Diagnose this...

[Oakie]

My bike ('92 CB400) has the following symptoms which until a moment of remarkable clarity on the way home Thursday, I didn't connect up (and perhaps I'm wrong anyway) they are:
A) Poor fuel economy (15 kml but should be 19kml to 23kml according to specs)
B) Won't run at all by itself if I use any choke.
C) Surprisingly good acceleration for a 400
D) Seems to run out of breath after several kilometres of a speed run with a pillion. (Doesn't quite misfire or sputter but doesn't quite run sweetly either)

Could the above all be caused by having larger than standard jets in the carbies?
A) ... increased fuel useage is a given with larger jets
B) ... plenty of fuel going through so using the choke only causes flooding
c) ... does accelerate very satisfyingly although I admit I've only ridden a few other 400s to compare it with
D) ... would larger jets suck use gas a little quicker than the pump can provide it when the bike's working hard?

I freely admit I'm not a mechanic's arsehole so feel free to put me straight if you think I'm barking up the wrong tree.

[bugjuice]

yes to most of the above.. you've pretty much answered your own questions.. bearing in mind the carbs could be running rich too, so may be get it all reset, if it was done a while ago..

they after market large jets that you had fitted?

[Oakie]

Cheers
I haven't actually touched the carbs since I bought it 4 months ago (it was tuned and carbs synchronised by the dealer). It still has the jets that it came from the dealer with but I have no idea if they're standard or larger. I think I may have to find out though.
How often should you resync the carbs?

[bugjuice]

about 10-15,000ks, depending on how hard you ride it. Normal carbs are probably ok for about 20ish... some people like to do it more often, I guess depending on how groggy the bike feels. Have you dropped it? Sometimes a good jolt can knock something out of whack and change something in there.. May be get it checked if you've noticed a change in the last 4 months..

[Oakie]

I've got a few ks to go before I need them re-synced then. Hasn't been dropped (touch wood) and I haven't noticed a particular change in performance. 99% of the time it goes fine ... and it's really only the fuel economy (or lack of it) that I'm concerned about.

[bugjuice]

well in which case, that might just be the way the bike is then. Larger jets will use more fuel, for sure. And it will alter the characteristics of the bike at lower and higher end revs. Their designed to be running at high revs, which you've found. At lower revs, there's a lot of fuel going in, most likely too much to burn. Hence the crapish milage. Just go round town in 1st gear like you own the joint

[What?]

What's the milage? Being a 92 it would be a fair bet that the needles are flogged out by now. Worn jets are effectively bigger...

[oldfart]

Check your air filter. If you have a manual, run through all the normal things, eg, fuel filters, lines, carb settings, air filter, air box & inlets, timing, valve clearance etc.

[FROSTY]

you want to look at your riding as well. You talked about pillion carrying and high speed runs both of which aren't going to help ya get good fuel economy

[unhingedlizard]

I realise this is flogging a dead horse but I had a GSXR4000 that did exactly the same, would run like a cut cat for 10 kms then go all limp. Turned out the carbs were way out of sync.

[MSTRS]

Other thing to check would carby float levels.

[Oakie]

What's the milage? Being a 92 it would be a fair bet that the needles are flogged out by now. Worn jets are effectively bigger...

Good point ... although according to the odometer it has only done just under 12,000 ks (probably genuine going by the condition of the bike)

[Oakie]

Check your air filter. If you have a manual, run through all the normal things, eg, fuel filters, lines, carb settings, air filter, air box & inlets, timing, valve clearance etc.

Air filter is 4 months old and the fuel line including filters has been checked because of a different problem. I suppose they would have done the timing but not sure about the valve clearances.

[Oakie]

you want to look at your riding as well. You talked about pillion carrying and high speed runs both of which aren't going to help ya get good fuel economy

Yeah, that's the funny thing. It doesn't seem to make that much difference how I ride the bike. My wife broke her leg 5 weeks ago so all my riding since then has been one up commuting and I'm still getting 15kml.
One up commuting, two up commuting, quick two up blast out to the 'Brew Moon' at Amberley for a sausage roll and a beer ... all 15kml. Shouldn't complain. At least I know I'm always going to run onto reserve at 218 km. (Scarily consistant)

[inlinefour]

I know enough (not much) to say leave it standard on my bike, besides for a CB400 they go fast enough, don't they (err, well not enough for me it turns out). Full riders tune at the local Honda dealership did my bike wonders Oakie. Maybe your bike deserves the same mate? :ride:
The way I've been riding my bike, its either time for another tune soon, or my preference-------->bigger bike :devil2:

[Toast]

Yeah, that's the funny thing. It doesn't seem to make that much difference how I ride the bike. My wife broke her leg 5 weeks ago so all my riding since then has been one up commuting and I'm still getting 15kml.
One up commuting, two up commuting, quick two up blast out to the 'Brew Moon' at Amberley for a sausage roll and a beer ... all 15kml. Shouldn't complain. At least I know I'm always going to run onto reserve at 218 km. (Scarily consistant)

Yeah they're strange like that. My old CBR250R was the same, it didn't matter if it was constantly at redline while chasing bigger bikes, or if it was in top gear commuting at any speed, 22km/litre almost exactly...everytime...pissed me off frankly.

[hondacmx450]

well if i got that 15 to the ltr i would be happy but speed dose pay and at the moment it is in the way we by gas

[FEINT]

I get 20km/litre on my CB400.

I find that it won't go past 150km/hr.... but i dont' find it sluggish up till then.