Misfiring 350 GP
 

I've been going through the newly acquired 76GP , and I can't figure out the missing.
The previous owner stated that it would miss on start up and then clear up. I did drive it 500 miles it had no power .
Now it pops and backfires, and no clearing up.

I have new spark plugs and leeds.
The timing is OK as I put a timing light to check it but I may need some advice on doing it properly.

The distributor cap and electronic ignition are new from the previous owner.

It will backfire through the carb and pops out the exhaust.

I know it's a *****ie 350 with a possible 120hp.
I will need to replace it in the future but for now I just want to get it driving.

The carbie is new and is made in China with no brand name.
It still has the big catalytic converter in place.
At one stage on the drive home, the engine had a loud ticking noise which did go away.

Do you suggest I remove tappet covers and have a look what might be going on and do a compression test next?
John L






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Do you have another carb to try..?

Yes I have 2.
I picked up some cores and have intentions on doing some research as to what carbie to use and parts required.
I can purchase a cabie from Firebird 350 supposedly rebuilt , Owner wants a premium price and is 200 miles away.

I will start asking the carbie questions next week as I plan to remove old china. Photo. Has a lot of silicone to seal to manifold, I am a bit scared what else I might discover.


I have plugged all the vacuum lines to AC and other things,not vacuum advance or brakes or automatic and the auto choke vacuum line that connects to a pipe on smog manifold is loose.
I did wire open the automatic choke as it was not going to open fully and was headed for fouling plugs.
JL

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Any tapping noise? Wonders if you have a wipe exh cam lobe? Back firing in carb could be cam/ cracked dist cap(easy ck) or very lean mixture. Do double ck your firing order too.

^^^^^^^^^^^^^^

Flat cam, and firing order would be my first things to check if I were troubleshooting it. There of course dozens of things that could cause the problem, but from my past experience those would be the standout things I'd check first. BTW you can have a flat cam lobe, and still have good compression in a cylinder, I would run it at idle with the valve cover off to confirm.

Check the distributor cap for carbon tracking. Also check the center of the rotor under the contact and on the under side. They will very often burn through if the plugs or the wires have had high resistance.

Thanks for replys, this weekend I will check the basics, firing order, inspect cap and button, then I'll remove tappet covers.

Am I going to see no movement or very little movement on a tappet ??

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Usually very little movement, over 50 years I've only seen one, or two that were not moving at all. Usually the rocker arm falls off when they get too loose.

If there's silicone sealing the carb to the intake. THAT could be most of your problem.

I don't think you mentioned how many miles you had on your motor, but I would also check out your timing chain as well, loose chain can cause all kinds of erratic engine issues.

So I went back to basics in me chasing a misfire.
Dizzy cap looks fairly new , no signs of ware on the copper points , dizzy button has no burn or heat stress, button looks fairly new also.
I did new plugs and wires already.

I am holding off from removing the carbie as it will mean spending good cash on a 2 barrel carb, which, if it's not the problem it a simple waste of time and money.

I have removed the valve covers, and everything is in place, I was hoping to find a broken something.

Now I have to check out if the timing has jumped a tooth.
So the bush mechanic in me says
Number 1 @ TDC should see the tappets rocking,

It can be on 1 of 2 rotations of the crank, the piston coming up closing the exhaust, and then opening the intake valve .
At that point, the timing mark on the balancer will line up with marks on the engine, am I correct?

Any tips on doing this single-handed ?
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Both valves will be closed is correct. Remove #1 plug wad up a ball of paper and sit it on the plug hole. As you turn the engine over by hand it will blow the paper off of the plug hole, you're on the compression stroke. While you're removng plugs you might want to look at all of them to see if there's one that looks particulary bad, narrows down your search to 1 cylinder.

Did you idle the engine with the valve covers removed? That's the best way to see if you have a lazy cam lobe, you're not going to see anything by just examining the rocker system with the engine shut off, unless the rocker arm has fallen off completely.

You're also going to be looking for push rods not slowly spinning. When they aren't spinning the cam, and lifter is going flat, also for sluggish movement as they aren't moving all the way to open the valve as the cam no longer has full lobes left on it.

Rebuilding a Rochester 2 bbl is pretty simple, follow the directions in the kit, or a manual detailing how to disassemble it and re assemble it, setting the adustments, and getting sediment is paramont, then adjusting the idle mixture screws to get the smoothest idle.

Pulling plug wires one at a time at idle will narrow down which cylinder is missing, then you have narrowed your search to usually 1 cylinder.

Hope maybe something helps you narrow it down.

Thanks Sirrotica
I have only turned the engine by starter motor, and appears to be fair movement of all rockers .
ok, so to determine if the cam chain has jumped a tooth, which appears to be a common problem , can I expect to see the timing mark on the balancer way past TDC?.

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You could remove the distributor cap, spin the crank by hand to where the engine in on TDC. From there, spin the crank by hand in the opposite direction and see how many degrees of play are in the chain. If no play, I doubt the chain hopped. While it's running you could also connect a vac gauge and watch the gauge at idle. If it bounces, you have a compression/valve/ cam issue.

Just thinking also, if the engine hasn't run well for a while, I wonder if you could have a clogged cat. That probably has the old pellet style cat where if they were over heated, they could melt and clog inside. You can drop the pipe and see if it runs better.

The chain jumping isn't going to cause a miss, but the engine will be way down on power, if that in fact did happen. When the cam is out of time, it kills all the cylinders power, not sellectively on one. Likely not what you're looking for.

Taking the distributor cap off, and turning the engine forward, and backward by hand is going to let you see if the timing chain has a bunch of slop when reversing directions.

If the engine is shaking at idle in gear, your going to be looking for a weak cylinder, not a jumped timing chain. Could be a fouled plug, or a bad plug wire, stuck/burned valve, etc. Valve timing being off doesn't cause a miss, just a loss in power overall.

Idling the engine, with a vacuum gauge attached to manifold vacuum is going to likely see a gauge with a twitch as it idles for a miss. Stock engine should have 15-20 inches of vacuum, and a steady needle.

A vacuum test, and a compression test, will tell you more about the engine health, and point you to your problem. Putting the engine on TDC isn't going to tell you if the cam timing has been affected because the cam timing is not shown with TDC, that shows the position ofthe crankshaft. The only way to verify cam timing, is to take the timing cover off to see if the timing marks on the cam, and crank are off. If a timing chain jumps one tooth, the engine will run, but very, very, sluggish. Manifold vacuum will also drop way down as shown on a gauge, it won't cause a miss as in one cylinder not carrying it's load.

Running a compression test is also going to verify that valves and seats, as well as ring seal is adequate.

From your description, I doubt your problem is timing chain related.

An update on my progress in finding the miss fire.

I did remove the valve covers and had the engine running. All pushrods are spinning, some more than others but all spinning, nothing obviously broken in valve gear as I was hoping to find.

I did check out the disy cap and have a good spark from each wire.

I did a compression test. The lower was number 6 at 85 psi, and the higher was number 1 at 110 psi.

The next test this weekend will be

1. Get engine running and spray around inlet manifold with carbie cleaner to see if a leak may exist.
2. Get 1 gallon of gas and set GP on fire.

3 .

To recap, a very noticeable misfire, pops, and backfires out the exhaust, new wires ,plugs ,has no name 2 barrel carbie.
I'm not giving up yet.
John L

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Good job working through this, it can be frustrating and there's no diagnostic computer spitting error codes to guide you.

You are getting good advice from very knowledgeable people and I'm sure they will comment on your results.

To me, 85 psi sounds low and the range from 85 to 110 sounds like a lot of variation.

I'd personally repeat this measurement before looking more at ignition and carburetor.

I also have a worn out 350, original 2-bbl from 1970. I don't think the compression ratio was significantly higher in my engine than yours and my compression test measurements are all between 120 and 130 with a Summit 2800 cam that has just a little more duration than the original.

That is way low,I'd get some 30w oil and a oil can pumper like this,
https://m.media-amazon.com/images/I/..._AC_UL320_.jpg
Pull the plugs again squirt a few pumps of oil in each cylinder,re-do the compression test with throttle held at wide open.
See how far up the numbers go.
That engine sounds tired and worn rings.
Good Luck

If there is no change look into worn valve or seats.
IE; valve job.
Also recommend a leak down test.To help confirm.
https://www.harborfreight.com/cylind...ter-64918.html
https://i.postimg.cc/MMdY6y54/64918-W3.jpg