Petcock and spark plug questions GS1000

[Jackbob]

Hi,

Mine is a bit of a mongrel 78 gs1000 EC with Aus spec ST larger port head and VM30 slide carbs (all refitted by me - originally was the VM26SS set up). I had the head reconditioned and the bike runs noticeably stronger through the midrange and up and a very nice induction tune.
The cams are the original from the VM26 head though and timed accordingly as per the manual (20 pins etc). The reason i did not use the slightly higher lift cam/s from the VM30 head was because one is slightly damaged, and also i did not have a set of springs for it and i read somewhere that the VM30 head has different springs (although i cant see this being much of a concern in regards to another 0.5mm or so lift). Standard airbox from a CV carbed model and standard exhausts.
Question 1.
If i was to fit the ST cams would this be a problem for the std springs?

The not original tank is the 1979/80 type with the small slotted fuel tap. This fuel tap is near new - SGP (expensive!). Anyway whether the tap is set in the prime position no fuel flows unless vacuum is applied.
Question 2.
Is this normal?? Same as when set to normal position as i would expect.

Also i have had some fun trying to get the air/fuel screws correct (has been running rich around town and fouling plugs). I think i have nearly nailed this after a strip replace o rings, replace coils and set up as per specs found in a search of this forum etc etc,
Question 3.
When a plug has fouled due to carbon buildup, whey do they sometimes not work again after a clean (light beadblast) - does the plug short out internally if its not sparking across the electrodes? Once i achieve a nice colour on the plug chops I am thinking of buying a set of iridium plugs - is this a waste of money? Plugs are standard NGK B8ES.

Thanks.
Jack

[mustangflyr]

Gs1000

Hi,

Mine is a bit of a mongrel 78 gs1000 EC with Aus spec ST larger port head and VM30 slide carbs (all refitted by me - originally was the VM26SS set up). I had the head reconditioned and the bike runs noticeably stronger through the midrange and up and a very nice induction tune.
The cams are the original from the VM26 head though and timed accordingly as per the manual (20 pins etc). The reason i did not use the slightly higher lift cam/s from the VM30 head was because one is slightly damaged, and also i did not have a set of springs for it and i read somewhere that the VM30 head has different springs (although i cant see this being much of a concern in regards to another 0.5mm or so lift). Standard airbox from a CV carbed model and standard exhausts.
Question 1.
If i was to fit the ST cams would this be a problem for the std springs?

The not original tank is the 1979/80 type with the small slotted fuel tap. This fuel tap is near new - SGP (expensive!). Anyway whether the tap is set in the prime position no fuel flows unless vacuum is applied.
Question 2.
Is this normal?? Same as when set to normal position as i would expect.

Also i have had some fun trying to get the air/fuel screws correct (has been running rich around town and fouling plugs). I think i have nearly nailed this after a strip replace o rings, replace coils and set up as per specs found in a search of this forum etc etc,
Question 3.
When a plug has fouled due to carbon buildup, whey do they sometimes not work again after a clean (light beadblast) - does the plug short out internally if its not sparking across the electrodes? Once i achieve a nice colour on the plug chops I am thinking of buying a set of iridium plugs - is this a waste of money? Plugs are standard NGK B8ES.

Thanks.
Jack

So far as I remember the U.S. spec "S" models were just a cosmetic change to the standard "E" with different paint and the fairing. The mechanical changes were between the 79 and 80 years due to the change to CV carbs (big port head), electronic ignition, etc.

The petcock should operate on prime when vacuum is applied then there should be flow through it until it is reset back to normal. Otherwise on normal it will flow only when vacuum is present.

I have been using ND IW24 plugs for quite some time and find they work well in mine.

[SqDancerLynn1]

1 Stock springs should still be fine for street use.
2 Prime setting fuel should flow all the time, do you have it on Reserve?
3 B8es Plugs are fine, if you want to spend the extra money fine..

[Nessism]

I have no personal experience (although I have a "slot" type petcock new in the box) but I've read here that fuel won't flow, even in Prime, with that petcock until a little vacuum is felt. Once Prime is engaged, it will stay on without vacuum.

[Jackbob]

OK Thanks all - good info here for me.

[chef1366]

mustangflyr has it right about the petcock. A second of vacuum applied to the line and prime will run until turned back to run.

[Matchless]

mustangflyr has it right about the petcock. A second of vacuum applied to the line and prime will run until turned back to run.

Chef, I have always wondered about this as I do not have one, but have read many posts where a slight blow into the breather gets fuel flowing and was wondering if this is related to that type of petcock?

....?

[Steve]

Chef, I have always wondered about this as I do not have one, but have read many posts where a slight blow into the breather gets fuel flowing and was wondering if this is related to that type of petcock?

....?

I think you are confusing a couple of concepts here.

Sucking on the vacuum line to start fuel flow through the petcock is normal. This one type of petcock (with the slot instead of a lever) also needs a brief application of vacuum to start it flowing in PRIme.

The concept of "blow into the breather" involves the vent tubes, not the vacuum tube. By blowing into the vent tubes, you pressurize the float bowls, forcing a bit of raw gas through the jets into the throats. When you then start cranking the engine, the moving air will pick up some of that gas, enrichening the mixture. Although this probably works, I find that my vent tubes are too dirty and too awkward to get to to make this part of the normal starting routine. Far better to clean the carbs properly and just push the starter button.

.

[Matchless]

I think you are confusing a couple of concepts here. Yes thats me, confused about many things!

Sucking on the vacuum line to start fuel flow through the petcock is normal. Not sure if this is normal, but will take your word for it!This one type of petcock (with the slot instead of a lever) also needs a brief application of vacuum to start it flowing in PRIme. I have seen this mentioned before and this design could really catch someone out that does not know this.

The concept of "blow into the breather" involves the vent tubes, not the vacuum tube. By blowing into the vent tubes, you pressurize the float bowls, forcing a bit of raw gas through the jets into the throats. When you then start cranking the engine, the moving air will pick up some of that gas, enrichening the mixture. Although this probably works, I find that my vent tubes are too dirty and too awkward to get to to make this part of the normal starting routine. Far better to clean the carbs properly and just push the starter button. Never had the need for this, may be thought to be seen sniffiing petrol fumes in public! So I rather keep the carbs and starter enrichment working properly!

.

Steve, thanks. This tells me that the screwdriver type vacuum petcock is not a user friendly type!

[chef1366]

Steve, thanks. This tells me that the screwdriver type vacuum petcock is not a user friendly type!

I sold mine and use a Pingle.

[hampshirehog]

The slotted tap is, IMHO, a pile of poo and who in Suzuki dreamed up 'the prime that needs a vacuum' idea should be ****ed. As everyone above has stated yours appears to be working as intended, though a Pingel is definitely an upgrade worth doing.

I didn't think the ST did have different valve springs, though couldn't swear to it worldwide. However, the same springs were fitted to many different bikes in the GS range (and others), so the extra .46 on the intake and .51 on the exhaust shouldn't be an issue.

The ST cams should be marked 451 on the inlet and 49S on the exhaust. If they are, you'll need to set the valve timing to 19 link pins instead of 20.

The Iridium plugs? You'll start and argument! Personally I've never found them any different to standard (except on 2-strokes where there is a noticeable gain in 'dirty' motors) but you'll find loads who say the opposite.

Does your CV airbox fit easily by the way? I had a look at doing something like this on my bike but found the airbox to carb rubbers were about 8mm too short and I would have had to mount the box slightly differently (more forward) to fit.

[Steve]

Originally Posted by Steve
Sucking on the vacuum line to start fuel flow through the petcock is normal. Not sure if this is normal, but will take your word for it! It's the engine that does the sucking, not you. This one type of petcock (with the slot instead of a lever) also needs a brief application of vacuum to start it flowing in PRIme. I have seen this mentioned before and this design could really catch someone out that does not know this.

.
Steve, thanks. This tells me that the screwdriver type vacuum petcock is not a user friendly type!

message lengthened to at least 10 characters.

[Jackbob]

Hampshire-
1) i have concluded that yes the fuel tap design "sucks" and will crank it over on prime for a couple of secs then let it do its thing if its been sitting for a while (only use i once or twice a week at best) to get the flow going, then set to run after running.
2) My cams - one was the correct number as you say and one was something else. We are moving house soon and i have them packed up in a box somewhere so cannot confirm the numbers, but remember that one was neither the ST number or the standard 26mm carb numbering . I know others here have reported variances in numbers etc.
3) The airbox - i picked up a gs850g air box i think (was for 34mm vacuum carbs) - the inlet tube inside diameter was about 5mm too big.
I therefore shortened and reversed them basically and bought a length of 48mm id x 5mm x section rubber hydraulic hose and cut to length. The spigot diameter on the VM30 carbs is about 48mm and seems a fairly tight fit in the airbox. Yeah a bit rough i know. If i may install CV carbs one day anyway as they seem to be a nicer carb from experience. Hopefully a picture will attach here now FYI. Cheers

[Jackbob]

Here it is?