1974 Bus - High Engine Speed Miss

[glasseye]

Running perfectly for nearly 800 miles from Creston to Maupin, the Dorper is sickly. 1800cc dual-carb, 1974 spec.

Under heavy power demand and at high engine speeds, it misses. It pops from the exhaust when the missing appears and is down at least 30% on power. At light-throttle cruise, the symptoms are nearly invisible, but reappear with wide throttle openings and high engine speeds, above 40 mph in 3rd or 55 in 4th. These symptoms appeared more or less suddenly while attempting a summit in the Wallowa National Forest and persist following diagnosis attempts by satchmo and I this evening.

It also pops and misses at idle and is fussy at the idle to off-idle transition. I did notice the off-idle fussiness before the summit attempt and I did notice backfiring in the exhaust system in the hours before it got really bad.

Removing high tension leads to cylinders 1 and 3 at idle changes nearly nothing, removing the leads to 2 and 4 results in near-stall.

Valve clearances are optimal on all cylinders.

Brake booster connections are tight.

Fuel filter shows a tiny bit of crud suspended in the chamber, the filter element is clean.

Spark is clean, regular and strong to all cylinders.

satchmo suggests a worn cam lobe as a possible cause of the symptoms. How can we evaluate this potential problem?

Fuel delivery deficiency also seems possible. How can we test this?

Comments? Suggestions? Advice? Jokes?

Thank goodness for a safe harbour in satchmo's driveway!

[bus71]

Sometimes a bad spark plug will misfire under load. Try a new distributor cap. Carbon tracks and or cracks in the cap can cause a misfire. It may have a good spark except when under load. I would try new points, cap, etc. Good luck! I hope it's a simple thing!

[vdubyah73]

Dual 2 barrel or stock dual 1 barrel? Popping out the exhaust under load is a lean condition. Vacuum leak or fuel supply. Jets plugged, fuel filter dirty, fuel pump getting tired, pinched fuel line, float saturated with gas and not floating anymore. Vacuum leak at manifold to head gasket, or carb to manifold gasket, or power brake vacuum leak, balance tube if you have one. In my limited experience a small vacuum leak will show up first as an annoying problem under high vacuum conditions, such as high rpm light throttle conditions. Maintaining speed on flat ground with a light foot on the pedal will create this situation. Picture the pistons going down their bore at 60 mph they wanna breathe, but you are hardly giving it any gas. The throttle plates in the throat of the carb are restricting the air flow through the carb and the engine is just begging for more. With the throttle plates restricting the air and those pistons slamming up and down wanting to be unleashed, the vacuum gets pretty high. That puppy is gonna suck air anywhere it can.

Compression test may identify a valve problem. I would look for vacuum leaks and fuel restriction.

Bill

[vwlover77]

You've got a lot of good suggestions in the previous posts and I don't want to add confusion, but one additional way to evaluate your ignition system is to see how it reacts at low rpm, high load. For example, upshift to 3rd at a low speed, as if loafing around town, and then accelerate hard. If the ignition system is marginal, it will buck and miss.

[Amskeptic]

Running perfectly for nearly 800 miles from Creston to Maupin,
Under heavy power demand and at high engine speeds, it misses.
Above 40 mph in 3rd or 55 in 4th.
It also pops and misses at idle
I did notice backfiring in the exhaust system in the hours before it got really bad.
Removing high tension leads to cylinders 1 and 3 at idle changes nearly nothing, removing the leads to 2 and 4 results in near-stall.

Valve clearances are optimal on all cylinders.
Brake booster connections are tight.
Fuel filter shows a tiny bit of crud suspended in the chamber,
Spark is clean, regular and strong to all cylinders.
satchmo suggests a worn cam lobe as a possible cause of the symtoms. How can we evaluate this potential problem?

Do a compression test. If cam lobe is going to show up, then your variations should be between 2/4 and 1/3 readings. Hopefully, all are near each other and above 85-95 psi.
Be extremely thorough with brake booster circuit integrity. It should hold a vacuum several minutes after shut-off. That covers the entire vacuum line from the one-way valve to the booster itself. Check the hose between the balance pipe and the one-way valve.

Fuel delivery symptoms should not be showing up at idle but crud in the carbs could show up at all times. Clean the idle cut-offs and the ports in which they insert.

Please keep me posted. I am near civilization in three hours, it is currently 4:00PM on June 17th I think.
Colin

[satchmo]

We did the compression test - 120 on all cylinders (after we spent a lot of time tapping and twisting on the number 1 spark plug that was frozen in place). Plug colors were all light brown. Brake booster line checks out okay and no change in engine speed with repeated tapping on the brakes at idle. Cut-off solenoids are normal functioning. Took out the other carb adjusting screws and sprayed the passages with gumout. Replaced a very old condenser. Replaced the cap and rotor too just for good measure. Reset the timing and dwell. Adjusted the mixtures with the reference hose. Did the 50 cent tune-up with a hand placed over the carb while reving the engine.

It's better, but still pops a lot on deceleration, and cylinders 1 & 3 still contribute little to the overal engine speed at idle. Glasseye made it home okay. He will report in later, I'm sure.

Other ideas appreciated. Thanks.

Tim

[glasseye]

The replacement condenser did it. We found or saw nothing else amiss during our diagnosis and with the new condenser The Dorper pulled strong and smooth again.
23 mpg at 65 mph indicated, all the way back to Creston.

Exiting Bonners Ferry, there's a mile-long grade, about 7%, I'd guess. I, like , totally smoked a newish Dodge van camper up the grade. You should have seen the look on his face as I ripped past.


(capacitor, if you like. It was a condenser back then)

[vwlover77]

Don't let all that horsepower and torque go to your head!

Glad to hear it's running well again!!!

[Westy78]

Way to go Glasseye. Now that you have that beast back on the road and your bus legs under you how 'bout a trip a little closer to home?

http://itinerant-air-cooled.com/viewtopic.php?t=3894

Sure wish I could've been there last weekend to meet all you.

[Amskeptic]

Exiting Bonners Ferry, there's a mile-long grade, about 7%, I'd guess. I, like , totally smoked a newish Dodge van camper up the grade. You should have seen the look on his face as I ripped past.

I believe we have loosened up our hitherto tentative driving technique. . . . .

[LiveonJG]

You know I've had those same symptoms on and off for as long as I can remember, on both my buses, but switching to pertronix about 6 months ago seems to have cured it.

-John

[glasseye]

I believe we have loosened up our hitherto tentative driving technique. . . . .

I got some practicin' to do if I'm gonna keep up with The Leadfoot Boys.

(shift, shift, vroom, vroom)

[Amskeptic]

I got some practicin' to do if I'm gonna keep up with The Leadfoot Boys.

(shift, shift, vroom, vroom)

I think engines acclimate themselves to their owners. My engine decided long ago that if it is going to start at all, it is going to be running at 3,800-4,000 rpm pretty much the whole day, heck summer. It just doesn't care.

However, I have met engines that tell me to take it easy. Dirty sickly vacuum leaky tired worn engines just want to trundle and they will for astonishing distances as long as you don't annoy them. You may leadfoot all you like when you KNOW your engine's every part and have been granted permission to exercise them. Unknowns slow me down real quick.
Colin