Carburation issues, Honda Hornet

[West Cork Paul]

Haven’t posted in here for a long long time  and truth be known I forgot Zi even joined here 3 years ago 😱 😱but it was suggested in AN Other Forum I frequent there may be more in here with the specific knowledge/experience than over there so here’s the issue….

It’s a 2000 Honda Hornet, 600, IL4, the issue being the revs ‘hang’ when the throttle is closed. It’s running too lean on the slow circuit I hear you say. My initial thoughts too. 

The carbs (Keihin) have been stripped & cleaned & rebuilt (the bike had been standing for some time), the old fuel drained, the tank flushed out with new fuel and drained again. There are no air leaks in the inlet manifolds. 

According to the workshop manual the standard fuel screw/pilot jet setting is 1 3/4 turns out, I’ve increased this to 2 1/4 turns. 

According to the workshop manual the std slow circuit jet is 40, that’s indeed what was in there, they were a bit blocked up with old fuel but were ultrasonically cleaned in carb cleaner, then reinstalled. I even put in new 40s just in case. I’ve since increased them to 45s as even the new 40s made no difference. 

The above actions have improved matters somewhat. When warm the bike starts and ticks over as it should, around, 1200 rpm. However, blip the throttle to say 5-6k and the revs rise and quickly fall back to 3k and then slowly fall down to 2.5k and just hang there.

Yes, the throttle valves snap shut with force, thereare no restrictions in their movement or that of the throttle cables. 

I’m at a loss now as to what else might be causingit, so any and all suggestions/advice/thoughts arewelcome. 
 

 

[dansp1]

Vacuum fuel tap Vacuum leak?

Cracked inlet stubs?

Edited July 22, 2022 by dansp1

[West Cork Paul]

18 minutes ago, dansp1 said:

Vacuum fuel tap Vacuum leak?

Cracked inlet stubs?

Thanks, inlet manifold stubs are are all perfect, no cracks and the vacuum fuel tap valve works as it should, fuel flows through to the carbs. I should know, the carbs have been on and off more times than a whore’s drawers and bowls have to be drained each time. 

[dansp1]

Not sure if a small vacuum leak in the tap diaphragm would let the tap work but upset carburetion, blank the hose off and run it on reserve to eliminate it?

[West Cork Paul]

1 hour ago, dansp1 said:

Not sure if a small vacuum leak in the tap diaphragm would let the tap work but upset carburetion, blank the hose off and run it on reserve to eliminate it?

Thanks 👍, I did have it running on a drip tube type rig set up to eliminate the tank and shitty old fuel in there as a possible cause. At that stage the inlet vacuum outlets from all 4 cylinders were attached to the vacuum gauge so I could also check the vacuum differentials across the 4 carbs once it was ticking over and running. 
 

 

its got to be an air leak somewhere but I’m buggered if I can find it 🙁 and it’s driving me insane. 

[Jenny Pryde]

Don't want to teach my granny to suck eggs, but if the bike has been standing for a long time, it may be worth checking the condition and operation of the choke cable and mechanism.

[El Gringo]

Has it still got the snorkel in the end of the air filter cover?

We had one that did similar when the snorkel was removed

[Thunderbolt]

Can you spray wd40 near the carbs, when it is hanging? 
This will help diagnose the air leak and locate it.

[svingel]

6 hours ago, Jenny Pryde said:

Don't want to teach my granny to suck eggs, but if the bike has been standing for a long time, it may be worth checking the condition and operation of the choke cable and mechanism.

^this. Check the choke closes properly 

[West Cork Paul]

On 7/22/2022 at 11:17 AM, Jenny Pryde said:

Don't want to teach my granny to suck eggs, but if the bike has been standing for a long time, it may be worth checking the condition and operation of the choke cable and mechanism.

Thanks👍, that’s all working as it should. 

On 7/22/2022 at 11:57 AM, El Gringo said:

Has it still got the snorkel in the end of the air filter cover?

We had one that did similar when the snorkel was removed

Thanks 👍 yep, it’s still attached.

20 hours ago, Thunderbolt said:

Can you spray wd40 near the carbs, when it is hanging? 
This will help diagnose the air leak and locate it.

👍I have done that in the early stages as part of checking the integrity of the inlet manifolds but there was no reaction ie no increase or decrease in revs but I will check again. 

20 hours ago, svingel said:

^this. Check the choke closes properly 

👍

It has been suggested to me that one or more of the tiny o-rings that seal the pilot fuel screw may have perished with age. This is a possibility, although I did remove the pilot screws to inspect the needles at their tip and did visually inspect the o-rings, and they appeared intact, to eliminate this Ive now ordered a set of new ones and will replace them, they arrive next Thursday. 

Edited July 23, 2022 by West Cork Paul

[firebeast]

Have you tried revving it up without cables using the fuel drop feed, just using the stop moving it by hand instead of the cables, anything is worth a go. 

Realise you did spray it, but are the vacuum take offs on the rubbers or below in the casting ? If below, worth a check maybe?  My now very old cbr4 rubber tips on the vac take offs went rock hard so just thinking but assume you have checked them tbh.. 

Other option is seeing if anyone can spare a set of carbs for a short while off a bike that's not playing up?  Worth a bit of postage if it helped.. 

Does that have any of the emissions shit on it? If so get rid anyway. .. 

[West Cork Paul]

26 minutes ago, firebeast said:

Have you tried revving it up without cables using the fuel drop feed, just using the stop moving it by hand instead of the cables, anything is worth a go. 

Realise you did spray it, but are the vacuum take offs on the rubbers or below in the casting ? If below, worth a check maybe?  My now very old cbr4 rubber tips on the vac take offs went rock hard so just thinking but assume you have checked them tbh.. 

Other option is seeing if anyone can spare a set of carbs for a short while off a bike that's not playing up?  Worth a bit of postage if it helped.. 

Does that have any of the emissions shit on it? If so get rid anyway. .. 

thanks, the emissions stuff, the PAIR device, was removed by a previous owner. The vacuum take off are metal nipples screwed into the metal of the inlet passages just below where the rubber intake manifolds are affixed. 
 

Another set of carbs is a good idea if only to eliminate anything outside the current carb set. 

[firebeast]

5 minutes ago, West Cork Paul said:

thanks, the emissions stuff, the PAIR device, was removed by a previous owner. The vacuum take off are metal nipples screwed into the metal of the inlet passages just below where the rubber intake manifolds are affixed. 
 

Another set of carbs is a good idea if only to eliminate anything outside the current carb set. 

Maybe put a thread up on the wanted section, I don't have any, but another option is if you have any decent local breakers who may be up for 'leasing' a set,for beers possibly? but obviously you don't have as much knowledge on the history.. 

Maybe block the vacuum take offs with little bolts to be 100% on those as well.. The more you take out of the equation the better and make sure you update us when you find teh culprit 👍

[West Cork Paul]

16 hours ago, firebeast said:

Maybe put a thread up on the wanted section, I don't have any, but another option is if you have any decent local breakers who may be up for 'leasing' a set,for beers possibly? but obviously you don't have as much knowledge on the history.. 

Maybe block the vacuum take offs with little bolts to be 100% on those as well.. The more you take out of the equation the better and make sure you update us when you find teh culprit 👍

Thanks, and yep, I’ll post the updates. First thing is awaiting the new o-rings 👍

[West Cork Paul]

As I hate threads that just die with no answer, and as I also know from using other forums its incredibly rude when someone pops in, asks a question, and is then never heard from again - here's the update.

Nada, rien, niet, zilch 😩

O-rings arrived, turns out two of the original ones did have tiny breaks in them, most likely due to age as they're hardly moving parts, 

and the new ones were inserted. Carbs back on, connected, started, slightly better but not perfect. Carbs off again, bowls off, 45 jets taken out and 40s put back in, reassembled, reinstalled, reconnected (Lordy Lord, this is tedious but at least I've got it down to a fine art now🤣), bike restarted. It now ticks over much better 👍, it clearly prefers the 40s as the slow jets. Great, I think it's done, hoo-fecking-ray.

Dream on kiddo.

I leave it to tick over and warm up, can knock the SE valve off, all good, ticking over at c1300 revs, nice. Blip the throttle, revs rise to 5-6K, release the throttle, it snaps shut, revs drop instantly but then hang around 2.3to3K. Adjust the throttle idol screw - decking fiddly to get to with the airbox on, revs drop, but too much, and bike stalls. Now I know readers might think I adjusted the idle knob too far but I didn't we're talking about a 30 degree rotation that's all, which has to be reversed in order to get the bike to start and idle again. 

Seriously, I give up. It's not even my bike, just trying to help out a friend. It's rideable, I know as I took it out for a test ride, as long as you can live with the fact the revs don't fall below 3K until they feel like it. 

It really is perplexing. It's symptomatic of an air leak but where?

I read on AN Other forum of someone who had a similar problem which his mechanic eventually traced (allegedly) to "a leaking oil seal in the engine" but I'm at a loss to think what oil seal or gasket could be leaking that would permit air to enter the intake manifold and lean out the mixture. 

 

[RonniB]

That sounds air leaky, or a choke circuit sticking open/closed. Try going over all the joints with diesel start while it ticks over

[West Cork Paul]

On 8/4/2022 at 3:23 PM, RonniB said:

That sounds air leaky, or a choke circuit sticking open/closed. Try going over all the joints with diesel start while it ticks over

I agree with you, it’s symptomatic of an air leak causing it to run lean on one or more cylinders. However, I’ve done the ‘spray an inflammable aerosol’ on it several times to no avail 🙁. My playbook is empty 😕

[Damnthistinleg]

If the carbs have done a fuckton of miles, it's possible that the butterfly shafts are passing a bit of air at the point they pass through the carb bodies. Unlikely but I have seen it before.

I think if it were me I'd be having them off for a complete strip and rebuild with new seals everywhere, paying very close attention to the fuel and air circuits to make sure nothing is amiss. Then I'd return everything to strict factory settings followed by a very careful check/adjustment of the float heights.

Sometimes it's best to adopt a sledgehammer* approach to old carbs and at least once done correctly, you can effectively rule them out if the problem persists.

 

 

 

 

* - No @theo, not that kind of sledgehammer approach.

 

[West Cork Paul]

48 minutes ago, Damnthistinleg said:

If the carbs have done a fuckton of miles, it's possible that the butterfly shafts are passing a bit of air at the point they pass through the carb bodies. Unlikely but I have seen it before.

I think if it were me I'd be having them off for a complete strip and rebuild with new seals everywhere, paying very close attention to the fuel and air circuits to make sure nothing is amiss. Then I'd return everything to strict factory settings followed by a very careful check/adjustment of the float heights.

Sometimes it's best to adopt a sledgehammer* approach to old carbs and at least once done correctly, you can effectively rule them out if the problem persists.

 

 

 

 

* - No @theo, not that kind of sledgehammer approach.

 

I think you have a good point👍. The o-rings the valves pass through are about the only thing that hasn’t been changed. The bike’s done just 20000 miles mind you (if I can believe the speedo) but it is 22 years old and it hasn’t seen much TLC in its life 🙁

[theo]

I fully approve of the suggested sledgehammer approach. 

Straight into the bank sat on the floor, from head height. Repeatedly.