Compression Test 85HP Force - Neutral vs. WOT

[Mark9740]

I put a new head gasket on my 85HP Force today and did a quick compression check with the shift lever in neutral. I noticed on this site that everyone recommends the shift lever be in the WOT position during these checks. Does that make a difference in the compression and if so, how? I just want to understand. Also, is it recommended to do compression checks after the engine has been warmed up? My results were 125 on all cylinders. Thanks.

 

[Frank Acampora]

Re: Compression Test 85HP Force - Neutral vs. WOT

Since the engine is a 2 cycle, the ports are open at the bottom of the stroke and the cylinder goes to atmospheric pressure. Therefore it doesn't matter if the carbs are fully closed or fully open. At cranking speeds. the same amount of air will be in the cylinder to be compressed.

THEORETICALLY, with the carbs wide open there would be slightly less resistance and the cranking speed will be just a smidge faster. PERHAPS compression will go up a point.

I never worry about throttle position when checking compression: If there is damage, the decrease will be quite noticable. It will either have compression or not. It will either be above 100 psi or less than 90, and throttle position will not effect this type result at all.

'Smatter of fact, when field testing an engine I simply put my thumb over the plug hole. It will either have compression and push off my finger or it will not, and that tells me all I need to know.

 

[moparman]

Re: Compression Test 85HP Force - Neutral vs. WOT

I agree with FRANK, it shouldn't matter as long as all are done the same way[that is the important part] . You need to have a good battery so that the cranking rpm's are consistent for all the cyl's to be checked,that way you are sure to get accurate readings. Hope this helps and keep us posted on your progress. MOPARMAN ROLL TIDE 9-0

 

[Mark9740]

Re: Compression Test 85HP Force - Neutral vs. WOT

Thanks guys for the explanation. If the weather clears up on Friday I'm going to sneak my boat (16' bowrider) out on the river and try it with the new head gasket. Last time out I was doing 31 on my boat speedo (pitot) at 4200 rpms pushing a 13 1/2 x 17 prop. I pulled the plugs and found the bottom one shiny clean and the other two carbed up so I figured the head gasket was leaking. Hopefully the extra power will give me the higher rpm's needed for it to run optimally. I'll let you know how it goes.

 

[Mark9740]

Re: Compression Test 85HP Force - Neutral vs. WOT

Well, I took it back out today and got the same exact result...it won't idle and it appears the bottom cylinder isn't firing. I'll post a new thread tonight laying out all the facts. Thanks again for your feedback. I would be lost without this forum.

 

[O.T.Water]

Re: Compression Test 85HP Force - Neutral vs. WOT

Mark, When you removed the old h/gasket, was it bad or did you see signs that it was blown? I think there are other threads on how water can get in the bottom cylinder from other gaskets. I did my h/gasket this weekend and it was blown letting water into #2 cylinder. I had to look close at it, the cyl block was corroded right next to the cyl liner and had to rebuild the edge with JB Weld. O.T.

 

[Mark9740]

Re: Compression Test 85HP Force - Neutral vs. WOT

OT - the old gasket looked fine but it was slightly different from the one I got from the marina. The gasket I took off had the oblong-shaped holes around the cylinders as if the water was to pass through them into the head. The only problem though, as we both know, water only circulates into the head through the top and bottom ports. The block and head both look good and the old gasket was in tact and didn't look blown. Someone else said I should look at water coming in through the exhaust cover gaskets and that those should be replaced. Have you ever run across that? Mark9740

 

[pnwboat]

Re: Compression Test 85HP Force - Neutral vs. WOT

Leaking exhaust cover gaskets are a very common source for water in the lower cylinder. Use caution when removing the exhaust cover bolts. It's not uncommon for them to snap off when loosening.

 

[john from md]

Re: Compression Test 85HP Force - Neutral vs. WOT

Pull your fuel pump and make sure you don't have a torn or stretched diaphram. The diaphrams they are selling today are very thin and tear if easily if the engine backfires. The engine still runs with the torn diaphram but it idles poorly and won't get top end.

John

 

[JB]

Re: Compression Test 85HP Force - Neutral vs. WOT

Your gearcase should be in Neutral for compression testing because the gears and prop place an extra load on the starter.

As for throttle, I don't think it matters. She will be turning far too slow for scavenging pressure from the crankcase to affect the readings.

 

[Mark9740]

Re: Compression Test 85HP Force - Neutral vs. WOT

Pull your fuel pump and make sure you don't have a torn or stretched diaphram. The diaphrams they are selling today are very thin and tear if easily if the engine backfires. The engine still runs with the torn diaphram but it idles poorly and won't get top end.

John

john from md,
I'm not sure the fuel pump is the issue because the line goes to the bottom carb first, right? The top two get plenty of gas. I did notice yesterday though that there was a lot of black soot around the fuel pump. I'm guessing due to a leak in the past? Is the fuel pump difficult to tear down and rebuild? BTW, where are you in MD? I live in Harford County and launch at Peach Bottom or Glen Cove Marina on the Susquehanna above the Conowingo Dam.

 

[john from md]

Re: Compression Test 85HP Force - Neutral vs. WOT

The fuel pump is easy. All it takes is removing six screws.

The carbon is from a leaking port cover gasket. The port cover is what the fuel pump sits on. It has four bolts to remove it if you have to change the gasket.


I live in Davidsonville, a little west of Annapolis. I launch out of Sandy Point State Park and usually fish from the Magothy down to the South River.

I look down on the Susquehanna whenever I drive to PA to see my relatives. I looks like it has some really shallow spots.

John

 

[Mark9740]

Re: Compression Test 85HP Force - Neutral vs. WOT

John,
Thanks for the heads-up on the port cover. I'll take a look at it and check out the fuel pump this weekend.

In my younger days, I spent many hours down your way on the Severn rowing for Navy. I love it down there...I think I know every inch of that shoreline. Have a great holiday.

Mark

 

[john from md]

Re: Compression Test 85HP Force - Neutral vs. WOT

Mark,

I still see some rowers and academy sail boats but not as many as in past years. They don't have the accent on seamanship at the academy like they used too. I guess it offends too many of the students sensabities.

These days, the YPs tend to **** me off. I assume to save fuel, they do much more manuvering in the severn than out in the bay. There is good fishing right off the academy bulkhead but you have to dodge YPs constantly.

Next year I'm going to see if I can get a schedule of when they will be out so I can fish on other days.

Happy thanksgiving to you also.

John