#1  
Old 04-10-2021, 04:37 AM
Ponty Ponty is offline
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Default 65 Tri power dead/flat spot on hard acceleration

Hi, I've just recently purchased a 68 Bonneville 400CI, TH400 with a 65 tri power system on it. The car starts fine, hot or cold, idles fine. No missing, popping or backfiring. I can drive around the streets with no problems, car doesn't stall or hesitate. As soon as I start to hard accelerate or floor the throttle I get a dead spot/delay happening. It doesn't backfire. The other day when I drove it, it had the slight delay there but would then drive through this & you could floor the throttle & the thing would hard accelerate. When you hard accelerate now the car will die down & sometimes stall, back off on the throttle & it will drive ok. The ignition system is a Street Fire CDI unit. According to the troubleshooting the LED light has no active faults. The vacuum advance for the dizzy is teed into the back lower centre of the centre carb as well as the transmission vacuum line. I think the problem is more a fuel issue than an ignition issue? I've removed all 3 air cleaners & opened the throttle linkage to fully open position & all 3 carbs are getting fuel when engine is off. Any advice would be greatly appreciated. Thanks.

  #2  
Old 04-10-2021, 12:30 PM
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Try the same acceleration with the slider removed, so you can see if the center carb is OK. If the problem went away, you have too small main jets in the end carbs or debris blocking fuel flowing through the jets on one carb. This won't be a difficult fix. PM me if you want to discuss the fix.

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  #3  
Old 04-10-2021, 12:51 PM
mike w mike w is offline
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Default Dead spot during hard acceleration

Check end carb accelerator pumps. Pull end air cleaners, engine off. Open throttle quickly while watching the venturi clusters for two strong squirts.

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Old 04-10-2021, 05:56 PM
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Mike is right, it could be accelerator pumps in the end carbs. Easy to check with the engine shut off. Operate the throttle and look for the squirt of fuel in the airhorn.

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Old 04-10-2021, 11:40 PM
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Quote:
Originally Posted by Ponty View Post
... As soon as I start to hard accelerate or floor the throttle I get a dead spot/delay happening...
My '61 SD Tri-Power (bought used in 1961!) originally had that problem. It turned out to be too much fuel from the end carbs' accelerator pumps, so I lengthened the arms (maybe 50%) to reduce the travel of the pump pistons. After 60 years those same arms are still in place and have worked great on 8 or 9 different engines (all 400 c.i. or smaller).

I don't want to mislead anyone- this may have been unique to that particular Tri-Power.

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Last edited by Jack Gifford; 04-11-2021 at 12:08 AM.
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Old 04-11-2021, 07:46 AM
Ponty Ponty is offline
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The front & rear pump rods on my set up are almost straight. With the engine off I can hold the throttle linkage wide open & I get 2 squirts of fuel from both front & rear carbs. Funny how the car didn't have this issue the last time I drove it & now it does. Have ordered some carb rebuild kits. Fuel lines & fuel filter have been changed.


Last edited by Ponty; 04-11-2021 at 07:53 AM.
  #7  
Old 04-12-2021, 12:14 PM
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Your test tells you the accelerator pumps are working. Rebuilding the carbs will not likely fix the problem.

Is the problem only initially when opening the carbs? Once the carbs are open, does it accelerate well? If not, it's possible that your jetting may be on the lean side, especially if the air temperature is low.

I would drive it a bit more, opening the end carbs several times. I've also had this issue in springtime when first driving my GTO.

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Old 04-12-2021, 08:21 PM
Ponty Ponty is offline
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When you are driving along in normal street driving conditions (obviously only running on the centre carb everything is fine). No hesitation, no backfiring no missing. Car idles starts & stops no problems. It's when you apply the throttle down hard there is a delay/hesitation there.( Engine doesn't "ping" on hard acceleration). It will then pick up & accelerate hard. Other times it falls flat on its face, sometimes it will stall. When it does stall you can restart the car no worries. I've changed out the fuel filter & fuel lines the other day. Replaced vacuum lines. I've noticed the front & rear pump rods are almost straight where as in photos I've seen on the tri power systems these rods have bends in them. I've ordered a set of pump rods as well. I'm in the process of draining & flushing the fuel tank, changing out the AC Delco R45S plugs & inspecting the carbs for any contamination. I've been told the centre carb jet size should be .062-.063" & the end carbs should be .066-.068". I live in Australia where the temperature is around 86*F (30*C) most year round. I haven't checked timing or fuel pressure yet. From what I've been able to find out from the previous owner the engine (400CI) is stock apart from a mild cam. But he doesn't know the specs of it. Someone at some stage has put a 65 tri power system on a 400CI 68 Bonneville. I'm wondering now if the carb jet sizes are still set up for a 389? Like you mentioned Dick to lean?

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Old 04-13-2021, 12:13 AM
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I would use .063" jets in the center carb. End carbs should have .070" jets.

Is it possible that the carbs are running out of gas? If there is a problem with the sock on the pickup in the gas tank, starving fuel could be the problem.

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Old 04-13-2021, 01:38 AM
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Quote:
Originally Posted by Ponty View Post
... when you apply the throttle down hard there is a delay/hesitation there... It will then pick up & accelerate hard...
If it's fuel-related, I'd say it must be a problem with end carbs' accelerator pump function; any other fuel fault would not allow it to "accelerate hard".
If not fuel-related, I'd look for a possible ignition fault when the vacuum advance drops out.

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Old 04-13-2021, 07:04 AM
Ponty Ponty is offline
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I'm just waiting on all my parts to arrive. Hopefully have them here in 10 days or so. I've ordered a set of jets as well just in case. I'll finish cleaning & inspecting fuel system. Will let you know how I go. Thanks for all your help guys. Much appreciated.

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Old 04-13-2021, 10:50 AM
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Make sure to use ethanol resistant fluoroelastomer (or leather, good luck finding these!) cups for the accelerator pumps.
Most kits still contains the buna type cups which swells and gets stuck in the bore delaying/preventing the function resulting in hestitation at WOT.

HTH

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Old 04-13-2021, 07:18 PM
Ponty Ponty is offline
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I'm buying all my parts from John & Mike from Pontiac Tri power.

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Old 04-13-2021, 11:05 PM
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Accelerator pump malfunction does not explain why the engine sometimes stalls after accelerating. Nor would jet sizing cause stalling.

The problem has to be either the carbs running out of fuel or an ignition issue. Rebuilding the carbs will not fix this problem.

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Old 05-06-2021, 02:11 PM
scott06 scott06 is offline
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Im going through somewhat similar issues on my 65 gto tripower. I would reccomend installing an Air Fuel ratio meter so you can see what is going on.

I had an issue where it ran pretty well except when accelrating. What I found was off idle on the transistion circut I was way lean like up in the 18-19:1 range, cause the engine to go flat for a second then kick in perfectly... With out the AF meter I was thinking this was the accelerator pumps etc, what I found I could adjust my idle to near perfect 14.5 AF then as soon as the throttle opened slightly I ended up way lead until I could see fuel coming off the main boosters around 2k rpm maybe a tad more. Pmnce the mains kicked in I was in the 13-14.5 range, lower under WOT.

I opened my idle tubes to .036 then .038 this helped some but Im still way lean on the transition circut so not solved yet but I never would have seen this much detail with out the AF meter.

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Old 05-17-2021, 11:57 AM
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Progress?

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