Makita 6401 with low compression

[bsearcey]

I have an HD Makita 6401 that I got about 2 years ago. I've never had any problems with it, but honestly know next to nothing about what to expect from chainsaws. My experience was limited to a 40ish cc Poulan before the Makita. I went and got a Harbour Freight compression gauge today and connected it up. Followed the instructions and got readings of 50 PSI!!!??? That can't be right? I mean the saw runs and cuts alot better than my Poulan. It'll bog down in oak on occasion if I've got the bar buried, but if it only has 50 PSI, I would think that my Poulan would be cutting stronger than it.

I checked to make sure the decomp valve wasn't in, and it wasn't.

Am I missing something, or is it definitly time to get the BB kit? I've been reluctant swap the P&C up to this point because the saw cuts and I thought didn't have any issues.

On that note:

I bought a tachometer from HF too, but I don't think it is what I need. Apparently you have to put some reflective tape on whatever you are reading to get the readings. I don't think this is doable on a chainsaw. You guys know where to get a tach that will work, but not cost more than I paid for my saw? If I do make the P&C swap I'll need to retune, and definitly don't think I can do it by ear. I would take the saw to the closest Dolmar place, but the guy said that I'm looking at 4 -5 weeks to get it back. I've got three big red oaks down in my yard from Irene that I need to get working on so I'm hoping to get this handled sooner than later.

Thanks

 

[watsonr]

It won't run at 50psi. Pull the muffler and look at the piston, if it's scored it may have lower compression. Does the compression tester have a schrader valve in the end? It looks like a tire valve.

Get a tach from Edge and Engine, one of the site sponsors, click there link in the upper portion of the screen, tiny tach is a good one, so is the fast tach.

 

[bsearcey]

OK. I admit I didn't read the directions. I just hooked up the tester and started cranking.

According to the instuctions on Baileys' for their tester:
Testing Procedures:
1. Run the engine for about 15 minutes until it reaches normal operating temperature.
2. Stop engine. Disconnect all spark plug wires one at a time and label for proper reconnection.
3. Loosen all spark plugs about one turn, then use an air hose or tire pump to blow all the dirt out of the spark plugs and gaskets. Place them on a clean, flat surface in the order in which they were removed. This will help to correlate any compression problems with the conditions of the plug from the particular cylinder involved.
4. Remove the Air Filter and block open the carburetor throttle plates to maximum. Remove the high tension lead from the center of the distributor and ground it. To disable electronic ignition system, disconnect the electronic ignition module or remove the primary battery terminal from the distributor cap. (On Ford V-8 and V-6, disconnect the primary lead from the distributor cap.)
5. Screw the spark plug adapter hose into the spark plug hole. Hand tighten only - DO NOT USE A WRENCH.
6. Now crank the engine for at least 4 compression strokes or until pressure stops rising on the tester.
7. Record the compression reading and repeat the test on all remaining cylinders. (Step No. 5 and 6).

I think steps 1 and 4 are the two steps that would make a big difference. To block off the throttle plate, can I just pull the choke out?

Anything else I'm missing besides a brain?

I feel like a moron.

 

[bsearcey]

It won't run at 50psi. Pull the muffler and look at the piston, if it's scored it may have lower compression. Does the compression tester have a schrader valve in the end? It looks like a tire valve.

Get a tach from Edge and Engine, one of the site sponsors, click there link in the upper portion of the screen, tiny tach is a good one, so is the fast tach.

Thanks.

Not sure about the schrader valve. This is the tester I got.

View attachment 200829

I'll also check of the E&E tachs. Hopefully, when I retest (after following the instuctions) I'll get a proper reading.

 

[watsonr]

the schrader valve looks like the valve in a tire stem on the car tire. If it doesn't have one, that tester wont work. The volume of a car cylinder is different than a chainsaw. Pull until it stops rising, not just 4.

 

[blsnelling]

My guess is you bought the wrong kind of gauge. You do not need to warm your saw or block anything off. Comp will be about 10 PSI less on a warm engine.

 

[bsearcey]

Maybe I did get the wrong kind.

The description of the tester says for testing cars, motorcycles, recreational vehicles, and boats.

The tester has a pressure release valve on it up by the dial. Is that the schrader valve or similar?

 

[bsearcey]

Do I need to have the throttle engaged? I just saw a youtube video of a guy testing compression (with a tester similar to mine) and he showed that the throttle was wrapped to keep it depressed.

Thanks guys. Also, just saw on craigslist a guy selling a Smartach TA100 tachometer for $65. Maybe I can get it for $50.

 

[nmurph]

I think all of the extensions have the valve at the foot.

Pull it until it stops climbing. The highest reading will be when cold, but a saw that has sat for several weeks will lose a little compression due to the loss of oil on the cylinder walls.

You don't have to open the throttle on a two stroke to get a correct reading. Even if you stuffed the intake full, you could still get an accurate reading bc of the negative pressure at the exhaust port caused when the piston heads to BDC.

 

[bsearcey]

You were all right...of course.

Sure enough I did have the wrong kind of tester. It did not have the schrader valve on it. Took it back and got one that did ($10 bucks cheaper too). Hooked it up and got a compression of 125 psi. Took 9 pulls for the needle to stop moving, so I'm pretty comfortable that the number is right.

So...based on what I've read in other threads, this is getting down pretty low. What is the range I should be looking for I guess for optimal power? 140-180?

Thanks guys. This site is the best.

 

[7sleeper]

125 doesn't sound fantastic but as long as it runs good what is there to worry about? If you want to maybe try a set of new rings before spending money on a new p&c. As far as I have understood the original Dolmar 7900 p&c are the way to go quality wise.

Good luck,

7

 

[nmurph]

I don't know the typical compression on a 64cc top-end, but the 79cc seem to run about 165psi.

 

[Andyshine77]

Sure enough I did have the wrong kind of tester. It did not have the schrader valve on it. Took it back and got one that did ($10 bucks cheaper too). Hooked it up and got a compression of 125 psi. Took 9 pulls for the needle to stop moving, so I'm pretty comfortable that the number is right.

So...based on what I've read in other threads, this is getting down pretty low. What is the range I should be looking for I guess for optimal power? 140-180?

Thanks guys. This site is the best.

It's likely your cheap gauge giving you a bad reading, even the better gauges are known to fail regularly. I'm sure your saw is fine, and it likely has more compression than 125psi, that's a low number for a 6400.

 

[bsearcey]

It's likely your cheap gauge giving you a bad reading, even the better gauges are known to fail regularly. I'm sure your saw is fine, and it likely has more compression than 125psi, that's a low number for a 6400.

Thanks for the comments guys. I can perform a test of sorts to figure out how this guage is reading. My neighbor just bought a new MS 311. I can hook it up to it and see what kind of compression it reads. Do you guys know what the compression should be on the 311?

 

[watsonr]

Compression should be between 140-180 and that depends on the saw. My Homelites run 165, Mac's 150-155, Stihls 140-150, old saws 85-120 so it just depends, 140 is a good number.