is this as good as it gets

[Guest]

Morning all. My '78 GS750 is now behaving herself and running without covering me in oil (which is nice).

However, if I describe how she runs can the experts here let me know if this is as good as I can expect or is there a lot of fettling still to be done.

I am running VM26 smoothbore carbs, stock jets (I haven't changed sizes from those recommended in the Haynes manual). I have a Marshal Deeptone 4:1 exhaust and individual foam filters. The carbs have been stripped, cleaned, blown through, stripped, cleaned, blown through, stripped...?..you know the rest.

From the first 1/8-1/4 turn she sometimes hesitates then revs fine, the rest of the throttle range is very good, pulls and accelerates fine.

When I'm toodling along at 70'sh it feels like I need to keep applying a touch of throttle to keep the momentum going, it also feels a bit jerky like the fuel isn't flowing smoothly into the carbs / engine.

I'm happy with how she's running considering how long she's been off the road (15 years! don't ask) but need to know if this is as good as I can expect or if I fiddle a bit more can I improve the running, not looking for superbike performance just a bike that I can ride with impunity.

 

[Steve]

I am not acquainted with "VM26 smoothbore carbs" to know how they compare with stock carbs. Just the fact that you have different carbs might require different jetting, but the fact that you have pods and a pipe pretty much guarantees that you need different jetting.

The main jets will be the biggest change, but you will also have to make some adjustments to the pilot screws and the needle height.

Have you read the spark plugs? Find a place where you can "toodle along at 70-ish", then pull over in a safe spot. Do your toodling, hold the throttle VERY steady for about a mile, if you can. As you approach your safe spot, pull the clutch and hit the "kill" switch, coast to a stop. Pull the spark plugs and note the color.

You actually need to repeat this process at different throttle openings to check all the circuits, but since you mention specific problems at 70-ish, start there. You have all the symptoms of running lean, which is typical of running pods and a pipe with stock jetting.

.

 

[Downs]

I was about to say with your mods stock jetting is going to be a problem.

 

[Guest]

Thought I might have to fiddle with the jets. Last time I checked, the plugs were running ok (brown tips) to slightly rich. I'm assuming that changing jets is a one step at a time job. Would it be better to sort the pilot jet first or the main jet or play with the needle height. Ref the needle height, higher or lower? Higher lets more fuel through the system and lower has the opposite effect ?

 

[allojohn]

VM26 Smooth bores, hmmm...
If you indeed have stock carbs there are a few of us on here running a '78 GS750 with 4-1 and pods.
Pilot: 15 or 17.5
Main Jet: 115 - 120
Needle: e-clip lowered 2 notches.
I ideal setup is in there somewhere....

 

[TxGSrider]

That jerky feeling while cruising at 70mph is probably "surging", which is usually caused by a lean mixture (or rarely a really rich mixture). Since that is part throttle position (between 1/8 and 3/4 throttle opening), the mixture would be controlled by the needle. Id raise it one notch if its adjustable or find a way to raise it slightly by using small washers, re-arranging spacers, or similar. It may even help your 1/8-1/4 hesitation which may be lean too.

Of course check for vacuum leaks before making these change or you'll be chasing your tail trying to get the mixture correct.

 

[allojohn]

Have a read of this too. (pods, pipe, panic.)

 

[Guest]

Thanks for the advise / info. Can anyone point me in the direction of a supplier of quality jets for the VM's here in the UK?

 

[allojohn]

Someone in the UK will need to chime in here....

Note: Do not buy a kit! Kits are known to be sub-par and are not genuine Mikuni....
As far as I'm aware, no-one on this forum has ever bought a genuine "Mikuni kit".

 

[Guest]

You could try NRP or Motocarb.

 

[Guest]

Thanks, will give them a try.

Just a quick update and a word of thanks to everyone who chipped in with suggestions.

changed the pilot jets to #17.5's and mains to #120's and dropped the needle clip 1 notch. She's running far better than before, kept the fuel mixture screws @ 1/2 turn and the air screws adjusted to the best revs. Might need to up the pilot jet one more size, she no longer hesitates when pulling away from a standstill but the only way to describe it is she's pulling away but her attention is momentarily elsewhere.

Going to give her a bit of a run this weekend to shakedown the adjustments and see how she runs.

Again, many thanks for the advise and info.