Fuel economy carb synch related?

Basic

Basic

Joined
Nov 13, 2012
Messages
45
Location
Netherlands
Bike
ST1100N '92
Hi All,
Last week, I had the bike on a testbench, with the results below: Click on picture to see larger


The top graph shows:
Max torque - 109NM at 4859 rpm
Max power - 87.8 Hp at 6661 rpm

The bottom Graph shows the air fuel mixture.
13 kg air : 1 kg fuel would be the optimal setting.
Clearly, the fuel economy drops below 12 between 3000 and 4500 rpm

My bike has a problem regarding fuel economy.
When it went worse than 1 liter/15 km (35.25 miles/gallon US), I replaced the carburettors with a set known to run very fuel efficient.
Now it's running 1 liter/17 km (40.00 miles/gallon US).
It should be doing 1 liter/18 km (42.34 miles/gallon US)(Highway speed is 130 Km/h = 80.8 miles/hour)
If not worsening, my current fuel economy is acceptable, though it should be better.

Now, I understand there are two possible causes regarding that dip in fuel efficiency between 3000 and 4500 rpm:
- The mixture screws may need adjusting (They are less than 1,5 turn open, which should be about minimum.)
- The carbs need to be synchronised.
I suspect that the last one may be the cause, since I have synched the carbs many times since I got the bike, but somehow, they seem to be off again pretty soon. I replaced the rubber boots, checked the vacuum lines for holding vacuum. The mechanics seem to run smooth.
Coming weekend I will again check the synch. and expect it to be off again.

Any suggestions on how to eliminate this problem?

Regards, Rob
 
It looks like you are running fairly lean from 3500 to 4500
you might want to increase idle mixture a bit on all carbs and then synch.
That's my amateur opinion, It' s free and worth every penny.
 
Or just the opposite?

Well, I certainly can call myself an amateur also and we seem to disagree on the interpretation of rich and poor running.
Please correct me if I am wrong on this:
Lean mixture would be an excess of air, or lack of fuel.
That means that when you get a mixture higher than 13.5 kg air/1 kg fuel, it starts getting lean. (Bad for the engine)
A mixture lower than 12 would be rich = bad fuel economy, lack of power.
In my case, that would be a rich mixture in the lower rpm range, with the need of closing the mixture screws, which should be allready on minimal settings.

Unfortunately, I haven't got the tools and equipment to set these mixture screws myself and a run on a testbench is expensive.
Setting the mixture would cost me about 150? = 200$
To me that is a lot, that might be wasted if the cause is elsewere.

Regards, Rob
 
Basic said:
Now, I understand there are two possible causes regarding that dip in fuel efficiency between 3000 and 4500 rpm:
- The mixture screws may need adjusting (They are less than 1,5 turn open, which should be about minimum.)
- The carbs need to be synchronised.
I suspect that the last one may be the cause, since I have synched the carbs many times since I got the bike, but somehow, they seem to be off again pretty soon. I replaced the rubber boots, checked the vacuum lines for holding vacuum. The mechanics seem to run smooth.
Coming weekend I will again check the synch. and expect it to be off again.
Click to expand...

The primary purpose of synchronizing carburetors is to get an even vacuum across all 4 cylinders AT IDLE, when the butterfly valves are barely open by less than a millimeter or so. As soon as you open the throttle, the butterfly valves are open much much wider than they are at idle, so that fine adjustment at idle pretty much means nothing at normal operating throttle settings. It has nothing to do with fuel economy at normal running speeds.

I've been synching my own carbs for over 30 years, well make that 20 years because I kicked over my mercury gauge about 10 years ago and haven't done it since. For the 20 years I was synching them, it appeared to me to be a total waste of time. I'd put the gauges on when the bike was idling normally, and they'd be off, every once in a while by quite a bit. I'd synch them back up and it still idled normally, nothing really changed other than the height of the mercury columns. There was no audible idle difference, or any detectable performance difference, before and after synchronizing on any bike I've ever owned. I've had my '97 ST for 16 years now and I'd guess I've synched them 2-3 times, probably at the 15/30/45k valve adjustments, the last of which was back around 2000. I haven't synchronized them in at least 10 years, and maybe 13 years. I routinely get 45-50mpg at freeway speeds.

If your mixture screws are in fact user adjustable (the EPA usually puts anti tamper devices on US models, but there may be folks here who have removed them and can give you advice) from the looks of your graph you could lean it out just a little bit and you might get a bit better fuel economy.

The other variables to consider are compression ratio, and spark quality. If your coils are getting old and not putting out as strong a spark as they used to that could lower your mileage, as would less compression than when the bike was new. And of course spark plugs need to be checked and/or replaced any time fuel economy is being questioned.
 
I don't know that i would go so far as to never sync my carbs again, but i agree with Dwalby that I don't think a carb sync is going to solve your fuel economy issue.

Sent from my Nexus 7 using Tapatalk HD
 
Don: A clogged airfilter will make your mixture noticably rich. The question the rises, whether tyou just did a recommende annual change, or waited longer with that change?

A note on carb synching: Some prefer to synch with the air filter mounted and others clearly without air filter. I synched them once without, then put on the air filter and found values completely off. Since then, I synch with the air filter mounted.
My bike idles at 800 rpm and when synching, I set it to 1200 rpm.
That the sync being off effects fuel consumption in the lower rpm range sounds logic to me. After all, not only the throttle valve position differs, but also the throttle needle.
It should however be mentioned again that I am an amateur and may see things wrong.
I am still puzzling about the cause for the sync to get off so soon.

Grts, Rob
 
I went from what appeared to me a clean K&N to an aftermarket OEM filter and synced at the same time. That's when the mileage went down. I used a SK Flowmeter to sync at 1200 RPMs and the air cleaner has to be off to do that.
 
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May be a silly question; Why did you get rid of the K&N. They are supposed to last a bike's lifetime. The OEM need to be replaced each year.
K&N should provide more air = leaner mixture. Might explain your difference.

Grts, Rob
 
Basic said:
K&N should provide more air = leaner mixture.
Click to expand...
Yeah... too lean on my EC spec model... plugs turned snow white within ~400km... (and with that foam ring installed the engine wouldn't even fire up properly...) FWIW am I syncing at 1000/1100rpm (with/out headlights on, lower idle/tick-over might lead to too low oil pressure while sitting at a red light...), air filter installed, air box properly closed, PAIR system checked for leaks and attached, fuel petcock checked for leaks and attached (utilize a T-fitting in vacuum line), second air-duct installed, engine properly warmed but not fully hot. Once that's done we check the bar charts also at ~2500 and ~4000rpm to see if there significant changes (there should not...) BTW besides a smoother run and more 'omp' I also see better mileage/less consumption on 98oct fuel whilst facing a noticeable lower range combined with slightly decreased performance/response and some vibes on 95oct or less...
 
Basic said:
Don: A clogged airfilter will make your mixture noticably rich. The question the rises, whether tyou just did a recommende annual change, or waited longer with that change?

A note on carb synching: Some prefer to synch with the air filter mounted and others clearly without air filter. I synched them once without, then put on the air filter and found values completely off. Since then, I synch with the air filter mounted.
My bike idles at 800 rpm and when synching, I set it to 1200 rpm.
That the sync being off effects fuel consumption in the lower rpm range sounds logic to me. After all, not only the throttle valve position differs, but also the throttle needle.
It should however be mentioned again that I am an amateur and may see things wrong.
I am still puzzling about the cause for the sync to get off so soon.

Grts, Rob
Click to expand...

Ya got it fairly right. Sync should be done at that lower rpm. If cooling fan comes on during sync, stop the sync process and bike and let cool, or use a fan in front of rad. Honda SPECIFICLY states that carb sync is done with filter in place.....and yup, makes a huge difference.
 
At 80 MPH the the carb sync and mixture adjustment will have little to no effect - since you are now running off of the main carb jet circuit, not the slow speed circuit. I have had mixed results with K&N filters. Sometimes they are better than stock paper, sometimes worse. No rhyme or reason for it that I can determine. Getting lots of fresh air to the intake box is the biggest help at high speeds. Do you have 2 fresh air intake tubes from the fairing to the rear of the air filter housing? Helps a little if you do. Double check for correct valve and cam belt adjustment, good spark plugs, wires, and coils. After that re-sync (I love the Flowtool vs factory method) to smooth out idle & lower RPM range driveability if you wish. I use a very accurate digital RPM meter to "finely" adjust low speed mixture screws at idle, but that is only getting me the last little "nit" of performance and mixture at my altitude.
 
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Brant said:
Ya got it fairly right. Sync should be done at that lower rpm. If cooling fan comes on during sync, stop the sync process and bike and let cool, or use a fan in front of rad. Honda SPECIFICLY states that carb sync is done with filter in place.....and yup, makes a huge difference.
Click to expand...

Yeah, doing it without the filter in place seems kinda like balancing the rim, then installing the tire, and claiming you did a wheel balance.
 
Basic said:
May be a silly question; Why did you get rid of the K&N. They are supposed to last a bike's lifetime. The OEM need to be replaced each year.
K&N should provide more air = leaner mixture. Might explain your difference.

Grts, Rob
Click to expand...
What I saw was grit or grime showing up in the top part of the snorkels, which told me some things were getting past the K&N air filter which was clean and oiled prior to that. I'm not convinced the K&N is the be-all some others ascribe it to be.
 
ST1100Y: Yes, Placing K&N is known to make the mixture too lean on some bikes. They supplied that foam collar for these cases, but it does not work becaus that collar blocks the air intake opening, reducing top speed with some 50%.
I experimented with a thinner collar on my previous carb set and that worked well. On this carb set, I don't seem to need it.
Might be that the bike could run little better on the more expensive fuels, but the price differences are so high over here that it is not even worth trying.

Brant: Glad to read that Honda advises the sync should be done with the filter in place.
Still, it seems to me that some people seem to note that it shouldn't make much difference whether or not the filter is in place or at what rpm is synched. Could it be possible that it does not matter on a bike that is 100%, but when something is not 100%, these differences occur?

Garysco: My bike seems to run fine on high rpm. It's just between 3000 and 4500 that it is running rich. Are you suggesting that the mixture screws control different rpm range?
Valves were checked and ok. Spark plugs seem to be in optimal condition.

dwalby: Agree completely, though others might say that when things are off at different rpm or with the filter on, "somthing is wrong".

DonMac: There are good reasons for taking the step towards K&N, though you seem to have a rock solid one for stepping back to OEM. I did read some reports on bad fit of the filter, which might explain the larger particles.

Thanks, Rob
 
Basic said:
ST1100Y: Yes, Placing K&N is known to make the mixture too lean on some bikes. They supplied that foam collar for these cases, but it does not work becaus that collar blocks the air intake opening, reducing top speed with some 50%.
I experimented with a thinner collar on my previous carb set and that worked well. On this carb set, I don't seem to need it.
Might be that the bike could run little better on the more expensive fuels, but the price differences are so high over here that it is not even worth trying.

Brant: Glad to read that Honda advises the sync should be done with the filter in place.
Still, it seems to me that some people seem to note that it shouldn't make much difference whether or not the filter is in place or at what rpm is synched. Could it be possible that it does not matter on a bike that is 100%, but when something is not 100%, these differences occur?

Garysco: My bike seems to run fine on high rpm. It's just between 3000 and 4500 that it is running rich. Are you suggesting that the mixture screws control different rpm range?
Valves were checked and ok. Spark plugs seem to be in optimal condition.

dwalby: Agree completely, though others might say that when things are off at different rpm or with the filter on, "somthing is wrong".

DonMac: There are good reasons for taking the step towards K&N, though you seem to have a rock solid one for stepping back to OEM. I did read some reports on bad fit of the filter, which might explain the larger particles.

Thanks, Rob
Click to expand...

You'd be surprised how syncing carbs on these can affect mileage(filter housing on or off), and a proper sync DOES affect mileage, showing improvement. My 92 gets 50-52mpg@75-80mph all day long on a long haul. That sync was done 8yrs. ago by a Honda tech friend with me watching for future reference. Set idle mix, sync....re-adjust idle mix, sync and repeat as necessary till nothing changed flippin' throttle seating sync screw threads each time with motor off.Had to repeat 4 times till he was satisfied. If you'll notice, Honda has put a thread coating similar the blue Loctite on the sync screw threads, which keeps them from vibration movement. That would be if carbs haven't been cleaned and rebuilt, which mine haven't. Last year had my carbs off replacing coolant elbows. No re-sync as the idle holds steady with no fluxuation whatsoever, and still gives the high mileage. So in this bikes case, looks like if it ain't broke, don't fix it. There's a bit more to it than just throwing a flow meter on the snorkels, if you expect premium results......just sayin'.
 
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