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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / June 2007Briggs & Stratton 18hp Twin
I have a B&S model twin cyl 18hp model 4217xx that has about 1000 hours on it. It is running poorly right now and among other things I thought I'd check the compression. Both cylinders measure about 50psi with the throttle closed and about 88-95 psi with the throttle wide open. There are no specs as to what the compression should be, but I would have hoped for something over 120psi. Anyone have any idea if these values sounds reasonable? TIA, Rich > I have a B&S model twin cyl 18hp model 4217xx that has about 1000 > hours on it. [quoted text clipped - 9 lines] > > TIA, Rich I have a Briggs and Stratton service manual, Rich, and will see if it says anything that might be of help. (If I can find the **@#/ manual, that is:>) >> I have a B&S model twin cyl 18hp model 4217xx that has about 1000 >> hours on it. [quoted text clipped - 12 lines] >I have a Briggs and Stratton service manual, Rich, and will see if it says >anything that might be of help. (If I can find the **@#/ manual, that is:>) I'm not sure a manual would help. I have the B&S manual for this engine and it does not give any compression numbers. I believe that the 88-95psi measurement is my 'real' compression and that the 50psi reading is due to some restriction in intake air at a closed throttle that I don't understand. A friend with a single cyl 13.5hp B&S with only 30-40 hours on it measures 130psi regardless of throttle position. That sounds much more reasonable, but it is a completely different engine with much lower hours. My engine normally starts well and runs strong. Right now I have some issue going on and I've put in new plugs, checked the carb mixture adjustments, governor adjustments, and idle/WOT speed adjustments. Changes were necessary in all of these adjustments, so maybe it will run better when I am finished tweaking. Really it is the low compression psi measurements and the much lower measurements at closed throttle that have me confused. I tried to find a small engine newsgroup but came up empty which is why I posted here. Do you know of a better forum? Rich >Really it is the low compression psi measurements and the much lower >measurements at closed throttle that have me confused. [quoted text clipped - 3 lines] > >Rich Rich.. Standard procedure for a compression test on any gasoline engine is to open the throttle wide when cranking.. If the cylinders can't get any air in on the intake stroke (closed throttle) , the compression readings will naturally be a lot lower. Your engine is probably normal for the hours on it. Bob Some of the references I found mention having to rotate the engine backward during compression check so that compression release devices (on some engines) do not interfere with the reading. In fact, on the Briggs and Stratton site, they mention a procedure to turn the engine backward to check for a pressure "kick", this being a useful rule of thumb test. This engine does have a lot of hours on it. It wouldnt surprise me at all if the valves are not sealing optimally anymore. The rings may also have relaxed. I hope you get to the bottom of this, and that you will post what you find. >>> I have a B&S model twin cyl 18hp model 4217xx that has about 1000 >>> hours on it. [quoted text clipped - 35 lines] > > Rich Your B&S engine has a compression relief bump on the cam which will drop the compression UNLESS you rotate the engine backwards and look for a spike in pressure that will occur just before that bump hits the follower. You also have to keep the throttle wide open on the carb. I would bet that you will see a spike of 120 or so if you rotate the engine backwards. That is the true compression. What is it doing? Oh and for folks with Kohlers you also have a relief valve BUT it is designed as a part that moves out of the way when you throttle up. One reason why the Kohler engines don't have much power at idle but get better with more throttle.  SignatureSteve W. Near Cooperstown, New York >>>> I have a B&S model twin cyl 18hp model 4217xx that has about 1000 >>>> hours on it. [quoted text clipped - 42 lines] >would bet that you will see a spike of 120 or so if you rotate the >engine backwards. That is the true compression. What is it doing? How do I spin the engine backwards to get a compression reading? It is an electric start and only turns forward. Rich >>>>> I have a B&S model twin cyl 18hp model 4217xx that has about 1000 >>>>> hours on it. [quoted text clipped - 46 lines] > > Rich The easy way is to wrap a rope around the output shaft. Then PULL. SignatureSteve W. Near Cooperstown, New York >>>>>> I have a B&S model twin cyl 18hp model 4217xx that has about 1000 >>>>>> hours on it. [quoted text clipped - 48 lines] > >The easy way is to wrap a rope around the output shaft. Then PULL. Can't do, there is no pulley to engage a rope. On the up-side, after making carb mixture, governor, and idle/WOT speed adjustments, and new J8C plugs, this engine seems to be back to it's former performance. A few more times of use will prove or dis-prove my "fix". Regarding the compression readings at closed vs open throttle, I do actually seem to recall that the throttle was supposed to be WOT when the readings are taken. It's probably been 30 years since I took a gasoline engine compression, and I am a bit fuzzy. What I can't explain is why on a friend's almost new B&S single cyl engine it made NO DIFFERENCE on throttle position for his compression readings. Maybe somehow on the new carbs they don't restrict the air but only add fuel to increase speed? I'm guessing..... Rich I own a 12 horse power horizontaly opposed two cylinder electric start Briggs & Stratton gasoline engine.Those light weight aluminum block aluminum cylinder head engines (Briggs & Stratton and or whatever brand names) are not built to last long term. cuhulin > What I can't explain is why on a friend's almost new B&S single cyl > engine it made NO DIFFERENCE on throttle position for his compression > readings. Maybe somehow on the new carbs they don't restrict the air > but only add fuel to increase speed? I'm guessing..... > > Rich See my previous post. If his engine has a governor, it may let the throttle be wide open at cranking RPM even if the throttle lever is in the idle position. >>> Rich >> The easy way is to wrap a rope around the output shaft. Then PULL. [quoted text clipped - 17 lines] > > Rich A LOT of the newer equipment doesn't have the throttle plate directly connected to the throttle lever on the machine. They hook it through the governor so that it makes the engine easier to start and also limits how fast it can run. The governor works in reverse of what a lot of folks think. It actually is trying to close the throttle plate when the engine is running and the the springs and throttle lever try to hold it open. When the engine isn't running at speed the governor keeps the plate open. SignatureSteve W. Near Cooperstown, New York >>>> Rich >>> The easy way is to wrap a rope around the output shaft. Then PULL. [quoted text clipped - 25 lines] >is running and the the springs and throttle lever try to hold it open. >When the engine isn't running at speed the governor keeps the plate open. Thanks, Steve. That makes sense. This engine in question does have a governor and I know the throttle is connected to it. Rich > I believe that the 88-95psi measurement is my 'real' compression and > that the 50psi reading is due to some restriction in intake air at a > closed throttle that I don't understand. What's not to understand? The closed throttle IS the restriction! All compression tests on any engine should be done with the throttle open. As for your friend's engine, if it's a governor-equipped engine, the governor may hold the actual throttle butterfly open even with the throttle lever all the way at idle. |
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