Problems with the engine

[mech0001]

Pumps and vapor lock are always prime suspects when engines stumble and die, but hoses, connections and the fuel selector valve should also be verified.

When I first got the Birchaven I had a number of things that were additive in causing the problems. One was the fuel selector valve was sucking from both tanks at the same time. I don't know if it was an intermittent problem or persistent, but higher fuel levels masked the problem. When the fuel level was lower and I was taking a left turn the problem would rear its head.

Another problem was the rubber fuel line on top of the tanks was partially flattened so it was oval, when under suction it would flatten more and be more restrictive. When fuel hose, any hose, is wound onto the spool at the factory it's done under tension and the hose takes a set. Pinch a piece of fuel hose between your fingers then rotate the hose 90° and pinch it again. You'll feel the difference in the amount of force it takes to pinch the hose. I don't think that this is a major factor, more a contributing one.

After I had the valve and hoses replaced I discovered that a couple of the hose clamps hadn't been tightened. They were on the suction side of the selector valve and didn't leak fluid, but it's possible that air was able to enter the fuel stream and create a bubble. Again, maybe not likely, but another easy thing to check.

The above AND the fact fuel has become more and more aggressive towards rubber products. That was my reason for going with AN6 line as the internal hose is plastic and has a high pressure rating when used with its (usually) proprietary fittings.

 

[ScottC]

Did you put thermal compound (heat sink grease) on the back of the ignition module before installing?

The wires on the distributor pickup fatigue and break from the vacuum advance moving them back and forth continuously causing intermittent ignition.

My coach would stumble, die or not start intermittently. I finally found an OEM crimp connector that was fully crimped on the insulation but the conductor itself was loose in the connector. Once I found that and re-crimped the wire tight it has run great ever since!

Just had a similar issue that had us on side of road but I knew it was ignition, assumption was module, coil or pick-ups, turned out it was the power wire to HEI, when the PO converted from points a new connection was made and failed. Check that power wire, I had to wiggle it pretty good to recreate the power failure.

 

[LQQKatJon]

Just had a similar issue that had us on side of road but I knew it was ignition, assumption was module, coil or pick-ups, turned out it was the power wire to HEI, when the PO converted from points a new connection was made and failed. Check that power wire, I had to wiggle it pretty good to recreate the power failure.

One reason I would hook up a timing light. Zip tie the trigger on and set it where you can see a flash as you drive. When it is stalling out watch that light. Can show if you are loosing spark.

some dire cases, put a test light on that wire going to distributor. See if your loosing power at all. .

 

[Keith V]

The above AND the fact fuel has become more and more aggressive towards rubber products. That was my reason for going with AN6 line as the internal hose is plastic and has a high pressure rating when used with its (usually) proprietary fittings.

I went to nylon fuel line for the same reason. But its cheaper and easier to work with than either steel or AN

 

[Matt L]

I had the very same issues with my coach. It would run fine on cool days as it warmed the coach would stall, after a few minutes would restart and continue to run for a short time. The condition continued to worsen over time to the point where it would only run for a few minutes once it was at normal operating temperature.

If you determine that you have spark but no fuel and all your associated components are in order, pour a little fuel into the carb and see if the coach will immediately restart. As I mentioned mine would run perfectly until it reached normal temps and then it would quit.

Having gone through everything in my fuel system including dropping the tanks, all new fuel lines, and senders I knew I had no fuel delivery issues. What it turned out to be was a small crack in the intake manifold between the primary and secondaries which was barely visible. This seems to be fairly common on the GMC’s as it was a problem caused by the ports in both heads supplying heat which controllers the choke.

You can pull the carb and look for the crack, you may or may not see it but it’s worth the effort. As I mentioned if everything else can be verified to be in working order this would be my next step. From what I understand if it is not currently a problem sooner or later it will be.

Applied GMC sells a gasket set with stainless blocking plates for the intake, there won’t be any need to change the manifold unless you find an extremely large crack Dick Patterson of Superior Ignition came up with the fix. Mine has been running flawlessly since I did the repair. 10,000 mile trip without any issues, knock on wood. Getting the old girl ready for the next big trip. Good luck hope this helps.

 

[RvRev2]

On the Fuel Tank Selector Valve under the coach, I read that the bottom most hose comes from the Auxiliary (Front) Tank, and the the hose above that comes from the Main (Rear) Tank. Is this correct?

Also, what would be indications of a faulty Fuel Tank Selector Valve?

 

[LQQKatJon]

On the Fuel Tank Selector Valve under the coach, I read that the bottom most hose comes from the Auxiliary (Front) Tank, and the the hose above that comes from the Main (Rear) Tank. Is this correct?

Also, what would be indications of a faulty Fuel Tank Selector Valve?

dont want to answer because I do not know what selector you have.

Sometimes they stick part way between and that can cause engine stalling.

The ones I usually replace with i believe one is marked for aux.

 

[RvRev2]

dont want to answer because I do not know what selector you have.

Sometimes they stick part way between and that can cause engine stalling.

The ones I usually replace with i believe one is marked for aux.

Does anyone have a part number? Where to get?

 

[LQQKatJon]

Does anyone have a part number? Where to get?

This is the one I have always installed.

https://www.amazon.com/Standard-Motor-Products-FV1T-Selector/dp/B005ETOVZM/ref=mp_s_a_1_3?crid=15NOUNMY36I46&dib=eyJ2IjoiMSJ9.Jv6p7kqprrPF5-uu9WE65cVk1OPRbZ3V-DPQsbRo2nchhe_jI4t43DRy284nlyTgvXafoJMfj6ieGEe2lziRRMUb3QlriZyRvZgy8CiSE-wA9Sr8QgmIEHn-dZvYhDOrQ1RGZciXTWpOSQP9U4rQ48ocp42pnUFU4TW8LqX-j2CbiTe8aNxTmMrspVdivNfWbtuer6B_AeNaZSvBduMVGg.nj3SWF7OMqtr1wzkWew1cuftZNkkXaMxUk-XuAWOa6I&dib_tag=se&keywords=fuel+selector+valve&qid=1779319908&sprefix=fuel+sele%2Caps%2C217&sr=8-3

 

[mech0001]

On the Fuel Tank Selector Valve under the coach, I read that the bottom most hose comes from the Auxiliary (Front) Tank, and the the hose above that comes from the Main (Rear) Tank. Is this correct?

Also, what would be indications of a faulty Fuel Tank Selector Valve?

Mostly unable to switch tanks when desired. Best test is always to bypass. As was mentioned, setting up a tank in the cockpit and running gravity feed to the carburetor is the best bypass test I know of.