VM carb blow back into airbox

[Adi]

I have a 1977 GS750, standard pipes and airbox / filter, jets and needles.

Starts and idles well, mid range and top end fine. Only niggle is when on idle there is a relatively frequent “ffffutt” of blow back through the carbs in to the airbox. Not sure if I spelt the sound right, but you get the idea.

I have not found anything on the threads about this. It does not detract from the pleasure of riding the bike but irritating and suggests that something is not set up properly.

Anyone have any idea what could be causing this?

[Big T]

Clogged pilot jet

[OldVet66]

Spitting into the air box is usually an indication of a lean condition. Have you checked the boot O-rings and boots, sealed the portions of the air box that require sealing, checked the boots to the air box for leaks. Sounds like you might be getting too much air to the affected carburetor or carburetors?

[Steve]

When is the last time you checked valve clearances?

You said it starts well, which goes against the idea of tight valves, but a single tight valve could blow back through its carb, while the rest of them let the bike start "easily".

The clearance on these valves is pitifully small, so when they go tight, they stand a chance of not closing at all, allowing the intake charge to get pushed back through the carb.

.

[Adi]

Hi Big T, Oldvet66 and Steve
Apologies for the delay in reply, the bike has been on the back burner while I have been concentrating on having proper flu!
I checked the valve clearances about 6 months ago and had to re shim a couple, the majority were fine. But you have sown the seed of self doubt so I will go back and check them again, particularly as it doesn’t take too long to do.

I thought about pilot jet but then as it is starting first press of the button I disregarded the idea, but maybe worth a check.

Now the age old carb boot issue could be a candidate I guess but I went to great length to seal them so I hope not. I cut a new sealing gasket for the air filters because the old foam had disintegrated. I used 5mm neoprene and it seals beautifully.

I guess if it might be caused by a lean mixture then I might need to tinker with the air screw a touch.

Many thanks for the suggestions. I will let you know how it goes once I have found a spare couple of hours to trail and error.

I am in my late forties and I have to say I am not a great fan of the internet, but I absolutely love the fact that I can sit in London mulling over an engine problem on my GS and get expert advice back from you guys all round America, marvellous!

Cheers, Adi

[Big T]

Be sure you are using the correct method to measure the valve clearances
Are you using the choke on start up?
Buy some new carb boots

[Rob S.]

Try to put the bike's model and year in your signature.

[Bortasqu]

Try to put the bike's model and year in your signature.

I have a 1977 GS750

By the way, what's niggle?

[Rich82GS750TZ]

nig·gle

/ˈniɡəl/

verb

• 1.
  cause slight but persistent annoyance, discomfort, or anxiety.
  [COLOR=#878787 !important]"a suspicion niggled at the back of her mind"

• synonyms:

  irritate, annoy, worry, trouble, bother, provoke, exasperate, upset, gall, irk, rankle with; More

  
  
  
  
  
  

noun

• 1.
  a trifling complaint, dispute, or criticism.
  

  synonyms:

  minor criticism, quibble, trivial objection, trivial complaint, adverse comment, moan, grumble, grouse, cavil; More

  
  
  
  

[/COLOR]

Feedback

[superawesome]

Hi Adi,

I had the same blowback issues on idle on my GS1000, until I realized the choke plunger (on carb #3) did not fully retract.

https://www.thegsresources.com/_forum/showthread.php?254877-that-*-amp-!-choke-plunger&highlight=

I think you have the same VM26 carbs, so try to gently push the plunger down with a screwdriver, and see if it moves. Might as well be something different, but it is a quick check.

Cheers
juergen