Briggs 18HP Vanguard Valve Adjustment

[Joker]

Hi all.
My Argo Response with 18HP Vanguard engine was running rough, had little power, backfired under load... I decided to check the valve clearances. At 1/4 inch past TDC the clearnaces all measured 0.012-0.018" - way too loose. I set them to about 0.005" and the engine ran a lot better (good power, no bogging on turns, smooth idle...)

Then, about 4 hours (run time) later, it started running bad again (same symptoms). So, I pulled the valve covers off again and found that there was no clearance in any of the valves. The springs were always somewhat compressed.

So, I adjusted them again. This time, before I put her back together, I pulled the starter cord a few times and then re-checked the clearances. They were too loose! I re-adjusted a couple times before I could actually spin the engine and not have the clearances change. Once I was able to do this, I put it back together and ran it, but it still runs poorly.

Tomorrow, I will recheck the clearances as well as try to pull off the black plastic cap over the carb adjustment screw (in case the cold weather is causing a lean condition that is confusing the matter).

I am wondering if I need new valve springs, or if the valves are sticking. Either way, I have never fixed either problem, so I would appreciate any suggestions.

By the way, one more oddity is that when I pulled the spark plugs, each one was half "dark gray" and half "new". These had about 4 hours on them.

Thanks in advance.

Joker

[Marc]

I once had a max II with a B&S 18 hp vanguard and it also runns rough and had backfire under load and i did not find the problem, then i checked the spark plugs and one was damaged, it had a spark but not each time. I only changed the spark plugs and everything was ok.

Hi all.
My Argo Response with 18HP Vanguard engine was running rough, had little power, backfired under load... I decided to check the valve clearances. At 1/4 inch past TDC the clearnaces all measured 0.012-0.018" - way too loose. I set them to about 0.005" and the engine ran a lot better (good power, no bogging on turns, smooth idle...)

Then, about 4 hours (run time) later, it started running bad again (same symptoms). So, I pulled the valve covers off again and found that there was no clearance in any of the valves. The springs were always somewhat compressed.

So, I adjusted them again. This time, before I put her back together, I pulled the starter cord a few times and then re-checked the clearances. They were too loose! I re-adjusted a couple times before I could actually spin the engine and not have the clearances change. Once I was able to do this, I put it back together and ran it, but it still runs poorly.

Tomorrow, I will recheck the clearances as well as try to pull off the black plastic cap over the carb adjustment screw (in case the cold weather is causing a lean condition that is confusing the matter).

I am wondering if I need new valve springs, or if the valves are sticking. Either way, I have never fixed either problem, so I would appreciate any suggestions.

By the way, one more oddity is that when I pulled the spark plugs, each one was half "dark gray" and half "new". These had about 4 hours on them.

Thanks in advance.

Joker

[rm25x]

Set intake at .005-.007" and exhaust should be set at .010-.012". Reason exaust is set with more gap is that the exhaust valves expand more then the intake due to the heat of the exhaust gasses. Also, when you set the valve lash, you need to roll the engine over until the intake valve opens all the way, at that point set the exhaust lash. Then roll it until the exhaust valve is open all the way and set the intake lash. You can't set them both with the piston at TDC on the compression stroke accurately due to the cam timing on the vanguards.
(Learned that at Briggs school, ex 6 year small engine tech)

[Joker]

Thanks guys, I'll pick up new plugs today.

As for the valve adjustment, does it matter which Vanguard we are talking about? I have read several posts here and on a couple different small engine sites describing the adjustment as "set both exhaust and intake to 0.004-0.006" at 1/4" past TDC." This would be the first time I have read to set them as you have described. I don't mind trying it, I just want to know that if the adjustments don't work, it wasn't the adjustments causing the problems.

Thanks again,
Joker

[superherojohn]

18hp Vanguard Service manual web-book link

This is on Google books, luckly most of the manual is viewable. Google posts a preview of a number of books in hopes that you will purchase the book. You can't print it or anything but it comes in handy for research.


Large Air-Cooled Engine Service ... - Google Book Search

[Roger S]

"set both exhaust and intake to 0.004-0.006" at 1/4" past TDC."

That is what George Herrin, certified briggs master mechanic, and performance engine builder will tell you. The 1/4" is piston drop from TDC.. make sure it's the compression stroke.. intake valve was the last to close.. Pull a spark plug and measure the 1/4" with a pencil, etc.

The compression release is a steel ball in the heel of a cam lobe, holding a valve off it's seat at below cranking speeds(lowers compression). This allows any 1/2 charged lawn&garden battery and weenie starter, (or your arm on the pull rope) to spin the vanguard. The 1/4" drop means the cam rotated just far enough to get off the steel ball to set valve lash. Excessive valve lash will make it hard to crank, due to lessening the effect of the release ball.

All vanguard twins 14-23 use the same stock cam. With aftermarket cams, none of the above applies.

[Joker]

I got home late from work tonight so I will have to look in to this another night. But I am sure I forgot to check that I was making the adjustments on the compression stroke, so I could have things messed up with that.

I obviously need a good service manual. Thanks for the Google link SHJ.

Thanks again for the help.
Joker

[Joker]

Ok. I checked the valve adjustment and it seemed right on. So, while putting the valve covers back on, I also installed new plugs per the book link SHJ posted (Champion RC12YC) versus what was in it (NGK BPR8ES) and cleaned the idle screw (the one with the black cap I have been reading a lot about). After running at high RPM for about 5-10 minutes, I got a backfire at full throttle, so I leaned the idle adjustment a tad and it has run good for about 45 minute of engine time.

Hopefully, either the plug change or the idle screw cleaning did the job and it will run good from here on... We'll see.

Thanks go out to to the suggestions here.

Joker

[Roger S]

Sounds like things are better. There are a couple other things to look at if it's not fixed. Weak spark can cause the backfire.. Looking back at your first post, it's clear that the on gain/off again valve clearance checking/adjusting was due to not always at being at TDC of the compression stroke. So that mystery is solved.