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Discussion Starter #1
So here's where Im at. 1972 cb175 k6 up and running but rough. I have synced the carbs but in trying to tune I have found the mixture screws don't really make any difference in the idle. Had done the timing, points are new and open dead on the F mark. Valve checked and adjusted to manuals specs .002. Carbs have all the factory jets, floats set to 21mm and carb have been cleaned. I have long straight pipe I fabricated; there are basic baffles. Accel coil making a very nice spark. I have pod air filters and I have added stacks to the carbs to give the filters a little distance and make them fit. Just bought a compression tester and found both cylinders are sitting around 90psi; bad I know, will do a top end rebuild over the winter.
The rough running sounds like both cylinders are missing at idle. Test rides it runs smooth and pulls not too bad for a little bike of its age. Check of the plugs have shown the left is black and sooty right is also rich but not as bad.
So would the bad compression be the issue.......
As I can't get a smooth idle I can see how the mixture screw isn't increasing or decreasing the RPMs, am I wrong?
I understand I will need to change up some jets and guidance there is strongly needed, what size where to buy and so on image.jpg
Suggestions and things to try would be appreciated.
 

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Earl, I understand that you are new to this forum and I will be happy to try and help you sort out the bike's problems....
It might be helpful to us to know where you are located (Please add that and your bike's specifics to the signature line on your profile).....

The smaller 360 degree twins are very finicky about intake tract length, and between that and low compression (IF your measurement is correct, although I doubt it's accurate) you MAY already be above the idle circuit in the carbs which is why the mixture screws have little effect, and you are running so rich.......
The pipes appear to be long enough and (as I have run several 175's with longer straight pipes) will probably be OK......

Feel free to phone to discuss the things you will need to do to get it "tuneable" and solve these.....
Meanwhile, read some of the existing 175 and 200 project logs here for reference..... 540-525-5199.....Steve
 

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Discussion Starter #3
I promise to spruce up my profile later. 66sprint When you said "intake tract length" are you talking about the fact that I added length between the carb and air filter? And sorry they are not in the photo posted. If that is what you are referring to please suggest a length.
And why the doubting of the compression reading, this ain't my first rodeo.
 

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As to my doubts, I'm NOT implying you ran the test incorrectly...
I am assuming engine Hot, choke open and throttle wide open (or carbs removed)....(90 is about what you'd get with throttle closed)
At 90 PSI, the engine will reluctantly start and run, but it won't pull very well, and you said it "pulls not too bad"......
Normal compression spec is 170, and 90 is WELL below the 150 Minimum spec......
I suspect your gauge has a long hose attached to it......
When dealing with about 20cc's of compressed dome volume, an 18" hose of 1/4" ID nearly doubles that volume....
Did you correct for the hose and adaptor volume or is 90 PSI (and PSIG or PSIA?) just what the gauge read?

As to suggesting a "stack" length, I'd need to know the size and shape of your aircleaners so I can calculate "still" volume and surface area.
Plus I'll need to know how smoothly the stack transitions to the carb bell....
 

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When dealing with about 20cc's of compressed dome volume, an 18" hose of 1/4" ID nearly doubles that volume....
Did you correct for the hose and adaptor volume or is 90 PSI (and PSIG or PSIA?) just what the gauge read?
Stephen, I'm not intending to hijack this thread, but I have to know, how do I correct for the hose and adapter volume? I've done compression tests on my bike (150 cold) and have used a long hose and connector for my gauge. I thought I was at the bottom end of the compression range, but as good as the bike pulls, I must need to compensate somehow? I've not seen any articles for that though.
 

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If you are getting 150 cold you're probably at least in the 160/165 range hot........
The explaination and calculations are long and have been covered IF you are still interested....Use the search.....I'm the author, but it was in someone else's posts..........
 

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If you are getting 150 cold you're probably at least in the 160/165 range hot........
The explaination and calculations are long and have been covered IF you are still interested....Use the search.....I'm the author, but it was in someone else's posts..........
I dug around and found where you talked about the volume of the hose being added to the displacement of the engine. I assumed that using the volume of the hose as a percentage of the cylinder, you can add the corresponding percentage to the compression reading. I did that and wound up around 165 so.....thanks for clearing that up for me.

Back to the programmed thread and thank you Earl for your patience.
 

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Discussion Starter #8
Thanks for the schooling. Retested compression, warm throttle fully open. I have a 15" hose on the tester and I have a reading of 120psi. image.jpg
66sprint, these are the specs on the pods I'm using on 5" stacks. You got me thinking that is far too long so I try a set of smaller pods with angled rubber mounts ( no stacks straight off carbs ) no change. I also sprayed water around the intakes while running it seems I don't have any leaks.
 

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Discussion Starter #9
Anyone suggestions as to where and size of jets I need to get for this cb175 k6 with pod filters and straight pipes with minimal baffles
 

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Earl....Those "pods" have less than half the original surface area, and less than 1/3 of the "still volume"......
I would try an approximately 3" diameter X 4" long cylindrical foam filter on a 3.5 to 3.75" "stack"......

I should think with that aircleaner change, and those longer pipes, it would be fine with stock jetting, but "Chop test" and read the plugs....
At idle they should be slightly rich, mid and upper ranges slightly lean.....you could need to go up one size on the pilots to 40's....
 

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Discussion Starter #11
Thanks. Oiled filters or no oil? Any suggestion on where to buy jets; have had some bad experience buying jets off ebay. Planning to open up the engine over the winter and deal with low compression.
 

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To tag onto this post, I have similar issues. 1971 CB175, did a "cafe mod". Used the stock rubber air cleaner connectors, connected to the Emco "wire cones" type filters.
Exhaust is a custom 2 into 1 header, with exhaust pipe/24" cone muffler, pretty straight thru. I have big issues with low rpm's, stumbles up to 3000 RPM then seems fine ( so far i have not gotten it off the bench) compression is 150, cold, using a direct tester, virtually no hose ( type you "push into" the plug opening)
Points/timing checked, all spot on, new plugs. I was all planning on buying several new jets (my mains are 98, slow is 38), but cant find the slow jets. i have been in touch with JETSRUS and they have the mains, but not the slows.
I am in Evansville, IN (very SW corner). carbs were rebuilt, floats spot on at 21mm. My rebuild kit did NOT come with the rubber O rings, and I am thinking this could be an issue, but I have not had another pair of hands around to spray anything at them while it was running ( takes a hand on the throttle or it dies)
When i get a little smarter, i will post a picture to this post showing the carb and exhaust set up. Meanwhile, does anyone have a source for the "slow" jets?. I have all the critical dimensions taken for reference.
 

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I am having similar problems, and I found that the spark advance was stuck on my bike (CL175K5). After freeing it up I was able to get really good running at mid range and higher, but still no decent idle. Sooner or later I'll get it right, but check the operation of the advancer (behind the points plate).
 
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