Hesitation

[69.5SuperBee]

Hello everyone I am new here. My name is Sal and I am speaking on behalf of my father Paul.

He isn't internet savvy so I would like to start by saying my Dad has a 1969.5 A12 Superbee that recently had an engine overhaul. Probably has over 500 miles into it now. The engine starts great and idles great. It was done in time for my Wedding last June as the limo and it ran great. However about a month after he had some intermittent issues when putting it into drive. The engine just stalls. BUT only sometimes. Besides that problem, when the car does idle good my father tells me that the car moves really great but around 65 mph the car hesitates. At first, I thought it would be detonation but he said it's not. It's almost like a light sensation of pushing and releasing the accelerator rapidly. I have yet to drive it myself but my Dad has been driving it since the 70s so I'll take his word for it.

The engine is pretty much stock except for an electronic ignition upgrade, a Mr. Sixpack cam installed straight up, and base timing set at 16 degrees. Full advance is 34 degrees at 3000 RPMs.

When looking for the problem, I did catch that the vacuum advance was hooked up to the manifold so I put it to the carb port. The correct one where vacuum signal is only achieved when the throttle is cracked open a bit or when engine is under load.

Any thoughts? Like I said the car runs great except at cruise. Vacuum advance is adjustable and we've tried that. We're stumped.

Any help would be greatly appreciated and I would love to see my Dad driving it again soon.
Thank you!

 

[Yatzee]

There are certainly a number of things to check. Considering that the car runs at 65 mph I would think that the ignition system is OK but depending on the type you have it's worth looking at the ignition box/points, if used, and all associated components like plugs and wires. But I'm suspecting that the fuel delivery is the culprit especially if it sat a long time with regular gas in it. Carbs don't respond well to ethanol.Other issues with the fuel system include a clogged filter, dirty fuel tank, partially plugged pickup sock at the sender and water condensing in the tank. Sometimes it's as simple as a faulty gas cap that fails to breath creating negative pressure in the tank that the fuel pump just can't overcome. Now not to antagonize anyone here, if you've got one of those damn "Edelbroken" carbeurators on the engine you may be chasing your tail for a long time - they seem to be riddled with issues these days. You'll have to spend some time hunting down the cause unless the obvious avails itself as the problem right off the bat, and, I wish you the best of luck.

 

[69.5SuperBee]

Thank you for your reply.

It's an A-12 Superbee (Lift-off hood 440 6-pack) No edelbrocks under this hood. My dad changed the ignition box, wires, plugs, and even the distributor. Still happens. Fuel delivery. Interesting. I should have him try it without the gas cap just to eliminate any vacuum in the tank as the culprit. I will check out his hoses.

I appreciate your input. Thank you Yatzee

 

[Yatzee]

Thank you for your reply.

It's an A-12 Superbee (Lift-off hood 440 6-pack) No edelbrocks under this hood. My dad changed the ignition box, wires, plugs, and even the distributor. Still happens. Fuel delivery. Interesting. I should have him try it without the gas cap just to eliminate any vacuum in the tank as the culprit. I will check out his hoses.

I appreciate your input. Thank you Yatzee

Well if it's a six pack, take a close look at the way the carbs are set up. Hesitation is usually related to fuel delivery at the manifold as well as from the tank. You may have to dismantle your carbs and have a good look for loose throttle plates and faulty internal items as well as worn throttle shafts and linkage. Lower than usual vacuum can also play havok with six packs especially when one of them is the cause.

 

[69.5SuperBee]

Interesting that you said that about low vacuum.

Before the Mr Sixpack cam my dad had a purple cam in it. Even with vacuum advance at the manifold he never had any of these problems but I am not blaming it on the cam. A while ago I discovered that since he now has 19" of vacuum I should put the vacuum advance line to the carb and change his power valve too. Engine has a 6.5 now. Is there such thing as a 9 or 9.5? Need to check.

 

[dieseldazzle]

How does it run at part throttle with no vacuum advance hooked up?

 

[lewtot184]

those mr. sixpak cams have relative short duration. the center carb will need #64 jets. do a fuel pressure check and make sure there isn't an in line fuel filter on the suction side of the pump.

 

[69.5SuperBee]

How does it run at part throttle with no vacuum advance hooked up?

I have yet to try that. I will see to it this weekend.

those mr. sixpak cams have relative short duration. the center carb will need #64 jets. do a fuel pressure check and make sure there isn't an in line fuel filter on the suction side of the pump.

Inline filter before the fuel the fuel pump? Hmm...never checked that. I will check the fuel pressure.

 

[Grady Cain]

Welcome to FBBO from another A12 'Bee owner in CT.

 

[oldbee]

Maybe you have a vacuum leak? GREAT ride for your wedding!

 

[RJRENTON]

With regard to the power valve in the center carb, the number stamped the hex indicates the opening point of the valve. Its normally open, vacuum holds it closed, then opens when manifold vacuum drops below that level. Go to the Holley Web site to see the availability of what you need. One caviet is. The fuel metering component in the POWER ENRICHMENT CIRCUIT IS: the power valve channel restrictions which do the flow metering. These are calibrated drillings....be very careful if you enlarge them....for there is no turning back if you make them too big. Measure the size with a drill rod and a micrometer. The power valve can be eliminated with a power valve plug by Holley, but the main jet size will need to be increased to compensate. REMEMBER: jet size is deiermined by flow. For exsmple, if using #64 main jet and you want to eliminate the power valve, you must determine the AREA OF THE CHANNEL RESTRICTION (A=3.14159 × radius of the restriction squared) which results in ______sq in. Convert back to diameter to add to the main jetsize. DO NOT JUST ADD THE DIAMETERS as it is an Area function...if you do, the jet will be too big. The calculation results in square inches.
Another consideration, is the idle mixtures of the front and rear carbs. Typically, they're too lean, this results in a transition stumble. You can dig out the lead sealing plugs, and unscrew each about 1/4 richer. Once set richer, that's all that you will need to do.
Fuel level in ALL the carbs is important....Holley carbs are sensitive to fuel level. Follow shop manual setting procedures.
This is what I've done to my 70 GTX. See my garage pix as RJ's Toy.
Bob Renton

 

[biomedtechguy]

Welcome. WOW! A-12 Super Bee!...
I have a 70 V code 440+6bbl Roadrunner. I usually post carb/intake questions in the "A-12" forum, you may want to copy and paste your initial post there.
I have a FBO upgraded electronic ignition system from Don in Oregon. He specifically instructs that his distributor vacuum line should be connected to constant manifold vacuum for his ignition system setup, not ported vacuum. Is the ignition system one of his? Try disconnecting the vacuum advance and plug the vacuum source when you do. A jet# check in the center carb would be my next step. I went from #62 (car came to me that way) to #65 (it has a 292° 509 purple stripe cam). With my 4 speed, that seems to work well in south Louisiana. Check for a vacuum leak with the red straw in the spray head of a can of carb cleaner. Spray a brief squirt only along areas where carbs meet manifold and manifold to cylinder head intake ports with the engine warm and idling. If the idle slows, you have found your leak at the spot where you just sprayed. Many other good suggestions previously listed. Focus on vacuum related issues, IMO, at a STEADY 65mph you are on the center carb "mostly" and the "hesitation" you describe sounds like a lean condition. The outboard carbs idle mixture screws in the front of the base of those carbs make a BIG difference in idle and running at light throttle performance.
Interesting too that this took a month to develop, like something may have "loosened". Carb to manifold mounting bolts? Intake manifold bolts? Clogged fuel filter? Float level check in all 3 carbs? At idle on a level surface, the fuel level should be at the bottom of the fuel bowl site screw at least or ever so slightly trickling out. Use caution if/when you check and adjust the fuel bowl floats and when you have the level screw plug out (1 at a time) due to fuel spillage.
I await your findings and solution(s).

 

[AR67GTX]

. Besides that problem, when the car does idle good my father tells me that the car moves really great but around 65 mph the car hesitates. At first, I thought it would be detonation but he said it's not. It's almost like a light sensation of pushing and releasing the accelerator rapidly. I have yet to drive it myself but my Dad has been driving it since the 70s so I'll take his word for it.

What this sounds like to me is if this pulsing sensation is when he's holding a steady speed, the motor timing is over-advanced. I had this problem with a BB Chev where the vacuum advance unit pulled in too much additional timing and when I was at a highway cruise speed where most of the centrifugal advance was in as well as the vacuum advance - I had a constant slight missing or pulsating type sensation that can feel like a lean surge. What's happening is the plugs are firing so far in advance of TDC that the motor is almost on the verge of kicking back as each cylinder fires and has a little hiccup. When I gave the motor throttle, the vacuum advance would drop out and the motor ran clean - it was sort of puzzling until I figured it out. It's easy to check for - just plug the vacuum advance and take it for a test drive. If it runs clean at 65 mph with the vacuum advance plugged, you have found your culprit. I ended up fabricating a small limiter to limit the amount of advance my vacuum can could pull in. But if he has an adjustable vacuum advance can on the distributor he should just be able to reduce the amount of advance with an allen wrench.

 

[dieseldazzle]

Yes I believe the surge will only happen at light throttle when engine is pulling vacuum and he will find that it goes away when vacuum advance unplugged and can tune from their with a limiter plate and or adj vacuum can or pull rod

 

[69.5SuperBee]

I believe this may be the problem. Smaller napa filter ?

 

[69.5SuperBee]

Replying through my phone is confusing. I may have responded on the wrong post. If i did sorry!
So we tried disconnecting the vacuum advance and plugged the carb port. Problem is still there.

 

[AR67GTX]

Replying through my phone is confusing. I may have responded on the wrong post. If i did sorry!
So we tried disconnecting the vacuum advance and plugged the carb port. Problem is still there.

OK - thought that vacuum advance was a sure winner from the description. Could be an old gas line filter - but running good under throttle and acting up at steady cruise is not typically symptomatic of a clogged filter. Should be just the reverse - runs OK at low speed/light throttle but falls flat with open throttle as the engine leans out.

I'm not familiar with Holley 2 bbls but I assume they have a transition circuit just like the front part of a 4160/4150. It picks up between the idle circuit and the primary circuit unless the throttle is goosed to kick in the accelerator pump. Possibly the transition passages are a bit gooped up and running lean. It might be worthwhile to just pull the center carb, get a can of carb cleaner with a nozzle and spray out the idle circuits & transfer slots, the transition passages, fuel inlet through the needle and seat, and any other port or office you can find.

 

[Lionized]

I believe this may be the problem. Smaller napa filter ?

If this filter is installed prior to your mechanical pump I will practically guarantee this is the source of your issue.
Edit;
Also, yes, you should have a 3/8" line from tank to carb with vapor separator fuel filter (return line).

 

[biomedtechguy]

prior to

"Prior to" being the bigger factor than filter "size" IMO.

 

[biomedtechguy]

I believe this may be the problem. Smaller napa filter ?

I'm sorry I didn't look closer when you sent this pic to me, but that definitely appears to be installed between the tank and the mechanical fuel pump, a definite no-no. The fuel filter should absolutely be between the fuel pump and the carburetors!