I have a 68 F-600 Dump truck with a 391 Ind and 2bbl carb. The 391 was installed by the previous owner. I am not sure if he used the 2 bbl and intake from what I believe was a 330 that the truck I think cam with originally. Is is jetted correctly ? It is backfiring when I take off, and also when pulling a load in the 1500-2000 rpm range. It starts and idles good. Shuts off fine, but when I try to acceralerate from a stop it will pop and backfire. I rebuilt the carb a year ago and it was fine. I imagine that it needs it again as I am thinking the accererator pump could be the culprit. It squirts fuel when I pump it and look, but maybe not enough? I changed the positon of the actuator rod to allow for max fuel, but it still pops. I used a NAPA Echlin kit #2-5585, not sure of their quality, is there something of better quality?
I installed new points and plugs, cap and wires seem fine.
Timing is @ around 8 BTDC right now

What are my timing settings ?

Any advice beyond it just being the accel pump ?

Should install a carb spacer ? Seems to me that carb gets very hot when hauling heavy loads. I a running a 190 thermostat.

thanks for any advice..............

 

I would do a compression check I had a 390 a few years ago with the same problem after a lot of time messing with it did a compression check it had a bad intake valve and a couple burned exhaust valves. Had the heads rebuilt runs great now.
Richard

 

Spark Plug Gap = .030
Point Dwell = 26--31,
Point Gap = .017,
Timing and Idle should be set to specifications on Engine Decal, but if it's missing, set timing at 6 Degrees Before Top Dead Center with vacuum advance disconnected and blocked and idle at 550 rpms. NOTE: If Auto. Trans, set idle at 500 rpm in Drive.

You might want to check out your vacuum advance and make sure it is operating as it should. Sounds like you might not be advancing as the rpms increase. Advance could be sticking or hanging up on condenser wire, etc.

Hope this helps a little..... Good Luck!

 

I would be inclined to think the vlaves might be an issue if it were not for the fact that the motor idles down to 500 rpm and hits on all 8 perfectly.

If I had valve issues I think it would not idle as it does.

When I pulled the vacuum line from the carb to plug and set timing I did not detect any vacuum from the carb to the distributor. I need to cehck that out.

I have a carb rebuild kit waiting for me. I'll post the results when installed.
Thanks for the replies..................

 

NO change with the new carb kit installed.
Despite having changed the points recently I am thinking there is an issue with the ignition system. How can I test the dist to see if it is worn out ? \
As far as the distributor goes, the points base plate moves more side to side more then I believe it should. It may be time for another. I guess going to electronic i s best if I am changing it out ?

Also the butterfly shaft on the carb seems to be worn also. I have a Motorcraft 2100 2 bbl. It has #57 jets in it. Is this correct for the 391 ?

I am doing a compression check next.

Any more suggestions are welcome.

 

If ther any side to side movement that you can plainly see in the Dist. its worn out. time for a rebuild of it or replacement. if you go to pull it, carefull of the oil pump shaft and that it doesnt drop out and into the oil pan. Ad for the carb, the 2100 is a dime a dozen. so finding a rebuildable one should be easy

 

Points are obsolete. Although not listed on our website, we do make our D.U.I. Distributor for the 391 motor.

info@PerformanceDistributors.com

 

While Point are obsolete, that work good and are cheap to replace, after spending over $250 on a Ing conversion and having it fail in the middle of no where. I had to strip it all out and put points back in.

 

I've been watching your progress since I posted the tune-up specs for you. It seems everything you have run into points to the distributor being the problem. At least, I would suspect it quite seriously.

If it were me, I would disconnect the vacuum line from the distributor and plug it with a bolt so as to be sure I wasn't introducting a vacuum leak to the carb. Then using my timing light, I would check the timing and watch to see if the mark seems to "jump" around in the light. It should remain in one location and not "dance" around. Slowly increase the rpm's up to about 2,000 all the time watching the movement of the timing mark in the timing light. When you see the timing mark "advancing" note whether it is doing so smoothly or if it is "jumping" around. Perform this test at the rpms you seem to be having the trouble at. If the mark is jumping all over the place, the advance probably needs repair or the shaft bushing, etc. Other than to spend a lot of time on rebuilding the distributor and not having a test bench to check the repairs on, go get an exchange from AutoZone or whatever discount parts house you have in your city. These distributors only run in the neighborhood of about $40 exchange. I know, $40 is $40! But this could very easily be the cure for your problem. Test It! Good Luck!

 

I will test the distributor as you have mentioned. I did notice that the advance rod in under the cap looked as though it had been rubbing on the point base plate. I bent it up enough so it can move without contacting the plate. Also I watched the advance when it revved up and it did seem to move smoothly. I called my local NAPA, they want $127 + $22 core for a rebuilt distributor there competitor wants to send in my old one for rebuild. Anyone have a part # for the correct dist ?

When I suck on the vacuum line to the dist it moves the base plate and appears to be working correctly.
I figure that as this truck had (I believe) a 330 in it originally, the replacement motor that was purchased as a long block may have had the old dist installed, and it is now worn out.
I am going to do a leakdown test to verify its not leaking valves.

It was mentioned that carbon build up near the valve set couldl be causing this also. I was told that as the carbon gets redhot the incoming fuel charge can ignite as it enters the cylinder and cause this condition. I was told to pour water slowly down the carb to see it it would help remove the carbon deposits. Not sure If I can beleive that.

More as I get the above noted items checked out. Again thanks for all suggestions/info.

 

We cannot give you a price for our DUI Distributor on this forum, but you can e-mail your request for a price to:

info@performancedistributors.com

 

Sounds a LOT like a bad distributor cap to me.

Check the rings around the center contact and see if there are any small lines running perpendicular across them.

I worked on points-type ignition for MANY years, and that was one of the most common problems.

You'll have to look very hard, because they are sometimes very hard to see.

You might want to just go ahead and replace the cap and rotor, as cheap as they are. At least that will eliminate the possibility that they're bad, and possibly save yourself a LOT of time and headaches!!!

 

It DEFINITELY sounds like ignition. Anything mechanical should show up at almost any RPM. It sounds like, based on what you say, that the problem is happening when the engine is under a load, and at specific RPM ranges.

You said you replaced points and plugs. How about the condenser?

Check the wire coming from the distributor to the points. Does it have any cracks in the insulation?

Check the top of the coil. Is it really clean? No dust or oil? Could be arcing between the coil primary wire and one of the terminals on either side.

If you can suck on the vacuum advance and make it move, the vacuum advance is probably bad, because it takes one helluva lot of suction to make enough vaccum to cause it to move. I've always had to use a hand-held vacuum pump to create that much suction. (Or, go down to a local street corner and pick up a hooker!!! LOL) Possibly a broken spring in the vacuum advance unit.

Worn breaker plate should show up with a dwell meter when you rev the engine, whether the vacuum advance is connected or not. If the dwell is consistent when the engine is revved, look elsewhere.

Just because wires "look" good, that means nothing with resistance-type wires, which is probably what you have (I haven't seen solid-core wires in many years). The core can have too much or too little resistance, and will cause a plug to misfire under a load when the demand for spark is increased. Replace them!!!

Eliminate the cheap-to-fix possibilities before you go chasing the expensive stuff.

Actually, you'll come out ahead if you just go ahead and bite the bullet and find someone with an older engine analyzer with a scope. The cost to hook it up to one of these should be low, and you can isolate the problem in just a few minutes, rather than spending many hours and many dollars guessing.

The only other causes I can think of have already been mentioned. A bad cam lobe on the intake side would cause backfire through the carb, but that would sound like a popcorn popper, and would be consistent under wide open throttle at most any RPM under a load. Bad exhaust valve should cause a misfire (skip) at most any RPM. A simple vacuum test should show you a bad or leaky valve that would be leaking enough to cause a problem. Bad accelerator pump would cause severe hesitation and probably a single backfire when you first push the accelerator pump, even sitting still.

 

Thru 1971, there were two point plates, upper and lower separated by a plastic bushing.

In 1972, Ford replaced those crappy upper and lower plates with a one piece assembly. On your truck...you might still have those two plates, or this one...

D2PZ12151A .. Point Plate (Motorcraft DB187A)

This plate, the points, condenser, rotor, cap and etc are the same for 221/260/289/302/330/351C/351W/352/360/361/390/391/400/410/428/429/460 V8's thru 1974.

The points are the same thru 1976....and are still available from Ford (as are the other parts) and every autoparts store: B8Q12171A (Motorcraft DP12)

 

I installed a new dist cap, new quality plug wires, new condensor, verified that all wiring on the distributor and coil was clean and tight. Rechecked and verified timing, installed new fuel filters.

It runs perfectly now with now hesitation or popping/snorting and wheezing like it had been doing.


I am very grateful for all the advice shared and given. Thanks guys.