Carb motorcycle conundrum ... I think ... help!

Experiment with different choke positions - it will vary with the weather. Full out might be too much unless it's really cold. On my GPz 550 it was full out, then back in ~2mm most of the time.

Can we have a picture of the bike please.

Sounds like a blocked idle or other jet. Modern petrol is utterly rubbish in carb engines & will settle out in layers or deposit rubbish at the first opportunity. Have you tried a fuel stabiliser - comma petrol magic worked brilliantly on my ktm 950.

Modern petrol is utterly rubbish in carb engines & will settle out in layers or deposit rubbish at the first opportunity

To be fair I've never had an issue with my carbed gsx750 and I often don't get a ride in on it for several weeks, sometimes months.

It starts pretty well usually but choke position is critical. It's easy to kill it with too much or too little.
Fine once warmed up.

OK, so went out this morning, tried to start it, it's 3 degrees now, and the bike is cold. Usual crank, fire, die with full choke. Then tried half-choke, crank, fired, grumbled, died. Then 1/4 chock, fired up, chugged and gently built up to a nice idle.

The bike's got an aftermarket free-flowing 4-1 exhaust so beginning to think it's not jetted/air flow correct for the exhaust...

Pilots don't generally do an awful lot on bike carbs, they tend to come most of the way "out" from the factory so you've only got a little movement there. Is it an actual choke, or an enricher? The latter are pretty prone to mechanically jamming up at the carb end and can stick open, shut, or anywhere inbetween.

I doubt it's anything to do with the needles or mains as a starting/idling condition, they really come into it more at meaningful throttle and you'd probably feel it if they were far off.

(this is far from universal, but, as a very general rule if you cruise at completely level throttle, then smash it wide open, undersize jets will often cause a stumble or flatness which you'll feel and hear- not a misfire, just weak running. Try the same thing every 500 revs or so from about 3000, if it pulls out cleanly then it's probably not terrible

Full systems tend not to do a huge amount unless the original was really restrictive, especially on un-catted exhausts, they just open up options for more tuning. You could do worse than to go up a couple of sizes on the mains just to see what happens though

I had a similar problem on my CBR1000F. Chucked a can of Seafoam in the tank, went on on a run, filled up with Super at next stop. Runs loads better. Starts much easier, but still a bit hesitant. Stripping the carbs out and cleaning then properly would probably be the next step.

Is it an actual choke, or an enricher?

Northwind - I thought a Choke was a fuel-enrichment device. Can you explain the difference for me? Just curious :o)

I've a ZZR11 with carbs and its night and day with fuel containing ethanol. The more ethanol in it, the worse it runs. I just buy ordinary "Super" 95 with the lowest ethanol rating. Not the posh 100 Ron which is another level again.

With the ethanol fuel it wont settle on a good tickover but its constantly hunting depending on how hot the engine is.

Thanks all.

OK, so this where I'm at ...

- Have removed main jet and pilot jet on cylinder no 4, the sooty one, thoroughly cleaned

- Replaced and bike started fine with 3/4's choke, idled happily (possibly a touch rough but didn't have time to mess with the mixture screws, all set at 1.5 turns at the mo, the recommended setting), rode well, pulled well ... so now I'm falling into a hole of confusion!!!

Assuming the carbs are fully A7 standard, and assuming no blockages, but I'm running a little rich with a 4 in 1 exhaust designed for A1-A6 bikes, would it be advisable to have a go at opening up the airbox as per A1-A6 and leave the carbs alone (the A7 airbox has one intake as the bike was restricted vs earlier models) - performance is plenty for me, but be nice to get mpg up?

I've filled it up with posh petrol, E5, plus added an appropriate amount of Red Line fuel cleaner additive, so really would like to do a 100mile run to see where I'm at.

Again, thanks

JAG
Full Member

Northwind – I thought a Choke was a fuel-enrichment device. Can you explain the difference for me? Just curious :o)

Some chokes are a literal choke, they reduce the amount of air coming into the engine which makes it richer. But on relatively modern bikes, the "choke" is usually actually doing nothing to the air, but adding fuel. It's not that important a distinction really, it's just that the old jargon doesn't really make sense, chokes enrich but enrichers don't choke. It's a bit like clipless pedals I suppose.

But, basically the "adds extra fuel" type tends to be relatively vulnerable to jamming or gumming up and so a mechanical issue presents as a fuelling issue.

Thanks all.

OK, so this where I’m at …

– Have removed main jet and pilot jet on cylinder no 4, the sooty one, thoroughly cleaned

– Replaced and bike started fine with 3/4’s choke, idled happily (possibly a touch rough but didn’t have time to mess with the mixture screws, all set at 1.5 turns at the mo, the recommended setting), rode well, pulled well … so now I’m falling into a hole of confusion!!!

Assuming the carbs are fully A7 standard, and assuming no blockages, but I’m running a little rich with a 4 in 1 exhaust designed for A1-A6 bikes, would it be advisable to have a go at opening up the airbox as per A1-A6 and leave the carbs alone (the A7 airbox has one intake as the bike was restricted vs earlier models) – performance is plenty for me, but be nice to get mpg up?

I’ve filled it up with posh petrol, E5, plus added an appropriate amount of Red Line fuel cleaner additive, so really would like to do a 100mile run to see where I’m at.

Again, thanks

I'd take it to a decent Dyno for a quick run. They may not be able to tell you instantly what jets, but you'll know quickly the what and the why, instead of guessing you'll have a solid baseline to start from in terms of diagnosis. You're chasing your tail in terms of 'maybe' ..

open the airbox sounds worth a try. It has helped on some of my bikes.

Are your carb sliders set up properly?

Great, and thanks again to all - it's proving to be a good education.

So to recap - my issues in this thread(!) were:

1) poor cold starting (I was using full choke)

2) very sooty no 4 plug, others all good

3) low mpg

Am I right in thinking ...

- The sooty plug carb had an incorrect (too large) pilot jet - ordered the correct one.

- Poor cold starting, or the need for v little choke when cold likely to be resolved by no 4 pilot jet switch??

- Mpg - still unsure but suspect it's running lean because of the exhaust ...

My bike has a Lazer exhaust designed for the A1 > A6 models (a straight replacement for the O.E system) so consequently I have an exhaust designed for A1>A6 air box and carb jetting of pilot 35/ main jet 135/ pilot air jet @ 2.5 turns out, so I've got a less restrictive exhaust added to an air box that is 50% more restrictive and a main jet of 100, which will be way too lean for top end performance along with the restricted breathing.
So I could take the carb jetting to A1 > A6 settings, and modify or replace the airbox to A1>A6 spec and then balance the carbs when refitted.

So my next steps, unless any more useful pointers, are:

1. Check my km odometer is accurate (after the wheel change) to make sure actual mpg

2. Check exhaust for restriction - don't know how but will Google. It also is very sooty from a visual look, so could it be 'bunged up' and restrictive because of collective soot? Any pointers on how to clean?

3. Look into main jet change, airbox change

4. Explore compression testing and carb balancing!

Thanks again!

Aus
Free Member

Am I right in thinking …

– The sooty plug carb had an incorrect (too large) pilot jet – ordered the correct one.
– Poor cold starting, or the need for v little choke when cold likely to be resolved by no 4 pilot jet switch??

This could be it, it's definitely plausible. But, don't move onto thinking about other things yet. Break it down, test this out, see if it helps, reassess everything once you've made one fix. It's a weird thing to find tbh, somebody's put some effort into doing that and presumably there's a reason.

I'll make an exception for carb balancing though since that's a full on "underlying issue" that will impact everything. If that's off then it needs fixed.

Hi, just a quick update ...

Replaced the oversized pilot jet yesterday. Struggled to start from cold - took 5 mins But, once warm, the idle setting was wayyyyy too high - 3000rpm! SO adjusted it, and did 30miles, running well.

Went out this morning to try cold start - full choke, no throttle, and YES! Coughed and chuntered briefly and then ran nicely. Did 50 miles, some 'rapid progress', some crawling along, a real mix and all very well.

Took the cylinder 4 plug out, and it's a little white - running lean? But will continue to try a few runs before any more adjustments! Staggered that that little pilot jet alone could be so troublesome!

Hopefully this week I can get a few 100s miles in, and then reassess ... again, thanks to all for the help/guidance.