Do i need to re jet

[funky]

Hi all advise and wisom needed.
This is the situation just got my rebuilt engine Thanks Smed Speed basically everything inside is new plus it a 707cc now not a 650. I am going to fit the standard Mikuni 34's do i need to re jet the carbs I have fitted strait through reverse cone cafe racer exhaust no baffles just wadding see pics

All the best Funky

 

[angus67]

Nice xs707 you have there.
Yes rejet. its a different motor now.

 

[Paulrxs650]

Hi, you will need to tweak the mixture and probably re jet. The best place is to start from stock then make single changes, then test and repeat. This guide will help you - http://www.amckayltd.com/carbguide.pdf

 

[funky]

Thanks Paulxs650 thats handy I have been thinking about this the Exhaust is strait through but the engine has increased in CC by 57cc to a 707 so I think the Carbs will be tweekable on standard jetting ... the larger cc will produce more exhaust gasses which will need to escape from the system. the increased cc will also produce more induction and draw more fuel air mixture ...I am thinking i may have to increase the needle jet by one notch probably so the plan is to put the engine in run it on millers running oil leave the carbs as is run the engine through the heat cycles once this it done,take it for a spin keeping it under 3000 rpm for about a mile then check the plugs and timming and adjust acordingly if i run out of adjustment then that will need re jetting there is no way of knowing the only way is by looking at the plugs and riding it sensably ... thats my plan any suggestions would be appreciated...

 

[MartinNC]

Funky - you've got BS34 and their needle position is not adjustable. Hopefully someone with more knowledge of your increased capacity may be able to suggest different needles/needle jets. All other carb jets can be had from http://www.motocarb.com/

 

[5twins]

Yes, it appears you're going to be using U.S. spec BS34s. I can see the mix screws are still capped over with the brass plugs in your 1st pic. You'll need/want to remove those plugs so mix screw access and adjustment is possible. The mix screw also has a small o-ring on it to seal it in the hole and that most likely will need replacing.

The U.S. spec BS34s came very lean from the factory. Many report improved running with one up on the mains even on a totally stock bike. I'm quite sure you'll want to re-jet. The displacement bump isn't that big a deal but the free flowing exhaust will be.

As mentioned, the needle is fixed and can't be adjusted. It can be shimmed but only richer because it is spring-loaded from the top. Adding a shim washer above the fixed e-clip will not change the needle height, only preload the little spring more. But, many are able to successfully tune the BS34s without having to adjust the needle. By increasing the mains and pilots, they bleed over into the factory lean midrange and make it right.

If that doesn't work out, there are a couple adjustable needle replacement options. MikesXS has what they call the "Canadian" needle. They also sell the matched "Canadian" needle jet and say these two parts should be used together. The other option, and one I think may be the better one, is a rebuild kit from eBay seller Cruzinimage. It contains not one but two adjustable needles. The taper profiles are quite different so there should be a noticeable difference in how the two needles work.

 

[grizld1]

The undrilled mixture screw caps indicate that those carbs have never been thoroughly cleaned; that would be a very good place to start.

 

[XSLeo]

On your engine, did you use the same intake and exhaust as you did when it was a 650? If you only changed the cc's then you won't need to rejet.
When I rebuilt my 75 as a 750 with a kit from Mike's, the only change I made was the increase in cc's.
I thought it would need rejetting. I played with the jetting for about two weeks. I ended up with the same set up as it was as a 650.
So if the only change you made was the increase in cc's I would start with where they were. Test what you have, it may be ok.
Leo

 

[5twins]

Yes, the TX750 twin that Yamaha produced in '73 and '74 had almost the exact same carb set as the 650 models for those years and it was jetted the same.

 

[funky]

Hi All the picture of the Carb are decieving you the Mixture screws have been removed as at the time i was waiting for two new replacments, which i have know fitted and turned out 3 1/2 turns as spec.
The valving on the engine it exactly the same all that has changed is the pistion size and bore obiously. I was told by the egineer that flat side carb would be better as they have a better throttle response but i opted not to do that as this ment i would have to chop about moving battery box and having quite large K&N filters.
I have cleaned these carbs internally they are clear of any obstruction and glazing all rubbers and plungers are in exelent working order, but have tried to maintain a patina externally to give a period look.

 

[peanut]

nice looking bike Funky Love the cafe seat . I nearly bought one myself a year back on ebay and passed up on it without realising how rare they are.

Do you know where the silver rims came from on the guages ? looks so much better than black plastic on a cafe

 

[funky]

nice looking bike Funky Love the cafe seat . I nearly bought one myself a year back on ebay and passed up on it without realising how rare they are.

Do you know where the silver rims came from on the guages ? looks so much better than black plastic on a cafe

HI peanut
The seat can be bought from MikesXS $249.00 the silver bezel rim on the clocks you already have mate Just use some wire wool on your black one and hey presto unless mine were changed for metal ones prior to me bying the bike all i dod was strip them .

 

[funky]

Heres some more pics of my carbs as they are now

Balanced

new mixture screws

inside venturi

I hope they are clean enough all circuits are clear and all jets..... floats are set to 27mm as per spec i freed up the drain screw so i can check floats and fuel levels when the carbs are fitted on the bike with two lenght of clear hose

 

[funky]

I am planning on putting the engine in the frame today ... I must say the information posted on this thread has made me feel better I am so worried about getting the mixture right and timming and starting the engine to avoid any problems I am a total virgin on bikes.
Thanks for all your good advise chaps and keep it comming it greatly appreciated ..... THANKYOU

 

[peanut]

did you check that the diaphrams are airtight ?

if you lift the slides with your finger and cover the diaphram air intake they should either stay up or fall very very slowly . If the slides fall within a couple of seconds or less then you'll need to reseat the diaphrams and check for air leaks .I had to flat my covers with wet and dry and use a little vasoline around the diaphram beads to get mine to seal properly.

The other thing I would do is to check that the throttle return springs are still strong enough to return the throttle valves (butterfly disks) fully closed. I found that mine had weakened and were not returning the disks quickly to close (idle) I simply bent the spring hook to shorten the spring by about 4mm and now they have plenty of strength around the critical closed idle position to prevent rising or hanging idle

 

[scabber]

Good to get it back on the road and have some fun testing. Nice looking ride by the way. You might go with a bigger pilot jet I'm thinking. But you are on the right track.

 

[funky]

yep the diaphragms seem ok no cracks plungers work fine thanks Peanut

 

[5twins]

Your float setting will differ depending on whether you have plastic or brass floats. The brass floats get set to 27mm, the plastic to 22mm.

 

[funky]

Hi 5Twins I have brass floats set at 27mm +or- 1mm

 

[zurek4]

My main is at 140 and my pilot is at 40 that I upped from 132.5 and 32.5. Take the exhaust off and run straight pipe with pod filters you would be glad u did it. I am