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Discussion Starter · #1 ·
I have a stock 70 Chevelle SS 396/350hp w/ TH-400. I recently installed a 1” thick phenolic spacer under my stock Q-Jet. It seam to give it better low end torque. It runs strong. However, it now shifts automatically at 5,100 rpm. Prior to installing the spacer it would shift automatically at 5,500. When I shift manually and push it to 5,500 I sometimes but not always have a substantial loss of power as I reach 5,100 rpm. If I let up on the gas and hit it again it pulls strong until 5,100. When it goes beyond 5,100 with out a loss of power it sounds different. Like it straining to hard to get to that rpm. I can’t figure it out. Could it be a vacuum thing? Bottom line is I need some expert advice. I checked manifold and carb vacuum at various rpm and it runs around 18-20. I run a stock distributor with a 1-year-old pre-tonix module. Timing is 15 initial, 7 vacuum, all in at 36 by 3,000. It’s almost like the carb fuel bowl runs dry. Any ideas would be appreciated.

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Texas SS
1970 SS 396 350HP
Unrestored & Almost Perfect
Houston, Texas
Take a Look http://www.geocities.com/MotorCity/Street/6320
 

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If all you did was add a spacer, I'd suspect a vacuum leak.
The carb. butterflies don't hit the spacer do they ?

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Make it the way you like it, forget what the other guys say!
 

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This might be a long shot, but it happened to me.

It is possible that the spacer under the carb doesnt seal completely. The "footprint" of the carb may not match the spacer exactly, if this is the case then you could have a vacume leak at that spacer.

Vacume readings on a no load condition at high rpm dont really tell us much. If you know what your readings where at 5100 at full throttle under full load when things were running good, then that could be compared to what you would read now. Otherwise I would concentrate on recent self induced changes. Which leads me to think vacume leak.

Run it with out the spacer and see if the problem goes away.

Good luck

Clark
 

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Discussion Starter · #7 ·
I've checked the fuel pressure and it's running around 7-8psi. Butterflies do not hit. Fuel filter is clean.
Clark, the footprint seams to match the spacer. However being the original carb and manifold the sealing surfaces may not be perfectly flat. If this is the case could I use two gaskets between the carb and the spacer and two between the spacer and the manifold to effect a good seal. Would this cause me any problems. I think both you and Fred are on the right track as I was. Vacuum leak.

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Texas SS
1970 SS 396 350HP
Unrestored & Almost Perfect
Houston, Texas
Take a Look http://www.geocities.com/MotorCity/Street/6320
 

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Since everything is normal under 5100, it sounds like you just changed your power curve. If it feels like it's starving above 5100, try richer metering rods and/or playing with timing.

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Dale McIntosh
TC Gold #92
ACES #1709
www.dalesplace.com
Restore it or customize it, but drive it and enjoy it!
 

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Fred, your right. I'm not sure where I was trying to go with that one.

TXSS, You might try replacing the gaskets between the carb a spacer and between the spacer and manifold. If you have one of those thin fiber gaskets under the carb I would replace it with a thick one that will compress alot when you torque down the carb.

I went down to to my local autozone and asked them to pull out all the 4 barrel carb gaskets that they had on the shelf. I ended up finding a really thick one with the right footprint that came stock on some chryslers. It solved my vacume leak problem. Holley also makes thick gaskets.

If you cant find thick gaskets then I cant see where using two smaller ones would cause you any harm. If it works, it works.

Funny thing though, most of the time vacume leaks are more of a problem with a closed throttle. If you car is running good at Idle and at part throttle then we might be headed in the wrong direction.

Did the addition of the spacer change your throttle linkage geometry? (does the accelerator pedal fully open the butterflys)

I hope you find it.

Clark
 

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Discussion Starter · #10 ·
Just a little back ground. After I installed the spacer it ran the best it had ever run. I week or so later I noticed my first power loss. I replaced both gaskets with thin gasket. That helped but did not eliminate the problem. Now it's intermittent. And It never auto shifts past 5,100. I ran it with out the spacer and it ran fine. No power loss shifts at 5,600. However, the spacer gives it noticeably more low end torque and it's alot more fun to drive.

Clark: The linkage is on the same plane only 1" higher and feels the same.

Dale: I was thinking about richer secondary rods and checking my alot level.

All recommendations are greatly appreciated

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Texas SS
1970 SS 396 350HP
Unrestored & Almost Perfect
Houston, Texas
Take a Look http://www.geocities.com/MotorCity/Street/6320
 

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I tend to lean toward the whole shift in the powerband thing. Maybe doing this has changed the amount of vacuum you have at certain areas of this new powerband. This in turn might cause the transmission's modulator valve to shift differently than it has before.
I would also check your floats and reset timing (i.e. it might like a bit more or less now than it did before). When you change the combo on your engine, usually you have to start the whole tuning process over. I'm as guilty as anybody about throwing a part on....and running it like it sits. I have to learn to take the extra step. I'm interested to see what you come up with. Keep us posted!


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Justin Wray

70 Fathom Blue Malibu...
Boulevard Bruiser, Heavy Hitter, Pavement Pounder, Street/Strip and Weekend Warrior.....

Bottom Line?.....
DAILY DRIVER!!!
 

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Discussion Starter · #12 ·
This may help someone in the future. I've discovered my power loss problem. After trying nearly every suggestions made on this thread, with no success I removed the spacer again and took it for a test drive. It ran great. However, on the way home I punched it and I had no secondarys. I thought they're right, it must be a vacuum leak. I punched it again and the secondarys opened fine but when I hit 5,100 rpm I had a substantial power loss. When I got home I realized that I had unconnected the choke rod when I installed the spacer and had left it un-connected when I removed the spacer. In both cases that caused the secondary lockout lever to sporadically lockout the secondarys and allowed the choke butterfly valve to flop around and in some cases close under WOT. After fabricating a new choke rod 1" longer it now runs perfect.

The devil's in the details.

Thanks for all the suggestions

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Texas SS
1970 SS 396 350HP
Unrestored & Almost Perfect
Houston, Texas
Take a Look http://www.geocities.com/MotorCity/Street/6320
 
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