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Little help diagnosing stumble? UPDATE: Fixed w/ New Dist.

A

Amtek

1/2 ton status
Joined
Feb 13, 2011
Posts
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Location
Alta Loma, CA
I'm at my wits end, no matter what I do the truck stumbles at around 1600 rpm under any throttle position (WOT or just cruising) when under any load. It does not stumble when revving in neutral, but if put it in gear and hold the brakes it will do it.

I've tried it with all vacuum ports plugged (running hydroboost) except for vac. advance, with no change. I've plugged the vacuum advance into ported and manifold with no change. Running at 6 degrees initial timing (for smog purposes), advancing or retarding timing doesnt seem to affect it. Holley 600CFM carb is new 5 years ago, truck sat for 2 years while I rebuilt the 350 which is mostly stock except for very mild summit cam, bored .30 over, edelbrock EGR alum. intake, and higher stall torque converter. No vacuum leaks, no loss of coolant. Stock T-stat temp of 195. New plugs, new wires, cap and rotor. During the rebuild, I took the distributor apart and cleaned and very lightly lubed the weights and put everything exactly the way it was. Vacuum guage needle sits at 15 inHg @ idle and vibrates very very slightly. I've adjusted the accelerator pump with no results.

I have taken a couple of videos to demonstrate the stumble/hesitation

The stumble is very noticeable at 0:27 - 0:34 but the truck is cold.


Here it is again at 6:00 - 6:10 and 6:27 - 6:33

[youtube]a8Z54p1-bOY[/youtube]

any thoughts?
 
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do you know what your timing is doing?
Check your base, and mech advance...it may be coming in to soon for a heavy truck. Not sure if I heard some detonation or not.
I would try for a base of 10-14*, and start the timing curve at about 1500 rpm, and slowly have 36* all in by 3500 rpm.
Also lock out the vac secondary by mechanically preventing them from opening..unhook the vaccum plunger...if your hesitation goes away the spring is too light..if no change then try a bigger acc pump squirter.
 
Sorry can't help but I was interested in your camera setup. Could you give some details on it? I've been thinking about something like this to diagnose an issue I'm having.
 
76Zimmer: I have no clue to be honest, my timing gun is very basic. the distributor/timing springs are all stock, other than a slight cleaning and lube it is untouched. The truck ran great before the rebuild Good response and power, no stumble or nothing but the rings were worn out and the valve seals were letting too much oil get through, it would barely pass smog (max HC ppm: 255 measured: 254). It also had a non-smog legal edlebrock performer intake manifold with no EGR. So I decided to freshen it up. Got one of those rebuild kits from summit, bored cylinders 0.30 over , edelbrock performer w/EGR, mild summit cam (1400-5000 rpm), and a 1700-2000 rpm stall converter.

Now here is where I made a big mistake, My stock 350 heads were cracked in several areas and I got a set of rebuilt 305 heads for dirt cheap, but I didnt realize they were 305 heads till after the engine was in. They looked identical and actually the intake valves on the 305 heads were larger 1.84" vs 1.74" stock. I decided to run them anyways cause I ran out of money. They worked OK but if I advance timing above 10* i'll start getting some pinging even with premium fuel. I had to run stock at 6* as required for smog testing and just left it at that.

I also think it was a mistake to get the higher stall torque converter with 3.73's and 37" tires...The engine rev's up to around 1500-1800 rpm to get moving and has also killed my gas mileage. It ws pretty nice when I mounted stock sized tires, it launched hard but gears arent going to happen for a long while. I'll be pulling the engine again to swap the heads and change out the torque converter back to stock soon

Rampage: I used a keychain HD camera Model 808 #16 from ebay, its the size of a car alarm key fob and records in 720P @30FPS. Very popular for its small size and awesome quality for only 38 bucks. I then made a mount using parts from an old erector set and some powerful neodymium magnets.

0528121322.jpg
 
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rampage said:
A nice price AND it gives me an excuse to play with my erector set. :thumb:
Click to expand...


My thoughts exactly! If you are interested in getting one, make sure to buy it from ebay user " eletoponline365" He is one of the few to sell the authentic 808 #16 for $38 shipped. The battery lasts about 45 minutes while recording in 720P HD. every 20 mins of video is 1.1gb. He also sells kits with a cigarette lighter adapter for infinite power and/or with memory cards included for cheaper than buying it all seperately. The camera can also record continuously to double as an inexpensive dash cam. Its small and light enough that you can attach it to any remote controlled thing you can think of, even the micro rc helicopters for some cheap aerial footage. :D
 
How is your coil? I believe you can test them, but I don't know how ... other than zapping the heck outta myself...
 
Jessie James said:
How is your coil? I believe you can test them, but I don't know how ... other than zapping the heck outta myself...
Click to expand...

Its still the original coil, never changed it just the cap. It ran good before the rebuild, guess that would be worth looking into..


Took the truck for another spin today :D Different angle and less vibration this time. Noticed the passenger side exhaust is a bit loose.

 
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Try the carb thing I mentioned and see what happens.
 
If you have HEI,the module has a dwell advanxce curcuit built into it that kicks in around that RPM --some cheap aftermarket modules dont even have that curcuit,and you'll notice a drop in performance and MPG's if you run one of them..I prefer using GM modules for that reason....your pick up coil may have broken strands in its wires too,that can interupt the spark when the vacuum advance moves it too...hard to diagnose,replacing it is about the only way to tell...weak spark can cause a stumble under a load also...
 
Jessie James said:
How to test your coil:

http://autorepair.about.com/od/troubleshooting/ss/coil-test-ohm.htm
Click to expand...

That article mentions needing your service manual to find the numbers you need for your particular coil- if you have a service manual, it should also detail the testing procedure, in case you don't have the old school coil like in the pictures. I mean, they're all pretty fundamentally the same, but sometimes it helps to see a diagram that looks like the coil you're actually testing.
 
My mistake, I should have stated in the OP that its a 1980 Chevy K-20 350sbc. Stock HEI system. i've only changed the cap and rotor once before the engine was rebuilt. I remember the little carbon button and the plastic around the coil where the spring touches was a little melted and charred. I changed the cap and rotor and used a new button/spring, used sand paper to clean up the contact and it ran well before the engine was rebuilt. It has the coil under the cap inside the distributor.

I'm going to try Zimmer's suggestion first of disabling the vacuum secondaries, Then I will remove and test the coil. Any one know off hand what the resistances should be between which terminals? My chiltons book is 30 miles away. AFAIK the HEI system is completely stock.
 
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With what you said here, have you checked your cap and rotor? If they are ruined, that's a simple check and fix.
 
Update: Stumble is fixed! It was something with the distributor... Wasnt sure if it was the coil or module or incorrect mech/vacuum timing advance and I didnt want to replace parts and have it be something else. So I bought a brand new-ready to run distributor from Skip-White on ebay for $60 shipped. Truck runs great now and no more pinging with more advance! So I think it was a combination of things inside the distributor, hard to tell with a 32+ year old truck.

This was a such an awesome deal, a new brass cap and rotor alone would have been $50, module $30, coil $30, vac advance $12. Not to mention new mechanical advance weights and springs, shaft, housing, bushings, and gear.

dsc08767u.jpg


Now my next issue is figuring out why its so hard to start when the engine is hot...Its a fresh battery with 1000 CA. Feels like the starter is not getting enough juice when hot, but cranks perfectly and without struggle when cold.
 
The starter solenoid is getting hot with heat sinc near the header pipe. I have the same problem. Gotta get a heat blanket on the starter or make a heat blocker for it.
 
"HELP" makes a solenoid heat sheild that will help that a lot,also painting the starter with aluminum paint will help reflect heat away from it and keep it cooler too...
 
D4Me's dead on. Dorman (aka Motormite aka "Help!") makes a heat shield for the starter solenoid.

http://www.dormanproducts.com/p-3269-45629.aspx

I used to replace starters bi-annually. I've since switched to using these things and I haven't had to replace a starter since. Obviously you should use good starters (mine are Delcos) and shim them properly and check your wire connections and so forth ... but the heatshield is an integral part of the process.

-- A
 
Hey thanks for the replies! I will have to pick one of those up then. Or I have some thin gauge aluminum to try and make my own, just didnt realize this would make much of a difference. I'm using the stock exhaust manifolds.

I've done the ford solenoid mod years ago and before I rebuilt the engine It would crank super easy hot or cold...but the engine was so worn that I was able to grab the flywheel or harmonic balancer and rotate the crank without much effort....even with all the spark plugs still in it.

The engine is still fresh, probably has 400 miles on it total. The hard cranking/hot start issue is only apparent when I stop for gas. Feels like the battery going dead, my headlights and dash lights dim out almost completely and the starter feels like its struggling hard to turn the engine, but once it manages to rotate the crank once or twice it starts right up. I just noticed a little bit of corrosion on the battery cables near the terminals too, gonna have to investigate.
 
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