RD350: Trouble after up-jetting

[dustDevil]

Guys, I own a 1988 Yamaha RD350, which I got about 6 months ago. When I decided to go in for modifications, I first decided to get rid of the stock air-box (which almost everyone has told me is pretty restrictive).
So I got myself a pair of cotton filters and upgraded to 150 jets (main).
The result was very pleasing in the begining, but with more city riding I noticed a rather irritating PROBLEM:
The bike sounds and moves well as long as it is accelerating, but as soon as it reaches a constant speed (at a constant throttle and mostly at moderate revs) it makes a very bad spluttering noise. It seems as if all systems have gone out of sync. There is no jerking though! But again, this does not happen at full throttle!
What might be the problem? Is it happening because I've left the pilot jets untouched? Help!

[straight6]

Contact rohit arora on 9820111242, he is very good with rd's & has an official yamaha service center. He will guide you and take care of this problem.

[hollywoodhogan]

spluttering at steady throttle means that the bike is running very rich. nothing odd considering the jetting you've done. if you are able to attain explosive acceleration and top end performance with the 150 jets then perhaps you have to live with the low speed constant throttle splutter. besides am sure that your mech knows what he is doing.(?)

an rd on stock jets itself splutters a bit in slow city traffic but once its given a few blasts of acceleration and the engine temperature is up the splutter becomes minimal.

[sharan666]

The fluttering sounds like your running rich around half throttle. Might want to try dropping the carb needle by a slot.

[dustDevil]

I was suspecting a rich mixture. But my mech says its not a problem.
The bike really is a pain in the city. Will try playing around with the needle!
Thank you guys!

[topend_hooligan]

Dude, are those 'jets' you've put the right kind? Mikuni follows 2 completely different numbering schemes for its round head and hexagonal head jets. What you wanna try is a original hexagonal head mikuni jet. You should get pairs of sizes ranging from 150 to 200.
I'm pretty sure that one of your pilot jets has dirt OR airscrew is set incorrectly. When you accelerate, you open the throttle and hence most of the fuel sucked by the engine is through the main jet circuit. When you settle into a 3000-5000rpm cruise, the throttle is barely opened past 25%. The pilot circuit plays a huge role in carburation till that throttle opening and hence any problems in the pilot circuit would show up at low thorttle cruising. Once warmed up does the bike idle well on both the cylinders?

[Jaggu]

what are the porting specs? what size pistons and whats the condition of the engine otherwise? whats the kind of ignition you are using and the timing set?

Its easy to blame it on jetting, but to set it perfect there are lots of other parameters that need to be corrected.

take it to a good professional workshop.

[dustDevil]

The bike is stock (porting-wise) and am running an RXG CDI-kit (atleast for now). The piston is also OEM (not an over-size)

The problem got worse yesterday. Bike ran as if on a single cylinder. The right cylinder was giving excessive smoke! Took it to the mech, and on checking it we found:
1) The bike was actually on one cylinder.
The right carb was flooding and also the oil-seal between the gearbox and the right side of the crank-case had leaked. The bike was sucking in oil! Eventually plugs got fouled, and the cylinder stopped firing!
2)The right carb was flooding because the float level was excessively high.
(I have no clue how that could happen on its own)
3)Put forward the idea of dropping the needle... but my mech was against it.

For now, the plug has been cleaned and re-installed. The float level has been corrected and the carbs have been re-tuned with the same jets.

On a short test-ride, the bike seems to run well (except a little extra smoke from the right cylinder). I have been advised to avoid using it much till the oil-seal is replaced.

[dustDevil]

@topend hooligan: Ya the bike did idle well after warming up! And after re-tuning the carbs, things are better!

[moralfibre]

Aalap, where did you get jetting done on the bike in Pune?

[dustDevil]

@MF: There's this guy near salisbury park! I got the bike from him, so now I get my usual servicing and repairs done at his workshop.

[Jaggu]

a) fix the air leak
b) ensure carb float level and all the circuits are in order
c) tune and time the bike (plugs, timing and syncing)

[moralfibre]

The piston is also OEM (not an over-size)

Original pistons were from ART. Are you sure you have the ART pistons?

BTW are you using the oil pump or have bypassed it? Its safer to add oil in petrol since using the oil pump means setting it on the markings everytime you set out for a ride.

[dustDevil]

No, no.... they are not ART. It was a mistake on my part! By OEM I meant the original size. The seller tells me they are TKR. I'll be opening her up by next month, when my exams finish. Only then I can confirm!

Ya, had bypassed the pump the day I got the bike! Have been adding oil to the fuel eversince!

[moralfibre]

Originally Posted by dustDevil

No, no.... they are not ART. It was a mistake on my part! By OEM I meant the original size. The seller tells me they are TKR. I'll be opening her up by next month, when my exams finish. Only then I can confirm!

TKR I believe were RD350 LC pistons? Anything other than ART used on bikes are prone to Piston slapping since you have to keep a slightly more clearance otherwise the engine packs up. This is little I learned when we restored a friends RD way back when I just got out of school. Give me a shout when you open up your bike. I know of a VERY renowned and silent guy who works on SBK's in the backyard of his house. Not many people know of him but he is the guy you must show your bike.