351c Won't RUN after Rebuild- carb backfire - Vintage Mustang Forums
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post #1 of 33 (permalink) Old 09-06-2019, 01:50 AM Thread Starter
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351c Won't RUN after Rebuild- carb backfire

So, this was a high school shop project, so the Lord only knows what could be overlooked.

351c 4v, 1970. We have a crane cam and hydraulic lifters, but otherwise stock valvetrain. Pertronix distributor with an msd 6al box. We have quadruple checked the timing. On compression stroke. I went to check valve adjustment but forgot that these aren't adjustable. They're the stock rockers.

One odd thing is that I can't seem to see any fuel coming from the accelerator pump on a brand new Edelbrock 600 cam carb. We get carb backfires. It's only ran for a few seconds before backfiring and dying.

Ideas for us for tomorrow? I may check compression soon, just for good measure, but it'll be a pain working around the headers.
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post #2 of 33 (permalink) Old 09-06-2019, 02:23 AM
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Joe, I know you quadruple checked your timing, but did you verify that the marks on the damper (and the pointer) actually are pointing to TDC? Forgive me if I hit anything that's way beneath your level; I'm just going to go over the basics here, so you can hit the check boxes for yourself and figure out what's going wrong!

If the wires are on in the right order (and keep in mind, 5.0 engines and 351's use 13726548 instead of 15426378), then I suspect your mechanical timing's just not right. It is *possible* to get the valve timing wrong if you have an adjustable timing set, but if you are off by even one tooth on a regular cam, then it's not likely to ever start. So, most likely reason for backfires? Wrong ignition timing.

With that in mind, why not verify that the piston's where the damper and pointer actually say it is. You might think there's no way this could be wrong, but there are tons of dampers with different markings, and the same holds true of pointer position. Plus, the damper ring can sometimes slip. Pull the plug out of #1, and use a dowel in the hole to see where the piston's actually at. You could potentially use other things in the plug hole, but avoid sharp and pointy metal objects, or things that can bind/break/fall into the cylinder where they are hard to get out. Keep in mind - even when you've found TDC, there are two TDC's for the crank. One is TDC on the compression stroke, and one is TDC on exhaust.

Once you've proven to yourself that you're really at TDC, then you can set your base timing to something between 6 and 12 degrees, while you have your vacuum advance off the dizzy, and plugged.

At this point, it should run well enough to start tuning it up.

If not, the only other thing I can think of that might be doing this to you would be improper mix, probably caused by vacuum leaks. Under the carb is a pretty typical place, especially if you're running an Edelbrock manifold without some kind of spacer. There's not much of a wall for the gasket to sit against, and it doesn't match up very well with the underside of most carbs, so you will usually get a leak. If you're not familiar with running down vacuum leaks, I'd be glad to explain how to do that too.

Hope this helps!
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post #3 of 33 (permalink) Old 09-06-2019, 07:07 AM
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Backfiring through the carb while trying to start is commonly caused by the distributor rotor being 180* out of sync. I know you've checked it but....
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post #4 of 33 (permalink) Old 09-06-2019, 08:01 AM
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I had a 351 that did same thing, someone unhappy with my asking price had switched plug wires around.

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post #5 of 33 (permalink) Old 09-06-2019, 09:09 AM
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When I get to this point, I stop and go back to square one on everything.

Since you seem to question the fuel delivery, pump the carb a bunch of times, you should see fuel out the accelerator pump. If not, fill the bowls with gas through the vents using a squirt bottle. Pump until you see fuel and then try again. If it fires off and runs, skip to the fuel diagnostic section below. Otherwise....

- Verify your timing mechanically - remove the #1 plug and turn the engine over with a remote bump switch (or by breaker bar). Hold a finger over the #1 plug hole until the compression pressure pops past it. Stop and slip a wood dowel into the plug hole and rotate the engine over with the bar. When the dowel stops moving, you should see the pointer indicating within ten degrees or so of TDC on the damper. If not, the ring on the damper is wrong or has slipped. Now check the distributor rotor, it should be pointing roughly at the driver's seat. Again, if it's way off, you need to rectify that. Verify that your rotor is also pointing at the #1 cap terminal and that the wires are on the correct terminals, in the correct firing order and direction of rotation.

Cleveland's firing order 13756548 in a counter-clockwise direction.

- If that's all good, rotate the engine backwards past TDC, then bring it back forward to 10* BTDC indicated on the damper, rotate the distributor body to align the rotor with the #1 terminal and snug it down.

- Once you're square there, squirt a little gas down the carb throat and try starting the engine - it should fire and run for a few seconds. If not, you have a spark issue.

- To verify fuel flow, remove the feed line from the carb and put it in a small bottle. Crank the engine over, if it pumps gas out, then the problem is the carb. If not, verify you have fuel in tank and all the lines are good. If it still won't pump, run a hose from the inlet side of the pump into a small gas can and try again. If it still won't pump, either the pump is bad, the arm is on the wrong side of the eccentric, or the eccentric is either missing or loose.
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post #6 of 33 (permalink) Old 09-06-2019, 11:13 AM Thread Starter
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I think I'm going to verify that the damper is marked correctly today. Well, we'll have the kids do that. It's an advanced project during a beginning class, so I can only go over to help periodically. The damper is aftermarket, so that is now suspect.

We pulled the valve cover to verify that the distributor was installed correctly. And we also have made sure the firing order is the correct 351c order.

We can visually see fuel filling our clear filter, but I'm wondering if maybe the filter could be internally collapsed? Because we didn't have any fuel out of the accelerator pumps when I checked, so that's a definite concern.

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post #7 of 33 (permalink) Old 09-06-2019, 11:48 AM Thread Starter
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Verified that the damper is correctly marked. Did that myself during a break, so hopefully by noon when the advanced kids get here, I'll have a new list of things for them to check. We'll probably verify that the fuel is making it to the carb.

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post #8 of 33 (permalink) Old 09-06-2019, 12:13 PM
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Whack on the top rear of the carb by the inlet crossover, needles might be stuck.


This is an excellent exercise in troubleshooting and critical thinking for your students.

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post #9 of 33 (permalink) Old 09-06-2019, 12:22 PM Thread Starter
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Whack on the top rear of the carb by the inlet crossover, needles might be stuck.


This is an excellent exercise in troubleshooting and critical thinking for your students.
We actually did do that on day 1, Friday, and then they took the top off and all the floats and needles looked good.

I was just thinking, we started the motor originally on starter fluid/carb cleaner. I need to just try starting it with the gas in the carb and see if it works at all.

This carburetor worked fine last spring, but we shall see...

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post #10 of 33 (permalink) Old 09-06-2019, 03:10 PM Thread Starter
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OK! So some progress. After rechecking and resetting everything, I got her to start and idle. We were getting some small pops out the exhaust. I've got to break in the new cam, so I raised the idle and let it ran... right up until the heater hose popped off the heater core inside the car. #HighSchoolerFail

So, we are currently mopping. All this said, I'm STILL not seeing any gas from the discharge jets for the accelerator pump. I guess if it runs, that's all we need for right now, but I don't know why a band new carb would have this issue. I guess we can take it apart again and inspect it more closely.

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post #11 of 33 (permalink) Old 09-06-2019, 04:12 PM Thread Starter
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OK, more good news. We started it. Ran it for 20 minutes going through the cam break in procedure. We eventually tried to set idle and it idled "ok". Checked vacuum and only had 10-11"hg. So we sprayed some carb cleaner and it died isntantly once I sprayed ont he intake near cylinder 5.

Afterwards it overheated some. Got to 230. It had been under 190 the entire break in procedure. So that snuck up on us.

351c Intake Manifold Gasket Questions- This is my first 351c. It has that big metal pan gasket. Is that necessary? I'm just honestly curious why that is needed, vs traditional paper felpro style gaskets.

Is that used in conjunction with paper gaskets? I believe the instructions that came with it were the metal gasket + RTV. Again, I had kids working on this last year, so my memory is foggy.

And regardless of all that, what's the best gasket scenario with a stock intake manifold? We also have an Edelbrock manifold, but we were waiting to put that on later after we had a baseline to compare with.

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post #12 of 33 (permalink) Old 09-06-2019, 06:52 PM
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Any intake, stock or otherwise, will benefit from a phenolic spacer to help keep heat out of the carb, and improve seal. Which heads are you running, and what radiator, which shroud and fan combo?


Grats on finding the leak at #5!

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post #13 of 33 (permalink) Old 09-07-2019, 12:39 AM Thread Starter
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We've got the stock 4v closed chamber heads. Running an aftermarket 3 row aluminum radiator, no shroud currently.

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post #14 of 33 (permalink) Old 09-07-2019, 02:27 AM
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That's most likely your problem then. Double whammy. The 3 core aluminum radiators have a lot of surface area. As you already know, they're really really thick, because each core is about 1.5x the width of a normal brass core. All those fins and all that distance makes it tough for air to make it all the way through the radiator. Without sufficient air being pushed through at decent velocity, the air either doesn't make it through, or heats up so much on the journey that it's not doing much by the time it gets about 2/3 through. This is why typically, a 2 core aluminum or 3 core brass is recommended, especially if the car has air conditioning.

If your car was going down the road, enough air would be shoved through the radiator that it would probably be fine. But at a standstill, it's easier for your fan to just thrash air around in the engine compartment and there's no real incentive for it to come through the radiator at all, if you have no shroud. If you want to run a belt-driven fan, I would very strongly recommend the 7 blade big-block version, with a thermostatic clutch, and the factory shroud. For electrics, the Contour double-fan setup works pretty well.

Glad you got the closed chamber heads - those open chamber Cleveland heads are really octane sensitive! I was afraid that might be a factor, but nope. =)

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post #15 of 33 (permalink) Old 09-07-2019, 03:31 AM
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Originally Posted by gijoe985 View Post
We've got the stock 4v closed chamber heads. Running an aftermarket 3 row aluminum radiator, no shroud currently.
shroud is a must on a Cleveland, i would look into derale high output dual fans if you plan to drive her plenty
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