Possible Ignition Box Failure, Need Help
03-09-2011, 05:57 PM
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Possible Ignition Box Failure, Need Help So heres the story,
I have a 90 Camaro, race built, 355c.i. 253/253 @.050 dur. 560/560 lift 110lsa cam. 210cc Aluminum heads etc. Thing cranks great idles fine and has a great sound but if I try to rev the motor it stumbles right as the carb hits the secondaries. I have to slowly rev the motor or it "hiccups". Ive changed carbs, added a spacer, done everything I can think of. I then decided to be sure it wasnt the timing curve in the distributor and locked the distributor and have it set at 36 degrees. No advance. it revs fast and fine until i hit that perfect spot around 3k rpm or when I blip the throttle back to the secondaries. I now think It could be the GM Ignition box (basically an MSD 6A). It sounds like its popping through the carb and throught the exhaust when the problem occurs. SO i put a timing light on it and found that it is retarding timing when I rev it which is IMPOSSIBLE, Unless the box is bad or there is a problem. Please anyone who has any idea chime in this has been going on around 8 months and I haven't figured it out. in my experience these boxes either work or don't work so I am stumped. Thanks in advance!
I have a 90 Camaro, race built, 355c.i. 253/253 @.050 dur. 560/560 lift 110lsa cam. 210cc Aluminum heads etc. Thing cranks great idles fine and has a great sound but if I try to rev the motor it stumbles right as the carb hits the secondaries. I have to slowly rev the motor or it "hiccups". Ive changed carbs, added a spacer, done everything I can think of. I then decided to be sure it wasnt the timing curve in the distributor and locked the distributor and have it set at 36 degrees. No advance. it revs fast and fine until i hit that perfect spot around 3k rpm or when I blip the throttle back to the secondaries. I now think It could be the GM Ignition box (basically an MSD 6A). It sounds like its popping through the carb and throught the exhaust when the problem occurs. SO i put a timing light on it and found that it is retarding timing when I rev it which is IMPOSSIBLE, Unless the box is bad or there is a problem. Please anyone who has any idea chime in this has been going on around 8 months and I haven't figured it out. in my experience these boxes either work or don't work so I am stumped. Thanks in advance!
03-09-2011, 10:29 PM
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What you are seeing with the timing light is common. At 3000rpm your msd is going from multiple sparks to one big spark. Thats why you have to set the timing at 3000rpm or above. Thats not your problem. I see you tried two carbs but I still think you have a carb problem. Is it a Holley carb? Do you have a double pumper or vacuum secondary. You hiccup is likely a lean condition.
03-09-2011, 10:31 PM
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Check over your wiring really well! Check to make sure your rotor is secure. I wouldnt be surprised if you had a bad distributor/ignition module if all else seems good.
03-10-2011, 01:57 AM
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Do some carb tuneing searchs on google. Get your timing set correctly first (although ive never had a problem setting my timing at idle). After that start running the idle air mixture screws out around an eighth of a turn at the time and see if it starts to help any. The best purchase ive ever made as far as tuning goes is a ngk wideband. My car gets great gas mileage for a 600hp carb'd engine and runs spot on everytime I drive it.
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03-10-2011, 02:58 AM
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Ive thought long and hard about a wideband. Maybe its time. thanks for the tips. Im thinking about going up a few jet sizes on the primary side just to see if it cleans it up at all. Just very odd that two carbs would do the EXACT same thing. One AED and the previous a quickfuel. Ill do some more work on it tomorrow. Thanks again
03-10-2011, 03:43 PM
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A wide band is nice for tuning a car on the street but when you blip the throttle,even on a well tuned setup, you will see a lean spike on the wideband. First thing I would check is the accelerator pump arm on the secondaries make sure there is no slack between pumpcam and the accelerator pump linkage with every thing at rest. Then pull off the pump cam and make sure it isnt worn out or installed backwards. The cam needs to be in a position that will move the arm and pump gas out of the squirter with only the slightest amount of throttle movement. If all this checks out try larger squirter till the problem goes away. What size squirter is on it now front and rear? Another thing to look at is the check valves under the accelerator pump diaphragm. If theyre the umbrella type and the carb has set up for a while they will have lost their shape and will not seal.
03-10-2011, 05:15 PM
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It has 35 front and 31 rear squirters. Ive noticed with this carb that the pump arm springs do compress some at full pressure so I want to replace those but I was instructed not to go over a 37 squirter on a 750 cfm carb. Same problem existed though even with the springs relaxing. I went today and raised the float level just a wee bit and It helped a little and also put my hand over the primaries and it revved good. My question at this point is do I need to raise the jet sizes slightly or cover it up with pump shot? Im thinking at this point of going up on the jets since it should be metering through those by 3000 rpm
03-10-2011, 07:41 PM
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When you laid you hand on the primaries you richened the idle mixture. How many turns out on the idle mixture screws are you right now? Those screws dont determine a/f ratio of the idle fuel but simply vary the amount of that fuel mixture going into the engine. The idle air bleeds and the idle fuel restrictions in the metering blocks determine a/f ratio at idle. What I am getting at is if the motor is recieving an already lean idle mixture then when you blip the throttle it will hiccup. You would be surprised at how high in the rpm the idle circuit has influence on fueling the motor. You can bandaid this with larger squirters and maybe a higher capacity accelerator pump. Long duration cams, high overlap, single plane manifolds, and that spacer agravate all this.
03-10-2011, 09:39 PM
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yeah I went with a 4 hole to try and help the problem and adding that spacer did help maybe a little bit. Ill check the screws tomorrow, I believe they are at about a turn and 3/4 out right now. I was instructed before that that is as far as they should ever need to go. I did the hand trick to test that richening method I figured thats what it would do. So if you were in my boat as of now would you go bigger on the squirters if the idle mixture doesnt help?
03-11-2011, 02:05 AM
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Thats exactly what I was getting at with the wideband suggestion. Getting them to idle with a correct ratio and also drive well at part throttle are the most tricky parts of carb tuning. The WOT stuff is the easy part. he is right all the way around. Ive fought this problem multiple times and its always been a lean idle condition. We like it cause it sounds choppier but its not correct. Once I got my car to idle at a healthy 13.7 to 14.0 in gear and around 13.4 in neutral everything else fell into place. My 406 feels like it might jump out of the car right off idle.
Get your timing set and make sure you have your butterflys showing the propper amount of transfer slot and go from there. I found out real quick about keeping the spread in jet sizes between primary and secondarys also. Just a jet size or two in the primaries will make your part throttle A/F extremely rich.
Get your timing set and make sure you have your butterflys showing the propper amount of transfer slot and go from there. I found out real quick about keeping the spread in jet sizes between primary and secondarys also. Just a jet size or two in the primaries will make your part throttle A/F extremely rich.
03-11-2011, 03:00 AM
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im sold that its a carb problem. Im just afraid it wont be enough. maybe bigger acc pump squirters? and how do I not know its not just lean all the way around. Thats my main fear I dont want to hurt the motor I just finished it
03-11-2011, 05:57 PM
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first off thanks so much for all the advice guys. Bad news is nothing so far has worked. I feel like its time to go to a bigger squirter. Swapping to the 35 in the front helped so much I'm considering going with a 40 or 42 until someone says no. My other question is about the acc pump arm springs. They seem to collapse alot and I noticed If I bring it to wide open (motor off) the secondary pump opens then it takes a few seconds for the spring to press the pump down all the way. The front collapses as well but doesnt press the pump arm all the way down.
THanks again!
THanks again!
03-11-2011, 07:13 PM
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I wouldnt be scared to try a 40 squirter on it, front and rear. I have a 750 hp that has 40's front and rear. Its a heavy car with a low stall and a big single plane manifold. Those situations need a lot of pump shot. But on the contrary I have a 1250hp carb on the car in my sig that has 36 squirters on it.
As for the pump arm springs, they have to compress because the fuel in that accelerator pump is not compressible. Something has to give. You say the front arm doesnt press the pump all the way, I would look at pump cam wear. Get a pump cam kit and try it. You would be surprise at what a new set of cams will do. If you have no clearance in the pump linkage at idle then the pump arms should be ok. Be patient you will get it.
As for the pump arm springs, they have to compress because the fuel in that accelerator pump is not compressible. Something has to give. You say the front arm doesnt press the pump all the way, I would look at pump cam wear. Get a pump cam kit and try it. You would be surprise at what a new set of cams will do. If you have no clearance in the pump linkage at idle then the pump arms should be ok. Be patient you will get it.