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 Post subject: Backfiring/ Timing issues on 20R in '77 Celica GT
PostPosted: Sat Feb 13, 2010 11:24 am 

Joined: Sun Feb 07, 2010 10:37 am
Posts: 12
Location: Lakewood, Co.
A friend of mine has a '77 Celica GT with a carbureted 20R and 5spd.

I went with her when she bought the car and test drove it with her. At the time, it was running just fine, but now it has developed a backfire through the intake side under hard throttle, acceleration or under load. This doesn't happen durring shifting or deceleration, so I don't think it has anything to do with the air pump or anti-backfire valve system.

I timed the engine with the distributor that was in it, but another friend of hers replaced it, because it was believed to be the wrong distributor, possibly from another R motor or A motor? I didn't look at it that closely. I'm not even sure if the A 4cyl motors in Tercels will swap distributors?

Anyway, the engine now has the correct electronic dist. that it should have and appearantly, the problem has progressed even worse. I haven't taken a look at the car, but I spoke with the owner last night and she told me that it was worse.

I plan on checking the distributor rotor placement against true TDC (pull #1 plug, check for compression, etc.), and retiming it myself, but I'm beginnning to think that the backfiring issue may be unrelated to the distributor and could be developing a valve issue. Possibly lash or valve seat related.

I wanted to ask you folks if there were any common causes for this condition with the 20R. Thanks!

Dave


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 Post subject:
PostPosted: Sat Feb 13, 2010 1:52 pm 

Joined: Sun Feb 01, 2009 5:10 am
Posts: 232
Location: Northridge, CA
If the advance mechanism in the distributor is working properly, the timing is set, cap and rotor are in good shape, then...

Pull the plugs...what color are they? Have they been replaced recently?

Check plug wires for high resistance...they may be breaking down under load.

Look for vacuum leaks. (BIG ONE)


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 Post subject:
PostPosted: Sat Feb 13, 2010 3:53 pm 

Joined: Sun Feb 07, 2010 10:37 am
Posts: 12
Location: Lakewood, Co.
I'll look and listen for a vacuum leak with some soapy water. I didn't even think of grabbing my vacuum gauge from my box at work. That would have been nice. I'll see if it is misfiring, but from memory it idles just fine. Who knows though, it could be some leak showing up under high vacuum only (high load), which it does seem like. That was my initial thought, but I was unable to trace it by ear.

I'll run a spray bottle full of soapy water and a can of penetrating oil around fittings and the intake to listen for leaks.

I did bring my multimeter and I'll check each wire for excessive impedance.

Thanks for the vacuum reminder.

If anyone else may have run into this problem or has any ideas, I'm all ears as well.

Dave


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 Post subject:
PostPosted: Mon Feb 15, 2010 9:26 am 

Joined: Sun Feb 07, 2010 10:37 am
Posts: 12
Location: Lakewood, Co.
Okay, so after I set the timing at zero with the engine warm w/o vacuum advance, it still pops through the carb when I give it throttle. It does backfire a bit through the exhaust as well. I couldn't hear any significant vacuum leaks. I compared it to my 22R in my truck, side by side and I could definitely hear backfiring, but no leaks.

I think the valvetrain may have tightened up on her when she was driving the car, between the time that we test drove it and now.

I'm going to do a lash adjustment and see if that fixes the problem.

Does anyone here have a link to a FSM or cold/ hot lash specs on a 20R?


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 Post subject:
PostPosted: Thu Feb 25, 2010 7:16 pm 
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Joined: Sat Feb 10, 2007 5:23 pm
Posts: 297
Location: Napa Ca
Sounds to me like the carb has gone lean. The cam timing might be a hair retarded, but I'd look at the carb. Double check the cam timing, its always good to be thorough.

Will

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 Post subject:
PostPosted: Sat Mar 06, 2010 1:01 pm 

Joined: Sun Feb 01, 2009 5:10 am
Posts: 232
Location: Northridge, CA
.008"/.012" on the lash, cold.


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