# 1970 GTO dies when braking and around turns



## michaelrj9 (Jan 25, 2010)

I have a a newly acquired 1970 GTO so I don't know too much about it.
It has a 400 in it an older Holly 650 DP, Headers & a different cam as it had a little lop to it. Not sure if anything else in the engine is changed.

About 1/2 the time or less it will die when braking (moderate) not slamming on the brakes but not coasting to a stop either. Same thing around turns.

Need suggestions on what you think could be causing it and where to start looking.

Idle is min 650 in gear when not dieing  

Thanks


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## geeteeohguy (Feb 2, 2008)

Check your float levels in the carb.


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## tiger13 (May 4, 2012)

Check also to make sure that none of the wires running to the coil are loose, or have a bare spot on them that are grounding out against anything should they move when you go around a corner that may be causing the trouble. I had this trouble one time on my circle track car, and it was the power wire to the coil, car ran like a bear until I got into the corner, then it would die, until it hit the straights. The wire to the coil, had a bare spot that would swing out and hit the body momentarily and kill the ignition until the wire would swing back, it had come out of the loom we had it in and was unnoticed.


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## michaelrj9 (Jan 25, 2010)

Checked the float and they were a little too high (1/2 turn or so for primary and 1 turn for secondary)

No loose wires or frayed that I can see.


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## BearGFR (Aug 25, 2008)

Power brakes? If your power brake booster diaphragm is leaking/bad, it can fail such that when you apply your brakes it creates a humongous manifold vacuum leak, leading to an extremely lean mixture. That can produce the behavior you're describing. Try disconnecting the vacuum hose to the booster and plugging the hose to see if that changes anything. ****CAUTION**** you won't have power brakes when you do that so braking pedal effort required to operate your brakes will be very high and your brakes aren't going to work very well.

Bear


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## michaelrj9 (Jan 25, 2010)

BearGFR said:


> Power brakes? If your power brake booster diaphragm is leaking/bad, it can fail such that when you apply your brakes it creates a humongous manifold vacuum leak, leading to an extremely lean mixture. That can produce the behavior you're describing. Try disconnecting the vacuum hose to the booster and plugging the hose to see if that changes anything. ****CAUTION**** you won't have power brakes when you do that so braking pedal effort required to operate your brakes will be very high and your brakes aren't going to work very well.
> 
> Bear



It does have power brakes. It seems good but I will end up replacing it anyway.

I do think it is good as I had the car running and and then shut it off and about one minute later I removed the power brake booster check valve and it was still under pressure and air escaped.

I ended up swapping carburetors with a car that did not have any problems.

I could not get it to run right. It wanted to idle at 400 RPM. I started adjusting the idle screw and 1/2 turn it would go to 1000k RPM'S.  The I would swat the throttle a little and it would go way too high.

That's when I was moving the PCV hose at the valve and it just seemed way too loose. I could not see if it was seated in all the way or not. So I tried to tap it in with a screwdriver and the grommet broke in half.

So waiting to get the new grommet at this point and go from there.


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## PontiacJim (Dec 29, 2012)

The stock engine idle is 650 for the automatic and 950 for the manual and may be higher when you use a bigger cam.

Have you played around with the idle mixture screws or idle speed? On a typical carb (Q-jet), here is what the manual says for 1970 cars:

Leave the air cleaner on and disconnect and plug the vacuum line to the distributor.
Plug the hot idle compensator on all automatic transmissions except Ram Air III & IV.
Using light pressure, screw in the idle mixture screws until they stop (don't force them).
Back the screws out 3-5 turns. 
With the car in drive (automatic) or neutral (manual trans) adjust the carb as follows:
Adjust the carb idle speed screw (should be on the side of the carb) until the engine is at 675 RPM for automatics and 1,050 RPM for the manual transmission.
Now lean out the idle mixture screw equally by turning them in to obtain an idle speed of 650 RPM for automatics and 950 RPM for the manual.
Reattach your distributor vacuum line. 


With a larger than factory cam, you may have to increase your idle speed.


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## BearGFR (Aug 25, 2008)

If you've got "some cam" in it you could be getting into a situation where the idle screws are ineffective because with the throttle plates "at idle" it can't get enough air. Turning up the idle _speed_ screw will be really touchy because once the blades begin to open and let in more air, it also begins to uncover the transfer slots and the car starts to switch off of the idle circuit and onto the transition circuit. Get it to idle as best you can, then see how far you can close the idle _mixture_ screws before they start to have an effect (if they do at all). Many QJets have a special "idle bypass air" circuit to address this phenomenon - Cliff Ruggles talks about this in his book. I'm not familiar enough with "Brand H" carbs to know if they do or not, but another way to address it is to drill one small hole in each of the primary throttle plates. Start very small, slowly go larger until you can get it to idle with the plates mostly closed.

Another good way to set your idle mixture is to use a good manifold vacuum gauge. Adjust the screws evenly until you find the point where you get the highest vacuum reading, then sloooowly close them evenly until the reading just begins to drop.

Still, none of the above would explain really why it does what it does under braking or around corners. That too makes me want to say "float level" or something related -- fuel sloshing around in the bowls is changing "something". If you haven't already, it'd be worth going through the carb and making sure everything is right. No leaking seals, good needle(s) seat(s). Are you sure it's fuel related? Perchance could there be an ignition wire that's flopping around when the car "moves" and grounds itself out?

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth". --Sherlock Holmes.

Bear


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