Hello all,
I'm taking apart my Concentric carb in order to check the pilot jet is clear, having been advised that a pilot jet blockage is probably the reason for my bike conking out about one second after starting. I've never done this before, so am seeking help please! So far everything looks generally pretty clean, although there was a little bit of gooey muck in the bottom of the drain plug. I've found the pilot jet underneath the pilot air screw, and I see two holes in the carburettor bore which I believe are the outlet holes. So I need to check that the passageways through the jet and through to both those two holes is clear. Please forgive this basic question: how do I check it? ( I have a can of carb cleaner. I have not got compressed air but have a foot pump if that's any use.) How do I actually do it?
Phil
Looking to answer my own question, I did a bit of carb-cleaner squirting. First I squirted some into the pilot jet hole, and it came squirting up through both the tiny holes in the carb bore. So I guess that's clear (and maybe the idea that the engine is stopping because of stale fuel blocking the pilot jet now appears wrong?) I then squirted some into what I think is the pilot air hole, ie the left hand one of three as you look from the rear of the carb, it came squirting out of one of the two tiny holes in the bore (the one nearest the centre of the bore) but little or none through the other (the one nearest the carb flange.) Is that what should happen? Phil
Hi,
Have you seen "Bushman's Carb Tuning secrets"? It's very informative and easy enough to folllow.
http://www.ntnoa.org/carb_secrets.htm
Click on the link to the pdf and print it off for some bedtime reading!
Thanks very much for the link, but it's not secure so I'm a bit reluctant to use it. When I say insecure, I mean it has an exclamation (warning) mark at the top left corner rather than a padlock sign.
You're just downloading data from the site not submitting any sensitive data back to it.
A secure connection is not necessary for that.
Cheers,
Bernie
But doesn't it mean my computer would be susceptible to hacking? (unlikely, I know, but...)
I downloaded it several years ago and haven't had any problems. If you are that worried I can send you direct my copy.
Cheers
Pete
Just download it.
I've down loaded many articles, manuals etc over the years. Never had any problems. You are not giving out any personal info, only downloading. I assume your computer has some anti viral program and if you are running the latest version of windows 10 there is some built in antiviral software.
Apple Mac systems rarely get any problems.
Quote from: Phil C on 29 Oct, 2021, 09:13
But doesn't it mean my computer would be susceptible to hacking? (unlikely, I know, but...)
No
Quote from: Pete C on 29 Oct, 2021, 12:25
I downloaded it several years ago and haven't had any problems. If you are that worried I can send you direct my copy.
Yes please Pete.
Thanks very much everyone. Pete emailed me the Bushman's stuff - really useful and interesting info. Phil
Interesting: I thought the two tiny holes in the bore were one for the pilot air and the other for pilot fuel, but no, they both take both, the fuel and air having been mixed just below, in the pilot mixing chamber. So I guess, unless I'm mistaken, any carb cleaner squirted into either of the pilot passageways should come up out of both those tiny holes. Phil
Ah, I might have been premature when I said the theory that the pilot jet was blocked might be wrong. Looking at the Bushman's information, and at other stuff, it appears the route that the pilot fuel takes begins near the rear of the carb, at a threaded hole where (pre-1968) there used to be a threaded jet. From 1968, in order to place the jet closer to the outlet holes, it was placed (pressed) in the gallery behind the pilot air screw. So the thread at the start of the pilot fuel's journey from the bowl is now redundant. As I understand it, the pilot fuel is drawn through the threaded hole, and along towards the front of the carb, and through the jet, near the outlet holes. So the holes I squirted carb-cleaner into a couple of days ago (as described in my previous post) were both part of the pilot air route. The pilot fuel route is different. And when I now squirt carb-cleaner into the threaded hole, ie the fuel route, none seems to appear at the tiny outlet holes. Which I guess might mean the route is blocked, as I was told! ( by Paul, I think - thanks again.) My bumbling efforts continue. Watch this space (well, if you're easily entertained and/or very patient!). Phil
Me again. Update: gave it a bit of a blow with a foot pump, and then squirted carb- cleaner onto the threaded hole and watched for it coming out of the two tiny outlet holes in the carb bore. It came out of one ( the one nearest the centre of the carb, but not the other. I think it should come from both? I've seen a diagram of the carb (Concentric) which labels that hole (the one I get carb-cleaner through) as "pilot by-pass" and the other as "pilot outlet". What's that all about? All help appreciated! Phil
By the way, the reason the carb-cleaner didn't come out of either hole first time was that I'd forgotten to refit the pilot air screw, so the carb-cleaner would just have shot out of there. Anyway, as I say, it's now coming out of one hole but not the other. Why are there two holes anyway?
Hi Phil,
The hole nearest the engine is the pilot the other is the pilot bypass. The pilot hole supplies fuel for idling, as the throttle is opened the pilot bypass supplies the fuel.
Best wishes...
I was at my local motorbike repair shop this morning. The (very experienced) mechanic had a look at my carb, took it into his workshop, came back and assured me both the tiny holes in the bore were clear. Back home, I squirt carb-cleaner into the fuel gallery and it shoots out of one tiny hole (like before) but little if any seems to come from the other. Any thoughts please? Phil
Obviously the mechanic will be right. I'll put the carb back together, stick it back on the bike, put some fresh fuel in the tank, and see what happens. Phil
May be when the engine is running the fuel is drawn in through the other hole when the slide is lifted, just a thought :-\
Hi Phil
The fuel will take the path of least resistance. Put your finger over the hole where the cleaner sprays out and squirt cleaner through the pilot jet. Hopefully you'll now see cleaner coming out of the other hole.
Fingers crossed.
Hi Paul, I did try putting my finger over the other hole, but still wasn't sure it was coming out of the suspect one. It's a bit difficult to fully block the hole with a finger, so I'm a bit unsure. I'll give it another go before I put it back together. What I was hoping for was the same as the "good" hole, ie like a tiny fountain of liquid coming up out of the hole. Phil
Tried again, same result. Frustrating and confusing. Phil
Hi Phil,
It sounds like a lack of fuel. Assuming that all the settings are as per the book the bike should start and run. Perhaps a bit erratically until it is warm.
Will it run if you keep the throttle open wider so that the slide and needle/needle jet are in play?
As soon as it fires keep the ticker down to keep the fuel level high in the carb so creating an over rich mixture and keep the throttle open - does it keep running?
Try raising the slide with the throttle stop to give a very fast tick over - does it keep running?
It is possible that there is still bit of an obstruction in the pilot jet system and getting the engine running at higher revs may help to clean it by making more of a vacuum.
Also if it will run at faster revs, remove the air filter and whilst the engine running quite fast almost completely block the cab intake making sure the pilot air holes are clear. This will cause all of the suction of the engine to pull through the pilot system and may pull through any bits remaining after cleaning.
Dave.
If there is a blockage, its whereabouts is a mystery to me. When I squirt down the pilot fuel route little if any comes from the tiny hole nearest the engine but a good little fountain comes from the other one. But when I squirt down the pilot air passage a good fountain comes from both. As I see it, if the jet/bush had a blockage then fuel would reach neither of the tiny holes. If there were a blockage at the pilot mixing chamber or at the tiny hole itself, then I wouldn't get a nice healthy fountain from both when I squirt down the air passage. Am I getting this right? I'm using the Bushman's sketch to analyse this. I'd like to understand exactly why this is happening. Phil
Have you tried blasting the holes from the other side I.e. the engine side of the carburettor. If there is some minor debris partially blocking one or both of the holes it may come out if you essentially back-flush it
You mean blowing down the two tiny holes?
Yes. Carb cleaner sprayed through the pilot air inlet should spurt from BOTH holes. If it isn't then one of them is blocked. The two holes are the narrowest part of the system and would therefore be expected to catch any detritus coming through. Back blasting it would push the detritus through the wider parts of the system. You will need a narrow flexible tube (like you get on a WD40 can) which can be held tight over the small holes while attached to a carb cleaner nozzle. If necessary you could do it with WD40.
Another alternative to clean the pilot ways is to leave the carb body soaking in paraffin for 24 hours, then try blasting it out with carb cleaner.
Carb cleaner sprayed through the pilot air inlet DOES spurt from both holes. But when I spray it through the fuel inlet it only spurts through one. It might come through the other - not sure - but it doesn't spurt. Phil
Hi Phil
I'm not sure it should come through both holes EQUALLY as they serve different purposes.
If you spray down the air screw hole and block off each of the exits in turn then i think you'll find a good jet should emerge from each hole in turn?
Pete
When I spray down the air hole I get a good jet from both holes. It's only when I spray down the fuel hole that I get a good jet from one hole but not the other. I think I'll assume it's okay and put it back together. Thanks very much everyone. Phil
Update and plea for advice: I put the carb back on the bike, put some fuel in the tank (same batch as before, so a couple of months or so old, but looked and smelt okay) and fired up the engine. It won't keep running unless I give it lots of revs. Plug looks a bit black but not too bad. Any ideas please? Trying to think what I might have done to cause it. I resoldered the wires on the alternator, but it couldn't be to do with that, could it? What could it be if not the carb? All advice/ thoughts would be much appreciated. Phil
Jj
Does it start relatively easily?
If so and you need to "give it some revs" to keep it running, it does sound like a fuelling problem.
Have you tried screwing up the throttle speed screw to keep it running?
Failing that, try and get someone to lend you a carb to see if it confirms fuelling problems.
Yes it starts fairly easily. I don't know anyone with a similar carb. I'll try altering the throttle speed screw - thanks.
Just a thought: couldn't be the plug breaking down, or the HT lead, could it?
Any other ideas very welcome!
Phil
Help! I am absolutely flummoxed by this. My bike starts but unless I keep the revs high it immediately conks out. It won't idle. As several of you have already said, it does seem like a fuel/carb issue (maybe connected with having the petrol tank lined recently?)But how can it be that when I squirt carb-cleaner in the AIR passageway it comes spurting out of BOTH the tiny holes in the carb bore, but when I squirt it in the FUEL passageway it only comes spurting out of ONE of them? How can that be? I'd be very grateful for any more advice to help me sort this out. Thanks again for your thoughts so far. Phil :(
Phil.
what bike?
what carb? numbers on the flange?
where are you?
cheapest option is plug and lead swap, one thing Ive tried is once running squirt easy start and see if it keeps running, a squirt when needed . this will rule out plug/lead or carb.
failing that, answer the above , and I will see if Ive got something similar to try.
So plug/lead breaking up could give those symptoms?
I would have thought that a plug/ lead failure would be noticed at high revs not tickover
You say the problem has started since you had the tank lined. Have you checked that you have full flow through the filter and tap on the tank and that there is no detritus/resin line partially blocking the flow. If there is a blockage it would explain why the engine starts but will not run (I.e fuel starvation).
presumably, tap was out when tank lined?
I trim around hole to get full flow, any thoughts on my offer?
If I remember rightly the tap was out but here are photos of it, I think it's okay, and a drill shank pushed up the hole in the tank goes freely into the tank. A drill bit slid through the tap goes through okay as well. So I think (!) that area is okay, although I don't know how quickly the fuel should pour from the tank when the tap is opened.I only had about a pint or do of petrol in the tank, but it was enough to flood the bowl when tickled.
Chaz, in answer to your questions, the bike is a 1965 B40F, the carb is a Concentric, on it is written 626 L301, and I'm in SW Devon.
If anyone has a similar carb on the bench and could see if carb-cleaner squirted into the FUEL passageway (threaded hole but no longer taking a jet, as the jet was repositioned I think about 1967 to be closer to the tiny holes and was pressed on place) see if it spurts out of BOTH tiny holes, that would be very helpful. On mine it only spurts out of one, but when squirted into the AIR passageway it spurts out of both.
I hope that makes sense and apologise for keeping repeating myself.
Phil
I've got the carb off again. At the bench, I took the bowl banjo off, peeped into the float needle seating hole and - maybe ! - saw a partial blockage, so just a small hole remaining, as though there were a jet in there (which of course there isn't.)Took my eye away to look for my new can of air, looked back and the hole was clear (I know, couldn't make it up.😁) So either I was getting muddled with what I was looking at, or there might have been a blockage which fell out. Unlikely I guess, given that there's a filter, but if there had been a blockage then I guess that might cause the bike to conk out rather than idle. Anyway, if the problem has gone when I put the carb back on then maybe that was it. But I think a blockage there would be very unlikely, given the filter is in place and looks fine. Phil
Anyway, please could anyone answer my query in my last but one post, about the two tiny holes? Phil
will check my carbs tomorrow,
Very kind of you, chaz. Look forward to hearing what you find. Phil
Hi Phil,
Reading through again have you had a go at your carb with eg ultrasonic cleaning?
I bought a small ultrasonic cleaner off EBay and it's just about big enough to fit a dismantled Monobloc. I still also go back to older methods like soaking in vinegar to get things clean too. It's a judgement call as to how long you leave things to marinade but it can be very effective at clearing out the tubes.
I know others resort to boiling their carbs in a washing powder solution!
A last resort is to drill out the pilot passageway as espoused by the Bushman.
Regards.
No I haven't done that. To my inexperienced eye the carb looks generally pretty clean, so I'm kind of thinking there's unlikely to be a blockage. But then I don't really know what to judge the cleanliness by. Maybe an ultrasonic cleaner is something I should consider - after all it can't do any harm (except a few more quid spent on something that would be used maybe only once.) I'll certainly keep it in mind, thanks Stubaker58 (unusual name you have there, makes mine sound pretty boring!)
You having a Monobloc makes me wonder what kind of carb would have been on my bike originally. From what I've read, the Monobloc was a good carb and the Concentric, whilst also pretty good, was built down to a price (I could be wrong.)
I'm keenly awaiting the result of chaz's investigation today - should help me narrow things down.
Phil
There is a YouTube video on unblocking pilot passageways on concentric carb may be useful - I can't work out how to copy the link!!
Yes, thanks Pete, I saw one by Paul Henshaw, where he describes a common place for a blockage is the right angle bend furthest from the pilot air screw. But, as I see it, if I had a blockage there then when I squirt down the fuel passageway I wouldn't get any carb-cleaner spurting out of either of the two tiny holes, but I do get it from one of them. Anyway, interesting video. If I remember rightly he uses nozzle cleaners and thin wire. Actually, I've just this morning received some nozzle cleaners in the post, but won't be prodding around with them unless I have to, in case I do more harm than good. Phil
Phil
pm sent, I would be wary on nozzle/jet cleaners as small ones can break off inside then totally blocked so take your time, a cuppa and the door shut for no interruptions.. I also use small drills but take my time and I have a good selection of small size ones.
Thanks Charlie. Did you try squirting carb-cleaner into the pilot fuel passageway to see if it came out of one of the tiny holes or both? Phil
Phil
the pair and single 300/301 showing blanked off holes. Its possible as these carbs are diy tuned items , someone removed a plug in the past but would only come out of one hole
So chaz, just to be sure I'm understanding you correctly, please could you tell me these two things: firstly, are you saying you squirted carb-cleaner into the hole shown on the photo but it only spurted out of one of the tiny pilot holes in the bore, not both? And if that's the case, then was the hole it spurted out of the one nearest the centre of the carb, rather than the one nearest to engine? Thanks very much chaz. Phil
Sorry Chaz, got my holes muddled. If, with the bowl off and put to one side, you squirt into the carb pilot fuel intake hole (threaded hole on the face which mates with the bowl top face) and watch the two very tiny holes in the carb bore, does it come out of one, or both? If only one, which one? Phil
PS am I testing your patience? I'm very grateful for your help!
Phil, here to help, if only I could get my motor right!!
squirting cleaner into either the pilot screw hole or the outside of the three (the opposite plugged) on the air filter side results in spray coming out of both small holes in the bore
Haha 😁, thanks Chaz, yes but (I think!) you're still not squirting into the pilot FUEL hole (bottom left hole on photo attached.) If you squirt in there, does it come spurting from BOTH the very tiny holes next to each other in the carburettor bore? Phil
Quote from: Phil C on 06 Nov, 2021, 17:04
Sorry Chaz, got my holes muddled. If, with the bowl off and put to one side, you squirt into the carb pilot fuel intake hole (threaded hole on the face which mates with the bowl top face) and watch the two very tiny holes in the carb bore, does it come out of one, or both? If only one, which one? Phil
PS am I testing your patience? I'm very grateful for your help!
thats where you threw me, the first time ive ever noticed the hole is threaded, I thought only the two bowl fitting holes are threaded,
just checked, only comes out of the slide hole not the engine side one.
note for future... next house... garage attached to house..
So mine is the same as yours, which I assume is in working order. So I'm now going to refit mine to the bike (well, not tonight, but maybe tomorrow) and I'll see what happens. I expect you'll be hearing from me soon! Thanks again for your help. I'll try not to cause confusion in future! Phil