Problem: On cold starts, it runs fine. Smooth idle (high idle from choke, but smooth). Right when it starts to get warm it begins to putt putt putt sputter. It feels like a misfire, and not just one every now and then, but 80% of the time. I haven't done a compression test since this started but I did one in the summer and got 150-155 psi on all 4. I have not done a wet compression test yet, only dry. The truck has had a slight hesitation/misfire or shudder for last few years and I thought it was just because it's old. It never has been the smoothest idle but has seemed to get much worse. I have checked for vacuum leaks but haven't found anything (one of my reasons for getting the offenhauser, to get rid of the base plate leak I had).

Truck: 1977 SR5
Engine: 20R (stock internals)
Trans: W50
Fuel/Air system: Weber 32/36 DGV (electric choke); Offenhauser Dual plane manifold, airtex aftermarket fuel pump (last I checked a year ago was pushing about 2 psi). Brand new fuel lines and fuel tank. NEW fuel filter.
Ignition: Pertronix Flamethrower 2 coil, Pertronix Electronic ignition, NEW NGK nickel spark plugs, NEW Oreilly house brand plug wires, NEW cap and rotor. Distributor is fairly new (rebuilt)

Exhaust: 22R stock manifold (4-2-1), stock muffler, 1-7/8" pipe

WEBER changes I've made:
Primary idle jet: 65 (was 55)
Secondary idle jet: 55 (was 45)

Other notes:
Battery is about 4 years old. Haven't tested it but it seems to do just fine. Alternator replaced about a year ago. Voltage regulator replaced about 3 years ago.

Timing is set at about 8-10 degrees.

The spark plugs looked just fine when I pulled and replaced them last night. Nice light brown dusting, no oil or glazing or burned chunks (they had about 8-10k miles on them I think. A year old).

Conclusion:
I think that covers everything so far. Anybody have any idea what could cause this misfire? I will do a compression (wet and dry) this weekend.

Thanks in advance!

 

Is it an intermittent putt...puttputt, or a steady puttputt. Ur symptoms could possibly be caused by an arc in ur ignition system somewhere. Not sure ifu know how to check that.

Also, what was the method u used to test for vacuum leaks?

 

Its an intermittent, yet still quite often, putt......... putt putt.....putt........put putt putt..........putt.......putt......putt putt.....putt putt putt..........putt.......putt putt.....


I'm not sure how to test the arc. I know that the truck has a weird connection or something because about a month ago when I try to start the car (not every time. about 50% of the time) I hear a click and then nothing. No start no lights.... nothin! Then if I pop the hood, walk to the front of the car and pull up little on the positive cable running to the starter..... just ever so slightly.... everything comes back on. Dash lights, headlights. Its as if there is one spot the cable likes to be for it to make contact and start the truck. I got stranded and had a zip tie on me so I thought to tie the power cable to something that would assist in it be elevated just right. Ever since then, not a single starting issue. Lazy solution, I know.

As for testing vacuum leaks, I just take some starting fluid and spritz it around areas like vacuum lines (I think there are only 2 on it now) and around the carb base and areas where gaskets may have failed.

I did think to try and spray it around the intake manifold gasket, and wouldn't ya know, the idle increased and smoothed out for a second. So all day today I spent replacing the intake manifold gasket. Welp. That didn't do anything to remedy the issue.

So I sprayed a little fluid around the same area, and it seems like it may be finding its way in by the valve cover? Are those gaskets known to fail? I have done valve adjustments and it seems like a pretty sturdy piece every time I put it back on. I did think to get new cork washers for the 4 valve cover bolts. The originals were dunzo.

I guess I'll try the valve cover gasket next.

What are your thoughts on the arc test?

 

An intermittent misfire like the one you're describing COULD be an arc in your ignition system somewhere. It's good that you've apparently sealed all the vacuum leaks, so probably you can rule that out. If i'm not mistaken, valve cover gaskets only seal the motor oil from coming out. So a bad valve cover gasket will not cause a vacuum leak.

To test for arcing, start the engine up in a DARK area, and spray all ignition components w/ some soapy water (cables, cap, etc...). Listen and look for arcing. If it's arcing you'll see a little blue spark and hear it too. Arcing (and hence the misfire caused by arcing) tends to increase intensity and frequency w/ engine load (and rpm).

The battery cable issue probably is not the cause, but you should consider inspecting the whole length of the cable, and making sure it's tight at points that its supposed to be tight at. If it's corroded, replace or clean it. Sometimes you cant see the corrosion b/c it's under the conduit. It's a long shot, but if for some reason your coil isnt getting enough voltage it could cause a weak or intermittent spark.

One other thing worth mentioning, i have an electric fuel pump, and its pushing about 4/5 lbs. Not sure if 2 lbs that yours has is enough pressure. You should verify that just in case.

 

Also try pulling each plug wire 1 at a time and see if any make a difference/no difference..

Weber like 3-3.5 psi, so your pump could have something to do with it. Do you have any problems like lack of power, or short powerband or something like that?

 

For the starter, check your grounds. A bad ground will show the problems you mentioned.

Clean all contacts where (-) cable attaches to the frame.

 

^ I haven't thought to run that test yet. I am going to actually test my fuel pump tomorrow. Just to see what it is pushing. I really don't want to have to put another fuel pump in the tank. It was a pain last time. (airtex pumps are junk)

^When I tried this, (when the sputtering was at its worst) spark was occurring on all cylinders yet there was very little difference in idle on cyl 2 and 3. Didn't make sense to me since my compression was good. I assumed somehow the fuel/air mixture was not getting to those cylinders properly (which would make sense if the intake manifold gasket was fouled).

As for lack of power it certainly feels like it at time. Like I have to floor it to keep up with traffic. That doesn't seem normal at all.

All of the ground points look so filthy I need to do that.




So today after driving around, the damn thing runs fine. Much smoother idle. Not putting or backfiring. And I have quite a bit more power it seems (at least probably closer to the power it should have). I really don't know how it could have changed so drastically.

Just to let anybody who is interested know, Offenhauser intake manifolds are a total waste of money, at least when put on a fairly stock 20r. Not because they are necessarily made poorly. Nice fit and look good. They just don't do anything good for performance. It might even hurt performance. Granted I did have a dual port and it may have worked better with a 38/38 as opposed to the 32/36, but the stock manifold is just great (as long as the weber gaskets seal properly).

I ended up going back to the stock manifold because on my second run taking the manifold off the engine, I noticed a little coolant leak around the thermostat housing. Not even trying, I sheered the heads off both bolts holding the housing on. Fortunate for me I kept my stock manifold and the base plates for the weber. A quick trip to Ace for some hardware and I was back in business. Maybe the mounting surface of the offenhauser wasn't flush and didn't mount to the gasket well enough, causing a leak? Oh well. I'll probably try and drill out those bolts and re-tap it. Maybe somebody would want it for a cool $100. Or maybe turn it into a cool planter.

Thanks to everybody who chipped in with great thoughts and advice. I'm always learning something new.