69' BB Tri-Power bogs @ take-off
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From: Rittman oh
69' BB Tri-Power bogs @ take-off
Recently finished a rebuild / install of the original tri-power set up on my 69' BB RPO L-68. It runs great at mid range and high rpms and idles down fine and smooth (which was a feat in itself) BUT- If I nail it at take off from stop it bogs / hesitates and would stall with out feathering the pedal. Automatic transmission also doesnt downshift into 1st on it's own ( which may be normal but it was recently rebuilt before I bought the car). If I shift into first it will launch with less / no hesitation but much tire smoke. Timing has stayed right on and plugs all check out normal. I'm still running the points ignition and everything appears ok in the distributor. Any help from the experts would be greatly appreciated!!!!Swamper
Melting Slicks
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From: San Jose California
If it bogs when you mash the throttle on take off you have too much air and not enough gas. A slighltly stronger spring in the vacuum pods will cure this. Is the accelerator pump/cam set properly on the center carb?
Chuck
Chuck
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From: Rittman oh
The pods are brand new, I did not replace the accelerator cam but set the tolerance as specified. It does appear that the cam does have some wear. Is there a way to check if the cam is wore too much?
Melting Slicks
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From: San Jose California
Even though the pods are new, installing a stiffer spring within the pod will cause the secondaries to open more slowly and avoid the bog. Holley sells a variety of springs just for this.
If this is more of a problem only off idle and not when you mash the throttle at over 2500rpm, then I would more suspect the accelerator pump/cam setting, and/or the size of the aquirts. The squirts may even be plugged and simply need to be cleaned out.
Chuck
If this is more of a problem only off idle and not when you mash the throttle at over 2500rpm, then I would more suspect the accelerator pump/cam setting, and/or the size of the aquirts. The squirts may even be plugged and simply need to be cleaned out.
Chuck
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From: Rittman oh
Chuck, finally got a chance to try your recommendations, I adjusted up the accelerator pump / squirt timing which cured about 50 % of the bog. then tried a no 25 squirt nozzle but it did'nt make a difference. Still stalls if you punch it hard. Did notice improved acceleration. I've always wondered about the vacuum secondaries on this set up. When I got all the stuff, some came with the car and and I had to buy the rest, I had to use a universal linkage kit which links the carbs in a fixed position. Thats how the linkage is shown in the assembly manual I have too. With this fixed set up what do the pods on the secondaries actually do? It seems to me the linkage should be different to allow them to " feather " fuel as needed to the outboard carbs.
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From: Beverly Hills/Pine Ridge Florida
Sounds like you have non original carb linkage. You should have progressive linkage. That way, the car runs on the center carb only. When you punch it, vacuum opens the 2 end carbs.
If I'm reading your post correctly, you have all 3 opening at the same time from the minute you press the gas pedal.
Think of it like a 4 barrel carb, where you run on 2 barrels only till vacuum "kicks in" the other 2 barrels.
Also...If I'm reading your post correctly, sounds like your trans is starting out in 2nd gear, not 1st. Do you have the vacuum modulator hooked up? Chuck
If I'm reading your post correctly, you have all 3 opening at the same time from the minute you press the gas pedal.
Think of it like a 4 barrel carb, where you run on 2 barrels only till vacuum "kicks in" the other 2 barrels.
Also...If I'm reading your post correctly, sounds like your trans is starting out in 2nd gear, not 1st. Do you have the vacuum modulator hooked up? Chuck
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From: Rittman oh
That's correct about the linkage, I know the linkage is wrong.
Also, correct about the vaccum modulator to the transmission. When I pulled the 4 bbl set up off there was no vaccum line to the transmission. I looked into it in one of the manuals I have and there was no mention of it just the electric detent switch at the pedal so I assumed that's all there was. I dug into the assembly manual I have and found the modulator. Hopefully the modulator is there, I will check and make operational.
Also, correct about the vaccum modulator to the transmission. When I pulled the 4 bbl set up off there was no vaccum line to the transmission. I looked into it in one of the manuals I have and there was no mention of it just the electric detent switch at the pedal so I assumed that's all there was. I dug into the assembly manual I have and found the modulator. Hopefully the modulator is there, I will check and make operational.
Race Director
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From: Taylor Michigan
The OEM tri powers require the use of the factory vacuum linkage to work correctly because the front and rear carbs do not have accelerator pumps. Only the center carb has one. Activating all three at once will cause your stumble. Too much air and no fuel.
Instructor
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From: Buffalo, NY
And remember you can't really swamp a motor with that much carb at 2000 rpms, especially under heavy load. Do try stiffer springs, it should really help. Don't open them all up until 3500+ rpm.
Last edited by 427TRI; Sep 26, 2004 at 09:44 PM.
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From: Beverly Hills/Pine Ridge Florida
The electric trans switch is for "passing gear" kickdown only. Low speed shifts are controlled by the vacuum modulator on a THM 400.
I have to ask why you're fooling around with non original linkage? It'll never work correctly with all 3 carbs opening. Wasn't designed to work that way. Chuck
I have to ask why you're fooling around with non original linkage? It'll never work correctly with all 3 carbs opening. Wasn't designed to work that way. Chuck
Melting Slicks
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From: Eastern US XX
Originally Posted by Chuck Gongloff
The electric trans switch is for "passing gear" kickdown only. Low speed shifts are controlled by the vacuum modulator on a THM 400.
I have to ask why you're fooling around with non original linkage? It'll never work correctly with all 3 carbs opening. Wasn't designed to work that way. Chuck
I have to ask why you're fooling around with non original linkage? It'll never work correctly with all 3 carbs opening. Wasn't designed to work that way. Chuck
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From: Rittman oh
Carb linkage corrected
First, thank you all for your help in correcting my linkage problem. The kit I bought was a "universal kit" but the bolt and washers to the center carb made it fixed to the center carb. I replaced it with a longer stainless sex bolt that allows it to work properly, all is well with that and it looks clean too. Found the vaccum line to the transmission was there and hooked up tightly to the manifold. Transmission still does not downshift into 1st upon stops. It seems to stay in second upon take off. How do I check to see if the modulator is working properly? Also, what is the number of the proper accelerator pump discharge nozzle for the 4055-1 carb? I have a no.21.
