing issue.

Fuel into a container from the feed just means that the pump is, well, pumping. /ubbthreads/images/graemlins/devilgrin.gif
 
If number 1 cylinder is indeed on compression stroke and the rotor is at 7 O'Clock the 'drive dog' gear is in 180 degrees out. Just use the wire it points to as #1 and go counter clockwise to wire in the others in the right order.

In truth it doesn't matter WHERE the dizzy is positioned mechanically, but rather that it trigger the spark at the proper time electrically, and in the right firing order. Mine's in bas-ackwards, and I've just been too lazy to correct it... FWIW. /ubbthreads/images/graemlins/smirk.gif
 
Simon, Like Steve, I think you have a timing problem also. Make sure the wires are right. Also, no cracks in the dizzy cap. Did the car run when you bought it? Did the PO have the timing chain cover off? (Wrong cam timing)PJ
 
I just got to ask, GOT FUEL?
 
She runs!!! Timing it was

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Or switch #1 and #4 wires and #2 and #3 at the plug ends and see what you get.

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Genius - thanks Scott /ubbthreads/images/graemlins/cheers.gif
 
Super stuff.
 
It's GOTTA be a timing problem if you're getting intermittant exhaust backfire. Boring as it is, I'd say start with TDC'ing #1 on compression stroke again and check the diz for rotor position to #1 wire. Should be at an approximate "One O'Clock" spot. And the dizzy spins ANTI-clockwise, 1-3-4-2.
 
How 'bout that!
 
Chopped liver AGAIN!!!

*sigh* /ubbthreads/images/graemlins/jester.gif
 
Thanks to all again for the help. Tomorrow, it is time for a little tune up and then jump into the brakes. Who knows, I may be able to drive it soon!
 
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Chopped liver AGAIN!!!

*sigh* /ubbthreads/images/graemlins/jester.gif

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/ubbthreads/images/graemlins/savewave.gif "No respect, no respect at all" /ubbthreads/images/graemlins/devilgrin.gif

In fact, we were all suggesting the exact same thing, just different ways to get there.
 
Actually, I'm just glad it runs. I don't want to see someone discouraged with an LBC over 'the little stuff'.

Always good to see th' "VROOMM!!" post. /ubbthreads/images/graemlins/wink.gif
 
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