spuker1
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| posted on 22/7/13 at 08:54 AM |
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Bike carbs problem - help needed!
Hi,
I'm completely new to the forum so on this occasion I would like to say hi! HELLO! I have nothing to do with building kit cars unfortunately but
because your magnificent forum looks like it has got loads of people knowing they way around bike carburetor setups I think it's the best place
to drop in and seek for help.
I'm building my 1986 Honda Civic for street use and I'm halfway through converting it into bike carburetors. My problem is, that it
won't rev on full throttle. It cuts out and my wideband shows massive drop in AFR. It will go up in revs when I throttle it gently though, idle
is beautiful too and it has no problems with starting etc. - two or three pumps of throttle, turn the key and it fires right up. I'm running a
stock fuel pump which is mechanical and a fuel pressure regulator. The reg is fully open at the moment and the gauge shows 0 all the time... The fuel
pump is self regulated anyway. I've been told by people from Honda forums that this pump should be more then enough for it. I was suggested that
the main jets could be too small... I wanted to verify that before I take the entire thing apart again.
Here's the short video of how it runs, you can see the AFR gauge readings there when I rev it gently and boot it:
http://www.youtube.com/watch?v=m6_7U31MzVk
Thanks a lot in advance!
Regards,
Tom
[Edited on 22/7/13 by spuker1]
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daniel mason
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| posted on 22/7/13 at 09:18 AM |
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dont think afr should be any where near 17 at idle. i thought around 14 was the norm but could be wrong!
nice car btw
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Norfolkluegojnr
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| posted on 22/7/13 at 09:21 AM |
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have you changed the jets to suit the engine or are they the standard ones for the bike?
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40inches
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| posted on 22/7/13 at 09:24 AM |
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The first thing I was asked, when I booked for a Dyno carb setup was " has it got the bike fuel pump fitted?" bike carbs don't like
car fuel pumps apparently. I think a Dyno set up is essential, the difference before and after is night and day, and well worth the £96 it cost, plus
you get a Dyno readout for bragging rights 
Welcome to the forum by the way.
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daniel mason
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| posted on 22/7/13 at 09:40 AM |
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14.7:1 is the gsoline afr. ratio to aim for at idle. 17 is way to lean so dont over rev it in its current state! hight egt will be occuring so go
steady!
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Surrey Dave
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| posted on 22/7/13 at 09:50 AM |
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Main Jets?
Hi,
Have you changed the main jets ?, I hope I'm right in saying that most if not all bike carbs need them enlarged when fitted to a car engine
roughly twice the size of the original bike engine.
For example my Suzuki GSXR carbs had standard jets around 112, the car would run but couldn't revout due to insufficient fuelling , even with no
load it would seem as thought the rev limiter was set to 4000rpm.
I ended up with the jets being 160, most people seem to end up in the 150 - 170 area.
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spuker1
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| posted on 22/7/13 at 10:02 AM |
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Thanks for quick replies!
I think AFR around 17:1 on idle is sort of ok for this car. I had my wideband installed when I was running stock carb/filters/exhaust and it would
idle around 17:1... it would run a bit hot when standing in traffic though.... The jets weren't changed at all and they are exactly factory spec
for the bike it came off (Honda CBX 550 F2). I think they are 90mm. My engine is 1.3, what jet size would you recommend then?
Thanks!
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whitestu
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| posted on 22/7/13 at 10:18 AM |
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If it was me I'd be starting at around 1.3mm and working up If I had to guess I would say 1.4-1.5 would be about right.
mine are 1.6mmin a 2.0 Zetec with ZX6 carbs.
It is quite possible to get bike carbs running really well without an RR session, just by using your wideband and a bit of trial and error.
Stu
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Surrey Dave
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| posted on 22/7/13 at 10:21 AM |
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What bore?
The bore of each carb will have bearing on this , it's not easy or advisable for anyone to guess.
But just looking at percentage increase you are probably looking to start at 125 -130, but its safer for your engine if you are a little over.
You don't have to buy jets , you can buy a selection of micro drills and enlarge them gradually your self.
From somewhere like this:
Drill Service ,Horley,Surrey
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spuker1
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| posted on 22/7/13 at 10:39 AM |
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Thanks!
I was always going to drill them, I just don't want to do it to my only set. I'd much rather get a spare set that's closer to the
diameter I need (they cost around 3 quid per jet, no biggy) and drill that if necessary.
Should I get 120mm ones then and take it from there? Or should I get 130-135mm to begin with?
[Edited on 22/7/13 by spuker1]
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blakep82
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| posted on 22/7/13 at 10:53 AM |
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I saw this video years ago, and I think its brilliant. Ok its for bike carbs on a bike, but it gives info on how to tune them.
For a 1.3 engine, I guess you could find that the jets might not be too far off already?
The video gives info on what and how to adjust for what rev ranges. I'd say try it first, then look at drilling jets out?
https://www.youtube.com/watch?v=y836ZPjvoMI
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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snapper
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| posted on 22/7/13 at 02:54 PM |
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Try the clip position
I think you need more fuel at idle 17 is to lean
Bigger jets
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
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spuker1
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| posted on 22/7/13 at 03:03 PM |
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Thanks, I will most definitely try bigger jets then. On another forum someone suggested this:
"Have you actually confirmed they slides are opening with your finger with the engine running. After watching your vids one to many times i am
still thinking your not getting throttle. The small pilot jet controls idle and just off idle, the Big main jet controls full throttle and the needle
attached to the bottom of the slide does most of the stuff in between. If that needle is not coming up fast enough you will go lean real
quick."
Could that be an issue too? If yes, what would cause the needle not to go up. Diaphragm is in mint condition and seals are just as good.
I think we're actually getting somewhere! It looks like there's a light in the tunnel with your help guys!!! Let's get it
sorted!!!
quote:
Try the clip position
what is the clip position exactly? how can I adjust it?
[Edited on 22/7/13 by spuker1]
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whitestu
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| posted on 22/7/13 at 03:22 PM |
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quote:
what is the clip position exactly? how can I adjust it?
Take the carb top off [being careful to note the position of any o rings etc] and lift the diaphragm, slider & needle assemply out of the top of
the carb.
On some carbs the needles are held in position on the slider with a circlip and have multiple grooves so you can adjust them. If it doesn't have
these you may be able to get the same effect with spacer washers.
Either way if you adjust the needle upwards in the slider your mixture will be richer between idle and WOT [As the needles are tapered, the higher
they sit the less they obstruct fuel flow thought the main jet].
Stu
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spuker1
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| posted on 22/7/13 at 03:36 PM |
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Oh, I see. I don't think that mine is an adjustable one, it just goes into the slider and I think it sits on spring in there. Not too sure how
to take it apart...
But I suppose that I could take it from there...
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scrappy_7
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| posted on 22/7/13 at 06:25 PM |
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Drill out main jets to 1.3 & fit a faucet electric fuel pump for a bike 1.5 to 2.5 psi sounds like you are possible forcing fuel past the float
needles is the exhaust smoke black when u rev it.
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Surrey Dave
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| posted on 22/7/13 at 06:31 PM |
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Pump
Best off with a bike fuel pump, they are low pressure ands self pressure regulating.
I use a Facet electric pump but you need a regulator , my carbs don't like much more than 1 psi before they flood.
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40inches
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| posted on 22/7/13 at 07:02 PM |
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quote:
Originally posted by Surrey Dave
Best off with a bike fuel pump, they are low pressure ands self pressure regulating.
I use a Facet electric pump but you need a regulator , my carbs don't like much more than 1 psi before they flood.
Spot on. Bike pumps are cheap, and work, they emulate the gravity feed from the tank, very low pressure.
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spuker1
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| posted on 22/7/13 at 08:55 PM |
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Ok,
So I've taped the filters up with good old duct tape and left about 0,5cm gap around the wider end of each cone filter clear. It idles
14,5:1-15,5:1 and it revs up all the way through rev range on between 12:1 - 14:1 (I think). It revs super quick and super smooth now and idle is
beautiful. All it was is too much airflow, restricting it helped a lot.
BTW. the needles were going up and down as normal. Just too much air that's all.
My mate who was helping me out, when I mentioned getting bigger jets instead of blocking it off, said that the fuelling is completely spot on and
it's normal on bikes to put restrictors in to make it right.
I will get the video up at some point tomorrow probably!!!
All I have got left to do is to reinforce throttle/choke cable bracket and get it all looking neater.
Thanks a lot and stay tuned!!! 
[Edited on 22/7/13 by spuker1]
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daniel mason
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| posted on 22/7/13 at 09:15 PM |
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thats a far safer AFR. around 14.7;1 a i said earlier. you were way to lean before. (too much air/not enough fuel)
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Surrey Dave
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| posted on 22/7/13 at 11:25 PM |
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Weak
Sounds like you've just proved how weak and under jetted it is.
Whats the point of putting 4 decent sized carbs onto your engine and then completely restricting them?
Rather than saying you have to much air, you should be saying I don't have enough fuel?
Or am I missing the point somewhere?
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x_flow57
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| posted on 23/7/13 at 12:28 AM |
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quote:
Originally posted by Surrey Dave
Sounds like you've just proved how weak and under jetted it is.
Whats the point of putting 4 decent sized carbs onto your engine and then completely restricting them?
Rather than saying you have to much air, you should be saying I don't have enough fuel?
Or am I missing the point somewhere?
Here here. Get those jets opened out. Loads more power to be had.
Find a good rolling rd and get the max from the engine and the carbs without risking blowing it up.
[Edited on 23/7/13 by x_flow57]
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blakep82
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| posted on 23/7/13 at 04:17 AM |
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And get a good filter, those cone things won't be doing much good ;-)
Unless I've read wrong (its 5am, and ive not had much sleep) you've taped up and sealed the filters to the carbs? But I don't see
you've mentioned sealing the carbs to the manifold?
If theres air getting in between the filters and carbs, you've stopped the air and thats impoved it, then yes, you have not enough fuel rather
than too much air. Those filters are very restrictive, and won't do much good.
You can't have too much air through the filters, just not enough fuel.
[Edited on 23/7/13 by blakep82]
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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spuker1
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| posted on 23/7/13 at 09:16 AM |
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Yes, I have taped them up and left a gap on each filter so the air can still go through. And yes, there's no such thing as too much air,
there's only not enough fuel. However for those jets it was drawing way too much air going super lean very quickly. I'm happy I've
got it running like that because I can make it to the meet next week that I reall wanted to go to and also because there seems to be a noticeable
improvement in performance already so when I get the restrictors off and get bigger jets in, it will be even faster!
And for the filters I think they are quite good, still loads of airflow and much more filtration then bare trumpets or trumpets with mesh.
Awesome!!!
[Edited on 23/7/13 by spuker1]
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mcerd1
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| posted on 23/7/13 at 09:42 AM |
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quote:
Originally posted by 40inches
quote:
Originally posted by Surrey Dave
Best off with a bike fuel pump, they are low pressure ands self pressure regulating.
I use a Facet electric pump but you need a regulator , my carbs don't like much more than 1 psi before they flood.
Spot on. Bike pumps are cheap, and work, they emulate the gravity feed from the tank, very low pressure.
I'll 3rd that
bike carb's work best with a pump form a carb'd bike!
personally I like the ZX6R / ZX9R ones (and other similar kawasaki ones) if you can get ones from the bike breakers for £30-50 and they come with a
nice little mouning bracket and fuel filter
quote:
Originally posted by spuker1
And for the filters I think they are quite good, still loads of airflow and much more filtration then bare trumpets or trumpets with mesh.
bare mesh is very old fashoned !
and those individual type of filters have been proven to restric flow much more than the more upto date methods...
what you really want is an air box around the carb's (with appropriate trumpets/bellmouths on the carbs) and then duct air to this from a good
quality large cone filter sitting somewhere it can get nice cold air
or almost as good you can get a 'sausage' type foam filter around all 4 carb's (and the trumpets)
both of these are far better than what you've got just now - look at the ITG filters, they get a very good name
[Edited on 23/7/2013 by mcerd1]
[Edited on 23/7/2013 by mcerd1]
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