How to reduce vapor lock / boiling fuel

[tomnj]

Ok, I'm not sure if this is a common problem with 850's, but I am experiencing vapor lock / boiling fuel with my car. I'm starting a thread about this because this is a complex problem with a lot of components involved and a lot of potential fixes. It seems that today's fuel has a lower boiling point, which is why this problem is more common today than it was in the 1970's when our cars were new.

My Vapor lock problem: When I stop the car after a long drive (more than 15 min) I can hear the fuel inside the carb boiling and there are bubbles in the clear fuel filter after the mechanical pump. This causes the car to be hard to restart and it will even cut out on me and run poorly around town when hot. If I let it sit for an hour to cool, everything is back to normal. This was on a 75 degree day, not very hot.

Unique to my car (making matters worse) is the car (originally a 817cc) is upgraded to a 903cc, but running the stock fuel tank, which doesn't have a return, so the 903cc carb had to be swapped out for the one on the 817cc (re-jetted) which does not have the fuel return line. Also, I am running the early 817cc radiator, which has a different upper radiator hose that sits closer to the carb, in my case touching the air cleaner assembly and likely heating up the carb (see picture). This may be the single biggest problem, but there doesn't seem to be any easy fix, except to replace the radiator with the one from the 903cc which would give a different hose setup, but its a lot of work to do this.

Questions: Is the mechanical fuel pump a source of hot fuel? Would switching to electric help? (I dislike electric pumps personally, but will try it if it will help). Would replacing the carb manifold gasket/spacer with a different style that insulates more from the heat of the engine be possible? How about running steel breaded fuel line? Maybe install heat shielding on the upper radiator hose so it won't heat the carb as much? How difficult is it to install an electric fan? Can you simply unbolt the mechanical fan from the water pump or do you need to change to a remote water pump too? Any other ideas?

 

[Jeff Stich]

Installing an 843/early-817 type thermostat housing lid & its corresponding short accordion-style upper radiator hose would take care of that big mess you currently have going on there. Or swap out your early radiator for a late one & get the correct upper hose for it.

As for the carb/tank fuel return issue, all you had to do was swap the early vs. late (30dic vs. 30dica) carb lids, not the whole carb. But you've already solved that problem, so no worries.

Installing a phenolic spacer between the carb & manifold might help with heat soak, but in your case, it would put your chrome air filter lid into the engine bay roof.

The mechanical fuel pump is not normally a source of hot fuel as long as the thick phenolic spacer is in place between the engine block & fuel pump, & the exhaust-side engine bay underpanel is installed. A fuel hose or inline fuel filter sitting on, against, or very close to the engine or cylinder head can be, though.

 

[RocketMonkey101]

I had a similar problem over the weekend. First time I've taken it up a very large hill by my house. Engine just cut out. Pulled over the emergency lane and was able to crank it back over, pulled off the next exit and it died at a stoplight. Managed to get it to start by popping it into gear while rolling down hill. Got home and tried to turn it off and start it back up and it just woudln't. Let it cool for a couple hours and voila started right back up, no problem. I'll have to see if any of my lines are sitting on the head. By the way I'm using an electric pump so I don't think the mechanical one is the problem.

 

[NM850]

I’ve never had vapor lock problem and I always run the single line of the early carbs and a mechanical pump. I sometimes drive when the temps are in the 90s so I don’t think it’s a an inherent 850 problem.
Is the engine running temperature normal and have you checked your timing?

 

[Pantdino]

How long has this been going on? Do you still have winter fuel I’m the car, which boils at a lower temp?
I had an 843 coupe in the 70’s that I drove every day and never had that problem, even on 100 degree days.

You might need a phenolic spacer under the carb, even if that means getting a shorter air filter

 

[tomnj]

Thanks for the suggestions guys. I am going to re-check my timing. I also have the wrong kind of hose on the top of the radiator, which rested against the air cleaner (which I just added, the PO was running no air filter). Also, the fuel line was resting against that radiator hose as well. I will fix all that and see. I have only had the car about 10 months and it was running straight water (Didn't know until the Winter about that) until 2 months ago so I don't really have a history of the car. It supposedly has a rebuilt motor and runs very well otherwise.