What next?

[rob249]

Truck is a 1994 GMC yukon TBI.

Finally got my new motor in, all new "gm genuine" sensors, Davis unified distributor, new balancer, new everything.

I primed the motor with a priming tool through the distributor hole, set to TDC on #1, stabbed the distributor with rotor pointed at #1.

No start. While cranking, it hits on one cylinder sometimes. If I hold the throttle down, I get a backfire out of the tbi.

Ive reset TDC 5 times now, still same results. Dont want to keep cranking this and flooding the cylinders.

Its acting like the timing is way off, but it cant be. I set TDC by putting a small screw driver in the #1 plug hole until the cylinder starts to rise, then lining up timing mark on balancer. Ive triple checked the plug wire routing using this diagram.

At a complete loss now. What should I be checking?

edit; fuel pressure is 11psi

 

[PlayingWithTBI]

I set TDC by putting a small screw driver in the #1 plug hole until the cylinder starts to rise, then lining up timing mark on balancer.

Are you setting it at TDC combustion stroke or exhaust? When it's coming up for compression stroke, if you put your finger over the plug hole, it'll puff against it. Take the valve cover off and watch to see if the valves are both closed on the stroke. If the exhaust valve is open on its stroke, turn the crank another 180° for compression.

 

[Road Trip]

Truck is a 1994 GMC yukon TBI.

Its acting like the timing is way off, but it cant be. I set TDC by putting a small screw driver in the #1 plug hole until the cylinder starts to rise, then lining up timing mark on balancer. Ive triple checked the plug wire routing using this diagram.

At a complete loss now. What should I be checking?

Greetings rob249,

Since (4) 180° strokes = 720° of crankshaft rotation, this means that
just using a small screwdriver in the spark plug hole only gives you a 50/50
chance of of getting this right. In English, the crankshaft has to rotate
2 full revolutions in order to complete a single 4-cycle operation.

So we need to know when the distributor rotor is pointing to the #1 spark plug wire tower,
are we on the top of the exhaust stroke (180° out {distributor}, 360° out {crank})
-or- are you at the top of the compression stroke? (correct)

The simplest way is to use your finger over the spark plug hole & feel for the compression. Or,
when working solo some like to use a TCD whistle like this guy demonstrates in this 1 minute video:

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EDIT: Looks like @PlayingWithTBI just beat me to this, but since we're in violent agreement I'm
going to go ahead and post this. I've used my thumb over the years, but I'm thinking about
adding a TDC whistle to the toolbox in order to make the point/entertain my new engine building
crew who will be in training. (3 grandsons :0)

****

Not to worry, once we get (timed) spark, (metered) fuel, & compression, it *has* to run. Let's
get this spark thing sorted out and go from there.

Best of luck!

 

[rob249]

I am 99% sure im on the compression stroke, I hooked up a chinese compression gauge, but cant get any reading at all. I can here hissing so im thinking its just a bad gauge, or cant make a good enough seal.

Using a finger isnt an option, no way I can turn the crank and keep a finger on the hole at the same time, doing this solo.

Going to rent a gauge at Oriellys and try that. I dont think the timing mark will line up if its 180 degrees out will it?

 

[Schurkey]

I am 99% sure im on the compression stroke, I hooked up a chinese compression gauge, but cant get any reading at all. I can here hissing so im thinking its just a bad gauge, or cant make a good enough seal.

Possible defective gauge.

Just as likely (maybe MORE likely): lifter preload adjusted WAY TOO TIGHT. Valves forced open, no compression.

Using a finger isnt an option, no way I can turn the crank and keep a finger on the hole at the same time, doing this solo.

God bless long-handled ratchets. Mine is 24" long, flex-head. I can turn the crank with one hand while I feel for compression with the other.

Going to rent a gauge at Oriellys and try that.

Worthwhile...but my money is on over-tight lifter preload.

I dont think the timing mark will line up if its 180 degrees out will it?

Yes, it will. TDC is TDC, whether it's on Compression or Exhaust is up to the cam timing. When it's on TDC-Compression of #1, it's also on TDC-Exhaust of #6--and vice-versa.

 

[Scooterwrench]

Remote starter button helps. Hook up the remote switch,stick your finger in the hole and bump the starter until you feel compression.

 

[rob249]

Yes, it will. TDC is TDC, whether it's on Compression or Exhaust is up to the cam timing. When it's on TDC-Compression of #1, it's also on TDC-Exhaust of #6--and vice-versa.

Going to verify im not 180 out tomorrrow, didnt realize the timing mark would line up on the exhaust stroke. Hopefully its just that

 

[Cadillacmak]

My favorite way to tell if I am on exhaust TDC or compression TDC is to go about 8 degrees before TDC and hit the cylinder with a puff of air from an air compressor. If its on exhaust you will hear it heading out the port, compression it will pressurize and blow back at you. Works every time and you can do it by yourself.

 

[Erik the Awful]

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[Sean Buick 76]

Stick a wine cork in the number one plug hole and turn it over by hand using a socket till the cork pops then line up TDC mark. Foolproof