Can't find vaccum leak. Backfiring, missing on acceleration.

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boeing7873
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Can't find vaccum leak. Backfiring, missing on acceleration.

Post by boeing7873 » Mon Mar 16, 2015 12:26 am

Engine still a mess.

After finding the hose for vacuum balance between the intake manifolds, I expected all problems to be solved.. but unfortunately i am still dealing with backfiring and low power on startup. I need to pump the throttle to get the engine to carry load to start the truck.
Idles ok, actually idles very smoothly.

Running pertronix - nkg bp6es plugs changed yesterday - blue Bosch coil
Carburetor rebuilt to the best of my skills and knowledge. New gaskets. Float level checked with the correct tool/technique with engine pump running.
New fuel pump.
New fuel lines. and transparent clean fuel filter.
Got a timing light gun and set it to my best guess of about 5 to 10 degrees advance (about 10 mm from block mark to pulley mark.)
Tested auto advance with RPM and it looks like it is working fairly well. Back and forth.
Idle screws are to 3 turns out now.
Adjusted all valves and found all exhaust valves to be tight. Now all back to 0.2mm.

I bought starter fluid. Sprayed the base of the carbs and the shaft ends. No noticeable increase in rpm.

Where else could the leak be? How do I test the base of the intake manifolds? Run it without the tin tunnel for a bit?
Could the leak be in the brake vacuum system??
Any ideas?
Jerome
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edzz
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by edzz » Mon Mar 16, 2015 12:16 pm

Is the hesitation and back firing when cold? Is it still present when hot? Is the backfiring primarily on deceleration? At one time I was experiencing backfiring descending hills under little to no throttle, turned out to be a bad exhaust gasket before the muffler.
Ed
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McCall Pinz
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by McCall Pinz » Mon Mar 16, 2015 12:45 pm

FWIW, my friend's 712 had a mystery issue like you're experiencing- idled fine, revved fine, died when applying a load and trying to drive. I believe the final answer was a distributor with a stuck and rusty advance mechanism.
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by Stekay » Mon Mar 16, 2015 6:46 pm

A thought. When you try to accelerate cylinder pressure increases because you open the throttle allowing more air in, which in turn is compressed. That makes it harder for the spark to fire than when you just check for spark. The energy is looking for the easiest place to go. One place is to fire another plug that's not on the compression stroke. One place that can happen is under the distributor cap. If your haven't already checked its possible your rotor is a tooth off and not aligned to the appropriate terminal under the cap and the arc jumping to an adjacent terminal. That would potentially cause a backfire. BTW, this has nothing to do with timing, just where that spark goes. Another thought is you just have a chaffed or old ignition wire that either arcing to ground on the block or inducing the spark into an an adjacent wire. You will find it, keep at it and Good luck!
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Jimm391730
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by Jimm391730 » Mon Mar 16, 2015 8:37 pm

Where else could the leak be? How do I test the base of the intake manifolds? Run it without the tin tunnel for a bit?
Could the leak be in the brake vacuum system??
Yes, you can run the engine at idle (for a while, maybe like 10 minutes on then 20 minutes off) without the air tins in place; just don't drive it without the tins. You can disconnect and plug the hose to the brake booster as well. With the engine running, spray WD40, carb cleaner (even ether if you are brave) around the base of the manifolds and listen for any change to the RPMs; spraying won't change a thing if there is NO vacuum leak. Any change to the revs or sound of the engine says there is a leak.
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by Buzz » Mon Mar 16, 2015 9:55 pm

I'm sorry if this is such a basic suggestion; but, have you checked the distributor cap and rotor?
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by boeing7873 » Tue Mar 17, 2015 12:03 am

So many thanks for the suggestions.
To answer some of the questions...

The engine starts well. During the first few minutes, with today's temps of around 70F, it did not need choke, and adding choke when cold make the idle a little rough and occasionally, a mild pop from the exhaust.
When hot, the engine idles like never before. But as soon as I load the engine, it starts coughing, and the carbs pop like hell. However, if I manage to increase the engine loading only the minimum for acceleration, in most occasions, it will not stall.
It is the loading at 1000 to 2000 RPMs when it wants to stall.
Something new or that I had not noticed is that at highway speeds, in certain load regimes, it has a faint missing.
It also backfires in certain engine breaking conditions.

Remember, I am still running with the idle screws at about 3 turns out.. I was expecting to go back to normal after fixing that hose...

I have checked the spark, the distributor cap and rotor, and I can't find anything wrong with them.

Will do the leak check at the base of the intake manifold and report back when able.

Cheers
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edzz
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by edzz » Tue Mar 17, 2015 1:25 am

when you rebuilt the carbs did you replace the accelerator pump seals?


http://www.aa1car.com/library/carburetor.htm


http://www.thecarburetorshop.com/Troubleshooting.htm
Ed
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by pinzinator » Tue Mar 17, 2015 9:16 am

Sounds like a bad condensor. Replacement is not expensive.

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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by Jimm391730 » Tue Mar 17, 2015 9:51 am

As Stekay said, weak spark can idle well but as you open the throttle the weak spark is quenched and is lost. Have you checked the coil + terminal for ~17 volts when running? I had very similar performance (or lack of performance) and it was due to failure of the 4500 RPM box making the spark weak. The box was intended to weaken or kill spark if the revs got too high, but as the modules age and fail they have a tendency to do this at any speed.
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by audiocontr » Tue Mar 17, 2015 11:47 am

Adjusting the carbs is trickier than it appears. Are you confident in your adjustments?

I'm decently mechanical proficient, yet I bashed my head for weeks on the carbs. I sent them to Jim and never looked back.
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Texas710
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by Texas710 » Tue Mar 17, 2015 8:43 pm

Could the carbs not be getting enough fuel when the throttle is open? Wrong or blocked jetting? Running lean. Maybe the fuel hose or pump cant keep up with the increased fuel demand?

The carb works different when it's idling than when you open the throttle, could be something you missed in there. Not sure how you cleaned them, but complete disassemble/rebuild may be what it takes to verify all passages are clear.

If you have good spark, good delivery of fuel, and it is timed right, it should work. Should...

My trucks previous owner told me his advance was up around 17, if I remember. Any consensus on what that should be?

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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by boeing7873 » Wed Mar 18, 2015 12:38 am

Texas710 wrote:Could the carbs not be getting enough fuel when the throttle is open? Wrong or blocked jetting? Running lean. Maybe the fuel hose or pump cant keep up with the increased fuel demand?

The carb works different when it's idling than when you open the throttle, could be something you missed in there. Not sure how you cleaned them, but complete disassemble/rebuild may be what it takes to verify all passages are clear.

If you have good spark, good delivery of fuel, and it is timed right, it should work. Should...

My trucks previous owner told me his advance was up around 17, if I remember. Any consensus on what that should be?

Truck runs quite well above 2000 RPM. I assume hoses, filter and pump all ok, if not, it would not run well at highway speed.
To clean the carbs, I took almost everything apart, and let everything sit overnight in a bath of lacquer thinner. Then blew everything with compressed air.
The spark looked great when I disconnected the main coil to distributor wire and let the spark discharge on the oil cooler. Look great means it jumped at least 2cm... almost an inch.
Advance is now set at about 7deg. Based on math caculations, that is about 1/3 of an inch from the bock TDC mark toward the drivers seat.
I am still betting at the leak at the base of the manifolds.
JC
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by boeing7873 » Wed Mar 18, 2015 12:40 am

Jimm391730 wrote:As Stekay said, weak spark can idle well but as you open the throttle the weak spark is quenched and is lost. Have you checked the coil + terminal for ~17 volts when running? I had very similar performance (or lack of performance) and it was due to failure of the 4500 RPM box making the spark weak. The box was intended to weaken or kill spark if the revs got too high, but as the modules age and fail they have a tendency to do this at any speed.
I will check the voltage ASAP, assumed ok since I saw fantastic spark visually, maybe it is not good enough...
Jerome
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Re: Can't find vaccum leak. Backfiring, missing on accelerat

Post by boeing7873 » Wed Mar 18, 2015 12:40 am

pinzinator wrote:Sounds like a bad condensor. Replacement is not expensive.
Would that mean bad spark?
Jerome
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