24HP V-Twin B&S - Fault driving me CRAZY!

OK .... New Carb fitted but the problem is still the same :frown: .... I think we are "barking up the wrong tree" any other thoughts what could be causing the random popping?
Also, yesterday I broke the drive belt an in my confusion I switched off the ignition and when the was at full RPM. When it had almost completely stopped there was a HUGE backfire (like a firecracker!) and flame flash on the starter motor side of the engine. Not sure exactly where the flash came from, but most likely the exhaust box :confused2:

Any thoughts gents.

Put a run of masking tape around the flywheel so you can make fairly accurate marks on it.
Stick it on your arms first to denature the glue so it comes off easily.
Remove the plugs, check the valve lash then mark the start of lift , max lift , end of lift , TDC & BDC for each valve & cylinder.
Use 2 runs of tape , one for each cylinder then use the TDC marks to align the two pieces of tape .
The lines for each action should align, the space from start of lift to end of lift should also be the same for each valve & each cylinder.
To measure start & end of lift, you rotate the push rod between your fingers and take the start & end as the point where the push rod just can not be turned.

I think you will find the duration of the exhaust cam is shorter than the inlet and if too short it prevents all of the exhaust leaving the cylinder so when inlet opens there is still pressure in the cylinder which blows back through the carb.
At high idle the volume of the fresh charge is quite small till you put a load on the engine & the governor allows the throttle to open up wide .
In this situation there is not enough time for the exhaust to fully evacuate the cylinder.

Animals dead in the exhaust system will do the same thing,
We had a muffler that just would not flow so the customer replaced it.
Cut it open & found the remains of around 2' of whip snake in there, oddly it did not stink like dead snake usually does.

Sounds complicated, but you can do both valves on both cylinders in less time that it takes to find somewhere to fit a degree wheel and pointer.
Use a bit of dowel to measure TDC & BDC and mark the position of both cylinders.
If it is not the same L to R then one rod is bent.

Wow Bert, Thanks for this. Will get around to doing it all in a week or so.... It will be a great learning experience if nothing else :laughing:

FWIW I look at the flywheel rotate a few times & find a place where I can use the side of a coil to mark the tape.
I use Stanley knife blade if I can, laid against the coil then run a 2B clutch pencil line along the edge of the blade.
Varies a bit from one engine to another.
When I kicked off I used to measure cams using a timing disc as one would do on a motorcycle engine.
Then found cam timing details almost impossible to find and when you did, you needed to reset the valve gap to specific sizes so all the numbers did was got confusing.
You can use the flywheel teeth to sort of get numbers ( second ) try .
Rang a friend who marked the flywheels with a paint pen but that just got too confusing with a twin & the pen marks were a bit coarse, then came up with masking tape.
The auto stuff is really good cause it pulls off really well but it s a bit thick so does not pass under the magneto legs particularly well, but the cheap K-mart stuff does.

AFAI can work out all of the common engines use the same cam profile inlet & exhaust.
Kohler actually use the same cam some of the moe qualified techs might correct this or provide lift & dwell numbers but these engines are really basic so just laying the tapes side by side is generally good enough.

All the posters who list as B & S Master Techs should know or be able to get numbers but I found numbers just clouded the matter.

Hi Bertsmobile1,

So firstly I compared how far all 4 valves get depressed and they are all pretty much the same 17mm.

I couldn't get narrow masking tape so I did things a bit differently. Using the inside sharp edge (of metal U) of the Left Coil I used a fine line pen to mark 6 points on the Flywheel TDC, BDC ExhOpens, ExhCloses, InlOpens, InlCloses. Then I compared that with the Right Coil, same measurement and the same point on the Right coil. Any differences I measured with a straight edge ruler ....

• Left-TDC compared to Right-TDC - Exactly the same
• Left-BDC compared to Right-BDC - Exactly the same
• Left-ExhOpens compared to Right-ExhOpens - Exactly the same
• Left-ExhCloses compared to Right-ExhCloses - Right Closes Earlier 25mm
• Left-InlOpens compared to Right-InlOpens - Right Opens Later 31mm
• Left-InlCloses compared to Right-InlCloses - Right Closes Later 13mm

From my reckoning this does not actually tell me which valves are out of time but it does prove that Left and Right Valves do have different timing points which I think means I need to replace the Camshaft?... Found this great Youtube Video ... you can skip the first 3 mins ... https://www.youtube.com/watch?v=fmbwImeiqaE

Let me know you thoughts guys?

All the best.
Neo

If you've decided to replace the cam, that video by Taryl is great !

In case ya never saw Taryl & sons out of their wacky video garb....

Hey Neo welcome back Mon Ami ....... I am catching up on some cleaning in my house for Turkey Day and I decided to check on the forum...... Shut up Boo Boo LOL.....

Neo if your valves are acting like the ones in the video then yes it is a possibility that it's the same thing as in the video....

Take your plugs out and spin the engine by hand to confirm it about 10 times before you even think about splitting the engine apart .......

Most of us on here do watch Taryl and his crazy antics..... Too funny most of the time and then corny a lil of the times LOL......

Yes a worn cam does happen and the ACR's on the single cylinder series go out a lot of times.........

Let us know Mon Ami ~!~!

don't know if anyone addressed this but shutting the ignition down at RPM fills the exhaust and muffler, as well as the air at the end of the muffler with a ready to ignite mixture of gas and air. All it needs is a spark of static, piece of glowing carbon etc to set it off. That is probably the flash you saw. I used to have an old ford falcon with a rusty old exhaust system (and generator, non locking column, & stick shift that allowed me to do this). I'd turn the key off for about 5 to 7 seconds when someone was tailgating, turn the key back on and the exhaust would send out a shower of carbon and rust flakes that guaranteed they would not tailgate. Sounded like a small cannon going off too when I'd do it. That was almost 50 years ago. I'd be afraid to do that on a modern car for fear of destroying some expensive sensor, computer, or the alternator.

Yes Taryl stuff looks good and the photo cleared up a few things too lol!... Thanks for that ;-)

This is 24HP engine (similar to all the other B&S V-twins) so I'm guessing this is a high lift cam which is technically more prone to wear?

Neo7 said:

Hi Bertsmobile1,

So firstly I compared how far all 4 valves get depressed and they are all pretty much the same 17mm.

I couldn't get narrow masking tape so I did things a bit differently. Using the inside sharp edge (of metal U) of the Left Coil I used a fine line pen to mark 6 points on the Flywheel TDC, BDC ExhOpens, ExhCloses, InlOpens, InlCloses. Then I compared that with the Right Coil, same measurement and the same point on the Right coil. Any differences I measured with a straight edge ruler ....

• Left-TDC compared to Right-TDC - Exactly the same
• Left-BDC compared to Right-BDC - Exactly the same
• Left-ExhOpens compared to Right-ExhOpens - Exactly the same
• Left-ExhCloses compared to Right-ExhCloses - Right Closes Earlier 25mm
• Left-InlOpens compared to Right-InlOpens - Right Opens Later 31mm
• Left-InlCloses compared to Right-InlCloses - Right Closes Later 13mm

From my reckoning this does not actually tell me which valves are out of time but it does prove that Left and Right Valves do have different timing points which I think means I need to replace the Camshaft?... Found this great Youtube Video ... you can skip the first 3 mins ... https://www.youtube.com/watch?v=fmbwImeiqaE

Let me know you thoughts guys?

All the best.
Neo

Click to expand...

Good now we are getting somewhere.
The lift & duration should be the same on all 4 valves.
If the valve lash was very carefully set then it looks like right exhaust cam has significant wear.
Closing earlier will choke the cylinder at higher speeds cause all of the exhaust might not get out.
Also looks like the right inlet has also worn.
If it intentionally has a lag ( oft done with V twins to smooth out the running ) then one would expect the lag to be the same for both opening & closing.

Measure the circumference of the flywheel & convert your mm to deg then check with a Briggs dealer because the data is always given as deg from TDC.
I do not have access to Briggs cam details but it does look like you have a cam wear problem.

Double check the height of the valves from the floor of the rocker chamber, they all should be the same.
However if a guide has slipped then the valve would open latter & close earlier ( or visa versa ) .
There are a couple of Briggs master techs on here, Walt is particularly good.
The right exhaust cam is worn