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#1
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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.
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#2
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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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BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
#3
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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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#4
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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.
__________________
BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
#5
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Quote:
I don't want to mislead anyone- this may have been unique to that particular Tri-Power.
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Anybody else on this planet campaign a M/T hemi Pontiac for eleven seasons? ... or has built a record breaking DOHC hemi four cylinder Pontiac? ... or has driven a couple laps of Nuerburgring with Tri-Power Pontiac power?(back in 1967) ... or has a Pontiac born the same year as Jim Wangers? (1926} Last edited by Jack Gifford; 04-11-2021 at 12:08 AM. |
#6
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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
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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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BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
#8
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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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#9
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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.
__________________
BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
#10
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Quote:
If not fuel-related, I'd look for a possible ignition fault when the vacuum advance drops out.
__________________
Anybody else on this planet campaign a M/T hemi Pontiac for eleven seasons? ... or has built a record breaking DOHC hemi four cylinder Pontiac? ... or has driven a couple laps of Nuerburgring with Tri-Power Pontiac power?(back in 1967) ... or has a Pontiac born the same year as Jim Wangers? (1926} |
#11
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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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#12
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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 |
#13
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I'm buying all my parts from John & Mike from Pontiac Tri power.
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#14
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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.
__________________
BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
#15
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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. |
#16
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Progress?
__________________
BONESTOCK GOATS '64 GTO Tripower Hardtop (Wife's Car) '64 GTO Tripower Post Coupe (My Car) '99 Bonneville SE Sedan |
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