Re-Jetting Carbs

[gregwillclark]

Afternoon all,


I finally got around to trying to diagnose an ongoing issue I have had since owning my Fazer, it seems that the bike runs great up to half throttle, as soon as I go past half throttle it starts 'spluttering' and missing. I originally thought it maybe something electrical, so I recently bought some bits from another foccer on her who was stripping his bike. I changed the Rectifier, coil packs & leads and finally the fuel pump/filter. If anything the bike now seems to pull stronger in every gear, but the issue at nearly full throttle still exists.


Im now thinking that since I had the bike, I fitted Motad stainless downpipes and can before getting the bike T&T'd, i did not however ever think of re-jetting the carbs to accommodate the increase in airflow. Would I be right in thinking that this may cause fuel starvation type symptoms at high revs/half-full throttle? If so, does anyone know how hard/easy the jets are to change?


Any help appreciated.


Greg

[Paul]

My experience with Motad downpipes

If you change your exhaust headers make sure the restrictor point of the new set is similar in overall opening size to the original set.
The original downpipe outlet measured: 35mm (narrowest point) to 43.5mm (widest point) externally at its most restricted point.
I’m not sure what the wall thickness is but I reckon it must be somewhere between 1.0mm and 2.00mm, my original pipes are still in good condition and I was loathe to saw them up to find out.
In any event the Motad replacement header system outlet pipe is has a clear open area of 1364sqm.  Whereas the original outlet is restricted to somewhere between 962sqm and 1075sqm.
When I fitted the Motad headers the power band almost evaporated and a gas analyser showed it be running far too weak, this being supported by the plug colour.
A further test was done by a lazer temp gauge on the exhaust clamp bracket of both original and new pipes and this showed the exhaust gas temp had risen.
To solve the issue the Motad pipe was sleeved down first to 1075sqm which improved things no end, and finally down to 975sqm, which seemed to be better.
Plugs were checked and exhaust gas analysed and the results were as good as I could get them.

[gregwillclark]

Hi Paul,

Wouldn't increasing the jet size have the same effect? I.e compensate for the weak mixture by allowing more fuel in to the engine?

Greg

[His Dudeness]

You could put the old exhaust back on to test the theory if you've got it but I don't think changing the exhaust will effect a Fazer enough to require rejetting. Nearly everyone swaps the exhaust and I've never seen anyone on here needing a rejet. You probably have another problem. Have you given the carbs a clean or the air filter a clean?

[Paul]

To Gregwill clark


Question: Will increasing jet sizes work?
Answer: Possibly.


Question that you didn't ask: Am I likely to get it right?
Answer: Unlikely.


Question that you didn't ask: Why am I unlikely to get it right?
Answer: You don't have Yamaha's engine test facilities at your disposal.

Summary:
If I were you I'd try sleeving the exhaust outlet down first.   
Because piss-malling about with jet sizes, often results in a significant amount of wasted time and effort. 

Attached is a photo of a the first piece of pipe I used to sleeve down the outlet.

[mtread]

Many of us have the Motad headers following the dreaded exhaust rot. No rejetting, no sleeving down, nothing changed other than perhaps a K&N filter. If anything, the bike runs better all the way up to full throttle.
I think your problem lies elsewhere. As said, dirt in the carbs, air leak, dirty filter etc etc

[gregwillclark]

Cheers all,


Cleaning carbs is the main task for the weekend, are there any good 'how to' posts on this??


Greg

[darrsi]

I don't actually know what make my down pipes are, as they were already on the bike, but a few years ago my bike was in PDQ having the carbs stripped down and cleaned, then Dyno'd, and he decided to re-jet my bike while he was at it but it never felt right to me so I had it back in and put back to standard which sorted it right out.

[MarkWales]

Mine has not lost power - quite the opposite since fitting the Stainless exhaust and the K and N I have seen increased performance and MPG


You are welcome to try the filter in yours - maybe a quick fix?


Carb cleaning may be the way forward


these are local - http://www.ultrasoniccarbcleaning.co.uk/

[/color]Ultrasonic Cleaning[/color][/font]

[/size][/color]An ultrasonic cleaner is a great device that uses ultrasound to clean delicate items. The ultrasound can be used with just water, but use of a solvent appropriate for the item to be cleaned along with the soiling enhances the effect. [/color][/font]
[/size][/color]Just using the ultrasonic cleaner will get rid of any blockage within the jets or small holes and the appreance of the items is improved but just this process alone wont get rid of any staining or rust. [/color][/size][/font]
[/color][/font]
[/size][/color][/font]

[/color]Ultrasonic Prices[/color][/font]

[/size][/color][/font]

Stripped & Rebuilt By the Customer[/color][/b][/size][/font]

[/size][/color][/font]

£15 Single Carb*[/color][/size]

[/size][/color][/font]

£5 Each additional Carb*[/color][/size]

[/size][/color][/font]

[/color][/size]

[/size][/color][/font]

Stripped & Rebuilt By Us[/color][/i][/b][/size]

[/size][/color][/font]

[/size][/color][/font]

£60 Four Carbs* / **[/color][/size]

[keratos]

I had a carb problem; cracks in the rubber manifold on the carb housings; allowing air to escape; mixture run rich and hinted by sooty deposits around the plug gaskets. tried silicone repair - no good - so bought a new manifold. Also put a guzzle of redeX down the carbs and in the fuel tank. Runs Great!

[His Dudeness]

That's good news that you got her sorted. I think cracked inlet manifolds would be more likely to lean out the mixture rather than richen it. The engine creates a vacuum to suck in the air fuel mixture from the carbs so if you've got cracked inlet manifolds it would be sucking in extra air through the cracks rather than letting air out so it would lean out the mixture. Sooty plugs might suggest your air filter could be a bit blocked so it would be causing a slightly richer mixture.

[keratos]

That's good news that you got her sorted. I think cracked inlet manifolds would be more likely to lean out the mixture rather than richen it. The engine creates a vacuum to suck in the air fuel mixture from the carbs so if you've got cracked inlet manifolds it would be sucking in extra air through the cracks rather than letting air out so it would lean out the mixture. Sooty plugs might suggest your air filter could be a bit blocked so it would be causing a slightly richer mixture.

yes, thats what the logic would point to. However, I had sooty plugs, clean (new) air filter. So replaced the plugs and ran it for a month - still sooty. Eventually diagnosed as cracked (hairline but cracked) rubbers so replaced this then the plugs. all fine now.

[unfazed]

Check the plug caps as dodgy ones can give the appearance of carburettor trouble dying and picking up over 7 thousands revs.

I have stripped a few as the internal resistor connection can become corroded and/or loose effecting the spark.