Itinerant Air-Cooled

There's been something that's been going on in my bus ever since I had the engine out for a new alternator and other miscellaneous stuff. That was in January of 2016.

Every couple of months, when I'm in fourth gear and in high revs, I get this stumble and lose power. It's almost as if there's a fuel delivery problem, with the engine temporarily getting fuel starved. It's been happening when I climb hills or accelerate on the freeway.

What's confounding me is that it's extremely intermittent ... every couple of months or so. I pull over, check the spark plug wires, look for anything suspicious under the hood, and like that. Then I start up the bus and find that the problem has gone away, and will stay away for another couple of months or so, up to a half a year.

I've replaced the fuel filter, even though there was no crap inside. Replaced the coil and the spark plugs and wires, too. The fuel pump was replaced when I did the alternator install, and that would be suspicious since that's more or less when the trouble started, but if it was the fuel pump, wouldn't it happen every time I loaded the engine? And why would it be fine for another couple of months?

Another thing I thought of was vapor lock, but nothing in the engine seems unusually warm, and it doesn't have to be hot outside for it to happen.

It's a Brosol carb, about nine years old with around 25,000 miles on it. SVDA distributor with Pertronix ignition.

I figure that it's time to consult the hive mind to see if there is anything I've missed, or if somebody has dealt with a similar problem.

Hey there John,

Can you IMMEDIATELY clutch-neutral-pull over safely next time the issue presents itself? At that point, you can check the fuel filter for fuel AND take the top of the carb off with a stubby flathead on those five screws. That would rule several issues out. Sucks waiting for it to happen, doesn't it? :)

Have you heard of the fuel pump rod getting stuck in the stand due to heat expansion? I only ask because I've seen it twice, and pouring a bottle of water on the fuel pump fixed the issue on the spot both times.

Robbie

[youtube]https://www.youtube.com/watch?v=L-QDI74acLY[/youtube]

asiab3 wrote: ↑

Wed Mar 07, 2018 10:49 pm

Hey there John,

Can you IMMEDIATELY clutch-neutral-pull over safely next time the issue presents itself? At that point, you can check the fuel filter for fuel AND take the top of the carb off with a stubby flathead on those five screws. That would rule several issues out. Sucks waiting for it to happen, doesn't it? :)

It sure does. I've done everything you described except for taking the top off the carb, which seems like major roadside surgery. Exactly what should I be looking for?

Have you heard of the fuel pump rod getting stuck in the stand due to heat expansion? I only ask because I've seen it twice, and pouring a bottle of water on the fuel pump fixed the issue on the spot both times.

No, I hadn't. And you may have hit on something, because all my previous experience with fuel pumps was with the ones that came with the generator setup, and the problem began when I converted to the new pump. I'll keep a bottle of water handy, and I'll also check the tolerances of the fuel pump shaft housing as was described in the YouTube video.

JLT wrote: ↑

Thu Mar 08, 2018 12:00 pm

the problem began when I converted to the new pump. I'll keep a bottle of water handy,

Fix it before then. Then you can tell us that you never had the symptom ever again.
You should see signs of binding if indeed this is the problem. I would pull the push rod and get out my coarse emery cloth (80 grit) and do a severe cross-hatch. Wash like hell with detergent and hot water. Grease the pushrod with moly grease-the-good-stuff, and slip the push rod on down then back up. Do you see a specific area where the grease got aggressively wiped clean? If yes, a ream of the plastic spacer is in your future. If no, enjoy.
Waiting for a spritz of water is also tempting the universe to have a crack/failure of the plastic spacer.
Colin
(p.s. are choke wire and electromagnetic cutoff wire both secure at their crimps and away from alternator housing?)

Sort of an update:

Due to a gimpy shoulder and other stuff, I haven't been working on the bus so I can't report any progress. But I thought I'd chime in to let you know that I'm not flaking. When the shoulder gets better, I'll check all that stuff out and, while I'm at it, I'll replace the fuel lines which are now five years old.

JLT wrote: ↑

Fri Mar 16, 2018 6:56 pm

Sort of an update:

Due to a gimpy shoulder and other stuff, I haven't been working on the bus so I can't report any progress. But I thought I'd chime in to let you know that I'm not flaking. When the shoulder gets better, I'll check all that stuff out and, while I'm at it, I'll replace the fuel lines which are now five years old.

Don't make me drive out there and do it myself . . . .
Colin

Amskeptic wrote: ↑

Tue Mar 20, 2018 9:42 am

Don't make me drive out there and do it myself . . . .
Colin

Gee, woodja?

The reason I posted, though, is that it's always been a policy of the board not only to post problems, but post remedies to the problem so that others may reap some advantage from the discussion. And even more important, to post remedies that worked, and remedies that didn't work. Since this whole issue is currently "hanging fire" and may do so for some indefinite time in the future, I wanted folks to know that I haven't forgotten them.

Case in point: I updated the thread on horns five months after my fix, to let folks know that my fix seemed to be working. That may be of value to somebody.

Another quasi-update:

Shoulder still gimpy (but slowly healing, I think) so I still haven't done any of the work I described earlier.

But when I mentioned my problem to Justin Campbell at Kombi Haus the other day, he told me that the "stumbling" he's seen in buses of my vintage usually was caused by the fuel screen at the bottom of the tank getting dislodged. He says that it's an easy fix, not involving dropping the tank. You just drain the tank, unscrew the connection to the fuel line, and feel around to see if the screen is in place. If it isn't, replace the screen. Has anybody else had issues with the fuel screen?

I haven't had that experience, so I can't comment on it, but I DO know that the EMPI tank screens sold in a blister pack these days Do Not Fit.

Good luck!
Robbie

JLT wrote: ↑

Mon Apr 16, 2018 11:22 am

Justin Campbell at Kombi Haus told me that the "stumbling" he's seen in buses of my vintage usually was caused by the fuel screen at the bottom of the tank getting dislodged.
Has anybody else had issues with the fuel screen?

Well, you will have to set up a scientific study if you want a definitive answer. Don't do the fuel screen check until after you have executed the push rod test. You would have to tell us that the bus stumbled again, that you drained the tank, that you *discovered that the screen was dislodged*, and that there were no symptoms subsequently. Up for this?
Colin

(p.s. never had a lick o' trouble with any fuel screen)

Amskeptic wrote: ↑

Mon Apr 23, 2018 8:02 am

Well, you will have to set up a scientific study if you want a definitive answer. Don't do the fuel screen check until after you have executed the push rod test. You would have to tell us that the bus stumbled again, that you drained the tank, that you *discovered that the screen was dislodged*, and that there were no symptoms subsequently. Up for this?

Well, I decided that my shoulder was healed enough for me to undertake the task. I removed the fuel pump and the pedestal/spacer thingy. First, I checked the pedestal fit the way it was described in that video that Robbie posted. It dropped in without a problem. So I followed Colin's suggestion and scored the fuel pump shaft with coarse emory paper, washed it three times with hot water and detergent, and coated it with moly grease. When I inserted the shaft into the pedestal, it slipped through effortlessly, going through the pedestal impelled only by the force of gravity, and the moly grease showed no signs of wiping away anywhere. I put everything back together, and checked the crimps on the wires that connect to the choke and the electro-magnetic cutoff valve. They seemed pretty tight to me.

(p.s. never had a lick o' trouble with any fuel screen)

As for that, I had Kombi Haus check the fuel screen when they replaced the fuel lines for me back in April. (It was easy then, because the bus was on their lift.) They found the screen quite intact, cleaned it, and put it back in. So score one for Colin. I have to add that in between the fuel line replacement and today, I took the bus out toward the coast and back, roughly a hundred miles each way. It stumbled twice, both times when the revs were high and it was pulling a load. When I slowed down and the road leveled off, the bus ran perfectly. That told me that whatever was causing the problem, it wasn't the screen.

So now we've eliminated the fuel screen and the fuel pump shaft and pedestal. What's next?

I'll be taking the bus on another trip next weekend, and to Maupin after that. We'll see how it fares.

Hi again, glad to hear you had a nice trip, and I’m sorta glad the issue persisted, as it gives you another chance to make things perfect.

Today I read that the fuel pump should have a minimum volume output, as well as a fuel pressure. I’ve always enjoyed the 3-5psi range for single carb buses, but check your Bentley for the exact volume test. It will require a measuring device that you can stick the fuel pump outlet in for an exact length of time, like 20 seconds. Multiply the amount by three to get your “amount per minute” which I think the specification is listed in.

If you’re on the low end of fuel pressure or volume, you could just have a slight lean out. Do you have a CHT gauge that would catch a fuel delivery issue like this? A spark issue won’t show up in a gauge but a lean out would show with higher temperatures leading up to the stutter and then subsiding when the fuel delivery returns to homeostasis.

Good luck!
Robbie

asiab3 wrote: ↑

Fri Jun 01, 2018 10:10 pm

I read that the fuel pump should have a minimum volume output, as well as a fuel pressure.

Oh heck yeah to that. Somewhere in the internet universe, is that lesson learned in Micanopy FL in 2004 with VWGirl.
We had perfect gauge pressure and her FI bus wouldn't move itself ten feet. The pressure test was with the AFM wiper engine off. Then we did the volume test into a Mason jar. Pressure would drop to damn close to 0 once fuel started flowing. We discovered a plugged copper pipe between the tank and the inlet to the square filter.

Yesh, yesh, never forgot that lesson, why, when I was a boy . . . . . . . .
Colin

JLT wrote: ↑

Fri Jun 01, 2018 4:42 pm

stumbled revs high pulling a load.

See Robbie's reply. Supply issue. Pressure and volume, follow the Bentley, report here.
ColinInCoalingaCA

asiab3 wrote: ↑

Fri Jun 01, 2018 10:10 pm

It will require a measuring device that you can stick the fuel pump outlet in for an exact length of time, like 20 seconds. Multiply the amount by three to get your “amount per minute” which I think the specification is listed in.

I don't have such a gauge. I also don't have a fuel line pressure tester. Maybe if Colin is at Maupin and has these things, he could loan them to me. Otherwise, I'll have to take it to Kombi Haus and spend a hundred dollars to let them do it.

Do you have a CHT gauge that would catch a fuel delivery issue like this? A spark issue won’t show up in a gauge but a lean out would show with higher temperatures leading up to the stutter and then subsiding when the fuel delivery returns to homeostasis.

No, the only temperature gauge I have is for oil temp, with the sensor that replaces the drain plug. I don't think that would be very helpful. I also have one of those laser thermometers, but I'm not sure that would be of much use here, particularly since the trouble is so transient.