Author Topic: Sporadic chugging when cruising  (Read 534 times)

WaltZ

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Sporadic chugging when cruising
« on: July 03, 2020, 07:22:57 PM »
Sporadically, my 302 chugs, almost feels like a misfire, when cruising on the highway.
Checked plugs and are burning clean.
It's been dyno tuned prior to reinstalling it back into car.
Running a Pertronix Flamethrower III.
Using 93 Sunoco with 1.5 oz per gallon of Klutz Octane Boost.

a buddy of mine has a z 302 with the same Pertronix and does the same thing.
Any thoughts
Thanks in Advance

67CruiseMaster

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Re: Sporadic chugging when cruising
« Reply #1 on: July 03, 2020, 08:03:31 PM »
I'm not and expert but seems to me that's a carb problem.
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SoCal805

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Re: Sporadic chugging when cruising
« Reply #2 on: July 03, 2020, 08:56:35 PM »
if this happens under load...might want to do the "look for sparks" aka plug/wire arc from plug boots/wires in the dark. A spray mist of water helps bring it out if that is your issue

If that checks out than look into carb tune...(your floats filling up to much?)

william

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Re: Sporadic chugging when cruising
« Reply #3 on: July 04, 2020, 02:28:05 AM »
Check for blown power valve.
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PHAT69AMX

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Re: Sporadic chugging when cruising
« Reply #4 on: July 04, 2020, 05:58:46 PM »
I would suspect fuel boiling, burping or dripping out boosters or bowl vent tubes, intermittent temporary flooding.
Rich chugs, lean backfires.
Would hesitate to suspect power valve(s).
Simple test for bad power valve, with it running, turn mixture screws in all the way.
If the Power Valve diaphragm is ruptured it usually will keep running, if not it will quit.
imho modern gasoline boils or evaporates a whole lot easier, and doesn't play well with materials.
From Holley get All Metal Needle and Seats rather than Viton Tipped.
Install a fuel pressure regulator as insurance of a constant steady fuel pressure to the carbs.
Slightly lower the Fuel Level in the Fuel Bowls, just at, or just below, the sire screw holes while, with everything HOT, up to temp.
And the hard part, confirm primary Transfer Slot, Idle opening setting AND ALSO the Secondary Side Idle Speed Screw Adjustment and sufficient secondary butterfly idle speed opening.
This ensures that even at idle there is some amount of flow through all 4 bores of the carbs to keeps things fresh, from setting there, getting hot, and boiling.
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JoeC

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Re: Sporadic chugging when cruising
« Reply #5 on: July 07, 2020, 11:22:30 AM »
are you running stock distributor set up?

I would look at the distributor vacuum advance  assuming your mechanical advance is working correctly

a lot of tuning can be done with the vacuum advance unit.

If you search on it , should find info on running ported vacuum vs non ported vacuum

The amount of advance is sometimes too high on a stock vacuum advance unit

I made plates for the vacuum advance unit where I can limit the amount of vacuum advance you get from the stock unit

crossboss

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Re: Sporadic chugging when cruising
« Reply #6 on: July 07, 2020, 04:44:06 PM »
Sporadically, my 302 chugs, almost feels like a misfire, when cruising on the highway.
Checked plugs and are burning clean.
It's been dyno tuned prior to reinstalling it back into car.
Running a Pertronix Flamethrower III.
Using 93 Sunoco with 1.5 oz per gallon of Klutz Octane Boost.

a buddy of mine has a z 302 with the same Pertronix and does the same thing.
Any thoughts
Thanks in Advance



Since you asked…
Pertronix has issues. Failure rates are over 50 percent across the board. Never liked it, never will use them. Some guys swear by it, yet they all say the same thing (before it fails): "I keep a set of my original points in the trunk…just in case". What does that say about the quality of Pertronix? Translation: it will fail. Since you also mentioned your buddy is having the exact same issues, its a no brainer what the cause is. Btw, Accel makes a superior electronic 'conversion' worth considering.
« Last Edit: July 07, 2020, 05:10:47 PM by crossboss »
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Kelley W King

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Re: Sporadic chugging when cruising
« Reply #7 on: July 07, 2020, 08:10:11 PM »
my vote would carb issues. Holleys can leak internally and you don,t see it. When you are accelerating the fuel just burns, cruising it will cause issues.
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KurtS

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Re: Sporadic chugging when cruising
« Reply #8 on: July 07, 2020, 08:29:40 PM »
At least we are unanimous. :)
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ko-lek-tor

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Re: Sporadic chugging when cruising
« Reply #9 on: July 07, 2020, 09:59:23 PM »
I do know that the a Flamethrower III states in the install instructions that it needs a power source that does not use any resistors or anything that would create an impedance.   Not sure if this info will be of any help?
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WaltZ

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Re: Sporadic chugging when cruising
« Reply #10 on: July 08, 2020, 02:10:00 PM »
Carb issues, probably caused by gas. After running about a half tank of 93 Sunoco with Klutz additive, mixed at 16 oz for ten gallons, it has cleared up.
Thanks for everyone's input, it is greatly appreciated!