# 21 HP Briggs & Stratton Runs Rough



## ranman

I have a lawn tractor with a 21 HP twin cylinder Briggs and Stratton that idles rough (and loud), will not rev, backfires through the carb and blows black smoke. I checked for spark and noticed the spark is much stronger on one cylinder. I replaced both plugs - no change. I thought maybe the coil was bad on the weak side so I reversed the two coils and spark remained poor on one side but not the other. Not sure where to go next


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## Rentahusband

Do you have high/low speed mixture screws on the carb. Try adjusting while running. Initial adjustment is 1/-11/2 turns from closed. Check for cracks on the plug wire of the side with weak spark.
Dean


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## ranman

*21 Hp B & S*

Thanks for the advice. Figured it likely isn't the carb because only one cylinder is running rough/ I pulled the valve cover (it has overhead valves) to check the valves and found a very bent pushrod. Not sure what would cause that...


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## 30yearTech

ranman said:


> Thanks for the advice. Figured it likely isn't the carb because only one cylinder is running rough/ I pulled the valve cover (it has overhead valves) to check the valves and found a very bent pushrod. Not sure what would cause that...


A sticking valve, loose valve guide, or simply out of adjustment valves can cause a bent push rod.

It's also possible on Briggs OHV twins to have an issue with the carburetor that only affects one cylinder. The carburetor on these engines feed each barrel to the cylinders separately, so if only one side is plugged, then the result is an engine that is only running on one cylinder.


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## ranman

Didn't know that about the carb, I'll clean it. I ordered the pushrod. After installing it, what should I check before starting it so I won't bend the new pushrod?


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## 30yearTech

ranman said:


> Didn't know that about the carb, I'll clean it. I ordered the pushrod. After installing it, what should I check before starting it so I won't bend the new pushrod?


Take a close look at the valve guides on the side that had the bent push rod and make sure they are both around the same extending up through the head. Make sure both valves open easily. After installing the new push rod and adjusting the lash, turn the engine by hand and observe the valve operation. Look for anything that may cause the valve to bind when it opens.

Best of Luck.... :thumbsup:


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## ranman

I've got problems now. I installed the new pushrod and adjusted the valves with the piston at TDC. I removed the plugs and turned the flywheel by hand and watched the pushrods for both intake and exhaust valves bend! I figured a valve was stuck so I took off the head. I am able to open and close both valves, so they are not stuck. I don't have a tool for compressing the spring to remove the valves from the head - could they be bent or would I be able to tell without pulling them from the head? The piston is moving easily, what else could be catching and bending pushrods? Oh, I looked at the valve guides and the guide on the exhaust valve sticks out of the head almost twice as far as the guide on the intake valve.


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## 30yearTech

ranman said:


> Oh, I looked at the valve guides and the guide on the exhaust valve sticks out of the head almost twice as far as the guide on the intake valve.


That's your problem!

At one time the exhaust valve seized up in the guide, causing the whole guide to move in the head. Now the guide is not in the proper position and the valve keeper bottoms out on the guide before the rocker arm is finished it's travel. This is what is causing the damage to the push rod. You will need to replace the cylinder head in order to get it to work properly again.


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## smallengineguru

another option is to just replace the said damaged valve guide. any good machine shop can do this. as a rule of thumb most times its an idea to replace both guides. this is usually more cost effective as the machinist has the gear already setup for the first guide.
i only replace heads if they are badly damaged from warping or cracked etc

cheers


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## 30yearTech

Well this is always an option, but since Most Briggs OHV engine do not have replaceable valve guides, a machine shop around here will almost always charge as much as a replacement head to repair the old one. But it's an option worth checking out, you never know, you might get lucky and save some money doing this.


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## smallengineguru

well for us here in oz a new head for a B&S ohv intek or the like is around $200-$300. and as you could imagine most customer won't want to pay that. so we try and find other repair/replace options for our customers.

cheers


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## 30yearTech

smallengineguru said:


> well for us here in oz a new head for a B&S ohv intek or the like is around $200-$300. and as you could imagine most customer won't want to pay that. so we try and find other repair/replace options for our customers.
> 
> cheers


Wow, is that in US currency? The head is sells for $130.00 here.


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## smallengineguru

lol no thats in aussie dollars.
we get a little more creative here in oz.....the shop i work at has a "try and repair it before we replace it" attitude. which works quite well 98% of the time.

it seems everything is cheaper in the states.....pity the same can't be said here in oz.

cheers


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