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PostPosted: Fri Sep 08, 2017 7:40 pm 
On the main road
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Joined: Thu Sep 29, 2016 4:53 pm
Posts: 139
Country: United States
Bikes owned: T500
tz375 wrote:
Massive reversion seems to be picking up fuel each time it passes the jets. I don't recall the inlet timing on that motor - it could be too long.

You could also try even smaller pilot jets but that may make the transition to the needle & jet a problem, but one step at a time. How big is the air "jet" that feeds the pilot jet? It should be a fairly large drilling. And air blockage in there would exacerbate the situation

Dang Richard I wish I lived near you so we could work through it together! I don't understand the air jet drilling, is that where the air screw goes into?

PostPosted: Sat Sep 09, 2017 5:22 am 
Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 5962
Location: Illinois
I checked through the pics of the build and found your carbs and they look OK in that department. You said it starts OK without choke and idles OK so that actually suggests the pilot jet is a size too large. Main is OK so that suggests one of two things.

Sounds like the needle and needle jet combo is way too rich OR the choke plungers are not seating properly. I found some on a GT750 where the rubber tip of the choke plunger was hard and did not seal properly. It would be worthwhile to check that out. If that's all OK, try a smaller needle jet or thicker needle to see if that makes a difference.

PostPosted: Sat Sep 09, 2017 12:24 pm 
Road race school

Joined: Tue Dec 16, 2008 11:37 am
Posts: 886
Location: Trowbridge UK
Country: UK
Bikes owned: T500R, SV1000S, TS125, Seeley T500
Hi john,

My 2c worth is that the fuel blow back you are experiencing is a normal factor for a piston ported engine. It will be more pronounced if you have advanced the inlet porting with a bit of tuning but will probably almost disappear when the revs rise and the engine resonances begin to work. My race engines do this and you learn to live with it, but it's never too bad. You could always convert to Reed Induction and if you make a few kits I'll be your first customer :up:

Cheers Geoff

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