# 1966 LeMans hesitating under load?



## ryanpitts (Aug 22, 2013)

I have a 1966 LeMans with a 1974 455 motor from a 74 Trans Am. It drives great and we've had it checked before (although not thoroughly) and it appears to be in good shape. We've done a lot to the brakes, suspension, and some to the engine area (new radiator, hoses, new carb, new headers).

It has always had a problem under load though. When sitting still (park or neutral) it revs fine, doesn't even flinch. No hesitation and no noticeable pings or knocks. However, when you drive it and punch it the car will pull great until about 3k rpm or so. Then it starts to hesitate, it's not a straight pull. I can't tell if maybe it's a timing issue or something. Anything past about 3k rpm under load the motor will just stating hesitating.

Anyone have any thoughts about what could be the cause or what things I can further do to diagnose the problem? Thanks for any help!


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## 05GTO (Oct 6, 2004)

Sounds to me like you may have a problem with the mechanical advance.


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## geeteeohguy (Feb 2, 2008)

Need more info, a lot more. Does it stutter and jerk? Or just flatten out? Could be: fuel delivery, spark advance/ignition timing, bad ignition wire, bad ignition coil, bad plugs, weak valve springs, restricted exhaust, etc. etc.


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## ryanpitts (Aug 22, 2013)

I really appreciate the replies. I do understand that I am not giving a lot of information and I apologize for that.

It does stutter and jerk, definitely does not just flatten out. I can say that we have a new carb (Holley 650 i believe), new plugs and new custom wires. The exhaust should be fine as well. It was re-done about 2 years ago and new headers in the past 3 months. It is using the points distributor and coil that was on it since we bought it about a year and a half ago. So maybe it's the coil (it's an MSD)?

What would be the best way to describe it? Post a video of it being driven? Is there anything I should try to do to rule out some possibilities? I am willing to do anything I need to in order to get well informed answers. I appreciate the help!


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## Space (Jun 17, 2013)

Holley IMHO.


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## ryanpitts (Aug 22, 2013)

Might be, but it is really brand new and we had an Edelbrock on there before and was still having the issue. Not saying it still wouldn't be the carb but seems unlikely at this point.


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## geeteeohguy (Feb 2, 2008)

If you can get it on an ocsilliscope, you'll be able to see if the ignition system is up to snuff and rule that out. Could very well be a plug wire or wires or a plug issue.


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## ryanpitts (Aug 22, 2013)

Ok, I did some initial testing on the coil. Let me know what you think of this.

I tried testing the primary and secondary coil winding resistances. The coil is an MSD ignition coil (not sure what model exactly). From what I could find online the primary resistance should be between .7 - 1.4 ohms (depending on the model) and the secondary resistance should be between 8.9k - 9.8k ohms (again, depending on the model).

I tested the primary resistance with a multimeter set to 2k ohms. I then placed the positive test lead on the positive terminal post on the coil. I also placed the negative test lead on the negative terminal post on the coil. The reading on the multimeter was 002 ohms. (see image)










Next, I tested the secondary resistance with a multimeter set to 20k ohms. I then placed the positive test lead on the center connection for the high voltage wire on the coil. I also placed the negative test lead on the negative terminal post on the coil. The reading on the multimeter was around 1223 ohms. (see image)










The secondary seems really low. First of all, does it sound like I am taking the correct readings properly? Secondly, does it look like the secondary coil winding may be bad? I'm going to continue to test a few more things with the ignition system but that definitely doesn't look right, unless I'm just missing something.

Thoughts?


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## ryanpitts (Aug 22, 2013)

I also just tested the ignition coil power. The specs say the max voltage should be around 58k. I placed the positive test lead on the positive terminal post on the coil. I also placed the negative test lead on the negative terminal post on the coil. I set my multimeter to 200 and got a reading of 055. (see image)










Does that look correct?


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## Space (Jun 17, 2013)

Not trying to turn you away from spark, but if you know of a way to check, you might could consider checking the intake manifold gasket to ensure it's not leaking. Knew of a similar situation once, and I believe it ended up being a warped aluminum intake that was leaking. If it idles, and drives fine, but only on hard acceleration it seems to be amiss, I'd guess it's more in the fuel delivery, rather than ignition. Could also check into fuel filter/pump. Vacuum lines?

Not sure what resistance readings would be good or bad, it does seem that you are getting higher than you should be, not sure if it's the culprit. You'd think if your timing was good, your spark isn't going to vary in intensity at different rpms/loads. 

Take all this with a grain of salt, I really don't know what Im talking about... Just tossin a few suggestions out.


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## Rukee (Feb 8, 2007)

Sometimes what I'll do is hook up a timing light and run the wires inside the car with the timing light on the dash. Some black tape on the trigger so it's always on and drive the car looking at the light to see if it's randomly losing spark.


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## geeteeohguy (Feb 2, 2008)

I've heard that about 90% of all carburetor problems are ignition system problems. Not saying it is, but experience tells me that you need to get your car on a scope. Ignition coils rarely fail. But they can. It sounds to me like you are dropping a cylinder or three under load. Could be plug wires, plugs, coil output, coil hooked up backwards, etc. A quick scope check will validate the ignition system and you'll find the problem or at least be able to move forward into the fuel system without chasing your tail....


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## Reuhkapelti (Sep 18, 2012)

My -70 LeMans started doing some irritating hesitation a few weeks ago when driving on a highway. Idle was fine, took rpm great, but feeled lazy when doing over 40 mph. I then noticed, that the power brake booster leaked air to the engine and made it run on lean mixture. Now the booster vacuum hose is plugged, and it runs great again... ofcourse braking asks some foot work  going to get a new vacuum booster soon. Could this be the case in your car too?


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## gjones (Oct 8, 2012)

You sure that the check valve on the booster just didn't fail, Reuhkapelti?


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## Reuhkapelti (Sep 18, 2012)

Sorry I jumped in the topic of ryanpitt's, but how can I test that booster check valve? I did suck it and I think it allows the air flow one direction? What is it supposed to do? What about the rubber seal under the valve? How thight it should be?

Edit. I went and bought a new vacuum check valve, installed it and voila! Good brakes, instant acceleration and a smooth ride atleast 80 mph. Thank you -gjones- for that good tip about that check valve. No need for changing the booster now. Also that weird schmuck-sound dissappeared from the brake pedal. Now I can concentrate to the poor starter and left front scueecking sound (when braking).


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## WhittP (Jun 19, 2013)

I had the same issue when I bought my 67 GTO a few months ago. I assumed it was fuel delivery and I couldn't ever get the Holly 600 to run right so I replaced it with an Edelbrock. The issue remained. What I should have checked first (with the benefit of hindsight of course) was the ignition timing and advance. I pulled the distributor and found the mechanical advance was in such bad shape it was not functioning. The vacuum advance was bent and also not functioning. Basically, I had my ~10 degrees initial timing and nothing else.

I replaced the whole unit with an Accel electronic distributor and coil. I'm not sure what particular part fixed the issue but the new set up did the trick. I hope this helps.


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## BearGFR (Aug 25, 2008)

I realize I'm coming to the party late, but just reading the high points of the symptoms you're describing, my first thought was spark plugs. Have you checked them?

Bear


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## gjones (Oct 8, 2012)

*Check 'em off*

My Cadillac limousine had exactly the symptoms that you're describing Ryan, and it turned out to be a bad fuel pump. (Not saying yours is, though. Lots of things to check, first). What kind of vacuum is the engine pulling? Do you know the condition of of your filter(s), gas tank sock, any marginal rubber hoses or connectors, etc?... what pressure is the fuel pump putting out? As geeteeohguy has said, if you can get it on a scope, that'll tell you a number of things, including if it has a bad coil. (Yes, a bad coil will do what you describe, also).


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## ryanpitts (Aug 22, 2013)

Finally got to drive it around a bit more and then decided to check my plugs since it was still stumbling pretty bad. This is what they look like.




























Those definitely don't look good to me. What do you think...replace? I am going to go ahead and get a new set anyway. I did a bit of research and it seems I may be running too rich. Would you agree with that?


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## gjones (Oct 8, 2012)

*A cool bil...*

You're running richer than Bill Gates! Make sure that choke is opening up when it's fully warm!!!- Make damn sure! (Or your floats are set too high, or wrong rods and jets, etc...)


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## ryanpitts (Aug 22, 2013)

I really appreciate all the replies on this thread! I didn't see some of them before my last post. (computer decided to show me a cached version of this page) I might try and get it on a scope. I did buy new plugs tonight and will put them in as well as check the float level, air/fuel mixture, etc with the carburetor over the weekend. Hoping it's as simple as it is just running too rich.

gjones, I will also double and triple check the choke on a fully warm motor. After checking the choke my first thing will be to check the float level. My dad looked at it the other day (briefly) and said he thought the float level was too high. Would there be a correlation between a high float level and it running too rich?


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## Rukee (Feb 8, 2007)

Absolutely if the float level is set too high, it could totally run to rich.


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