Topic: Smoke on choke (cough, splutter, gasp)

[martin99]

Hello folks
I’m new to the forum and looking for some advice on getting my newly rebuilt 750F2 to run right. This resto has taken the best part of 18 months on and off (full time job, three kids and a wife who seems to think decorating the house is more important than motorcycles-ah well, you can’t win ‘em all!) and I’m hoping to use it over the summer to get away from the aforementioned irritants every now and then.
The problem is that she’s very smokey on choke(and I mean very – I’ve even had the neighbours coming round to complain and illuminate me as to the hazards of CO2 poisoning!) but as soon as she is able to come off choke the smoke all but disappears. The smoke is white (ish) in colour, and smells very rich.  She idles reasonably well, although every now and then engine speed increases of it’s own accord by a couple of hundred rpm or so. On a short test run – she’s not registered yet so I have to be careful not to get nicked – throttle response is poor to start but picks up after a few seconds hesitation.
Now for some info. Engine rebuild included new main and big end bearings, primary chains, camchain, all tensioners, clutch, honed bores with standard new rings fitted. Compressions were all over the place when I initially got her going, but after rechecking the valve clearances they are now 1) 145, 2) 147, 3) 144, 4) 153. From what I can make out these are pretty acceptable, so I’m suspecting a fuel problem myself although I’m happy to be corrected by any of you aficionados out there!
The bike has a new ignition backplate and new points and condensers. HT leads and plug caps are also new and the plugs give a good spark. It is timed using a strobe at 900-1100 revs. Dwell reading for both 1 and 4, and 2 and 3, are 48 degrees.
Carbs have standard 120 jets and have been meticulously cleaned. I have set the float levels at 14.5mm and bench-synched the slides with a drill bit. The mixture screws are out 1 ¾ turns as per Mr. Clymer’s instructions. I confess that I haven’t got a clue what the clip position is, but I’m assuming it will be standard. The carbs came from a donor bike with a standard airbox fitted. My bike has pod filters fitted but not proper K&Ns and are not oiled. I have checked for air leaks around the inlet rubbers with WD40 and can’t detect any. The exhaust is a 4 into 1 Marshall Deeptone.
Phew! I think that’s everything. Here’s hoping that someone out there can help me sort this out so I can relieve my stress levels with the wind on my receding hair rather than reaching for the Valium!

[scondon]

Welcome, cool looking bike from what I can see. Any more/bigger pics for us? Sounds like you ruled out everything but carbs and Id say thats exactly your problem. If you want to keep the pods youre gonna have to put bigger jets in. Youve certainly created a lean condition by letting the carbs breath more. Do a search on this cuz alot of people have dealt with it and Im sure you could figure out what jets, clip position etc. to get it running right. For a quick fix, just put the original air box back on an Ill bet she'll be alot happier. The engine changing idle speed after a warm up points to the need for a precise carb sync with a manometer, or you have an air leak you haven found yet.

   Good point in starting with the stock airbox to troubleshoot carbs as it's much easier to pinpoint problems when starting with stock set-up. The stock main jets for the F2 bike are 105's so the 120's are plenty big for just adding pods,IMO. Needles in the F2 carbs do not have adjustable clip positions, at least for the '78 model('77 might have them but not sure).

   Oldskool, each mixture screw should have a spring, washer, tiny o-ring on it. If any of those are missing it could possibly make a carb over-rich at idle.

   There is a "fast idle" adjustment screw that relates to the choke. As the choke cable is pulled a cam on the carb bank activates the fast idle and dumps more fuel in, at the same time the butterfly valves are choking off your air supply.

   With your poor throttle response and white,smelly smoke when choke is applied I would guess that your too rich on one or more of your carbs. If it were me I would pull the choke 3/4 of the way and let it idle for a minute(smoke and all), kill the engine and remove all four spark plugs to determine which ones are running lean, or too rich. A little black and sooty is to be expected since the choke was applied, but white(lean) or wet and heavy black(rich) plugs would tell you which carbs aren't operating right. Don't do this test when the engine is already warmed up, try it first thing or after the bike has been sitting a couple hours at least.

   As Wykydtron points out, a vacuum synch might be your first order of business before fiddling with carb settings/jetting.

[Jerry Rxman Griffin aka MuthaF'er]

My shop manual says your bike had stock 105 main jets with the stock carbs. You have donor carbs with 120s. Are these the correct carbs for your model year? IE accelerator pump carbs or are they earlier carbs without the accelerator pump. If they are earlier non pump carbs changing the mains is easy. What about the slow jets? Also easy to change on non pump carbs. You may need to adjust the clip position too. This is the biggest PITA. I have not dealt with pump carbs so I'm not sure about the difficulty of changing the jets. You have pods which, if you search this subject, will find they require great attention to getting the jetting right. Bench syncing is a start. My thoughts are to sync the carbs with the correct tool first then jump into jetting. Some folks say to start with the big jets and work your way down. You have a good start so far but you're going to need to put some attention and time into the carbs. It isn't any fun but it's necessary.      

[martin99]

Wow!
You boys certainly know how to deliver!
Wykydtron - thanks for the compliment - I hope to add some pics in the gallery section soon. As for your suggestion, the airbox will no longer fit as I have set all the electrics in a Santee type box and reduced the width of the bike considerably, by repositioning the oil tank in the frame and fabricating a one off battery box to fit tightly next to it. It's reassuring to know I may be on the right lines tho'
Thankyou Scondon for the tip, I'll try that test in the morning - it's nearly midnight, cold, wet and generally flaming miserable in my neck of the woods at the moment but I'll come back tomorrow with what I find.
The donor bike was a late 750K with accelerator pump type carbs which look identical to the ones on the F2. Are there any internal differences? I don't know. Thankyou Rxman for enlightening me to the fact that clip positions don't apply in this situation, one less thing to worry about and I would have been bloody embarrassed if that had turned out to be the problem seeing as I obviously hadn't thought about it at stripdown (well, I am getting on a bit)! Another confession - the jets to which I refer are the air jets, I don't know what mains are in there but my manual agrees with yours that they should be 105s. Oooer, this is getting more embarrassing by the minute!

[martin99]

Me again
Sorry Scondon I didn't credit you for the clip position info, want to thank you all for your input I'll sleep soundly tonight in the knowledge that I have some new things to try tomorrow. Don't own a manometer, local hardware shop now Tesco's so can't make a Clymer special either! But I know a man who can, just might take a day or two. I'll check the faqs again too now it seems it is definately pointing towards carbs
Martin

[eurban]

A few thoughts . . . . . . .I am running a set of 78F carbs on my 78K with good results.  The air jets are supposedly different between between the F and K carbs but I have not discovered any other functional differences that can't be changed by switching the main jets or adjusting the idle mix screws.  Basically I doubt there is an issue with using 77/78 K carbs on a 77/78F motor although a bit of carb tuning may be involved particularly with your modified setup . . . .Most of the 77 carbs have adjustable main jet needles while most of the 78 carbs do not.  Certainly it would be prudent to find out what type of needles you have and what position the clip is in.  Frankly I can't see how you you could have meticulously cleaned your carbs if you didn't remove the main jet needles during the cleaning.  . . . .These carbs have pressed in idle jets which can be carefully removed (gentle twisting motion while pulling) and cleaned.  You seem to have a rich idle condition which would typically not indicate a clogged idle jet, however this is an area of carb cleaning that is over overlooked and source of many headaches . . . .In case you didn't know, the idle mixture screw on the 77/78 carbs controls the bleed of fuel rather than air so this means that clockwise leans the mixture and counterclockwise richens it.  This is the opposite of the earlier carbs. . . . . .  I know that you say that you measured the float levels but it is tricky sometimes and the float needles (even new ones)  can have issues with sealing.  I typically do a "reality" check of the float levels which involves connecting clear tubing to the brass overflow nubs, and while holding the tubing up high along the carb body, open up the drain screw allowing fuel to flow into the tubes. The level of fuel in the tube is the actual level of fuel in the bowl.  Compare all four carbs; fuel levels should be just a bit below the bowl to body seam and all should be the same.  The carb rack must be basically level for this test to be valid! . . . .Although this is in all likelyhood unrelated to your problem, you may want to visually confirm that the acellerator pump is working as its function is critical to good performance with these carbs.  Remove your pods and with fuel in the bowls, whack open the throttle and look for a squirt of gas from each brass nozzle located on the bottom of the carb bore near the choke plate.  . . . .Hope some of this helps!

[kach_me]

What is proper compression for 76 750k?  I noticed your numbers there and I think that mine are lower with one cylinder being significantly lower less than 100.  My bike also smokes on start up but quits after warm up.  I have found several folks on here who say that's par for the bike, but I'm wondering if the compression issue isn't related as well. 

[750goes]

compression at factory - zero miles probably 160-170
after run in period 165-170

after 30 years of motoring with unknown previous owners and total lack of history on bike servicing

anything over 100 is probably workable but has issues somewhere....figures of mid 140's bloody good

 

[Hop on a Honda]

Check your air filter for age or it being over oiled (Too much Vaccume)
It is possibly RS or clogged Thats when the carbys will over fuel through brunellis therom, and may be the engine oil is being sucked through the rings momentarily.
This will give the exhaust a look of white maybe a touch of black if your lucky.
Dave Australia.

[martin99]

I've not made any headway at all today I'm afraid, too busy trying to service my car inbetween the rain storms. Eurban, thanks for your thoughts, you have obviously sussed that my meticulous is different to everybody elses! In my defence (violins at the ready?) it's been a hell of an 18 months and with Spring in sight I kinda adopted a 'That'll do attitude' - now corrected! I had read somewhere that the air screws on the 78F worked the opposite way to the earlier ones - might it be a good idea to turn them in a bit? I can't do a float level check as you describe easily 'cos the electrics box gets in the way. It's a carbs-off job to take the bowls off but I suppose I could hold them in a vice and fill the bowls that way. As it's looking more and more likely that they're coming off anyway this is one check I will definately do. I still have the carbs that were originally on the bike, I changed them because the carb bodies were so badly corroded. As far as I remember, the air jets for these were 120s too but they were brass whereas the jets in my current (K) carbs are made of a white nylon-looking material. I will take the originals apart and see what mains they have. I've checked the pump is working by doing just what you have suggested. My plan now is to have them synched properly before I take them off, still clinging to the hope that my prayers might be answered (I know, there's one born every minute)!  Kach me - I know this is presumptuous from a newbie such as myself, but as I mentioned earlier adjusting the valve timing made one hell of a difference to my compression readings - worth a try?

[supersport_CB400F]

You should move up North the air’s cleaner and SOHC bike‘s run sweet……seriously been there and done that with my 400 carbs, I wish you luck and I know it’s a pain doing the on off thing with them but it has to be done, once sorted you can get payback time and thrash the living daylights out of the thing, once the bike is run in obviously. 

[kach_me]

I've not made any headway at all today I'm afraid, too busy trying to service my car inbetween the rain storms. Eurban, thanks for your thoughts, you have obviously sussed that my meticulous is different to everybody elses! In my defence (violins at the ready?) it's been a hell of an 18 months and with Spring in sight I kinda adopted a 'That'll do attitude' - now corrected! I had read somewhere that the air screws on the 78F worked the opposite way to the earlier ones - might it be a good idea to turn them in a bit? I can't do a float level check as you describe easily 'cos the electrics box gets in the way. It's a carbs-off job to take the bowls off but I suppose I could hold them in a vice and fill the bowls that way. As it's looking more and more likely that they're coming off anyway this is one check I will definately do. I still have the carbs that were originally on the bike, I changed them because the carb bodies were so badly corroded. As far as I remember, the air jets for these were 120s too but they were brass whereas the jets in my current (K) carbs are made of a white nylon-looking material. I will take the originals apart and see what mains they have. I've checked the pump is working by doing just what you have suggested. My plan now is to have them synched properly before I take them off, still clinging to the hope that my prayers might be answered (I know, there's one born every minute)!  Kach me - I know this is presumptuous from a newbie such as myself, but as I mentioned earlier adjusting the valve timing made one hell of a difference to my compression readings - worth a try?

Don't let my "enthusiast" rank fool you.  I am a technical noob.  I can change oil, air filters, spark plugs and the like. I possess neither the place, nor the know-how to do serious repairs.  thanks for the feedback though... I'll suggest it to the mechanic whose doing the real work for me. 

[martin99]

Hello Guys
Hope you don't mind me keeping this thread alive cos I haven't resolved it yet - I'm waiting for a set of vacuum gauges I bought off EBay so I should be back out to play at the weekend. A couple of things though - on re-reading the other day's posts I can see how I may have confused the issue somewhat with my drunken contradictions about the jet sizes as the session progressed....hmmm, perhaps that's why it's taken me so long to get the damn thing finished...

Anyway, to clarify the issue the main jets on the currently fitted carbs are a kind of white nylon material and stamped '120'. The main jets on the original carbs (and I have no way of knowing if indeed they really are original or parted out ones, but the bike was running with them when I bought it) are smaller in overall diameter, brass and have no numbers stamped on them. Would it be reasonable to assume these are the standard 105s? And would the others be pattern replacement larger ones?

Is it possible to have too large a jet size for my current set-up and would turning in the mxture screws in any way compensate for the oversized jet?

If anyone has yet to fall asleep reading this I'd really appreciate some thoughts, It'll help me troubleshoot as I attempt to fix the bike inbetween stopping my kids from fighting!

All the best

[martin99]

Just pulled another of the old jets and has 105 stamped on the side, so I reckon that's sorted that bit

[750goes]

I have a 750f with pods 4 into 1 headers, baffled muffler and it runs well with 105 mains & 40 slows combination, and 1 1/4 turns out on each carb  -it even runs a bit rich (have never adjusted float height but they are within specs +/- 2mm by eyesight)

I also have some 120 & 130 mains I am going to experiemnt with in the future....

[martin99]

Very grateful for your continued support, and with all your help I hope to be able to report some progress soon!
My mixture screws are engine side, so I understand it as clockwise to lean it out, anticlockwise to enrich.
Gauges are from a UK EBay seller 'onlineoffroad' and are the usual ones you see on EBay mounted on a blue backplate. They've got a 'Made in Taiwan' sticker on the back and I haven't got a clue who made 'em. They probably wouldn't be robust enough for a shop, but for the occasional job they look fine to me. Can't say what they're like in practice yet, let you know when I try them. They have all the hoses and adapters included and cost about 75USD with carriage. Anyone else got anything good/bad to say about these?

[martin99]

Ok, here's what I did today. Started the bike from cold, ran it for a minute before pulling the plugs, which were black and swimming in fuel. Put them back, warmed up the bike and balanced the carbs. Wykydtron, thanks for the tips regarding the gauges they were as you said and seemed to have worked ok. Took it for a short ride (as I said, not legal yet) and it is now noticably smoother through the gears, although I've yet to give it a decent test to see what its like through all the ranges.

There is now a backfire/spit from carb No.4, intermittent but pretty frequent and accompanied by a jump in rpm, so I decided to recheck the ignition timing. 1&4 can't be timed correctly, remains too advanced and I'm at the end of the adjustment. 2&3 however are spot on. This is a new backplate assy with new points and condensers - should I suspect the advance mechanism or could it all be related to the ongoing fuel issue? The bike will also only sustain idle for approx 30 seconds or so before it gradually fades and stops. Could the balancing have had this effect do you think? I took the plugs out again when the engine was hot and they are neither sooty or grey - they just look 'clean'apart from blackened electrodes (not sooty tho)  and are still wet with petrol. All are the same condition. Time to turn those mixture screws in?

Any help greatly appreciated!
Martin

[scondon]

  How are you checking your timing(static, strobe?). Your points gap will affect your "F" timing but not your advance timing, so if your advance timing is good then adjust your 1&4 points gap until your "F" timing is good.

  Carb synch was one step, now get the timing right before going on to any next steps. Spitting from the carbs, sooty plugs, idling then fading, too rich-too lean.......no way to tell for sure until you've set the timing since this could cause all of the above.

[martin99]

Thanks for that. Before I balanced the carbs, I thought I had the timing pretty spot on, with dwell readings of 48 and the marks lining up across both sets. I hadn't realised that balancing could affect the timing in that way..but then could this be explained by having set it on unbalanced carbs in the first place?
I'm using a strobe for timing, and a shop-type tacho to keep it around 900/950rpm. It's the F mark I'm refering to, that is it's approx 2mm to the left of the timing mark on 1&4. The gap had been reducing by clockwise turning of the backplate. I'll set it static again tomorrow so I have a fresh starting point and go from there

[scondon]

 Unbalanced carbs can make the revs rise and fall which make setting the "F" timing kinda difficult. One thing to note is that the "F" timing is for idle only and once you get up to 2700rpm(or so) you should be completely advanced and reading between the 1[]4 advance marks.

  When you spin the backing plate you change both the "F" timing AND the 1[]4 advance timing. When you change the point gap it will affect "F" timing but NOT the advance timing. So, if the point gap is off(too much or too little gap) and you spin the backing plate to get the "F" mark lined up then chances are that the advance timing will be off. And often you will run out of room to spin the plate before lining up the "F" mark.

   It's not in the books but this method has worked well for me. With a strobe/timing light set timing to the 1[]4 advance marks by spinning the backing plate(engine at or above 2800rpm). Set the 2[]3 advance timing by adjusting the 2&3 mini-plate. With the advance timing set, adjust the points gaps until both 1&4 and 2&3 are lining up on "F" at 1000rpm or below. Once the "F" timing is set, double check the points gap and it should fall between the .012-.016 that Honda calls for. The gaps are usually right at .014 when I use this method.

   Having the timing set perfect might not make your troubles vanish, but it's best to start troubleshooting other causes AFTER it's set or you might end up making changes that get you further from where ya want to be.

[martin99]

Hello All.
I've spent a few hours taking heed of all the great advice I've been getting here, and making some progress I think. Carbs have been balanced and ignition timing has been set with a strobe at 1000rpm (haven't checked the advance timing - how does one do this? I mean, do you have to have someone holding the throttle open for you whilst you check it, or do you just wind in the throttle stop screw until it gets to where you want it to be? It all seems like a three-handed job to me-tell me I'm missing something, please!).

A short blat up the road and throttle response and acceleration are definitely better-she pulls as good as the last one I had twenty years ago! It's still smokey on choke although not as dense as it was before all the tweaks and appears to clear quicker, but it's still significant. At idle the bike smells fumey and rich but doesn't appear to smoke, however a blip on the throttle will produce a short burst of smoke. The fuel level in the tank does seem to fall a lot quicker than it should.

Just to recap, mains are 120 and mixture screws are 1 3/4 turns out. It has pods and a Marshall 4 into 1. I'm tempted to try adjusting the mixture screws now to lean it out at idle, but when I've tried this in the past (following the Clymer manual procedure) I've found the rise in revs hard to discern, and have made it worse! Would it be sensible to turn them all in at the same time, say 1/4 turn at a time? At least I could then stand a chance of getting it back to how it was, should I manage to make a pig's ear out of it again. My mixture screws are absolute sods to get at due to the electrics box, and to do anything on them accurately you really need to take them off...I know, seemed like a good idea at the time...

Here's hoping you can come up with your usual words of wisdom people, couldn't have got this far without 'em!

[eurban]

My experience with these carbs is that if turning the mixture screws makes no discernable difference then the pilot jets are either plugged up or the setting is quite a ways off.   Obviously if they are plugged then don't waste your time with carb tuning until they are cleared.  You can certainly try all of the screws at 1 1/2 turns out and see how she runs.  If there is an improvement then keep going clockwise 1/4 turn until the improvement has stopped.  Ride the bike and note changes to the crispness of the aceleration.  You are on the idle jets quite a bit during stop and go driving so getting them sorted out should effect a good bit more than idle quality.  In reality the best way to adjust idle mix screws is to set them individually for the highest idle speed at a given throttle setting so being able to turn them with the bike running would be best.  You can buy a tool that is designed to facilitate the idle screw adjustment that is basically a long flat head screw driver that bends 90 degrees at the very end.  The flat head tip is turned by a flexible metal wire that is inside the screwdriver's tubular shaft and is operated by a little lever on the outside end.  The 90 bend is pretty tight so it might be able to clear your electrical box setup.  I think Motion Pro offers this tool but I am not sure.  Good luck

[Gordon]

Once you get everything else sorted out (timing, valves, mixture), then go back and balance the carbs.  This should be the last thing you do in your tune-up procedure since everything else will affect the carb balance.