350 Driveability With Factory Performanc Mods.

[davis]

Working in parallel building a show 350 GTR and a rider. Slow going given available time and energy. The show version has most parts polished, chromed, vapor blasted, zinc plated and most new parts purchased, but am dreaming about the rider. That one will have an expansion chamber of my design (using the Factory dimensions provided) but am wondering about the porting and other modifications suggested by Bridgestone. I wonder if anyone who had done these mods would tell us how it effects drivability.

I plan to polish the ports, but I have read pros and cons about polishing intake ports. Any thoughts on this question too?

[old smokey]

I would guess you could search for old posts from OldSwartout and others for experience and opinions on factory mods.

[RebusCom]

If you want a street rider, I recommend not changing any port profiles nor altering the rotary valves.  The valve and port timing is already pretty aggressive and changes that increase output in the upper rev range will be offset by a greater loss everywhere else.  I would focus on proper carburetor jetting as stock is far from optimal and it's a simple change that makes a big difference.  Really about the only performance upgrade that is worthwhile if you aren't concerned about absolute originality and don't mind a bit more noise would be expansion chambers.

Scott Noga

[slawsonb]

Could you elaborate on your jetting comments. Which to change to what? I know this will be dependent on atmospheric conditions and altitude, but would be interested in your recommendations.
...bert

[disc_valve]

Hi,

The 350s all came from the factory fitted with #140 main jets. These were on the rich side, and intended to help cool a new engine during  break-in only. There was a Dealer Service Letter that suggested a change to #130 main jets after break-in which makes a huge difference to the running of the bike.

Whilst you are changing the mains, it might be worth while going down a step on the Pilot Jets (from the factory #30 to #25 or #20 pilots). Other than that, just take great care in balancing the two carbs - one carb opening further than the other makes light running and pick up from closed throttle horribly lumpy.

Hope that helps,

Graham

[MitchB]

Graham -

From my perch, your input helps a lot. Very good info for us new owners.

For the greater group -

Where can these different jets / sizes be sourced..?

[slawsonb]

Good to know, Graham. Thanks
...bert

[disc_valve]

Hi, Mitch,

Richard may have some #130 mains in stock. Alternatively, try on eBay. There was a guy selling early type mains in a variety of sizes a few months back. Search for "Mikuni Jets".

You need to be careful, though. The Bridgestone carbs used pre-ISO threads (the pitch is different to modern jets) so modern jets will not fit. In the case of mains it is fairly easy to tell as Mikuni changed the style of jets at the same time as the threads changed - from Hexagon head to round head witha screwdriver slot. You are looking for #130 jets of the same shape as fitted in your carbs.

In the case of the Pilot jets, things are a little trickier. Modern pilot jets are different thread, but externally identical apart from a distinguishing groove machined around the edge of the head. All very well, but you need a magnifying glass to see it! I was lucky in that I also run a couple of early 1960s Suzuki 50cc models which came with early type #20 jets as standard, so I just raided some old Suzuki carbs from my rubbish box. I think I tried a pair of #17.5 jets as used on the 90/175 Bridgestones. but they were a bit too weak for the 350 motor. Maybe you could talk to your friendly neighbourhood Old Suzuki Dealer or Collector and see if he can help with pilot jets.

Graham

 
 

[farmerdl]

Sudco has jets for these older mikuni carbs.  Not that expensive either.  A couple of years back I picked up a handful of primaries and pilot jets as out of the dozen 350s I had picked up over the years only three had the correct jets.  Try Richard first as he may still have these jets, but if he doesn't go to Sudco.

dl

[farmerdl]

As an afterthought,  Where I live and ride the altitude runs pretty much from 700 to 1300 feet above sea level and 130 mains with 22.5 pilots and the needle clip in the third position from the top works pretty well and is probably pretty close to most of the country.   To people in Denver and similarl places I wouldn't have a clue.  Back in the mid 70s had a T500 that had been built fairly close to Art Baumann's T500 Daytona bike and correct jetting would change with the weather.  Lived with the bike on the street for two years and traded it on a GT750.  Highly modified engines for street use can be a lot of fun but they can be difficult to live with day to day.

dl

[davis]

Thanks guys. Everybody here had great advice including OldSwartout who returned an email inquiry. Your efforts make it easier and more enjoyable for those who follow. The jets are in Sudco catalog 37, p. 212 "VM22/210" section for the pilot jets and p.214 "Large Hex 4/042" section for the mains. Checked on the main jets and they are in stock.

[Gerrit]

For what it's worth..... to put things in perspective, here are some GTR and Kawasaki A7/A7-R figures as a comparison:

Inlet timing: GTR 118-62 (180 deg open), A7 112-65 (177 deg), A7-R 130-70 (200 deg).
Transfer timing: GTR boost 135/main 125 deg, A7 120 deg, A7-R 126 deg open.
Exhaust timing: GTR 183 deg, A7 182 deg, A7-R 197 deg open.
Comp. ratio: GTR 9.31:1, A7 7.0:1, A7-R 7.7:1.
The GTR specs are from the tuning info at the end of the GTR service manual, the A7/7-R figures from the Kawasaki A series shop manual.

The GTR tuning sheet specs increase the intake timing to 124.5-65.5 for a total of 190 deg. Kawasaki issued a 7-page tuning bulletin for the A1 and A7 (which I have but can't find at the moment) but one of the tips mentioned was fitting the A1-R steel disc valves. Alternatively one could of course trim the fibre A1/A7 disc valves to get the 130/70 timing. The A1 and A7 had a 5-speed gearbox and the GTR has an advantage here with its 6-speeder.
The GTR exhaust port is only 35.5 mm wide, or 58% of the bore. The Kawasaki H1's exhaust port is 39 mm wide (60 mm bore) and this gives no problems, suggesting the GTR exhaust port's width can safely be increased to 39 mm. The radius of the top edge of the H1 exhaust is 60 mm, bottom edge 40 mm, corners are all 9 mm radius. Incidentally, Roy Bacon's “Two-Stroke Ports for Power”, a booklet published in the early 1960s, mentions a maximum width of 62% of the bore, a figure the standard H1 happily exceeds at 65%.
Marcel Ankoné, one of our 1970s GP riders, started his racing career in 1971 on a converted A7. The only changes to the engine were fitting expansion chambers and the steel A1-R disc valves. The engine was otherwise bog standard, not even the gearbox had been changed to the A1-R close ratio cluster, though the parts were probably still available from Kawasaki at that time. With that bike he became Dutch national 350 champion that year, beating riders on Yamaha TR2s.
I'd try modifying a spare set of discs first before carving up the cylinder, gradually increasing the timing to the figures in the Bridgestone tuning bulletin.

One figure I do question in the Bridgestone bulletin is the diameter of the GTR road racing spannie's header of only 33 mm diameter. The standard GTR exhaust passage is 36.5 mm according to my vernier caliper, meaning that the header is actually a restriction,especially when compared to the scrambling header diameter of 40 mm. Anybody care to comment?