OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines briggs quantum 35 knocks when under load

hey guys picked up this old rover craftsman with a brigs quantum 35 engine, i had to lap the valves and increase lash cause they wernt sealing properly, while i had the head off i noticed two deep gouges inside the bore identical on each side almost like a piston pin had come lose but they wernt inline with the pin so dident inspect i just put it back together and its a runner apart from this knock under load,
has anyone els had the same problem ? and might know what it might be b4 i rip it apart again
would a worn crank bearing cause this and maybe send the piston off into the walls of the cylinder under load??

Steptoe, the Briggs engines normally have a full-floating gudgeon pin, which means it is located axially by circlips at both ends. The pin can't move toward the bore unless somebody had left out the circlips, or put them in so badly that they popped out, during a previous overhaul. (Most of us have probably seen this happen, however - I certainly have, with car engines.) Hence end float of the crankshaft, or lateral slack of the big end bearing on the crankpin, should not result in score marks down the bore.

If it has a scored bore, I can't understand why you don't just salvage a few parts and throw the rest away unless it is a rare model of interest to collectors. The cost of reboring and buying an oversized piston greatly exceeds the cost of picking up a better Briggs engine from a nearby nature strip. It can't operate satisfactorily with deep scores down the bore.

thanks grump.. i relise it couldent of been the pin cause the gouges are not in the same alignment as the piston pin !!! but when i said gouges in the bore you say scoring its actualy not scored these 2 gouges on each side are canyons and after looking at it internaly id prefure to stick to rebuilding the two strokes full crank victa's (atleast the they have removable barrels and no cam alignment) and yes i do agree with you i should just scrap and get a new motor or something , but if it only costs a few bucs for a new bigend bearing then ill do it the mower was a freebi.... the experiance is worth more than mower plus i recon the newer motors from briggs isent up to same quality as it was 20 year ago ... poor quality alloy id asume and poor workman ship ect. to keep cost down

but if you recon its not lateral slack in the crank bearings what could be causing the knock under load the thing sounds great at 3500rpm until you hit some ruff stuff, would to much valve lash cause it to happen ?

I would prbably blame poor maintnance for the gouges, it seems quite common when grass has been allowed to collect in the cooling fins of the engine. If its not using oil, I would run it a few times and leave it be. I hava a 5hp quantum that has the same issue for over 3 years in one of my main fleet that get used all summer and still has not got any worse.

The crank bearings and conrod bearings are not replaceable in these, they are cast into the block/conrod.

Hope that helps

If it only knocks under load, not with no load at the same speed, it sounds as if it may be detonation. That shouldn't happen in a Briggs unless the flywheel key is sheared and it has ended up with the ignition advanced. Does it ever kick back on the pull-start? If it isn't detonation it is probably a loose big-end bearing.

cheers guys thanks for the replys
that sucks how the crank and conrod bearings are not replaceable just like it sucks how you have to split the crank case just to get the piston out there is no kick back on the starter rope but ill check the flywheel key just to be sure thanks fellas

Hi Steptoe, we are starting to build a good archive here... lets get to the bottom (no pun intended) of this prob and see what causing this; grumpy is the Briggs guru, probably the most simple explanation will be the correct one and with his help, we will find it.

After you've checked the key steptoe, if it turns out to be intact, it would help both your question, and our archive, if you take a look in the crankcase. That way everyone will know what it was, even if the engine is beyond cost-effective repair.

To help clarify the issues, severe detonation usually develops suddenly after an adjustment or mishap and makes a sharp clacking sound that is rather distinctive when you've heard it, but hearing it is usualy followed immediately by a broken piston and no further sounds at all. Loose big-end bearings normally start off with a slight rattle which is less noisy than normal Briggs tappet noise, but it gradually gets worse if you don't attend to the cause, which if it is an older engine, is nearly always lack of oil in the sump. If an irresponsible owner just ignores the noise and keeps running the engine at low speed and low load, it may advance to the stage of a loud rattle that dominates other engine sounds.

An anecdote that probably won't help but will make me feel as if I've explained. Long ago my grandfather had a little OHV Morris Minor. One day my father happened to drive the car, after no one but grandfather had done so for months. There was an appalling clatter of big end bearings - really loud - and no oil pressure. My father remonstrated with his own father, saying "Surely you must have noticed it developing that noise?" Grandfather replied, "It's all right if you're careful not to open the throttle much". The previous owner of your Quantum may have been a person like my grandfather.

A supplementary anecdote, about high speed detonation and how fast it kills the engine. I was driving on a Melbourne urban freeway some years ago when a Brock or HSV Commodore went past, flat out in third I think. Just as he passed me the car disappeared in a cloud of exhaust smoke - he instantly shut it down and coasted into the emergency lane. Looked like he had just been working on the distributor - that's how it usually happened, some genius decided he needed more centrifugal advance, and made his first test of the concept at full throttle, maximum engine speed. As I explained to my wife at the time though, in those days of fairly cheap, carburetored homologation-model racing Commodores, the young guys who bought them got most of their jollies from bragging to each other about their blow-ups. (High speed detonation normally breaks at least one piston within one to two seconds, before most people can identify and react to the sound.)

I have a 4Hp here with a pretty good knock.

Runs ok its just rattly. Best i can work out its a loose cam drive gear. Or something in the valve train.

If it sings sweet and full song but its knocking at low revs i would give it a good test. Mow a good area of heavy grass and see how it does. If it ok it will be a good mower for someone.


Bob.