Author Topic: How much compression on rebuilt 350? (Read 3299 times)

Crashly · « **on:** June 29, 2006, 02:14:17 AM »

A couple of weeks ago we picked up a 350 in a bunch of boxes and crates. It has been a great father and son project. We were able to bore the cylinder 1 over and install a new weisco piston. I did a compression test and Found that I was only at 100 PSI, (with wide open throttle) (the 350 has been run a total of 12 minutes) My machine shop inspected my installation work and found everything to be perfect. They called Triple E and they said that I needed to use the stock Honda head gasket with just one of the plates. By doing this they said that it will raise the compression. FYI the gasket kit that I used is the cometic kit. I checked the gage on my fl250 that I rebuilt at the same exact time as I rebuilt the 350 and found that it has 145 PSI (it has been ran about 2-3 minutes) Anybody have any thought or ideas on the low compression?

Moskito · « **Reply #1 on:** June 29, 2006, 12:53:28 PM »

Welcome to the site and welcome to the world of Pilots & Oddys.

I don't know what the compression should be (I don't know squat about 350s) but I would imagine it's probably close to the 150 psi range.

Have you broken in the 350 engine? If you have, how did you do it?

Breaking in a two stroke: (or any engine, for that matter)
Start and warm to operating temp
RUN THE SNOT OUT OF IT.

None of that pussy foot half throttle for 5 miles, easy this, easy that.

You have to build combustion pressure to force the rings to seat. You build combustion pressure by running it HARD.

If you did the easy, slow method, you may have glazed the cylinder slightly and that would aid in lack of sealing ability of the rings.

Since you need to switch back to an OEM head gasket, pull the head and see if the jug looks like it's developed any sort of discoloration (usually a yellowish/brown) around the exhaust side of the jug. If you see this, then it's glazed. - Lightly hone it if you can.

A jug with 10 - 20 minutes of running shouldn't have any signs of anything yet.

What oil and what ratio are you running?

Should be any name brand oil at 32:1. (I prefer 40:1)

PilotSniper · « **Reply #2 on:** June 29, 2006, 02:21:03 PM »

Quote from: "Moskito"

Breaking in a two stroke: (or any engine, for that matter)
Start and warm to operating temp
RUN THE SNOT OUT OF IT.

None of that pussy foot half throttle for 5 miles, easy this, easy that.

LOL!!! :lol:

nuke em · « **Reply #3 on:** June 29, 2006, 09:34:25 PM »

Yes, Welcome to the board! Feel free to ask questions.

Skeeter is giving good info. I have been working with 350s since the mid 90s so I have alot of good info for you, but I still do not know everything. Ed and Sahara at EEE are right about the head gaskets too. All I use now is the stock Honda 3 layer ones, ofcourse using only one layer. If you want to PM me with your address, I'll send you some for free(I have bunches). :wink:

As far as compression, what elevation are you at? Air temp does make some difference too, but alot less of an amount. My stock set-ups are 105-115 lbs here at 6,000ft. Do you have any mods done to the machine yet?

Did you double check the readings with another guage? Just ruling out the easy stuff. LOL

Again, welcome!

Gary

Crashly · « **Reply #4 on:** June 30, 2006, 12:54:35 AM »

Thanks for all of the advice, I like this board. Its nice to know that there is so much help avaiable. My elevation is around 1400 feet and I am using Mercury quicksilver 2 stroke mix at a ratio of 33:1. I have been using the Mercury in the fl250 (at 50:1)for a couple of years and in my kx500 (at 50:1). So far I really like the oil. I talked with Ed at Triple E today and he was quite helpful.---man he has been doing this odyssey thing a long time and it seems like he knows his stuff. According to him with the stock gasket and stock head, one can only expect 125 PSI. He attributes the difference in the manual; 138 and the 125 PSI is due to how many changes Honda made with this motor. Different jugs and heads etc. One thing that I noticed while researching is that the 250 runs 6.6:1 and the 350 runs 6:1. This could partially explain why the 250 is running 145 PSI and the 350 is running 100 PSI. Ed also bought up the fact that the weisco piston is different than the stock ART piston and that also explains some of the difference in the lower compression. Rather than messing around with the motor to much more I am having the machine shop put the motor together today and they will be doing a leak down on it tomorrow along with a compression test.That way there should not be any questions asked if there is low compression. If it turns out OK then I am going to follow Eds advice about breaking in the motor and see how this project turns out. I will post the results tomorrow about the difference in the PSI with the different gaskets. Once again thanks for all the advice and help!