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Author
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Topic: Engine shakes on accel
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'69PonyRider
Gearhead Posts: 141
From: Hawthorne, CA
Registered: Sep 2001
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posted 08-20-2002 10:35 PM
   
I had this problem even before i rebuilt my carb. Its a holley 650 double pump. When i start accelerating, at about 1500 rpms the engine feels like it starts to shake then it calms down when it gets going. But if i give it gas at the point when it shakes it will backfire through carb. If I floor it from standing still it will also backfire. This problem seems to go away when the engine is warmed up. Ignition is set at 12 with MSD 6al, on 351C, with 6.5 power valve. What could be causing this??
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RonnieT
Gearhead Posts: 637
From: Port Allen, La. 70767
Registered: Jun 99
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posted 08-20-2002 11:09 PM
What are you using to fire the MSD box? If you still have points it could be a problem in the distributor. My car was doing a similar thing and I found that the dwell was changing as the vacuum advance moved the breaker plate. Bought a MSD Distributor and haven't had a problem since.------------------
Ronnie
69 mach1 351W-4V
70 Torino GT 351C-4v with a "shaker"
Mustangs and More Member #23
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'69PonyRider
Gearhead Posts: 141
From: Hawthorne, CA
Registered: Sep 2001
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posted 08-20-2002 11:15 PM
i have an MSD distributor also with MSD blaster 3 coil. no vacuum advance.[This message has been edited by '69PonyRider (edited 08-20-2002).]
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69 Sportsroof
Gearhead Posts: 1214
From: Valley, Alabama, USA
Registered: Mar 2002
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posted 08-21-2002 07:26 AM
Sounds like your advance may be kicking in a little late. Try an initial timing of 14 and see what happens. What cam are you using?
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mustangboy
Gearhead Posts: 670
From: Ont, Canada
Registered: Mar 2002
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posted 08-21-2002 09:28 AM
Sounds like the engine might be lean.A cold engine needs more fuel and when you step on it you're getting a lean backfire.Do you have a functional choke?You say you rebuilt your carb but if it was lean before,unless you rejetted it, it will still be lean.I just noticed you have a 6.5 power valve,if your engine is fairly stock you might need a higher numbered one to give you some extra fuel a little sooner.[This message has been edited by mustangboy (edited 08-21-2002).]
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'69PonyRider
Gearhead Posts: 141
From: Hawthorne, CA
Registered: Sep 2001
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posted 08-21-2002 12:22 PM
I have the stock 351C cam. I didnt rejet the carb. Ill try to put a higher power valve and see what happens. Is it higher # opens sooner?? I have access to a vacuum gauge, how can i use it to help tune better? Thanks
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69 Sportsroof
Gearhead Posts: 1214
From: Valley, Alabama, USA
Registered: Mar 2002
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posted 08-21-2002 05:42 PM
Connect the vacuum guage to a manifold vacuum source. Should be one on the base of the carb. Starting with one of the idle mixture screws, screw the needle in until the engine starts to go dead. If it doesn't try to quit, the carb is too big or the butterfly's are open too far. Start backing out on the screw a little at a time until you get max vacuum. Then do the other screw. You'll need to do this a few times until its right. Then, screw each needle in about 1/8th of a turn. Also, with a stock cam you should get about 18 inches of steady vacuum. If the needle is bouncing around, you have some mechanical problems (burnt valves, worn guides, etc).
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'69PonyRider
Gearhead Posts: 141
From: Hawthorne, CA
Registered: Sep 2001
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posted 08-22-2002 12:12 AM
I took it out for another test drive and even when engine was warm and i was moving at 2300 rpms, i floored the gas and it still backfired. Could it be that the secondaries arent opening up soon enough or too soon?
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69 Sportsroof
Gearhead Posts: 1214
From: Valley, Alabama, USA
Registered: Mar 2002
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posted 08-22-2002 09:09 AM
Just my opinion, but I believe a 650 dp is too big for a stock 351. Those mechanical secondaries dont care if the engine needs the extra flow or not, they just open when you press the gas. You may want to try a vacuum 600 (Holley 0-1850).
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indyphil
Gearhead Posts: 877
From: Lafayette, IN, USA
Registered: Jul 2002
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posted 08-22-2002 10:53 AM
Im not convinced its not an ignition problem. While you may have a high power ignition you still need to play with timing and check all the plug wires etc...Heres another place something could go wrong;
sometimes when you hit the gas the engine twists in the mounts and that can cause odd things to happen. Sometimes it pulls on the grounding strap and causes problems, other times if you have bad spark plug leads when the engine moves it shows up. its hard to repeat while you are looking under the hood because you cant get the engine to "twist" on its mounts without driving it. check plug leads and electrical leads, make sure there is lots of clearance so nothing can short out. I had a friend whos car would sometimes stall without warning. He saw smoke one day too. Every time he popped the hood he couldnt see what was happening. It turned out the hood stick was shorting across the battery (he had the battery in back to front).
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69 Sportsroof
Gearhead Posts: 1214
From: Valley, Alabama, USA
Registered: Mar 2002
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posted 08-22-2002 02:30 PM
I was originally thinking timing because when he mentioned the MSD and said it has a 650 dp, I *** -u-me-d that he had a radical cam which would require more timing. When he floors it at 2300 rpm, he's dumping a ton of gas into a stock engine and it cant handle it. Not enough duration to allow enough air into the cumbustion chamber to create a decent air/fuel ratio.
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69 Sportsroof
Gearhead Posts: 1214
From: Valley, Alabama, USA
Registered: Mar 2002
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posted 08-22-2002 02:32 PM
quote:
Originally posted by 69 Sportsroof:
I was originally thinking timing because when he mentioned the MSD and said it has a 650 dp, I @ss-u-me-d that he had a radical cam which would require more timing. When he floors it at 2300 rpm, he's dumping a ton of gas into a stock engine and it cant handle it. Not enough duration to allow air into the cumbustion chamber to create a decent air/fuel ratio.
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69 Sportsroof
Gearhead Posts: 1214
From: Valley, Alabama, USA
Registered: Mar 2002
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posted 08-22-2002 02:33 PM
oops....I was suppose to be editing, not replying with quote... 
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'69PonyRider
Gearhead Posts: 141
From: Hawthorne, CA
Registered: Sep 2001
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posted 08-22-2002 04:15 PM
I used the vacuum gauge to tune the carb. Before i moved anything the vacuum said 20 then i got it up to 22 and the probklem has gone away i can floor it any time and it wont cough anymore. My engine IS fairly stock except MSD, edelbrock performer intake, flowmasters, and the carb. There is one thing that im not sure of. One of the idle mixture screws is in all the way, thats how i got max vacuum, will i have any problems with that? I went on a semi-long trip today and fuel mileage was A LOT better than before. Will i be fine like this now?
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68DARKHORSE
Gearhead Posts: 378
From: Austin, Tx
Registered: Aug 2002
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posted 08-23-2002 11:44 AM
quote:
Originally posted by 69 Sportsroof:
Connect the vacuum guage to a manifold vacuum source. Should be one on the base of the carb. Starting with one of the idle mixture screws, screw the needle in until the engine starts to go dead. If it doesn't try to quit, the carb is too big or the butterfly's are open too far. Start backing out on the screw a little at a time until you get max vacuum. Then do the other screw. You'll need to do this a few times until its right. Then, screw each needle in about 1/8th of a turn.
My 351C would bog and backfire everytime I punched it. Now it raps out quickly with no hesitation or backfire. The troubleshooting guide in my Hayes Holley book had the same suggestion. I didn't think I had a carb issue till I read your post. Thank you. ------------------
Alton 68 GT FASTBACK J code
http://www.bullittclub.com/phpBB/Uploads/1020432917.jpg
01 GT BULLITT
http://www.bullittclub.com/phpBB/Uploads/1020440453.jpg
70 SR 351C
96 OJ BRONCO
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69 Sportsroof
Gearhead Posts: 1214
From: Valley, Alabama, USA
Registered: Mar 2002
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posted 08-23-2002 02:26 PM
quote:
Originally posted by '69PonyRider:
There is one thing that im not sure of. One of the idle mixture screws is in all the way, thats how i got max vacuum, will i have any problems with that?
You're running on half of your idle circuit. Once you open the throttle, your idle mixture screws are out of the picture. Try counting how many turns it takes to lightly bottom out the working side and split the difference between the two screws.
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'69PonyRider
Gearhead Posts: 141
From: Hawthorne, CA
Registered: Sep 2001
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posted 08-23-2002 07:49 PM
So if it bottoms out with 2 turns (theoretically), unscrew the other side one turn, and the other one turn also?
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69 Sportsroof
Gearhead Posts: 1214
From: Valley, Alabama, USA
Registered: Mar 2002
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posted 08-23-2002 11:56 PM
Sorry so long to reply, but that is correct. From there you alternately adjust each side 1/8th of a turn until you get max vacuum.
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'69PonyRider
Gearhead Posts: 141
From: Hawthorne, CA
Registered: Sep 2001
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posted 08-24-2002 03:07 AM
Thanx a lot 69 Sportsroof and everyone else also, so far youve been A LOT of help.
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