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Old 12-13-2010, 04:25 PM   #15
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80,000 miles, i put my bet on timeing gear. i go with keyair, seen it alot of times when i worked at chev dealer years ago.
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Old 12-13-2010, 05:44 PM   #16
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also look under the distributor rotor. they are know to burn the rotor bottom and even burn through the rotor.

check the plugs. a fouled plug can cause a backfire under load.

look for any broken or disconnected vacuum hoses. i like to check the basics first.

is the idle rough or smooth? is it worse when cold or wet?
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Old 12-13-2010, 07:23 PM   #17
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My money is on a bad rotor, very common to burn through on the HEI distributers. I'm thinking new cap & rotor. Adios, John
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Old 12-13-2010, 07:32 PM   #18
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Quote:
Originally Posted by Pop Rivet View Post
My 1990 250 MOHO will backfire through the carb when under power - accellration - going up hill - -
Any ideas - timing gear/chain?
So when was the last service and complete tune-up and what was done?
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Old 12-13-2010, 08:00 PM   #19
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I hope it is electrical or fuel related, but these weak sprockets are an issue..

Mine started to misfire a few days before it went bang..
And here is what I found..
Every Intake pushrod is bent..



Every intake valve is bent, and you can see the mark on the larger Intake Valve head where it contacted the piston..





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Old 12-13-2010, 08:47 PM   #20
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Ours backfired when the coil was going bad.
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Old 12-13-2010, 10:26 PM   #21
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Keyair,
These are some of the scariest pictures I've seen on the forum
. How many miles was on the engine when the timing gear gave up the ghost? From your experience do believe it would be worthwhile to be proactive and replace the chain and sprockets at a given mileage. Thank you for sharing.
Regards Bill
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Old 12-13-2010, 10:54 PM   #22
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If I am right it had 160k on it at the time.
Stupid this was this issue is well known, and I had replaced the waterpump a few weeks before, so i could have done it... they cost under $50 for a stout one.
I was told, 80k miles is a good time...
Yup, it was running a little off the week before and there was a ticking noise... I had a buddy listen and he felt it was a lifter going...
The pickure speak volumes, and I was shocked that they put an Aluminum/nylon sprocket on it!
Would you believe the bore wear was within tolerance, and you could see the honeing marks on most of the bores(Note the color of the internals... regular oil changes). The crank pins were good, but oddly the crank thrust was really bad. I elected to swap in a used, but good crank, new bearings, new rings on the original pistons and swapped some used heads on I had...
Now has 175k and pulls hard! Transmission is toast tho!
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Old 12-14-2010, 12:05 AM   #23
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Hi, you have been given a lot of ideas on possibilities, But one common one that hasn't been mentioned yet is that the accelerator pump in your carburetor might have quit working. With the engine off, and the air filter lid removed, move the linkage and you should see two tiny, steady streams of gas, squirting into the primary venturi's. Do all of the simple tests first, before ripping your engine apart.
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Old 12-14-2010, 10:49 AM   #24
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Hey pop, it never hurts to check the timing. LJH
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Old 12-14-2010, 01:14 PM   #25
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With so many different solutions how about letting us know what the actual fix is.
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Old 12-14-2010, 01:18 PM   #26
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My suggestion is check the timing first... that will show you if there is a timing issue. If the timing is correct, it rules out the timing chain having jumped too.
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Old 01-12-2011, 06:42 PM   #27
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Pop Rivet you ever solve this problem ??????????
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Old 01-15-2011, 12:31 PM   #28
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Rotor burn thru is a common prob on HEI ignition. I always carry spare rotor for this reason. And yep when burn thru occurs, it causes backfire thru carb
KMP
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