Vapor Lock?

Information and questions on GL1000 Goldwings (1975-1979)
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Posts: 14
Joined: Sat Aug 10, 2013 10:46 pm
Location: Magnet Cove, AR
Motorcycle: 1975 GL1000

Vapor Lock?

Post by BaronBigDog »

I have been working on a 75 GL1000 project for over a year and have it almost running. It runs OK until it gets into the normal operating temp. range and then it doesn't want to idle. Virtually everything on it is new. New electronic ignition--the spark is good when it quits. I checked this when restarting it. It starts fairly hard after a stall, but easily after cooling down. New plugs, new wires, new coils and a new starter. I have installed an auxiliary relay that is triggered by the original coil supply wire that provides voltage directly from the battery to the coils. The carbs were rebuilt by Old School Carbs in California and are synced with the bike at normal operating temp. The valve timing is correct as is the ignition timing. It has a new rectifier and I replaced the voltage regulator as the old one was suspect. I could not get a new regulator. I purchased a good used one on Ebay.

I found that enriching the idle bleed screws when it was hot helped a bit, but has not cured the problem.

The voltage does drop off a bit when the alternator gets hot. It is around 14.3 cold and 12.8 hot, but the Dyna-S ignition is supposed to operate well in this range, and it does have a hot spark when I crank it after it has stalled.

I have tried popping the gas cap when it starts chugging and dropping cylinders at idle and this had no effect. It is not running too hot coolant wise as the gauge just goes into the normal range and the fan cycles normally.

Compression runs a spread of 147 to 153 on all four.

I have a Vetter fairing with the lowers and the carbs are getting hot, but I have seen other bikes with the lowers and they seen to run OK. I can tell you that the carbs get too hot to touch which leads me to think the fuel is boiling in the bowls.

It will run liked a striped ape, but wants to chug and stall at idle when hot. It still runs OK hot, but the idle is a killer.

Any suggestions other than removing the lowers. I am going to try that next.

Please note that I am not a purist. I have made some changes to the wiring and have removed the emergency lighting circuits as well as adding the auxiliary relay for the coils. I have also replaced the 75 spokes with 79 stars. That was another education as the fork width seems to have been narrower on the 79. Also the speedo drive changed from right to left somewhere between 75 and 79.

It also has an electric fuel pump and I checked the output when it stalled and it is not the problem. Plenty of gas.

I have a bunch of work and money in this thing and am determined to get it running. I have had several favorable comments on it, but at the moment it is more or less a very heavy paper weight.

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Re: Vapor Lock?

Post by virgilmobile »

Great details....
With all you've done,there's only one test left....

When it gets warmed up and won't idle well,I suspect that it may be running rich...
You could pull a plug or 2 and look.A rich idle should soot up the plug....

Another easier test is when it's idling poorly,reach over and turn OFF the fuel lever...Let it run for a little...30 seconds to a minute or so....You might find that it will start to smooth out for a little while before it runs low on fuel...

The reason I mention this......
I had a single cylinder Honda that exhibited the same problems and then my 78 did the same thing....After many days,I finally started the bike cold when it was idling fine,shut off the fuel and pulled the fuel line off the carb...
I drained the fuel from one carb and measured the amount...

I put it all back and ran it till it happened again...poor idle...I did the same procedure and measured the fuel again...surprise...there was at least 30% more fuel in the carb...

What I found...The composite black float would absorb fuel like a sponge and sink in the bowl...this filled the bowl with too much fuel....Let it cool off and evaporate some fuel and it would return to normal....
My cure....I removed all 4 floats and dipped them in Polyurethane...2 coats,drying 24 hours...They worked perfectly after that....viewtopic.php?f=4&t=4776
Posts: 14
Joined: Sat Aug 10, 2013 10:46 pm
Location: Magnet Cove, AR
Motorcycle: 1975 GL1000

Re: Vapor Lock?

Post by BaronBigDog »

Thank you for the input. I will give the fuel cut off a try to see what happens and go from there.
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Motorcycle: 1977 GL1000
1980 KZ750 LTD
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Re: Vapor Lock?

Post by wjnfirearms »

Vapor lock is not as common a thing as it was a long time ago. It is basically when the fuel in the line at the engine gets so hot because of the proximity of the line to the engine that the fuel vaporizes in the line and stops the flow of the liquid. When manufacturers used metal lines at the carb, this was the main culprit. The most common solution back then was to wrap the line to insulate it. When flexible lines replaced metal it seemed to quell the problem. You really don't hear much of it any more.
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Gus Riley
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Motorcycle: 1985 Honda Goldwing LTD, and 2004 1800 Goldwing, with Bushtec Quantum GL trailer

Re: Vapor Lock?

Post by Gus Riley »

My bad, I thought I was posting in a 1985 LTD vapor lock issue. I cannot delete it, so here it is, maybe someone will find it here too.

Ok, in researching the forums for the same issue(s) one forum poster wrote that he had once poured cold water on the fuel line right where it enters the fuel pump. He stated that the engine leveled out and ran well or much better. I got to thinking, maybe an line of some sort could be routed to that fuel hose section and cool it somewhat with ram air? I did just that. I took some aircraft air cooling hose(scat hose) and routed it. At 83 degrees and humid it WORKED!!! I test rode for half an hour. I could bring the engine close to red line and it never missed a beat...not once. After my test ride I shut it down and felt the fuel line at the fuel pump...I could comfortably touch it no before it was hot! So I am waiting for a 90 degree plus day to test it again.
hopefully my pictures will post here. It isn't pretty, but it doesn't look too awful.

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