'70 318 BBD falls flat under load

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Robj

Robj

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I was running a '74 BBD on my '70 318. It has been running great even though the BBD had not been rebuilt before I installed it. I grabbed it off the shelf from my son's car.

I wanted to go back to the correct carb as far as the bowl vent was concerned and to get away from having so many blocked off, unused vacuum lines on the '74 carb. It had also started to leak and seep around the bowl and everywhere else it could.

I just put a commercially rebuilt BBD on. She fires right up and idles beautifully. When pushing the throttle the engine is instantly responsive and runs great.

I took it for a test drive and got around the driveway fine but at the end of my driveway is an uphill. As soon as I gave it more throttle to go up the hill, the engine immediately died. It started right up again and I drove back to the garage. The engines responds well to the throttle as long as it's not under load.

I can put the brakes on and put it in drive and put it under load and it will stumble and try to die when applying the throttle. As soon as I release the throttle before it dies it returns to a perfect idle.

I can see that the accelerator pump is supplying fuel when the throttle is pushed and at throttle under load it will continue to stumble long after the accelerator pump has delivered it's load.

This is the first time I've had an under-load drivability problem with a carb. Any ideas where to check? Thanks.
 
Accelerator pump needs adjusting, that would be my first thought.. With the car not running remove your air cleaner and look down in the throat of the carb and open the throttle while watching for gas, you should see a squirt instantly..
 
Looked it over tonight. The rod on the throttle lever was not in the medium hole so I moved that. The linkage to the accelerator pump is different on the rebuilt carb. It's the one in the left on the photo. I'm thinking I'm going to change them out and put the original on the rebuilt and then adjust the pump travel.

Any comments or cautions?
 

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Adjust the carb to the correct specs for your year and tweak from there. BTW, just out of curiosity, did you hook up your vacuum advance to the correct port?
 
just to throw it out there but compound problems are the biggest head scratchers if it were me i would check the timing........it might be coincedence when things happen but like i said just to throw it out there.....

i would try advancing the timing to see if it helps any .....if it gets stronger the more you advance, that means the chain stretched or skipped ....

im just throwing it out there bcoz i had a timing problem with same symptoms ....and it might just be coincendental that you changed the carb at this time.

just to rule things out bcoz compound problems are noodle twisters
 
Yup. Got the vacuum advance to the correct port. My son and I were just talking about exactly what everyone is saying. Adjust things up and check the timing again. Thanks.
 
Changed over the linkage and lever and adjusted the pump to 9/16". Checked the timing and everything is well. The engine still starts perfectly. Idles perfectly. Is nicely responsive to the throttle without any load. But as soon as I give it a little load either by setting the brakes and putting it in gear or by driving it, the engine will stumble and die or try to die when I give anything other than a very very careful, slow and steady acceleration.

Looking in the carb, there is plenty of spray from the accelerator pump down both throats. It's a nice atomized spray but there is a good amount. I can also see a good amount coming into the venturi when I flip the accelerator lever.

Is it possible the accelerator pump is flooding it out with too much?

What else might I look at with this carb?

I had no acceleration under load issues with the old carb and the only thing I did was change the carb..
 
Robj said:
Looking in the carb, there is plenty of spray from the accelerator pump down both throats. It's a nice atomized spray but there is a good amount. I can also see a good amount coming into the venturi when I flip the accelerator lever.
Click to expand...

Not to be repetitive, but when you do this is the shot instantaneous, as in right now, or do you get some movement before you see the shot?

Some people confuse seeing gas and seeing gas instantly. As soon as you move that throttle you should see a shot.

IF you are getting some movement of the throttle before you see gas that could be your dead spot. If the problem didn't exist before you replaced the carb then your looking in the right place IMO.
 
Badsport said:
Not to be repetitive, but when you do this is the shot instantaneous, as in right now, or do you get some movement before you see the shot?

Some people confuse seeing gas and seeing gas instantly. As soon as you move that throttle you should see a shot.

IF you are getting some movement of the throttle before you see gas that could be your dead spot. If the problem didn't exist before you replaced the carb then your looking in the right place IMO.
Click to expand...

Good point so I just went out and looked.
It is like right now. It sprays instantaneously.
 
Badsport said:
Interesting, have you checked for vacuum leaks?
Click to expand...

I have. I don't hear anything and the only unused port is capped as it was on the other two carbs. I've physically checked the other vacuum lines and everything is intact.
 
Put some water in a spray bottle and spray around vacuum lines, base of carb, anywhere there is a vacuum line and see if you catch a stumble in idle. A vacuum leak might not be "audible".

Some people use carb cleaner as well but it could damage paint. If your engine isn't all painted up nice and fancy you could use that as well.
 
Badsport said:
Put some water in a spray bottle and spray around vacuum lines, base of carb, anywhere there is a vacuum line and see if you catch a stumble in idle. A vacuum leak might not be "audible".

Some people use carb cleaner as well but it could damage paint. If your engine isn't all painted up nice and fancy you could use that as well.
Click to expand...

I can do that. When I lean the grill up next to my Weber as the charcoal lights our two cats, Duster and Fury, appear from nowhere to lick on it. I just happen to have a spray bottle of water on the counter next to the patio door for such occasions and I will put it to use on this.
 
.......................power....................valve................... (piston)
 
More clues. I took it out on the rural two lane highway our road is off of. If I ease into it, once the throttle is open and I'm accelerating, it takes off like never before. But if I get off the gas and punch it, it stumbles and when it catches itself it takes off again like a new engine. If I just partially let off on the gas and punch it, it takes off great. It's only if I release it all the way and try to accelerate at anything other than easing it open. Again once opened up I can punch it and go.
 
67Dart273 said:
.......................power....................valve................... (piston)
Click to expand...


You would think on a recently purchased rebuilt carb you wouldn't have to worry about that, but I guess anything is possible.
 
Adjustment? Replacement? What am I looking at to try here? I've read up on it and it looks like maybe bringing the needles up a little?
 
This IS a "rebuilt," right? These things have a HORRID track record, I"m afraid. Hell you might have a main jet that is out of something completely different. But that's what I'm thinking, main jet or the power circuit. Is this the small bore (318) bbd, or the large (360)?

But I'd be really tempted to tear it down and at least check the piston/ rods for free movement. Hell they may have left the spring out!!

For the record, you CAN DRIVE a car with OUT an accelerator pump. They of course can be a ***** to start, and may need a squirt down the carb. I used to know "an old guy" (LOL, now I'm older than he was then) who used to UNHOOK the pump link on long trips!!!
 
Pulled the airhorn off and looked around. The spring under the step up piston is there and it all moves freely. The accelerator pump looks new. I can only assume that it's the correct part. It's the leather type.

Checked for vacuum leaks with carb cleaner and water. Everything on the exterior seems to be ok as far as that goes.

I bought the rebuild from National Carburetors. A couple buddies recommended them based on their experiences. I had planned to install it as soon as I got it but then got rear ended and it got put on the back burner so it's just past the warranty now. I'll still call them on Monday but if anybody else has some ideas I can check.

I was hoping to go for a drive on Sunday. Beautiful fall weather. I can drive it but I'd rather not kill it at an intersection and have some joker hit me in the rear again.

Thanks.
 
My 69 Dart ran like that for years, thru many carburetors, even a new long block. Turned out it was the carb and ran great with no stumble with the 4th carb. I think all the others idled too lean. I always wondered why I couldn't turn the idle screw to get too rich. That was a 225 w/ 1 bbl, but same concept. Indeed, I have a Holley Pro-jection TBI w/ O2 sensor on my Newport. If idling lean (LED indicator), it will stumble off the line. The accelerator pump (or function) worked in all these carbs (and TBI). If you install a rich/lean indicator (Holley or MSD), you would see what is going on.
 
I had the same problem with my daughter's 1974 Dart Custom 318 as what you describe. I ended up adjusting the step up piston little by little until the carb performed under all conditions. I wrote down my starting position of the adjusting screw and then noted which way I turned it and how much and what the result was.
It took me a lot of fiddling around with it (it was an off the shelf reconditioned carb) until I was satisfied.

A little while later, my daughter ran the car into a utility pole and bent the floors and A-pillar so bad I had to get rid of the car. But the carb was adjusted! LOL

Get an Edlebrock intake and put a 4 barrel on it.
 
I don't see an adjustment on that screw. I was looking at it. It's screw with a long shoulder on it but it seems to just tighten down. I know the 74 BBDs went to the solid fuel design. I wonder if it became adjustable at that point.
 
Just found a BBD manual online and it says, "The metering rods in the solid fuel unit are both mechanically and vacuum operated and must be adjusted."
 
I officially raise the white flag on this BBD. Put a rebuild kit in the reman carb and took missing parts off my old carb. Bowl vent, spring and clip, bowl baffle and it had no gasket under the step up valve. Adjusted the float a tad and it still runs exactly the same. Figured I might find a gross problem while taking it apart but no.
 
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