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Chipster27
09-25-2008, 02:04 PM
Does anyone have any idea what the cylinder compression should be for a genny? I have my Yammi 2800, motor is 171cc and from what I gather it has about 5.5 hp. The thing is 6 years old, but I've used the piss out of it the past 3 years. It's noisy (I've taken a db meter to it and it's much louder than is spec'd) and burns oil. Leads me to believe the rings are shot (that and some serious neglect on oil changes).

I ran a compression test the other night and it was about 50psi, I'm used to most engines being closer to the 150 mark than 50, but I thought that since this is a smaller (probably lower compession engine) the reading might be lower.

Anyone have any idea what the compression should be for a motor like this?

shadow
09-25-2008, 03:29 PM
50 seems really low - I think thats considered a dead cylinder on a V-8.
I know my honda 650 had a 6.5:1 compression ratio, so that would be low too. Maybe call the Yammy shop and see if they can look up specs.

Might just be time to get a new one :D

pronstar
09-25-2008, 04:12 PM
If you're really smart...or recently stayed at a Holiday Inn...you could guesstimate the cylinder pressure using the formulas on this Wiki page.
http://en.wikipedia.org/wiki/Compression_ratio

I'm neither of the above, and my calculator only does simple math.

Chipster27
09-25-2008, 04:28 PM
If you're really smart...or recently stayed at a Holiday Inn...using the formulas on this Wiki page.
http://en.wikipedia.org/wiki/Compression_ratio


i stayd at a hooliday in and finuly mak it to dubl degit smartnes. it were phun wile it last it....

When did they start using the alphabet in math???? The metric system failed, just as the alphabet/numbers system will fail!

Chipster27
09-25-2008, 04:32 PM
Maybe call the Yammy shop and see if they can look up specs.

Call the shop...:dez: :rotfl::rotfl:

I thought about that, I even know the guys at Mission...:soap:

For grins I might just pull the cylinder and have them clean it up and buy new piston/rings and muffler. Yea at that point why bother, should just buy a new genny. :ph43r:

Derwud
09-25-2008, 05:13 PM
Call the shop...:dez: :rotfl::rotfl:

I thought about that, I even know the guys at Mission...:soap:

For grins I might just pull the cylinder and have them clean it up and buy new piston/rings and muffler. Yea at that point why bother, should just buy a new genny. :ph43r:

I'll give you $50 for it as is...

jws
09-25-2008, 05:24 PM
I'll give you $50 for it as is...

i'll give a hundy:vib:

Chipster27
09-25-2008, 05:59 PM
i'll give a hundy:vib:

It's on ebay; like new, low hours, runs strong, provides electricity, never off road, waxed twice a year, adult owned, low hours, only used to run the coffee maker, no shipping for bidders in Houston and New Orleans :rotfl:

Like I said, I'll probably put a new top end on it and call it a day, cheaper than a new one and for some reason I just can let this one go :rolleyes:

jws
09-25-2008, 06:15 PM
Like I said, I'll probably put a new top end on it and call it a day, cheaper than a new one and for some reason I just can let this one go :rolleyes:


good call:vib:

Derwud
09-25-2008, 09:52 PM
Okay Chippy, $101.34....

J.T.
09-25-2008, 10:01 PM
get that thing running , we will need it for the rhino tunes.

shadow
09-25-2008, 11:11 PM
get that thing running , we will need it for the rhino tunes.

he just needs to show up, we can use my honda.

pronstar
09-25-2008, 11:41 PM
Since the engine is close to a rebuild anyways you might want to try one of those miracle cures in a bottle like Rislone or Restore. I'd never run that shit in a good engine but you have nothing to lose.

jackxclan
09-26-2008, 12:44 AM
I've ran restore in an old motor that shit does work.

Chipster27
09-26-2008, 05:26 AM
get that thing running , we will need it for the rhino tunes.

It runs, I'm just not sure it's 100%, the low compression concerns me, but it still puts out the power and it's never let me down. It will power the Rhino for the tunes :)

pronstar
09-26-2008, 05:32 AM
It runs, I'm just not sure it's 100%, the low compression concerns me, but it still puts out the power and it's never let me down. It will power the Rhino for the tunes :)

Right on!
Plus, there's a few little gennies in camp these days so you'll have plenty of backup power :buford:

shadow
09-26-2008, 12:03 PM
It runs, I'm just not sure it's 100%, the low compression concerns me, but it still puts out the power and it's never let me down. It will power the Rhino for the tunes :)

you can bring it by the house, and we can throw it on the meter and load it down - see if its putting out what its rated for.....easiest way to tell.

Big Buford
09-26-2008, 03:34 PM
Chip, from what I remember 80 psi is concidered the lower limit.... 50 is no bueno :buford:

Jay
09-26-2008, 04:12 PM
Chip, from what I remember 80 psi is concidered the lower limit.... 50 is no bueno :buford:

Really? A well tuned, single cylinder motocross engine is only putting out ~58 psi.

Derwud
09-26-2008, 04:39 PM
Really? A well tuned, single cylinder motocross engine is only putting out ~58 psi.

2 STROKE OR 4? You also have consider cranking speed into that as well.. A car, using a starter vs. a Motocross kick starter vs. a Gennie with a pull start.

pronstar
09-26-2008, 05:10 PM
Plus you'll have less cylinder pressure if the compression ratio is very low.

Jay
09-26-2008, 06:20 PM
2 STROKE OR 4? You also have consider cranking speed into that as well.. A car, using a starter vs. a Motocross kick starter vs. a Gennie with a pull start.

A 4 stroke CRF450X is ~58 with an electric starter. Don't get me wrong. I know nothing about this, just curious when comparing it to the motors I do know about.

Big Buford
09-26-2008, 06:22 PM
What compression should my engine be reading?...

Question
What compression should my engine be reading?

Answer

The compression number is not a published number due to several reasons (compression release mechanism, strength of pull, etc.). We use a tool called a leak down tester (part number 19413), which uses color gauges and a low-pressure air (of about 70 psi) to determine if there is any loss of compression.

The cylinder leakdown tester is placed in series with a compressed air source and the engine being tested. The crankshaft must be locked at TDC between the compression and power stroke before compressed air is introduced into the cylinder leakdown tester. The overall condition of compression components is determined by the amount of leakage after the combustion is filled with compressed air. Air continues to fill the combustion chamber until air pressure in the chamber equals the regulated supply air going into the tester. Engines normally have some combustion chamber leakage, and some air may continue to flow. An engine having satisfactory compression displays a reading in the green range on the leakdown gauge with a minimum of audible leakage. A reading in the red/green or red range, along with high audible leakage, indicates a problem with compression components.

All internal combustion engines leak some air when tested with a cylinder leakdown tester. The best way to determine the location of a leak is the audible noise from the source as it passes through any engine opening. The engine is inspected for noise with the air cleaner removed at the muffler and the oil fill dipstick or breather tube. Head gaskets, hose, and fitting should also be checked for leaks.

Compression loss can present different symptoms. For example, an exhaust valve leak can cause an engine to appear to have little or no fuel for cold starting. When the piston moves toward the crankshaft, maverick air can enter the combustion chamber through the exhaust valve. The maverick air takes the place of the air-fuel mixture drawn through the carburetor. Without the proper air-fuel mixture, the engine has an insufficient amount of fuel vapor and does not start.

With a leaking exhaust valve, an engine with less than optimal compression can sometimes be started by priming the combustion chamber. Less fuel is required to keep an engine operating than is required for initial cold starting. The engine may operate acceptably when warm, but the leaking exhaust valve still affects engine operation. Maverick air continues to enter the combustion chamber through the carburetor. The lean air-fuel mixture produces higher combustion chamber temperatures, which can cause damage to the valve train and other compression system components.

This test can be performed by a Briggs & Stratton Authorized Dealer. This Service Dealer has all the information relating to Briggs & Stratton warranty information, engines, short blocks, parts, prices, service, specifications, proper fit, etc.

To find our "Best-of-Class" dealerships, look for the Master Sales & Service Dealers (red lettering).


If you wish to perform this test yourself, our Service Tools Catalog (PDF) contains illustrations and part numbers of our specialty tools.
Selected Service Tools can be purchased through the Repair Parts section of our website. Service Tools can also be ordered via phone by calling our Customer Contact Center at (414) 259-5262 (48 Contiguous United States orders only).