[70CB175]

I finally finished (or so I thought) my 1970 CB175, I got a VT plate and reg, got my motorcycle permit and then this happens.

I can't get the thing to rev past 5000rpm, it just cuts out and barely runs on the left cylinder while the right dies completely. With the air filters off it runs perfect right to redline which obviously points to a rich mixture but I can't ride it without filters so I'm stumped. The bike has aftermarket pipes off an XS650 my dad had laying around, basically straight thru megaphones, they are proportionate to the bike and they do extend slightly past the rear axle.

I know plugs and ignition are all ok as it pulls great w/out air filters

So far I have done the following

-tried 3 new and used throttle needles
-tried the needles on 2nd, 3rd, and 4th grooves up from the bottom (runs best on 3rd worse on the other two)
-tried another throttle slide I had laying around just for kicks
-tried a CL #92 main jet in place of the #98 factory (only had one laying around so put it in the rich running rh cyl)
-I know how to adjust the air screws and have them set right (only helps from idle to 1/4 throttle anyway)

Before I assembled the bike my dads buddy (very meticulous guy worked at Honda dealers since these bikes were new until the late 90s) rebuilt the carbs for me with all new o rings and complete carb kits I purchased.

Motor does have good compression (I just did a light rebuild w/ new rings, all valves & seats cut and new intake valves) I wanna say it was about 140 with a long hose on the gauge which I know skews the reading

Any help with this issue is greatly appreciated, I am stumped and really want to get some riding in!

 

[JamesPal]

What air filters are you using?

 

[Simo]

Also What's your float height, what's your idle speed set at and will it idle without feathering the throttle, and did the guy who rebuilt the carbs clean the old brass or use what came in the kit? Which kit did you use?

 

[mike in idaho]

Why not source some clean air filters instead of messing around with carb jets?

 

[70CB175]

1. I have a set of new rebuilt air filters with new foam, plus a set of originals that are clean (and I blew them out w compressed air for good measure) and bike acts the same with either set. The bike just seems to want *zero* restriction. I know something is wrong with the carb as there is actual liquid gas laying on top of the slide when I pull it out.

2. Float height is correct, there was gas coming out overflow until I set that right w the new floats

3. The brass parts are a combination (mostly new) but some old like an old throttle needle and an old needle and seat bc the new ones were trash

4. Carb kits are aftermarket I think I paid about $20 each on eBay, definitely lower quality to Honda stuff. Switching to honda jets does not improve it

 

[70CB175]

Forgot to mention, bike idles great around 1200rpm as it should and revs up cleanly until 5k

 

[Simo]

1.the filters shouldn't be restricting the air flow that much just providing laminar flow to the carbs

2. But what height? If you set them to the wrong height all other setting will be off. The thickness of the bowl gasket is enough wrong for everything to be off. The overflow isn't a measure of float height since the stem can split or a sticky valve can flood the carb

3. Almost all aftermarket brass is lower quality than oem jets don't wear out
Float valve parts being the exception
So go back to as many oem parts as possible

4.kits normally have badly cut float bowl gaskets that hang up the float bowls and bad brass parts with unknown jet sizes

???what height are your floats set at and what face did you set them from???

 

[70CB175]

1.the filters shouldn't be restricting the air flow that much just providing laminar flow to the carbs

2. But what height? If you set them to the wrong height all other setting will be off. The thickness of the bowl gasket is enough wrong for everything to be off. The overflow isn't a measure of float height since the stem can split or a sticky valve can flood the carb

3. Almost all aftermarket brass is lower quality than oem jets don't wear out
Float valve parts being the exception
So go back to as many oem parts as possible

4.kits normally have badly cut float bowl gaskets that hang up the float bowls and bad brass parts with unknown jet sizes

???what height are your floats set at and what face did you set them from???

Float height set at 21mm taking into account the aftermarket bowl gasket which is kinda cheesy and doesn't fit perfect but does a better job than an old brittle one.

Already changed aftermarket needle and seat out for a good used genuine Honda one, I understand jets don't go bad but it runs the same with Honda jets or aftermarket jets, they're still the same size

Left carb is setup fine with all the same settings

Thinking I just have a bad carb body at this point as when I pulled the bike out today there was gas in the throat of the carburetor, gonna try and steal the carbs off my dads cb200 this weekend and see if it runs better (I know 200 carbs are 2mm smaller) and I will report back

 

[Richard_Pitman]

I've got CB200 carbs on my CL175 and it runs fine.

Carbs have the standard CB200 88 main jets etc, mounted on the 175 alloy inlets, rather than on the CB200 rubber ones.

(I know 200 carbs are 2mm smaller)

I personally think that this is an urban myth. I'll leave it for you to decide when you have the CB200 carbs and 175 carbs side by side, see if you can detect the alleged 2mm difference. I know I can't ...

 

[kennedy759]

where do you measure the float height from? getting different data, manual says from carb edge, but also told at the gasket, also do both floats need to be even?

 

[70CB175]

Well the issue seems to be solved. My dad checked the bike over (and found I had the float height perfect) but noticed there was a little ball kind of plug that goes in the side of the carb that was missing. I sprayed some carb cleaner in there and found it goes straight thru to the main jet. Blocked off the passage with a little screw and went over the LH carb for good measure and bam she revs to 8k on the stand quick and easy. probably some new Chinese replica carbs will be ordered in the spring (slide bores are bad and leaking a little) but hoping I can put a few miles on this fall now (16,892 now so if I can get it to 17 even I'll be happy lol)

Also as a side note Richard thanks for the tip on the 200 carbs, they did indeed appear the same upon comparison, however I didn't end up putting them on the bike

I'll try and get a picture up once the weather clears up (3 days straight rain) and get a test ride once it dries up a bit

 

[70CB175]

where do you measure the float height from? getting different data, manual says from carb edge, but also told at the gasket, also do both floats need to be even?

Yes both floats should be even, I believe factory spec is 21mm or somewhere around there

I went off the bowl gasket as that is what the bowl will be resting against, but my dad who worked on these when new says to go off the edge of the carburetor, so it looks like your manual is right I would go off the edge of the carb