Compression and Carbs


HondaJohn
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Compression and Carbs

Post by HondaJohn »

I have lowered the motor of my "new" 81 cbx project and conducted a compression check. I have readings from 133 to 103. the book says it is supposed to be 170psi plus or minus 12psi. in your experience, what is the average compression readings? It seems that 170 is high and as I remember, my readings of my two previous cbx's, they were never that high. I squirted some oil in the low cylinder (#2)and retook a reading. It came from 103 to 106 so I really don’t think rings are the problem. I think I need to adjust the valves first in number 2 and redo the compression reading in that cylinder. This may determine whether or not I have to pull the top end. Would appreciate input on this problem..... valve shims?? Where do I get them??



Also between carbs 2&3 and 4&5, there is a breather tube connecting them. this tube is rotted and broken where it connects the carbs. What will it take to fix this? If it is a major problem, then I have to make a decision on how to get fixed or look for another set of carbs… As always, would appreciate advice on these issues. NOTE: this bike came from original owner and he did not ride it much in the past 5 years but did ride it some. carbs are very dirty. private email is cbxjohn@cox.net

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Post by alimey4u2 »

John, The first thing I would do would be to check your pressure gauge. They have been known to be out.....

Then check your valve clearances......

If you rotate the engine over slowly (put a wrench on the crank end...Ignition OFF.) Remove the carbs & exhaust, with a long plastic tube, stick one end in your ear & the other end in an open port. You will be able to hear valves passing. It will be easy to find out which are bad by the volume of the noise.......

With the crankcase vent tube off, you may also listen to piston blowby there when rotating. Need peace & quiet to do this so send the kids off to play........ :lol:

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Re: Compression and Carbs

Post by Chris »

HondaJohn wrote: Would appreciate input on this problem..... valve shims?? Where do I get them??



Also between carbs 2&3 and 4&5, there is a breather tube connecting them. this tube is rotted and broken where it connects the carbs. What will it take to fix this? If it is a major problem, then I have to make a decision on how to get fixed or look for another set of carbs… As always, would appreciate advice on these issues. NOTE: this bike came from original owner and he did not ride it much in the past 5 years but did ride it some. carbs are very dirty. private email is cbxjohn@cox.net


John,



I have got valve shims from local dealers (not just Honda dealers either). I have bought shims from TIMS and have exhanged shims with CBXMan. Its hard to get just the size you need sometimes.



The vent tubes are still available from Honda. The shop manual says that you need to disassemble the carb racks completely to replace them and this is not true. The new rubber "T" fittings can be pushed back onto the carbs with a blunt pushing tool (such as a Popcicle stick). Use some silicone lube on the rubber.

HondaJohn
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Carbs & compression

Post by HondaJohn »

thanks for those replies. the compr gauge is new and not a cheap one so I guess I have to put my trust in it. thanks for the comments and i will do further investigation, adj valves etc....

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Post by broook »

To the best of my knowledge you should check compression with the engine "warm" and with the "throttle wide open". I think you will get a higher reading. Hope this helps.

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Post by alimey4u2 »

I forgot about the WFO throttle...... :oops:

Thanks for the reminders broook.......... 8)

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Compression and carbs follow up

Post by HondaJohn »

I did remember to have the throttle wide open. Just did all this on my 87 Hurricane 1000. The compression numbers on it were astounding for a stock 87 bike. book says 180 psi and the bike had 175-185 in all with throttle open. closed, the readings were in the 135-145 range, so this gives an example of the difference in closed and open throttles. The previous owner of this 81 cbx did his own valves and his valve adj tracking sheet noted that #2 was tight. He had 3 abbreviations = T (tight), S (snug) and L (loose), but all being within tolerance. #2 was the only one that was marked as tight and now I am thinking it was probably "too" tight and with the passage of time, it may have gotten tighter, with 4k miles and 5 yrs since valves were adjusted. I am going to adjust them and then take another reading, after cleaning and reinstalling the carbs. THis used to be old "hat" for me but I have not touched one of these beasts since the early 90s, when I owned a 79 and an 82, both bought brand new. My thanks to those that took the time to reply to this problem.

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Re: Compression and Carbs

Post by retrex »

HondaJohn wrote:I have readings from 133 to 103. the book says it is supposed to be 170psi plus or minus 12psi. in your experience, what is the average compression readings? It seems that 170 is high and as I remember, my readings of my two previous cbx's, they were never that high. I think I need to adjust the valves first in number 2 and redo the compression reading in that cylinder.



Also between carbs 2&3 and 4&5, there is a breather tube connecting them. this tube is rotted and broken where it connects the carbs. What will it take to fix this?


On the compression, pressure is also determine by air temp, see the Boyle's Law equation PV=nRT where T = temp. On a cooler day you should have lower compression than on a hotter day. That being said I think that Honda's number of 170 is still a bit optimistic. I have consistently recorded readings of 135 to 150 for a 9.5 to 1 compression engine in good condition using a professional grade tester. The CBX is a 9.3 to 1 if memory serves me, so therefore I would think that an average range of 130 to 150 psi might be considered a good ballpark figure.



Go ahead and adjust the valves and retest. Recently having done a topend job on my CBX, I think that carbon buildup on the valve surfaces might be a likely suspect if you don't see significant improvement after the adjustment. It also might be a burnt valve in which case a top end job is in order.



As for the rubber T like tube, this is the overflow tube from the carbs if I am following what you are describing correctly. They are only in use when the carbs overflow due to bad float valves. My tubes were also shot. I did not want to take the time, not to mention the high cost, to replace them so I did the following.



After situating the rubber of each tube so that the cracking was at a minimum, I wraped a thin wire around the tube to stabilize it. Stripped twist tie wire and a pair of thin needle nose pliers are the trick here. Slip the wire around the rubber and joint and give it a couple of twirls to secure the rubber from spreading. After securing the tube with the wire give it a quick cleaning with a shot of brake cleaner and either blow the rubber dry or let it dry naturally. Once the tubing is dry use some type of silicone sealer on the rubber to fill and seal the cracks. I used the red stuff from Permatex. Someone may have a better suggestion for a sealer.



What this fix does is keep debris and water from entering the carbs. I was more worried about that then having fuel leak onto the engine case should a fuel needle valve go. Good luck.
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1981 GS1100EX - Undergoing rebuild

1981 GS1100EX - Rolling rebuild

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compression and carbs continued

Post by HondaJohn »

I agree with you on everything. I polled some people and several have said the same thing - compression usually comes in between 12-135 at best, cold engine, throttle open. Who can do this on a hot motor?? Not me!! Going to do the valves, redo the compression check and see where I am. Concerning the broke air vent tubes, I am going to do something very similar to what you have suggested. Having been in the offshore fishing for several years, I was introduced to liquid rubber for protection of electrical connections and anything else one would want to keep salt out of. I am going to use liquid rubber to build up the broken areas, (lightly), reinstall and then apply liquid rubber in thin coats over them. It is thick and if used sparingly, it will not interfere with the air passage. One other CBX expert said that he threw his away and that they are not needed. the vent holes will be ok without the tubes. THese 81 carbs have a lot more "stuff" on them than my 79 did - wonder if it is needed. I really want a set of 79 carbs for this motor. I adapted a set of 79 carbs on my 82 years ago and after figuring out the jetting (dropped motor 3 times!!), down two sizes on main and one size on the slow jet, the motor and jetting was right on target with a nice grey color at the end of the pipe. Also improved the mid-range for the 82. Someday..... Thanks for taking the time to respond to this dilemma.

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Compression and Carbs continued with other stuff...

Post by HondaJohn »

And, before I ran into financial problems in the early 90s, I owned a 75 Gold wing - built motor to 1140cc, polished the heads, 4:1 gear in rearend, 35k volt coils, dnya ignition etc... and it would humble a KZ1000 with no problem; 79 CBX - blew motor and rebuilt to some ungodly cc level, dont remember; MTC pistons, MTC rods, 35k volt coils, dyna ignition, polished head, rejetted carbs, K&N, etc... and yes it was a beast. Then the 82 CBX came along and i did nothing to it except ride it, polish it and put a K&N filter in it. Two Honda 750 fours, and lastly an 87 Hurricane (blk and red) 1000. All of these bikes were bought brand new except for on of the 750s. Sold all due to money and divorce problems. Now I have collected this 81 which will turn out very, very nice and will be 99% original. My 87 Hurricane is nothing other than pristine!! I just finished it. THis is part of my near 40 year bike history but it would take a day to get into the really early days of Dream 305, Superhawk 305, etc.... etc.... Now, just hoping to collect a few of my old favorites before I check out to motorcycle heaven.... or wherever....

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Post by Jeff Bennetts »

The vent tubes on the carbs are to equalize the carbs to the outside atmospheric pressure, the only reason they have the tubing is to keep any outside contaminents from entering the carbs.



The tees are still available from Honda and are very expensive.



jb

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Re: Compression and Carbs continued with other stuff...

Post by Chris »

HondaJohn wrote:And, before I ran into financial problems in the early 90s, I owned a 75 Gold wing - built motor to 1140cc, polished the heads, 4:1 gear in rearend, 35k volt coils, dnya ignition etc... and it would humble a KZ1000 with no problem; 79 CBX - blew motor and rebuilt to some ungodly cc level, dont remember; MTC pistons, MTC rods, 35k volt coils, dyna ignition, polished head, rejetted carbs, K&N, etc... and yes it was a beast. Then the 82 CBX came along and i did nothing to it except ride it, polish it and put a K&N filter in it. Two Honda 750 fours, and lastly an 87 Hurricane (blk and red) 1000. All of these bikes were bought brand new except for on of the 750s. Sold all due to money and divorce problems. Now I have collected this 81 which will turn out very, very nice and will be 99% original. My 87 Hurricane is nothing other than pristine!! I just finished it. THis is part of my near 40 year bike history but it would take a day to get into the really early days of Dream 305, Superhawk 305, etc.... etc.... Now, just hoping to collect a few of my old favorites before I check out to motorcycle heaven.... or wherever....


HondaJohn,



You sound like a good fit with a number of X'ers on these forums. Keep us posted on your repair work. We'll provide whatever help we can.



Chris

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Post by Chris »

Jeff Bennetts wrote:The tees are still available from Honda and are very expensive.



jb


I'm a life long motorhead and currently have 3 X's. I'm way past getting upset about many OEM and aftermarket part prices but...........



I was a little surprised when I bought a set of these "Joint Set D" parts for my most recent restoration. PN 16036-422-004. Price from Service Honda is $22 each. Ouch :shock:

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compression, carbs and other bs

Post by HondaJohn »

I may have been welcomed to this club as a new member in a very professional and informative manner, but the real news is that I was a very early member of this club o/a 1980/81 but my original membership number could not be found. I corresponded with Stan Carpenter many times and spoke to him several times, especially during the period when he built the black, beautiful CBX. When I bought my Hurricane in 1987 (still owned the 79 and 82 CBX's), Stan got mad at me, (or indifferent) and disappeared from communication. I was so in love with the Hurricane that I did fade from the CBX club but still owned them up into the early 90s when I ran into money problems. I know a lot about these bikes but need memory refreshment and am getting a giant dose of it right now. THanks for all the comments....

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Post by Rick Pope »

There are two things which were brought up earlier in this thread that I'll address. I may be wrong so please help me out here.

First was the relationship of compression to cranking pressure. The two may or may not correlate. A high perfomance engine will have a high compression ratio, but it will also have more valve overlap in it's cam timing. This makes for some of the stroke to be wasted at low rpm, thus a low cranking pressure.

Second was the suggestion to use silicone on the carb tees. Don't do it. In fact, don't use silicone anywhere it will come in contact with gasoline or it's fumes. It will turn to a gooey mess (Don't ask...), unless there's a specific formulation of silicone I've not used.
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