Been doing little runs in my modified out on my dead end dirt road and fine tuning things a bit before I venture further from home. Today I figured it was time to get on it a little bit and found that the 401 breaks down at higher revs. Not exactly top end but I would say somewhere above 3000 - 3500 rpm (I don't have a tacho). I'm not much of a mechanic. I can get by to assemble stuff and can usually make an engine run but systematically diagnosing a problem is not my strong suit. The plugs and leads are all new, plugs correctly gapped. The dizzy is rebuilt (by a pro) and the coil is new. My limited knowledge says check the points gap, try a different coil (the one in it is a NOS Bosch SU 120). I can't even find a listing for this coil so I am suspicious of its suitability. I pulled the plugs out and I think if anything it's a bit on the rich side. Any suggestions on how to work through this? Thanks, Pete
Pete You have not supplied enough information 1. What type of carb 2. How many carbs 3. Is it popping? 4. Is it just laying down/Nosing over 5. describe in detail what the plugs look like. 6. What is the inital timing set at 7. What is the total lead at the crank.
See what I mean. I'm no mechanic. Some of your questions are in another language. I'll try: 1. Triple Stromberg 48s with progressive linkage 2. See above 3. Not sure exactly what sort of popping you mean but it's like the sound on the Aussie V8 supercars when they have their pit speed limiters on. Kind of a popping. 4. There's that other language. 5. Fairly even coverage of black carbon except for a dark bare metal area directly under the "electrode?" 6. Need to check. Just set by ear. 7. Wrong language again. Thanks for asking the questions. Just don't know if I can give smart answers. Pete
Any chance you have an MSD with built-in rev limiter? Usually the 6AL model. They come with a 3000 rpm chip for test purposes. A total shot in the dark . . . but ya never know. Fuzzy has a good comment about the timing. You need a light or at the least a vacuum gauge. For now, try advancing it about 10* and see if it does better.
No MSD. Stock Delco dizzy. Got a light and will check tomorrow. Last time I used it the light only confirmed my ear was pretty good so I don't bother usually. Maybe I should.
My first thoughts (after all the other posts) are: Spark sounds to be too weak. This is probably caused by the coil, or not enough voltage going to it. Check the coil voltage while running by connecting a voltmeter to the + terminal and to ground. Let us know what that measures. If it is between 6 and 12 volts it probably points out the coil may be defective. Having carboned up plugs tells me that the engine is running rich and is not burning the fuel completely. Try a new coil after doing the voltage measurements.
Shot in the dark, These kind of problems are hard to diagnose even if your present let alone going by a description.. Check to see if your carb choke plates are not partially closing during acceleration. I am leaning into a timing issue. Check to see if your advance is even working.
Sounds like a reasonable suggestion. Thanks I think checking the timing in the morning is the quickest and easiest thing I can do. Good place to start. I don't think it's the choke. Only have one on the centre carb anyhow. I know what you mean about working from a description. I was more just hoping to get a bit of a sequence of things to check that seemed most likely to least likely. So far, so good.
Not really going for performance as per stock motor. Just a good reliable runner with some style. BTW, Jere Jobe built me the carbs after much discussion. He thinks it's enough for a stock 401 and I am nobody to argue with him.
OK pete Here we go. If the plugs are sooty then you are toooo rich. Try this - unhook the 2 end carbs and block them so they can't open and go drive it again. If it improves greatly then you may have to drop jet sizes. Do this and we will see where to go from there. Inital timing should be around 14 Degrees with about 32 total.
Double check your point gap while you're at it. Worn points with a too-close gap often mimic carb problems.
Thanks guys. I'll try to get onto it in the morning (it's late here now). I'll need some help from above because it just started pissing rain. C9, the points are new but I'll check the gap just the same. Fuzzy, I'll check the ignition issues first and then move on to the carbs. Seems like ignition I think. Wouldn't running rich be less of a problem when wide open than lower down? thanks again Pete
I just thought of something else ??? What are your Air/Fuel screws set at?? Should be no more than 2 turns out. I would probably start at 1 1/2 turns out but check and see.
When you say it breaks down on top end, the first thing that comes to my mind is some kind of ignition problem. At 3500 RPMS you can eliminate the idle circuit. In the early days this was a big problem that started all the HP ignition companies but it happened at higher RPMs. If the plugs are fouled they can break down at higher RPMs. Especially if you had some bugs to work out when you first got it started. You said that they look rich. When you say it breaks down do you mean it all of a sudden falls on it's face or does it pick up a miss? I was told that 2 94s would run out of poop at roughly 4500 RPMS on a 350. If the outer carbs are not working you may have reached the limit on a single 48 on 401 CIs If you could better define "breaks down" it might be easier to find the right path. Without being there it's hard to tell if it's ignition or carb related.
The soot and popping are both signs of spark issues. I don't think you're going to be flooding out 400 cubes at 3000 rpm with a couple strombergs. If your advance curve is not advancing, it'll do what you describe If your spark is weak, it'll do what you describe If your spark has a path of lesser resistance and can go somewhere other than the correct spark plug, it'll do what you describe. If it's running out of fuel, it just kinda noses over like you're letting off the throttle. Not usually a popping sound to go with. good luck
Shifty Shifterton If your advance curve is not advancing, it'll do what you describe x 2 sounds like you need to give the dist. a twist.
Well, it's no fixed yet, but here's where I'm at. The timing appears to be OK. Seems the balancer has moved but if I extrapolate where it should be it appears to be advancing about the right amount. The points are set at .015 which is slap bang in the middle of the acceptable range. The distributor cap looks to be in good nick...no cracks The mixture screws on my centre carb are out 1 turn. The front and rear are out 1.5 turns. I have driven it again as well as running in the shed and it appears to start popping much earlier than I thought, maybe around 1500 but gets quite bad at maybe 3000. A tacho would be real handy right now. I don't have a volt meter and the parts shop was closed by the time I had a chance to go there this afternoon so I haven't checked the coil. Maybe tomorrow I'll get a volt meter and also a good coil and a new ballast resistor. If I can rule out the coil etc then I guess I'm onto the carbs...jet sizes etc. Thanks for all the help so far. I'll keep you posted. Pete
The choke plate was partly closing so I put a spring on it. That didn't actually help but at least that is sorted out
Yes but this may be a lean situation. Usually when a 4 stroke motor is lean it will pop as your is. Pete I should have asked this in the begining but is this a new combo for you or has it been running and just now started to give you trouble. Fuzzy Also How does it run at WOT ?
It's not likely to be lean based on the blackened plugs and it smells rich too. It is a new car. Just finished (almost). I haven't had a chance to get to WOT yet. I started this thread having just started to get into it a bit. Up until then it had been fairly tentative drives up and down my road and the problem hadn't shown itself.
Pete All the carbs should have an idle circuit so try this. Put the end carbs at 1 1/4 and the center at 1 1/2 then do a test drive. You may also want to advance the dist a bit. My old rule of thumb was to slow ly advance it until the engine picks up a little rpm and try that. But do each change seperatly. By that I mean set the screws and test then set dist and test. Good luck Fuzzy
If your balancer ring is slipping you have no idea where TDC is. Just replace it now before the intertia ring decides to make a hasty exit. Then time the car with a known TDC. The other possibility is one of your advance mechanisms is sticking just a little and making base timing unrepeatable. But if you can't get a timing light to read the same thing, repeatably, start there before you go moving the carb settings away from baseline. Don't dig yourself into a hole. Good luck
Changed the coil and resistor today. No different. Not to ignore your advice but I don't think the idle circuits will have an effect at higher revs. Tomorrow I'll try a bit more advance and see how it goes. It's currently at the sweet spot at idle but maybe a bit more will help up top. You're right. I definitely need to rebuild the harmonic balancer but it has to go interstate and I put it off in favour of getting the thing running. As I drive it more and get more confident and familiar with it I have been pushing a little harder and tonight I'm wondering whether I'm just pushing this old, stock engine too hard and getting valve bounce. It sure goes like stink in that little car but they were never built to rev, were they? Maybe I just expect it to rev harder than it does.
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