oiliver 770 misses

we have replaced the plugs,wires,coil points,condensor & had the carburetor rebuilt we cannot find this miss???? it is driving us crazy

And it's not a burned valve?

First thing to do is find which cylinder it is. Once you do that check the compression on that cylinder and see if it is lower then the others. Could well be a valve that need to be adjusted or a bent push rod so the valve does not open etc.

we just had the head gone through by a machine shop

When you find out which cyl it is, switch
spark plugs with a different cyl and see if
the miss moves

Just replacing parts ?willy nilly? is not usually the best way to sort out a problem.

You ideally first need to know if it is ignition or fuel related.

Faults which you have likely overlooked could be the distributor bearings, the distributor cap, air leaks into the manifiold, broken valve spring,
sticking valve or even a worn cam lobe (distributor or engine). Even a loose spark plug or electrical connection could be the culprit. Loose coil
connectors or a low tension break in continuity can cause intermittent misfires, which can often develop to a full ignition failure. Even corrosion
build up between connectors can cause problems.

You don?t mention whether this misfire occurs at idle, full throttle, or over the whole range. Whether off or on load, cold or hot engine. Is the
miss intermittent or regular? All these details can make diagnosis from a distance somewhat easier.

Most certainly, ?Old? has a point re checking where the fault lies. Checking the plug insulator colour is a good starting point if the engine can be
run steadily for sufficient time to allow differences to show (if indeed it is only on one cylinder).

I would not be surprised if a valve is sticking, as it appears to have become apparent after the head has been overhauled.

Been there & fixed it. Bad distributor bearings. Sometimes it idled good but at other speeds it had a nagging miss. What found it was an old ?70s tune-up scope.

You have a lot of excellent replies to your post, follow them out and you should be OK.

If that is the case good chance you have bent push rod or one of more valves that you got the tappet setting wrong or you got a push rod installed wrong seen all 3 happen to people when installing a rebuilt head

I'm not sure it's the most accurate method, but we use the ol' grab the exhaust manifold test at startup to figure out which cylinder is cold and therefore missing.... They have neat little laser guns for that stuff now.

Last non-electrical miss I saw was caused by a pushrod that had popped/slipped/moved/somthin'd from the rocker...

Hey Old,

We don?t know whether the misfire is on one pot or not, whether continuous or intermittent. I am guessing it is intermittent or it would be so easy to sort out which pot was the offending item.

OP is not helping the diagnosis process. Guesses are pot luck without corroborative evidence. I gave a fairly exhaustive list of possibilities, but there may be more, and no response of further info.

I?m not one for imposing a fix without actually being as confident as possible (and certainly not infallible, either!) that the fix fits the symptoms reported. It could yet be fuel related....

RAB

My 1550 gas started missing and backfiring,got a good used Distributor off ebay put it on started it up and ran great.

Yes something along the lines of the manifold not install correctly or the gasket not being on correctly and causing it to suck air

That works on some engine but on many the exhaust ports area is shared with 2 cylinders. The better way is the hot plug test. Cold plug means plug not firing or cylinder not firing

Gotta love the parts swappers. Parts sellers love them too.

Actually it can be the best way to trouble shoot sometimes,I keep a box full of used but I know are good ignition parts and when I have a problem I can't figure out quickly I just exchange
one part at a time.Old part goes in the box if that's not it and leave the part out of the box on the tractor.Total $$$ a little time and $0.00 out of pocket until I find the part that's actually bad.

A vacuum gauge would be a good place to start.