[Big Drag 49]

I recently installed a big bore kit on my 1978 GS750EC and I'm looking for suggestions on baseline jetting with stock 26mm Mikunis, Vance and Hines 4 into 1 header, and stock air box with UNI filter. I have been told the stock carbs will work but I may have to go down on jet sizes which didn't make sense to me. Can someone help me get started in the right direction?

 

[Unkle Krusty]

There is a Mikuni tuning guide on line. Is the bike running? You can start by checking the plugs. Better IMO to do that and follow the tuning guide. You did not say what the engine size is now. However, more power usually means more heat, and more fuel required. Start with the main jet. Buy some jets, starting with the next size bigger, and maybe up to 3 or sizes. Also find out what was stock for the 750. In can happen that there are different main jets in the carbs. It can depend on the stock air box, and the cooling function of the inner and outer cylinders. It should pop under load if it is too lean. And the engine will run a bit hot.

UK

 

[Tobacco Road]

In the OP's subject line it says 750 to 850. I would assume that he made a 750 into an 850 cc bike. UK's info is spot on. Keep your eye on the plugs, they can tell you a lot..

 

[Unkle Krusty]

Now I see it. Big letters up there. I was just reading the post. TY TR.

UK

 

[Big Drag 49]

There is a Mikuni tuning guide on line. Is the bike running? You can start by checking the plugs. Better IMO to do that and follow the tuning guide. You did not say what the engine size is now. However, more power usually means more heat, and more fuel required. Start with the main jet. Buy some jets, starting with the next size bigger, and maybe up to 3 or sizes. Also find out what was stock for the 750. In can happen that there are different main jets in the carbs. It can depend on the stock air box, and the cooling function of the inner and outer cylinders. It should pop under load if it is too lean. And the engine will run a bit hot.

 

[Big Drag 49]

Thanks for the input! I guess I need to do a little more research. Right now it won’t even idle properly. Carbs have been cleaned and rebuilt using ultrasonic cleaner and stock jetting. Timing and points set. It has good spark, fuel, and 145 to 150 compression in three cylinders. Cylinder #4 however only has about 130. I did not check valve clearance when I reassembled it, assuming it should still be good since it was a running engine when I took it apart. I know, never assume. Is it possible I mixed up a couple of shims and the valves are too tight? I ordered a shim removal tool and will check when it comes in.

 

[Porky]

More displacement=more air being sucked in, which means more fuel and it will push more exhaust too. Unless the kit came with dished pistons, a bigger bore means higher compression probably needing more octanes.

I'm glad you are going to adjust the valves as that may eliminate some of the running problems.

Of all of the seemingly easy 'Hop-up' ideas, big bore kits must really be thought out and it's not for the inexperienced.

Sam

 

[Unkle Krusty]

Good point what Porky said. Larger bore increases the compression. More swept volume. 145 to 150 is what I was thinking. That is about stock for my XS1100. Even tho it may idle rough, start with the main jet first. Isle is over rated. Smooth WOT is desirable. Some engines have a natural tendency to bog a bit below there maximum torque curve. This is a function of a hi lift long duration cam, that most modern bikes have.

Up the steep hill here, my XS400 can do it in top gear. But if I go WOT it will cough a couple of times. Down one gear and screaming it is fine, and about 10 mph faster. When bikes are put on a dyno, they are under load, similar to a very steep hill.
UK

 

[Big Drag 49]

More displacement=more air being sucked in, which means more fuel and it will push more exhaust too. Unless the kit came with dished pistons, a bigger bore means higher compression probably needing more octanes.

I'm glad you are going to adjust the valves as that may eliminate some of the running problems.

Of all of the seemingly easy 'Hop-up' ideas, big bore kits must really be thought out and it's not for the inexperienced.

Sam

More displacement=more air being sucked in, which means more fuel and it will push more exhaust too. Unless the kit came with dished pistons, a bigger bore means higher compression probably needing more octanes.

I'm glad you are going to adjust the valves as that may eliminate some of the running problems.

Of all of the seemingly easy 'Hop-up' ideas, big bore kits must really be thought out and it's not for the inexperienced.

Sam

Thanks for the input Porky. I have been working on bikes for over 50 years but never did a big bore kit until now. I'm just trying to sort it out and thought I would ask people who may have done this for some tips. I am hardly inexperienced. I know I should have checked clearances and will update you guys when it is done.

 

[Porky]

Big Drag, don't take offense! Lots of Folks read things on this forum or ask absolutely silly questions and then admit they don't even have a bike or their ride is a 49cc pedal moped or someone contemplating his or her first bike, a 200HP superbike--believe me, we have seen it all BUT we always help, when we can.

I built some big bore HD bikes, all aftermarket and I also did big bore kits for single cylinder, 4 stroke desert, Baja and track bikes and it gets fairly technical as in Deck height, Piston and valve clearance interference, compression ratio, Jetting, exhaust flow, on and on, so for the average person, it can be daunting. If that piston at either top or bottom dead center even slightly touches the CASE or the Crankshaft, or the Cylinder head, the fun is over and the real expense begins!

Have fun and let us know how it all turns out!

Sam