Irish69427

I am having a peculiar hesitaton/bog with my tripower carburetion when I slowly advance the throttle to full. When I quickly floor it, it is fine. For some reason it is leaning out when the end carbs begin to open. I have verified this through my LM1 wideband oxygen sensor. I have done all of the following without success.

1. Rebuilt and cleaned carburertors
2. Reset float levels
3. Verified and adjusted fuel curve at idle, light throttle, cruise, power mixture (power valve) and WOT with my LM1. All are right on and the car runs great except for this problem.
4. Changed accerator pump cam to orange (stronger) from red.
5. Verified timing (38 degrees mechanical all in by 3000 rpm)
6. Checked coil. Wires are new and ignition is pertronix.
7. Went to a stronger vaccum spring (black - this is the strongest there is). This improved it but did not make it go away. I went back to brown spring as the full throttle performance degraded with this change.

I noticed that there is an idle circuit and transition slot on the end carbs (not adjustable). I am assuming this transition slot provides some fuel as the butterflies begin to open. This all appears clean and operating right so I am mystified.

I wonder if anyone else has had this problem? Any input would be appreciated.

0Paul Ruggeri

Sounds like you've done everything correstly so far. I think at this point I would try richer jets in the outside carbs and see what kind of A/F numbers you get.

Good Luck, Paul

WESCH

Hi

Or weaker 2ndary vacuum pod springs to open them a touch earlier.

Rgds. Günther

Capella

Are you running vacuum advance?

38 degrees total timing is a bit much. With vacuum advance also in at light throttle you may be too advanced. Try backing off the timing a couple of degrees at a time and see if part throttle hesitation goes away. If it does, try reducing vacuum advance the same number of degrees you backed off total timing and then put the total back in, BUT, I would consider keeping total timing at no more than 36 degrees.

Irish69427

I did try this awhile back. Went to a purple spring (two steps lighter). Seemed to be about the same.

Irish69427

I am running vacuum advance (about 12 degrees for a total of 50). I will try backing of the mechanical and see what happens. I was under the impression that these old closed chamber heads like a little more timing (38-42 mechanical) as compared to later open chamber designs (36) but not sure. I should verify it on a dyno.

Can't say it is really a light throttle condition. By the time the end carbs come in even advancing the throttle slowly I have felt the power valve come in and the vacuum is below the threshold for the vacuum advance.

Also there is no indication of pinging at all at any throttle position and throttle response it great.

Matt Gruber

like 63mako says
guys that run tripower successfully convert it to mechanical dumpers

Capella

36 - 38 degrees is usually about max as you say, but dependant upon fuel grade and everything else being in perfect shape. You might also try changing the timing curve so that total advance is all in by 2500- 2800 rpm - ensuring once again that vacuum advance is adjusted accordingly.

Be careful about perceived pinging. If you can actually hear it you may be way past the onset of actual pre-ignition. Remember too much advance results in negative torque.

Assuming timing curve is correct, check vacuum readings at the throttle setting that is resulting in the hesitation and then check that you have a power valve to match. If you can "feel" the secondaries come in, the power valve may be activating a bit late resulting in a lean condition. If set up correctly there should be a relatively smooth transition through secondary activation. The only thing you should feel is exhilaration!

As an aside, are you running a vacuum balance tube between the two secondary diaphrams?

Capella

Well, mine runs pretty good.

I'll see you're double pumper and raise you two vacuum secondaries!!!

mudbone64

I don't want to be a hi-jacker, but since we're talking about tri-power set-ups I'm wondering where your vacuum advance is hooked up. Isn't the stock vacuum source a ported vacuum source? I am currently working on my friends '68 427 400 h.p. car and noticed the ported vacuum going to the vacuum advance. I would like to change the vaccum advance to a manifold vacuum source and was wondering if anyone else has done this.

Thanks,

Mudbone64

Capella

Manifold vacuum should help throttle response and fuel economy as it will add advance at idle - allowing you to lean the idle mixture a bit. It may also help reduce engine operating temperatures. You will probably need to adjust your complete spark advance curve to get optimal results though, once you make the switch. There is some good info on this subject in the tech section. All engines used to be set up this way before the smog police took over - retarding advance at idle by using ported vacuum was done only as a means of increasing exhaust temps to reduce emissions. I have not done this to my Vette yet as I have only had it a week and am still chasing Bubba out of other areas, but I have done it to other vehicles with expected results.

Ironcross

i`m curious, how can you do this without an accelerator pump?

WESCH

Hi

I would like to change the vaccum advance to a manifold vacuum source

I can't see how to do this because the vacuum pods are internally connected to the carb's ported port.
For a manifold source, you need to block the internal port and route it somehow externaly to the manifold.

Günther

Capella

To connect the vacuum distributor advance to manifold vacuum is a simple reconnect to the manifold port behind the carburetors and then plug the vacuum port on the center carburetor - this does not affect the outer vacuum actuated carburetors.

Matt Gruber

the WHOLE idea is a TRIPLE PUMPER.
i'd buy 2 center carbs, remove choke, turn in idle screws. mount on ends.

Capella

Hmm.

Not sure I would do that - trying to force feed nearly 1300cfm to her will likely lead to a bigger bog/drivability problem - unless of course you are planning to launch at high rpms to start off with.

I'm looking at this from a street drivability standpoint. As to the original problem, I think I would try going up a couple Hg on a power valve that would open a bit sooner and see if that eases the part throttle lean condition.

Nothing wrong with properly tuned vacuum secondaries. One thing I would seriously consider though would be to install the Holley vacuum secondary covers #20-28. These have a hose nipple that allow you to connect a balance tube between the two vacuum actuators - helping them to open together more evenly.

Matt Gruber

HOT ROD mag did a test 750cfm vs950 cfm.
the 950 was worth 1.3 extra HP.
so if 1300 is too big, a throttle stop should fix that.
And drive the car, dont let it drive you.(apply power as needed, not WOT unless needed)

Capella

Yup, and if you read between the lines on most of those tests they also find that throttle response and low speed drivability is often degraded by going with an oversize carb - not worth it for a couple HP in my opinion. Seems most everybody thinks bigger is better - not usually the case.

A good book on Holley carburetors: 'Super Tuning and Modifying Holley Carburators', Dave Emanuel. Available at Barnes & Nobles, Amazon, etc. A good source to further your understanding of how these things work and should help you smooth out the problem.

ML67

I'm curious as well. I ran a 67 Tripower for several years and one time had an impromptu run against a 69 Tripower who had converted to mechanical linkage. The race was not close -- I ran away and hid from him. When we spoke after the run, he said he could not whack the throttle w/o bogging -- undoubtedly due to the lack of accelerator pumps on the outboard carbs. Ironically, given the topic at hand, it ran best when he slowly accelerated.

You may have to enlage the idle feed restrictions on the outboard metering plates. It sounds like you need more fuel during the transition from idle to main circuit activation. You could also drill and tap the outboard carb idle air bleeds and drop the IAB orifice a few .001s" which has not only the advantage of richening the outboard carb idle/transfer, but will have a tendency to richen this circuit as a function of rpm.

Good luck,

Mark

Capella

Is this a stock motor were talking about, or has it been modified with cam, head work, headers, etc.?