flat spot when accelerating in 2nd and 3rd (1 Viewer)

Joined
May 23, 2010
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707
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Near Charlottesville, VA
I have recently put in a new to me motor in my 87' 60 and it really runs great. It start great, idle very smooth and drives great. It also has been fully desmogged. However, I have been noticing a very slight flatspot when accelerating in 2nd and 3rd gear. It is very minor. I wouldnt even call it a hesitation, but when I near 3000 rpms, for a second or two before up shifting it just seems to go flat but continues to accelerate then I will shift and its fine. Hopefuly this description makes sense. Again it seems very minor but I do notice. I dont know if this is normal or not. Has anyone had similar observations, or does anyone have any suggestions for adjustments I should look into? I have checked my timing and have it slightly retarded ( timing mark on flywheel is just to right of the needle on bellhousing) I have not had my dizzy recurved and am wondering if this my be the issue. I am planning on doing a valve adjustment today, but I doubt this is my issue as there is not much valve noise. Any thoughts or recommendations welcome. Thanks
 
Joined
Jan 18, 2011
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1,300
Location
Seattle
My first thought is fuel delivery- when was the fuel filter last changed?

Then I'd put a fuel pressure gauge on it (temporarily) to make sure
pressure's holding.

After that, I'd put in smaller air correctors... if it was a Weber carb.

hth,

t
 
Joined
Sep 18, 2010
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326
Location
St. Augustine, FL
I may have a similar issue if I'm interpreting your description accurately.

I am desmogged, Aisin carb tuned by Toyota, valves adjusted a few months ago (all prior to my ownership)

truck starts, runs smooth and strong (for a 2f) everything excellent but feel a flat spot, almost like I'm at peak rpm and need to shift but if I just put the pedal down it will get over the flat spot and accel on up to 3500rpm (not comfortable going over that)

curious to see what others think
 
Joined
May 23, 2010
Messages
707
Location
Near Charlottesville, VA
I may have a similar issue if I'm interpreting your description accurately.

I am desmogged, Aisin carb tuned by Toyota, valves adjusted a few months ago (all prior to my ownership)

truck starts, runs smooth and strong (for a 2f) everything excellent but feel a flat spot, almost like I'm at peak rpm and need to shift but if I just put the pedal down it will get over the flat spot and accel on up to 3500rpm (not comfortable going over that)

curious to see what others think
Yep... That is the exact same thing that I have going on. I would love to get some feedback on this.
 
Joined
Feb 6, 2002
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15,139
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OC, CA
 
 
 
 
Sticky centrifugal advance weights? Try a little spray lube in the distributor.
 
Joined
Dec 7, 2010
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113
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ark city ks
mine does it too..PINHEAD good call on the distributer weights,i will try it. mine is more of a lagg and then takes off rpm. post up if you find the problem
 
Joined
Feb 6, 2002
Messages
15,139
Location
OC, CA
 
 
 
 
The weights are under the plate that has the pickup coil. Spray some WD40 in there and wiggle the rotor back and forth. It would be nice not to have to take the plate out, but it is not hard.
 
Joined
May 23, 2010
Messages
707
Location
Near Charlottesville, VA
Did this yesterday, and it seems to have helped. I only had time to drive up the street and back, but it does seem to go though that rpm range better. I also found that my rotor cap was damaged. I had another on hand and swaped it out well. Thanks for the recomendation. I never would have thought to do that. Thanks again.
 
Joined
Feb 6, 2002
Messages
15,139
Location
OC, CA
 
 
 
 
The detailed symptoms you described suggest that it is a centrifugal advance problem, rather than a fuel problem. If it was a fuel problem, it would tend to be load sensitive, showing up on hard acceleration or going up hill. The fact that it revs fine after working through the flat spot suggests it is not fuel related. It is RPM sensitive and one of the major factors in RPM dependent problems is centrifugal advance.
 

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