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Occasional backfiring, how to eliminate?
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beetleman217
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PostPosted: Thu Dec 19, 2013 8:02 pm    Post subject: Occasional backfiring, how to eliminate? Reply with quote

My newly rebuilt 36 hp starts right up at a touch of the starter. But as soon as I push the gas pedal I get sort of a hesitation and sometimes a backfire. I rev up the engine a little and it runs well and sounds good.

But then when driving, I also get backfires more than I would like to admit. This usually happens when I shift gears up. For instance, I drive on second up to the point where I need to shift to third, and when I do, as soon as I give it a little gas, the engine sort of coughs and backfires, sometimes multiple times.

I have a rebuilt 28 PCI carb in there, not really sure if it's tuned properly. Should I try to tune it, or is the problem distributor related?
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wayne1230cars
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PostPosted: Thu Dec 19, 2013 9:14 pm    Post subject: Reply with quote

What distributor are you using? Are the points in good condition set at .016 and timing set to 7.5 degrees BTDC and advance working? Point gap and timing are simple things to check and the ignition system is where I would start.
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beetleman217
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PostPosted: Thu Dec 19, 2013 9:45 pm    Post subject: Reply with quote

I am using the original VJU4BR8 distributor which I restored with all new parts, the points are set with the correct gap. As for ignition, I set it up with a test light. I just ordered a strobe light and will soon be able to do it more accurately. Could the timing be so off as to cause this backfiring? I've never had backfire due to timing (this is my third rebuild), so I immediately suspected the carb.
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wayne1230cars
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PostPosted: Fri Dec 20, 2013 12:10 am    Post subject: Reply with quote

I have the same distributor in my 36 hp and I have always just timed it statically to 7.5 BTDC. I also have a strobe light and I have used that to verify correct advance. It wouldn't hurt to double check points and timing again and also valve adjustment. Of course plugs and wires, rotor, condensor, cap and coil are all part of the equation. If you have the original pulley, the notch indicates 7.5 BTDC. Sometimes pulleys get changed over the years and that can create confusion.

I also have the 28 PCI carb, rebuilt by Ed Fall. Adjustment is pretty simple - idle speed and mixture. Dirt in the fuel lines and carb can be problematic. I had a fuel pump that was putting out about 6 psi and that caused some problems but not really backfiring. Once I got the pressure down to about 2.5 psi, all was well.

Bad exhaust leaks can cause some problems as well.
Quite a few possibilities. Make one change at a time and that way you can be sure what the problem was. Good luck in getting this figured out.
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Da TOW'D
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PostPosted: Fri Dec 20, 2013 10:52 am    Post subject: Reply with quote

when you say backfiring
is it coming out the carb or exhaust?

sticky valve or weak valve spring?
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PostPosted: Sat Dec 21, 2013 1:55 pm    Post subject: Reply with quote

I would aim for a firing (so timing/ignition) problem, or otherwise a substantial leak in the exhaust system.
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Tee Dub
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PostPosted: Mon Mar 24, 2014 8:28 pm    Post subject: Reply with quote

How did you make out with this ? What knowledge have you got to pass along ?
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beetleman217
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PostPosted: Mon Mar 24, 2014 9:04 pm    Post subject: Reply with quote

I checked the timing with a strobe light and it was perfect, I also checked at 3000 rpm and the advance is working good. Haven't had a chance to check more stuff as the weather was so bad this winter (car is outdoors). I run the car occasionally and have noticed that when ambient temps are warmer it almost doesn't stutter. But yesterday it was really cold and when I drove it it coughed a lot...

I may have described it wrong - it's not really backfires (those are loud 'booms') but more of a stutter. When it stutters I momentarily feel no power from the engine.
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Tee Dub
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PostPosted: Mon Mar 24, 2014 10:12 pm    Post subject: Reply with quote

Da TOW'D wrote:
when you say backfiring
is it coming out the carb or exhaust?


I may have missed it, but did you answer or determine this above ?
Tow'ds question above is one of the keys to the solution....

I understand the cold man and just noticed your home 20...... I'll give you a couple more months to work through this physically, but the mental part can go on all year ! Smile

Good luck with this pita problem..... hang in, someone here will have the answer. I am interested in the solution myself.
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Snort
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PostPosted: Tue Mar 25, 2014 8:43 am    Post subject: Reply with quote

Those symptoms are typical of distributor advance, accelerator pump, and heat riser problems.
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Tee Dub
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PostPosted: Tue Mar 25, 2014 8:52 am    Post subject: Reply with quote

Snort wrote:
Those symptoms are typical of distributor advance, accelerator pump, and heat riser problems.


Quote:
Heat Riser Problem
....... as in a leak ?

I like the dizzy theory and what, the accelerator pump dumping too much gas ?
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Snort
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PostPosted: Tue Mar 25, 2014 9:32 am    Post subject: Reply with quote

As in plugged with carbon deposits.
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Tee Dub
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PostPosted: Tue Mar 25, 2014 9:58 am    Post subject: Reply with quote

Snort wrote:
As in plugged with carbon deposits.


Thanks for the clarification. That would be a lot of soot on a new engine non ?
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Snort
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PostPosted: Tue Mar 25, 2014 12:00 pm    Post subject: Reply with quote

New engine would not necessarily be the source of much carbon. Rather, the heat riser tube would have suffered from years of accumulation during operation, and is commonly overlooked during a motor rebuild.

As to the accelerator pump, the problem would typically be not enough or not any, rather than too much fuel.
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Tee Dub
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PostPosted: Tue Mar 25, 2014 12:52 pm    Post subject: Reply with quote

Snort wrote:
New engine would not necessarily be the source of much carbon. Rather, the heat riser tube would have suffered from years of accumulation during operation, and is commonly overlooked during a motor rebuild.

As to the accelerator pump, the problem would typically be not enough or not any, rather than too much fuel.


Thanks Snort, I hear ya on both counts.

TW
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PostPosted: Wed Mar 26, 2014 7:59 am    Post subject: Reply with quote

Check your transaxle ground. If you have an ohmmeter, check the coil body to chassis ground connection. I would also check the generator body to chassis ground connection.
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PostPosted: Wed Mar 26, 2014 4:16 pm    Post subject: Reply with quote

Check and recheck the dwell= points gap.
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PostPosted: Fri Mar 28, 2014 3:20 am    Post subject: Reply with quote

sounds like to me the inlet manifold heat risers are blocked and your getting carb freeze.take the inlet manifold off and clean out the inlet pipes with a old cable on a drill. typical symptoms are its better on warm days and worse on cold days.other things to look for are a leaking exhaust system/worn carb spindle shaft causing air to draw in making it run lean (open throttle slightly at carb end and wiggle shaft up and down for play) but my moneys on first option.
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PostPosted: Fri Mar 28, 2014 9:48 am    Post subject: Reply with quote

If the elimination of other possible causes leads you back to the carb and possibly the accelerator pump circuit, check to make sure the check valve ball in the carb is free to move up and down in its passage. A must when rebuilding the carb; you should be able to hear the ball rattling to the ends of its passage when tilting the carb.
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PostPosted: Fri Mar 28, 2014 3:06 pm    Post subject: Reply with quote

Did the OP ever say if it was back firing from carb or muffler?
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