500 carb accelerator pump settings

G

Giles C

New member
Location
United Kingdom
Hi again,

When I wind on my throttle gently my 500 will spin up nicely, but when I twist it aggressively the engine bogs, then will respond if I throttle back a bit.
The bike seems low on power (from memory of 20 years ago mind).
After reading up I have concluded it may be the accelerator pump. (The rest of the carbs are stock, no air leaks, standard plugs and air filter, standard exhaust. Plugs look pretty good, one cylinder slightly leaner than other but not much).
After reading the manual it mentions the pump, but doesn't say which way the screw should be turned.
Questions are:
Am I even right? Is is something else?
Which way should I turn the screw for more/less fuel?
Should I look for any other symptoms?
Thanks a lot
Giles C
 
Pull the air filter and look into the carb,wacking the throttle open should result in a water pistol type stream of fuel squirting into the engine.Easy to see .If the bike has sat for a while these tend to stop working.
 
Giles C said:
Hi again,

When I wind on my throttle gently my 500 will spin up nicely, but when I twist it aggressively the engine bogs, then will respond if I throttle back a bit.
The bike seems low on power (from memory of 20 years ago mind).
After reading up I have concluded it may be the accelerator pump. (The rest of the carbs are stock, no air leaks, standard plugs and air filter, standard exhaust. Plugs look pretty good, one cylinder slightly leaner than other but not much).
After reading the manual it mentions the pump, but doesn't say which way the screw should be turned.
Questions are:
Am I even right? Is is something else?
Which way should I turn the screw for more/less fuel?
Should I look for any other symptoms?
Thanks a lot
Giles C
Click to expand...

Perhaps the pumps are delivering too much fuel... ::)

3cc @ 20 strokes iIrc.  Winding out the adjuster screw increases the pump volume.

Increasing the pump output won't have much effect on usable power, they are only meant as an aid during hard acceleration.  Engine should pick up cleanly and smoothly on a slow throttle opening.  If it doesn't, review idle jet, slide, needle jet and needle, at best renew the whole caboodle.  Check float heights, incorrect heights will influence fueling over the entire range!  Make sure the choke slides are seating correctly.  The newer choke slides(with the plastic button instead of brass) available for the 500 carbs require a little reworking of the actuation mechanism to let them seat completely.

piet
 
You might want to check your jetting as by what you are saying sounds to me that the mains are too big also if the bike has been untouched for years then chances are not set up for modern fuels.
There are some very good articles around on setting up Dellorto's try here

http://www.thisoldtractor.com/guzzitech.dk/gb_en_technical_carburetor-dellorto-manual.htm
 
Thanks for the advice all.
That site's fantastic..
The bike certainly hasn't been set for unleaded. In fact I think it's not even on super unleaded at the moment.
Do you lot add an additive?  If so what's best?
 
Forget additives. Most of them are a rip-off that achieve nothing. Just tune the bike to perform the best it can on the fuel available. You may need to spend a bit of time on a dyno playing around with jetting and ignition advance to see what works best. If you have a high compression engine, go for the highest octane pump fuel available.
 
Hmm, snake oil eh?

I substituted the unleaded for Super unleaded this afternoon, and it's made problem more noticeable. It will still spin up all the way but only on about 1/4 throttle; -anymore and it's stuttering.

I did a plug chop after hard acceleration and both were very rich.

I recently refurbished the carbs with a new gasket set and the right jets (the original ones were for the Monty exhaust). So I think the float height / needle type and position and emulsion tubes are OK

So I thought before I rip the carbs off again I'd look at the other options. I checked the ignition timing and the RH cylinder is a fair bit (about an inch, sorry Yorkshireman accuracy here..) advanced. I went to move the block to find it's as far anticlockwise as it can go.
Should I open up the mounting holes and move it back or do you think it's a symptom of something else?

I did wonder about the air filter. I cleaned it out, but didn't oil it as apparently they're dry filters. Has anyone substituted something else? I reckon mine's about 30 years old.

Thanks again

GC
 
I'm not familiar with the ignition setup on the 500s, but you really need something a bit better than Yorkshireman accuracy. A strobe and degree wheel would be useful. If there's not enough adjustment in the pickup plate then the rotor may have shifted on the crankshaft.
 
Giles C said:
Hmm, snake oil eh?

I substituted the unleaded for Super unleaded this afternoon, and it's made problem more noticeable. It will still spin up all the way but only on about 1/4 throttle; -anymore and it's stuttering.

I did a plug chop after hard acceleration and both were very rich.

I recently refurbished the carbs with a new gasket set and the right jets (the original ones were for the Monty exhaust). So I think the float height / needle type and position and emulsion tubes are OK

So I thought before I rip the carbs off again I'd look at the other options. I checked the ignition timing and the RH cylinder is a fair bit (about an inch, sorry Yorkshireman accuracy here..) advanced. I went to move the block to find it's as far anticlockwise as it can go.
Should I open up the mounting holes and move it back or do you think it's a symptom of something else?

I did wonder about the air filter. I cleaned it out, but didn't oil it as apparently they're dry filters. Has anyone substituted something else? I reckon mine's about 30 years old.

Thanks again

GC
Click to expand...

"Thinking" is not "knowing"...

The mounting holes of the ignition pick-ups can be elongated slightly to get the marks on the flywheel to line up at full advance if needed.  Equally important is to set the pick up/rotor gaps equally for both cylinders.

If you're still running the stock ignition system, you might find one or both cylinders not advancing correctly, quite a common ailment of the old Bosch ignition boxes.  Get a substitute or replace the ignition system with a DMC or similar.

piet
 
Dellortoman said:
I'm not familiar with the ignition setup on the 500s, but you really need something a bit better than Yorkshireman accuracy. A strobe and degree wheel would be useful. If there's not enough adjustment in the pickup plate then the rotor may have shifted on the crankshaft.
Click to expand...

Haha I'm not that much of a luddite! I used a strobe.. what I mean is that the timing mark is about an inch before the crankcase mark. That's at 3000rpm.  (Full advance). What my manual recommended anyway.
The pickups have limited movement across but virtually no movement up and down. I don't think I can adjust the gap. It was 1mm though so all good there.
The float height I set according to the manual. 18mm I think.

 
OK, are you any good at arithmetic?

Divide 360 by the perimeter of the rotor (diameter x pi) then multiply that by your advance measurement of one Yorkshire inch  :D

As an example: if your alternator rotor is 4" diameter, that's 12.57" around the perimeter. Divide 360 by 12.57 and you get 28.65 degrees per inch.

I suspect your alternator rotor is bigger than 4", so you might only be getting around 20? advance. You may find you want more advance rather than less. I don't know what the optimum advance should be, but if the engine is reasonably un-molested, just set it to the spec given in the manual.
 
Not much use Cam...

The 500s have the pick-ups bolted directly to the clutch cover, no rotating mounting plate that could be adjusted.  The pick-up mounts have a tiny amount of adjusting range, for both the gap and the timing, a bit fiddly.

The timing marks on the flywheel are usually quite precise, I suspect one or both boxes are not advancing sufficiently (1 1/4" is about the distance from full retard to full advance).  Pick-up gap has a small influence on WHEN the advance occurs, but at 3000rpm all should be evened out.

Perhaps the fit of the flywheel to the crank should be checked, not unknown to find a partially-sheared locating key... :D  If TDC(PM) marks line up with piston TDC (simple screwdriver-down-the-plughole check), all should be OK.

piet
 
Giles C said:
I did a plug chop after hard acceleration and both were very rich.
I recently refurbished the carbs with a new gasket set and the right jets (the original ones were for the Monty exhaust). So I think the float height / needle type and position and emulsion tubes are OK
I did wonder about the air filter. I cleaned it out, but didn't oil it as apparently they're dry filters. Has anyone substituted something else? I reckon mine's about 30 years old.
Click to expand...
Sounding that you are way too rich...also premium unleaded burns slower so if you are running rich it will cause you fouling issues for sure. Better not to use premium unless you are sorted on your jetting etc..
Easy way to test if the air filter is the issue, take it out and take it for a run without it. What size mains did you put in, what are the needles and what are the needle settings.
500's were around 115 mains when they rolled out of Breganze.. nowdays you would be looking at around 100-105 maximums I would think.
 
That's very interesting. I had assumed the higher the octane the better.
Everything is stock alpino s. Slaters sent a kit. After your reply I called Richard and he suggested the exact same thing. Go down 5 to 110.
I will try the old no filter trick. Even just for the intake roar
 
Brett said:
...also premium unleaded burns slower.
Click to expand...

I think that's what is supposed to happen Brett!
If I am right (happens once in a 100 times) If fuel burns too quick you don't get the full benefit of the expanding gas over the down stroke. Quick burning can lead to pre-ignition in extreme circumstances?
 
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