Electrical or mechanical fuel pump? Regardless, I'd disconnect the fuel line at the carb and measure the volume of fuel the pump can deliver. I'd plumb a "T" into the fuel line at the carb to hook up a fuel pressure gage with a long hose so I could tape it to the windshield and go for a test drive.
What type of fuel pump/regulator, fuel lines are you running from the tank to the carb ? Just wondering if the regulator is set to low or if the pump might not be putting out enough fuel at higher RPM's ? maybe the electrical to the pump ? Fuel line collapsing, just some thoughts. One other thing does that electronic module require a ballast resistor or one of those ceramic ones ? I believe it depends on how its wired whether you need it or not just remembered that.
Good thoughts guys. The carbs are 350 cfm Holleys. The fuel pump is an internally regulated (7psi) electric pump and the fuel line is steel 7/16 line all the way to the fuel block. From the block to the carbs its 3/8 rubber hose. The ignition does use a resistor and I bypassed the resistor with no avail. The part that I am having trouble figuring is that the engine runs so good while idling and reving while not moving. I spoke with my old man and he said that he has had trouble with a wet cell battery mounted sideways that was grounding internally while in motion. I have another battery that I will try today and see what happens. I will also try the no alternator thing too. I really appreciate the suggestions on this subject guys and I will update my findings. If anything comes from this deal I hope it will help someone else in the future Hack
From what you decribe, it seems that the problem only shows up when under a load. I have had similar issues in the past with different solutions. Once I had this issue when I had a muffler that has collapsed inside. It looks like you are running open pipes but do they have any baffles in them? If so and they are removable, I would try that to see of they are too restrictive. One other time I saw this happen, it was a problem with the choke on the Holley carb. The choke flap would suck closed under hard accelleration but it didn't happen when standing still. They guy had the choke held open with a spring. We wired the flap open and that cured it. I can't tell from your pictures if you have the choke flaps in place. The only other thing I can think of would be a venting problem on the gas tank. A plugged vent tube, or no vent tube and a non-vented cap, or even a problem with a vented cap.
Please keep us updated. I have the same problem with my hemi at 4500 rpm the ignition breaks up but it clears up if you keep your foot in it. You might be on to something with the wet cell battery. My my wet cell is mounted sideways in the trunk also.
I'd put my money on low fuel pressure, and going lean under load. Seen it before, runs fine in the parking lot, even revving up hard, but put it under a good load, and it misses, falls flat etc.
Did I miss where you tried a different coil? Try a different one if you have not. My dad had an old Dodge truck that would just shut down whenever it felt like it, sometimes it would fire right back up, sometimes it would have to sit 10 minutes. After many people chasing it for a couple years, one time while fighting with it once again, the wrench got caught up in the wire leading to the + side of the coil and snagged it. the terminal came off the wire. There as only one strand still making contact. Apparently when it would get hot, it would create too much resistance and break the connection until it cooled. Just for kicks, I think I would give a hard jerk to every wire connection in the ignition, fuel and charging system. if the connection is good, giving it a jerk won't hurt it, if it breaks, the connection was suspect. Gene
Post #21, he changed the coil. I know you said the valve springs are good to .5 lift, but they could still be weak, and not able to deal with the inertia of a heavy valvetrain. If all else fails, this could be the issue. Are they new? Chinese?
I totally bypassed the ignition switch by running a jumper wire from the battery straight tho the ballast resistor first, then to the coil. No change. The valve springs are from Hot Heads. They told me that they were the ones to use with the cam I have. They sell 2 dif sets. I bought the better of the two, or so I thought. I didnt get a chance to work on it last night but I will give it hell tonight. I ran it down the highway on sunday and its missing all over the rpm range when its in high gear and my foot was in it. It will gradually gain rpm but its missing like fucking crazy the whole time. Or if you are at a dead stop and rev it up its clear but as soon as you drop the clutch its scatters all over the place no matter the rpm.
I also re jetted it from 61 jets up to 70's just to rule that out. The plugs are coloring dark now so its def a little fat. Hack
It really sounds like a weak spark, or spark scatter, but not sure where to point you, since it seems like you've tried about everything. What kind of cam drive do you have, and is it new? A sloppy chain will cause the timing to roam around a bit, but doesn't normally manifest itself as a bad misfire.
What is the plug gap? Is the engine completely stock internally? If the problem is in fact ignition related, then the reason the problem shows up under load is probably that when the throttle is open and the the engine is loaded, the vacuum is lower and the charge air is denser. Air is an insulator. We don't usually think of it because it's everywhere, so we perceive it as empty space, but it is actually an insulator. The denser it is, the harder it is to pass electricity through it. That is why supercharged cars need a relatively smaller plug gap ... the dense air under boost makes it too hard for the spark to jump through a long gap. So if any part of the ignition system is marginal, it will probably show up first under heavy load. High RPM adds another challenge into the mix, as there is limited time to deliver the spark.
Ok, you say in post 1 that this problem "just started one day" - ok what did you do if anything immediately prior to that "one day" ? Where you messing around, cleaning, changing, adding, altering, adjusting
Sounds like you've replaced/swapped/tried/isolated every component in the ignition system. If it were here, I'd run a compression test on it first. This is very important and eliminates much guessing and head scratching. Your problem seems to be erratic misfire so I wouldn't expect to find anything abnormal but I'd still do it. Takes 10 minuets and a plug inspection is a valuable un intended consequence. Next would be to check your battery voltage and charging system. Engine grounds and distributor housing to block ground. Next would be a driving fuel pressure check. Install a temp gauge lashed to the windshield in some fashion and monitor the fuel pressure while its breaking up/missing. For some reason this is overlooked while trouble shooting possible fuel related drivability problems. There is no way on Gods green earth to efficiently trouble shoot this without knowing exactly what's going IN to the carb. The carb is designed to work with a set amount of pressure and volume going in. If that's off it can't possibly provide the correct output. After those tests, something should be at least questionable to provide further direction. Now this is based on the fact that it ran PERFECT until "one day". If there is no history I'd take a further back to basics approach.
I agree completely. It was like a one day thing. And is progressively getting worse. I will do a compression test for sure but like you said it sounds like an electrical thing. I have experienced spark scatter before and that is exactly what it sounds like. As far as the control box to coil proximity as an issue, it should have gone away when I put a points distributor in it with the exact same result.
The engine is stock internally except for the valve train upgrade stuff. I set the plug gap at .035 as recommended. I really appreciate all the thought going on here. Hack
Spark scatter , as in quote " spark scatter" unquote is a variation in timing caused by mechanical lash. The entire stream from the timing chain, cam gear, intermediate shaft or distributor gear, distributor drive dog, distributor bushings and even oil pump/pressure have a part in this. This can be seen with a timing light at upper rpm and load or no load makes no difference. This generally put the power out put down but not so bad as to cause audible missing. If the missing is from mechanical lash problems the offending part will be obvious upon inspection of the parts in the stream. Heard of a timing chain going bad within 20 miles because of a hopped up valve train and the chain being installed dry out of the box (vs soaked in lube)
I never said it sounds electrical, I said it sounds like you've eliminated the electrical end of this by swapping everything. You need to do a fuel pressure road test before you do anything else
Another freebie... run it in a dark garage and see if anything is arcing. I've seen weirder shit wind up to be something simple. The compression and running fuel pressure tests are good ideas. I'd still like to see it on an old time 'scope to see if anything shows up. Bob
You can isolate the ignition with a stress tester. Put 2 small nails thru a piece of Styrofoam at 45 angles in a spread V. Place the points exactly 9/16 of an inch apart. The purpose of the V is to allow you to adjust the sharp ends close together and keep the other ends far apart. Using a jumper wires ground one nail and hook the coil wire to the other. Prop it up so the nails are not touching any metal. As you crank the car you should see a blue spark jumping the 9/16 gap. This puts a stress on the ignition system of about 18 Kilovolt. If the system can fire this gap at idle ,then move on to a plug wire. Start the car and raise the rpm to the 4500 mark. If it can fire this gap at 4500 RPM it's not the coil, condenser etc, If it can't then we are narrowing down the fault. Or buy one of these;
When i got trouble with my 53 Dodge that it start to misses under heavy load it was the fuelfilter that had colapsed so it didn't get enough fuel.
Sounds more like the floats hanging...ON ACCEL. (that's what's changing...engine revs strong at rest...then get the vehicle going forward and the problem arises) Fuel is either 'restricted' or shutting off on accel. I would look for an 'inner-carb' problem. A 'knowledgable' customer had a Deuce with a 354 Chrysler Hemi, brought it in with similar symptoms to yours...had the cap off the distributor, set the points a few times, changed the timing, checked the coil...he had just changed the plugs before washing the car. I hooked up my air nozzle with the 12" length of 5/16" brake line tube...blew out all 8 plug tubes, gave him a bath. "Try it now..." This was my first thought here, but quickly dismissed it. I'm still watching...
That's about where I'd be heading by now. Tear 'em down and do a rebuild on both. Look for the obvious like a float hung up or debris in the float bowl. Bob
And then - after you've torn the carb apart - then will you check to see that it has the correct fuel pressure going into it in the first place? Help me understand why we tear carbs apart. For practice ? There's no way , none ,zero, zip , zilch, not going to happen kind of way to tell what's wrong with a carb or if anything is wrong with it unless you know 100% for a fact that the correct pressure is going in. Would you say, my fuel pressure drops to 1.5 psi on WOT - maybe it's inside the carb? My fuel pressure goes to nothing after a burnout and my car stalls , I need a bigger carb? My fuel pressure falls off at 3000 Rpms and the car starts to miss, rebuild the carb. My fuel pressure bounces all over and is running like crap, change every part of the ignition. Sorry pet peeve of mine due to seeing the needless and wasteful discarding, dismantling of carbs, ignition parts and fuel injection when actually being fuel pressure problem. Rant over.
Hackman, with all due respect I think you should start testing things in an orderly fashion. A critical Test is the Fuel Pump Pressure and Volume Test. You can go to an Autozone or similar and borrow/rent the tester. The directions are here; http://www.autozone.com/autozone/re...epairGuideContent.jsp?pageId=0996b43f8038ed16 The Spark stress test I described earlier is a logical test to determine if the ignition can pass a stress test. I compare this to the left wall maze instructions, hand on left wall and go, you will get there. Following an orderly set of diagnostic tests will get you there. I have seen a thousand odd ball fixes (high oil pressure pumping up lifters, weak exh valve springs from heat), but you start, with, fuel, spark, and compression. Compression is not normally a high RPM High load fault so I would TEST the first 2. Hoop
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