Just installed a new 351M into my 78 F-150. It has the edelbrock Mainfold and 600 CFM carb. I'm having a problem with the engine sputtering right after I accelerate off-idle. It feels like the engine is jerking the truck back as I try to accelerate. It also seems to do it more frequently after the truck has been warmed up. I've timed it to 12 BTDC and it has a steady vaccuum of 17 at idle. It seems to idle smoothly, but as soon as I accelerate it start it starts to sputter (At least thats how I interpret it, not really sure how else to describe it). I've tried adjusting the idle mixture screws according to the edelbrock manual, but not sure if I've done this correctly. I have them about 2 turns out from totaly seated. I have also tried to moved the accelerator pump lever to give a longer squirt, but that didn't seem to help. One more thing, it sputters regardless if I have the vaccuum advance hooked to the port, full, or plugged off. The vaccuum advance also doesn't change the timing at idle when it was hooked to the ported or manifold vaccum, so I'm thinking the vaccuum advance mechinism in the distributor my be damaged, but would that cause this problem, even when it is plugged?

Any suggestions would be greatly appreciated.

Thanks,

 

[updated:LAST EDITED ON 11-Mar-02 AT 04:49 PM (EST)]Smitty, the vacuum advance is working correctly; it should not advance the timing at idle. In fact, you can remove the hose, and block the port temporarily. The engine will run very well without it, untill you get the bogging fixed.

I would try backing off the timing a bit. Try 6-7 degrees btdc. That is the stock setting, and at least will tell you if timing is the problem.

If retarding the timing dosen't help, then the hesitation is in the carb; probably the accellerator pump circuit.

EDIT: I just re-read your post, and the manifold vacuum should indeed advance the timing at idle. But having the v.a. disconnected will not affect the hesitation. Maybe the advance weights in the distributor are sticking at low rpm. Does the timing advance correctly (using timing light), when the throttle is advanced?

 

Thanks, I'll give that a try tonight. Could it be that the timing is too far advanced and the sputtering is really pinging or pre-detonation? Also, If I did have the vaccuum advance hooked-up, when should it kick-in?

Smitty78

 

Hi Smitty. I don't think it's pinging, just a hesitation problem. The vacuum advance should come on at about 1200 rpm or so. But like I said, just disconnect it for now, until you get the hesitation fixed.

If the timing is advancing OK, then the carb is the culprit.

 

Get a vacuum pump tester and check to see if it makes the
vacuum part of the advance work.That's for piece of mind.
Make sure static timing is right on spec.

That rod linkage for accelerator pump should be correctly indexed per
that application.
Normally the second hole from the end.
That's what usually gets rid of hesitation.


Dennis https://www.ford-trucks.com/user_gallery/sizeimage.php?&photoid=1733&.jpg
[i][font color=red]Calgary,Alberta,Canada[/font][i]




[font color=blue]Please Don't Ask Me Any Tough Questions,
"I'm Saving My Memory For When I Develop Alzheimer's" [font/]


78 F-150 429CJ,Silver, Explorer Pkg.
641/2-Mustang 260,Pre-World's Fair Car.
64-Fairlane500 S/C waiting for a 390-4spd.
68-Mustang 289-Sunlit Gold 80,892Mi
78-Buick LeSabre 403 4V
84 Volvo DL Wagon

 

I set the timing back to about 7 deg BTDC tonight and took the truck for a test spin and it seemed to hesitate even more. I noticed alot of "pop" sounds (kind of like a backfire) as I was trying to accelerate down the highway.

I got it back to the garage and then tried to test the mechanical advance. Since I don't have a tach I have to kind of guess the rpm. Just had the wife increase the rpm slowly, while I gunned the timing light. I tested it with the truck in park and in gear (as the problem is alot more noticable when the truck is in gear). I also had the vaccum advance pluged off for the test.

IN PARK
idle - 7 deg
- 12
- 16
- 18
- 19
- 20 (started to sputter and would not increase any more)

IN GEAR
idle - 7 deg
- 11 (started to sputter and would not increase any more)

This has me really confused, could it be that the mechanical advance is not correctly advancing far enough and that is causing the sputtering at the higher RPM and under load? Not sure about this particular engine, but my Chevy, uh..I mean my other car, advances up to 40 deg BTDC. :-)
Also, I set the accelerator pump linkage back to the second hole and it didn't really have any effect on the sputtering.

Thanks for the help
Smitty78

 

Condensation in the cap?
Carbon tracking inside the cap?
You said you plugged the advance.
Do you mean you plugged the Vacuum Hose going to the advance?
It's the hose to the advance that needs plugging.
If you run it in the dark can you see any like a lightning show under the hood?

Dennis https://www.ford-trucks.com/user_gallery/sizeimage.php?&photoid=1733&.jpg
[i][font color=red]Calgary,Alberta,Canada[/font][i]




[font color=blue]Please Don't Ask Me Any Tough Questions,
"I'm Saving My Memory For When I Develop Alzheimer's" [font/]


78 F-150 429CJ,Silver, Explorer Pkg.
641/2-Mustang 260,Pre-World's Fair Car.
64-Fairlane500 S/C waiting for a 390-4spd.
68-Mustang 289-Sunlit Gold 80,892Mi
78-Buick LeSabre 403 4V
84 Volvo DL Wagon

 

hi
have you tried looking at the valve timing? if your time chain is bad it could cause it to do the same thing. choke is also another factor, if it is closing or opening to much it might not run right. normally if it is warned up than you shouldn't need the choke, well try putting the choke on just a little bit and if the problume stops than your bottom gasket for the carb is bad or your intake gaskets are bad. That's another thing if your intake gaskets are bad it could cause this problum. you said you had good vacum but it could still be the gaskets. well that's all i can help, if any. Good luck!

 

I would like to give my two cents worth of advice. First off, I noticed you said you connected your vaccuum line to various ports. full ,ported,etc. It has to be connected to a ported vaccuum source. ,in other words ,it should have no vaccuum at all at idle and then should increase the vaccuum as engine rpms increase. Otherwise, why set your intial timing at x degrees and then hook up a vaccuum source that changes the timing immediately? secondly ,You can run off mechanical advance only . It doesn't do as well as a dist. that is calibrated to run solely on mechanical advance but it will run as you have already discovered. I would suggest connecteting the ported vaccuum line directly to the dist. and not running through a bunch of timed vaccuum lines/solenoids/checkvalves that barely worked new and by now haven't worked in a decade or more. Secondly , While you never named your carb,I'm guessing it is a Holley. It is somewhat common for a part called the power valve to fail during a backfire unless you have a pretty new carb with the backfire preventing checkvalve/ ball in the baseplate. so I would check it out. It seems like you are having the exact same problem I had with my 351m. It ran so lean at idle /just off idle that it stumbled/backfired like crazy . I ended up putting bigger jets in the front of my Holley to help it idle better and come off idle without stumbling and backfiring ( which is a sign of a lean mixture as well as other problems). Just keep setting your carb.to obtain the highest manifold vaccuum at idle and dial in the intial advance around 10-12 . Look at it this way if the only thing you changed was the carb and intake why would the dist. be bad ?

 

You mentioned a popping noise "like a backfire." This advice may or may not help, but I had a similar problem with my 400M. My problem was that the air bypass valve was stuck open. I had to hook up all the smog stuff for inspection and then it would take a long time to warm up and would "pop off" when giving it gas and when going off throttle. I took the air bypass valve off and out came a bunch of white dust. I took a length of vacuum hose and hooked one end to the bypass valve and the other in my mouth and blew and sucked until it came unstuck and worked properly. This took care of a number of oddities with the performance of my engine that I could not figure out. I don't know if you have any emisions stuff on you truck but it may bo worth a look at.

Jeff

 

Thanks for all the good advice. This is really a great site. I should have mentioned, but it is an edelbrock carb, and there is no longer any emissions equipment attached. Are there any parts on the edelbrock similar to the power valve that could blow out?

There is only one vaccuum line from the distributor to the carb. When I took the line off, I plugged the hose going to the distributor and the vaccuum outlet. I intially had the vaccum attached to the ported outlet, but tried the other one to see if it had any effect on the sputtering.

The rotor, cap, spark plug wires, and coil are all new and I didn't see any sparks last night when it was dark.

I don't think it is the valve timing, because it is a new engine. But who knows, I sure hope it isn't. Would a bent pushrod cause anything like this?

I'm going to try the choke idea tonight. Since it is an electric choke, would I just force the choke plate down while the engine is running?

79F150 mentioned that this sputtering might be a condition of the fuel mixture being too lean off idle, I did adjust the idle screws to get maximum rpm, but the engine was still sputtering when as the rpms increased. Also, the sputtering makes it almost impossible to drive.
Also, I'm not really sure if the distributor is good or not, as the engine was blown when I got the truck.

I might stop by the parts store and see how much a reman. ditributor runs. If it is reasonable I'll get it and see what happens.
Has anyone seen a distributor with a faulty mechanical advance mechnism that caused a problem like this?

Thanks Again.

Smitty78

 

[updated:LAST EDITED ON 12-Mar-02 AT 04:47 PM (EST)]Hi,

If it was me, I would hook the distributer vac advance to the ported connection on the carb, I would try a little more timing (just a bit 2-4 degrees!) and less on the accelerator pump (go down to the hole that provides the smallest shot of gas when you step on the accelerator).

I suggest this because I never entierly trust the stock harmonic balancer (they can slip on the rubber!) and often the pointer can move or get bent to provide an inaccurate zero reading. As long as there is no pre-detonation (pinging) it should be OK.

Just another idea to try, let me know how you make out.

PS I doubt the distributor is bad!
-Brents.

 

I got it fixed tonight!! :7 It ended up being the distributor. I put the new one in, timed it to 10 deg BTDC and boom, it fired up without any sputtering. Took it for a test drive and boy does it run good. Was able to get a total advance of about 35 or so.

I examined the old one and the vaccuum advance defintly was not working and the whole advance mechanism looked kind of loose. I'm guessing that it was just not advancing at the higher RPM. And that is why it ran fine at idle, but started sputtering as I accelerated.

I did accidently put the vaccuum advance on the full manifold vaccuum and I timed it at idle and had 37 deg of advance. Oops.


Anyway- thanks for all the advice.

Smitty78

 

I knew it, I just knew it.

You're more than welcome.

Excellent Smither's !

Dennis https://www.ford-trucks.com/user_gallery/sizeimage.php?&photoid=1733&.jpg
[i][font color=red]Calgary,Alberta,Canada[/font][i]





[font color=blue]Please Don't Ask Me Any Tough Questions,
"I'm Saving My Memory For When I Develop Alzheimer's" [font/]


78 F-150 429CJ,Silver, Explorer Pkg.
641/2-Mustang 260,Pre-World's Fair Car.
64-Fairlane500 S/C waiting for a 390-4spd.
68-Mustang 289-Sunlit Gold 80,892Mi
78-Buick LeSabre 403 4V
84 Volvo DL Wagon