Misfire from hell

[85 Cutlass Brougham]

For some odd reason, my 350 is missing one one cylinder regularly, but not every cycle...if that makes sense. I checked the header tubes using my infra-red heat gun to see which one was not running, and found one significantly colder than the others, so I figured that was the one. I pulled the plug and found it looked rich, but not wet. So, then I ran a compression check on that cylinder and found 150psi of cranking compression which is plenty for it to run, and it also proves the cam and valvetrain are not at fault either. I replaced the plug wire with a new one with no change in performance. I also tried fine tuning the idle mixture on the Quadrajet with no change to the better. The vacuum gauge vibrates back and forth rapidly over a span of around 4 in/hg, but that only verifies a misfire. The plugs are almost new too, so I did not change the plug in the offending cylinder. I also pulled the distributor cap ( which is almost new) and checked but found nothing wrong. Now the car has been sitting for a while ( about a month since I last ran it) and it didn't have an issue last time. It also has fresh gas as I ran it out last time that it was run, so I needed to put a gallon in to get it started. I have checked the vacuum hoses and found none that are broken either. The car is also popping through the carb on heavy acceleration, so I may try to stiffen up the secondary air valve spring adjustment and see if that helps. The carb has stock calibration for a 1979 350 but with a K hanger instead of the stock one. I want to put in a new distributor eventually, but really don't want to put the money into that until the car is painted and mostly assembled. (There's no sense putting that money into it while the car is not usable.). I want to drive it to the paint booth this Thursday, but it's 30 miles away and I don't want to drive it if it is not going to run right. Any ideas?

 

[pontiacgp]

are you sure the valve on the inlet side is opening on that cylinder?

 

[85 Cutlass Brougham]

Well, I haven't pulled the valve cover to verify it, but if anything I would think it would be stuck in the open position and not closed after sitting. It also is not a dead miss, it's more like every 3-10 cycles it misses. I replaced the cam about 2,000 miles ago and with it the rockers and timing chain, so most of the valvetrain is relatively new and recently adjusted.

 

[JESSE]

miss

the first thing that i would look at is the valves.....since you confirmed compression and fire...if you can elimanate fuel it has to be mechanical.. at least thats what my old t-top mullet flying in the breeze ,cheap beer drinking brain says to look at if you ran your carb dry you may have picked up some trash, sometimes you can take a rag an hold open the throttle an stuff the rag down the throat until it almost dies and then remove the rag and rev the motor a little an see if that helps i've done this in the past and it has worked for me... let us know what you find...jesse

 

[dan2286]

Re: miss

at least thats what my old t-top mullet flying in the breeze ,cheap beer drinking brain says to look at

Thats got to be the funniest comment I have seen in a long time! :lol: :lol: :lol: :lol: :lol: :lol: :lol:

 

[megaladon6]

i'm thinking a sticking valve. maybe an exhaust valve that's not fully opening, trapping burnt A/F in the cylinder. that would keep it from firing, therefore a cold header tube. it might take a couple of cycles to trap enough gas to prevent firing.
that could also explain the backfires as hot burnt gasses try to flow out of the cylinder and/or the hot surfaces may ignite the incoming air charge.

 

[85 Cutlass Brougham]

I fixed it. It seems that 11 year old plug wires are not a good thing. I borrowed a timing light and checked all 8. One was not firing at all, and the others were not firing all the time ( except for the one new one I had installed-that one worked perfectly). I replaced two of them that were generic ( 6 are MSD, 2 were cheapos) with a new one I had left from 11 years ago, and one off my AMC that has been converted to HEI cap terminals. The rest still do not fire perfectly, but it runs better now. There is a 300foot long stripe of rubber in front of my house and two of my neighbor's houses to attest to that! ( I didn't mean to do it, but I was just trying to duplicate the pop through the carb). I guess I will be ordering a new set of black MSD wires for an 85 MCSS from Summit!

 

[megaladon6]

i thought you changed the wire?

 

[85 Cutlass Brougham]

Well, there was more than one that was bad, apparently. The two front wires on the passenger's side were generic ones because I needed a straight boot wire and the MSD set I bought was all 90's. So, as a "temporary" fix (11 years ago) I had used these two leftovers from a previous set of cheap wire I had tried to run instead of the MSD's. The other cheap one was also bad, so it was also missing. I have no idea why it read higher with the temp gun than the other one I originally changed, but I wound up changing two wires to fix it. In the end, I will get a new set of wires (that I had intended to buy all along) right after paint, but at least now it drives good enough to get it to the paint booth in the next few weeks. I have since found that if I get a set of MSD direct replacement wires for a 305 Monte Carlo, they come with the two straight boots and 6 90's I need for my stock wire looms. Plus, they are available in black to match my blingless engine bay.

I also need a new distributor in the future as this one is old and the mechanical advance is in poor condition. I plan on an MSD replacement unit as it has an adjustable mechanical and vacuum advance so that I can run the extra initial the engine likes while still keeping it around 36 total.

 

[pontiacgp]

if you looking for a distributor check ebay for one that came out of a ZZ4 Crate Engine. There's a bunch of guys buying that engine to drag race and they sell off the components that have upgraded. I bought a new ZZ4 distributor from a local speed shop for $150...