Boatguy95

First, thanks for your indulgence - I realize these are three distinct issues but I've been chasing the same elephant for a few months now and am pretty new to the hobby. I searched the forum and found several examples of solutions to each of these individual issues but each post had slightly different circumstances.

Background: I have an 71 smallblock 350, numbers matching original with original Qjet that has been rebuilt at least once. Mallory distributor installed about 3 years ago.

Driving scenario is similar to this -- yesterday evening, clear skies, 68 degrees, drive about 15 miles to a place for dinner (about a 20 minute trip door to door). Park. Car sits for about an hour. Come back, starts right up, start the drive back on the interstate and about 5 minutes into the trip down the interstate it starts lurching like a brahma bull. High speed stumble lasts about 1/4 mile, I safely pull over to the side of the road and it finally dies out. I let it sit for about 10 seconds, and then start it back up -- it has no problem turning over. Put it back in gear, and start easing down the side of the road at less than 5 mph just to see if it starts skipping again, which it does. That lasts for about 100 yds.

At this point, I stop, pop the hood, and remove air filter. (Background -- the first time this happened we discovered the PO had put cheap(er) hoses on the fuel lines and the pump was actually sucking the hoses closed. We replaced the lines between the pump and the carb.). I check the hoses for integrity and they seem fine. I visually inspect the carb and cannot see anything readily wrong. Elapsed time - maybe 3 minutes. I get back in, and she starts right up. I step out and manually throttle the car and can see fuel squirting into the chambers freely and easily. I have fuel. Hmm.

Now, with a running car, I let it run at idle for about 3 minutes. It never falters and has good idle. I decide to see what I can get -- put everything back together, and start easing back out. I get quickly to 55 mph and ease it to about 70. I run the remaining 10 miles home without incident. Total elapsed time since the first 'lurch' -- about 10 minutes.

So, to summarize -- quick easy driving trips in decent weather, park for a bit, start back up without issue and about 10 minutes into a return drive it will have a stumble. Side note - last summer after installing a Dewitt's I took a trip with my friend and our wives to NC in the heat of the July summer, and never had a single issue. Same equipment then wth no changes.

This gets me to my title question - I can see the possibility of each of these, but can also find reasons to rule out each of the 3. I am new to the hobby, and love the self-sufficiency aspects, but here will ask for the opinions of those much more skilled and experienced than I am.

Any other suggestions or ideas? Thanks in advance for your time.

Jarrett

Alan 71

Hi Jarrett,
Is the carb getting gas when it needs it?
There should be no rubber lines between the fuel pump and carburetor inlet. Did you replace the hose with a steel fuel line?
The 2 rubber lines that run from the steel fuel lines on the frame to the fuel pump have very tight bends in them. Are you using a pre-formed hose at that point? The bend is so sharp that it can pinch off the supply/return of fuel to the pump.
???
Regards,
Alan

ChattanoogaJSB

For posterity since I'm the buddy that got him into this hobby/obsession:

take is that it isn't hear soaking at the carb, leading to percolation and resultant hard starts after sitting.

Im also suggesting that it isn't vapor lock- correct me if I'm wrong- but with low engine temps on a cool day, a problem like this would not occur with such randomness AND he would not be able to walk around and find fuel coming from the primaries on the roadside just after failure/shutdown

my thought is that it's probably ignition related - my first check will be for resistance at the coil (of indeterminate age) cold in the garage and then immediately after it's been subject to full engine heat.

I wanted to postulate these initial thoughts to gain the wisdom of the group.

I should also note that when he car exhibited this before and we replaced the fuel line, we prophylactically replaced the fuel filter at the car which was clean - and the tank is clean.

Boatguy95

Hi Alan -- thanks for the reply!

Yes, I have steel lines there. I wish my compartment looked as good as yours, but here's a pic:

KEZ06

Jarrett
I have had the same thing happening to my 65 cobra replica and just cured it this weekend! First I checked the fuel pressure to make sure the in line fuel filter was not clogged. Then (Holley carb) I took out the needle and seat to the primary float bowl and cleaned it with carb cleaner. The needle and seat stuck a little in its movement when I had it out. Then cleaned the float bowl itself which had some minuscule sediment. Then cleaned the primary metering block by shooting carb cleaner thru the passages. I did not have to remove the carb from the intake. Then I reinstalled the metering block the float bowl the needle and seat set the float level while running and bam no more problem! Best if all I spent ZERO dollars and it literally took 15 minutes. Talk about satisfying when you chase down this type of gremlin!!

MelWff

as others have said I would look at ignition first. You installed a Mallory distributor, which one, points, electronic, need more information. If points, I would look at their condition and regardless of condiditon replace the condensor. I then would start taking resistance measurements of the coil and if you have a spare coil swap it out.

Alan 71

Hi Jarrett,
Thanks.
The consensus seems to indicate an ignition problem, so I'll wait to read what happens.
Regards,
Alan

CA-Legal-Vette

Just to be contrary, I'd suggest starting with replacing the fuel filter. Always do the easiest, cheapest thing first and go from there.

KEZ06

Totally agree. I started at fuel filter then carb clean. I have an old Detroit line mechanic help me and he constantly reminds that 80% of problems will be fixed in the first 2-4 steps.

Boatguy95

Thank you all very much! Your responses have been both encouraging and helpful.

To clarify, the first time this happened we did replace the fuel filter and verified pressure at the pump -- all were good.

To KEZ06's comment, the 2nd time this happened I did a good carb scrub (but did not remove it from the intake) and that seemed to fix it.

Just now, I took some measurements on the coil and have the following:

-- Between the two posts (primary), I am showing 1.7 Ohms resistance. A quick search tells me this is at least 2x the expected amount.

-- Between the center and the posts, each side shows ~ 10.5K Ohms. This appears to be within normal limits.

It looks like I have a coil problem. Yes?

Boatguy95

I can also find several sources that say my 1.7 on the primary side is good, but that my 10.4K on the secondary side is off.

Thoughts appreciated. I just took her out for about a ten minute trip and she ran fine with no issues - measurements when i returned were very similar, although I might not have let it get hot enough to really make a difference.

KEZ06

Please post what seems to work when you get it nailed down. Interested to see what it is.

ChattanoogaJSB

based on the intermittened nature of this problem at cruise- and not under high fuel demand, we are taking the marginal signs from the coil and replacing it with an Echlin from NAPA (best of the commonly stocked brands IMHO)

this forum is a tremendous asset.

Benton

68notray

Interested to hear what the culprit was...my guess would have been fuel filter, then fuel pump, then carb clogged with ethanol gunk, then sock in fuel tank, then fuel lines. In short, 100% fuel issue

Boatguy95

I will absolutely let you all know - thank you for weighing in with your guidance and thoughts! Makes the new guy feel good about our troubleshooting thus far.

ChattanoogaJSB

all reasonable responses. But the filter is new and replaced recently even though the old one was clear. Fuel pump pressure was strong at the time, carb never has ethanol has sitting in it- the car is driven multiple times per week and several fresh tanks per month, fuel sender and sock are new and tank is clean. Just to cover some bases.

The only plausible fuel issue I come up with is a stuck float but based on the timing of this issue (always at cruise, always when warm, not under high load, and fuel available immediately on shutdown) I'm going ignition.

I have been wrong before!

benton

MelWff

Without telling us who made the coil nobody can tell you what the correct readings are.
You still havent told us which Mallory distributor?

Boatguy95

I can't tell the manufacturer of the coil - markings are too worn. It's a Mallory Unilite electronic distributor.

Boatguy95

All - I have installed a new Napa coil - primary resistance of 1.7 ohms and secondary settings at 9400 ohms. Car started right up just fine and did a cursory spin through the neighborhood. Will report back after a few trips to see if this was indeed the culprit.

ChattanoogaJSB