Fuel starvation?

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

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yeah, I too thought we had that ruled out as the answer months ago

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.

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Do as he suggests. This exact thing happened to me. I bought these same carbs from him and he told to lower the needle and it cured my same problem. It's been a couple of years and it hasn't happened again.

Fast Eddie said:

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

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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!

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

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At times I suffer from a bad case of rectal cranial inversion, just saying! Not afraid to admit it either!
John in Texas

Quite often what you see is not what you get. If you look at your bike, it is all there looking beautiful however it can have an inherent flaw. The tank breather problem is not obvious and it often happens when you start racing using methanol fuel. One of the things I've mentioned on this forum has been steering geometry - why would you ever change it ? However as far as my bike goes, getting it to tighten it's line in corners has been a game changer. You can brake up to a corner then get straight back onto the gas and accelerate all the way around it.

jseng1 said:

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!

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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).

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It is at 1/2 + turn out and seems to be running good. Took it out for a 40 mile loop ride. Started to hesitate and if I held it steady on a straight road it didn't want to go past 65. Not that I needed it but I stopped for gas and topped off the tank. With an extra 3 gallons of gas it would pull 70+ and then some without any problems. I do love my Interstate tank! So, with extra fuel and the weight of it the symptoms of my problem went away. Fuel pressure?
I did a fuel flow test and in 30 seconds I could fill a 12 oz bottle before the carburetor but after the filter. With the drain plug out of the bottom of the bowl it takes about 1 minute 15 seconds to fill the same 12 oz container.
Don't know if this has anything to do with what is going on but, when I start the bike up I use the enriching lever, if I shut it off "to soon" before it is warmed up the bike will die. It takes a minute or more to get to that point.



John in Texas

N0rt0nelectr@ said:

with extra fuel and the weight of it the symptoms of my problem went away. Fuel pressure?
I did a fuel flow test and in 30 seconds I could fill a 12 oz bottle before the carburetor but after the filter. With the drain plug out of the bottom of the bowl it takes about 1 minute 15 seconds to fill the same 12 oz container.




John in Texas

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switch to a higher flow fuel filter.

I checked my Interstate tank with Paoli petcock to see how it compares to your measurement.

I got 12 ounces in the same 1 minute and 15 seconds. This measurement is one petcock open and one carb bowl plug removed to drain into a volume marked container.

So maybe your flow rate is not a problem.

Does this clear up and go away at times, my bike, single mikuni , runs exactly like that same distance out of town every day for about a mile then clears up, it happens after a high vacuum 65mph cruise, popping out the exhaust, missing , choke improves running doesn't cure, after it clears its good for the rest of the ride.
My assumption is that the primary by pass is freezing up leaning the mixture out coming off the slide, carb comes up to temp and clears up.
At 65 cruising I'm under 1/4 throttle so how much fuel does it need.
Whats the opinion of the fuel flow through BAP taps the main stand pipe is tiny, when you run the fuel through clear hose it never fills the hose and has to fight the air bubble in the line

A vacuum created above the fuel in the petrol tank does a lot of things to the tuning. Some tanks only have two small holes in the cap to vent the tank, and if the seal rotates it can sometimes block the holes. Most racers have at least a 5mm ID tube venting the fuel tank. If air doesn't enter the tank at least as fast as the fuel leaves, you get starvation at the float bowl. The easy way to check is take the lines of the carbs and put the ends into a measuring cylinder, then time the fuel flow and use a bit of judgement. If the venting is really poor sometimes you will even see the fuel stop flowing.

Funny how no one considers the obvious. ... Simple and easy to check things..... FIRST.
High throttle settings fuel consumption??? 30 miles per gallon?? Whats that work out to in pints per mile?? Go work it out. Now put a pint pot under your tank and see how long it takes to fill it up. Your fuel cap could well have a blocked breather causing a vacum to occur in the tank restricting your petrol flow OR your soper dooper petrol taps could be employing a seal that does not like the fuel we use theyse days and could be partly blocking the fuel supply.
I spent years with my old Dommy going slower and slower at times of high throttle settings such as over the Mountain Mile in the Isle of Man when pretending I was a TT rider!!...till I checked my fuel flow rate from the tank and found it was causing the level in the carbs to drop causing increasing weak running. Of course as I closed the throttle or stopped the level rose back up again!! On my Morris Minor a lot of decades ago I EVENTUALLY used my brain and rushed round the car and removing the blocked breather filler cap heard air being sucked into the tank...... and thought AHH how bloody obvious....why didnt I think of that FIRST!!

J. M. Leadbeater said:

Funny how no one considers the obvious

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Funny how it took you five and a half months.

Triton Thrasher said:

J. M. Leadbeater said:

Funny how no one considers the obvious

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Funny how it took you five and a half months.

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Oh so true! My wife has some health issues that come way before the bike!

Better men than you and I have felt the effects of a blocked tank breather. At least you were not leading a race when you experienced fuel starvation.