Topic: Vapor lock on a sohc four?

[jcarthel]

Vapor lock on a sohc four: I have never seen this topic discussed here.  There have been lots of discussions that the current gasoline leaves a lot to be desired, but nothing ever on vapor lock in particular and ways to prevent it.  Has vapor lock ever been an issue to anyone with an sohc four?  If vapor lock occurred to your sohc four, what did you do about it?  My k2 750 so far has not been affected by vapor lock, but if it is, can I pour something into the gas tank to combat vapor lock?  Vapor lock in the summer, frustrates me to no end when it comes to my two carbureted classic cars!  I have a '55 Buick and a '69 Dodge, both with electric fuels pumps in place of mechanical pumps and insulated fuel lines.  I pretty much have given up driving them on a hot day because today's fuel is crap!  On a hot day, both cars vapor lock to the point that sometimes they stall, and I have to cool things down in the engine compartment by pouring a jug of water on the insulated fuel lines in order to get them running again.  Thoughts, anyone?

                                                                                                                                  Justin from Texas

[supersports400]

Modern fuels have a lower vapor pressure than the older fuels, so vapor lock risk is lower. Older cars are prone to vapor lock because of the way they are build.
Isolate the carbs and fuel lines from engine heat (put a heat reflective shield under the gastank, avoid fuel lines running next to heated surfaces etc), and check if the floats chambers are well vented. If you use winter fuel in the summer, changes to vapor lock are also higher.

But you did not encounter vapor lock on your K2 jet, so is it a question "what if"?, maybe there is a reason for the fact that you never seen a topic about the subject, maybe looking for issue's which are not existing to the older fours ?

In the 20.000 km on my CB400F I encountered an AIR lock in my fuel lines once, which isn't a vapour lock, and somehow caused by a leaking connection between fuel line and fuel cock.

I'm living in the Netherlands, sea climate, so maybe I'm not the best to answer the question.

[70CB750]

On CB750, the fuel line needs to be routed to be horizontal at the worst, never uphill. 

And no, I don't remember anybody talking vapor lock on here before.

[Grabcon]

From Wikipedia
Vapor lock is a problem that mostly affects gasoline-fueled internal combustion engines. It occurs when the liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult.

I don't think you have a vapor lock issue I think the vent on your gas tank is plugged.

[Bodi]

I haven't heard of this ever happening on one of these bikes. You would have to be stopped for an extended time with a really hot engine - idling in hot weather with no wind - to get the temperature around the fuel lines and carbs high enough, in my opinion. That would be a bad practice at best. I'm not sure vapor lock is even possible in a gravity feed system: this problem occurs around the fuel pump. A bubble of vapor should just float up in the fuel line like air bubbles do as the liquid flows past them down to the carbs. Wikipedia claims gravity feed is not immune to vapor lock but their explanation makes no sense: a vapor bubble in the fuel line can not "reduce the pressure" from the fuel tank above - the laws of physics are quite clear on that, if it was possible I could scuba dive to the bottom of the Pacific ocean with no pressure problems just by keeping an air bubble over me.
The proscription on uphill fuel line routing puzzles me, I think pretty much every bike I have seen has the fuel line go uphill somewhere and they all appear to work super great. A recurve, with the fuel line going up then back down, sounds like a bad idea. Again, I have seen many fuel lines do that with no issues. If the top of the curve stays below the tank bottom and carb bowls it shouldn't make any problem. If the top of the loop is ever above the fuel level it can stop flow - but even then some air has to be trapped at the top to break the siphon action. I do avoid such up/down curves.
I can't really argue with those who say to make all fuel runs direct without uphills or to never use fuel filters because that will ensure good fuel flow. That works for them. I can only attest that I have worked on countless bikes that break those rules and only saw one problem when a fuel filter (installed because of a flaking rusty tank) clogged up: replacing the filter solved that temporarily at least.

[70CB750]

From the double petcock down, across 1 (cylinder 3 and 4) or 3 (1 and 2)insulators, down to the hole(s) in the stay plate and connects to the T piece between carbs.

No need to route up hill on CB750.

[flybox1]

downhill routing of your fuel lines, cleaned out gas cap vent hole, and properly vented carbs will prevent any vapor lock.

[eigenvector]

The only time I have run across this is when the vent hole on the carbs were plugged.  It can be a real pain to track down because that hole is not obvious and certainly not large.  It can gum up real quick.  When it's plugged fuel will NOT flow into that carb

[dusterdude]

I run mine down between carbs 3 and 4 between the rack plate and the head,then turn them up a bit and feed from the bottom.never have an issue with flow.also,look for a non venting gas cap too,could be a problem as well

[katman]

I had gas cap issuses on my 75 750k. Would be going along and would act like it was running out of gas. Lines, filters everything good. I could open the gas cap for a few seconds and close it and it would start and run like normal. Suspected the vent so I replaced the cap, and no more problems

[eigenvector]

Incidentally - what did you replace the cap with?

Mine's pretty scaly from rust, but I've never found a way to replace it given that it isn't designed like a modern gas cap.

[HondaMan]

The closest thing to "vapor lock" problems of any sort that I have ever seen on these bikes is when the fuel boils in the carb bowls.

Personal experience: when running my K2 with the Vetter fairing and Lowers in place at high altitude (6000 ft) on a 110+ degree day in heavy city traffic, I have 3 times boiled ethanol-laced fuel in the carb bowls. The result: wet spots under the bike from the bowl overflows when sitting at stoplights, and a marked stumble when moving from stop to slow speed. But, the engine never stalled, and as soon as I reached 25+ MPH the boiling stopped every time.

Way back when these bikes were young, racers would sometimes install a baffle plate in front of the bowls to deflect the hot engine air away from the bowls, so as to cool the fuel. Cold fuel atomizes much faster when it hits a hot piston (as many experienced dragracers will attest), providing faster fuel burn. One example of this I have seen successful in early 1970s Mustangs: at the strip, the racers would pack dry ice into a charcoal (emissions) canister with a coil of copper tubing inside so as to cool the fuel enroute to the carb(s). It noticeably improved their ET, like tenths of a second!

[01Thomas]

And the cold fuel is more dense as is cold morning air, which is why engines tend to perform better in the mornings than around noon.

The closest thing to "vapor lock" problems of any sort that I have ever seen on these bikes is when the fuel boils in the carb bowls.

Personal experience: when running my K2 with the Vetter fairing and Lowers in place at high altitude (6000 ft) on a 110+ degree day in heavy city traffic, I have 3 times boiled ethanol-laced fuel in the carb bowls. The result: wet spots under the bike from the bowl overflows when sitting at stoplights, and a marked stumble when moving from stop to slow speed. But, the engine never stalled, and as soon as I reached 25+ MPH the boiling stopped every time.

Way back when these bikes were young, racers would sometimes install a baffle plate in front of the bowls to deflect the hot engine air away from the bowls, so as to cool the fuel. Cold fuel atomizes much faster when it hits a hot piston (as many experienced dragracers will attest), providing faster fuel burn. One example of this I have seen successful in early 1970s Mustangs: at the strip, the racers would pack dry ice into a charcoal (emissions) canister with a coil of copper tubing inside so as to cool the fuel enroute to the carb(s). It noticeably improved their ET, like tenths of a second!

[strynboen]

the few cbx 1000 vho vas raced in 1978..have big problems..specily vith "racing fuel"..air bubbels bloked for flow..and stopped 2 of the 3 cbx in 1978 castrol 8 auers race..make the suzukies vil the race..
it killed the cbx..and make the sales goes back..as peapel vanted the farstest bike..and that vas the suzuki..so vaper lock make hondas historry a sad one..it killed the cbx as the spedyest machine it vas..and like the goldving it vent touring machine...but bad fuel økonomi..then killed it as a touring bike..
the end for cbx 1000..