Author Topic: How do I improve compression?  (Read 5778 times)

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Offline edward

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How do I improve compression?
« on: August 02, 2014, 07:15:31 PM »
  Is compression only a measurement of pressure inside the cylinder? OR does it some how include the rest of the compartment? Like, the seal around the disc valves?
  I am measuring very low compression in both sides of the DT 175 (@ 60). I have new pistons, rings and cylinders from Richard. Everything fits perfectly and the rings are fitted in properly around the small set pin.
   I did not see the torque measurements for the bolts on top of the heads anywhere. Does anyone know about what it should be? I have downloaded all 3 manuals (tech, service, and owners) and do not see the specs.
   Actually I received the parts over a year ago and just now got to a point of putting it all together. I have noticed that the pistons and cylinders are both aluminum and the manuals states that the aluminum cylinders should have aluminum pistons designated by an (EX) on the top of the piston and a flat spot in the middle so, that seem to be in order.
   Any help with improving the compression would be greatly appreciated, especially if it solves the problem.
               Thank you so much in advance.

mqtsteve

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Re: How do I improve compression?
« Reply #1 on: August 02, 2014, 08:05:34 PM »
Edward, this is a thread with torque specs.  Hope it helps, mqtsteve

http://bridgestonemotorcycleparts.com/index.php?topic=755.0

Offline edward

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Re: How do I improve compression?
« Reply #2 on: August 03, 2014, 04:09:53 PM »
Thanks. Where was this found? I thought I would find it under cylinder and piston assembly/ re-assembly section of the manuals but I couldn't find it.

mqtsteve

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Re: How do I improve compression?
« Reply #3 on: August 03, 2014, 05:08:50 PM »
edward, I did a search for "torque specs" from the menu.  I'm not sure if they apply to all models?  This is a photo of the torque specs from a manual from Fox Corp. for the smaller bikes.  Hope it helps, Steve  (50sp,60sp,all 90s,175dt,200mIIrs and 200mIIss models)
« Last Edit: August 04, 2014, 05:57:43 AM by mqtsteve »

Offline OldSwartout

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Re: How do I improve compression?
« Reply #4 on: August 03, 2014, 10:48:02 PM »
It will take some break-in running to get the rings seated, so your compression will be somewhat low to start with.  I assume there is some oil in the cylinders from assembly. If they're pretty dry, the compression will definitely be low.
Karl Swartout
Mooresville, IN
BS175 Roadracer. BS200RS, BS350 GTR

Offline edward

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Re: How do I improve compression?
« Reply #5 on: August 03, 2014, 11:41:35 PM »
Thanks. I put a layer of engine oil on the cylinder to get the pistons and rings in. It was probably not enough.

Offline edward

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Re: How do I improve compression?
« Reply #6 on: August 04, 2014, 09:26:49 AM »
I am really new at this so this may seem like an odd question but, should I be measuring the compression of only 1 kick of the starter or several (like 3 or 4 kicks)? Because after 3 or 4 kicks of the starter I have about 120 PSI and it stops building up after that. The first kick is 60 PSI the 2nd is 90PSI and the 3rd and 4th are up around 120 PSI. The 5th and 6th stay at 120 PSI.

Steve S.

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Re: How do I improve compression?
« Reply #7 on: August 04, 2014, 06:51:56 PM »
I always measured with several kicks and throttle wide open.

Steve S.

Redrust

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Re: How do I improve compression?
« Reply #8 on: August 04, 2014, 08:38:53 PM »
3 to 4 kicks is wat I've always done. Its how I found out that mine had low compression at 70psi. After I put new rings in I got all the way back up to 120 like yours.

premiumjo

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Re: How do I improve compression?
« Reply #9 on: August 04, 2014, 08:58:15 PM »
yeah when the gauge stops increasing, that is your compression reading. And like Steve said, you need to hold the throttle open (or have the carb removed). Some people say you should measure it when it's hot, but i find screwing a compression tester into a hot engine annoying. Also, if you are trying to troubleshoot an issue (like no start) it's kinda tough to make it hot. The compression test IS indeed a measurement of pressure in the cylinder. To test for leaks in seals, etc., you would need to do a leak-down test which is basically pressurizing the bottom end to about 6 PSI and seeing how long it takes to leak down (which is a good idea on a 47 year-old 2 stroke). Google how to build a leak-down tester. Everything can be had from home depot for about $20.
rd

Offline Mopar5426

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Re: How do I improve compression?
« Reply #10 on: August 04, 2014, 11:10:31 PM »
  I Just Sold My 350 GTR To A Customer Who Asked For A Compression Test  I Installed A Compression Gage And Kicked It Untill It Stopped Riseing It Stoped At 130 Lbs   It Runs Great & Stars On A Couple Of Kicks Hope This Helps   GENE

Offline edward

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Re: How do I improve compression?
« Reply #11 on: August 04, 2014, 11:37:51 PM »
WOW! Guys thanks for getting back to me so quickly. It looks like I don't have a problem there after all.
I have already fashioned jigs for the carburetors at 18mm and the set pin and drilled out bolt for the timing and they both look good. The points have a 0.35mm gap when the pistons are at the top. The carbs?....well I'm still learning about the carbs. I did however get a 6 inch flame to shoot out of one of them the other day!  :o

Redrust

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Re: How do I improve compression?
« Reply #12 on: August 06, 2014, 02:32:18 AM »
Check your timing and don't be stingy about having to move the generator. Mine did that once while I had it running trying to get a solid tune. Damn near started myself on fire. Ended up going step by step to get the point gap perfect at exactly spec. Still not sure if its got a great tune but it runs like a champ. I'll have to get a video of it running now that I ride it almost daily.

 


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