manifold problems

[Crusher]

Hi new 2 forum and have just brought a 750 paso.It has come with some head aches ie,flatside mikunis have been installed which is good but the manifolds are of unequal lengths, the rear one been standard and the front one in a c shape to keep the carbs level and more or less side by side.The c shape one back fires when applying throttle as im looking down both carbs 2c if the barrel slides r moving simultanlously.So i just want some advise on where 2 go, is it the manifold or carb balancing or both as im new with all this but have the ability 2 do the job myself and no more about my lovely machine as this great forum helps people like myself 2 have a go and if all else fails take it in 2 the pros

[Fraser]

Hi Crusher,

Any chance of some photos so we can see the problem?
Cheers,

Fraser.

[Crusher]

I hope this photo comes out ok and thanks 4looking Fraser will appreciate any advice as i cant wait 2 get back out on the road

[ducinthebay]

Yikes!!
That's just wrong on so many levels. I don't know those carbs off the top of my head. Check with the manufacturer and see what angle of installation they will tolerate. Flat slide FCR Kehins are down draft carbs, and won't work at all in that orientation. Dellortos will tolerate up to 30 degrees or so.

Most important is that both intake lengths need to be the same length. Generally, shorter manifolds are better. A mix is horrific.

Rubber elbows between the carbs and the air cleaner are good, but a big elbow between the carb and the intake, bad.

I would imagine that it runs funny.

Cheers,

[Mc tool]

Never thought I would say this but I think a webber would be a vast improvement . for a fault finding mission ' I think you would be waisting your time bothering with that set up . The best engine tuners in the world couldnt get that to run acceptably

[jcslocum]

OMG!!!!! My EYES have been injured.

That will not work. Not at all. You need to find the proper manifolds first. One carb should point to the front and one to the back. That's OK.

[Tamburinifan]

Rubber elbows between the carbs and the air cleaner are good, but between the carb and the intake, bad.

???
That`s how all CV & FCR are mounted.

Otherwise, I agree, this setup won´t work, impossible to sync f e.

[ducinthebay]

Yeah, Gert, my previous post was a bit confusing. The rubber mounting boots are essential, yes. But that funny shaped intake manifold has got to go. I'm going to look in my garage tonight to see if I have spare manifold to contribute to the cause. Sometimes, bad is bad.

Cheers,

[Crusher]

Thanks 4 the replys every 1,it confirms my thoughts that u cant have unequal length tubes 4 manifolds, just wish i had picked it up when buying the bike but they say love is blind so any thoughts on where 2 go would b good. Do i keep the short manifold and replace the c shape 1 with a simirlar 1 facing other way with carbs in the same upright position or go back 2 a weber. Just trying 2 keep costs down as i paid 3500 nz$ with 75000klms on clock, average condition and also 2 new tyers and also looking at swing arm mods 2 allow the new rear tyer 2 fit .

[paso750]

there are MALOSSI manifolds for Dell ortos. They might work for the Mikuni TM, too.

The original Weber is not the best carb but it will work too so if you have it incl intake manifold that would be the cheapest cure at least for moment.

[Tamburinifan]

Flatslides are just lovely carbs so I would try to make them work.
Maybe the manifolds f the Mikuni CV on Monster & SS would work?
The CV fits w those on a Paso 750 w not too much angle.

[ducinthebay]

The Mikuni carbs you show I think are side draft carbs. Check the Mikuni website to confirm. The FCR carbs and the stock carbs from SS and Monster are downdraft carbs. They will need the stock manifolds from an SS, Monster, or possibly even the stock Sport/Paso.

Since you already have the carbs, and one correct manifold, I think your best bet would be to get the correct manifold for the front cylinder. What size are the Mikunis you have?

You will want to add some air cleaners also. I put rubber elbows on after the carbs and put some nice big air cleaners above in the space where the old airbox once lived. works great.

Since the engineering on your setup is now extremely suspect, you should take those carbs apart and confirm the jetting between them. In general, both carbs should be jetted the same, with the exception of the main jet, which should (generally) be one step richer for the rear cylinder since it runs hotter. (less airflow)

Cheers,

[Crusher]

Yes think u r right they r side draught carbs, TM38 85A. I will check jet sizes later as i need 2 down load info as ive never stripped down a mikuni b4.And yes the idea of retaining 1 of the manifolds and modifying the other or obtaining 1 from a monster or ss750 seems the cheaper and easier way 2 go so thanks again 4 the imput of information and pointing me in the right direction. One thing i did notice with the carbs was they did not like been tilted 2 far as fuel ran out of overflow so fingers crossed on that one as play with angle on the c shape manifold

[ducinthebay]

Hmm, again I wrote a confusing post. The manifold you want is not a stock manifold from a Monster, SS, or Paso. Those are all for down draft carbs. You want a stock manifold from a Pantah engine, or one of the Malossi manifolds for side draft carbs.

Is there any writing on the rear manifold? Pull off the back carb and see if the inside diameter matches the port in the head well. The bore of the carb should match the bore of the intake manifold, and should match the bore into the head. All the same diameter, and nice and smooth.

You may be able to ask the previous owner if he has it. You never know. Maybe he made this modification just to prove his plumbing and brazing skills. I'm guessing he either used an old P-trap from under the sink, or found a spare exhaust pipe to cut up. What did the guy do for a living (besides torture defenseless Italian motorcycles)?

38mm carbs are a touch big for the 750 Sport. Generally, a 36mm carb is about max. It will run, but it may not run well off idle. But then again, they are flat slides, and the Kehins run 39mm for the 750. Once you get a correct manifold, you can assess the situation more completely.

As for fuel running out of the carb when tilted, check the float bowl settings, and see that they match the spec from Mikuni. All carbs on bikes are tilted to some degree, so they are generally pretty tolerant. Check the specs to see for sure. Since all is suspect, check to see if that carb was made for a 4 stroke, or a 2 stroke.

Is there a fuel return line in the Paso like there is in the Sport? I wonder if the fuel pump is overwhelming the floats?

Fun project though.

Cheers,

[ducapaso]

BTW, does anyone remember the name of the producer of rubber elbows? there was a catalogue matched some time ago...