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Discussion Starter #1 (Edited)
When our Magnum 325 died and would not restart we rebuilt the carb since we knew the float shut off needle valve had fail. After it would not start after the rebuild I pulled on the rope starter and noticed the compression just much lower than in the past.

Today we pulled the compression reading cold and got 40 PSI then added some motor oil into the cylinder and PSI came up to 45 PSI. Opening the throttle made no difference.

Then we jumped over to the 2002 Magnum 325 that we bought as a project bike knowing after an oil cooler line failed it started to knock. The PSI read 45 on it but with throttle open it would go to 95 PSI and did not add oil into the spark plug hole and recheck PSI.

The project 325 will start and idle well but will bog if you give it gas and knock.

I will need to check the valve clearance. Do auto compression release fail on these engines cause low compression and if so know to you determine that to be a fact?

Thanks for thoughts.
 

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It is possible the ACR valve is the problem.

Any untoward noises when she cranks over like chunks of ring?

Do you have access to a borescope to peek through the sparkplug hole?


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Discussion Starter #3
I will turn it over the next time I work with it and listen but with the starter noise I did not notice it as of yet. We were warming it up to see if we could get the idle adjusted from when we had it apart earlier. This was before I ordered the carb kit and it just died and would not restart. It would hit a little but not start that day.

Why does this engine have to have the throttle open to make max PSI in a compression check? With a car or most engines that I have seen will give a good reading after about five compression strokes into the gauge.

Both of our 325's have ballpark similar PSI readings without the throttle open but the knocking engine will make near the expected with the throttle wide open and the one that just died on me when riding will not make more when throttle is wide open.
 

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Discussion Starter #4
If the compression is low due to the auto compression release being open where would the air be leaving on the compression stroke. I mean what is the path way it takes?

Since it was running how be it poorly then just died something major had to change.

Anyone every pulled a Polaris to start it by towing it?
 

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Harley routes the ACR into the exhaust port...

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Discussion Starter #6
Thanks for that info. I just found Polaris does the same. It as to do with the ball not letting the exhaust valve fully close or something. Do think I am going to grasp the concept until I pull the cam cover. :)
 

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The proper way to do a compression test is with the throttle wide open and no more than 3 compression strokes of the engine. It should also jump close to 90 on the first one if it is in good condition and the compression release is working properly.
 

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Discussion Starter #8 (Edited)
Thanks for that readout number. If the rain holds off I want to pull the top cover and look at the cam and see if I can figure out if the compression release is working by spinning without the spark plug being installed.

Hate to peel the project bike since it will be setting outside so I can remove the gas tank so I can see the cam in it that does permit 95 PSI readout. Since I have to tear it down at some point to check for damage from no oil when the oil cooler line failed I could take the top end and put it on the one with the good bottom end I guess if the cam on it has failed.

Since both engines give the same PSI if the throttle is NOT open and it was stating and running then just died it could be compression release related because we know the timing change is intact. I guess it could have jumped timing so the valves do not seal right on each stroke.
 

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That sounds plausible. Prolly way cheaper to fix faulty ACR than broken rings or valve train issues.
 

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Discussion Starter #10
I found a used head for $100 and it came in the mail today. Did not dig out the mics but per the digital caliper it measures in the middle of factory min/max specs on both lobes and journals O.D. The cam sprocket teeth show no wear. I did not check the I.D. of the journal bore with just an oil film it was sucked into place nicely.

The automatic compression release shows no sign of wear as well as both lobes look new like so at this point I am happy.

Now that I have been able to study the moving parts I expect the compression release may be stuck in the 'release' position but I do not see how that could have happened in a running engine. When we pull the valve cover that should rule that in or out.

After we set the tank off again we will check the valve clearance again to make sure nothing changed there. If not we will break out our new two pound Harbor Freight brass hammer if we cannot hear the automatic compression release flopping inside when turned with the rope starter. By the cam position we will know when the release should flop from one position to the next by having the nice used head in our hand.

The goal is to get it running without turning a tap. :) On the other hand we know we have good compression on the top end of the project 325 Magnum that has a knock due to the guy blowing an oil line without knowing it until it started knocking. If the cam on the running engine that just died and will not restart shows a lot of wear I think since this head looks good we will just swap out heads and keep the mated parts together.

We have a top end gasket set in hand but no new ring set. I we wind up pulling the head it would be tempting to put in a new set of rings being that close. It did however run with good power but the carb was loading up so it was hard to tell if it was smoking any.

Well I would like to work on it over the holiday but tomorrow the shop temp is going to be 31F and 19F after dark so we will be waiting for warmer temps to do any real wrenching. :(

At least we are learning while it is too cold outside to work on it. Having a good complete head in hand is a plus to lower our learning curve.
 

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Discussion Starter #12
It is to be in the upper 40's or low 50's Fri/Sat and all we have to do is to move the gas tank to the side and take of the box that bolts to the top of the head to check the tappets and get some view of the cam.

I looked at one 7x18' 1987 enclosed trailer that had been used years ago to haul a mini race car with the broken tail and heavy ramp a few weeks ago that was already wired but it was red and rough. If it had been white and not needing the $1650 to go to medical deductible I would have loved to had it but it would have stuck out if we parked it close enough to plug it in to 120v. Maybe this summer we can make some progress on hauling dirt to make the fill so we can build a real shop at some point.

It really helped getting the used head so we know how the ACR works now and can start there. It would be nice to get it back to running without going deep. The project 325 Magnum may have to wait till spring since the rod knock means we have to go deep on that one.

We do have the heated/cooled 7x14 enclosed trailer that we use for tool storage and work bench area but there is no free space in it plus it has swinging doors. We could get the engine inside it if we wanted to tear into it however.
 

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Discussion Starter #13
Cam and ACR looked OK on inspection.
small compressor can only bill to 30 psi continuously running. Filled cylinder with seafoam same results. Built to 75 psi with Motor oil until it ran out into the cold case. Had to leave house so we covered to next time.
 

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Discussion Starter #14
Well I am back home and on a 'real' computer.

Just wanted to say using an air compressor to trouble shoot is awesome. When we got the cam cover off I told my son about how some uses air to find the cause of low compression and he said why do not we do it then since you have the compression tester hose in the spark plug hole and it has a quick connect that should fit the compressor. :)

In the process of getting the piston at the bottom of the cylinder and valves closed at the same time we experienced what it was like to have leaking intake/exhaust valves. I was hoping it was a bad valve and we just put on the used head we got. We did check the exhaust cam lobe and the reading we go was within normal limit specs from Polaris. The valves were right on .006 as well and the automatic compression release (ACR) was working fine we determined even before we pulled the valve cover so it was clear we were going to have to go deeper.

What blew my mind was how fast the rings went to not having enough compression to run unless there is a hole in the piston or other major failure.

While the carb over flow drain was stopped up and the needle valve let the engine fill with gas we did not start and run it with the gas in the oil and double checked when we drained the oil and found pure motor oil ran out first. We then turned it over for 30 seconds a few times with the spark plug removed which should have flushed the ring groves with new oil.

Has anyone here ever just replaced rings without boring the cylinder and using a new piston ever recovered much compression after being down to about 35-40 reading on a compression test?

Since we do not have a shop to put ATV in I would like to get it at least running so we will have it this winter around the place. Since the project 325 tests at 95 PSI but has a rod knock I am tempted to just moving its piston and cylinder over to the one that was running and when we get to the project engine (guess we actually have two now :( ) we could just bore out this cylinder and put it on the project machine that is a much tighter chassis. The first one we bought and now has no compression just has more wear all away around but still is a solid machine it seems.

Maybe most any 10-15 year ATV that has seen a lot of use is ready for a rebuild.

The head bolts look like they have the spline head so since I was 2/3's the way to Harbor
Freight I ran by this evening and picked up some spline type sockets so we can get the head off. If the rod feels tight on the crank we will just do a top end on this one.

Thanks for any thoughts. It seems like these are not hard to work on. While we do not have a place to get an atv in a heated space we are going to built a 2' x 4' work bench in the rear of our 7' x 14' enclosed trailer that is wired and has AC and make it where mounting one of the 325 engines will be like an engine stand. :)

I really want to get a good build job on the project ATV engine since the rest of it shows little wear. The chicken farmer would have kept it for years had it not been for the blowing on the oil cooler line and messing up the engine.
 

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If the ACR ball is moving freely and fully retracting you may have a damaged piston or cracked compression rings. Kindof a WAG without some exploratory surgery. One of those Horrible Fright bore scopes would be nice.

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Discussion Starter #16
Yes a bore scope would be nice for many things since I have so much physical movement limitations from arthritis.

Well I finally came up with a plan to get into the dry and have a little heat for working in mild winter temps (40/20F temps norm winter for us). The old open on one end garage that was built about 50 years ago and we are planning to tear it down and build a nice shop but we need to move a hill to this hill so we can build a nice shop near the house. A couple years ago when we got the backhoe we got the place cleared off where we are going to get the dirt and this winter will try to cut off more of the area where we are going to make the fill so we can have a nice location for a shop and be able to drive around it on the outside as well.

The problem is we need to keep the old garage as long as possible because it is wired with heavy wiring to the building and houses our back up generator. After my FIL passed about 5 years ago it got used to put a lot of stuff out of his old shop but is really mostly junk the brothers did not want. I have a great wife and two kids but she never wants to through anything away because it all has meaning. The plus side after all of these years she has not tossed me into the trash. :)

The simple plan is to organize and move the stuff with meaning to a building on the other side of the house and throw away a lot of my stuff like old broken or fogged up head light assemblies, cardboard boxes, some long since dead leaf blowers, etc.

While it is a gravel floor it is about 12' x 22' but the back 4' is on a platform that is raised about 1'. The open end is like 7' high and 11' wide so a 8'x12' tarp will make a good temp door to keep out the wind chill and a couple tall IR heaters can raise the temps so mild days one could work. Anyway the next three days is going to be near 60 so that will give us a window to get it cleared out for the most part so we can get the two Magnums on the inside before we get deep into the engines. Having to up tarp and retarp is no fun. Sometimes I have 15 minutes to knock out a task but dragging out the tools and all just is not worth it.

Again I will have to open up the bike that lost down to about 40 PSI compression while riding it. I first thought it could be a seeping head gasket but if that was the case I do not think the Seafoam would have moved down to crankcase.

Well if I can them them in the dry before the ice that may come on Thursday I will be happy. Just getting a plan to be able to work on them some this winter has already helped the mind. :)
 

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Discussion Starter #17
If the ACR ball is moving freely and fully retracting you may have a damaged piston or cracked compression rings. Kindof a WAG without some exploratory surgery. One of those Horrible Fright bore scopes would be nice.

From orbit via Android.
Hey the HF flyer came and the cheap one is $67 but the head is 16 mm vs 8.5 mm for the one that is about $100 more currently. Did make some progress in a couple hours on cleaning in the old garage and took a load to the dumpster at the office but it was mainly card board. The scrap metal is going in the scrap pile to be sold later.
 

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Glad you're making some progress on a place to work. In the rering, you won't know if you can get away with just rings until you are in there and measure the cylinder and old ring gap. You might be able to just put on rings, but I would recommend a piston at the same time.
 

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Discussion Starter #19
Thanks. We have the shop manual and will step through the measuring the ring gap and bore. I guess the piston skirt is going to wear more than the steel cylinder since it is more soft.

We did get the old garage emptied out into the drive way so we have our work cut out for us to get the drive way cleared before the rain starts tomorrow night.

Will we will not be able to work in really cold weather the old garage should be OK on a typical day. We are to see 50F's Christmas week so that will be nice working with some heat in the dry and out of the wind. Being able to leave out tools and work when it is raining will be a great help. We still have the nice wired, heated/cooled 7x14 enclosed shop trailer where we can like rebuild a carb or even more and it is just five foot from the old garage. At least we have a plan until we can haul all of the dirt and fill for a nice shop building site.
 

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Discussion Starter #20
After two very long days I think we are ready if the predicted ice comes. We got the ATV's under roof late today, hay for the horses, new plugs for the RV generator and got it running as well as the house.

Sunday evening I picked up some spline sockets at HF that hopefully will address the head bolts just fine.
 
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