Diagnostic help please

[wirenut]

Hello All,
At the track on sat. the car developed a problem. The symptom is this.
After the car is in high gear, shift light on, so its turning like 5600-6000 the engine shuts down for a second until the rpm drops enough for the shift light to go off, so like 5400 then the engine comes back on, and it repeats the process like that for as long as I stay on. It did this the last two runs of the day. This did not happen during any other point where the rpm was in the same range. All other stats 60', 660' etc. were normal
I dont have a fuel preasure gage where I can see it from the drivers seat, but the gauge under the hood seemed to read a bit low back in the pits. Normal would be 7-8 lbs, but was reading 5-6 lbs..
My first thought was a fuel problem. But thinking back it did seem like someone was flipping a switch on off on off. A very clean and clear on off not a sputter or miss, very rythmic. I thought if the ignition was cutting on and off I would have heard or felt some kind of back fire in the headers from unburnt fuel being ignited. The second time it happened I had time to notice that the power in the car was still on, and voltage was like 11ish..
Somebody watching said he thought he saw faint black smoke, but being methanol I didnt think unburnt fuel would shaow any color. Its difficult for me to get to the track easily and where I live lighting it up in the street or driveway to push it to the shift point would not be an option. Any Ideas would be appreiciated.
Gary
The car;
440 steel heads
Carb is 750 holley vac secondary. runs methanol. 2 in line filters.
msd distr. and 6al w/ adjustable rev limit
1- mallory 250 fuel pump with return line

 

[Mrmopartech]

First install a fuel guage on the cowl to watch the fuel pressure as you drive and adjust it and check the pump for loose wires or bad connections.Disconnect the shift light and re-try,then check all the dials for the MSD and at the end swap for another unit. MSD do no like low voltage under 12 volts,they have a internal shot off in them,mrmopartech

 

[wirenut]

First install a fuel guage on the cowl to watch the fuel pressure as you drive and adjust it and check the pump for loose wires or bad connections.Disconnect the shift light and re-try,then check all the dials for the MSD and at the end swap for another unit. MSD do no like low voltage under 12 volts,they have a internal shot off in them,mrmopartech

Thanks MrMT, I wondered about the voltage / vs the MSD

 

[Demonracer]

You might check the RPM chip in your MSD. Change it out with one with the same RPM, or a notch higher. Just my .02.

 

[moper]

You might check the RPM chip in your MSD. Change it out with one with the same RPM, or a notch higher. Just my .02.

x2. Verify ignition feed voltage and good grounds (check with an ohm meter), and chips have been known to be bad and do odd things.

 

[wirenut]

x2. Verify ignition feed voltage and good grounds (check with an ohm meter), and chips have been known to be bad and do odd things.

it uses an adjustable rev limiter plugged into the msd. Dosnt feel like the rev limiter activating, its not doing that stuttering.
If it was ignition at that speed and with the car in gear ie motor still turning,pedal to the floor, wouldnt it make a hell of a bang when the ignition cut back in and lit the unburnt fuel in the headders?

 

[racerhog]

#8 line to the Reg. ?
#4 or #6 to each side of the carb?
High flow needle and seats?
Floats adjusted to low....
Sounds like your saying the car is "Noising over"

Or dose it just do the hucky-buck if you keep your foot in it?

And I would say....turn the limiter up for 1 pass to check it... Tach might be reading wrong?

 

[wirenut]

#8 line to the Reg. ?
#4 or #6 to each side of the carb?
High flow needle and seats?
Floats adjusted to low....
Sounds like your saying the car is "Noising over"

Or dose it just do the hucky-buck if you keep your foot in it?

And I would say....turn the limiter up for 1 pass to check it... Tach might be reading wrong?

Doesnt feel like its nosing over. Hucky bucy may be a better description, but slow and rythmic if you stay in it. Started going through filters and fuel system over the weekend, but havnt gotten as far as the carb yet

 

[rmchrgr]

Think we need a little more info on the car and combination - cam, gears, trans, shift points, tire size etc.

Is the fuel system designed for methanol? Might want to verify timing at a higher rpm or at least see if its correct to fire the mixture at a higher rpm.

Also, a 750 vac. secondary might be on the small side for your combo. Have you messed with the jetting? Secondary spring rate? Power valve? Throttle linkage adjusted properly? Float levels in the bowls correct? Where are the filters in the delivery line? Why do you need two filters? What size is the return?

What's the rest of the electrical system? Battery in the trunk? Alternator good? Belts good? Dampener good?

Is it an automatic? If so, maybe your converter is inefficient/slipping badly at that rpm? Not likely though, I think you would see that on each shift. Just a thought.