Engine stopp pulling around 7000RPM

[Viensal]

Hi,

I am a new owner of a GS750 1981 and I have (i Think) a exhaust or carb problem. the engine is running smooth and pull great until around 7000K Rpm.

When I purchased the bike the 4/1 exhaust was terribly choked(1.5'' ID with baffles), so I pulled out the baffles and build a new insert with a 1''3/4''ID perforated pipe with glass packing without baffles . So it help so to live up the engine but still it ''saturate ''aroud 7000RPM, like if it can't inhale or exhale properly.

Is 1x 1''3/4'' ID  is enough to flow out ?

Could it be jetting problem ?(no clue what is the carb set-up)

The plugs are new, so I can't really read them.

* My Tachy needle is a bit crazy, so I can't be sure of the 7000Rpm but surely I can' t reach the 8000+ of the hp peak.

 

Any ideas ?

 

Alex

[Gixer1460]

It would have to be a pretty atrocious exhaust to have that sort of affect so it may run badly but should rev out with virtually anything, so that means carbs! What are you using for carbs and filtration and what state are they in bearing in mind the state of the exhaust!

[Viensal]

Hi,

I have used sea foam last tank, to help but other than that I cannot tell anything other than idle and rev is smooth and linear from idle up to 6-7k rpm.  That said I am the new owner so no clue if it should be even better, I have the stock airbox with a washable foam filter inside.

I did my homework, so my understanding is that carbs needs to be re-jetted with almost any exhaust change, so I assume that is the next step

 

Thanks for the guidance !

 

Alex

 

Edited July 24, 2020 by Viensal
update

[dago]

Sounds like it needs a bigger main jet in the carbs, you need to know what size is in there now it might never had been jetted for the exhaust or might need bigger now as more free flowing. Is it running std airbox & filter?

[wraith]

As above main jet, also is the air filter clean? 

Is it on points or electronic ignition?

Start at the beginning, clean air filter, check main jet/clean carbs, check if points the gap is right and the spark is not a big spark (if need be you can use z650 points if you cannot gat Suzuki ones).

And welcome to OSS 

[jensvonbustenskjold]

Hi mate 

If you're the new owner, and this hasn't been a problem earlier, then I would go for complete cleaning of carbs.it sounds fizzy if you suddenly needs new jets. Might need that anyways,

Check parts for wear and tear. Specially rubber and gaskets 

[BigT]

http://members.dslextreme.com/users/bikecliff/

 

Factory service manual and carb cleaning tutorial here

[Mouse]

reading this thread i am interested in your comment about making sure,

On 7/24/2020 at 8:01 PM, wraith said:

the spark is not a big spark

Reason i ask is i thought the bigger the spark the better, i am having problems getting my own gs750 to start and have a small spark, so maybe this is not such a problem ???.

Mouse

[wraith]

45 minutes ago, Mouse said:

reading this thread i am interested in your comment about making sure,

Reason i ask is i thought the bigger the spark the better, i am having problems getting my own gs750 to start and have a small spark, so maybe this is not such a problem ???.

Mouse

How to describe a spark  if the spark on the points is to big you will see the contacts getting burnt, that means the condensers are f#cked, check the gap (RTFM ) spark should be no bigger than you would see at the spark plugs with standard coils.

[Gixer1460]

13 hours ago, wraith said:

How to describe a spark  if the spark on the points is to big you will see the contacts getting burnt, that means the condensers are f#cked, check the gap (RTFM ) spark should be no bigger than you would see at the spark plugs with standard coils.

The spark at the spark plug on the other hand, ideally, should be blue and 'fat' - a thin weedy yellowish spark ain't gonna light anyones fire, reliably!

[Mouse]

Thank you both for that 

Should have said that i have Piranha electronic ignition and it's the spark at the plug that crap, it's very small and you can just about see it in a dark garage, seems i have another problem to deal with.

I will start another thread, as i don't want to hijack this one.

Mouse.

[BigT]

10 hours ago, Mouse said:

Thank you both for that 

Should have said that i have Piranha electronic ignition and it's the spark at the plug that crap, it's very small and you can just about see it in a dark garage, seems i have another problem to deal with.

I will start another thread, as i don't want to hijack this one.

Mouse.

Mouse - start with unscrewing the spark plug caps from the wires, trimming 5-6 mm from the wire end and reinstalling the cap. Known corrosion/resistance area

Are you running resistor plugs?

[Mouse]

Hi

 will try that, the plugs are brand new B8ES , no R so not resistor, not sure what  difference it makes though.???  but also non resistor caps