CB750 rejetting... not a new topic

[mgy66]

Hi, this is my first post as I am going through some rough time with my 750k1 (1970).

Question: Bike has been fitted with 810cc kit. Do I need to resize the pilot jets #40?

Here is the long story: I have bought the bike a year ago and I never got to start it properly.

I have refitted new coils with a professional and have inserted new pilot jets.

Diagnostic is that choke (up=open/choke activated) would not help at all the start up. I have to open the idle by one turn to hope getting the bike to start. If successful (probably after half load of a battery), then the idle is about 1000rpm for say 30sec then moving up to 3000 rpm. Once running and hot, I have to turn the idle screw back to get to 2000 rpm but very sensitive and can not keep the 1000 rpm (would stall).

Bike is running ok apart from start and idle. Carb balance checked and air screw checked (velocity stacks now but also tried original filter).

Checked the carb bowls, clean no residue. I have also tried new pilot jets #40 but redrilled at .60 mm hoping that the increase in cc would require an increase in the pilot tip... Could be to much though (black residue on the spark plugs). Having said that, the increased piston diameter should create more depression and increase the flow throuth the tip, hence not sure if I need to increase the tips in the same proportion.

Tryied original spark p. and platinium type. All are rather black on the outer diameter but the center is brown and when running, the bike has good response.

Any suggestion?
Thanks
(Pictures to come)

[Trigger]

Welcome to the crazy world of the SOHC. Here is a past thread that you may find useful >>http://www.sohc.co.uk/index.php/topic,7690.0.html 

[philward]

Hi and welcome
I started that thread and question on jetting for my 836 std compression big bore kitted K2.  Bikes done about 250 miles now and running perfect on 110 mains, std pilot and airbox/exhausts. Air screw is 1 1/8 out. All new Keyster carb kit (jets/needles). Still running in so don't know how its like higher up rev range.

Trigs your man with the experience of building big bore kitted engines I think but have you checked all obvious stuff like air leaks, balancing, timing, etc.

Good luck anyway

Phil

[mgy66]

Thanks Trigger and Philward... BTW very nice pictures... I was thinking blue too until my wife said orange. Link below to some pictures (though now my mudgard was nicked on the street )
https://www.dropbox.com/sc/66bhwlgzbi6et68/AAA68LOuRkIR92VGzSyyGOwGa

Balance checked, clear fuel and fuel filter, air screw at 1. Had some fuel leakage into the airbox when replacing the pilots and changed the tightness on the floatters... resulting in some low level at start up or idle causing troubles. Do you know how to adjust the floatters? From Haynes manuel, I recall it says 26mm but not sure from where to measure. Is it from the joint where the bowls sit to the bottom of the floatters? (i.e. floatting valves in close position).

Tomorrow I will start the bike from main stand (I usually have it on the side which can decrease even further the imersion of the pilot)... It's got to be a 'no fuel in' situation...

Cheers
Mark

[mgy66]

Ok, so i dug a bit more on the carb levels and bent the floaters to get 26mm. I understand that it means 26mm when the floaters are up (valves closed).  I have now a much better success at starting it though i still need to open the idle screw by 1 full turn.

Once it has started, the engine move from 1000 rpm to 3000 rpm in the first 5 to 10 seconds at which point i turn back the idle screw to adjust the rpm to 1000. I have to further turn it down after few minutes when the bike is hot. It seems the choke is pretty useless to start the bike.

Bringing the levels up helps also steadying the idle, i can almost last a full red light without reving up the engine...

I have also increased the tip of the jets both idle and main. My view is that as the combution chambers increase by 10%, the volume of fuel to feed needs to increase by the same ratio. Given the bore have also increased by 10%, i have increased by the same ratio the inner diameters of the jets. I have noticed that the pilot jet no 40 are actually 0.35. So I have used .40 drilling rod for the pilots. The air screw are now at 2 turns but may still be to lean (I have a bit of a gap when opening the throttle from idle).

Next plan is to replace the breakers by electronic ignition. That may help further the tick over but I leave it for another post...
Best,
Marc

[martin_uk]

I will be going through the delights of finding best set up on my 836 soon.

Carburation is a complex issue and acheiving best set up will vary depending on preferred use.

The main reason electronic fuel injection is so good, is the ability to vary fuel delivery to achieve the best fuel ratio, using an oxygen sensor in exhaust to provide a constantly changing mix to suit the varying throttle openings.

If you are using pods or open intakes, mixture will weaken and will need to be richened.

Worst case is a very weak mixture that causes piston overheating and can burn a hole in it, so better to err on richer side, especially at high sustained revs !

[mgy66]

Thanks Martin, I actually went through redrilling up by 50micron the pilot and the main jets due to big bore. In addition, I have reset the air screw to 2.0.

I am checking the spark plugs and response (still have a slight gap so probably still too rich).

Actually, I also went through buying a infra-red thermometer. Did anyone tried that?

The idea is to measure the heat at the exhaust connection with the engine. I have set the air screws to get 180C but not sure if that is the right temp. I can see that it definitely reach the 220 which I am trying to avoid. I have actually noticed that despite having all the air screws at 2.0, one exhaust was running at 220C!

I think the OEM book provides temp indication for the oil but not so much for the engine. In all case the temp measure is very quick and repeatable without having to open up the spark plugs which can on the long term also damage the alu threads. Any idea what should be the combution temp?

Cheers,
Marc

[martin_uk]

Thanks Martin, I actually went through redrilling up by 50micron the pilot and the main jets due to big bore. In addition, I have reset the air screw to 2.0.

I am checking the spark plugs and response (still have a slight gap so probably still too rich).

Actually, I also went through buying a infra-red thermometer. Did anyone tried that?

The idea is to measure the heat at the exhaust connection with the engine. I have set the air screws to get 180C but not sure if that is the right temp. I can see that it definitely reach the 220 which I am trying to avoid. I have actually noticed that despite having all the air screws at 2.0, one exhaust was running at 220C!

I think the OEM book provides temp indication for the oil but not so much for the engine. In all case the temp measure is very quick and repeatable without having to open up the spark plugs which can on the long term also damage the alu threads. Any idea what should be the combution temp?

Cheers,
Marc

Depending on the carb, the air screw/mixture screw may weaken mixture as you unscrew as it controls air not fuel. The PD carb fitted to late SOHC 750's is the opposite 

Weak mixture causes high exhaust temperature.

Without cooling fans you are limited how long you can run statically before overheating occurs.

An infra red thermometer is good for giving rough idea, but engine revs increase is the best indicator for adjusting at idle, with maybe a slight richening to help transition from idle/slow circuit on take off.

[Spitfire]

When I fitted a 900cc kit to my CB750K1 I upped the main jets one size, raised the needle one notch and ran without a filter in the airbox worked great (plus a pretty noisy Dunstall 4 into 1).

Cheers

Dennis