A little help diagnosing please

[747mopar]

I've got a little issue to work out of my Motor that I'm sure I'll solve but might take a while, with all the combined knowledge onboard I thought I throw it out there and maybe someone will save me some time. The issue is a slight miss, if I accelerate very and I mean very gradually it runs perfect but if I accelerate like you would normally when driving it has a little miss. I can gradually work up to 2,000 rpm and cruise without any issues at all but try doing the same at a normal pace and it misses. I have an 870 Holley Vacuum Sec, I've checked the float level and it pulls 17 lbs of vacuum. It's not the accelerator pump because if you blip the throttle I get a strong squirt and great throttle response. Not sure where to look next but it seams like a transition from one carb circuit to the next so I'm thinking about just pulling the carb and going threw it. What about timing? I'm leaning away from that seeing how it runs perfectly at any rpm and only misses when accelerating normally. Thanks in advance

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P.S The miss seams like it would be right after the idle circuit, low rpms.

 

[snakeoil24]

Do you have a Mopar Distributor? If so open the hood after dark and mist the distributor and wires with a light spray of water. if anything is arching th en you know to look at Electrical. Take cap off and look for carbon arching. If this is good check gap in Solid state Ignition. if this is not it then put vacuum gauge on it and drive arounfd blocka fter warm upo and record the readings when it misses. let us know. Also take a can of carb cleaner and spray hoses, base of carb and intake and see if idle changes then it might be a leak. Don't assume carburator untiol you eliminate the basics. Check base timing also. You might have to check Powervalve to see if it has a torn diaphram!

 

[HEMI-ITIS]

Right on SNAkE!!

 

[RedHot67]

Sounds like a vaccum leak that leans out the mixture under load.
Or the timing advance curve is too aggressive, try some heavier springs.

Apart from all that, to pick the correct accelerator pump cam is also very important, often only one of the entire cam set really works!
I have the best experience with orange or green cams in hole 1 position.

Good luck,

Dan

 

[747mopar]

Do you have a Mopar Distributor? If so open the hood after dark and mist the distributor and wires with a light spray of water. if anything is arching th en you know to look at Electrical. Take cap off and look for carbon arching. If this is good check gap in Solid state Ignition. if this is not it then put vacuum gauge on it and drive arounfd blocka fter warm upo and record the readings when it misses. let us know. Also take a can of carb cleaner and spray hoses, base of carb and intake and see if idle changes then it might be a leak. Don't assume carburator untiol you eliminate the basics. Check base timing also. You might have to check Powervalve to see if it has a torn diaphram!

Good advice, last time I had the cap off I thought the rotor button looked funny but it was still making contact (no carbon). It's a Summit brand billet distributor (magnetic pickup, mechanical adv.) and I already checked the gap in it. Today was my first day of putting some real miles on it so I'm early in the diagnosing, I'll try all that I haven't already but I've covered most of those bases. What heat plugs do they usually like??? It's a moderately built 456" with a 501" lift cam and a Mallory 6AL ignition for ignition. Thanks, I'll keep taking all the tips I can get and then tear into it tonight and try to get it resolved.

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When I say "funny" I mean it was very squared off looking, I'm used to the buttons being a little more rounded and spring loaded but couldn't tell if it was supposed to be that way or not. I've got a pile of factory Mopar distributors laying around that I might try out.

 

[Moparsmitty]

Just another idea as long as you are checking everything. Many aftermarket carbs come set up as a kind of compromise so that they will work on the most applications right out of the box without any tuning. I find that they are sometimes jetted a little too lean on the primary side for some setups. The only way to tell for sure is to weld a bung into the exhaust collector and install an O2 sensor so you can get a read on the fuel mixture while the car is being driven. The cheap and easy way (especially on a Holley) is just to go a couple of jet sizes richer on the primary side and see if that helps. It's worked for me on a couple of cars I've worked on. Worth a shot.

 

[Hemi Rebel]

Sounds like Ignition. Breaking down under load. Is this a fresh engine or have you been running it for a while?

 

[nm9stheham]

Sounds like a weaker spark combining with a mixture distribution issue to me. If you have a voltmeter and the plug wires are resistor types, then check them for end to end resistance. New, they will typically be in the 1k to 3k ohm range; if they get over 5k they may effect spark, and if up towards 10k ohms, then discard and replace.

Mist around each intake runner with something combustible to see if you have a small vaccum leak on one; that would fit the symptoms: lean mixture on one cylinder at moderate vacuum with a load on the engine.

 

[747mopar]

Thanks again, yes it's a fresh motor with very little run time. Earlier when I was out in it I had company with me and was taking it easy (embarrassing to have an out of tune motor) so I didn't get the full effect. Just got back and I'm with you guys for sure now, it definitely seams like ignition. I matted it and left it there for a few seconds and it missed the whole time it was under load even in the higher rpm's so it doesn't seam to be isolated to a certain function of the carb. I'll check the wires (Taylors), cap, distributor, vacuum leaks, etc and see what pops up. Thanks

 

[hunt2elk]

Any chance #5 & #7 wires got switched? It would probably run rougher than it is, but still wouldn't hurt to double check.

 

[747mopar]

Think I found it but unfortunately I'll have to call Summit tomorrow and see how quickly I can get another because I really don't want to have to drive up there again. The button in the cap that I mentioned earlier is showing a little more sign that it isn't right now. The button looks like it may have gotten the tip broke off leaving a squared off edge, look where I'm pointing on the rotor and you can see a very small half moon shiny spot just outside of the dark circle. It looks to me that the button is barely contacting the rotor with only a little of it's now squared off edge and then arching and making the center dark. I'm hoping that they used a popular cap on this distributor and it's not a Summit only cap so I can just pick one up locally. I did test for arch's, vacuum leaks and loose connections.......all good. I'll let you know once I get another cap if that does it. Thanks

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Any chance #5 & #7 wires got switched? It would probably run rougher than it is, but still wouldn't hurt to double check.

I've checked them multiple times haha.

 

[charger360]

sometimes a plug will go bad as well has happened to me a couple of times. I always check the easy stuff first. is that vacuum reading in neutral? is the car an auto or stick? if it is an auto check the vacuum in gear to be sure you have the right power valve. is that a street avenger 870 carb? they are a bit funny to tune sometimes but at least they have power valve blowout protection

 

[747mopar]

sometimes a plug will go bad as well has happened to me a couple of times. I always check the easy stuff first. is that vacuum reading in neutral? is the car an auto or stick? if it is an auto check the vacuum in gear to be sure you have the right power valve. is that a street avenger 870 carb? they are a bit funny to tune sometimes but at least they have power valve blowout protection

Yes it's a street avenger and I'm running an auto as well. I ordered the cap and rotor today and will have them tomorrow so I hope that does it. I've heard 2 different ways to determine the correct power valve, one like you mentioned and the other while driving only I believe that one requires a meter in the car. I tore the carb apart last night just because I bought it used and wanted to see the condition, very clean like a new carb. The power valve is a 4.5 I'll have to check it while in gear if I ever have someone to help but it pulls 17 in neutral.

 

[charger360]

if it does not drop off much like say to 15 lbs or so I would say you could go up to a 6.5 easy. what was it jetted at? those carbs were a bit lean imo. have you had a look at the plugs? what kind of burn are you getting? don't want to be too lean!

 

[747mopar]

if it does not drop off much like say to 15 lbs or so I would say you could go up to a 6.5 easy. what was it jetted at? those carbs were a bit lean imo. have you had a look at the plugs? what kind of burn are you getting? don't want to be too lean!

I changed plugs not long ago and they looked pretty good but I'd say it could stand to go a little richer. I really don't see any point in doing anymore tuning to the motor until I get the new cap and rotor because I'm certain that even if it isn't the problem it is still a factor in the tuning. It should be here tomorrow, once installed I'll see what happens and go from there. 6.5? Idle vacuum in gear divided by 2 I'm assuming is where you got that which is the method Holley suggest.

 

[wedge69]

Thats a Mallory rotor.

 

[nm9stheham]

Go through the igntion and plugs, etc., and then run a compression test on all cylinders; with a steady miss like that, it is either ignition on one cylinder or a leak in a cylinder or instake runner. Is there any miss at idle?

Even your 4.5 power valve should have cut in with the pedal floored; changing from a 4.5 to 6.5 power valve won't change/cure a flat miss on one cylinder.

 

[IQ52]

What is the initial timing and total? What is the camshaft duration @ .050"?

 

[747mopar]

Thats a Mallory rotor.

Very good Wedge A+. I was a little worried when the Summit sales tech said "that distributor has been discontinued and we (Summit) no longer offer the cap and rotor) but then he said it's just a Mallory unit so he's shipping them. It is good to know that for the most part all of Summit's brand name parts are sourced from respected manufactures like Mallory.

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What is the initial timing and total? What is the camshaft duration @ .050"?

Uh Oh, I'm in trouble now! I tuned it by ear and haven't even put a light on it yet. I've been waiting for somebody to be around to help check the advance (operate the throttle) but it ran great right where it was at before and I haven't touched it. Duration is 274 Ex, it is comp cams 21-224-4. While your on here and since I know your a pro why don't you tell me what you think about the whole setup. 400 BB, 440 crank, ported 452's, the comp cam 21-224-4 cam, Holley 870 Vacuum sec, Edelbrock rpm intake, Mallory 6AL, Summit Mechanical Advance distributor, 9.8:1 comp, Hooker Super Comps and a 3" X pipe. I like hearing what the guys that actually get to spend dyno time have to say about setups. This was never intended to be a drag motor but instead one that is happy cruising at 2,000 rpm, makes plenty of power and most importantly is reliable (not tall gears and allot of stall).

 

[Budnicks]

I was going to say something in "ignition, my 1st wild as guess" {like your cap &/or rotor or even air gap if it has one} or advance curve even, or initial timing maybe,

valve-train somewhere {I know you don't want to think it's a bad rocker adjustments too tight/too loose, sorry},

possibly bad wire arching {need ultra-low resistance wires with a hot ignition system like less than 100-ohm's per ft loss, you could be getting cross arching between wires if not separated, adequately/properly}, wrong heat range plugs ??,

also with an aftermarket cam, they generally like allot more initial timing like 16*-20* BTDC easily,

do you have a vacuum advance & if so, is it adjustable ?? {a little allen head screw in the center of the pod thru the vacuum line port, limits the amount of added advance}, if so limit the amount of advance & time it to have between 32*-38* max total timing, usually towards the lower side...

Fuel pressure/supply, Jets, accel. cam & squiters could be lean/too small, if it was the acc. pump charge it should go away after initial hit, but if it's lean {jets or even wrong or bad power-valve} it will surge kind of, under a light load or when your on it hard, under a load... what kind of fuel pressure & pump etc. ??, is it "flowing 5-7psi" everywhere ??

sorry I'm not much help guessing here