new oem vacuum petcock won't work-HELP-

[kurtj]

Hi guys,

Just finished fully putting my gs 750et back together. I put on a vacuum petcock from z1. It works GREAT when it works.
I finally dialed in my carbs to a "T". Why does the valve fail sometimes?

My question is, yesterday and today the bike won't start from sitting all night. It will just crank and crank on the starter. It will try to start a couple of times, but no go.

I have to prime it for 30 seconds and she finally fires. I switch over to vacuum and she runs great.

If I try to restart the bike after it's been run, it's fine. Fires right up. Only issue is when cold from the night before.

Vapor lock?

Time for a pingle?

All help in this is greatly appreciated. I REALLY need to get riding (great stress relief), and I don't want to have to worry any more about starting issues.

By the way, Carbs, coils, wires, plugs, coil relay mod,air filter, valves, etc. all done or rebuilt. Plenty of spark, no or little fuel.

[Billy Ricks]

Try priming it before you ever crank the motor. The carbs should have enough fuel in the bowls to not run dry overnight. Check your float heights. Did you check the screens that are located above each float seat? They are part of the seat.

[chef1366]

Sounds like your bowls are emptying when the bike sits. That's why you need to prime. Probably isn't petcock but float needle and seat.

[bmac]

float levels

I have to agree with the above answers, Had same problem on my 1100g. Problem with hard start after rebuild. Took carbs off rechecked float levels, reinspected pilot jets, rechecked bench sync. all off marginal (float and beanch sync). Checked pilot jets to see if restricted, checked out okay. I Reinstalled the carbs and starts great and idles perfect. Minor adjust in float and bench sync. made all the difference. bmac

[BassCliff]

Hi,

Where is the gas from the float bowls going? Evaporating? Maybe into the crankcase?


Thank you for your indulgence,

BassCliff

[kurtj]

I have to agree with the above answers, Had same problem on my 1100g. Problem with hard start after rebuild. Took carbs off rechecked float levels, reinspected pilot jets, rechecked bench sync. all off marginal (float and beanch sync). Checked pilot jets to see if restricted, checked out okay. I Reinstalled the carbs and starts great and idles perfect. Minor adjust in float and bench sync. made all the difference. bmac

Thanks bmac, I'll do the same checks on the floats and bech sync.

kurtj

[kurtj]

Hi,

Where is the gas from the float bowls going? Evaporating? Maybe into the crankcase?


Thank you for your indulgence,

BassCliff

Hi BassCliff, to answer your question on the fuel, I just don't know. There's no leaks anywhere. I have been there before with fuel in the crankcase. You could smell it once you opened the oil fill cap. I opened it today and didn't smell any fuel. Doesn't mean it's not there. Oil level seems a little too high but could be I'm just paranoid. It's been getting real frustrating. I'll get there. I really enjoy this bike going down the road.(when she runs, she runs great!)

[themess]

You shouldn't need to prime it in this weather unless it's been sitting at least ten days.

Time-consuming suggetion: Remove the carbs, set them on paper towels, and fill them with gas through the regular fuel lines until the floats shut off the gas flow. Then watch for leaks.

[49er]

You say that the engine restarts good when it's hot. How does it run under load? If your float levels are too low, the bike should be staving fuel under load. If it runs good under load, the float levels aren't too far out of spec.
There's no way that your fuel will evaporate from the bowls overnight. Even leaks from the bowls would be obvious, with a strong smell of fuel in your garage the next day!

You mentioned that the valves had been "done or rebuilt" along with other items. Did that include setting their clearances. The symptoms you describe are often experienced when an engines valve clearances close up.

[kurtj]

I checked all 16 valves. all were within specs except 1 exhaust valve on #3. I set that one to specs.

thx

[Larry D]

Did you check for fuel in the bowls in the morning when it won't start or after you've got it running and it starts right back up ? I agree the bowls shouldn't be empty in one night.
The carbs floats in my '81 750 16 valve were off 5 mm each. I'd look at that again.
Kinda sounds like the choke isn't working well, how's your choke cable ? Attached correctly and moving freely ?
Weird......

[chef1366]

Did you feed a wire throught the choke jet in the float bowl when cleaning? (the hole the brass choke tube goes into)

[SPARKSS]

Hello Kurt. First of all - vapor lock is a condition occurring during a hot static soak (engine not running.....carbs unable to relieve the resulting vaporizing fuel......carb problem.

Your condition sounds like not enough fuel cold. Because you say the engine runs normally warm it's most likely not the petcock. It's possible that the petcock vacuum line leaks slightly or that the engine doesn't develop sufficient vacuum on that line to enable the petcock when cold.........If engine compression is good, and all rubber associated with this vac circuit is intact, then it's likely not the problem either.

Does your cold start enrichment circuit work correctly? This is a very common issue, often overlooked in warmer riding areas if the rider isn't used to using it to start cold (the "choke" knob or control). If your carbs use this, then you might review whether the individual valves work and are clean inside the carb. Cold startup without this working correctly can be pure misery and just may not happen. Warm running will be unaffected .
Using the prime position on the petcock should result in very quick fill of the bowls......because you say that it takes awhile for this to seem to make a difference, I'd wonder that what is happening is that one or more carbs are actually flooding slightly into the airstream, enriching the mixture and allowing you to startup. This isn't something that should happen.

DO check to see whether you have a gain to the oil level..........as others have indicated , carbs that flood easily (carb fload/needle valve problem coupled with a leaking petcock) will dump fuel into the sump. This does 2 things.......ruins your oil (change it when this happens) and it washes down the cylinder walls (cylinders flooded) which will hamper attempts to start due to reduced compression and increased friction at this point (bad stuff).

What the geez is a pingle?

[CodyLHen]

This might be silly but is it possible the spring in the petcock is to strong or binding not allowing the diaphragm to open. I agree with sparkzz, it could be a leak in the vacuum line. The fact that it starts and runs on prime than works on run suggests it can not get sufficient vacuum to release the fuel. I hope it is this simple. Good luck.

[themess]

Very easy to check. Suck on the vacuum line and see if gas comes out of the gas line. (I'd probably be very careful doing this, don't want a mouth full of gas.