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Prime Time (ing)

open1mind

open1mind

Forum Apprentice
Past Site Supporter
How long does it take to fill the bowls in carbs once you flip the petcock to "Prime"? Are we talking 5 seconds? 1 Minute? 5 Minutes? Does it vary much by bike?

Some background
I have searched with some diligence but I only find the prudent warnings about taking care to avoid sending fuel past the carbs and into the cylinders.

Per my ealier posts, I had some problems with a bad petcock and other issues this summer. A British bike shop did some checks and got it sorted after a few false start repairs at a Japanese shop. It rode home from there strongly, after starting without issue. I let the bike (79 GS 1000e- mint, stock) sit for about 20-25 days before trying to ride it to my winter storage spot. When all is in working order, it usually fires after 1 second of cranking. It cranked well, but sounded gas-starved. Could all of have the fuel in the bowls evaporated? After some extended cranking, I ended up push starting it to try to save the battery.

The idea of PRI position only occurred to me late, and it fired after 2 pushing tries while on prime. There may be a related issue in that the choke seems to need adjustment, and that may have been the starting problem. Once it fired and warmed at a low idle (choke seems not to be closing all the way- max at full choke is only 2500 rpm), it ran great for about 12 miles to indoor storage for the winter. I just drained the bowls for winter, but I might be brave and try a choke adjustment as a winter project.
 
Prime takes about 10 - 30 seconds or so. The bowl start to dry out right away, after several days it will start better with a quick prime. After two or three weeks it needs it for sure.
Every bike is different, as is every climate. Probably gas evaporates quicker in a hot place.

There's not really any choke adjustment, as there's not really any choke.
 
In answer to your first question, I'd say seconds but it will vary bike to bike.

You don't mention that the carbs have been cleaned or rebuilt so I wonder about that. Have they?

Gas will not evaporate from the bowls that quickly (unless it is in a super hot climate) so while I suspect there is some issue with the carbs and they likely do need a cleaning and rebuilding, the other thing that jumps out at me is the amount of cranking needed to fire. That tells me voltage is not strong enough to both crank and fire the coils.

You need to check the resting voltage on the battery, then the voltage at the coils. If the battery voltage and voltage at the coils differs by 1 volt or more you likely will be having problems creating a powerful spark. The more it cranks before firing, the harder it becomes to actually fire. Some bikes I've owned wouldn't fire if the battery got down to even 12 volts.

To correct this, you need to try for full battery voltage at the coils. First, you should try and clean up all connectors from the battery through to the positive on the coils (orange and white wire) including those up to and into the right hand controls (and back out again). If that doesn't help then you will need to put in the "coil relay modification" which will help send full battery voltage to the coils. Look it up in the archives.

In addition to these primary culprits you should be sure that your valves are in spec. since this is another big problem with hard starting.

check these and let us know what you find.
 
tkent02 said:
Prime takes about 10 - 30 seconds or so. The bowl start to dry out right away, after several days it will start better with a quick prime. After two or three weeks it needs it for sure.
Every bike is different, as is every climate. Probably gas evaporates quicker in a hot place.

There's not really any choke adjustment, as there's not really any choke.
Click to expand...


I dont follow your point about no choke adjustment/no choke? I see it in the service manual....?
 
spyug said:
In answer to your first question, I'd say seconds but it will vary bike to bike.

You don't mention that the carbs have been cleaned or rebuilt so I wonder about that. Have they?

Gas will not evaporate from the bowls that quickly (unless it is in a super hot climate) so while I suspect there is some issue with the carbs and they likely do need a cleaning and rebuilding, the other thing that jumps out at me is the amount of cranking needed to fire. That tells me voltage is not strong enough to both crank and fire the coils.

You need to check the resting voltage on the battery, then the voltage at the coils. If the battery voltage and voltage at the coils differs by 1 volt or more you likely will be having problems creating a powerful spark. The more it cranks before firing, the harder it becomes to actually fire. Some bikes I've owned wouldn't fire if the battery got down to even 12 volts.

To correct this, you need to try for full battery voltage at the coils. First, you should try and clean up all connectors from the battery through to the positive on the coils (orange and white wire) including those up to and into the right hand controls (and back out again). If that doesn't help then you will need to put in the "coil relay modification" which will help send full battery voltage to the coils. Look it up in the archives.

In addition to these primary culprits you should be sure that your valves are in spec. since this is another big problem with hard starting.

check these and let us know what you find.
Click to expand...

Soon before this problem, I showed about 12.5 volts on the battery. It was definitely cranking pretty well- sounded gas starved to me. If the valves are out of adjustment, it sure runs strong once fired up. The dealer (insert grain of salt here) did check them less than 1K miles ago. At the same time, they put in the Dyna ignition and coils- so my guess is that those things are ok.

It just turned into pure tundra in upstate NY, so I am off the road until spring anyway. My plan is to check out the choke adjustment and some of your electrical advice over the winter. Combined with some chanting and animal sacrifices, and the use of the PRI function in the spring, I may get lucky and it will fire right up? We'll see. First step is to find the time and expertise and bravery to check out the choke cable.
Thanks
 
open1mind said:
I dont follow your point about no choke adjustment/no choke? I see it in the service manual....?
Click to expand...

There is an enrichment device, nicknamed a choke. No real choke. Some have a cable adjustment, other than that there is no adjustment at all.
 
The VM carbs are notorious for evaporating the fuel away over time.

Some are worse than others

Always give it some Prime if you haven't ridden for a week or more

Be sure to drain the fuel from the bowls for winter storage and put some Seafoam in

I take the fuel line off the petcock and fill the carbs with Seafoam

It beats cleaning out the jets every year
 
I give it a couple of minutes with the bike on the centerstand so the carb bowls will fill completely.
And that works well for me, engine doesn't turn over much before it fires right up, less load on the battery.
 
I do have the carbs drained and the bike in warm storage for the winter... so spring will be interesting. Im still wrestling with how long it takes to prime the carbs. Note that some of you say just a few seconds, and sedelen (who has the same bike,yes?) says a few minutes?

Very senstive to pushing gas down into the motor- see my other posts about a petcock problem. So, I dont want to overdue it.

thanks
 
No, it's not like that. The fuel fills the bowls, then stops when they are full. A few seconds will do, a few minutes is OK as long as the needle valves (float valves) in the carburetors are working.

Prime will continue to flow fuel if one of the needle valves sticks open, or if one of them doesn't quite seal perfectly. A grain of rust keeping the valve from sealing, or maybe a worn needle or seat. In this case it will dump on the ground if you have a bike with VM carburetors, or fill the crankcase with fuel if you have CVs. It is usually a long slow leak that fills the crankcase.

Really from bone dry carbs, 30 seconds is more than enough. Try using clear hose and watch it as you go to prime. In six or eight seconds the flow slows, in fifteen or twenty seconds it stops.
 
An actual choke usually is a butterfly that cuts out air (hence choke).
On these bikes it opens up air flow to pull fuel from the bowl. See the long brass tube that goes into your float bowl? That's the enrichment tube or jet.
 
tkent02 said:
No, it's not like that. The fuel fills the bowls, then stops when they are full. A few seconds will do, a few minutes is OK as long as the needle valves (float valves) in the carburetors are working.

Prime will continue to flow fuel if one of the needle valves sticks open, or if one of them doesn't quite seal perfectly. A grain of rust keeping the valve from sealing, or maybe a worn needle or seat. In this case it will dump on the ground if you have a bike with VM carburetors, or fill the crankcase with fuel if you have CVs. It is usually a long slow leak that fills the crankcase.

Really from bone dry carbs, 30 seconds is more than enough. Try using clear hose and watch it as you go to prime. In six or eight seconds the flow slows, in fifteen or twenty seconds it stops.
Click to expand...

All good thoughts. I do recognize that the the needles/floats should hold the gas back while on prime. However, I am reluctant to trust them. Per my other problems from earlier this summer, a failed petcock combined with leaking needle valves dumped gas into the motor. No harm done, and I am back in shape thanks to a new petcock installed by the dealer. However, I am not convinced that anything was done to address the leaky float valve. No smell of gas in the oil, but I am reluctant to tempt fate by ever putting the bike on PRI longer than I have to.

It probably merits pulling the carbs, and having someone rebuild them (well deserved after 35 years I suppose). No hope of having the time and setting to do it myself this winter, so we will see how things sound in the spring. In the interim, I will find a few minutes to see if the choke (enrichener!) on this bike is adjustable in any way. I bought a new cable just for the heck of it, and we'll see if that is a major headache to install. If there is no adjustment, the cable must stretch over time and not fully "enrich", yes?
 
Update from a novice mechanic

Update from a novice mechanic

I did get around to checking the "choke" cable per the exchanges in this thread. It seemed very loose, and with some fumbling it seems that I was able to adjust it to "normal". I did so through a combination the bottom adjustment down by the carbs, and also some fiddling with the adjuster up at the bars.

Tempting to fire up the bike and to see how it responds, but since it is winterized I know that I would only be hurting it. I'll hold for another month when I can actually ride it when starting it. We'll see...
 
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