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jrhone

Stumble when shifting at high RPMS...float level??

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Hey guys...I have a few days off and I decided to look at tuning the car better.  A bit of history.  Months back, the car just seemed to die on me...then a few hours later, started up but ran VERY poorly.  Checked everything and it pointed to fuel delivery.  I replaced the carb with a new weber 38.  The car ran much better and the problem went away.  I now think I know what in the old carb went bad and why it died.  The only thing I DIDNT do when installing the carb was a float level adjustment.  So I opened up the carb and did what I thought was the proper adjustment.  The car runs much better and is stronger throughout the RPM range but I still have a stumble when shifting If I rev it close to redline.  It doesn't stumble until I shift into the next gear then stumbles and then takes off.  From a stop it revs to redline in first with no issues, then shift to 2nd and I get a stumble, then it catches and revs all the way to redline, then shift and again stumble then catches and revs all the way to redline.  This was doing this before the float adjustment just ALOT more.  I also felt I wasn't getting all the power I am getting now.  Should I keep adjusting the float level and dial that in?  Look at fuel mixture?  Timing?  I tried to tune it before with mixture and timing, but I never got the stumble out of there.  I pulled the old carb out that had the car running well until it stopped and tried to look at the float level on that to match it and noticed the spring valve that the float opened and closed was stuck.  I think sometimes it stuck open and sometimes it stuck closed and this is why the car just stopped and then I sprayed a bunch of carb cleaner into carb and it then started working again...my theory..make sense?  

 

 

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I vote fuel delivery... and recommend a fuel pressure gauge to prove me wrong.  You can just tee it in temporarily.

And it MIGHT be caused by junk in the tank.  Have you looked at the screen on the pickup?

If it's not there, the junk can interfere with fuel pump operation.

 

hth

t

Edited by TobyB

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Is your dizzy's vacuum advance working properly and hooked to a proper vacuum source?

 

Currently I have a Tii Mechanical advance distributor.  No vacuum.  Part of why I am tuning is I have a new 123Tune distributor sitting in the garage and I wanted to install it with things working well.  I don't want to introduce a new variable into th equation since it WAS running great before I had the car stop a few months back.  Car has been dead reliable since swapping the Weber 38 for a new one...just now has the stumble when shifting at redline...when sifting at 5k it doesn't have the stumble.  

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How's your fuel pressure at redline?

 

t

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Likely not the same problem but over the summer my wife was driving our 76 (fjord just like yours jrhone) back from RI and encountered what she described as a weird stumble. I did some over the phone troubleshooting and the vacuum line to the dizzy was cracked. She fixed that and it seemed to get much better but still had a weird stumble when revving up or shifting but would run fine. I thought fuel, plugs etc.

Checked the plugs and fuel delivery and everything was fine. A few days later I decided it could be the rotor and cap (which were less than 1000mi old) and when I pulled the cap I noticed one of the wires was chewed up. The metal tab on the firewall right behind the dizzy had bent down and was cutting into the wire shorting out one plug but only at shifting or hard acceleration as the engine moved a few mm and allowed the short.

Sent from my SAMSUNG-SM-G890A using Tapatalk

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Rev- limiting rotor ftw!

 

t

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I have changed the plugs, cap, rotor, fuel filter, carb and coil. The only things I have not changed were the fuel pump, the distributor and the pertronix module. I do have a 123Tune distributor to install and that would eliminate the distributor and the pertronix unit and I do also have another fuel pump I got from blunt. Those will go in soon. My thoughts though was it more tuning or was it something failing or failed? I'd hate to keep throwing parts at it when it really was just a better tune on the new carb. The old carb literally dripped fuel and had a stuck float when the car died. So that had to be replaced anyway.

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Solved. I replaced the distributor with a 123Tune Bluetooth unit and my stumble is gone. Not gonna blow the tii distributor for the problem but maybe the pertronix unit went south or was going bad so it was just enough to run bad when stepping on it. So it was either the distributor or pertronix unit or the timing itself but I could never get the timing right. It was always erratic.

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Could have been the apparent dwell of the Pertronix. I had a Hot Spark knock-off unit that only gave about 30 degrees dwell. If this was short then your coil may not be saturated before the Pertronix opens the circuit at high rpm.

Not a problem for you now!

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I still think it's your fuel pressure...

 

t

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Wouldn't it still be happening then? Since the new distributor there is no stumbling all the way to redline. Where before it would stumble anytime over 5k. I do have another fuel pump I can throw in there but I think at this point the stumble had something to do with maybe a worn distributor or a bad pertronix ignotor module.

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