vapor lock ??

YING · Mar 22, 2012

74 Roadster,std. Amal carbs,Straight down fittings off fo gas tank with in-line filters.
Starts great,runs nice
Stop for gas and a soda for 10 minutes then starts right up but immediately starves for gas for about 20 seconds.I have to
sit there and keep twisting the throttle to keep it running.After that it runs fine.Any suggestions would be appreciated.
YING

bill · Mar 22, 2012

I would say it is a fuel boiling issue. the fuels today are NOT formulated ( vapor pressure) for carburetors as we not are not running the 30+ pounds of fuel pressure of modern fuel injected automobiles. when you stop on a hot day it will heat soak the carbs and boil the fuel, as soon as you get on the move the air cools everything back down. you have several choices, don't stop for as long a time, stop for a longer time for it to cool way down or cool it down with it's own cold drink :lol:

debby · Mar 22, 2012

I've been having a similar problem with my 850. Stop for gas, doesn't want to restart. Ok if I let it cool down for a half hour or so. Worse if I have to do a lot of stop and go riding across town on a hot day, especially if there are some slow lights and the bike sits idling for a while. One time it happened after a long run down the freeway at 80+ mph (just keeping up with traffic). I pulled off, stopped at the red light at the end of the off ramp, and the engine died. Didn't want to restart.

For some reason my 750 is not nearly as likely to do this. Both bikes run Amals. 750 has points, 850 has a Pazon Sure Fire. I don't know if the ignition has anything to do with it though. At first I was thinking the Pazon had a heat-related problem, but maybe it 's fuel boiling as Windy is suggesting...

Debby

Diablouph · Mar 22, 2012

I would think this would stop the heat soaking.

850-carburetors-t12255-15.html#p143263

AussieCombat · Mar 22, 2012

YING, does it also happen on a cool day or night ?
I too had a similar problem, not wanting to start and dying at the lights.
Try wrapping a damp towel around the carbs just before you try starting the hot motor, that might answer the boiling question.
However, if mine wouldn't start by the second kick, when it was riding temp, it needed a tickle.
I raised the float level a little and it was better, not perfect but better.
I believe it is an 'Amal Idle circuit thing' (non adjustable), and float level needs to be right.
Lots of people suggested to me that the symptoms suggested are a 'Boyer thing', but Debby has Pazon, and I fixed mine completely with PWKs.
AC.

batrider · Mar 23, 2012

Never saw this problem. Are the heat insulating washers between the carb manifolds and the head there?

Also what about fuel line routing? There was a change to the way the fuel lines were made. If it has a crossover hose connecting the two petcocks in front of the carb it is the older way (H configuration). I believe it was considered a bit of a safety issue if that crossover hose gets melted from the heat of the cylinder barrel. The later way was to go to double banjos on the carbs and have a very short hose right between the float bowls.

Just throwing out some ideas.

Russ

stockie2 · Mar 23, 2012

Ying,

The insulation gaskets could be an issue, I noticed my carbs would get really hot after the engine was shut down, I only had asbestos type gaskets between the manifolds and the head, so I made up some 5mm thick Tufnel insulators, the carbs run much cooler. Also I took the trouble to set the fuel levels in the carbs, search for the thread for more details. I run a 850 and do not suffer your symptoms. Trust this helps.
Richard

lrutt · Mar 23, 2012

Regarding the fuels. Actually my understanding is that ethanol does not vaporize as easily as regular gasoline so would be harder to 'vapor lock'. That said, Even in florida I've never had that problem, with and without ethanol fuels. Could it be a problem with tank venting or something else possibly?

debby · Mar 23, 2012

AussieCombat said:
YING, does it also happen on a cool day or night ?
I too had a similar problem, not wanting to start and dying at the lights.
Try wrapping a damp towel around the carbs just before you try starting the hot motor, that might answer the boiling question.
However, if mine wouldn't start by the second kick, when it was riding temp, it needed a tickle.
I raised the float level a little and it was better, not perfect but better.
I believe it is an 'Amal Idle circuit thing' (non adjustable), and float level needs to be right.
Lots of people suggested to me that the symptoms suggested are a 'Boyer thing', but Debby has Pazon, and I fixed mine completely with PWKs.
AC.

Yup, a tickle is what mine wants also. I usually try tickling just one carb, with the idea that one cylinder or the other should fire, then pull the other one into life. Maybe I should try those carbs too...

Debby

bill · Mar 23, 2012

There is more to this issue than ethanol. Like I stated they have lowered the vapor pressure. It is formulated for 30+PSI of fuel pressure NOT the low pressures ANYTHING with a carburetor will run. mine will do this sometimes in weather over 85 deg. Same thing with older automobiles, I have fixed a return line to constantly circulate fuel from the carb to tank and it will 99% of the time fix this issue but we don't have this option.

lrutt said:
Regarding the fuels. Actually my understanding is that ethanol does not vaporize as easily as regular gasoline so would be harder to 'vapor lock'. That said, Even in florida I've never had that problem, with and without ethanol fuels. Could it be a problem with tank venting or something else possibly?

comnoz · Mar 23, 2012

bill said:
There is more to this issue than ethanol. Like I stated they have lowered the vapor pressure. It is formulated for 30+PSI of fuel pressure NOT the low pressures ANYTHING with a carburetor will run. mine will do this sometimes in weather over 85 deg. Same thing with older automobiles, I have fixed a return line to constantly circulate fuel from the carb to tank and it will 99% of the time fix this issue but we don't have this option.

lrutt said:

Regarding the fuels. Actually my understanding is that ethanol does not vaporize as easily as regular gasoline so would be harder to 'vapor lock'. That said, Even in florida I've never had that problem, with and without ethanol fuels. Could it be a problem with tank venting or something else possibly?

Click to expand...

Bill is right about the vapor pressure being higher. Carburated vehicles do suffer because of it. Especially in the spring in this part of the country when winter blend fuels are still in the system. Winter blended fuels have even higher vapor pressure than summer blends. And it is not because of the ethanol. Jim

debby · Mar 23, 2012

So is it higher vapor pressure or lower? Higher seems to make sense, for compatibility with pressurized fuel injection systems. That could also explain the shorter shelf life. It also seems to me that modern fuel requires leaner jetting. In the old days I ran the stock jetting (260 main jets for example) in my 850 and it ran fine at all altitudes. Now that would be so rich the bike would be unrideable here at 5000 ft. I currently have 210 main jets in my 850 and 200s would be better.

Anyway, it would seem there is no point in throwing expensive new parts at the problem, unless it's one of Jim's fuel injection systems. Or maybe some of that $18 per gallon race fuel is still made the old-fashioned way...

bill · Mar 23, 2012

I had it backwards :roll: . and I had to lean out my combat from a 220 to 190 with today's fuel. after i dropped to 190, I tried to step it back up to a 200 and it did not like it either so a 190 it is

debby said:
So is it higher vapor pressure or lower? Higher seems to make sense, for compatibility with pressurized fuel injection systems. That could also explain the shorter shelf life. It also seems to me that modern fuel requires leaner jetting. In the old days I ran the stock jetting (260 main jets for example) in my 850 and it ran fine at all altitudes. Now that would be so rich the bike would be unrideable here at 5000 ft. I currently have 210 main jets in my 850 and 200s would be better.

Anyway, it would seem there is no point in throwing expensive new parts at the problem, unless it's one of Jim's fuel injection systems. Or maybe some of that $18 per gallon race fuel is still made the old-fashioned way...

comnoz · Mar 23, 2012

bill said:
I had it backwards :roll: . and I had to lean out my combat from a 220 to 190 with today's fuel. after i dropped to 190, I tried to step it back up to a 200 and it did not like it either so a 190 it is

Yeah, I knew what you meant. It is hard to get fuel to gravity feed down a hose from the tank to the carb when the escaping vapors from warm fuel are trying to go up the hose.
I have seen better luck using 90 degree fittings from the fuel tank so the line is farther away from the cylinder. If you have a fuel filter besides the screen in the carb banjo, get it back away from the cylinder or get rid of it. The screen in the banjo does a pretty good job of filtering the gas by itself. Jim

AussieCombat · Mar 24, 2012

Yep, I understand exactly what you'r saying Bill, and Jim, but we don't have to run Ethanol over here, at this stage
we can still get reasonable quality 95 or 98 Octane at the pump but have that same problem.
Only real problem over here is cost... 95 or 98 is about $1.53 per ltr (4.5 ltrs to the gallon).
If I'm lucky I can fill the Roadster tank when on reserve, for under $12.00.
Prices can vary here by .20c ltr. Thieveing bast...s.
AC.

stockie2 · Mar 24, 2012

Next time it happens, just check if the carbs are hot!

Richard

YING · Mar 26, 2012

Pulled float bowls to check float level and made very slight adjustment
Pulled both taps off of tank and cleaned screens
Removed in-line plastic fuel filters that I have used before
Installed new fuel line
Started fine after 25 mile run and letting it sit for 15 minutes
Seems like problem solved !!!
Thanks for all the input
YING

vapor lock ??

YING

Pittsboro,NC

bill

debby

Diablouph

AussieCombat

batrider

stockie2

lrutt

debby

bill

comnoz

debby

bill

comnoz

AussieCombat

stockie2

YING

Pittsboro,NC

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