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Thread: New rebuilt TBI, won't start
10-27-2008, 10:08 AM #1
New rebuilt TBI, won't start
I've got a new/rebuilt 350 in my 1989 suburban. I Just finished buttoning her up this weekend. Put in oil and coolant, went to start her and she won't catch. Just spins the motor.
Here are the facts.
1) Starter sounds funny...like it is spinning too easy...I don't get the grrr--rrr-rrr of normal starter, I just get a high-pitched spinning sound. It is turning the engine over....but it just sounds wierd to me. I did drop the starter on the concrete while installing it...but if it is turning the engine over that is all that should matter right? Could a slow starter prevent her from catching? I did notice that the starter drive gear doesn't retract back into the starter? It stays engaged with the flywheel...but I kind of remember that the force of the engine starting is what sends it back into the starter housing? So since my engine hasn't fired, it is just staying engaged with the flywheel...is that correct?
2) After I try starting her...if I let it sit for a few minutes...then bump the key she backfires out the exhaust?? It won't back fire while I"m trying to start her...only after I let it sit and try to start it again. Can't figure this one out...makes me super suspicous of the timing. The only way it could back fire is if it is sparking with an exhaust valve open right?
3) I do have spark...checked against the body of the car while cranking.
4) I do have fuel...it is spraying into the intake from the throttle body.
5) I'm 99% sure my primilinary timing is set correctly. I've got the rotor very close to the #1 contact in the cap when #1 is at TDC. I've messed around moving it a few degrees left and right from there without any change.
I had my girlfriend crank it over while I watched the timing light, and I couldn't see the timing mark when the timing light flashed, but I couldn't see very well period. But I know for a fact, that my distributor is NOT off 180...
Any ideas?? What is the most typical part of ignition system to fail?
All these parts were on my old engine and it was running when I took it apart.
Thanks all ideas and help!!
Last edited by Big6ft6; 10-27-2008 at 10:11 AM.
10-27-2008, 11:20 AM #2
I know you said it is not 180 degrees out, but that's exactly what it sounds like. If you can't see the timing marks when it fires, it is firing at the wrong time.
Make sure you are on the compression stroke when you check TDC. Pull the #1 plug, have someone put their finger in the plug hole and turn the engine by hand until you reach TDC. If it pushes air by the finger, you are on the compression stroke, if not go around again and it will. At this time the rotor should be pointing at #1 plug wire.Gary
99 K 1500 Suburban
78 Silverado Big 10 w/factory 454
00 S-10 Blazer
1976 Trans Am
1980 Honda CB900 Custom
10-27-2008, 11:39 AM #3
but this just dumbfounds me....I was careful when I originally set up the dist...and I've double and tripple checked!
but you've given me hope...that will be my goal tonight to check into this for sure.
10-27-2008, 11:59 AM #4
I'm gaining confidence in the idea that i AM in fact off by 180 degree.
I just looked at this cam timing graph from comp cams,
I never realized that both valves are so close to being fully closed at TDC of the exhaust stroke. I thought that the exhaust valve was wide open at the top of the exhuast stroke, so anytime I saw the timing mark come around on the balancer and both valve's looked closed I assume this was top TDC of compression....but now after seeing that both valves might appear basically closed during the "overlap" at TDC of the exhuast stroke this makes it more likely that I do have everything off by 180 degrees!
I'm so excited I can hardly work...I can't wait to go home and try switching everything around!
10-27-2008, 01:12 PM #5
The quick way to switch things around is to swap plug wires on the distributor. Switch 1 and 6, 8 and 5, 4 and 7, and 3 and 2.
10-29-2008, 09:00 AM #6
Alright, well, now I'm 100% sure that my timing is (and was) correct (I tried swapping plug wires just in case, and nothing changed, very similar behavior with no start and occasional backfire). But I was very careful this time, watching valves, and I'm positive I'm at TDC of compression.
So I'm back to square one.
Here is my question now. Could a messed up starter cuase this?
Here is my theory. I dropped my starter while carrying it to the truck to install. Something get @#$(*& up becuase the starter sounds really wierd now, more like a high pitched electric motor sound, not like typical cranking sound.
It is turning the motor over, but maybe too slowly?? And by turing to slowly it isn't drawing air into the cylinder fast enough to get a properly mixed charge. And maybe what I'm hearing isn't a "backfire" at all, but actually a single cylinder firing after the excess unburnt gas volitizes enough while sitting between starting attempts to get lit-off by the spark plug.
I don't have any exhuast hooked up yet, so how would I tell the difference between a cylinder firing and a backfire?
10-30-2008, 01:31 PM #7
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- Oct 2008
It could possibly cause that by the reason you just mentioned. Try taking apart the starter and make sure that the brushes are in place and the communtater bars are spinning on them also make sure you didnt crack or damage the pole shoes other than that good luck
10-30-2008, 01:51 PM #8
Still stuck, replaced the starter....no change. She just cranks over, occasional backfire, but more often nothing.
Starter still sounds terrible (I think it is poor alignment between starter gear and flywheel), I'm at a loss.
There is gas coming out of my injectors, there is spark (blue) my timing should be pretty close to 0 BTDC with ETC wire disconnected.
What should I try next? What is the TBI "checklist" after the things I've alredy checked?
I'm half temped to do a compression test.
I just bought an ignition module for the distributor, but if I'm getting a blue spark...what else could be wrong?
10-30-2008, 08:43 PM #9
I don't have much experience with the GM motors (haven't had one opened up in over a decade), but your symptoms sound like what happens to a Honda when the cam timing is off. Just a thought.
10-31-2008, 08:32 AM #10
Now, I'm really freaking out. ct9a, your message may be unfortunately timely..
So last night, I finally got a buddy over to bump the engine while I'm under the hood.
I pulled a spark plug, put my thumb in the hole, had him crank the starter and......nothing. I didn't feel any pressure through the hole.
This blows my mind..this engine was assembled at a machine shop, I have the reciepts...I've never done the finger in spark plug hole test before...but from what I understand the pressure build up during compression is pretty darn obvious.
I'm going to pick up a compression tester tonight just to make sure I"m not insane....
But if it is true and I don't have any compression...does that almost gaurantee that cam timing is messed up? Or could I have just overtightened the rockers while adjust the valves? I have a hydraulic cam so it seems over-tightening the rockers is a long shot.
I can't believe this is happening. But it would make sense...it would explain the odd starter noise like the engine is turning over too easily without the "rythm" normally associated with starting an engine.. If there was no compression I'm assuming the would change the sound of the entgine turning over huh?
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