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The Vintage Japanese Motorcycle Owners Group: Discussion Forums

Vintage Japanese Motorcycle Owners Group :: View topic - Cold compression values, T20?


Cold compression values, T20?
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fodder
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PostPosted: Wed Sep 27, 2017 11:41 am    Post subject: Cold compression values, T20? Reply with quote

The engine on my T20 has been rebuilt with new seals, bearings etc., and new piston rings. The cylinder bores looked fine. It does run better than previously, once started, and doesn't burn transmission oil now (yippee, no huge clouds of white smoke from the right hand cylinder!)

Problem is, its very hard to start - 20+ kicks and use of the throttle. Just trying to start on the starter plungers with no throttle as advised for these bikes doesn't work. Having checked the ignition circuit and timing, replaced the coils and condensers just in case (points are new), and cleaned the carbs several times I have ruled those out for the time being.

I did a compression test just now on the cold engine and got 70 psi on one and 75 on the other cylinder. I know it should be done hot, but I think even a cold engine should be higher than that. Can anybody comment?

Any input will be gratefully received Smile

EDIT: I have just read elsewhere that a probe shorter than the spark plug length will give a low reading and one longer will give a higher reading. The probe I used is shorter so this would explain some of the lack of pressure. The tester has an alternative, long probe but its longer than the plugs and I don't want it hitting the piston.

I also dropped some oil in to the bores and got readings up to 80 - 85.
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05c50
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PostPosted: Wed Sep 27, 2017 2:12 pm    Post subject: Reply with quote

Doesn't sound like very good compression to me. Did you hold the throttle and choke open while checking the compression? The length of the hose on the tester can cause the readings to be off, but yours sounds a little too far from normal.

......Paul

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fodder
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PostPosted: Thu Sep 28, 2017 2:03 am    Post subject: Reply with quote

Hi Paul, thanks for the reply. Yes, I did hold the throttle fully open. I didn't have the choke on, though. I can try again with that open.

I looked at the difference in probe length and couldn't see that it would make a massive difference (2mm shorter, on a 14mm plug is 0.6cc volume difference. Which in a 125 cc cylinder is 0.5%) so I'm sure you're right, the compression is too low.

This may be over simplistic but the T20 has a compression ratio of 7.3:1. Atmospheric air is 15psi, so that squeezed x 7.3 suggests it should be 109 psi, call it 110, which is where I think it needs to be for good compression.

I'm getting mixed messages on the compression test. A couple of guys I've spoken to, including a bike workshop guy, reckon its of limited use in a 2-stroke, although didn't explain why. Whereas everything I see on various forums says its very important and low compression = no-start.

Nick
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05c50
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PostPosted: Thu Sep 28, 2017 5:05 am    Post subject: Reply with quote

I have limited experience with two stroke engines, but the ones that I've worked on with low compression were usually harder to start than ones with proper compression. let us know what you find.

...........Paul

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Rizingson
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PostPosted: Thu Sep 28, 2017 7:08 am    Post subject: Reply with quote

Just a couple of comments here.
1. Starter plunger open or closed will have no effect on compression readings.
2. You said the cylinders looked fine. Did you not hone them to break in the new rings? If not compression will never come up as it should when rebuilding because rings won't seat.
3. Carb were cleaned how? Did you confirm the adjustment on the plungers to close and open properly.
4. Starting method is throttle closed (with proper idle adjustment) and plungers open to start a cold 2 stroke. If this doesn't work something else is wrong. Air leaks at carb mounts or from crankcase, wet plugs, timing, enrichment circuit plugged.
5. Compression readings on two strokes are normally 100-110 psi, so there's not much room for loss without encountering difficulties in starting, but it should still fire off with 75, provided it can create enough vacuum to the carb when kicking over, that's the key.

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Billybobbigbear
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PostPosted: Thu Sep 28, 2017 11:34 am    Post subject: Reply with quote

Sound like a weird problem so i give a weird answer

Maybe piston was not put in the right way around usually there is an arrow on top of piston which is meant to face in the exhaust port direction.
may have broke a ring and scored the bore ????????????????

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4 - Yamaha RD250 dressed as RZ
5 - Suzuki GSXR250R 1991
6 - Suzuki 1985 gsxr750
7 - Suzuki 1984 GSX-R400
8 - Suzuki1984 RG250
9 - 1987 Nsr 250 mc-16
10 - Honda vfr400r 1991
11 - Suzuki rm250 1996
12 - Suzuki gsxr 750t 1996
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05c50
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PostPosted: Thu Sep 28, 2017 5:34 pm    Post subject: Reply with quote

Rizingson wrote:
Just a couple of comments here.
1. Starter plunger open or closed will have no effect on compression readings.


Sorry, Wink I was thinking choke as in the butterfly kind, not the plunger kind.

......Paul

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tmass
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PostPosted: Fri Sep 29, 2017 10:55 am    Post subject: Reply with quote

When I was putting my engine back together all I had done was Suzuki hone the cylinders and put new rings on old pistons that were to spec. The cylinders were ever so slightly over but I still got ~140 psi when cold in both cylinders.
The Suzuki dealer said I'd need at least 90-100 psi to get the thing to start reliably and run right. And yeah there are arrows on the pistons that should point forward to the front wheel.

The problem I had with starting issues turned out to be the ignition. points wrong, timing wrong, cam lobe wrong. After that correction the thing starts second kick from cold.
I just use the enrichment circuit and not the throttle like they said.
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dorT500
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PostPosted: Fri Sep 29, 2017 12:37 pm    Post subject: Reply with quote

tmass wrote:
...............The problem I had with starting issues turned out to be the ignition. points wrong, timing wrong, cam lobe wrong. After that correction the thing starts second kick from cold.
I just use the enrichment circuit and not the throttle like they said.
I suspected there was, at the least, a points gap issue. I've found that as a problem before. Sometimes it the simple things that can drive one crazy wacko and less we forget........never assume anything when trouble shooting. Have fun.

2thumbs

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fodder
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PostPosted: Sun Oct 01, 2017 12:40 pm    Post subject: Reply with quote

Thanks for all the suggestions.

I have checked, re-checked and checked again the points and timing and they seem spot on. Tmass, I haven't checked the cam lobe. What was the issue with yours, is it something I should check? If so, how?

Adrian Baker, who has the website http://www.t20suzuki.com/ site and is very clued up about these machines came over yesterday and had a look. He had helped with (i.e. did most of !) the engine rebuild so I'm pretty confident that it was done correctly. But no, we didn't hone the bores so that could be an issue. Or maybe I need to get some more miles on it to bed the rings in.

Quote:
Just a couple of comments here.
1. Starter plunger open or closed will have no effect on compression readings.
2. You said the cylinders looked fine. Did you not hone them to break in the new rings? If not compression will never come up as it should when rebuilding because rings won't seat.
3. Carb were cleaned how? Did you confirm the adjustment on the plungers to close and open properly.
4. Starting method is throttle closed (with proper idle adjustment) and plungers open to start a cold 2 stroke. If this doesn't work something else is wrong. Air leaks at carb mounts or from crankcase, wet plugs, timing, enrichment circuit plugged.
5. Compression readings on two strokes are normally 100-110 psi, so there's not much room for loss without encountering difficulties in starting, but it should still fire off with 75, provided it can create enough vacuum to the carb when kicking over, that's the key


Thanks for those thoughts:

1. Thats good to know!
2. No, we didn't hone them. I'm thinking I may need to get the barrels off and take another look.
3. Carb cleaner, compressed air, and latterly an ultrasonic cleaner. Plus the jets are all new - mains, idle, needle. New plunger on one of the starters. Since my first post I have today uncovered another blockage, which was in the float bowl. The starter circuit has a pipe going to the bottom of the float bowl, which locates inside a cylinder in the float bowl. The cylinder has a feed from the main part of the float bowl and this was blocked. So I have now cleared that.
4. Yes, I discovered that from reading the Mikuni manual and that method definitely hadn't been working, which was why I went through the starter circuit again - see 4. above.
5. Adrian reckoned compression was OK, by the feel of the kick over, and it did eventally fire. But yes, I feel things could be better if the compression were higher.

Bilybob, good thought and yes, the pistons do have the arrows facing the exhaust ports.

Adrians conclusion yesterday was that the carbs are not right - the starter circuits weren't working properly and there is also something else causing fuel starvation. He did balance the sliders for me, which improved the responsiveness when I got out on a ride today.

Having sorted the blockage in 4. above, I tried the bike again this morning. The bike started on the chokes, fourth kick. Hallelujah, I thought, and took it out for a 10 mile run. It was pulling well, I didn't go OTT but kept at about 60 mph. Then when I came to a halt at a trainline it died on me. A minute or so later I kicked it into life again and it started OK, and ran OK again until I got home. It had felt like fuel starvation when it died.

Then later this afternoon I tried starting it again. It took probably 20 kicks again and then wouldn't settle down to a decent idle ( Crying or Very sad ) .

My next steps will be:

1 Take carbs off. Again!! Re-inspect, blow through every orifice, see if any dirt has crept in.
2 Do a leak down test. I have made myself a tester.
3 Take the heads/barrels off and have a look at the cylinders. When you say "hone" do you mean just go over the walls with fine emery or similar to get the glaze off?

I won't have chance to anything much for a couple of weeks but will post back when I find anything. Thanks again for the input, its all gratefully received Very Happy
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tmass
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PostPosted: Tue Oct 03, 2017 1:21 pm    Post subject: Reply with quote

I the bike started, then the cam lobe I suspect is fine. So the PO put the cam lobe back on and not in the keyed slot so the timing on mine was off and never started.

I had the cylinders honed meaning a tool that smooths the bore out. It looked like stones on the tool that fit in the bore and was turned at a slow rpm while running some sort of lube in the bore.
Suzuki told me should have at least 100psi on compression chk. Said it needs that much to start and run right.... I don't know. But my tests showed 140psi cold.

"Everything" that I'm trying to correct on my bike now is I believe is fuel related....

Hows your gas tank? clean? I thought my tank was good, but put inline fuel filters in place and yeah, dirt showed up.
Gas cap vent is messed up, working to fix that. And you need to sync the carburetors up
I'm sure the crank and case seals are beat, but the bike doesn't have any problem starting and running.

Thing runs strong though, we laugh every time we ride it.
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kbts250m
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PostPosted: Wed Oct 04, 2017 8:34 am    Post subject: Reply with quote

Your compression is too low. You should have around 100 psi cold, a little higher than that hot. Throttle has to be open. Good point about the length of the probe that screws into the spark plug hole. If you knew the precise combustion chamber volume at TDC you should be able to mathematically correct for that short probe but it would be way easier to just shim the long probe so the remaining thread length equals that of your spark plug. You can use spark plug washers or even rubber o-rings for shims.

I suspect your problem is those new rings. The cylinders should have been honed. Even a cheepo ball hone or doing it by hand with 220 grit wet/dry sandpaper would be an improvement. Also, are you positive you have the right size rings? Standard rings will fit an oversize piston/bore, but in that case the ring end gap will be too big. Did you check the ring end gaps before re-assembling the engine? It should be between .008" - .016" (0.2mm - 0.4mm) for new rings and anything over about .025" will be an issue. Too much end gap will definitely give you low compression, low intake vacuum, and hard starting.

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fodder
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PostPosted: Wed Oct 04, 2017 8:35 am    Post subject: Reply with quote

Tmass, yours sounds pretty good! The seals can't be too bad if it is performing like that.

My tank is very clean. I gave it the POR15 treatment - acid etch clean to get rid of rust, then neutraliser, then the coating which protects against rust,ethanol etc.

I'm gradually getting through the carbs. They are synced. I am going to get them off again to check the pilot jets (again!).

It does sound like I should get the barrels off and get them honed, though. 80 psi is not a good starting value.


Last edited by fodder on Wed Oct 04, 2017 8:47 am; edited 1 time in total
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fodder
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PostPosted: Wed Oct 04, 2017 8:47 am    Post subject: Reply with quote

Quote:
I suspect your problem is those new rings. The cylinders should have been honed. Even a cheepo ball hone or doing it by hand with 220 grit wet/dry sandpaper would be an improvement. Also, are you positive you have the right size rings? Standard rings will fit an oversize piston/bore, but in that case the ring end gap will be too big. Did you check the ring end gaps before re-assembling the engine? It should be between .008" - .016" (0.2mm - 0.4mm) for new rings and anything over about .025" will be an issue. Too much end gap will definitely give you low compression, low intake vacuum, and hard starting.


Thanks for the thoughts. Yes, I'm reluctantly coming to the conclusion that the heads/barrels need to come off!

The pistons are standard size, as are the rings. Well, I say that, because I ordered standard size rings but I didn't check the markings on them.

No, we didn't check the end gaps. How do you do that - do you have to squeeze the rings in and then measure?
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kbts250m
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PostPosted: Wed Oct 04, 2017 10:34 am    Post subject: Reply with quote

Cylinder off, rings off piston. Put ring in cylinder right up near the top and check gap with a feeler gage. Below is a link to a screenshot from the service manual for my own bike. The procedure is the same for yours.
https://i.imgur.com/C2gIVVa.jpg

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Previously - 70 Suz T350, 71 Suz TS250, 72 Suz GT380, 72 Suz TS125, 72 Suz TS250, 75 Yam YZ360B, 72 Suz GT380 diy cafe racer, 80 Suz GS750, 73 Suz TS185, 82 Suz GS1100
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