The Stovebolt Site Forums The Parking Lot Big Bolts 348 diagnosis

I have a 348 in my 59 spartan 100 that I finally got on the road. The problem is it tried to stay there permanently! I got a little way down the road and it would no longer accelerate and every time i put it in gear it bogs down and died if i tried to push through it. Now that its home, ive done some testing and it idles great, and will rev slow, but if i hit the gas real quick at least half throttle, it hesitates real bad and backfires through the carb if i hold it. It looks like its getting plenty of gas from the pump, and the timing seems fine but i can not find a vacuum advance. It has the 4g carb with a governor. Does it have a vacuum advance? Just replaced points and condenser to eliminate a possibility. Cant find a vacuum leak anywhere and governor carb diafram seems to work. Ive stumped a few mechanics and car guys already, any ideas?

Disconnect (or loosen) an exhaust pipe at one of the manifold flanges. If it accelerates normally, find the exhaust restriction. (Maybe a squirrel has been using the muffler to store his winter supply of acorns?) Don't laugh- - - -I've seen it happen!
Jerry

There is no vacuum advance on the engines in the larger trucks. It sounds like your carb. is lean or the timing is real slow.
I quick test on for an exhaust restriction is to stand in back of the tail pipe. Have someone rev the engine up & let up on it. If it makes a hissing noise out the pipe it is probably restricted. If it has a nice rumble at idle it probably is ok.
Try advancing the timing some & see if it improves anything. You probably can run the timing a few degrees more advanced then the specs show.
Its possible the governor is causing a problem. You might consider removing it but you would need to replace the carb & dist. to do so. It is not needed on light duty trucks.

OK I see that is not a small light duty truck. You may want to keep the governor on it.

Put a vacuum gauge on the intake- - - -full vacuum, not "ported". If the exhaust is restricted (I guarantee it is!) the vacuum will drop to virtually nothing and the engine will try to stall as soon as you get a few RPM above idle. The backfiring is caused by the exhaust leaving the engine the only way it can- - - -through the intake! "Been there- - - -done that"- - - -many times!
Jerry

Guess i will try removing the exhaust this afternoon. I also didnt state too well but it will rev up pretty high as long as i dont hit it hard off idle or if i hit it while its already revved up a bit. My timing seems to already be more advanced than the specs say, i think the book says 5 and im around 8 if i remember right? Last full line on the pointer.

Two things I ran into when working on big gas powered & high mileage trucks in the old days was worn throttle butterfly shafts in the carb which caused hard to find vacuum leaks. Also made sudden throttle changes act like a bad accelerator pump. Another was worn bushings or shaft in the distributor. Usually timing & dwell would check out ok at idle but would go off as the rpm’s increased . Just a simple wiggle test will often let you know if there’s a problem in those areas. If the exhaust check doesn’t pan out, check the above items. Another possibility is exhaust lobe going flat on the cam, causing the intake back fire. Just a couple ideas to check out if needed. As an fyi, I’ve always been a fan (admirer?) of the 348/409 engines.

The big trucks with the combo centrifugal/vacuum governor does not have vacuum advance as noted by wrenchbender. There are several locations within the governor system that could be leaking vacuum. The vacuum for the governor originates in the carburetor and is directed through passages in the casting to the diaphragm, then through a steel tube to the distributor. A leak anywhere along that path could result in a flat spot in acceleration. It does not appear to be a problem with the governor operation but could be a leak. My Rochester 2G with governor has ball bearings on the throttle shaft, after 67 years it still has no slop in the throttle shaft. I suspect the carburetor is in need of internal cleaning.

The truck was a firetruck so it only has 16k miles. I have also rebuilt the carb twice and it seems clean and tight. Maybe ill pull the distributor and check those governor parts if the exhaust doesnt check out.

Inside of the distributor is a centrifugal valve, it opens and closes based on engine/distributor speed. That valve is a vacuum bleed and could be gummed up. It is underneath the electrical section, mine has a strap that covers a window that is the access for governor speed adjustment, the strap should have a paper gasket to prevent vacuum leaks.

Make sure the accelerator pump in the carb is properly installed and fitted with a spring. Many kits come without the pump spring installed and it is up to the rebuilder to either install a new spring or use the old one.

Took the exhaust off and it behaved the same. Bad hesitation on hard throttle. It does pop a bit when letting off after being revved (successfully and slowly). I took 2 videos, is there a way to post them? Could a sticky valve do this? It rarely does a tiny pop at idle. Ill pull the distributor and clean the governor once it cools down a little

If it is popping and sputtering even at idle, I'm wondering if you have a burned exhaust valve.

A compression test should confirm if your valves and rings are properly sealing.

I had a similar issue with my '57 fire truck years ago (322 Nail Head)...I lost all power going up a mountain and had to nurse it to the top riding on the shoulder. Ended up getting towed home. After days of head scratching I accidently touched the 3/8 main vacuum hose that stabbed into the intake and saw it move. Turns out it was just sitting on the pipe nipple and would float up and down causing a big leak randomly.

I also had burnt exhaust valves with only 20k miles on the engine, so vacuum leak and burnt valves were my issues. Fixed both issues and drove it another 20k miles before I sold it.

Mike B

While you've got the distributor out, change the points. Put one of Jon G's improved condensers in while you're at it. The offshore-made ones available commercially are two types- - - -they're either bad right out of the box, or they fail after a few minutes of running.
Jerry

Got new points and condenser in it already. It idles great and only occasionally has a tiny pop that isnt really noticeable with the exhaust on. Would that be burned valves?

Do a cylinder leakdown test and find out for sure. A compression test is good- - - -a leak test will pinpoint where the problem is- - - -assuming there is one.
Jerry

Sounds to me like what happened to my 327 when the governor failed. You could look down the carb and open the throttle all the way, could even see the blades move, yet the engine would just idle. Back in the '70s, Dad's truck (same year C60 just a flatbed vs a bus) did the same thing and he just bypassed the governor. Then it ran fine. I think I did the same thing, and installed an HEI distributor while I was at it because even back then (early 1990s), SBC HEI distributors were a dime a dozen and it was no thing to bypass the resistor wire. Then I swapped in a Cadillac V8, however if my bus had come with the 348 or 409, I would have rebuilt that instead.

These older governors are vacuum actuated, the throttle linkage from the peddle to the carburetor does not "open" the throttle blades, a spring inside the diaphragm housing opens the throttle blades, the peddle linkage allows the spring to act on the throttle shaft. With the engine not running, the throttle shaft/blades are allowed to open fully with movement from the peddle. With vacuum applied to the diaphragm, the spring tension has to be greater than the diaphragm pressure in order for the throttle to open. If the engine speed reaches the set point of 4000 RPM, the centrifugal valve on the distributor shaft will close and allow vacuum to pull the diaphragm against the spring tension and limit the amount of throttle opening. Under a heavy load with little vacuum, the throttle shaft is allowed to be opened by the spring tension, as the engine gains speed and the load is reduced due the engine making horsepower and torque, the vacuum will rise and the centrifugal valve will come into play and allow vacuum to act on the diaphragm. The governor is designed to limit the RPM of the engine so as not to scatter the engine-that is its sole purpose. In the case of the OP, I don't think the governor is the issue UNLESS there is a vacuum leak in the system.

Alright, i messed around a little. I can't find a distributor gasket that fits. All the ones online are too small. Anyone got a part number? I put the distributor back in without one for now and advanced the timing a bit past where it was before (without a timing light). It seems to have helped some with the hesitation but still not perfect. If i went any farther, it was too hard for my poor starter. Without knowing exactly what the timing is, it is way past spec. I started at probably 8-10btdc so its probably around 15ish now (wild guess). How far can i push the timing? Is this actually my problem? Unhooking the governor line doesnt seem to make any noticable difference.

GM Heritage specs call for 4 degrees btdc at 550 rpm, make sure the dwell is correct first at 28-32 degrees at idle, then adjust the timing. If you advance the timing too far, the idle speed will be too high. I have mine at 6 degrees and things work pretty well. When you disconnected the vacuum line between the diaphragm and the distributor, did you plug the end of the line when test driving?

My shop manual says 6deg at 475 rpm. I started there and increased it to about 8 and went up from there today. The dwell was set to 30 when i put in new points. I didn't actually drive it today since im afraid it will die again but i revved it plugged and unplugged (going to disy) and it didn't seem to matter. Plugging the carb side makes it idle only since it pulls the diafram.

I would trust the shop manual if it has specs for your specific truck. If the dwell and timing are correct, it still sounds like it's a carburetor issue. Have you disconnected and plugged the vacuum for the brake booster? that can be a large source of vacuum leak.