Chasing down a misfire

Buzzmobile · #1 (**permalink**) 06-21-2005, 12:39 PM

Ok, I am just about to chase down a misfire. Here is what I have. New MSD ignition box and blaster 2 coil. New MSD dizzy. New engine build. When we first fired it, RICH was an understatement (my brain fart). So now the rich has gone away. (Yeah! no more smoke!!!!). Now I have a definite stutter / hesitation when I am MODERATELY accelerating. When I jump on it, it usually just takes off. When I am cruising at street speeds with a constant speed...it does ok for a little bit then stutters and hesitates, and if I just add a little gas to it, it planes out, then repeats. It almost feels like slightly lugging the engine slightly, but at speed. Idle seems OK...well hell, how can you tell about idle with a radical engine? I guess you can say it does idle, kinda! Hesitation does it both at cold and warm engine temps. Seems to be getting worse. Also, note that I have the stalling at stops that was just addressed in the other thread recently, so carb floats also on the aggenda to work on. My first thoughts were that I fouled the plugs running rich. But right now, the engine is so danged hot, I am waiting to pull the plugs till it cools. Any other things to check while I am out there. MSD box...anything to do diagnostically? Distributor?

Thanks guys....I really appreciate your help as always. She is getting there....but more help is always welcome!

Buzz

blykins · #2 (**permalink**) 06-21-2005, 01:02 PM

When it was really rich, what color was the smoke?

Buzzmobile · #3 (**permalink**) 06-21-2005, 01:25 PM

it had several colors. mostly black/grey. I have a 2x4 system and when I put the air cleaner on the back carb, I hit the choke on the carb by accident and the butterfly kicked over. Since it was a new engine build, there was all kinds of smoke coming out of the engine compartment, off the headers, out the pipes...

when I got the chokes put back and got it rolling, no more smoke! and the other stuff on the exhaust has burned off.

Mike Simard · #4 (**permalink**) 06-21-2005, 02:31 PM

Plugs could still be on the list of things to check. Even though your plugs look OK they could still be fouled, especiallly if you have individual cylinders that the exhaust port is cool instead of a random misfire that affects all cyls.

Excaliber · #5 (**permalink**) 06-21-2005, 02:41 PM

This REALLY sounds like the main jets are just a little bit to small. When you "step on it" what is happening is the power valve opens. Thus enriching the system. When your main jets are "dialed in" you can STILL feel the power vavle respond but the hesitation and slight sputtering of which you speak goes away. I move up by two steps on the main jets. Sounds like the power valve is just about right. It SHOULD kick in with a slight increase in throttle application.

Bear in mind when the main jets are just about perfect for cruising, no sputter, smooth, they will be on the lean side when you remove the air cleaners and/or sidepipes. Normally you would change jet size by 4 or more points, but it sounds like your pretty close to correct size right now.

Buzzmobile · #6 (**permalink**) 06-21-2005, 03:54 PM

more info:

pulled 5 of 8 plugs (ran out of time, had to go to work)...and they were black with some brown...not the tan everyone talks about. Cleaned them up, checked the gap (Keith put them at 40)...they all were the same. wires look good. One was not seated well on the plug. Got it on, and the idle got much better, but the hestitation/stutter on acceleration is still there and maybe worse. The situation feels like it is getting progressively worse when driving the car. The one thing that I notice...when I am just parked in the drive way changing RPM without any load on the car, it does not hesitate one bit. no gurgle, burp, stutter...nothing. Put it in first or second and cruise down the neighborhood, it sputters all over the place, and then when I nail it now, it really sputters...

Whadda think? Carb, ignition, vacuum leak? I know this one is gonna take a while. BTW, what are the torque specs for plugs....dang, I am having a time getting them out. They really cranked them in there, and I am not sure some are gonna come out...

BTW...EDIT: When Keith came to Tulsa and I was having a brain fart with the choke, he took the carbs off and we found the jets were I think 36s.....what does that tell us?

EDIT #2: well that does not sound right on the jet size does it. Once again, the brain has farted....can't remember. I do remember it was on the small side per Keith. Keith, do you remember? Any suggestions?

Buzz

Buzzmobile · #7 (**permalink**) 06-21-2005, 07:09 PM

OK, after doing some reading, what I am feeling is similar to what people are calling a "lean stumble" It got worse when I lowered my idle the other night. I was stalling when braking hard, so I had the idle up to compensate. Then I thought I would try and lower it back down to see what happended and then this stumble started. Does this mean the I just need to jet the thing up some? Any suggestions?

HighPlainsDrifter · #8 (**permalink**) 06-21-2005, 07:38 PM

Hi,
You say you have a msd dizzy, probably just mechanical with no vacuum advance. If so you need more initial timing, say 16* to start. Set timing then set idle to 800 rpm and see if the stumble is still there. If it's ok you can play with the timing,try for 36* full in by 3,000 rpm.
If it's not timing,put a vacuum gauge on it and see what your idle vacuum is. The power valve in the metering block has to be less than the vacuum. IE: if the vacuum is 9 inches, then the power valve has to be more like 4.5 or no more than 6.
Good luck,
Perry.

Buzzmobile · #9 (**permalink**) 06-21-2005, 08:40 PM

yeap, mechanical advance..and here is how it went. I got the engine built by Keith and asked that they change the distributor from my old mallory unit to the MSD. Well, I think Keith forgot and they did the dyno runs with the old unit. They changed the dizzy right before shipping it up here without running it. Keith had me start it up and then run it up to about 3000 RPM and set the timing to 38 degrees (which a friend did THANK YOU CLOIS FOR ALL THE HELP and Keith checked when he was here for midamerica)...SO to answer your question...I have no idea what the initial timing is...

So what is the procedure for doing the low RPM timing and then the total timing?

Buzzmobile · #10 (**permalink**) 06-21-2005, 08:42 PM

BTW, I am checking other threads and when looking at my pump, it does not come in immediately when I activate the accelerator on the primary carb. The second carb pump moves immediately...So I think this needs worked out as well.

2KWIK4U · #11 (**permalink**) 06-21-2005, 09:52 PM

Buzz
I have/had the same problem, I think, as you are having. I think we have to bear in mind that we're talking about 650+ HP in a 2300# car and they aren't going to run real smooth at low RPM or no load. I have a problem at constant throttle between 1500 and 2250 RPM, like when I'm trying to get out of the neigberhood without breaking any windows. It's almost like I'm running on 4 cylinders. Anything above that, no problem. From idle to WOT no problem. Just low RPM under no load. To start with, Keith had my timing @ 38 all in by 2800. I changed the springs and changed it to all in by 2300 and it really helped, but not completley. I thought it may be vacuum leak, but can't find it if there is. Maybe power valves, don't know what the 950 Divincia has in it of the shelf. Just kind of learned to live with it so far. Let me know if you figure it out.

Brad W..

Buzzmobile · #12 (**permalink**) 06-21-2005, 10:12 PM

if I am in the car driving 30-40 MPH and mash the gas pedal...she spits/sputters/coughs...and then runs poorly until pretty high rpm. She has run way better than this with the new build. Not sure what is up, but it has changed

392cobra · #13 (**permalink**) 06-22-2005, 10:03 AM

Buzz,
Mine doesn't start to smooth out till 2,200 rpm as well.

If Keith didn't change your MSD from the stock factory settings you have a blue bushing ( 21*) and two heavy springs. The stock springs don't allow full advance till 4,000 rpm.

http://www.cobralads.com/gear/msd2.gif

Rick Parker · #14 (**permalink**) 06-22-2005, 11:25 AM

Adjust the accelerator pump levers, you should be getting similar fuel discharge at the nozzles with very little movement of the throttle. Are the carburetors syncronized. Is one open more than the other???

Rick

Mr.Fixit · #15 (**permalink**) 06-23-2005, 10:57 AM

Sounds like fuel delivery, not ignition.

Too lean: coughs/barks, wants to backfire through carb
Too rich: stumbles/gurgles, chokes on itself.

With that radical camshaft you mentioned, you may have it richened up to compensate for poor booster signal at low speeds, then it is too rich at higher speeds.

SCOBRAC · #16 (**permalink**) 06-23-2005, 11:07 AM

90% of all electrical issues on a fiberglass car is ground related. Your engine needs to be grounded to the chassis, your MSD box should similarly be grounded.

Buzzmobile · #17 (**permalink**) 06-23-2005, 11:58 AM

Here is where I am. I adjusted the pump lever and got that working correctly. I am thinking that I had a lean problem at low RPM because I made it a little richer and *most* of the problem went away. I can cruise at a steady speed without out hiccups. When I get on it, it does not hesitate. Total timing is 38, still haven't checked on the idle timing, but I think a little more advance of initial timing will help. All of what I am feeling is below 3000 RPM. When I am up in the power band a little, no problem.