Vapor lock most likely

[Tahoe]

Since I bought my car I always knew it had an electric fuel pump running through the mechanical pump and on to the carbs. I always wanted to bypass the mechanical pump but I keep forgetting about it under the carbs, but now I'm sure I should. The last couple of days have been hot here (87 today and 90 tomorrow) and at 6500 ft elevation I'm seeing vapor lock after I drive about 10 miles in traffic. Now I have had that issue a little in 70 degree weather but hardly noticeable. Today I drove the last 4 miles home while it surged and it never quit but it was close. Seems like a no brainer to me to bypass the mechanical pump for good but thought I'd get your opinions. As a side note I drove it in the garage and it returned to a smooth idle in about a minute and I let idle for another 5 minutes without any problems. As soon as I drove it down the street with a slight load it started again. I'm 99.9% sure it isn't electrical.

[bob_rich]

Hi Russ

Firstly I am not expert in this but why was the electric pump fitted? Is the mechanical fuel pump in fact getting, or is, a bit weak? and the PO did not wish to get at it so fitted a second electric pump. I think fuel pumps used in carburetor based cars used pumps that provide fuel in pulses rather than smoother continuos flow of a high pressure fuel injection type pump. If the pulses of fuel got out of phase could the second pump pull when the first one was closed or getting ready to pump so the suck of the second pump would perhaps produce a vacuum or vapour lock?

I think I would only use one pump. On my seven that uses the cross flow engine (similar block to the twink) I fitted an electric pump with a safety switch so in the event of fuel leaks from the carbs I can kill the fuel flow and keep the engine running until the fuel in the carbs is used up.

If your car was in the UK at present the current temperatures here are such there would be no problem at all with vapour locks!

hope this helps best of luck

Bob

[Tahoe]

The PO told me left the old pump in place and ran the fuel through it when he added the electric pump. At the time I didn't think much of it and never had an issue, but with sudden heat have and the higher elevation it just isn't working, although I drove it at least 10 miles in heavy traffic before I had n issue. Must be at the mechanical pump because I have see thru filter at the electric pump and everything seems fine at that end. I'm sure the mechanical pump was no good or there won't be any reason to add the second pump. I'll bypass it tomorrow and we'll see if that solves the problem.

[Bud English]

Russ-
Summit Racing has a blanking plate with gasket to cover the hole when you remove the mechanical pump. Less than $5 if I remember correctly. It's chrome but that's easily taken care of with a squirt of paint.

[gjz30075]

I'm sure the mechanical pump was no good or there won't be any reason to add the second pump. I'll bypass it tomorrow and we'll see if that solves the problem.

Usually the failures are bad diaphrams, which put fuel in the crankcase. I think it would be easier to bypass the electrical pump by simply pulling a wire. The car should run fine without vapor lock on the mechanical pump.

Greg Z

[Tahoe]

Russ-
Summit Racing has a blanking plate with gasket to cover the hole when you remove the mechanical pump. Less than $5 if I remember correctly. It's chrome but that's easily taken care of with a squirt of paint.

Thanks Bud, I'll check Summit. I was going to block it off anyway and that's a good price.

Greg,

Not sure why I would want to disconnect the electric one when I'm pretty sure it's the mechanical one giving me issues. Besides I think the mechanical pump would have trouble pulling through the electric pump so one way or another I'll be pulling a pump. I'll pull the mechanical one when I block it off, and check it out. I'll want to keep it if the diaphragm is good or bad for the future just in case I go back to original. Hard to go wrong with the electric pumps and they're easy to change.

[nomad]

Are you running ethanol blended fuel by any chance. Vapour lock occurs much easier with alcohol.

Kurt
26/3754

[Tahoe]

Are you running ethanol blended fuel by any chance. Vapour lock occurs much easier with alcohol.

Kurt
26/3754

Actually no I'm not. Still didn't bypass it but have been driving in the cooler weather this week without any issue. I'll buy a blocking plate and bypass the pump when the weather forecast shows hotter weather again.

[YellowS4DHC]

Russ,
I didn't see any mention of where your electric pump is located. An electric pump positioned near the tank should be able to keep the line pressurized all the way to the carbs and thus preclude vapor lock.

If your elec pump is in the boot near the tank, I would definitely rig a by-pass around the mechanical pump and eliminate the latter as a variable. It sounds like the mechanical pump is creating a pressure drop and allowing the fuel to vaporize in the line between the mech pump and the carbs.

By the way, re-plumbing the fuel lines is easily done, and doesn't require you to remove the mechanical pump if you don't have the time.

Regards
Rick


'72 Europa TC
'69 Elan S4 DHC
'67 S800 Coupe
'57 T1 Speedster

[Tahoe]

The electric pump is in the trunk near the tank. It is easy to bypass the lines at the mechanical pump, but I figure if I'm at that point I might as well remove the pump and block it off rather than going in a second time. I hear you though, I could just bypass it and wait until WInter to pull the pump and block it off.