350 Chevy Problems

[48bill]

The 350 Chev in my 48 Chev Convert if giving me a problem  the past two weeks and has me puzzled.  

It is a 1972 motor with a Quadrajet and a Mallory Unilite.  Carb rebuilt new cap, rotor, plugs and wires early summer 2009.  Probably 3500 miles ago.  This is backed by a 350 turbo transmission and a 10 bolt Nova rear.  The fuel tank is a "Tanks" 48 Chev replacement resin tank installed 5 years ago.

The engine was rebuilt to 355 cid with OEM cam, valve train, intake and later model heads for hardened valve seats 40+ thousand miles ago.  All rubber fuel hoses were changed 5 years ago to "viton" which is alcohol resistant and still seem fine.

Over the past two  weekends and the week between I have driven approximately 450 miles.  It started the weekend before this last one.  With no warning it acts as if it is running out of gas.  It cuts out, stumbles, catches with feathering the gas pedal or engaging the starter and then will backfire out of both tailpipes.  It seems to happen at speeds of 40-45 mph.  It has done this pulling a hills and on the flat.  Mostly on the flat.  It has happened at 1/4 tank and a nearly full tank of gas.  This has occurred five times over the 450 miles.    

The first time it happened I added "dry gas" as it had been very humid and I thought it acted liked there was water in the fuel.  It has also on three occasions on start up after sitting for an hour of so, started and then stalled when placed in gear.  By restarting, left foot braking and racing the engine I got it moving.  Once it got moving all was OK.   We have 10% Ethanol here in Connecticut.  
Any thoughts?
Thanks in advance for your help.
Bill

[enjenjo]

Check the fuel filter behind the inlet on the carb.  Some of them have a built in check valve that can stick shut cutting off the fuel flow intermitently. There are two types of filters possible, a pleated paper filter, and a sinistered bronze filter, I prefer the bronze filter. The bronze filter has no check valve.

[Mikej]

Sounds like ignition to me. If it was running out of gas, I wouldn't think it would back fire. Does it have a ballast resistor? Engaging the starter would bypass it.

[ACEMAN419]

sounds like my 89 caprise did, when the ign. coil was bad   al

[348tripower]

One other thing to check is the advance in the bottom of the distributor. Just came across a Mallory that the bushings on the counter weghts were so bad that they would go out and rub on the body of the distributor.  
Don

[Carnut]

My vote would be for fuel filters. My last $450 headache solution for an entirely new ignition system turned out to be a fuel filter problem.

[wayne petty]

with a COLD engine.. start it.. let it run 15 seconds... 20 seconds...

shut it off....   stick a rag under the fuel fitting at the carb....  holding the carb filter nut.. with usually a 1" wrench.. use a flair nut wrench.. loosen the line fitting...

do you still get a squirt of fuel pressure?

if not......    take the fuel line completely out of the carb fitting..  

extend the steel line with a rubber hose..  insert the end of the rubber hose into a clean 2 or 3 liter soda bottle...

DISABLE THE IGNITION SYSTEM...  so that NO SPARK will be created by the coil at ANY TIME during this test...

while you observe the end of the fuel line in the bottle.. have somebody crank the engine for 10 to 15 seconds...   you should get close to a pint of fuel.....

i want you to carefully examine the individual fuel pulses from the end of the hose...  they should all be full and  equal...   not starting out full and diminishing..

this is a full flow fuel pump test... this verifies the fuel pump can pump at full flow.. and the system can handle it..  

before dumping the fuel back into the tank... please use a strong magnet around the bottom of the bottle and see if it attracts any debris...

even through you have a plastic fuel tank.. you still have various steel components ... and you can get contamination from gas stations. old gas cans.. rusting gauge sending units..

if you have a fuel pump pressure vacuum gauge .. (15 pounds of pressure/ 30 inches of vacuum)  can you hook that to the end of the fuel line.. using clamps at the steel line and the gauge.. crank the engine one more time.. verify that it holds fuel pressure..

why..... it should hold pressure..

there are 2 check valves..  in the fuel pump.. one inlet.. one outlet..

if the seats of the check valves get dirt or rust embedded in them.. they won't hold pressure...   i have had cars and truck come into the shop that on an overnight sit.. they would run for 5 to 8 seconds and die.. need to be cranked for almost a minute to get it to start..

when i was testing.. i noticed that the fuel pump did not hold pressure.. i stuck a clear hose on the line at the carb.. hung the end on the hood latch..   cranked the engine to fill the hose.. when i stopped cranking. the fuel drained backwards through the pump..  all the way to the tank... i could blow bubbles in the tank by blowing on the hose..


you might also want to carefully examine the vent hole and the area on the block where it might be spraying gasoline..   if the diaphragm leaks.. it will squirt fuel from the side of the pump... it can also pull air in causing reduction of fuel flow to the carb...

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i would like to share a huge tip for installing chevy fuel pumps..

find TDC #1... either by pulling the number one spark plug or pulling the distributer cap...   as that is the position the fuel pump eccentric lobe is in the proper location for pump installation..

disconnect the battery..   pull the old fuel pump...   glue the gasket to the new fuel pump with something like super weather strip adhesive.. so it will stay in place...

now.. when you have scraped the pump plate on the side of the block..   pull the engine oil dipstick.. stick it straight down from the top through the opening.. you can work the end of it behind the tip of the push rod...

now.. put the pump in place.. the pump lever will push the dip stick and push rod back so the lever will drop down in front of the push rod..

i know it sounds like a junk idea.. but it really works.. takes the pain from installing chevy pumps.. you don't need to take the upper bolt out to hold the push rod up.. as you still have to hold the push rod up somehow..

the reason to disconnect the battery...  you are going to be sticking the dip stick down past the back of the alternator.. really close to the output terminal on the back.. and thats not something you want to short out with the dip stick..

you can also .. just set the timing marks on the damper.. see if the push rod is easily retracted with the dip stick end..   if not.. turn the crank ONE rotation..

[48bill]

RRT'rs- enjenjo, wayne petty, Mikej, ACEMAN 419, 348tripower & carnut THANKS for the great suggestions.  This site has been so very helpful over the years.  I can take apart and put back together but as a diagnostician I am a C- student.
I will use your suggestions this weekend to get to a solution to the problem.  I'll post the results when I get it done.

THANKS again.
Bill

[junkyardjeff]

It kind of sounds like way the 460 Ford I have was doing when it had the unilte,when the motor was pulled and put in my 66 f250 I installed a duraspark system and never had anymore problems so I would look at the distributor if the fuel system is ok. I dont know if it was the way it was installed or other problems with the donor truck but it would act like it was running out of gas and then would backfire when I tried to start it and after I let it sit a little it would start right up.

[48bill]

I checked the fuel flow as Wayne suggested and the distributor cap & rotor and they seem OK.  Replaced the filter. Again OK.  With work and crappy weekend weather I haven't taken it for a good long ride.  That's when it happened after 20 -30 miles of straight drive.  Hopefully check this weekend it the weather clears.

Thanks again for the help.

[wayne petty]

bill...  

do you have a digital volt meter?  hand held version..

this is a two minute test..

start the engine..  turn on the head lights..

set digital volt meter to 20 volt DC scale...

1.  test positive POST to negative battery POST.   14.1 to 14.7 volts...

2.  test Negative battery POST to engine block..   0.04 volts is expected.

3. test Negative battery POST to the body..         0.02 volts is expected

4. test Engine block to body...                          0.02 volts is expected..

if you get 0.00 on tests 2, 3, 4..  change the meter setting to 2 volt DC and retest 2, 3, 4  

post results by number..

try it on all your cars and trucks..  

if you get more than the 0.04 or 0.02 volts DC . you have a bad ground connection..

print this and try it.. ..









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on the ignition .. unilite..   is it a whole distributer.. or a conversion kit in an GM housing???

and.. somebody will know.. does unilite modules use heat sink paste under the module to transfer the massive amount of heat generated...

can you also test coil output..    got a 7 buck coil output tester.. or can you position the coil wire 3/4 of an inch from the engine block, NOT holding it.. and have somebody crank the engine..   sparks should be bright blue and snappy..  

you might also want to ohm the coil..

+ to -
+ to the output term
- to the output term  

post a coil  brand and model also..  coil windings do break down with heat..

if you have a timing light.. and some way to clip it to the coil wire.. hook up the clamps to power and wedge it under a wiper or tape it to the hood.. so  you can see coil flashing while you take it for a ride..    if the coil starts flickering..  randomly..   or goes out before the engine quits..   instead of flashing slower and slower until the engine comes to a complete stop...    might indicate if its an ignition failure problem or a fuel problem...


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on the fuel system.. were you able to verify that the fuel pump held pressure when not operating??

[alchevy]

I had a similar problem on my '40 Chevy when we first put it on the road. I had waited 3 years to finally get to drive it to Louisville for the nats and they were a few weeks away. Got the car cranked for the first time and since I did not have headlights installed yet, I waited until the next morning to drive the car around the block. It was so much fun I did it again! and again!
A few days later I drove the car about 2 miles in my neighborhood with everything hooked up. That went well so I decided it was time to drive the car to work for the first time. That was a very long trip. Since this was the first long drive, it acted differently. It would go for a few miles and then it would stall, acted like it was going to quit, so I would pull over and let it idle for a few minutes, sounded stronger, so I got back on the road. This was repeated all the way to work.
After a friend of mine went through the carb several times making sure it was fine the problem was still there.
Finally we tracked it down to the pressure regulator which was a dud. It was regulating the fuel alright....since it only let a little bit of fuel into the carb at a time. While I was on the shoulder of the road letting it idle, the carb would get some fuel. When that fuel ran out, it would start to quit.
We had used pressure regulators in the past with no problem, but this one was bad. Took it out and the problem was gone!
My friend looked at the regulator and found that the knob would just turn without stopping.
AL

[PeterR]

I struck a problem with a boat engine that would idle OK but only run for a few seconds at full throttle then progressively lose power.  After ticking over at an idle for a minute or so it would come good again and then repeat the cycle.

The small pleated paper filter behind the large nut looked perfect, but ready to try anything I gave the motor a run without the filter and it behaved perfectly.

[48bill]

I haven't had the chance to take the convert for a long drive to see if the problem was cured.
We got hit on the 29th of October by an early snow storm with 8-10" of wet snow with a resultant power loss/blackout of 6 days.  All the leaves were still on the trees so it was a severe storm where without the leaves it would have been a "ho-hum".
We were personally more fortunate than others as we had over the years removed hazardess trees and had a generator for some power, but still a mess.  Hopefully I get to drive this weekend.

[48bill]

Finally got the chance to take a good drive 5 miles of secondary roads to 15 of highway and another 8 of secondary with a couple of stops back to the interstate.  Problem occured again just as I accelerated on the entrance ramp to the interstate.  Feathered the gas and made it home without another occurance.

I'm stumped.