Engine Bog

[Bennett]

I am going to differ on the vacuum gauge diagnosis. When bog occurs and you look at the manifold vac reading, if it is above your power valve rating, it means the pv is not open and the engine is probably lean.
ie you are at 2200rpm cruising, you press the gas to increase speed, the vac falls to 8.0, the engine goes flat, this is where you need the pv to open. If this example was the actual readings I would put in 8.5 pv. On the dist, it may need attention, but I like get one thing right before adding more medicines to the illness.

This one of the tests that I tried tonight. I attached a vacuum gauge to a manifold port and wired it to the air breather so that I could see it while driving.
At curb idle I have approximately 11 to 12 in/Hg.
Cruising I have between 15 & 20 in/Hg. Usually equal to or less than 2200 rpms.
When I give it more throttle and try to go past 2200 rpms It first drops the less than 10 in/Hg and then comes the bog and vacuum drops to zero like a stone.
Once I get past the bog (usually by re-applying the accelerator pump) it picks back up and the vacuum returns as well.

 

[Bennett]

Check the advance works. Measure the circumfrence of the damper from the TDC mark. Divide this number by ten. Measure this against rotation from the TDC mark, make another mark. This is 36degrees advanced. Warm it up. Take the springs off the weights, disconnect the vacuum advance. Fire it up, it may buck a bit but take it up to 2000rpm or so should be smooth enough. Set 36deg advance with timing light. When you pop the cap off to replace the springs, the weights should be all the way out. Replace the springs carefully. Fire it up again and note the idle timing. If it's for instance 12deg that means you have some 22deg of advance weight timing which is about right. Replace the vacuum line. I like to see all the advance in by 3000rpm latest

I also performed this test tonight.
I measured the circumference of the damper and calculated the distance for 36* just as Wild Mango had described. I then produced a timing sticker to scale using a Avery/Staples label on my printer. See below.

Actually I made several just in case I wrecked a couple. I then applied it to my damper in the proper location.

This was easily observed with the timing light. I did everything as WM said, and when it was all said and done I had set the timing to 14* BTDC.
If I understand the instructions correctly this means I have 22* of advanced weight timing and is good.

 

[railroad]

If the vac goes below 6 and stays your power valve will open, if this is too soon you could be too rich. If the bog is coming in above 7 or 8, you are leaning out, need ing the power valve to open. The gauge may not be metered the same as your pv ratings, but should be within one whole number. If it is difficult to determine where the engine vac is when the bog starts, try to get some indication of whether it is a lean or rich condition. Someone watching your smoke, looking at a plug, modulating the pedal a little in the area of the bog. Easing into the bog may help isolate the vacuum range. I am out of town, but trying to monitor and advise. Keep feeding me info. good luck,

 

[railroad]

If I had to make a change on what I think, I would put 2-8.5 pv's in. I think a 6.5 would be fine with opening one set of primary butterflies, but by opening 2 sets of primaries approaching 390 cfm, it is just too much and needs the fuel earlier. I do not think pv's come in whole number increments anymore, so you will be looking at .5 steps in whole numbers.

 

[2old2fast]

Nice job on your timing tape , so now you know timing should be O.K. . I tried to see which position you had your accelerator pump cams in but wasn't able to tell. If they are in position 1, try 2 & vice versa. One other thing , I see that your carbs have the old style acc. pump brass transfer tubes , you were very careful putting new "O" rings on them & during reassembly ?? Also , what sec. vac springs did you use??

dave

 

[Bennett]

Nice job on your timing tape , so now you know timing should be O.K. . I tried to see which position you had your accelerator pump cams in but wasn't able to tell. If they are in position 1, try 2 & vice versa. One other thing , I see that your carbs have the old style acc. pump brass transfer tubes , you were very careful putting new "O" rings on them & during reassembly ?? Also , what sec. vac springs did you use??

dave

White cams in position #1. I can try position #2
Was real careful with o-rings.
Silver vacuum secondary springs.

 

[2old2fast]

Hmmm, I guess what's got me bugged is why you'd be "running out" of fuel @ 2200 rpm ?? I wonder if it's running out or if it's going rich?? Man these problems are hard to diagnose not being "hands-on" !!!
dave

 

[Bennett]

Hmmm, I guess what's got me bugged is why you'd be "running out" of fuel @ 2200 rpm ?? I wonder if it's running out or if it's going rich?? Man these problems are hard to diagnose not being "hands-on" !!!
dave

Well for what it's worth in my non-expert opinion, I think it's running out of fuel because if when it bogs, if I let off the throttle briefly and re-apply (reactivating the accelerator pump) it smooths out and picks up RPMs. I'm thinking the squirt of fuel from the accelerator pump is giving it the extra fuel it's looking for.

Trust me...I'm not sure I can ever give you guys enough accurate information so you can tell if it going lean or rich. This topic eludes me. Same goes for adjusting the metering block needles. These subtle adjustments kick my @$$.

I think my next step is to try 8.5 power valves and maybe mess with the cam position a bit. I also need to get you guys some better information regarding exactly when and what the vacuum is doing exactly at pre and post bog.
Mike

 

[2old2fast]

This is where trial&error diagnosing can get expensive. Too bad you don't know someone who has a bigger set of accel. pump shooters , maybe 31 or 35 to try. Also ,I don't know when you're total advance is all-in , maybe you could find some lighter adv. springs to get it in sooner , or take 1 spring off & try w/1 . getting that little extra timing in earlier may be just the ticket! If you do this , I'd try w / the vacuum adv. disconnected. Can you "ease in" to the throttle without having the bog?
dave

 

[railroad]

Mike, be sure to make ONLY 1 change at a time. I do not think you are opening the secondaries, no need to change springs yet, I do not think the accel pump is creating a flat big bog like you have at that rpm. Power valves are about $10 each, I hope we hit the nail on the head with the 8.5s and do not need to buy 7.5s, but I still feel like we will not be breaking the bank??? On the idle mixture jets, just try to rely on the vac gauge. If you do not have fuel dripping in the venturi or out the shooters, nor a vacuum leak, just make small adj and balance your turns between jets, aiming for highest vac. If the idle speed increases, reset you idle speed back to the original rpm and adj again.

 

[Bennett]

This is where trial&error diagnosing can get expensive. Too bad you don't know someone who has a bigger set of accel. pump shooters , maybe 31 or 35 to try. Also ,I don't know when you're total advance is all-in , maybe you could find some lighter adv. springs to get it in sooner , or take 1 spring off & try w/1 . getting that little extra timing in earlier may be just the ticket! If you do this , I'd try w / the vacuum adv. disconnected. Can you "ease in" to the throttle without having the bog?
dave

I have no problem trying (buying) bigger shooters if you think that's the thing to try. Based on the the experiment (Wild Mango test) I have 22* of advanced weight timing (all in at 2000 rpm). I think before I start changing components to this ignition, I will install a new ignition. I am unable to ease into the throttle without the bog. I have to modulate the throttle to get through the bog.

 

[Bennett]

Mike, be sure to make ONLY 1 change at a time. I do not think you are opening the secondaries, no need to change springs yet, I do not think the accel pump is creating a flat big bog like you have at that rpm. Power valves are about $10 each, I hope we hit the nail on the head with the 8.5s and do not need to buy 7.5s, but I still feel like we will not be breaking the bank??? On the idle mixture jets, just try to rely on the vac gauge. If you do not have fuel dripping in the venturi or out the shooters, nor a vacuum leak, just make small adj and balance your turns between jets, aiming for highest vac. If the idle speed increases, reset you idle speed back to the original rpm and adj again.

Don't worry "Controlled Experiments" are my game (DOE). One thing at a time.
I also agree about the secondaries....If you look closely at my pictures...every time I go out I have paper clips on my secondaries to tell me if they come into play.

So far no movement on a normal street run. The car flies even without the secondaries so it is tough to get them to come into play.
I ordered 8.5 power valves today. I hope to have them by early next week to try. In the mean time I plan to experiment a little by trying to identify the exact point of vacuum when the bog starts.
I will also play a little with the linkage accelerator cam as 2O2F previously suggested.
Later, Mike
Oh, by the way....Guys, thank you all for the help. Even if this is the best I get this tunnel ram to run...It's getting scary to drive. (Just the way I like it) LOL

 

[2old2fast]

Just for kicks , you might take those 4-#49 jets you have & drill them w/ either a #54 or#53 drill [.055 -.059] & run those to see if making the primary high speed circuit a good deal richer helps ?!?
I know that sometimes when I was searching that a "shot-in-the-dark" was sometimes successful [plus I felt that it was a "learning" experience.
dave

 

[Bennett]

The sun came out today so I had an opportunity to go out on the road and collect more data.
The bog comes in above 8 in/hg. I can ease through the bog by feathering or modulating the peddle through it. once I get beyond the bog, I can go WOT.
Even if I lay into the throttle, I can get through the bog by pumping the throttle as well. It doesn't sound very impressive but does help. My guess is it's going lean at the point of the bog.
I ordered two 8.5 PVs and should be here on Monday. I'll put those in as soon as I can and hopefully it will help or at least point us in the right direction. I may experiment with the cam position and maybe even try a larger shooter but not until after I try the new PVs first.

 

[2old2fast]

Well you're a patient soul , I'll give you that

dave

 

[railroad]

Your information makes me more confident we are heading in the right direction. I hope the 8.5s are high enough. With variations in gauges and power valves we should be right in the sweet spot. I'll wait for the seat of pants report before we address any future tuning. Don't let that hot rod get away from you.

 

[Bennett]

Your information makes me more confident we are heading in the right direction. I hope the 8.5s are high enough. With variations in gauges and power valves we should be right in the sweet spot. I'll wait for the seat of pants report before we address any future tuning. Don't let that hot rod get away from you.

I put the 8.5 PVs in today and went for a test drive. It's much better than with the 6.5s although it not totally gone. I think I need to try a couple of 10.5s. The bog comes in right around 10 in.Hg on my vacuum gauge.I didn't have to worry about over heating....today high was 51*
When would you use a dual stage power valve? I see Summit offers a 10.5 / 5.5 dual stage, although at a cost.
P.S. I'm also in position #2 on the white accelerator cam.

 

[2old2fast]

As I understand it , the 2-stage PV's were designed for heavy [motorhomes,etc.] vehicles so my guess is that's not the way to go. I still feel that bigger shooters [ to cover up the fuel "hole" on initial acceleration is the way to go..... by changing the PV's -yes you're getting more fuel- but you're getting more fuel "throughout" the time you're below the PV vac. number... this is probably not needed because, as you said- if I pump it twice [ i.e. more accelerator pump fuel] it goes .. just some more to think about... & experiment with .... hope this helps...

dave

 

[Bennett]

As I understand it , the 2-stage PV's were designed for heavy [motorhomes,etc.] vehicles so my guess is that's not the way to go. I still feel that bigger shooters [ to cover up the fuel "hole" on initial acceleration is the way to go..... by changing the PV's -yes you're getting more fuel- but you're getting more fuel "throughout" the time you're below the PV vac. number... this is probably not needed because, as you said- if I pump it twice [ i.e. more accelerator pump fuel] it goes .. just some more to think about... & experiment with .... hope this helps...

dave

You may be right about the bigger shooters, I guess I should try it. One thing that still puzzles me is that even when I ease into it (i.e. approx. 10 in/hg) I get the bog. Do the shooters really come into play when gently easing the throttle forward? I guess as long as the cam is still activating the accelerator pump it's still in play. I'm learning a lot here.

 

[2old2fast]

I'll try to explain .. as I understand the situation... this is as it applies to all carb systems... but it's especially true of tunnel-rams... when you advance the throttle the vacuum drops off which slows down the air velocity in the runners , the fuel/air charge drops to bottom ,resulting in a lean condition above that is normally covered up by the accelerator pump shooting in additional fuel until the boosters get enough vacuum signal to start supplying fuel , since the fuel the pv adds has to come thru the boosters , it's solely up to the accelerator pump to cover this "hole" in delivery. This is why a dual-plane short runner manifold is "better" for the street, less runner volume,less "hole". From a performance standpoint you can't beat a long runner manifold because of the ram-effect that's created. If you setup a tunnel-ram w/ individual port injection & just an air valve on top , you have the best of both.. long runners w/ fuel at the bottom where you need it... finger hurts again.... dave