calling all engine gurus

[dougimes]

This one has me stumped for the moment.

Engine runs rough at idle, chugging, very un-even. Smoke out the exhaust that smells like unburned gas.


It was running fine, has been for years. I made a 10 mile back and forth run to the gas station, by the time I got back, I had the above symptoms. The gas station sells pure gas, no ethanol.



this is what I have done so far to isolate the problem


1. took the cover off the carb, looking for stuck float. Gasket was deteriorated. Gave the carb a visual inspection, replaced the gasket and put the cover on. Started it, no change.

2. Tested voltage to the coil. 24vdc

3. Pulled plugs , and cleaned and gapped them .020 . they were carbon fouled, all 6 pretty evenly.

4. started the engine, no change

5. opened the distributor, adjusted points and changed the condenser

6. started the engine, no change

7. checked the timing (with a light), 5 btdc (where it has been, when performing well)

8. visually checked the strength of the spark. great spark, jumps a quarter inch.

9. I drew off a gallon of gas and ran it in a pu286 (flat head 4cyl genset) ran fine.


in summary, problem arose while on a short drive after going to the gas station. Runs rough at idle and poorly at all speeds.

ideas?

 

[NDT]

Trash under needle causing flooding?

 

[rchalmers3]

Any way you can peer down into the carburetor while it's running at idle to see if there's fuel pouring out of the main nozzle? There should be nothing coming out the nozzle until you are well off idle.

Rick

 

[jeremymx26]

Well I am a fuel injection guy so my carb knowledge is low. Possibly when you got the gas it stirred up the tank allowing some debris to make it to the carb sticking a needle. With the carbon fouled plugs and good spark excessive fuel is the issue.

 

[foxtrk2]

long shot also any carbon tracks in dist cap had a similar running eng and cracks in cap was culprit

 

[Coffey1]

Can't remember the symptoms of a truck I had doing same thing I think.
Could it be sticking valves just throwing that out there .

 

[74M35A2]

Crimp off or otherwise interrupt the fuel supply to the carb. Let it then idle until it starves itself out. If it clears up just before it falls on its face, then it is likely an overly rich condition.

 

[61sleepercab]

One old timer carb trick I have used is this: Take air cleaner off....... Start truck and rev up to full throttle and immediately put your hand over the carb throat and hold till engine almost dies...... repeat 3 or 4 times. This causes high vacuum in carb and will sometimes pull dirt or water out of jets,ect. I have done this and straightened out several carbs. Mark

 

[dougimes]

OK guys, Round 2

Here is an update-
checked ignition, 24v to coil, ok, points set ok, timing ok, good healthy spark


checked bowl valve- appeared to be leaking slightly, resulting in bowl too full, thought I found it, replaced it, checked it. no change
Replaced the low speed (idle) orifice. no change

Found that step up piston and control rod were not sealing off well at idle, replaced. Slight improvement


I am too rich at idle, bottom line. What can cause this. I am thinking some passage somewhere is not right.

 

[rosco]

My 2 cents: wrong fuel. It can happen. Like diesel in the tank, but that will really make it stink. Saw a DC6 crash because somebody put JP4 in it. My Dad dumped 5 gallons of diesel in the tank - the fuel gauge didn't work in the M37, & it made it run about like your describing.

 

[R Racing]

Is your choke sticking? ? How is your float level. Is it set to high? Also check power value. If it's ruptured a carb will run pig rich.

 

[M543A2]

Since all else apparently has failed, you might try removing all of the fuel from the tank, go get fuel from another station and try it.
We had a real problem like you describe with an M715. We drained the fuel, about 2 gallon left in the tank, put in new fuel and the problem went away. We thought the drained fuel looked weird, smelled weird. I dipped a paper shop towel in it and tried to light it. I had to really heat a corner of the towel to get it to burn! Something had settled out from the fuel over time, finally getting high enough in the tank bottom to be picked up by the pump. Fortunately these trucks have a tank drain. This new fuel sucks!!!!!
Regards Martin

 

[dougimes]

http://youtu.be/Q8_8PfvTLjw

Pulled the carb for the last time, checked the idle circuits and swapped a few parts, cleaned passages.

As you listen, filter out the sound of the fan, and machinery, the constant sounds and listen for the power stroke ignitions. an occasional PA, PA , then steady

What do you think?

 

[glassk]

Sometimes a vacuum leak will whistle or hiss and make itself obvious. But oftentimes, a vacuum leak will disguise itself as an ignition or fuel problem that defies diagnosis. Either way, an engine vacuum leak is bad news because allows "unmetered" air to enter the engine and upset the air/fuel ratio.

 

[Squirt-Truck]

Check your OIL, what you describe sounds like a damaged fuel pump diaphragm.

Level ok? Smell?

 

[dougimes]

next installment

Things checked-

No vacuum leak
Choke ok
I drained the gas and replaced with new unleaded pure gas 89 octane
played with idle mixture and idle speed to make sure settings were normal
Drove it at speed for 10 miles to clear it out

Still idles poorly.
I'll check the oil, as suggested. What am I looking for? low level.
I get a little smoke out the exhaust, very little. smells rich, not oily

Incidently, it run best with the idle mixture all the way closed, or nearly so.

R-Racing- whats a power valve?



It runs at speed great. Better than ever. Idle still gallops and misses

 

[bigbird1]

Is the float fiber or brass?

 

[Jeepsinker]

Pull the top of the carb cover and lay a straight edge across it. Sometimes old carbs warp and allow flow between passages and cavities in the carb where it shouldn't despite new gaskets.
Power valve is also known as accelerator pump.

 

[R Racing]

next installment

Things checked-

No vacuum leak
Choke ok
I drained the gas and replaced with new unleaded pure gas 89 octane
played with idle mixture and idle speed to make sure settings were normal
Drove it at speed for 10 miles to clear it out

Still idles poorly.
I'll check the oil, as suggested. What am I looking for? low level.
I get a little smoke out the exhaust, very little. smells rich, not oily

Incidently, it run best with the idle mixture all the way closed, or nearly so.

R-Racing- whats a power valve?



It runs at speed great. Better than ever. Idle still gallops and misses

A power valve is a rubber diaphragm or sometimes a piston. That has a spring behind it. When a engine is idling it has vacuum to hold it closed. When you apply throttle the intake looses vacuum and the power value opens to increase gas to the engine to run higher rpm. I have seen a backfire put pin holes in the diaphragm and when it does 2 things happen. 1 a vacuum leak. And 2 the valve will not close at idle and add gas thru the venturi.

 

[Squirt-Truck]

Power valve on these has a brass piston operator. They can stick.
In the oil, gas contamination will make the ventilation valve pull vapors that make the mixture rich, So that closing the idle mixture screw attempts to lean it out.
Yes the exhaust will smell rich.
If your oil level is high then you have gas contamination. (Or you have made a way to create oil.. . . .)
What is the vacuum reading on the engine at idle? Should be 16 to 20".