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#80353 - 06/23/05 12:42 pm Compression test questions  
Joined: May 2005
Posts: 57
AMERIBRIT Offline
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AMERIBRIT  Offline
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Joined: May 2005
Posts: 57
Pennsylvania USA
1968 Triumph T120 650.

I did a compression test but I want to make sure I did it correctly because it's silghtly different then a car or truck.

When checking compression, should I have the throttle closed or open when I kick? I have heard both.

I got 145 on the r/s and 140 on the l/s. I think this is low besides the differences in cylinder compression. Shouldn't compression be around 170-180?

The bike still runs and I sprayed the engine with soapy water and didn't find an air leak. Maybe a valve issue? I have not checked my clearances. I hear that can be hard to do accurately.

R/S pipe is still running hot. I cheked the float. .080. All three needles are clear and open. This may be a dumb question but am I not getting enough vacuum to pull in fuel?

Thanks RJH


RJH
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#80354 - 06/23/05 1:43 pm Re: Compression test questions  
Joined: Aug 2004
Posts: 82
flatrack Offline
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Joined: Aug 2004
Posts: 82
Arizona
RJH, Throttle should be open, if air can't enter there will be nothing to compress. The 5 psi differenceis so little I wouldn't worry about it. You could check again 2 months from now and find the side that had 140 now has 145 and the side with 145 now has 140. Paul

#80355 - 06/23/05 2:42 pm Re: Compression test questions  
Joined: Apr 2005
Posts: 662
Riff_Raff Offline
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Joined: Apr 2005
Posts: 662
Arizona Territory
Hopefully, others will offer their input, but your compression readings sound fine to me.

I check mine with throttle closed, both plugs removed during testing, but it's a personal call. As flatrack mentioned, the 5 lb. variance between cylinders is normal. Up to 10 lbs. variance would still be O.K. & again, as per flatrack's post, the readings can change up or down with either cylinder.

I've heard that compression readings for these old Triumph 650's can be acceptable anywhere's from 90-140 psi, so again, your readings sound pretty dang good.


"Factory Stock Is A Suggestion Only"
#80356 - 06/23/05 3:15 pm Re: Compression test questions  
Joined: May 2005
Posts: 57
AMERIBRIT Offline
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AMERIBRIT  Offline
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Joined: May 2005
Posts: 57
Pennsylvania USA
Thanks for the response guys. And yes, all replys are welcome.

Any Ideas on why the r/s pipe may be running hot. See above for explaination.

RJH


RJH
#80357 - 06/23/05 3:48 pm Re: Compression test questions  
Joined: Oct 2003
Posts: 11,110
RF Whatley Online content
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RF Whatley  Online Content
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Joined: Oct 2003
Posts: 11,110
North Georgia, USA
RJH -
There's nothing different about the compression proceedure for a car, truck or motorcycle. You remove all the plugs, open the throttle, and kick rapidly (or use the electric starter) until the gauge needle stops climbing. On a Triumph this usually happens in 4 to 5 kicks. (Or 4 or 5 compression strokes on an electric start vehicle.)

145/140 is great compression in that most "daily riders" have between 140-150 psi readings. It's generally considered that the highest and lowest cylinder should be within 15% of each other. So there again you're OK.

"I check the float. .080"?? I suppose you mean that you checked your carb float levels, but did you mean to say that the plastic float is .080 below the bowl gasket surface, or did you mean the fuel level is .080" below? You were neither clear nor specific.

"All 3 needles are clear." Huh?? All 3 of what "needles".

Replace the balance tube with BLACK 1/4" ID FUEL-rated tubing and renue the 2 o-rings where the carbs mount to the intake flange. That will cure any vacuum issue for under $4. At that price it's not even worth diagnosing, "just do it".

There are generally 2 reasons for different running temps, which I assume you measured at idle in your shop and not hanging off the bike going down the road. The first is a partially clogged pilot jet which is very common on these old bikes. #2 is timing not being the same on both cyclinders.

There is also the issue of the reflectivity of the chrome on your exhaust pipes which will cause different temperature readings from the laser temp gun you are most probably using.

I believe the bike is OK and running fine. You simply need to stop buying the fancy tools and start applying some more common sense.


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#80358 - 06/24/05 12:53 am Re: Compression test questions  
Joined: May 2005
Posts: 57
AMERIBRIT Offline
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AMERIBRIT  Offline
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Joined: May 2005
Posts: 57
Pennsylvania USA
Not all compression test are the same. A very common procedure for European cars is a running compression test. As for newer fuel injected Japaneese cars you cannot do a compression test with an open throttle or it puts the PCM in clear flood mode. So I do not agree that compression tests for cars trucks and motorcycles are the same.

Sorry for being so vauge, but my lack of knowledge on carberators does leave me at a loss.

As for the float, yes, I meant .080 is the level the float is below the float chamber. I believe too low of a level causes a lean condition an visa-versa. Once again my lack of knowledge leaves me at a loss. I did not know that one could measure the fuel level in the float chamber for a lean condition.

"The three needles."What I meant to say was the main jet, needle jet and the pilot jet screw were all clean and clear.

Finally for the fancy tools. I do own a laser thermometer. It was rather helpful finding dead buckets on forigen engines without pulling plugs. Differences in temps on the exhaust manifold and all. But the high tech gagetry I used to determine one cylinder was running hotter that the other were my eyes as they looked at the glowing red pipe at the end of the exhaust port.

As for applying my common sense, I guess when I have 400 years of experience with motorcycles I will be able to ask better questions.

Now excuse me while I return to the shop and see why my fine running bike's r/s exhaust glows redder than coals in my bbq pit.


RJH
#80359 - 06/24/05 2:01 am Re: Compression test questions  
Joined: Oct 2003
Posts: 11,110
RF Whatley Online content
BritBike Forum member
RF Whatley  Online Content
BritBike Forum member

Joined: Oct 2003
Posts: 11,110
North Georgia, USA
RJH -
Let me answer your questions one at a time, now that you've supplied the missing specifics...

Europeans do everything different. That's how they've managed to start 2 world wars. So we can discount running compression tests right off. I never suspected you were considering Jap fuel injection on your Bonnie, so we can discount that too. wink

Your float IS too low. That .080" deal was for one particular model. On your bike, lower the float seat until the top edge of the float (opposite the float needle) comes flush with the gasket surface of the bowl. You can actually go a little higher, but you run the chance of fouling the bottom of the tickle button. Check it by re-assembly and holding the bowl up-side-down, or simply connecting the fuel line and letting it fill. Either way you have to manually hold the hinge pin down.

The 3 needles you mentioned are always clean. It's what you can't see way up inside, behind the pilot screw that always gets you. Stick a very small guitar string (.015" max) up in there and twirl it around.

I find that spitting on exhaust manifolds gives me just as much feedback as a laser temp gun when looking for "dead buckets". smile And is somewhat less expensive.

Thanks for the input on the color of your pipes. Not to split hairs, but there's a smallish difference between "running hot" and pipes glowing red. Sometimes people trying to help you need that type information to ascertain the severity of the issue at hand.

BTW... If you'd really like to know how a carb works I'll be glad to explain it.


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes
#80360 - 06/24/05 2:12 am Re: Compression test questions  
Joined: May 2005
Posts: 57
AMERIBRIT Offline
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AMERIBRIT  Offline
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Joined: May 2005
Posts: 57
Pennsylvania USA
Redneck,

How'd you know I played guitar?? laughing


RJH
#80361 - 06/24/05 2:23 am Re: Compression test questions  
Joined: Oct 2003
Posts: 11,110
RF Whatley Online content
BritBike Forum member
RF Whatley  Online Content
BritBike Forum member

Joined: Oct 2003
Posts: 11,110
North Georgia, USA
Maybe it was the calluses on your left hand. laughing


Don't hide 'em, Ride 'em !!

RF Whatley
Cornelia, GA

"Shop Boy" at Rodi British Bikes

Moderated by  John Healy 


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