seeking answers to endless surging issues

[keakar]

i have 3 engines all doing the exact same thing and i rather not scrap them if at all possible

they are 19.5hp intek, 17.5 hp intek, and 15.5 hp metal top and all are B&S

on all 3 i have used fresh new gas in all of them to be sure it wasnt bad gas, i replaced the carb with a new one (more then once in case ebay carbs were bad) and then i swapped with a known good working carb from a smooth running 18hp intek and it was still surging with the known good carb. i tried with or without the air filters as well as partially blocking the intake with my fingers to richen the mixtures yet nothing changed and the surging continues

on all 3 i have checked the oil levels, flywheel key, and reset the valves to 0.004 intake and 0.006 exhaust and replaced intake manifold gaskets and/or rubber rings and reset the governor arm setting yet the surging continues

at this point i have resorted to just pure guesses that all it could be is a worn cam causing it since the only known cures that i am aware of were tried but dont fix the issue

in the past i just threw engines like these in the scrap bin because i felt they werent worth the trouble to spend any more time on but i hate throwing money in the trash if someone can clue me in on what i am over looking here because i have no idea what the issue is but it seems to be getting more common i see it

update: i got compression readings of 100 psi on 3 separate tests i ran today on the 15.5 hp metal top engine 28n707-1173-e1

 

[Scrubcadet10]

Intake tubes free of cracks.. Not sure if those have plastic or metal intakes...

 

[Hammermechanicman]

Start engine and hold throttle butterfly closed. Will engine run at idle? May have to speed idle screw up a bit. If engine won't idle you have a air fuel mixture issue running on the low speed circuit. If engine will idle smoothly holding butterfly SLOWLY open the butterfly and see if engine smoothly advances RPM up to around 3000 RPM release the butterfly and see if the engine RPM bounces (surges)on the govenor. If it does check linkages and govenor arm adjustment. If engine does not run smooth holding butterfly steady you have an air fuel issue or valve issue.

 

[slomo]

Slow down and revert to the basics like some air, fuel, spark and a dose of compression. The guys above gave some great tips.

Head gaskets good? Those Inteks are bad about those. Sticking valves in the valve guide? Remove a valve spring and see if the valves run smooth in the guides.

slomo

 

[keakar]

Intake tubes free of cracks.. Not sure if those have plastic or metal intakes...

the 15.5 hp has the cast aluminum intake, the others plastic

i checked them all carefully and even tried swapping it with a spare i had and can find noi leaks on the intake system anywhere on any of the 3 engines
i just cant figure this out

 

[keakar]

Start engine and hold throttle butterfly closed. Will engine run at idle? May have to speed idle screw up a bit. If engine won't idle you have a air fuel mixture issue running on the low speed circuit. If engine will idle smoothly holding butterfly SLOWLY open the butterfly and see if engine smoothly advances RPM up to around 3000 RPM release the butterfly and see if the engine RPM bounces (surges)on the govenor. If it does check linkages and govenor arm adjustment. If engine does not run smooth holding butterfly steady you have an air fuel issue or valve issue.

the engine will die at slow idle holding butterfly as per your instructions, anything short of full throttle makes it die.

by holding the governor arm it runs smooth and steady but only at a fast full throttle rpm and holding the governor arm

if the engine stops, even when i "think" its warmed up, it still must be choked to start it again

im limited for space so i can only setup to test one engine at a time and the 15.5 hp is the one i need right away so it is the one currently hooked up to test. if you need me to test the other motors, i need to pull the 15.5 hp out of the mower in order to do it

 

[keakar]

Slow down and revert to the basics like some air, fuel, spark and a dose of compression. The guys above gave some great tips.

Head gaskets good? Those Inteks are bad about those. Sticking valves in the valve guide? Remove a valve spring and see if the valves run smooth in the guides.

slomo

thats what im trying to do here slomo, so i came to the experts because its a bit over my head on this one

i confirmed spark, compression (100 psi), and fuel, sealed intake system with no leaks, they all start up easy and run fine, albeit surging while doing it, and

i operated the engine by hand when setting the valves and the travel and operation of the vales seems perfectly normal

 

[Rivets]

You say that it is happening on three engines. If that is so, what would be the most common thing each engine has? Your fuel. I would take a dry gas can and fill with fresh non-ethanol fuel. Remove all existing fuel from carb and hook up a remote tank. Run each of the engines this way and see if there is any change.

 

[keakar]

You say that it is happening on three engines. If that is so, what would be the most common thing each engine has? Your fuel. I would take a dry gas can and fill with fresh non-ethanol fuel. Remove all existing fuel from carb and hook up a remote tank. Run each of the engines this way and see if there is any change.

thought i had mentioned that as well, yes i used fresh new gas in all of them to be sure it wasnt bad gas

also its the same exact gas i am running in several other engines with no issues at all

and just to clarify, this isnt just something all 3 engines started doing, these are 3 out of about a dozen engines i have worked on recently and these engines had to be pulled and replaced because of the surging issues and the replacement engines ran fine with the same fuel and fuel systems and in most cases used the same carbs from the surging engines that ran fine on a different replacement engine. so whatever it is, it seems to be engine specific not related to anything having to do with the actual mower or its fuel systems

i really am stumped on this

 

[keakar]

update: i got compression readings of 100 psi on 3 separate tests i ran today on the 15.5 hp metal top engine 28n707-1173-e1

hope that helps