The elusive surge

[400 Cafe Racer]

Hey Dennis, thanks for all that sweet sweet info, I think I will do the vacuum gauge sync next time working on the bike. Seems like it will clear up more questions and hopefully solve my issues.

I was actually wondering if there is a proper methodology for the mixture screw settings. I know that the look of the plugs is a telltale sign but I was wondering how precise it gets without requiring an IR exhaust gas probe.

Cheers!

Hi  thanks Alex, you are welcome,

I would say that the if your bike is standard spec regarding air intake, air filter design and exhaust system, that the standard procedures and guides documented for carb settings are the best to use. The plug colour can be a useful guide for normal settings.

In any event the matching of equal slide heights from idle to full throttle is desireable to ensure equal fuel/air volume to all cylinders and in theory deliver equal power from each cylinder. There inevitably could be small variations on position of the idle mixture screw position between individual cylinders for final setting, due to small variations in manufacture. This is fundamental to all tuning.

On my 400F the air filters and exhaust are non standard and I have an Innovate Air / Fuel Ratio guage already fitted which was very useful during re-jetting and setting up under actual running conditions on the road. The settings I have used are to deliver best power, torque and engine temperature, with no specific regard to economy or plug colour.

I have assumed that all is OK with the Ignition advance unit and that if you have standard ignition, that all your contact breaker points, condensers and other factors are correct.

As Ted has hinted, its worth checking that all is OK with the static timing and centrifugal advance unit. Look for anything that could cause variation. Seized or stiff action on the spindle or weights, preload on the springs with no looseness, correct full advance as Bryan says at 2500 RPM

I am sure with patience and perseverence you will nail the problem - and keep us up to date with how you get on. 

Dennis

[daz-j]

Thanks Alex, yes I have checked the float heights previously but I'm thinking maybe under-estimated how accurate they need to be, so will be revisiting that when I reassemble the carbs with the new O rings that arrived today (thanks Julie).

Really interesting to read the viewpoints on carb balancing thank you.  I've bench sync'd previously before refitting, pretty confident that I did it correctly but will go through it carefully again and report back. 

[philward]

On my 400F the air filters and exhaust are non standard and I have an Innovate Air / Fuel Ratio guage already fitted which was very useful during re-jetting and setting up under actual running conditions on the road. The settings I have used are to deliver best power, torque and engine temperature, with no specific regard to economy or plug colour.

[/quote]

Dennis, I'd be interested to hear how the Air/Fuel physically fits on a 4 cylinder 4 carb bike?

[Lobo]

… the easiest way to rule out an issue wrt fluctuations in the timing would be to ‘jam’ the advance mechanism to fully regarded and run the bike at idle.

[400 Cafe Racer]

On my 400F the air filters and exhaust are non standard and I have an Innovate Air / Fuel Ratio guage already fitted which was very useful during re-jetting and setting up under actual running conditions on the road. The settings I have used are to deliver best power, torque and engine temperature, with no specific regard to economy or plug colour.

Dennis, I'd be interested to hear how the Air/Fuel physically fits on a 4 cylinder 4 carb bike?
[/quote]


Hi Phil,

To avoid distraction from Alexs' post I have put a link to my post showng recent guage additions. I have just added a final one to show where the sensor is fitted in the intermediate pipe.

Dennis
http://www.sohc.co.uk/index.php/topic,26987.0.html

[philward]

On my 400F the air filters and exhaust are non standard and I have an Innovate Air / Fuel Ratio guage already fitted which was very useful during re-jetting and setting up under actual running conditions on the road. The settings I have used are to deliver best power, torque and engine temperature, with no specific regard to economy or plug colour.

Dennis, I'd be interested to hear how the Air/Fuel physically fits on a 4 cylinder 4 carb bike?

Hi Phil,

To avoid distraction from Alexs' post I have put a link to my post showng recent guage additions. I have just added a final one to show where the sensor is fitted in the intermediate pipe.

Dennis
http://www.sohc.co.uk/index.php/topic,26987.0.html
[/quote]
Thanks Dennis - seems its not pracrical for me - My 500/750 have standard exhausts and my CR750  has 4 into 4 race exhaust. (all of which I'm not prepared to weld bits on obvious reasons) - thanks anyway

[daz-j]

Just to report back I seem to have cured the problem  - I took the carb bodies to work and gave them a good blast with compressed air, fitted the new seals, and reset the float heights.  So one (or a combination?) of these made the difference - my gut feeling is the float heights were the culprit.

Thanks Alex and everyone, hopefully I won't need to take those carbs off again anytime soon!

[Nurse Julie]

Just to report back I seem to have cured the problem  - I took the carb bodies to work and gave them a good blast with compressed air, fitted the new seals, and reset the float heights.  So one (or a combination?) of these made the difference - my gut feeling is the float heights were the culprit.

Thanks Alex and everyone, hopefully I won't need to take those carbs off again anytime soon!

I'm pleased you have them sorted Darren, well done.

[400 Cafe Racer]

Just to report back I seem to have cured the problem  - I took the carb bodies to work and gave them a good blast with compressed air, fitted the new seals, and reset the float heights.  So one (or a combination?) of these made the difference - my gut feeling is the float heights were the culprit.

Thanks Alex and everyone, hopefully I won't need to take those carbs off again anytime soon!

Hi Alex,

Pleased it was straight forward to find and that you got it fixed .

All the best

Dennis

[Alexrayz]

Hey everyone,

Just giving an update on the situation: Still not working atm.

The last actions taken were as follows: Did a vacuum sync which gave good results, smoother idle and no surges (or at least none happened while I was testing in in the shop). I still had revs hanging after giving the slides a quick lift. Actuating the slides the other way helped bringing it down. New issue though: Even without the idle screw acting on the slides the tick over is 2500RPM. No good.

My guess while vacuum syncing I lifted the slides too much. So I decided to get the carbs off again and redo the bench sync.

Did just that, also reajusted the choke's butterflies as they were not even. Put the carbs on again and it ran like crap again. Surges crazily and when it doesnt it can barely hold tick over at 1000rpm. Acting on the idle screw results in instant surges to 2500/3500 rpm. Still managed to sort of sync it at 2500rpm but without improvement.

Tried to lift the slides with the cable grip and it killed the engine immediately. I gave up after that.

I am starting to consider seeing a mechanic as I feel this is beyond my capabilities and knowledge.

I really struggle to see a consistency in the behavior and the right actions to take.

[K2-K6]

Its this proceedure referred to above http://www.sohc.co.uk/index.php/topic,17769.0.html have you worked in this sequence as it deals with exactly the problem you describe ?

Effectively, it resets the initial relationship between the slide position in the carb venturi and where the main idle speed adjust screw interacts with the throttle lift system components.

If the two conflict, then you'll never get it to run consistently.

[Alexrayz]

I did indeed! Even found an actuall drill bit that is 0.15mm (1/8")

[Bryanj]

1/8 inch is 3mm not  0.15mm

[Alexrayz]

1/8 inch is 3mm not  0.15mm

Right. 1/16" then. Still not accustomed to imperial measurements, but it was 0.15mm thick measured on the engine side.

[K2-K6]

I did indeed! Even found an actuall drill bit that is 0.15mm (1/8")

Apologies for labouring this point, but it's fundamental to identifying why you are having difficulty.  If you complete this successfully you should be able to wind the "idle" screw down far enough for there to be no real gap under the slides when you look through the carburettor, this is correct and should be able to effectively almost stall the engine when it's running.

It's then important that you leave them alone (don't now synchronise with guages) to further assess the problem. This effectively separates different causes to let you find the real problem. 

Then start the engine when reassembled for 20 seconds and switch off. Now check to see if all cylinders have hot exhaust which would indicate they all fire straight away. Any cold pipes and you'll need to find why that cylinder is not firing. All ok ? Move to next check.

Then, with it running, complete a spray test on inlet tracts to see if the engine speed alters (spray wd40 or similar around carb to head rubbers) this should not change the engine speed,  but if it does it will confirm there's a leak somewhere that is allowing air to enter the engine after the throttle slides which are supposed to be the only control.