GTX TROU­BLE

FIND­ING WHAT AILS AN ORIG­I­NAL 1970 PLY­MOUTH GTX

Mopar Muscle - - Contents - TEXT AND PHO­TOS BY CAM BENTY

Find­ing what ails an orig­i­nal 1970 Ply­mouth GTX

Di­a­monds in the rough still hap­pen. Lost mus­cle cars are dis­cov­ered in barns across the coun­try ev­ery day. Most are good cars gone bad, for­got­ten un­til some­one re­al­izes their value. Re­gard­less of where you find them, fig­ur­ing out how to get them back on the road takes at­ten­tion to the ba­sic el­e­ments of en­gine op­er­a­tion. With great reg­u­lar­ity, these cars of­ten have sim­i­lar is­sues that cause them not to run due to some sim­ple rules that any novice me­chanic can ad­dress.

If there’s one motto any me­chanic will re­cite, it’s the three rules of proper en­gine op­er­a­tion: fuel, air, spark. Take any one of these el­e­ments away and your en­gine will fail to not only run right, but could cease to op­er­ate as well. Stir in the need for a prop­erly timed spark ar­riv­ing at the point where the fuel and air car be ig­nited and you have the most com­mon mal­ady that causes frus­tra­tion with ve­hi­cle projects — re­gard­less of whether the car came from a barn re­cently or not.

Such was the case with this 1970 Ply­mouth GTX. A pris­tine ex­am­ple of the breed, this 440-cid–pow­ered orig­i­nal black GTX was run­ning very rough and needed a class in Troubleshooting 101 to get it run­ning right. While the prob­lems found with this car may not be present on yours, it’s a great ex­am­ple of some of the most com­mon things that hap­pen with these clas­sic mus­cle cars. In ad­di­tion, we rec­om­mend giv­ing your car a thor­ough safety re­view. It’s crit­i­cal if you in­tend to drive your ve­hi­cle, re­gard­less of whether that’s across the coun­try or across the street.

An­other im­por­tant tip for all clas­sic mus­cle car own­ers: Buy a fac­tory ser­vice man­ual for your ve­hi­cle. These man­u­als are very help­ful not only for ba­sic main­te­nance, but also for larger projects in­volv­ing dis­as­sem­bly or re­pairs on your ve­hi­cle. While a lot of this in­for­ma­tion can be found on the in­ter­net, a huge amount of mis­in­for­ma­tion will only cause you more frus­tra­tion. These man­u­als will save you countless hours by show­ing you the proper way to take things apart. Trust us — they’re well worth the in­vest­ment and places like Year One and Clas­sic In­dus­tries of­fer re­pro­duc­tions of these orig­i­nal man­u­als, so you can or­der one on­line and have it at your door in a cou­ple of days.

Fol­low along as we get our GTX back on the road!

We be­gin our re­view of the 440-cid en­gine and an in­spec­tion of the Carter AVS carb. This orig­i­nal car showed 96,000 miles on the odome­ter and was run­ning rough es­pe­cially upon ini­tial throt­tle tip-in.

Re­move the carb top screws and lift off the top of the carb, mak­ing sure not to drop any screws down the carb or into the open in­take man­i­fold. Once on the work­bench, turn the top up­side down to quickly re­veal if the float lev­els were set evenly. Check the floats don’t have any pin­hole leaks, caus­ing them to sink once in­side fuel bowl. Re­plac­ing these floats is a cheap way to en­sure this isn’t the case with your car­bu­re­tor.

The first prob­lem was ob­vi­ous and dan­ger­ous. This bat­tery ca­ble is orig­i­nal equip­ment, but ran its course, and in need of im­me­di­ate re­place­ment. Even if your ca­bles look great, re­move them and clean the bat­tery posts with bak­ing soda (make sure to wear skin and eye pro­tec­tion), brush­ing the ter­mi­nals and ca­ble eye­lets with a wire brush, and retighten to make cer­tain they de­liver full power to your en­gine. Make sure all ground ca­bles (neg­a­tive) are prop­erly in­stalled and none are re­moved.

The 440-cid en­gine was com­pletely stock right down to the orig­i­nal-style plug wires, which can be a com­mon weak link in the ig­ni­tion chain. Each plug wire was in­spected for breaks in the in­su­la­tion, and we ul­ti­mately just changed out the en­tire wiring sys­tem to be safe. Each of the spark plugs were taken out, the car­bon re­moved with a me­dia blaster, cleaned with sol­vent, and then rein­serted af­ter re-gap­ping of the elec­trode.

The ac­cel­er­a­tor pump link­age is re­moved in this man­ner. The ac­cel­er­a­tor pump was con­sid­ered the prime sus­pect in the ac­cel­er­a­tion prob­lem, since clearly the car had been sit­ting for some time.

Be­fore re­mov­ing the top of the carb, the rear bracket and fuel line need to be re­moved. Dou­ble check the car­bu­re­tor is cor­rectly tight­ened to the in­take man­i­fold and the in­take man­i­fold to the cylin­der heads.

Since the en­gine was ex­hibit­ing a stum­ble off idle, we started with the car­bu­re­tor. First, the me­ter­ing rods were re­moved af­ter re­mov­ing the screw caps that cover them from the top of the carb. They should be re­moved and cleaned be­fore tak­ing the top off the carb. Make sure to note from which side of the car­bu­re­tor each of the me­ter­ing rods were orig­i­nally lo­cated.

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