Vaporlock revisited

[Keoke]

Snowridge, you are right that mortorcycles and almost all small engines run on less then 1 psi at carb inlet. Also, there is no need to over design a system if it's not needed.
However, the higher the pressure in the fuel line the less likely it is to vapor lock. One psi raises the boiling point of water almost 3 degrees but it raises the boiling point of gasoline at lot higher depending on it's volatility.
Hey Snowridge, the word that got beeped out of my post is the one that describes where the fires burn down below and most likely where I will end up. Minus the virgins of course.
Next time I will use the word "HECK".

take care and have a good one!!!

 

[MossRoad]

I have heard that folks will be visited by the spector of dimness before they are sent to heck. Gosh! /forums/images/graemlins/tongue.gif /forums/images/graemlins/wink.gif

 

[Gravy]

Keoke,

First, let me welcome you to the forum. You're already contributing useful stuff. Based on my experience of 20+years as an auto mechanic specializing in driveability and electrical/electronic diagnosis, I agree with all the points you make.

Cutting to the chase, I have verified that the problem with my 425 was simple fuel starvation. After cleaning the tank and lines, replacing the restrictive factory tank outlet elbow with a straight brass hose nipple, and relocating the pulse fuel pump to the fuel tank supports (well away from the exhaust), I still had the problem. I then disconnected the fuel hose from the carb and cranked the engine. Virtually no flow I tried MR's suggestion of straight gravity feed with the same results. Since the bottom of the tank is below the carb inlet, I wasn't very surprised.

I then installed an elecrtic fuel pump that I had laying around, bypassing the original pump. I have no idea what the pressure and flow specs are. I had no way to test the pressure today at the job site I jumpered it straight to the battery and got enough flow to run a Chevy 350. I connected the hose to the carb and started up. Problem solved! I ran for an hour or more at full throttle with no problems.

Before I commit to properly installing the electric fuel pump with the attendant relay, fuse, and bracketry, I plan to disassemble and inspect the OE pump. It's possible that it just needs to be cleaned. I will also test the fuel pressure. I had no signs of flooding, but if I find much over 5-7psi, I'll take appropriate measures.

That's where I'm at for now. I'll be working on it further this weekend. If I find anything new, I'll pass it on.

Gravy

 

[MossRoad]

Good job of sticking with it! /forums/images/graemlins/laugh.gif

 

[Keoke]

( quote) Keoke,
First, let me welcome you to the forum. You're already contributing useful stuff. Based on my experience of 20+years as an auto mechanic specializing in driveability and electrical/electronic diagnosis, I agree with all the points you make. (end quote)

Gravy, based on my 34 yrs of full time teaching auto tech at a Junior College; let me say that you look like a super wrench to me.

Have a nice weekend,

 

[J_J]

You must have overlooked my earlier post about a fuel gage at the carb. It's hind site now but it could have saved you a lot of frustration. Just don't let that electric fuel pump overpower the float in the carb, and flood the engine. Your original fuel pump might only need a new diaphragm, or replace the factory unit.

 

[Gravy]

JJ,

I didn't overlook your post. You had a good idea - I just forgot to bring a pressure gauge with me to the jobsite yesterday, so I couldn't take your advice then. Now that I've got the machine back home, I plan to check the pressure, and disassemble & inspect the OE pump. Since it ran well for over an hour with the electric pump, I'm pretty confident that the pressure isn't excessive, but I'd prefer to run the stock pump and save the electrons for the battery.

Thanks,

Dave

 

[Keoke]

Gravy, here is a site with a low pressure pump at a decent price.

Click here for link.

cheers,


EDIT: Shortened long link.

 

[Gravy]

Latest update:

I disassembled the OE pump and found oil behind the diaphragm. IIRC, JackRobin has said this results from a high oil level and/or operating a lot on steep slopes. Since much of my work is on fairly steep slopes, I don't have much confidence that it won't happen again.

I tested the fuel pressure from the electric pump I used for testing. It puts out 4psi, and pulls 1.1 amps, so I feel comfortable using it in a permanent installation. I am leaving the OE pump in place so I can switch back and forth just by swapping hoses. That way, I should always have a backup.

I still don't have a good explanation as to why the OE fuel pump worked fine for the first 20 minutes or so, then, lost virtually all flow (I did test that by pulling the fuel hose at the carb and cranking the engine - virtually zero flow when hot, so I knew it didn't really matter that I'd forgotten my pressure gauge).

Thanks to all for all the suggestions. Now I need to fix my leaking quick-attach cylinder /forums/images/graemlins/mad.gif

Gravy

 

[Keoke]

Gravy, if OEM pump is of the crankcase type (only a guess) then don't expect much from this pump while turning engine over with the starter. These guys need rapid pulses on the back side of the diaphragm to move the front side of the diaphragm and get the fuel moving.
Bottom line, it looks like the electric pump is the sure way to go for a once and for all repair.
Sometimes the OEM design simply cannot be repaired and a design change, such as you did, is in order.
Unfortunately, this happens often when a machine comes with various OEM engines, not all of which went through hard field testing in their individual configurations.

take care,