GM 250 Inline 6 - Won't start

[Will Fish]

I've finished the install of my freshly rebuilt motor. After a few minor quirks, I've got everything hooked up and connected and ready to run, so I thought.

#1 cyl was set at TDC. Distributor set, with rotor pointing to #1. #1 Wire on the cap pointing to #1. All things as they should, except she won't start. Motor turns over but will not start. I did get some back-fire thru the exhaust and the breather on the carb. I have spark on the coil. I've tried advancing and retarding the timing with the same results. This has me baffled. I should mention the Dist, Cap, Rotor, and Wires are not new, but are only 2 years old. I am going to replace all of these with new and give it a shot. Any advice? It's been a long 2 years, and just want to get her back in the water and enjoy it.

 

[ziggy]

Re: GM 250 Inline 6 - Won't start

#1 cyl was set at TDC.

was #1 set to tdc on the compression stroke? also, i don't see in your comment where ya set dwell to spec. then set timing to spec. in that order. as dwell affects timing.

 

[RogersJetboat454]

Re: GM 250 Inline 6 - Won't start

Save your money on replacing the wires, cap, and rotor. Honestly IMHO people around here seem to change these items too often. If they worked fine before the rebuild, there's no reason they just decided to fail now.

Out of curiosity, did you replace the balancer when you rebuilt the motor? I've seen several of those 6 cylinder balancers where the rubber has broken down, and the outer ring has slipped (making timing the motor correctly impossible).

Also as Ziggy has asked, when you set the motor to number 1 TDC, you set it to compression TDC, and not exhaust TDC, right? The mark on the balancer will line up at 0 on both. If you have trouble finding compression TDC there are a couple methods you could use to find it.

Screw a compression tester into cylinder 1, spin the motor by hand (ratchet with a socket on the crank pulley bolt), watch for when the needle rises (this should be compression TDC), and set the balancer mark to 0?

Or

Pull the valve cover, spin the motor with the ratchet whilst observing the rocker arms for cylinder 1. When you see cylinder 1 intake rocker open then close the valve, you will be on compression, set the mark to 0?.

 

[Will Fish]

Re: GM 250 Inline 6 - Won't start

Save your money on replacing the wires, cap, and rotor. Honestly IMHO people around here seem to change these items too often. If they worked fine before the rebuild, there's no reason they just decided to fail now.

Out of curiosity, did you replace the balancer when you rebuilt the motor? I've seen several of those 6 cylinder balancers where the rubber has broken down, and the outer ring has slipped (making timing the motor correctly impossible).

Also as Ziggy has asked, when you set the motor to number 1 TDC, you set it to compression TDC, and not exhaust TDC, right? The mark on the balancer will line up at 0 on both. If you have trouble finding compression TDC there are a couple methods you could use to find it.

#1 was set to compression TDC by using a compression tester in #1. The balancer is brand new.

I forgot to mention I have the electronic ignition conversion, so no points.

 

[ziggy]

Re: GM 250 Inline 6 - Won't start

I have spark on the coil.

how about at the spark plugs?
if so, sounds like ya've eliminated the electric side of things.
since it's a new engine. ya'd think engine would be fine. though there's still valve adj. that would be needed. assuming that's done. only thing left if the fuel side of things.

 

[Howard Sterndrive]

Re: GM 250 Inline 6 - Won't start

crank with the dist cap off to verify the rotor is turning
although if you have spark it sounds like it is.

pretty simple motor to get going - air, fuel and a well timed spark and vroom
have you dumped any gas down the carb? Is the gas fresh?

 

[Don S]

Re: GM 250 Inline 6 - Won't start

When you adjusted the valves, did you have the engine at #1TDC on the Compression stroke to start adjusting those. If you installed the cam and crankshaft gears and have everything lined up, that is one revolution of the crankshaft off of being #1 TDC on the Compression stroke.

 

[Will Fish]

Re: GM 250 Inline 6 - Won't start

Plugs are new and gapped to .035.

I didn't do the rebuild myself, but the cam is new. I was told by the rebuilder I may have to adjust the valves 1/4 turn once running to eliminate noise.

 

[Silvertip]

Re: GM 250 Inline 6 - Won't start

Plugs are new and gapped to .035.

I didn't do the rebuild myself, but the cam is new. I was told by the rebuilder I may have to adjust the valves 1/4 turn once running to eliminate noise.

Yikes! The rebuilder gave you incorrect information. If the valves are noisy on a rebuilt engine, they need to be adjusted properly -- and 1/4 turn to silence them is not the way to do it. If you have the engine running, turn the nut SLOWLY until the noise "just quits". You then turn the nut 3/4 - 1 turn additional. He should have adjusted them properly during the rebuild. That process does not require the engine to be running and is very easily done when things are nice and clean. It also takes just a couple of minutes to do leaving nothing for the customer to do. Since he was wrong about simple valve adjustment, one could question the accuracy of the camshaft installation which then means timing issues (mechanical and electrical) enter the picture.

Since you've had some backfiring, have you tried rotating the distributor a bit CW and CCW to see if it will light off? Do you have the firing order correct.