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Old 04-28-2010, 04:51 PM
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Hesitation on acceleration in gear only

I did my share of searching, but it's kind of awkward to word without finding completely off the wall subjects so I was hoping I could just ask...

Basically, I just installed this first gen. SBC in my 1990 Wrangler, I installed a new MSD distributor, wires, plugs and a few other misc. things. When I have it in park and rev the motor up, I get a continous 6 psi fuel pressure, and it does not break up or hesitate at all; however, when I took the vehicle around the block and try to even gently accel, it completely breaks up and bogs down, but does not die out. I'm assuming I set my timing properly due to the fact that it revs up great in park, I also have my vaccuum advance connected from a line on the carb. Any help, advice, direction would be greatly appreciated.

-Jay
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Old 04-28-2010, 05:12 PM
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Sounds to me like you need to advance your timing. I like to turn (counter clockwise) until it Pings just a bit , then back it off ( clockwise) a little. That usually gets you pretty close. Then make sure it doesn't ping under load. That's how I do it. I am sure some guys will disagree with my primitive method though.
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Old 04-28-2010, 05:21 PM
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Quote:
Originally Posted by Fast 4 Door
Sounds to me like you need to advance your timing. I like to turn (counter clockwise) until it Pings just a bit , then back it off ( clockwise) a little. That usually gets you pretty close. Then make sure it doesn't ping under load. That's how I do it. I am sure some guys will disagree with my primitive method though.
I wouldn't disagree. I've never done it your way, but if it works for you, that's most important. There's a few on here that would "slam" on unusual techniques (and I've got a few of my own), but if it works for you, more power to you.
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Old 04-28-2010, 05:44 PM
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So even though my timing seems good when reving in park, it may be off when put under a load? Also, what exactly is the "pinging" noise being made from?I appreciate you shedding some light on my situation and be able to test this tonight or tomorrow.
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Old 04-28-2010, 05:52 PM
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Jay, Did you also replace the transmission, and if so to what?

Is Your Vacuum advance hooked to a manifold vacuum source?

What is your manifold vacuum reading, at idle, and in gear?

What carb are you using, and does it go to wide open throttle when you push the pedal to the floor?
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Old 04-28-2010, 06:46 PM
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Quote:
Originally Posted by carsavvycook
Jay, Did you also replace the transmission, and if so to what?

Is Your Vacuum advance hooked to a manifold vacuum source?

What is your manifold vacuum reading, at idle, and in gear?

What carb are you using, and does it go to wide open throttle when you push the pedal to the floor?
I did replace the trans with a TH350, Im pretty confident the transmission is good, I got it from a good friend. My vacuum advance is hooked up to carb. I have not checked the vacuum pressure, I'm assuming it's good bc my brake booster works good, along with the transmission shifting properly EDIT: (however, those vacuum lines are connected to the manifold). The throttle does go wide open when using the pedal.
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Old 04-28-2010, 08:12 PM
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Quote:
Originally Posted by JeepingJay
I did replace the trans with a TH350, Im pretty confident the transmission is good, I got it from a good friend. My vacuum advance is hooked up to carb. I have not checked the vacuum pressure, I'm assuming it's good bc my brake booster works good, along with the transmission shifting properly EDIT: (however, those vacuum lines are connected to the manifold). The throttle does go wide open when using the pedal.
Among other possible problems, I suspect a final gear ratio problem. The stock gears in your Jeep could be in the 2.80 gear ratio range. With your engine/transmission change you may need to change the axle gears to 3.73 or higher. This will allow the engine to get more into the lower 'power' range quicker, thus eliminating the bog on take off. You could bandage this problem with a higher stall speed torque converter, and still need to re-gear your axle/axles to get it to perform.

The reason I say "or higher", is if it is a daily driver for you also. With a lot of highway driving you may want a gear in the 3.50 range.

What camshaft, and or engine modifications were made, or changed?
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Old 04-28-2010, 08:33 PM
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That makes sense, because I find that if I kind of hold it or accel real slowly, it kinda gets past the hiccup and accels farely normal. The engine doesn't have any modifications. It's also running a stock Rochester 2 barrel carb. I may have to do some further testing though, because if I remember correctly, when I FIRST got it moving, my trans was stuck in first gear due to the lack of the vacuum line, and I don't remember having this hesitation problem.

EDIT: From what I gather in research, my Jeep should have had something ranged in between 3.07-4.10... It had the 4.2L I6 with a manual trans behind it.

Last edited by JeepingJay; 04-28-2010 at 08:41 PM.
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Old 04-28-2010, 08:50 PM
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Quote:
Originally Posted by JeepingJay
That makes sense, because I find that if I kind of hold it or accel real slowly, it kinda gets past the hiccup and accels farely normal. The engine doesn't have any modifications. It's also running a stock Rochester 2 barrel carb. I may have to do some further testing though, because if I remember correctly, when I FIRST got it moving, my trans was stuck in first gear due to the lack of the vacuum line, and I don't remember having this hesitation problem.
I'd be willing to 'bank' on that being the problem also. Sometimes it helps to ask questions. Thank you for your response's.

The amount of manifold vacuum you have right now will help me diagnose your ignition/engine timing closer to where it should be, for performance.



Keep the information coming as best as you can.

Stephen
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Old 04-28-2010, 09:13 PM
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Unfortunately, I do not have a vacuum gauge, I will have to try and invest in one tomorrow. If I remember correctly, my trans vacuum line is also connected to an open line on the carb itself. I will have to disconnect the line and try a run, however it's a little late to drive around with open headers so it'll have to wait.

Part of me thinks that some vacuum lines are not properly connected. This is my first motor swap.

Thank you for all your help and suggestions.

-Jay
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Old 04-29-2010, 05:17 AM
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Quote:
Originally Posted by JeepingJay
So even though my timing seems good when reving in park, it may be off when put under a load? Also, what exactly is the "pinging" noise being made from?I appreciate you shedding some light on my situation and be able to test this tonight or tomorrow.


The "ping" is pre-ignition The plug fires before tdc. Not what you want. but can be an effective way to get your timing close . I don't even use my light anymore.
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Old 04-29-2010, 09:29 PM
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Quote:
Originally Posted by carsavvycook
I'd be willing to 'bank' on that being the problem also. Sometimes it helps to ask questions. Thank you for your response's.

The amount of manifold vacuum you have right now will help me diagnose your ignition/engine timing closer to where it should be, for performance.



Keep the information coming as best as you can.

Stephen
Well I checked the vacuum and was getting a steady 20inHg at idle and dropped to around 15 while reving up the motor, then leveled back to 20 with the increased throttle.
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Old 04-30-2010, 09:19 AM
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Your vacuum indicates your timing is pretty close to right on. Your problem lies elsewhere.
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