Bike carb issues
Discussion
Hi all,
Total novice when it comes to bike carbs, so apologies in advance if this becomes frustrating!
Got a Saxo 1.6 8v engine, used to be MPFI now running a set of Fireblade carbs, on a Bogg Bros manifold. The whole kit came from them, though I didn't purchase it - it came with the car. Ignition is coil packs powered by Megajolt.
The carbs were supposed to be plug & play, but we found them to be set up what we deemed to be completely wrong (we have a sprinkling of knowledge with car tuning, and access to a dyno). The AFR was up in the 18:1 plus region, and the engine wouldn't pull on anything more than 1/3rd throttle load.
Since then, we've swapped the jets numerous times and fitte adjustable needles. Last run on the dyno, AFR was ok, not perfect (13:1 or so at full throttle) but engine behaved well, pulled smoothly and produced a decent power & torque curve. All seemed well, finally.
Skip to now. You can start the car, fine. You can drive off in the car from cold, and it'll behave fine. Then after an indeterminate amount of time, it begins pissing around. It will still idle fine, and if you accelerate very gradually, it'll drive with no issue, but a prod of any sort on the throttle pedal brings on bogging down and misfiring, with the occasional pop and bang coming back out of the carbs. If you slowly increase the throttle until you're driving along at around 4000rpm, and then smoothly floor it, it'll pull fine. It seems to be the low to midrange point that it doesn't want to pull cleanly through, but the really odd thing is that it'll drive without issue beforehand. On the weekend, I was 20miles into a journey before it starting playing up, and I wasn't hanging around prior to that. Other times, it'll begin 5mins from home. It's seemingly random.
The only other possible symptom, is that once the issue has kicked in, it becomes worse if you start throwing it into corners. The engine is mounted in-line, the fuel pump is electric with a manual adjustable regulator. The carbs are mounted at the correct angle, and we believe the floats are set correctly.
There's a good chance there's an element of tuning we've overlooked, but what I can't get my head around is why the problem takes ages to manifest, and how it drives perfectly before that. If it was a carb issue, surely it either works, or it doesn't (assuming they're not full of dirt, which we know they aren't).
There is always the chance it's something completely different, like ignition. The coil pack is new, the plugs are new. The only thing that isn't is the HT leads.
Anyone know what I'm missing?
Total novice when it comes to bike carbs, so apologies in advance if this becomes frustrating!
Got a Saxo 1.6 8v engine, used to be MPFI now running a set of Fireblade carbs, on a Bogg Bros manifold. The whole kit came from them, though I didn't purchase it - it came with the car. Ignition is coil packs powered by Megajolt.
The carbs were supposed to be plug & play, but we found them to be set up what we deemed to be completely wrong (we have a sprinkling of knowledge with car tuning, and access to a dyno). The AFR was up in the 18:1 plus region, and the engine wouldn't pull on anything more than 1/3rd throttle load.
Since then, we've swapped the jets numerous times and fitte adjustable needles. Last run on the dyno, AFR was ok, not perfect (13:1 or so at full throttle) but engine behaved well, pulled smoothly and produced a decent power & torque curve. All seemed well, finally.
Skip to now. You can start the car, fine. You can drive off in the car from cold, and it'll behave fine. Then after an indeterminate amount of time, it begins pissing around. It will still idle fine, and if you accelerate very gradually, it'll drive with no issue, but a prod of any sort on the throttle pedal brings on bogging down and misfiring, with the occasional pop and bang coming back out of the carbs. If you slowly increase the throttle until you're driving along at around 4000rpm, and then smoothly floor it, it'll pull fine. It seems to be the low to midrange point that it doesn't want to pull cleanly through, but the really odd thing is that it'll drive without issue beforehand. On the weekend, I was 20miles into a journey before it starting playing up, and I wasn't hanging around prior to that. Other times, it'll begin 5mins from home. It's seemingly random.
The only other possible symptom, is that once the issue has kicked in, it becomes worse if you start throwing it into corners. The engine is mounted in-line, the fuel pump is electric with a manual adjustable regulator. The carbs are mounted at the correct angle, and we believe the floats are set correctly.
There's a good chance there's an element of tuning we've overlooked, but what I can't get my head around is why the problem takes ages to manifest, and how it drives perfectly before that. If it was a carb issue, surely it either works, or it doesn't (assuming they're not full of dirt, which we know they aren't).
There is always the chance it's something completely different, like ignition. The coil pack is new, the plugs are new. The only thing that isn't is the HT leads.
Anyone know what I'm missing?
E-bmw said:
Try stripping & cleaning the carbs & replacing the needle valves.
I have had similar issues with carbs before, which were caused by separate issues.
Dirt getting pulled up into the jets when cornering.
Needle valve wear causing "steps" in the flow of fuel to the carbs.
Cheers. As mentioned though, we've fitted new needle valves (the adjustable ones). We've stripped and cleaned the carbs quite a few times now, never find anything in them. The fuel tank, pipes, lines and pump etc were all brand new too.I have had similar issues with carbs before, which were caused by separate issues.
Dirt getting pulled up into the jets when cornering.
Needle valve wear causing "steps" in the flow of fuel to the carbs.
Check that the float level isn't slowly going down over the first part of the journey. This could mean there isn't sufficient fuel available to support the big demand when you come to accelerate*.
Many, many years ago I had this happen on my Mk2 Escort: it regularly conked out after about 45 minutes of motorway driving. 
jeremyc said:
Check that the float level isn't slowly going down over the first part of the journey. This could mean there isn't sufficient fuel available to support the big demand when you come to accelerate*.
Good shout, but what would cause them all to sink? They can't all be perforated Many, many years ago I had this happen on my Mk2 Escort: it regularly conked out after about 45 minutes of motorway driving.
Kitchski said:
Good shout, but what would cause them all to sink? They can't all be perforated
It may not be all the float levels that are a problem - just a few of them being too low could cause the spluttering you experience.Check that they can maintain the maximum volume of fuel in the chamber at full throttle.
Warning: I have no direct experience of bike carbs nor of how exactly they work. Conventional car stuff I'm fine with - they can't be that different.
jeremyc said:
t may not be all the float levels that are a problem - just a few of them being too low could cause the spluttering you experience.
Check that they can maintain the maximum volume of fuel in the chamber at full throttle.
Warning: I have no direct experience of bike carbs nor of how exactly they work. Conventional car stuff I'm fine with - they can't be that different.
A few? There are only four carbs! Check that they can maintain the maximum volume of fuel in the chamber at full throttle.
Warning: I have no direct experience of bike carbs nor of how exactly they work. Conventional car stuff I'm fine with - they can't be that different.
Good shout though, I will look into it. There is always the chance they're not set correctly, or something's jamming them after prolonged use. Sadly you can't see them once they're fitted to the engine, so no way of observing them.
Of course, it might not even be a fuel issue, but that it was it feels like.
dom9 said:
Could the fuel be getting hot and vapourising in one of the lines?
That's actually a really good shout! I've just taken the engine out, and when I put it back in I'll double check the routing of everything. The fuel lines and radiator pipes run on totally separate courses, but there's always a chance something's getting too close somewhere. Would explain the symptoms.Thanks!
Its a bit iffy what with fire risk etc but the easiest way I have found to keep an eye on the float bowls is clear pipe to the bowl drain suspended up.
They will or should be the same at all times if the supply system is good.
When it acts up kill it immediately so the pump stops and check the levels.
I have found a few with issues this way.
They will or should be the same at all times if the supply system is good.
When it acts up kill it immediately so the pump stops and check the levels.
I have found a few with issues this way.
chammyman said:
Its a bit iffy what with fire risk etc but the easiest way I have found to keep an eye on the float bowls is clear pipe to the bowl drain suspended up.
They will or should be the same at all times if the supply system is good.
When it acts up kill it immediately so the pump stops and check the levels.
I have found a few with issues this way.
I'll try that if the problem persists once the engine goes back in, thanks They will or should be the same at all times if the supply system is good.
When it acts up kill it immediately so the pump stops and check the levels.
I have found a few with issues this way.
If its out just now then do it now to be sure all the floats are at the same height.
That way you know your starting from the right place.
Add some 2 stroke mix to the fuel you put in the carbs to make it easier to see the level.
I use a squeezee bottle to actually build up some pressure. The valves should stop the fuel flow easily enough but if theres debris or damage to the float needles then they will just keep filling up.
That way you know your starting from the right place.
Add some 2 stroke mix to the fuel you put in the carbs to make it easier to see the level.
I use a squeezee bottle to actually build up some pressure. The valves should stop the fuel flow easily enough but if theres debris or damage to the float needles then they will just keep filling up.
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