Hemi AFB accelerator pump issue - initial bog on opening throttle

[Fran Blacker]

From a slow roll your flooring gas right? The the accelerator pump spot is good also? Goofy idea to top all goofy ideas (dangerous?). Windshield washer tank with a little gas rubber hose run to front carb vacuum advance port, need to put a fitting there. Purge the line block or tie in 2nd outlet. Just before you hit the gas start WW pump. Bog or no bog. Would do it on my car, good question.

 

[AR67GTX]

I had .046 and .052” bits and the .046 slid in fine In the economize bore. The .052” would not.

On the idle jet, a .035”bit went in snugly. One had a bit of resistance and I had to rotate it a little.

The idle screws aren’t great but also not bad.

That shaft looseness I measured was lateral movement via dial gage and not an attempt to measure shaft or bore diameter.

 

[AR67GTX]

From a slow roll your flooring gas right? The the accelerator pump spot is good also? Goofy idea to top all goofy ideas (dangerous?). Windshield washer tank with a little gas rubber hose run to front carb vacuum advance port, need to put a fitting there. Purge the line block or tie in 2nd outlet. Just before you hit the gas start WW pump. Bog or no bog. Would do it on my car, good question.

Uhh - thanks Fran but I think I’ll let that one simmer awhile before I try it.

 

[AR67GTX]

Primary gaskets are correct, the secondary are wrong. The alternate you show is correct for secondary. Just 5 round holes. They come in 2 sizes make sure it is the larger one and fits properly on venturi.

Shaft tolerances are in spec for carter. The outside dimensions on the body are larger. The inner bore at the throttle blades is reduced and tighter tolerance to shaft. If you have some numbered small drill bits I would check the economize size that I showed in the earlier pictures .0492". Idle jets on primary should be .035".

What do idle mixture screws look like.

I replaced the secondary Venturi gaskets, rodded all the Venturi passages out with bits and blew them out again. Rodded the idle and transition ports out with bits. Installed the .043 shooter in the rear carb for a test. The front carb shafts are noticeably tighter than the rear carb. Rain moving in so a test is a few days off.

 

[Geoff 2]

Front carb shafts tighter is to be expected, since most of the time driving is done with rear carb.
Be interested to hear what the bigger pump nozzle in the rea carb does.
0.043" is not unheard of. 043 is listed in the small block MP bulletins for a Holley carb, & that is a smaller engine.

 

[AR67GTX]

duplicate

 

[AR67GTX]

Front carb shafts tighter is to be expected, since most of the time driving is done with rear carb.
Be interested to hear what the bigger pump nozzle in the rea carb does.
0.043" is not unheard of. 043 is listed in the small block MP bulletins for a Holley carb, & that is a smaller engine.

What about the metering rod springs? I don’t know their origin - may be original, may be replacements. My sense is the bog is too immediate and before the primary metering system has a chance to be brought in but so far I’m stumped anyway. I have an assortment of Edelbrock springs

 

[Fran Blacker]

Would think a heavier spring would let open sooner.

 

[AR67GTX]

Yes, but I’m not sure if the idle/transition system can flow any more fuel than the rod/ jet can flow on the lean rod position anyway (?). I don’t know. I have some Edelbrock springs I can experiment with if the bigger shooter and the additional cleaning/ blowing out the ports and bleeds doesn’t help any. I don’t think the secondary Venturi gaskets will make any difference.

 

[Geoff 2]

By all means try the springs. Who knows what previous owners have done? Maybe at one time it had a low vacuum cam in it & soft springs were used.

I would try the larger pump nozzle. And here is a twist you will not hear often: if the larger pump shooter helps, try enlarging the shooter nozzle tip on the side that has the deep plenum.

Springs: start with stiffest in the pack. Have the piston covers slightly swivelled so that you can see the piston but it is still captured. At idle, [ in gear if auto ], pistons should stay down & not move. If they move, try a softer spring. Object is stiffest spring that does not move & stays down all the way.

 

[AR67GTX]

OK - I’ll try the larger shooter first and if no change, experiment with the springs. That way I won’t have too many changes going on at once and not understanding which one is helping.

I’ve noticed a strange effect with my new Innovia timing light too. When I wing the throttle it actually shows a loss of initial advance via the timing marks while the engine is accelerating and once it peaks or stabilizes as I hold the throttle, the advance marks advance and stabilize. I know the distributor cant be advancing backwards but it’s a strange effect. I’ve never noticed my older dial back timing light behaving like that. I’m going to dig out the old timing light and check it with it. The Innovia timing light with dwell, voltage, tach and timing all built in is really nice but only seems to only read timing accurately when the engine speed is steady.

I went back and checked my notes when I mapped my advance curve and it’s a little slow, peaking at 3500 - 3600 rpm which I typically like on my other cars, but maybe it’s a little too slow for this engine? It probably doesn’t start advancing until somewhere above 1000.

 

[Dragon Slayer]

You could have shaft slop in distributor or plate assembly. Is vacuum in and can pulling before you wing it causing some retarding of timing? Are you doing these accelerator test with vacuum plugged? I have asked about how these repro cans work, but have not heard much. Sometimes you can chase carb issues that are tied to the ignition.

If the Cap distributor you have is marked LU it is 15deg at distributor so adds 30 at crank if not worn in slots which can add more. Realize you are using limiter. I still think getting a good set of index gauges with .049 rod to check the economizer is in order. Can't remember to you have heated manifold via stock set up.

 

[AR67GTX]

George, the vacuum can pulls in about 21/22 additional degrees and is hooked up to the ported outlet. I can sense vacuum as soon as the throttle is opened a little So I’m pretty sure rolling along in first at 10 mph it is starting to pull in some vacuum advance. I haven’t tried any tests with the vacuum plugged yet.

I have an FBO plate limiter on the distributor so it’s maxing out at 33 deg with 15 initial. I’ve also used one of these on the dual point in my GTX without any side effects.

I do have the heat tubes hooked up and choke tubes but the heat riser assembly was removed in the past so it takes a while for the manifold to warm up but the choke functions well. This car is kind of like my 427 Corvette with L88 heads/intake - until the engine builds some heat and warms the intake, it idles kind of rough like it’s not getting good fuel vaporization.

I can gauge some wire and see if I have anything close to .o49” but may be tough getting something exact. A .052” bit definitely won’t go and the .046 one goes in fine, one Venturi had a bit of crud that hung it up slightly.

 

[Dragon Slayer]

The stock springs in the 67 cap distributor would give 30 deg mechanical at 3100 rpm. The vacuum can would give 19.5deg at 15". Crank values. Arm stamped 10 had heavy spring internally by prestolite standards. Initial for cap was 0 deg

If your rolling along with slight throttle your at 35 deg advanced when you punch it, but vacuum can is probably taking that 22deg out as the throttle blades go wide open.

The standard 66-67 hemi gave 17 deg mech at 2800 rpm with same vacuum can. Had 6-8 deg initial if I remember correctly.

Your carbs are 4139/40 right.

Before swapping any more carb parts, I would disconnect vacuum and plug it. Check timing at idle and total. Then do the acceleration test.

 

[AR67GTX]

I should have added to the 15 degrees idle/[email protected] rpm, that that‘s with vacuum advance disconnected. I have a set of vintage Mopar Performance springs in it from Halifaxhop. I could reset it for 17 degrees initial/32 or [email protected]. I have the stock direct drive starter and seems it sometimes turned a little slow when hot when I tried 17 before but it’s always started.

I think you’re exactly right about low speed bringing the vacuum advance in and as soon as I go full throttle it falls back out. But the original cars seemed to work around that so I don’t know. I’ll try a test with plugging off the vacuum advance.

I found a micro pin set with a .049 gage but it’ll take a few days to get here.

A question on vacuum gage reading. I thought I read 17 a couple weeks ago but I must be mistaken as it‘s reading 14 now. I remember thinking at the time 17 seemed a little much but didn’t double check it. But I have some very small fluttering of the gage needle - not much - probably max amplitude of 1/2” or less. I’ve never hooked a distributor or vacuum gage directly to an intake manifold port before that I remember - always to the carb. Is that slight flutter a function of the intake valve flow pulses feeding back to the plenum? Or possible valve seal issues? Haven’t noticed any initial puffs of smoke on a cold start indicating oil pooling there.

 

[Dragon Slayer]

I think you are ok with 12 initial and going to 33 but I would have it come all in around 3000rpm. The 14 make sense with mopar spec and maxing around 19.5 with vacuum. How that repro can is curved versus an original I do not know. But eliminating it for the test maybe helpful. Some vacuum variation is probably acceptable reading at the carb with a dual plane at idle.

 

[AR67GTX]

I think you are ok with 12 initial and going to 33 but I would have it come all in around 3000rpm. The 14 make sense with mopar spec and maxing around 19.5 with vacuum. How that repro can is curved versus an original I do not know. But eliminating it for the test maybe helpful. Some vacuum variation is probably acceptable reading at the carb with a dual plane at idle.

By all means try the springs. Who knows what previous owners have done? Maybe at one time it had a low vacuum cam in it & soft springs were used.

I would try the larger pump nozzle. And here is a twist you will not hear often: if the larger pump shooter helps, try enlarging the shooter nozzle tip on the side that has the deep plenum.

Springs: start with stiffest in the pack. Have the piston covers slightly swivelled so that you can see the piston but it is still captured. At idle, [ in gear if auto ], pistons should stay down & not move. If they move, try a softer spring. Object is stiffest spring that does not move & stays down all the way.

My testing was delayed a couple days from throwing my back out but I resumed this morning.

With a .032 shooter in front and a .043 in back and ported advance, I could tell some improvement but still had a stumble.

I disconnected the vacuum line and plugged it next. If anything it seemed slightly worse without vacuum advance but not much change.

Next I watched one of the rod/ piston units operation while winging the throttle and it looked pretty normal. It did take a pretty good yank for the piston to rise. So I pulled that spring and compared it to a spring set in an Edelbrock strip kit I have. I finally went through and measured the wire diameter, length and number of coils on all the colors and on the one out of the Hemi. The Hemi one was a perfect match for the Edelbrock silver spring ( they call it plain) which is shown as the strongest one and starts at staging vacuum of 8” Hg. Having a hard time wrapping my mind around the springs and if I should swap them out but for now I left them alone.

Next, I hooked up the vacuum advance to a manifold port and decided to put the accelerator pump on the rear carb in the inner hole. I didn’t try to adjust the rod so the pump is probably a little high but seemed to squirt OK with the higher float level. This actually made a difference. I would say 90% or more of the stumble is gone, which is probably about as good as can be expected from opening all that carburetor from 10 mph and about 1200 rpm. I put the vacuum advance back on ported advance and didn’t notice any difference as I expected.

So not sure where to take this from here - it’s driving strong but I’m wondering if the .032” shooter might work if I leave the rod on the inner pump arm? Or should I also put the rod on the front carb in the inner hole? And I need to get my mind straight on the metering rod springs.

 

[Dragon Slayer]

Is there a stamped number on the accelerator pump stem?

 

[AR67GTX]

Mikes Carburetor shop calls it AP237. I’ll have to look to see if it’s actually stamped on it.

I looked, there aren’t any stamps or ID marks on it.

 

[Geoff 2]

AR,
You need to use the method I posted in #110 to ensure you have the stiffest met rod springs that the engine will accept.

Further thoughts.

Using Man Vac Adv should have significantly increased idle rpm [ unless the timing was locked ] If not it might be a stuck/bad VA unit or internal spring is too stiff & is not adding any timing.
I would try this:
- disconnect & plug VA.
- engine idling, adv timing about 10*, not critical [ turn dist ] & test drive to see if flat spot is gone.
- why would this help? Because the engine should make a little more HP with the extra timing. So now when you nail the pedal, the HP is already 'there' to take the increased load, the engine does not need to suddenly produce the extra HP. Easy to try & worth doing.
- report back.

Shouldn't need all this, because engine is stock, right? But something is not right so here we are....