Hard starting from cold

Just wondered if anyone could shed light on this issue.

I have three running 175s. The first is a CB175 fitted with the original ( cleaned ) Keihin carbs. This starts easily, even using the kickstart, as of course it should. Idles nicely hot or cold, and generally runs well.

The other two engines have Chinese carbs fitted, after I gave up trying to get them to run well with the original carbs, no matter how well I thought I'd cleaned them etc. Plus leaking floats, sticking float valves, you name it. I replaced the Chinese main and idle jets with the correct sized Keihin jetting. On the CL and SL 175s, 38 pilots and 92 mains.

Both bikes are hard to start from cold, needing to be churned on the electric start multiple times until they stumble into a very low idle speed. Touching the throttle at this point kills the engine. Choke open or closed makes no effect in this hot weather. Once the engines have warmed up for a couple of minutes they respond well to the throttle and generally run well. Hot idle is raised somewhat, maybe 1800 rpm. Not much chance of starting them from cold on the kickstart, which leads me to the issue with the non starting rebuilt SL175 engine, which was also on these carbs.

In general terms, what causes poor cold starting, carburettor wise, assuming engine condition ( ignition, compression ) is otherwise OK ? I'm sure float levels are correct, float valves operating correctly etc. Needle clip position should not be relevant for cold start ?

I am running CB450 carbs on my CB360 and I noticed that it was easier to start when I lowered the float height. I kept it at the "correct level" for the 450 and still manage to kickstart it, but with some difficulty.

Have you tried the Chinese pilot? It looks longer than the OEM jet and I wonder if that would help it draw fuel. Do you give it any throttle while e-starting or kicking it?

Hot idle @ 1800 means two things.....
#1... Your slide valve is too open and thus admitting way too much air (too lean) for start-up...
Multiple crankings eventually build up enough liquid fuel in the cylinder to fire, but it doesn't run well because the petrol is not properly "aerated/mixed with the air"....
Once the engine has some heat, the petrol evaporates quickly into a more easily mixed form and it runs better......

#2... @1800, you are already starting to advance the ignition timing......

OF COURSE, the main needle notch DOES have some bearing....How much depends on which clip slot you employ, needle taper, and fitment to needle jet.......

Thanks, food for thought. The Chinese pilots ARE longer, but as both they and the Keihins are submerged in fuel at correct float level it ought not to make a difference.

I take the point about the throttle stops. For me, the critical point is screwing them in just enough the lift the slides for a cold start without the engine then immediately stalling from a too low cold idle.

The other thing that I didn't think about last night was the position of the air screws. Maybe they are too far out at 1 1/2 ( one and one half ) turns out, less turns out should richen the idle mix ?

Lower throttle stops....apply choke (or partial choke) until engine warms......

Personally, I would not have put Chinese carbs on my Honda. Why not just properly rebuild the Keihins with genuine parts?
I know it sounds easy to just replace the carbs but not always the best solution.

What is your cold start procedure?
My Hondas like full choke and no throttle, til it fires, then a little throttle to keep it running and reduce choke until warm.

Personally, I would not have put Chinese carbs on my Honda. Why not just properly rebuild the Keihins with genuine parts?

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If genuine parts were readily available then yes, I'd agree. A lot of the rebuild kits contain dubious pattern parts, and even genuine parts can't rebuild a worn out carb casting, ovalised emulsion tube, idle circuits that continue to behave as though they were blocked despite rigorous cleaning etc.

The Chinese carbs are internally clean and unworn, and are good quality copies of the right hand Keihin. The floats and float valves are superior ( in my opinion ) to the Keihin items, nylon floats and captive viton tipped float needles. Just the brassware lets them down, and this can be swapped for Keihin parts. For example, they sell one carb as being suitable for the CB175, CL and SL 175s, while the OEM carb settings are different on each of those models. I suspect that as delivered they come with a 98 main and 38 pilot, although the actual jets are not marked with sizes.

Had a quick fiddle this afternoon. Screwed the throttle stops out a bit, and screwed the mixture screws in to just one turn out. Full choke, no throttle and the CL175 started and ran after a couple of blips on the starter.

Same procedure on the SL175, but it would only run on one cylinder. To cut a long story short, float bowl was dry on that side. No obvious reason, fuel at the tap float bowl, needle valve working correctly. No fuel at hose at the carb end. Blew through it and fuel flowed again, bike then started and ran correctly.

More fun with carburettors today.

Took the SL175 for a ride this afternoon. It started first go from cold, full choke, no throttle, idled almost straight away. Warmed it up a bit and rode off. While the engine was still relatively cold it ran well throughout the rev range. However, once full hot it was fine 5k rpm onwards, half to full throttle. But on closing then reopening the throttle there was a distinct flat spot, which cleared as the revs increased again.

When I got back home, bike would barely tick over, had to keep blipping the throttle to keep it going. Seemed as though it was running far too rich at this point in the range. I'd adjusted the mixture screws down to one complete turn out, which I guess richened it up for the easy cold start. Adding another half turn with the hot engine saw the rpms shoot up to nearly 3k, which I then adjusted down to 1200 rpm using the throttle stops. So I now have it idling correctly with a (very) hot engine.

It'll be interesting to see how it starts from cold next time. One thing about these Chinese carbs, the mixture screws actually do something !

I also wonder if dropping the needle one notch might help with the off on throttle flat spot, but best not to introduce too many variables at one go.

Mike Nixon has a good test that's quite a bit more reliable than what you're currently doing. The less air test. See https://www.motorcycleproject.com/text/the_60-40_rule.html for more info, but basically put some duct tape on the air filter duct, see if it runs any different/better. If it does then there's some kind of vacuum leak or ignition issue. If it's worse you can try removing the filter.

As early as these bike are I assume they are points and not CDI? If yes, I've noticed if the batteries in them are on their last leg, but still allow you to crank about 5 or 6 times before they get very weak then it may start but idle poorly and may not run well. You probably know that already, but just make sure it's not that I've chased my tail on that before.

I had an issue with idling and starting on my 77 CB550 recently and really suspected the carbs, despite me cleaning then 4 years ago and being pretty religious about fuel stabilizer and ethanol free gas, etc. In the end, it was the timing was way too advanced and one of the caps was arcing and grounding to the fins but only after the bike warmed up. When I replaced the caps I found out they were the wrong resistance and one of them was a no-name aftermarket one that performed somewhat poorly on that cylinder. After fixing the timing and replacing the caps and plugs it runs great now. Almost always starts the first time on choke pulled, if not then easily a second time. Warms up quickly, only about 3-5 minutes (before it would take at least 10 minutes). Just make sure it's nothing stupid like this. I really thought something was funny with float height and/or pilot mixture screws and/or idle circuit and it had absolutely nothing to do with any of that.

Are you using the original air box and filter with your clone carbs? If you're using pods it may make it difficult to tune the bike. It may even change in it's performance from day-to-day.

Since my last report, it has been starting first or second go, and idling well when hot. The only thing that's still bugging me is the transition from full throttle to closed then open again, feels too rich in the midrange, hence my thoughts about dropping the needle one notch. Above 5k the engine is a little screamer, very enjoyable to ride.

The 175s have points ignition as standard, although this engine has a Charlies Place contactless ignition. SL 175s have foam filters as standard, rubber intake runners from filter to carb dimensionally the same as the CB and CL variants. Only variance is the RamAir filter foam I've used, might vary in performance to the original Honda foam
boots.

Richard Pitman said:

Since my last report, it has been starting first or second go, and idling well when hot. The only thing that's still bugging me is the transition from full throttle to closed then open again, feels too rich in the midrange, hence my thoughts about dropping the needle one notch. Above 5k the engine is a little screamer, very enjoyable to ride.

The 175s have points ignition as standard, although this engine has a Charlies Place contactless ignition. SL 175s have foam filters as standard, rubber intake runners from filter to carb dimensionally the same as the CB and CL variants. Only variance is the RamAir filter foam I've used, might vary in performance to the original Honda foam
boots.

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Understood, just wanted to make sure you didn't miss obvious things that only seem obvious in retrospect . Yes, you can try adjusting the needle a notch or two and see what you get. Another trick you can try is to shut the petcock off and then test your range. If it magically becomes better right before it runs out of gas then you are too rich. It's quite possible that because it's a clone the float height is a bit off from spec. You may have to try raising or lowering 1mm or 2, go for a ride, see what it's like.

Never tried the Charlie's Place stuff. Assuming you already verified the timing with it with a timing light?

Good point about checking the dynamic advance, time to dig out the timing light and this cover to reduce oil splash !

Charlies ignition just replaces the points, mechanical advance is retained. Made life more difficult for myself by fitting CB200 rotor, different static and dynamic timing marks than the 175 engines. Timing currently set statically on transposed 175 timing marks, using a test lamp.



Regarding ruling out the unexpected or overlooked factors, a couple of years back I had an issue with my CB600. Wouldn't rev much past 6K, rather than screaming up to 12k +.

'Obviously' a blocked jet. Much effort involved in squeezing the air box out from the frame, followed by the bank of carbs. Stripped and found to be spotless. Refitted, problem still there. Eventually traced to the most obvious ( with the benefit of hindsight ) thing, a mislocated boot on the wiring loom was obstructing the air box snorkel when the seat closed. That sorted, back to bouncing off the rev limiter again.

Nice one on narrowing down the pinched snorkel. Probably was lots of fun to narrow that one down!

^^^Yeah, it's a bit busy in that area, isn't it? You'd have to zip tie everything in place with that little clearance. Damn modern motorcycles...

ancientdad said:

^^^Yeah, it's a bit busy in that area, isn't it? You'd have to zip tie everything in place with that little clearance. Damn modern motorcycles...

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:lol: I've never owned a modern bike, unless you count the early 90s Yamaha Enduro 100 I had as a kid. Just from working on modern cars and seeing how everything is tight together I can only imagine how silly it is on the bikes as well.

Amusing to see my 1998 naked bike referred to as 'modern' ! Access to most things really isn't too bad, just need to drop the radiator or lift the fuel tank to get at most things. Ignition system is the only black box on the bike, no fuel injection, ABS, trip computers and so forth.

Later sports bikes are far worse, have to strip half the bike down just to change the spark plugs, fairings off, tank off, then remove air box, etc etc. And as for the electronics they come with these days, sheer nightmare.

Modern cars are a complete mystery to me, took me a while to even work out where the battery was located on mine, I just take it to the Ford dealership once a year ....

Richard Pitman said:

Amusing to see my 1998 naked bike referred to as 'modern' ! Access to most things really isn't too bad, just need to drop the radiator or lift the fuel tank to get at most things. Ignition system is the only black box on the bike, no fuel injection, ABS, trip computers and so forth.

Later sports bikes are far worse, have to strip half the bike down just to change the spark plugs, fairings off, tank off, then remove air box, etc etc. And as for the electronics they come with these days, sheer nightmare.

Modern cars are a complete mystery to me, took me a while to even work out where the battery was located on mine, I just take it to the Ford dealership once a year ....

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And all of the above is why I was happy to leave the industry behind in the early '90s before things got so much more complex and cramped space to work in. I remember doing the 600 mile service on my '84 VF700F and much as you described above, tank, radiator and fairing panels had to come off to adjust the valves. First and last involvement for me.

ancientdad said:

And all of the above is why I was happy to leave the industry behind in the early '90s before things got so much more complex and cramped space to work in. I remember doing the 600 mile service on my '84 VF700F and much as you described above, tank, radiator and fairing panels had to come off to adjust the valves. First and last involvement for me.

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Had a neighbor see me out starting a moped a few weeks ago and asked me to look at her scooter. It's some 2019 Vespa clone. Was a lot of "fun" to remove all the fairing and brackets just to get at anything to test and diagnose.

My dad chose to stay in auto body instead of mechanics early in his career around 1982 because of the same reasons you stated.

Maraakate said:

My dad chose to stay in auto body instead of mechanics early in his career around 1982 because of the same reasons you stated.

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And when I left the City of Tampa job in early '89 I knew I didn't want to continue with mechanical for that reason as well, ended up getting into car accessories about a year later and though it was a bit more challenging for similar reasons to some extent, it was way better than trying to squeeze my hands into the tight under-hood areas. The only part of that job that equated to mechanical were the cruise control installations and after engines had fuel injection the linkage parts (mostly) fit with little modification. Had I not been mechanically inclined from the previous jobs I would not have been as adept at small fabrications on the fly, and most of my co-workers were just electronics guys who didn't know a vacuum hose from a small coolant hose. However, as often happens in those situations, when the employer figures out you can overcome obstacles other employees can't, you know what happens... so when the GM Quad 4 engine came out and none of the parts in the kits fit, you know who got to do them all. It took me 3 hours to do the first one, bending up crazy throttle pull brackets out of leftover parts, then the next one took me about 1.5 hours and I got them down to 55 minutes after a few more. And this was all on brand new cars, the company I worked for had agreements with many Tampa dealerships to do their add-ons, so at least it mostly clean work.

ancientdad said:

And when I left the City of Tampa job in early '89 I knew I didn't want to continue with mechanical for that reason as well, ended up getting into car accessories about a year later and though it was a bit more challenging for similar reasons to some extent, it was way better than trying to squeeze my hands into the tight under-hood areas. The only part of that job that equated to mechanical were the cruise control installations and after engines had fuel injection the linkage parts (mostly) fit with little modification. Had I not been mechanically inclined from the previous jobs I would not have been as adept at small fabrications on the fly, and most of my co-workers were just electronics guys who didn't know a vacuum hose from a small coolant hose. However, as often happens in those situations, when the employer figures out you can overcome obstacles other employees can't, you know what happens... so when the GM Quad 4 engine came out and none of the parts in the kits fit, you know who got to do them all. It took me 3 hours to do the first one, bending up crazy throttle pull brackets out of leftover parts, then the next one took me about 1.5 hours and I got them down to 55 minutes after a few more. And this was all on brand new cars, the company I worked for had agreements with many Tampa dealerships to do their add-ons, so at least it mostly clean work.

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It's nice when it's all new and comes off easy!