I recently replaced my timing chain and gears, distributor, wires and plugs trying to get rid of a nasty backfire coming from the carburetor. I also replaced the fan with an electric fan and am wondering if this may be throwing off the balance on the cam and causing all my issues. I say this because she idles nice but when I give it a lot of gas she backfires out the carb. I set the timing dead on and made sure all the wires were right.
I appreciate any and all help before I pull the engine and rebuild her.
Did you check the all in or total timing advance as well as setting the timing? If the advance mechanism in your distributor is not functioning properly you might be getting too much advance at higher rpm.
Its a new vacuum advance. Maybe I am not connected to the proper vacuum on the engine . How can I test to see if the vacuum advance is giving me too much advance? disconnecting it and plugging it up, shouldn't this cancel out the vacuum advance?
Replacing the fan with an electric unit will not throw your camshaft off balance.
I've found most cases of backfiring out the carb is caused by valve timing or a lean mixture. Replacing the timing set should have cured the valve timing, if that was the problem. Your crank hub may have spun, giving you inaccurate timing marks to begin with. You can check the timing marks on your harmonic damper [balancer] with aid of a piston stop. Check your induction system for vacuum leaks.
Pull the vacuum advance hose off the distributor and plug the hose. Start the engine and adjust carb for a smooth Idle around 700 RPM. Attach a timing light, set the timing to 12 deg. BTDC. Readjust the carb if necessary. Rev engine to 3000-3200 RPM and observe change in the spark timing. Should go up with the mechanical advance, if so you know that is working. Return to idle then reattach the vacuum advance hose, the timing will go up to about 19-24 deg, if you have it on a manifold pressure port of the carb, depending on the advance can and the amount of vacuum you have available at idle. The idle speed should also increase. If the idle speed and timing do not increase, remove the hose from the carb and check if you are getting vacuum at idle, if not you have the hose attached to a stepped vacuum source. Attach the hose to a manifold vacuum source, that is a source that has vacuum at idle with the throttle plates closed. The idle speed and timing should increase with the hose attached to a manifold vacuum source, if not your vacuum can may not be functioning. With the mechanical advance working and the vacuum advance working you should get a total advance of between 29-50+ deg. Before I modified my distributor I was getting 9 degrees base, plus 20 degrees manifold plus 20 degrees vacuum for a total of 49.
If you're valve timing and your ignition timing are all working properly you may be getting a cross fire at the distributor due to a cracked cap or a cap with a carbon trail enabling the spark charge to cross from one tower to another and fire the wrong cylinder, similar to plug wire crossover which you should have eliminated with the new wire set.
Since you've changed the timing set, distributor, plugs and wires and still have the same backfire problem you had before, and assuming the new equipment is installed and functioning correctly and you have no vacuum leaks, I would say you may have an internal mechanical problem such as a camshaft with worn lobes or a spun crank hub. Or perhaps a carburetor or fuel delivery problem.
Thanks Al. I think its a carburetor problem. I hooked up a vacuum gauge to the manifold and it jumps around a lot. It wont settle. Vacuum is working to the vacuum advance, vacuum advance works, timing is set.
When I spayed carb cleaner in the carb it seems to stop the backfiring when I revved it up. This is why I suspect the carb.
Do you know how I set the fuel mixture screws? I know on other cars it was turn all the way in then back it out 2.5 turns. Is there a formula for a 1968 Pontiac 350 with a 2 barrel carb?
Jeff, I just read your other post about the difference between a 350 and a 400 distributor. I did not read it earlier as I don't know what the differences are other than the timing curve as others have stated. Seems you have two questions [Posts] regarding the same problem.
So far, if I read things correctly, you have a problem with backfiring through the carb, a classic timing chain scenario. I once spent from Christmas eve to Jan 2nd on the side of the road north of Campbell River waiting for the parts stores to open so I could buy a timing set for a Chev truck. Same problem, backfiring out the carb. Luckily we had a camper on the box, some fly rods, Salmon to catch and plenty of Christmas cheer to pass the time. So far you have: Changed the distributor to a used one off a 400; Put in new plugs, and wires; Installed a new timing set; Installed a vacuum gauge and have a jumping needle situation. You still have the same backfire problems you had before any of the above was done; You are considering a Moroso part to possibly alleviate your problem.
Please do not buy any more parts until you have diagnosed your condition and know, within a certain amount of confidence, what the actual cause of the condition is.
If your vacuum gauge is showing a needle that drops and comes back to the normal reading every couple of revolutions, that could be a sticky valve situation, The needle jumping around and not settling could be an indication of a blown head gasket, as well as camshaft lobe wear.
Have you done a cylinder compression test? Do you have any oil in your coolant, do you have any coolant in your oil?
Setting the idle mixture is different depending on the carb and what you have for accessories. Some start with two turns out, some with five turns. You are able to start the engine and let it idle so, set the idle speed to 700 RPM, turn the left mixture screw in or out until the highest vacuum in obtained. Reset the idle. Adjust the right mixture screw as you did the left. Continue adjusting the two mixture screws and the idle screw until you obtain the highest vacuum possible.
Eliminate the easy stuff as possibilities. Your timing, vacuum leaks, harmonic balance slip.
Also your idle screws only help with good idle not with the transition circuit. You can verify a good pump shot looking down the carb and twisting the throttle open. It wouldn't hurt to rebuild the carburetor and check that you don't have any plunged ports. Might be worth while to just pull the carb top off to see if you have a bunch of crap in the bowl.
However from what you describe with the vacuum gauge I'm with Al. Pick up a compression gauge or rent one at Autozone and see what you get the motor should be up to temp and do the compression check with the carb wide open.
I would also suggest a leak down test. Harbor freight has a cheap one that will do the job. Check out some youtube video on how to use it if needed.
The valve spring looks good and the push rod is good. I swapped the rocker arm, and push rod next to it and same thing. To take the spring off to see if its the problem will be a job as the AC is butting up against the head, had to the rear one....
So final thoughts before I tear the engine apart? Cam for sure?
Hmmm, Can't tell for sure on the video but it looks like the pushrod is spinning but not moving up or down. The cam lobe will rotate the lifter every time it sweeps the bottom, if you have standard/hydraulic flat tappets. When you swapped the rocker and rod from the intake was there any wear on the bottom of the rod? Did you adjust the rocker with the engine running? When you pushed the rocker down with your finger did the rocker then move up and down? Are you sure the valve isn't stuck open. You could remove the rocker and tap the top of the valve with a dead blow mallet and see if the valve opens and then closes again. Only do this with the piston on the bottom of the stroke as to not hit the top of the piston with the valve. 'used to do it with running truck engines but they had gobs of clearance at TDC. Any oil coming out of the top of the rod?
If the rod, valve, spring and rocker are all good and adjusted correctly, one can only assume the problem is the camshaft or the lifter. The camshaft lobe may be worn down to next to nothing, the lifter may not be holding oil, or the lifter may be stuck in the lifter bore and not dropping back down with the lobe. The lifter may be cocked in a worn-beyond -limits lifter bore or maybe a piece of flashing or some other foreign matter is caught in there keeping the lifter up. That's kind of a long shot, I'd say a worn camshaft, but like I said I'm not an expert, these are just my opinions. Most likely someone with more knowledge of these matters will diagnose the cause for you.
The push rod is not moving up and down. When I pushed the push rod down towards the lifter and it did moved up and down but only very slightly. Push rod is not worn at the bottom.
I did adjust the rocker with the car running. The looser I made the rocker the more it backfired out the carb. Is that a sign of something?
Ill check if oil is coming out of the top of the push rod. If the cam is not pushing the pushrod up, the valve being stuck should not be suspect yet correct? Since is not engaging the valve. I think my next logical step should be to remove the intake manifold and take a peak at the lifter and cam. Correct?
That looks familiar! Just went through a similar situation this summer. Mine did not back fire but sounded like light pinging or valve float. The rocker beat the valve spring retainer so bad it barley held the spring. Could have destroyed my original block in a heartbeat. Dodged that bullet. Improper cam break-in will round a lobe pretty quickly. Make sure you follow the cam manufacturers break-in procedure or you'll be right back where you started. Good luck
I have a stock 1968 Pontiac Firebird with a 350 engine. I believe someone replaced the cam in the past and did not break it in right. The #8 exhaust lobe is rounded and the lifter is mushroomed. It appears the rest of the lobes have excessive wear as well. I want to put an as close to stock cam with maybe a little more performance keeping the rest of the car stock and lifter kit in the car. I believe the previous owner may have installed a bigger cam and did not compensate for it and/or break it in properly and so this cause the rounding of the lobes and mushrooming of the lifters.
Anyone have any recommendations? I may add headers later but for not I have stock exhaust
