Carb tuning with Wiseco big bore kit

[Anonymous]

I recently put an 844 kit into a rebuilt 79 GS750 and I would appreciate any advice regarding carb setup, jetting etc. This particular bike has had a very heavy head skim job done by its previous owner, so compression is now more than the 10.7:1 as touted by Wiseco.

All carb settings such as float levels, air screw positions are measured and correct but bke runs very rich with 115 main jets and needles in centre position. (4 in 1 pipe is fitted).

I have recently tried dropping needles by two notches to correct the mid-range richness but I would like to hear from someone who has actually done this mod so that I can reduce all the experimenting.

PS: Is it normal to be able to feel a pronounced pulsation on the fuel line from the tank after such a conversion?

thanks

Rob

 

[Billy Ricks]

This may not address your problem, but Mikuni recommends the same jet settings regardless of state of tune when it comes to their RS series flatslide carbs. You might actually have an ignition timing problem. Retarded timing will produce a rich condition. As for a pusating fuel line, that probably has to do with engine vibration or some other condition. There is no fuel pump and the carbs are gravity fed so no reason for fuel flow to cause pulsating.

 

[Anonymous]

Theory states that it is unlikely you should need to change jetting as a consequence of changing capacity. What happens is that because of the change in capacity, there is a slight increase in gas velocity through the carb choke. This will probably be within the efficient range of the carb so no changes will be necessary.
The bigger pistons are drawing in more mixture, not just more air.
I have experience in this fitting 1085 Wiseco kits to GS1000's. No changes to jetting are necessary (unless other mods are done at the same time.)
The change in CR again is in theory not likely to require jetting changes.
It is no wonder the bike runs rich with 115 mains. Although mains are mainly full throttle, there is some overlap. Large changes in MJ will affect the mid range too. It is common to have to change the needle position after setting up main jet size, despite the fact that theory says it isn't necessary.
I would go back to stock settings and start there. There is a small chance that you may need a small change to compensate for all the changes you have made.

The other thing sounds like you are looking at the vacuum line from carbs to fuel tap. This will pulse. If you are looking at the fuel line and that is pulsing, then your float bowls cannot be vented to atmosphere as they should be.

Hope this helps.

 

[Anonymous]

Carb tuning with Wiseco big bore kit

Thanks for the replies!

The standard jets are 100 for the 79 750. The previous owner fitted a 4 in 1 pipe and put in 102.5 jets which sounds reasonable. I then made the (rash) decision to up the jets to 115 after the 844 conversion but I think you are probably right in your suggestions that I return to original settings and try incremental changes from there as required.

As far as the pulsing on the fuel line is concerned, this is very apparent on both the tank-to-carb vacuum line as well as on the tank fuel outlet line. The vacuum line connects to carb#3 if which I assume is correct.

I can't find any reference inthe Haynes manual about venting the carb bowls to atmosphere. How would I go about checking this situation?

regards

Rob

 

[Anonymous]

Work your way from the top!
Increase in cc:s may not need other jets, but increased comp.ratio most definetely does. You should begin with the main jets. A perfect mixture at full throttle and RPM is what you want to start with, then you worry about the midrange. Adjusting the needles until you get the right mixture and finally the low range. I have found that the best thing to do is to set the screws to factory default and change the idle jets(dont know exactly what they are called, but the sit next to the mainjet) until you have the right mixture there too. I have spent many summers trying to get my carbs right and this is what gave me a very good result even good rideability with the old Mikuni SB33:s wich are killers to adjust right in all ranges

 

[Anonymous]

Increase in cc:s may not need other jets, but increased comp.ratio most definetely does.

Only to prevent detonation. i.e. as a side effect, not because of the change itself, but because of its undesireable consequences. I did say in theory. In practise, it is often convenient to richen the mixture beyond the perfect stoichimetric to increase piston crown cooling and reduce burn temperature.

For relatively small changes in CR, jetting changes shouldn't be necessary. For example I have run a GS1000 with 1085 Wiseco kit (10.25:1 nominal, probably slightly less in reality) and Vance and Hines 4:1, stock airbox, which was slightly rich on stock main jet. 89 bhp at wheel on the dyno.

As for the pulsing; (a) don't worry about it too much but (b) check that your float bowl vents are clear, there are 2 brass spigots outboard of the vac line for the fuel tap, on carbs 2 and 4 (I think) . Just blow through them.

 

[Anonymous]

Carb tuning with Wiseco big bore kit

Thanks again for the advice.

I have now returned to 105 main jets and needles inthe centre position. The 115 jets with fully lowered needles did cure the richness problem (plugs are now light brown), but I think that returning to first principles is the right way to go. ( I haven't yet ridden the new config).

The previous setup of 115 jets and lowered needles gave the bike a distinct 3 cylinder sort of roughness which was definitely not right.

I Would like some clarification on one point:

The vacuum line from the tank is connected to a nipple on carb#3 which pulls a very high suction (hence my mention of pulsating vacuum and fuel lines previously).

Carbs 2 and 4 have a similar nipple which pulls no suction at all.

Carb 1 has no corresponding nipple

If I open all 4 float bowls thereare holes in the top of the bowl in carbs 2 and 4 which are open. (I can blow through them and air exits the abovementioned nipples)

Carb 3 has a plugged hole in the same place

Carb 1 has a hole but it is blocked (no nipple)

Is this configuration correct and standard? This is my first attempt at fiddling with carbs in any detailed way so if this is a stupid question please bear with me.

(The carb bowl vents are very definitely open on all 4 carbs btw)

thanks

Robert

 

[Anonymous]

The set up as you describe it is correct. If memory serves, 1 and 3 will be vented via some link to 2 and 4.
If the bike runs OK I wouldn't worry about it.