350 has bad misfire/shudder over 2000RPM
 

Just put my '79 C30 service truck on the road recently. After a long 6 months of building it in the driveway, I was pretty darn excited to start working with it... until I discovered a really bad misfire/shudder over 2000RPM when under load. Which brings me here!

Everything has been serviced/gone through on the engine... plug wires, plugs, cap, rotor, distributor all rebuilt with new parts, carburetor rebuilt, vacuum lines all replaced, PCV valve replaced, etc.

-Has a Rochester Quadrajet carb
-Timing is currently set at 10deg (with dist vacuum like unplugged)
-AC R45TS plugs gapped at .045
-Dist power wire is good and getting full current from the fuse block
-Truck grounds are good and I added extra cables frame to engine
-While running, touched each plug boot with a screwdriver going to ground and nothing arced
-Previous owner installed an RV cam (whether it makes a difference...)

Took it on a service call this AM and had to turn back. At highway cursing speed (55-60) it was awful. Goosing it will cause the misfire will go away briefly, then come back once you let off. Climbing a big hill, mat the pedal and it goes away for about 20-30sec, then gets really really bad... stumbling all over, won't catch up with itself. The fuel pump did come to mind, but even if the truck is sitting there at an idle and you take off hard, it will still stumble once over 2000rpm or so. Any rpm under that and it's fine.

Any thoughts? Have some work scheduled in the next week+/- so would like to remedy this soon.


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Re: 350 has bad misfire/shudder over 2000RPM
 

I would think more about fuel pump, fuel filter in carb. Just spitballing here could be something else. Just a thought.

Re: 350 has bad misfire/shudder over 2000RPM
 

I’m thinking the ‘dreaded flat cam’.
Have you checked all the rockers move the same?
Was this a used motor?
Or a rebuilt.

Re: 350 has bad misfire/shudder over 2000RPM
 

Is the vacuum advance working? should be about 34-36 degrees around 3,000 rpm.

Re: 350 has bad misfire/shudder over 2000RPM
 

Possible vacuum leak around intake gasket or carb base gasket. Do the spray with carb cleaner test.

Does it still have the exhaust riser valve? Those can stick shut and force half the exhaust through the intake crossover which kills higher rpm performance and can overheat the carb.

Does it still has the original Y pipe? Some of those had double walls and the inner wall would collapse, mostly blocking the exhaust, Also killing performance.

Check the rubber lines from the tank(s) to the frame. If those are original, they are probably crumbling away and letting air into the fuel system. Same for rubber lines from dual tank valve to frame if equipped. Even if hoses recently replaced, mice sometimes like to sample taste the hoses, also letting air in.

Nice looking service truck.

Re: 350 has bad misfire/shudder over 2000RPM
 

Thinking more on it, might not be cam.
Could be fuel pressure problem.
Too much pressure in your case.
Who rebuilt the carb?

Re: 350 has bad misfire/shudder over 2000RPM
 

I did start considering the fuel pump thinking of it more.... Going to pull the line and see if it's got a good stream when cranking (I don't have a gauge to put on it).

Yep. When I rev the engine the timing mark moves right up to the 30deg/+ area.

Thanks Parrot. Good idea, I'll check the intake for vacuum leaks. The passenger side around the intake manifold does look a bit iffy, so I wouldn't be surprised if it's sucking air. If it means anything, this can be hard to start too when hot. Sometimes cranking for a bit and when it catches, it won't idle well... or catch then die again... have to keep the throttle cracked... I suppose that could signify a vacuum leak?

Riser valve is still there, though I think the PO cut the butterfly out because you can almost make a full turn with the bell-crank.

Y-pipe is gone. Currently has dual straight pipes with glass packs, no crossover pipe.

Gas tank lines have all been re-done too (and filter).

It's only happened twice, but when starting this, the float has stuck. Couple taps to the side of the carb and it stopped. Not sure if the float hung, or if the pump was pushing fuel past.

I rebuilt it. Very familiar with carburetors, but this was my first time rebuilding a Q-jet, so wouldn't be surprised if something didn't turn out right...

Going to check for vacuum leaks and fuel pump functionality today...

Re: 350 has bad misfire/shudder over 2000RPM
 

If the clip on the needle is on wrong or not used it can cause flooding like that.

Info in here on that.



http://www.crankshaftcoalition.com/w...4MV_carburetor

Re: 350 has bad misfire/shudder over 2000RPM
 

Vacuum advance is working but does mechanical advance move freely? When your foot is into it, no vacuum will be there to advance so it will be relying on the weights. Because you say it is only over 2K RPM under load, this makes me think it is not advancing. But check your fuel system first like everyone else stated.

Re: 350 has bad misfire/shudder over 2000RPM
 

Thanks Geezer! There's a ton of good info on that page...


The mechanical does move well, though I should check it again. The guts were swapped in from another 350 distributor because the original weights and shaft were very badly worn (see pic). Assembled nice with the good used parts and moved as it should. Worth another look though just to be sure.


-So, fuel pump is good.
-Turns out the carburetor wasn't bolted down tightly, never re-checked after I installed and ran it.
-No vacuum leaks around the manifold or carb base (after tightened)
-Set the carb idle screws by the book with a tach and it sounds much smoother at idle and starts better

After that, it still misfired when shifting up, even going down hill. So I pulled a plug wire and hooked up one of those inline spark testers. When running it jumped the 3/16 gap, but the spark was orange at best. Leaning towards weak spark now... Never replaced the coil when I rebuilt the distributor, did everything but... dang! Going to pick one up tomorrow.



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Re: 350 has bad misfire/shudder over 2000RPM
 

Difficult to start when hot is also a symptom of tight valves. If you still have the older style heads without hardened seats, you may have valves receding into the heads.

Also, to somewhat mirror one of the suggestions above, aside from checking for flat cam lobes, also check the lobe that drives the fuel pump for wear.

Re: 350 has bad misfire/shudder over 2000RPM
 

Might double check that the plug wires are in proper firing order. Some swapped pairs run surprisingly well at idle and near idle speeds.

Check that all the screws on the carb top are tightened. There are many and it is easy to skip one. Especially the pair inside under the choke flap.

Run it for a bit and check plugs for color. Might be able to find the problem cylinder(s).

Re: 350 has bad misfire/shudder over 2000RPM
 

The 454 in my Dually is doing pretty much the same thing and I need to get it sorted out. It started on the way back from the dump one day and spits back when I accelerate.

Carb bolts coming loose isn't something you usually think about though. I figured the cam went flat on it.

The smoke from the fires is so bad here that I haven't been wanting to stay outside long enough to fool with it.

Re: 350 has bad misfire/shudder over 2000RPM
 

Pour some Carb/fuel injector Cleaner down vent tube in carb. You could have some carbon or debris in carb bowl. I did this and idle smoothed out nicely

Re: 350 has bad misfire/shudder over 2000RPM
 

Hopefully that mess comes to an end sooner than later over by you guys. Aside from the devastation of homes being wiped out... seen a lot of pictures of burned up old trucks too.


Thanks for all the help and suggestions guys. Luckily it was just a bad ignition coil. Replaced it this AM and that 350'll wrap up tight now with no misfire. Glad it wasn't cam or valves related, time's been too short lately for major engine work!

Re: 350 has bad misfire/shudder over 2000RPM
 

How did you figure out it was the coil. I would never of thought of looking there and how would the coil do that. I thought coils worked or didn,t

Re: 350 has bad misfire/shudder over 2000RPM
 

Dad said "put the spark tester on it and see what happens"... turns out it was orange rather than blue or white.

It does make sense, I run into that with Kohler engines and Gravelys. The old Kohler K series will idle fine on a weak spark, but try to put a load on or rev up and it falls on its face. Gravely's with Wico magnetos will start and run fine, throttle up okay, but one sudden load and it immediately stops firing and dies. Some will run but have choppy intermittent fire (like the 350 at high RPM's).

Re: 350 has bad misfire/shudder over 2000RPM
 

Glad you got it solved. Great work!