I've had similar problems with those BBD carbs so I went ahead and moved your post to the General Technical Support forum since there's a lot more traffic here. 
'70 318 rebuilt carb dies when accelerating
I was running a '74 BBD on my '70 318. It has been running great even though the '74 BBD had not been rebuilt before I installed it. I grabbed it off the shelf from my son's car because my original BBD was not working at all.
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 '70 style 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 engine 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 changed over the accelerator pump linkage and lever from my original '70 BBD because the new one was a little different and adjusted the pump to 9/16" as per the factory manual. I 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 accelerating 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.
Thanks.
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 '70 style 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 engine 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 changed over the accelerator pump linkage and lever from my original '70 BBD because the new one was a little different and adjusted the pump to 9/16" as per the factory manual. I 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 accelerating 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.
Thanks.
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[sub]There's a slim chance that any carb will come off the shelf, whether new or re-built, and work on an engine without being tuned to that engine's specific needs. You will need at the very least a vacuum gauge. Do you have a shop manual for your car? Same year and model?[/sub]
I would have left the accelerator pump and linkage as-is. It's probably flooding out. You can try adjusting it to give less of a squirt and see if that helps, before tearing into it further.
Had much the same problem with Thermoquads on big blocks a long time ago. The bottom plate on these warped, possibly from overtightening the mounting bolts. Also had re-occuring problems with the base gaskets - not sealing after installation. One possible check to make - fill a pump style oil can with gas, idle the engine and squirt gas around the base of the carb. If the engine speeds up, you've got a vacuum leak under the carb. Other than that, sounds like the engine may be starving for gas under acceleration - possibly a leaking check valve (intermitant) in the accelerator circuit.
i Never use gas to check for vacum leaks,guess why?beep said:Had much the same problem with Thermoquads on big blocks a long time ago. The bottom plate on these warped, possibly from overtightening the mounting bolts. Also had re-occuring problems with the base gaskets - not sealing after installation. One possible check to make - fill a pump style oil can with gas, idle the engine and squirt gas around the base of the carb. If the engine speeds up, you've got a vacuum leak under the carb. Other than that, sounds like the engine may be starving for gas under acceleration - possibly a leaking check valve (intermitant) in the accelerator circuit.
nothing like an engine compartment on fire.....been there done that.as a licenced technician i would never give the unsuspecting that kind of advice.call me paranoid,but gas evaporating off a hot intake is a disaster waiting to happen.
best ,safest is water in a squirt bottle.
it will cause engine to stumble,or stall.
on many occasions it will make a slurping noise which is usually easy to find also.
those e carbs have a power valve if i recall correctly,and if not set exactly right it will not perform as expected.
i put a rebuilt on my 80 power wagon and discovered the rebuilt was worse than what i had.
proceeded to rebuild original and tune it exactly as oe manual specified.
been like that for ten years,untouched since.
dont forget the thick base gasket.....
If the carb was stored with fuel in it, you may have clogged the vents in the low speed circuit and also the power valve. Not likely clogged on the main jets. even if it was stored dry, I have seen stuff get into the jets (low speed jets/vents are very small) and that would lean you out causing the issue you are talking about. It does not sound like the accelerator pump is your issue. It works and would take a lot to flood the engine which then would be hard to start right back up without some excess cranking.
Have you tried putting the '74 carb back on? Maybe something else got knocked around while you were replacing it. I'd put the old one back on and see if the problem goes away. That would at least verify the problem is in the carb and not something else.
http://www.carburetorfactory.com/expvw03.html
Assuming it is the fault of the new carb, see the picture in the link. IIRC, it has a piston (#23) that lifts (via a spring #25) two tapered needles (#24) out of the jets (#49) to richen the mix At lighth load, high vacuum pulls it back down to lean it out (it's been a long time since I was into one of those...). When you load the engine, vacuum decreases, the spring pushes the piston up, and the tapered needles allow more fuel to pass.
You might pop off the cover (#11) and see if it's free to move up and down in the bore. With the engine shut off, the whole assembly should be up, but I think you can still push it down against the spring if it's free to move. If the spring is wrong or missing, or if it was stuck, it would be at the bottom of the bore and couldn't rise to allow more fuel through the jets when the engine vacuum signal decreases (as in under load). There is also some linkage to the throttle for that piston (items 20, 21, 22), but I sure can't remember how that was set up.
I'm doing this all from memory and I'm sure someone will correct me if I recall things improperly. But it sounds to me like your motor is going lean when you try to put a load on it.
How's your ignition? I know you have electronic ignition but I still hear my father and grandfather preaching to me (about breaker point systems), "Son, 90% of all your fuel problems are ignition-related."
http://www.carburetorfactory.com/expvw03.html
Assuming it is the fault of the new carb, see the picture in the link. IIRC, it has a piston (#23) that lifts (via a spring #25) two tapered needles (#24) out of the jets (#49) to richen the mix At lighth load, high vacuum pulls it back down to lean it out (it's been a long time since I was into one of those...). When you load the engine, vacuum decreases, the spring pushes the piston up, and the tapered needles allow more fuel to pass.
You might pop off the cover (#11) and see if it's free to move up and down in the bore. With the engine shut off, the whole assembly should be up, but I think you can still push it down against the spring if it's free to move. If the spring is wrong or missing, or if it was stuck, it would be at the bottom of the bore and couldn't rise to allow more fuel through the jets when the engine vacuum signal decreases (as in under load). There is also some linkage to the throttle for that piston (items 20, 21, 22), but I sure can't remember how that was set up.
I'm doing this all from memory and I'm sure someone will correct me if I recall things improperly. But it sounds to me like your motor is going lean when you try to put a load on it.
How's your ignition? I know you have electronic ignition but I still hear my father and grandfather preaching to me (about breaker point systems), "Son, 90% of all your fuel problems are ignition-related."
1 - 8 of 8 Posts
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