I have a 305 v8 1988 with a Rochester 4 barrel carberator. Rebuilt has 10 hours on it. I say hours because it is a boat engine. Was running smooth but seems to be missing. Brand new cap and rotor. And I have had it timed by a registered mechanic before this issue. When I give it throttle it moves but after a minute it makes a popping sound like it is back firing. I tried removing the a spark plug wire to see which cylinder was missing but could not tell a difference in engine sound or performance. It was worth a shot. Hooked up a timing light to each cylinder to search out a faulty wire coul not find out anything. All wires were showing a light but there definitely was inconstant in all cylinders. Hmm I’m at a loss. Any ideas or thoughts.
Hmmmm, fresh rebuild. I’d look to see if the distributor has turned and it’s out of time again. Does an '88 inboard engine still use points? Were the valves properly set? Has one of them tightened up? Loosened up? Has a lifter gone bad? Were the camshaft and lifters replaced as they should have been?
Is it popping back through the carburetor or out the exhaust? If it’s popping back through the carburetor under load you may have a burnt or sticking intake valve. In this case you need to do a compression test. You don’t set the valves in these. The lifters are hydraulic.
Edit: Correction - you do have to adjust the hyd. lifters.
Yes they are hydraulic, but the rockers still have to be properly screwed down during assembly. If not, the lifters won’t open the valves correctly.
Popping out the intake or exhaust?
There’s also the issue of rebuilt and rebuilt correctly.
Popping (either direction) can be caused by ignition timing, sloppy timing chain, timing chain not timed correctly, air leak, fuel system malfunction, timing not advancing correctly, weak valve spring, etc, etc.
What would I do as a first step? Put a vacuum gauge on an intake nipple. That’s cheap, quick, easy, and will likely give a hint as to the problem without having to spend a lot of time searching around.
Just a word on the rocker arms. There is no adjustment. You just torque down the rocker arm nut.
Edit: correction, you DO have to adjust the hyd. lifters. My error.
Detail adjusting rocker arms with hydraulic lifters.
Boats are different animals, maybe go on a boat forum for checking ignition module if you have one, as I conclude you seem to think you have an intermittent spark issue.
If you have or can borrow a infra-red temp gun or a digital volt meter with a thermocouple you can check each exhaust pipe’s temperature while running. This is easier with a boat than a car because you can put it in the water and run it under a bit of load. Pick a speed where the misfire is the worst and then start checking. The coldest cylinder is the problem child. Start there.
Were the cam and lifters changed at the rebuild? When the engine was first started did you run it at 1800-2200 rpm for about 20 minutes to break in the cam? If you didn’t, you may have wiped out the camshaft or a lifter or 2. If you idle the engine with the valve cover off of the bad-cylinder side, you may be able to see this if it is bad enough. If you even think this has happened, pull the oil filer and cut it open to look for metal shavings that a magnet will pick up. Heck, do that anyway at the first oil change.
I had the engine completely rebuilt top and bottom. All valves and lifters cam piston everything. The heads were install at the rebuild shop. The rest I assembled my self. This engine does not have a vacuum line. I did run the engine under load when first starting. If I blew a valve what is the easiest way to assess that is the most likely problem.
@Warrenfff does this engine had roller lifters? If so, you should be OK with the cam part. If it has a valve problem, a compression test is the easiest way to determine a valve problem. Unscrew the sparl plug and put your thumb over the hole and crank the engine. Compression should be obvious with a big POOF out the hole.
if it had the flat style lifters you probably damaged the cam with your break-in procedure. You never break in a new cam under load. It must be run for 20 minutes or so at 2000 rpm under NO load so the cam and lifters can wear-in together. If you have damaged a cam lobe, the valve won’t open far enough and the engine will run rough, like a mis-fire. Pull one valve cover and start the engine, don’t rev it as oil will squirt everywhere. Keep towels around anyway. Look at the rocker arms to see if one isn’t moving as far as the others and listen for clatter noises like a loose rocker. If you don’t find it on one side, replace the cover and repeat on the other side. The noisy rocker is your problem cylinder. The next step is to remove the intake manifold and pull that rocker and remove the lifter so you can see the underside and the cam lobe. You’ll know immediately if you wrecked the cam.
Check the fuel system throughly and make sure the system vents properly and are the plugs gapped properly?I hope you are using fresh fuel.
I would start by adjusting those hydraulic lifters, and yes they do have to be adjusted. With the engine running, you back off the rocker nut until the lifter starts clattering, then tighten down slowly 1.5 turns.
If you did not follow the correct pre-start procedure for the lifters, then you rotate the crankshaft aligning each piston at TDC for the compression stroke ad while at TDC, you loosen each rocker nut for that cylinder until the pushrod can be spun between your fingers, the tighten down 1.5 turns. Start the engine and readjust.
It helps to buy a set of shields for the oil hole in the rocker or you will get oil all over the place.
I STAND CORRECTED!
Yes, the hydraulic lifters on the small block V8s do have to be adjusted. It’s many of the 4 & 6 cylinder GM engines where no adjustment is required. You just torque the rocker arm nuts or bolts down, usually to around 25 ft/lbs. My apologies for the bum steer.
Here’s what I’d do as a start. Of course I know nothing about boats, so take that into consideration.
Measure the intake manifold vacuum. If it is not around 18- 20, post what it measures here.
Measure the ignition timing. I presume you’ve already done this at idle, but does it advance like it should as you bump the rpms?
Remove all the spark plugs. Are any of them different looking at the tip area than the others?And signs of cracks in the ceramic part in any of them? Look carefully, under a magnifying glass.
Check the compression. If 130 to 180, and all about the same, that’s not the cause. If not, post the results here.
It’s configured w/all new plugs, wires, distributor cap, and ignition rotor, right? And this engine doesn’t use points and capacitor, right? There’s no signs of carbon tracking on the inside of the distributor cap, right?
Run the engine in the dark, see if you see any sparks jumping around.
If accelerating the engine causes it to backfire in a regular rythmic pattern it is very likely that the camshaft has a worn out lobe. In a freshly rebuilt engine a worn cam lobe is indicative of poor procedures in prelubing and initial start up.
Another possibility is a rocker stud pulling out of the head.
Removing the valve covers would confirm or eliminate both those possible causes. Most parts stores have oil deflectors for small block Chevrolet engines which allow starting the engine with the valve cover off. Adjusting the valves is really easy with the engine running and a problem with the camshaft, lifters or rocker studs is easily seen when the engine is running while watching the rockers.
edit-of course rocker lifters are preferred in marine engines where sustained high rpm operation is expected. Cam lobe failure is rare with roller lifters.
Surely there’s a place on the intake manifold, throttle body or carburetor, etc where a vacuum gauge can be tied in.
A vacuum gauge will tell you in seconds if there’s a mechanical fault in the engine involving low compression/weak valve spring/cam lobe problem, timing chain mistake, ignition timing fault, etc.