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Old 10-20-2011, 10:42 AM   #15
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Workhorse Chassis Owner
Join Date: Dec 2006
Location: La Quinta California
Posts: 523
What/how many miles is on your carb?


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Old 10-20-2011, 11:06 AM   #16
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Newmar Owners Club
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Posts: 462
JimElliot: I like your idea with the propane and will try that next time.
SeniorChief: I think your backfire problem and fuel eruption from the carb air horn are related. I'd be running compression on all cylinders first.
A poorly seating intake valve caused by carbon, a weak spring or sticky lifter will allow fuel mixture to leak back into the intake on compression stroke, thus the backfire. If you shut the motor off and that particular cylinder is on compression stroke it can push the fuel mixture back through the carb.
A stuck float or needle and seat can also drop some extra fuel past the bowl. Normally you'd notice excessive black smoke with that scenario.

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Old 10-21-2011, 02:40 PM   #17
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Backfires when?

A few things, yes it is long...
If it idles OK, then you can proceed, if not that must be fixed first.

Check your connection for the vacuum guage, if it GAINS when the throttle is pushed it may be connected to the timing advance port.

This too may be why it backfires.

Advance port is above the butterfly in the carb, when you stomp the gas the increased airflow creats a vacuum here that advances the timing.

If you distributor is connected to intake manifold then when you load the engine the vacuum drops, retarding the timing reducing the power and possibly causing the backfire.

Vacuum guage will help. but just look at the connections, they both may be real close at the carb the one further from the engine is where the distributor shoud go.

The distributor goes on the one that increases when you romp the gas, the guage goes on the one that drops when you romp the gas.

You already changed the normal and a few extra items.

Unless it has a lot of miles or was greatly abused I would discount the internals until you have checked everyting else.

You did not state what the plugs looked like, they would indicate what is going on inside the engine, if all are same then problem not likely a valve, if one different then it may be specific to that cylinder.

All plugs should be dry, if real white the heat range may be too high, if wet looking could be rich or oil, if you do not know what to look for post some photos or take them to someone who does.

If it backfires on rapid throttle while parked in park, then the timing may be way to late, excess fuel in cylinder is still buring when intake opens causing backfire.

Vacuum leaks will lean it out, I prefer water spray from a old windex type sprayer, not going to blow up in your face, and it will still cause a change in engine sound and be safe, WD40 also works and is somewhat safe.

Next, confirm the timing, if just a little too late will act ok but will RUN HOT and have loss of power.

Next, connect vacuum guage, idle should be greater than 17, 20 to 22 is good, more the better, if low you may have a leak or late timing.

Timing, mixture many things change this, idle adjust for best number here.

Advancing the timing will increase this too, too much advance will cause knock or ping under load.

SLOWLY increase throttle and watch the guage and listen to the engine, the engine speed needs to come up smoothly and the vacuum will also be smooth and fairly steady.

If you have a chunk of crud in the carb or a bad EGR or other accessory there will be "flat spots", the engine will sound rough or odd, isolate the cause and repair.

When you are completed the engine will have smooth acceleraton from idel upto higher RPM, no need to go real high here, no more than 2000 RPM, that will get you past the low speed circuits in the carb and any emmisions equipment will be in the driving range and should not be affecting the engine.

The trick is SLOW acceleration, lost of folks romp it, that only proves the engine can suck a lot of fuel, the low end is where you can tell most problems.

Once you have it running as you think it should be, get someone inside, hold the brake to the floor, block the wheels, put in drive and do the same thing, but only give it enough gas to have it try hard to go, this is placing the engine under a good load, if you have missed anything it will spit up here.

Now get it on the road, all should be as good as it is going to be.

If it runs hotter than normal insure belts, fan clutch are OK, if it does not ping when floored you could advance the timing a degree at a time until it starts to ping, then back off 2 degrees, this will increase the power and fuel mileage while also lowering the engine temperature.

Tony & Lori
1989 Country Coach Savannah SE
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