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Old 05-22-10, 02:15PM   #1
dutch71formula
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Default need help with 1971 formula 400 engine

hi guys,

i cant get my bird running good, when cruising at low rpm there is backfire in the exhaust.
i tried it at home with regulating the idle up a bit say till approx 1100 rpm.
strange enough the backfire will come every few seconds at one exhaust side
only, its the driver side exhaust.

i few weeks ago i installed a new mallory HEI distributor including carbon core
sparkplug wires and new NGK X4 sparkplugs because i could not set the timing right on 12 degrees where it should, lots of misfires etc with the stock distributor.
on that occasion i also cleaned the edelbrock 1406 and fitted new gaskets and discovered the prev. owner had the adapter plate between edelbrock and intakemanifold up side down (at that point i thought i fixed the bad running)

with the mallory HEI and the cleaned carb the engine ran as smooth as a whistle, no backfires, no hesitation etc.

then after about 50 miles ( and some riding with full trothle) the thing began running bad, the problem got worse and worse, even starting to stop every
5 miles or so. end of story, Mallory Hei Died and there was no spark anymore at all.
The mallory was fried at that point (electronic modul where the rev limiter switches are located)

then i figured that a bad voltage regulator could be the cause of the fried Mallory elctronic part so i took the dynamo to a shop which confirmed this (strange enough from the Haynes manual the 1971 bird should have an external regulator on the firewall, mine however had a type 10 SI with internal regulator so i installed a new regulator and got on partial warranty a ProForm HEI in exchange which i installed. the ProForm runs, but not as smooth as the new mallory. i also changed the spark plugs into AC delco
because a mecanic told me to get rid of the jap crap NGK (strange enough on all my motorcycles i always prefered NGK sparks, but hey all my bikes where/are jap bikes ;-) i also installed again new Axxel carbon core wires
just to be sure.

The car runs and did not die on me for the last 100 miles, but i dont trust it at this point, the backfire from one side of the engine will not do good to it i guess ??.

for the setup of my bird.

- engine 400 YS (seems stock for the bird)
- twin inox headers and pipes (double system till tail, with only a connector tube between the 2 pipes under the tranny
- Edelbrock 1406 manual chocke (choke is set fixed in wide open position, means choke off)
- K&N air filter in edelbrock box
- the TCS (little part valve with electric cables) next to the carb is disconnected from the system. Strange here however is that the electric connector that is disconnected from the TCS valve has some kind of purple resistor build as a bridge in the connectors two wires done by the previous owner(any explanation for this resistance ??) the vacuum tube from the distributor is directly connected to the edelbrock tube at the passengers side (looked from front of the carb.
driver side of the carb attachment has the vacuum tube from the tranny TH400.
at the midle (larger diameter) connector is the (i think it is) cranccase ventilation tube from the front middle with the metal valve connected between.
- because the distributor + wire from the ignition is most likely a resistor wire
for the mechanical point ignition system i took a wire with switch and fuse from the battery to provide switched clean 12 volts to the H.E.I. i also renewed the Mass cable from battery to the engine en from engine to firewall and even made a mass cabel attach to the housing of the H.E.I. and an extra mass to the mass of the coil, just to be sure.


just for the record, any questions i direct to the previous owner will not be answered at all, not much of a help i know so i now seek help with you guys.

a lot of stuff i wrote, but maybe it will help to diagnose the problem. please forgive my errors in english language, dont forget i am Dutch ;-)

many thanks for your help in advance.

Frans
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Old 05-23-10, 10:26AM   #2
GREATGTO
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Hard to really nail down what it could be because so many things have been changed recently. I would check the the carburetor , I would go through it and set all adjustments.
You can find a lot of info on google just type in ( edelbrock 1406 troubleshooting)
Another thing you can check is the rubber hoses at the gas tank, sometimes they crack and suck air which will cause backfires and or misses at certain RPM when the draw is strong enough like under full throttle or mid level cruising.

It has to be fuel or electrical related, the only mechanical possibility is that you have a bad cam lobe. Check all wiring,make sure coil is good and has a solid connection.Also check fuel lines/filters

If you know anyone that has an identical carb try swapping the carb out real quick to see if it cures the problem. Just bolt it on and hook up needed fuel and vacuum lines, dont worry about throttle hookup if the carb varies really far from the 1406, just work the throttle by hand for the test. If it still does the exact same thing you will at least know its electrical not fuel related. Check intake for leaks as well where it bolts to the heads/ re-torque intake bolts. If none of this helps pull the valve cover off the side that backfires and check the rocker arms/pushrods/valve springs for any possible sings of damage.
Adjust rockers at this time if needed.

You also have a slight chance of having a burnt exhaust valve on the side you say always backfires but this is less likely than the other things discussed.

Last edited by GREATGTO : 05-23-10 at 10:51AM.
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Old 05-23-10, 07:05PM   #3
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First off, 1971 Pontiacs came with internal regulator alternators. The other GM companies didn't start until 1973.

"Strange here however is that the electric connector that is disconnected from the TCS valve has some kind of purple resistor build as a bridge in the connectors two wires done by the previous owner(any explanation for this resistance ??)"

This sounds like an anti-voltage spike diode. What it does is when the valve is turned off[coil releasing], a voltage spike is released through the electrical system, traveling backwards from normal flow of electricity. The "purple resister" you refer to is a diode that allows this voltage spike to travel backwards to ground, thus relieving the voltage spike so no electronic components are damaged from it. You will also see these diodes on the plug of an A/C compressor. Since the normal flow of electricity can't flow through the diode, there is no short in normal operation.

Causes of backfires in exhaust.

Bad or broken valve springs.
Vacuum leaks
Exhaust valve not sealing. Possibly from burned valve or old or bad valve job, or from valves being adjusted too tight.
Bad spark plugs or plug wires. NGK plugs are fine by the way. Read this post[http://www.pontiaczone.com/forum/sho...d.php?t=19759], about 1/3 the way down the last paragraph, about what happened to us with plug wires.
Bad coil.
Cracked distributor cap.
Bad cam lobes, but the backfire is usually up the carb.

Good luck. And take a close look at the valves springs. Or do a compression test to see if some valves are leaking.
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Old 05-24-10, 05:58AM   #4
dutch71formula
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thanks guys for the numerous replys her. i trie to give extra info here on each hint from you where needed.

@GREATGTO rubber hoses at the gass tank (this is fuel thank i assume?)
for the cam i was at a US car meeting 2 weeks ago and because of the little ruff idle of the engine (at idle it does not run smooth, but moves irregular as when missing a spark) he says definatelly need new cam, rockers and no need looking into the heads he found the rockers tightend and no possibillity to adjust them, after that i found out that the 1971 pontiac 400 has no valve lash adjustment possibillity by tighten or loosen the rockerarms as with chevy, so i went of the idea defective cam. but to be sure i need to take the thing apart.

no possibillity to get a carb, have the old stock quadrajet lying here, but i think the prev. owner also put his fingers into it and needs to be refurbished completely.

i did a compression test 2 weeks ago with following procedure:
Engine was cold, all sparkplugs taken out and had similar readings on all 8 cilinders between about 130 psi and 140 psi, was this the right procedure?

@ fireball, have to make one and yes this is the setup i am using, believ it or not the spacer on the right pic was mounted upside down by the prev. owner, carb mounting bolts just through the spacer into the manifold. i cleaned everything and bolted it on as it should with new gaskets and bolts.

@gtofreak, voltage spike diode ? this part i cannot really follow due to my lak of tecnical english, i disconnected it and put it back in place afterward because nothing changed, i hope i put it in back the correct way, because i assumed it to be a resistance i did not think of direction, diodes work in one specific direction as far as i know, for the compression test see above, will do it again if i used the wrong procedure (thanks for any input in how to do a compr. test correctly, by the way i have a simple 20 buck gauge tester)

i will see if i get to it, looking at it today and then i will again look at everything suggested in your replies.

and guys, thank you very much for your input allready.

Frans
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Old 05-24-10, 07:09AM   #5
dutch71formula
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hi there,

some extra info/questions:

i hope i am right with my understandign of valvelash adjustment on pontiac 400, i know the chevy system, because old Opel engines had the same principle, valvelash adjustment by tightening or loosen the rockerarm bolts to adjust, i never seen a system with rockerarms tightend to torque and set. i assume there is no need to adjust valvelash on pontiac, and when yes there is something worn out, ?? can i check if something is worn out at looking at the pushrods if they are loos at closed valve?? i noticed you can twist the rods a little with force with your fingers, does this mean that no lobe is worn, or the lifters are OK, because when something has worn out, ther will occur extra space between either, rockerarm, valve, or rockerarm, pushrod ??
or could this be camouflaged by valves wearing in into their seats deeper and deeper ?
i will check/visualy inspect the upper head section such as rockers, look for signs of broken valvespring, etc as far as i can with the valvecovers of, any info on where to look is welcom offcourse


thanks alot
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Old 05-24-10, 01:35PM   #6
dutch71formula
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so i took a look and started at simple stuff as carb ignition etc.

when i took out the metering rods i thought its strange with the springs on the brass rods
going around the bend. i installed them this way when i cleaned the carb a few weeks ago and did not think anything worng, just put them in as i too them out.

to be sure i took a drawing from edelbrock and noticed that the springs should be under the aluminum parts instead over the rods. you can figur out that i thought...yeeess i found the problem........ but....nothing, maybe i little more power at reving, but also this maybe subjectiv.

so i took the hei cap of and looked at al the connections there. i scraped the paint off the coil where the contact is made when assembling it, i also made an extra wire to supply 100% negative ground on the coil.

i also put some dielectric grease on the rubber washer underneat the coil, it was dry (hey, it was a brandnew distributor) and the manual said to put some supplied dielectric grease so i did. again..... not much better, i rode it and it died on me after one full rev. (now this could be that i had to low idle, about 700, now i put it up to about 800 till 850, i know a little high for a automatic, but i will test it that way.

again on the other hand, not much of an improvement (i think, also very subjective with little or no change)

so next time i will take a look at under the valve cover, too less time here with the fam. around.

@ fireball, i upload some engine pics in the gallery, if you need specific pics of a part, just whistle, i take them.

kind regards

frans
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Old 05-24-10, 01:36PM   #7
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Fastest way to find out if it is in the valvetrain is hook up a vacum gauge, if the needle is bouncing fast back and forth between numbers it could be a valvetrain problem.
The rocker arm studs you described you have are the factory type bottle neck studs, they have no adjustment. The nut gets torqued down and thats it. There is nothing wrong with that as long as factory specifications are followed when completing a valve job and the tips of the valves are set at the correct height.
I have a few questions.
Is there any valvetrain noise?
Did you pull any spark plugs and if so do they all look like they are burning the same?
Is the timing still jumping all over?
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Old 05-24-10, 01:46PM   #8
dutch71formula
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i dont have vacuum gauge, but where to measure?

OK, so the valvetrain with the fixmounted rockerarms is as i understood from the web and the haynes manual.

valvetrain noise? yes when warmed up a little tikking sound, not loud, but present for a good ear, both sides

i will pull the sparks out again and make some pics of it, but i tried to understand the gallery, but seem to miss something to upload pics.

-Is the timing still jumping all over?

dont know if correctly undestood by me, but i set timing with a timing light and it sets just fine, but offcourse i wont notice if out of 10 flashes one or two are missing...
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Old 05-24-10, 01:50PM   #9
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You would want to hook up the vacuum gauge to a constant manifold vacuum source off the carb or intake manifold itself.
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Old 05-24-10, 06:12PM   #10
dutch71formula
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OKAY, so i need to get my hands on a vacuum gauge, i hope that next weekend i can
get a look under the valvecovers. just hoping finding not to much more like, upside down adapterplate under the carb, or metering rod springs located wrong.

i update some pics in my galerie from the engine.

frans
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Old 05-24-10, 06:24PM   #11
dutch71formula
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i forgot, i also measured the ignition coil with a multimeter.

between negative ground (black) and center pin to rotor (bottom of coil) aprox. 10 K-ohm
between Batt terminal and center infinite
between Tach terminal and center infinite
between negative ground and Batt terminal aproxx 1 Ohm

hmmm, i hope i wrote this down correctly from memory today ??
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Old 05-24-10, 10:52PM   #12
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What fuel pump are you using? You may need to check the fuel pump, the fact that you can rev it and then it dies could be a sign its running out of fuel, this could also be because of the float level setting in the carb
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Old 05-25-10, 06:54AM   #13
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@GREATGTO it is the stock fuel pump i am using, but i think that the engine died after the quick rev was the idle beeing to slow, it only happened once on my test ride yesterday.
now i am running idle at about 800 to 850 and it keeps running, allthough rocking nervousily around in the mounts, this rocking also remains when reving the idle up a bit.
with fast idle it still bangs in the left (driverside) exhaust with un regular time distance.

the float level setting is good as shown in the videos at edelbrock.com
floatneedles and seats are also good, changed them when i took the carb apart last time, because they where in the overhaul kit.
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Old 05-27-10, 07:22PM   #14
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i took out the sparks and they (AC delco r45S) all look the same. i uploaded some pics in my gallery. can anybody give me some info on how they look? the core and the most of the electrode is very light (too lean ??) the outer ring upto the first few millimeters of the outer electrode looks fogged with carbon.

another question to the sparks, the 1971 bird should have 0.030 sparkgap, from 1975 the sprkgap should be 0.060.
since i now run a H.E.I. ignition one should assume that i can put the sparkgap at 0.060 because the H.E.I. should be powerfull enough too bridge the larger gap with a stronger spark?? (the haynes manual give 0.035 for pre 1975 pontiac400 and 0.060 for after 1975 engines with H.E.I. ignitions)

Next thing i noticed is when i leave the car standing a few days it is hard too start, just as when no fuel is left in the carb, or the fuel have to come from a long way.

when i start the engine when warm or after lets say 5 to 6 hours (so cooled down enough) it will fire up right away. how can i test the fuel pump for good function?

@mb125 i took my carbtune II vacuum tester that is intended for motorcycle multi carb
syncronising and hooked 1 line to the driverside small port of the edelbrock, 2nd line to the passengerside small port of the edelbrock and the 3rd to the vacuumport on the intake manifold where the fuelvapor canister goes to.
i think the problem with the carbtune is that it can not go any higher than 42 cmHg so the driverside vacuum port on the edelbrock and the port on the intake manifold go straight up to max. the vacuum port on the edelbrock where i have the distributor advance hose (timed vacuum port ??) does not move, so zero vacuum at idle.

when i rev it up just a little the vacuum on the passengerside edelbrockport where the vacuum advance of the distributor is hooked will go up to 20 cmHg. at the same time the readings of the other 2 ports will drop a little to about 36 cmHg the readings are quite steady and not jumping around. one has however to be aware of the carbtune II build in dampening function to give more accurate readings at syncronising carbs and prevent jumping around as clocklike gauges tend to do.

hope this helps to diagnose a bit more for me and my bird.

frans
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Old 05-28-10, 03:34AM   #15
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.045 is good for a stock HEI or with an aftermarket coil and module. If your using any aftermarket CD type ignition box like MSD go with the .060 gap.

the vacuum port on the edelbrock where i have the distributor advance hose (timed vacuum port ??) does not move, so zero vacuum at idle.

You should hook up the vacuum advance to a port that has manifold vacuum, so it pulls in more timing when the engine has the highest vacuum under no load. Or you can just not use the vacuum advance at all. It's mainly for better gas mileage on a factory engine.
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