flat spot

Post by **ebdl** » 26 Jan 2006 08:24 pm

I've mentioned various problems with my carb before. I bought a service kit from Volvo and I replaced the filter, needle valve and gasket and removed, cleaned and refitted all the jets. I also cleaned up the contact on my idle solenoid. I also removed and replaced the mixture screws in a highly non-technical manner. What I now have is a car that idles nice and smoothly without too many revs (no more problems getting the car into gear), but I still have a flat point just as I open the throttle. Unless I give it a lot of welly the car stalls. Traffic jams are a real PITA because I cannot creep along at low revs - it's a bit all or nothing. Any ideas?

Cheers, Ed

Post by **Andy's-Old-Beast** » 26 Jan 2006 08:51 pm

Well, I'm not really an expert on carbs, but I have 2 theories. (I'm sure people with a bit more knowlege will confirm or laugh at them).

1) When the accelerator is pressed a little pump should squirt an extra bit of fuel down the carb, to compensate for the sudden increase in air. Open the throttle by hand at the carb and see if it's working.

2) In my experience a leak in a vacuum hose can affect the vacuum advance on the ignition timing so the timing can be out until the engine catches up speed.

That's where I'd look first, but I'm sure the others will have input.

Hope it helps, cheers.

Post by **5lab** » 26 Jan 2006 09:57 pm

no power at realy low revs sounds like an air leak somewhere. if you boot it does the power suddenly thunk in suddenly? if so the carbs loose (unlikelY) if not.. i'd think its most likely somewhere like the rocker cover

Post by **huskyracer** » 26 Jan 2006 11:50 pm

How did you adjust the idle mixture? as a start, get the engine hot, and switch it off. wind in the mixture screw untill it stops, (dont go mad, just lightly) then wind it out 2 full turns. start the engine, and with it idleing turn the mixture screw out slowly untill it idles best/highest. now slowly wind it back in untill the engine "just" starts to slow down. now set the idle speed to the right setting on the idle speed screw, and carry out the above steps again. once you have done that see how the car drives. you may need to wind the mixture screw out slightly to get it just so, but no more than half a turn, any more means a problem else were, ie as 5lab said, an air leak etc.
By the way, pm me your address if you still want that carb !

Post by **pettaw** » 27 Jan 2006 12:07 am

huskyracer wrote:How did you adjust the idle mixture? as a start, get the engine hot, and switch it off. wind in the mixture screw untill it stops, (dont go mad, just lightly) then wind it out 2 full turns. start the engine, and with it idleing turn the mixture screw out slowly untill it idles best/highest. now slowly wind it back in untill the engine "just" starts to slow down. now set the idle speed to the right setting on the idle speed screw, and carry out the above steps again. once you have done that see how the car drives. you may need to wind the mixture screw out slightly to get it just so, but no more than half a turn, any more means a problem else were, ie as 5lab said, an air leak etc.
By the way, pm me your address if you still want that carb !

What a perfect way of describing how to tune a carb by ear! Can't add to that

Make sure also that all the vacuum hoses are put back exactly as they were before.

Also does this carb have the constant CO idling system with the volume screw or whatever its called? If it does have this system, you'll need to do a basic setting up first. Screw in the volume screw lightly, and then set the idle to be 600 RPM. Then readjust to 850-900 with the volume screw. Then reset the mixture again.

Post by **ebdl** » 27 Jan 2006 12:24 am

Thanks for the replies.

Quick q's:

does 2 full turns = 720 degrees (I presume it does)?
After I've set the idle speed screw do I go right back to the start (i.e. mixture screw all the way in)?

If Haynes is to be believed, then this carb should have the constant CO idling system. I'm now a bit confused because I thought this screw was the idle speed screw

There is no idle speed screw marked on the Haynes diagram, just mixture and volume.

Ed

[edit - in answer to "how did I set up the idle mixture..." erm... I just reinstalled the carb, crossed my fingers and started the engine.

And it ran better than it ever has before

]

Post by **pettaw** » 27 Jan 2006 12:33 am

The idle speed screw will have a tamper proof cover on it and will be just near throttle linkage where it contacts the carb body.

Post by **5lab** » 27 Jan 2006 09:15 am

if you're getting the idle screw (the haynes manual is crap at its diagrams), you will see the accellerator cable be pulled slightly when you screw it all the way in.

Website · Post by **antiekeradio** » 27 Jan 2006 09:58 am

if you have a CO-constant system on it, best bet is to bin the carburettor and put your renewed parts in a more modern type... these run much better