Fuel starvation?

N0rt0nelectr@ · Dec 5, 2015

I was out on a nice long ride and my 74 850 acted like it was fuel starved a couple of times. Pulled in the clutch and revved the engine and it would clear up for about 5 miles then do it again. When I got home I pulled the carburetor and checked for any obvious problems. I sprayed carb cleaner thru the passages and blew them out with my air gun. I am running a 34 Kehin flat slide carburetor that has been on the bike for 5 years. Checked the clear in-line fuel filter to see if there were any obstructions but didn't see any.
Took the bike back out today and it didn't bog down but still has a momentary hesitation, almost a "hiccup" if you know what I mean, when running at a constant speed. I plan on replacing the in-line fuel filter to see if that improves the situation.
The battery is fully charged with a Pazon ignition.
Crap gas maybe? I don't know.

Any ideas?

John Ebert
Texas

htown16 · Dec 5, 2015

Could be the ethanol in the gas has seperated out some water, especially if the bike has sat for awhile during humid conditions. It could be laying in the bottom of the tank and getting pulled up into the petcock occasionally.
Cheap enough to pull the tank off and drain it completely. I put Stabil in all my bikes all the time because they might sit a month or so without riding. Even then stale gas seems to cause the occasional hiccup until a fresh tank is put in. Besides the ethanol/water problem, gas also loses its more volatile compounds as it sits.
Another thing to look for is an intermittant short. I've had similar symptoms from a faulty ignition switch.

Rohan · Dec 5, 2015

N0rt0nelectr@ said:
I plan on replacing the in-line fuel filter to see if that improves the situation.

Try it without the fuel filter at all.

I fitted a larger automotive one at some stage to something.
While the tank was full, no problemo.
Once the tank level got low though, it was quite hesitant.
Removed it, no more problems.
Apparently they are meant for cars, with fuel pumps and fuel pressure to force the fuel through them.

Since found those itty bitty little clear filters with a bit of mesh looking stuff.
You can see any large bits that have been stopped there.

Nortoniggy · Dec 5, 2015

Try turning both your fuel taps on too. Depending what sort you have, some don't flow much fuel. I had some that would cause the bike to hesitate at a constant 80'ish mph.

Ian

o0norton0o · Dec 5, 2015

I'm not familiar with the carbs you run, but the term hiccup is usually a fuel delivery related issue in my experience... This summer I had a clogged idle circuit which I found and fixed that went pop, pop pop. I also had a cracked intake manifold which caused an intermittent air leak that gave me an occasional pop, pop, pop. Both fuel related...

Try the carb cleaner spray on outside of the carbs and intake to search for an air leak...

Triton Thrasher · Dec 6, 2015

Gauze strainers on the taps and carburettor banjo fittings get clogged.

N0rt0nelectr@ · Dec 6, 2015

I have a new in-line fuel filter and plan on replacing the old one today. I will drain the tank and check the strainers on the petcocks.
Being in north Texas it is very humid and we have had a lot of rain this year. Most recorded rainfall since records started being kept, quack!
Used to live in Arizona and we never had this sort of problems, "it's a dry heat" as we used to say!
So water being in the fuel isn't a surprise. I have been adding Sea Foam to the gas to stabilize it, maybe not enough.
I will follow up on your suggestions and keep you posted.

Thanks
John Ebert
Texas

Torontonian · Dec 6, 2015

New plugs. Clean up kill switch. Pull HT leads and inspect for oxidations. Check fuse holder for same. Clean up coil connections. Retighten battery bolts at posts. Have fun. :lol:

dero · Dec 7, 2015

I have had a similar problem with my bike , 74 920 twin 35mm FCR:s , 99% of the time it runs fine , but every now & then it suffers as you describe , I was wondering if they are designed to run from a fuel pump .

concours · Dec 7, 2015

N0rt0nelectr@ said:
I was out on a nice long ride and my 74 850 acted like it was fuel starved a couple of times. Pulled in the clutch and revved the engine and it would clear up for about 5 miles then do it again. When I got home I pulled the carburetor and checked for any obvious problems. I sprayed carb cleaner thru the passages and blew them out with my air gun. I am running a 34 Kehin flat slide carburetor that has been on the bike for 5 years. Checked the clear in-line fuel filter to see if there were any obstructions but didn't see any.
Took the bike back out today and it didn't bog down but still has a momentary hesitation, almost a "hiccup" if you know what I mean, when running at a constant speed. I plan on replacing the in-line fuel filter to see if that improves the situation.
The battery is fully charged with a Pazon ignition.
Crap gas maybe? I don't know.

Any ideas?

John Ebert
Texas

Tell us more... "Hiccup", meaning momentary intermittent misfire?
"acted like fuel starved"?
Test... will it pull strong at WOT? Third gear? Fourth? As long as you can safely hold it pinned? If yes, then NOT fuel starvation. Max fuel flow is at WOT...
You may have a steady state mid range lean mixture that occasionally misfires... to test, turn your idle mixture overly rich ( increase idle speed screw to compensate), see if the hiccup at light throttle cruise is gone. Next test would be needle clip position change.

As mentioned above, fresh plugs to be sure.

An intermittent electrical issue could act like that, bypass the key switch, momentary pushbutton kill switch with a jumper wire from the battery to the ignition unit to test.

YMMV :mrgreen:

Nater_Potater · Dec 7, 2015

N0rt0nelectr@ said:
...I am running a 34 Kehin flat slide carburetor that has been on the bike for 5 years...

I had a similar experience with my used Mikuni, and I believe the Kehin uses the same style of float needle. Check out the short thread carby-issues-are-clutch-t22739.html and you can see the ultimate cause towards the bottom of the page.

'Hope this helps,
Nathan

jseng1 · Dec 7, 2015

If this is a flatslide PWK/Keihin type then I am familiar with that carb and that problem. The problem is the opposite of what you think. You are running too rich and it stutters because its loading up. This happens at cruising speed in top gear. All you have to do is lower the needle a notch (raise the clip). Your problem will disappear. This happens a lot with those carbs because they go out jetted a little rich for safety sake. The customer can dial it in but the supplier doesn't want to be responsible for a seized piston. If your needle is already all the way down then you need a smaller needle jet. If so PM me.

If it pings and feels whimpy its too lean. If it cuts out momentarily its too rich and the plugs will be dark.

Torontonian · Dec 7, 2015

Now that's good advice. Can we see a photo of both plugs ?

acotrel · Dec 7, 2015

If it is too lean, it should cough. Have you checked the tank breather ? It might be the hole in the tank cap - might be blocked.

N0rt0nelectr@ · Dec 10, 2015

jseng1 said:
If this is a flatslide PWK/Keihin type then I am familiar with that carb and that problem. The problem is the opposite of what you think. You are running too rich and it stutters because its loading up. This happens at cruising speed in top gear. All you have to do is lower the needle a notch (raise the clip). Your problem will disappear. This happens a lot with those carbs because they go out jetted a little rich for safety sake. The customer can dial it in but the supplier doesn't want to be responsible for a seized piston. If your needle is already all the way down then you need a smaller needle jet. If so PM me.

If it pings and feels whimpy its too lean. If it cuts out momentarily its too rich and the plugs will be dark.

I will do a plug chop in the next day or two and see what I have. Up until recently I haven't had an issue and one would think that it would have happened all along? I did lower the needle back about 4 years ago on my first trip out to Lake of the Pines from the mountains of eastern AZ, a drop of over 6,000 feet.
I didn't raise it once I got back and had no issues until recently. Think I might have one more notch on the needle.

John Ebert
Texas

concours · Dec 10, 2015

Plug chop is useful for MAIN jetting, and using leaded fuel. The partial throttle cruise issue will not show up. Unless you are hog rich, (obvious black smoke, guzzling fuel economy, dopey throttle response) the plug chop with unleaded pump gas will not serve.

N0rt0nelectr@ · Apr 6, 2016

N0rt0nelectr@ said:
I was out on a nice long ride and my 74 850 acted like it was fuel starved a couple of times. Pulled in the clutch and revved the engine and it would clear up for about 5 miles then do it again. When I got home I pulled the carburetor and checked for any obvious problems. I sprayed carb cleaner thru the passages and blew them out with my air gun. I am running a 34 Kehin flat slide carburetor that has been on the bike for 5 years. Checked the clear in-line fuel filter to see if there were any obstructions but didn't see any.
Took the bike back out today and it didn't bog down but still has a momentary hesitation, almost a "hiccup" if you know what I mean, when running at a constant speed. I plan on replacing the in-line fuel filter to see if that improves the situation.
The battery is fully charged with a Pazon ignition.
Crap gas maybe? I don't know.

Any ideas?

John Ebert
Texas

OK, went out again the other day and had the same issues! Cleaned the carb and blew out the passages with compressed air. A friend asked if I had checked the gas cap vent! Sure enough it was pretty much blocked. Cleaned it out with a 1/16 drill bit. Runs like a champ with no hesitation.
Did a 50 mile loop ride and didn't have any issues until I was almost home. Rolling off the throttle it would die or almost die. I could down shift pop the clutch and she would start right back up! Now what!

jseng1 · Apr 6, 2016

N0rt0nelectr@ said:
OK, went out again the other day and had the same issues! Cleaned the carb and blew out the passages with compressed air. A friend asked if I had checked the gas cap vent! Sure enough it was pretty much blocked. Cleaned it out with a 1/16 drill bit. Runs like a champ with no hesitation.
Did a 50 mile loop ride and didn't have any issues until I was almost home. Rolling off the throttle it would die or almost die. I could down shift pop the clutch and she would start right back up! Now what!

Check your idle speed setting and tune in the idle mix (close till it slows down, then open it and stop when it speeds up (or 1/8 more turn) - should be about 1/4 up to 1-1/4 turn).

Craig · Apr 6, 2016

have you looked at coil connection from coil to plug wire .... in humid conditions moisture can find its way into that rubber boot and make a mess with no sign on outside
Craig

Fast Eddie · Apr 6, 2016

N0rt0nelectr@ said:
N0rt0nelectr@ said:

I was out on a nice long ride and my 74 850 acted like it was fuel starved a couple of times. Pulled in the clutch and revved the engine and it would clear up for about 5 miles then do it again. When I got home I pulled the carburetor and checked for any obvious problems. I sprayed carb cleaner thru the passages and blew them out with my air gun. I am running a 34 Kehin flat slide carburetor that has been on the bike for 5 years. Checked the clear in-line fuel filter to see if there were any obstructions but didn't see any.
Took the bike back out today and it didn't bog down but still has a momentary hesitation, almost a "hiccup" if you know what I mean, when running at a constant speed. I plan on replacing the in-line fuel filter to see if that improves the situation.
The battery is fully charged with a Pazon ignition.
Crap gas maybe? I don't know.

Any ideas?

John Ebert
Texas

Click to expand...

OK, went out again the other day and had the same issues! Cleaned the carb and blew out the passages with compressed air. A friend asked if I had checked the gas cap vent! Sure enough it was pretty much blocked. Cleaned it out with a 1/16 drill bit. Runs like a champ with no hesitation.
Did a 50 mile loop ride and didn't have any issues until I was almost home. Rolling off the throttle it would die or almost die. I could down shift pop the clutch and she would start right back up! Now what!

You do realise that Acotrel suggested you check the tank cap vent way back in December !?

Fuel starvation?

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