The Stovebolt Site Forums The Shop Area Engine Shop Really strange engine problem

Have been posting in Big Bolts with the re occurring problems for the engine in the 2 ton.
Details:
'48 235 (low pressure)completely rebuilt.
Modified with dual W-1 carbs and cast dual exhaust. Carbs rigged with dual chokes.
also has 700R4 transmission.
Ran well for local road tests for all truck systems.
First interstate speed test failed during a long uphill pull. blew headgasket between 3&4 cyls.
R&R head, all tests and checks found nothing, installed NOS gasket. Everything fine.
A month of frequent driving later ( heavy loads, no loads, traffic, highway) It dropped an intake valve.
Valve face broken in half and stem groove looked like it had been pulled through the keepers.
Sent head to the engine shop. Replaced everything for the dropped valve, removed all others, inspected and all new keepers. found nothing else wrong.
Found nothing serious with piston or cylinder wall.
When it happened, driver reported engine noisy, ran really rough, temperature hotter than normal, wouldn't idle, exhaust sounded funny.
Put everything back together and started engine to warm up for valve adjust and head retorque.
Problem:
Runs rough, won't idle!
Won't run at all below 1200 rpms, above that have to keep wiggling the gas pedal to pump more gas to keep it going. Will idle but only with Full choke (runs better at higher speeds). Vacuum low 14-15 at steady speeds. Temp is higher than normal, used to run at 170F (160 thermostat) now 185F.
Reset the valves and retorqued while pondering the problem.
Ignition timing good. Cam timing looks good.
Firing order correct.
Good fuel flow. Eliminated fuel quality by running out of gas can.
All vacuum lines to the manifold removed and blocked.
No external vacuum leaks detected (would have to be a big one). Vacuum advance is good, reconnected.
With throttle holding at 1200-1500 rpms running rough, used hand to completely block off 1 carb, ran smoother and faster, same with the other.
Removed manifold to inspect for cracks, etc, found nothing.
No matter what, just sounds off.

Hate to say, I'm almost out of ideas.
What do you guys have????

How dd you set the mixture on each carb?

How did you balance the carburetors?

OK,but don't think That's a problem here.
Get the engine to where it is warm and will idle.
Disconnect the linkage to the carbs. Use springs, weights, elastic bands to hold the throttle plates in a closed position.
back one throttle plate screw out till it's not touching and the butterfly is completely closed (assuming the butterfly is properly fitted in the throttle shaft). close the idle mixture on that carb.
Start the engine and adjust the 2nd carb to the proper idle RPM and set the idle mixture using a vacuum gauge. When it's perfect, carefully mark the position of the idle air and throttle plate screws. shut off engine and close the idle air and back out the throttle screw till the plate is closed.
Repeat the procedure on the first carb.
When done return the idle air screws to their set positions. Return the throttle screws to their set positions. start the engine.
back out each throttle screw in small moves until the idle speed is correct.
I usually do a final check with a piece of hose up to my ear and listen to the air flow in each carb in case it needs a final tweek.
Carefully link the carbs together without changing anything.

So it dropped a valve and you rectified just that item.

I suggest another valve is bad , maybe bent and not seating properly ??

How fast was the driver revving this engine ?

#2

Inadequate valve stem oiling , causing valves to shut late or not properly . ??

Dusty, The head had been done by the engine shop short hours ago, seats, guides, everything. After it dropped the valve they took everything out, replaced the guide and all the other components for the bad valve, inspected everything else and reinstalled with new keepers. Probably under 2000 rpms.
At this point the assembly lube hasn't worn off yet.

OK, an update to focus on where we're at.
Cam timing is right on, rechecked the transition in valve overlap at TDC.
Ignition timing is good, maybe slightly advanced.
No leaks or cracks anywere in the intake system found so far.
WHY SUCH A LOW VACUUM??? Vacuum is measured at the center tap on the manifold and 2 different gauges were used.
Apparently this condition happened simultaneous with the dropping of the intake valve but hasn't been affected by the valve repair.

Did you replace the intake/exhaust manifold gasket?

This is just off the cuff and more than likely you have already checked, but, ignition problem? Points, condenser, rotor, cap, bad plug wires or plugs themselves. Just suggesting a direction other than what's being discussed. Good luck.

Yes, as well as the head gasket of course.
Detected no leaks and the gasket was fine when I just removed the intake manifold again.

What else changed?

Martin brings up a good point about looking elsewhere for an additional problem. Sounds like you have have a highly modified setup and are asking a lot out of a stock engine. Any slight issue with any given component will be amplified, given all of the mods. I know that I am not offering much as far as specific help, but just offering a suggestion for a different approach.
Carl

I'd try removing the dual intake and split exhaust as a troubleshoot approach, put it back to stock

Bill

As for the ignition, I did check the gap when I had the cap off to remove the side cover. I did look at the distributor as a possibility because I was sent a video of the engine running(valve pieces clattering inside) before it was trailered down. It showed the distributor snapping back and forth as the manifold went between vacuum and pressure each time that cylinder went into compression. But nothing was found.
Nothing else has changed. It was driving along fine. The driver thought he heard a clicking sound but before he could focus on it, everything went to he**.
Did a compression test (5-130+, 1-0) lifted the valve cover and saw the valve parts. Tied the valve stem up out of the way (didn't know it had broken up)and pulled the spark plug wire. Started it again. Should have run like an engine with a miss, but wouldn't idle, sounded rough and like the timing was retarded.
R&R the head and put everything back.

The mods are pretty mild "old school", just letting it breath better. Fenton headers and Offenhauser manifolds have been around since the 50's. Everything was left stock. Point is, everything was working fine until ..........
There are only so many things that can cause the vacuum to drop (and remain steady) and I must be missing one.

Remove the intake manifold and all the pushrods. Fabricate a flange which will allow you to apply a vacuum to the intake ports. When you find which one of the intake ports will not hold a vacuum, remove the head and find where it's cracked.

If the engine was mine, I'd use that 2-carb manifold as a big paperweight.
Jerry

The manifold is off (and checked).
I was thinking of the same thing kind of thing for the morning.
Vacuum is a good idea, easier to use. I was thinking of unbolting the rocker shaft to close all the valves and using air pressure, figuring it would have to come out somewhere if there was a crack.
We magnafluxed 3 other heads before we found one Without a crack, guess it doesn't mean there was wasn't one waiting to form.
How big would the crack have to be to cause this much of a problem? The original head showed a crack when tested before it was rebuilt, but the engine ran fine.
Thanks Hotrod.

I've got a sneaky feeling the head is cracked in the area of the guide for the valve that got battered. It won't take much of a leak to affect the mixture of both cylinders fed by that port. Be sure the valves are fully closed and use a Mighty-Vac or even a vacuum pump used for evacuating an air conditioner. Obviously, tap into the port adapter with a nipple to accept the hose from a vacuum gauge.

I'd suggest using a rubber freeze plug with the center bolt drilled and tapped for a vacuum pump nipple. You can pick a plug that's close to the diameter of the intake port.
Jerry

Have you done a compression test since it was put back together after the dropped valve? I'm guessing you have a broken piston or a crack somewhere else that is resulting in a vacuum leak.

Looks like we ruled out the cracked head.
Made the vacuum fixture from a #8 rubber stopper with a piece of tubing through the center, compression T and the gauge and vac pump.
All three ports were the same at 19 inches.(pump will do 22 with a closed line) So I assume some guide leakage since there is no oil to the stems.

Compression is:
135,135,120,125,125,135

Sometimes a crack in a cylinder head will not open up and leak until it gets hot.

Try going back to a single carburetor, or at least block off one carb on the dual manifold and see how it runs. It's beginning to look like that eye candy carburetor system is the culprit. Unless you're planning to run wide open throttle at top engine speed all the time, the duals are as useless as a screen door in a submarine!
Jerry

If there is a crack that only opens when it is hot, it isn't the problem because it doesn't run right when started from cold. no noticeable difference when it gets hot.Jerry, I've been working on multi carb engines for more than 50 years. Most of them factory.
They do work very well. Take the early Corvette with the inline 6 and multi carbs. It cruised along main street on Saturday nights just fine.

You have ignored 3 pages of well-intentioned suggestions from other very experienced people- - - -go ahead and fix it yourself!
Jerry

Actually, haven't been able to work on it in 2 days.
I will steal a little holiday time tomorrow and start putting it back together after the cylinder head vacuum test.
When I do, I'll put a block plate under one of the carbs. Wouldn't want you to feel ignored.

Good luck, Jim

I've been reading along and scratching my head, seems like you're doing a good job with troubleshooting and process of elimination.

Not knowing exactly how the modern automatic transmission was adapted, could there be trouble with that connection, loose, and out of balance or something? Wouldn't cost anything but a few minutes to pull the cover and take a peak at converter and flywheel.

You visually checked cam timing by turning crank past TDC and watching valves. Could they be off enough to matter but not enough to see? Would it be worthwhile to use a degree wheel and dial indicator, and if you lack specs at least compare to another Stovebolt.

Could it have a broken camshaft that is fractured such that it still turns but results in slipped and or erratic timing for a few cylinders?

Grasping at straws,

Grigg

Sorry I'm late to the party. I've read this entire thread and assuming (yes I know it's not good) that the shop checked the head for cracks and found nothing, yes I get Jerry's point of internal cracks, I'd like to ask a few questions about your dual carb setup since you said it runs better when one carb is blocked.

I saw where you said you have both chokes hooked up. All of the single barrel-dual carb setups I've ever done I hooked up one choke (the rear carb). Have you checked to see if both chokes operate correctly?

Also how do you have you vacuum lines setup? I just use one, on the front carb to go to the vacuum advance.

Finally, we're both carbs rebuilt together? Meaning they have the EXACT same jets?

Grigg, great ideas.
I'm sure the trans is fine,Standard flex plate connection only modification was to machine the pilot bearing space out to the later large diameter to clear the nub on the converter.
I have been thinking the same thing about the cam timing. I was trying to remember how many teeth were on the cam gear. I think 1 jumped tooth would be enough to show. I've never seen a twisted cam, is it possible?
And thank you for the crankshaft idea, that will produce nightmares until I can eliminate it.

Sweet
Yes I built the carbs together and matched jets.
The fact is it runs better with either one blocked (at full choke). I think it is the reduced air and increased mixture.
I like the dual chokes for better New England winter warmup.
I used a choke cable from a Triumph sports car, single knob, dual cables (very popular on many European cars).
Works great.
Vacuum lines: the middle tap of the manifold supplies the vacuum wipers through the vacuum fuel pump and the Southwind gas heater. A big tap in the spacer under the rear carb supplies the brake booster and the 2 speed rear end. All lines (except vac advance after diaphragm was tested) were removed and blocked.
I will keep the cracks that only open when it warms up on the table if nothing else is found.

Took a little time today to reassemble everything back to where it was when the head was reinstalled.
Started it up and it ran better but not great. Vacuum went up from 14-15 to 17, still too low, but now it will idle with no choke. It is still running rough.
Did the same vacuum leak tests as before: piece of hose up to the ear and checking for leak sounds, followed by the propane torch check, flooding the area with gas to be sucked into the leak.
original test found nothing. This time detected a SLIGHT indication with the propane at one of the head/manifold gaskets. Also found a small exhaust leak at one of the header ports.
Seriously doubting the sealing ability of the NOS gaskets that I put on. Ordered a set of new stock from FLAPS, will be here Tuesday morning. Ran it long enough to reach full heat and reset the valves because I had lifted the rocker shaft for the vacuum head check.
That showed up a possible red herring.
The new replacement temp gauge no longer goes to ) when the engine is at ambient (85) instead it is offset just about the distance to move the normal reading of 170 to 185. More reproduction junk to complicate things, or a real symptom??

Congratulations on finding a vacuum leak! I have done many tests with the unlit propane torch and have never found one (never had any leaks to find). My only satisfaction from this method was putting the torch over the top of the carb just to hear the engine rev.
Hopefully your only problem is the vacuum leak, although your description of your symptoms reeks of multiple problems, the most difficult scenario to diagnose.
Carl

It can help to remove the fan belt while doing the propane vacuum leak test, works better without the fan blowing it around.

Yeah, I got that about the fan.
It would be too easy if it was only some porous dried out manifold gaskets. They may be leaking a little bit in a lot of places instead of one big leak.

Let's hope it is so simple. I figured you new about the fan bit for the vacuum leak test but others reading and trying it sometime might not have picked up on that part, or thought of it themselves.

I use a #0 or #1 heating tip from an Acetyline torch for vacuum leak checking. It's much easier to pinpoint the area being tested instead of using a relative wide stream of Propane.
Jerry

Everything put back together with new FelPro manifold gaskets and it runs like it's old self. Re balanced the carbs and it runs fine at all ranges.
Those gaskets might as well have been pieces of screen. Unopened original box that was not too worse for wear. You never know.
Vacuum is now a solid 19 at idle.

Note:
I earlier mentioned one of the symptoms was that the engine was running hotter, Not.
The new temp gauge I put in when I rebuilt the gauge cluster has gone out of calibration, doesn't return to 0 when cold. Today, cold engine (75 in the shop) gauge reads around 120. 185 at running temp (160 thermostat), used to hold at 170.
Another piece of reproduction crap.

Hy Jim Sears, I am so glad you finally found and cured the problem and thank you for allowing us to tag along and spitball ideas for the cause of the problem, I'm sure we all learned something, thanks again!

that is good you found the problem with the gaskets..
i installed a set of (NOS) head gaskets in one of my truck's a little while ago..they have been on the shelf for many years and looked good.. when i ran the engine after a hour or so they started leaking both compression and coolant.. i replaced them with some new fel-pros and the engine now has 10,000 miles and no problems.. so i will never use (nos) gaskets again..

http://i202.photobucket.com/albums/aa84/rcplumley/72%20flat%20bump%20project/IMG_9238_1.jpg